





Network Working Group                                    P. Calhoun, Ed.
Request for Comments: 5415                           Cisco Systems, Inc.
Category: Standards Track                             M. Montemurro, Ed.
                                                      Research In Motion
                                                         D. Stanley, Ed.
                                                          Aruba Networks
                                                              March 2009


      Control And Provisioning of Wireless Access Points (CAPWAP)
                         Protocol Specification

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
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   Please review these documents carefully, as they describe your rights
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
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   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.









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RFC 5415             CAPWAP Protocol Specification            March 2009


Abstract

   This specification defines the Control And Provisioning of Wireless
   Access Points (CAPWAP) Protocol, meeting the objectives defined by
   the CAPWAP Working Group in RFC 4564.  The CAPWAP protocol is
   designed to be flexible, allowing it to be used for a variety of
   wireless technologies.  This document describes the base CAPWAP
   protocol, while separate binding extensions will enable its use with
   additional wireless technologies.

Table of Contents

   1. Introduction ....................................................7
      1.1. Goals ......................................................8
      1.2. Conventions Used in This Document ..........................9
      1.3. Contributing Authors .......................................9
      1.4. Terminology ...............................................10
   2. Protocol Overview ..............................................11
      2.1. Wireless Binding Definition ...............................12
      2.2. CAPWAP Session Establishment Overview .....................13
      2.3. CAPWAP State Machine Definition ...........................15
           2.3.1. CAPWAP Protocol State Transitions ..................17
           2.3.2. CAPWAP/DTLS Interface ..............................31
      2.4. Use of DTLS in the CAPWAP Protocol ........................33
           2.4.1. DTLS Handshake Processing ..........................33
           2.4.2. DTLS Session Establishment .........................35
           2.4.3. DTLS Error Handling ................................35
           2.4.4. DTLS Endpoint Authentication and Authorization .....36
   3. CAPWAP Transport ...............................................40
      3.1. UDP Transport .............................................40
      3.2. UDP-Lite Transport ........................................41
      3.3. AC Discovery ..............................................41
      3.4. Fragmentation/Reassembly ..................................42
      3.5. MTU Discovery .............................................43
   4. CAPWAP Packet Formats ..........................................43
      4.1. CAPWAP Preamble ...........................................46
      4.2. CAPWAP DTLS Header ........................................46
      4.3. CAPWAP Header .............................................47
      4.4. CAPWAP Data Messages ......................................50
           4.4.1. CAPWAP Data Channel Keep-Alive .....................51
           4.4.2. Data Payload .......................................52
           4.4.3. Establishment of a DTLS Data Channel ...............52
      4.5. CAPWAP Control Messages ...................................52
           4.5.1. Control Message Format .............................53
           4.5.2. Quality of Service .................................56
           4.5.3. Retransmissions ....................................57
      4.6. CAPWAP Protocol Message Elements ..........................58
           4.6.1. AC Descriptor ......................................61



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           4.6.2. AC IPv4 List .......................................64
           4.6.3. AC IPv6 List .......................................64
           4.6.4. AC Name ............................................65
           4.6.5. AC Name with Priority ..............................65
           4.6.6. AC Timestamp .......................................66
           4.6.7. Add MAC ACL Entry ..................................66
           4.6.8. Add Station ........................................67
           4.6.9. CAPWAP Control IPv4 Address ........................68
           4.6.10. CAPWAP Control IPv6 Address .......................68
           4.6.11. CAPWAP Local IPv4 Address .........................69
           4.6.12. CAPWAP Local IPv6 Address .........................69
           4.6.13. CAPWAP Timers .....................................70
           4.6.14. CAPWAP Transport Protocol .........................71
           4.6.15. Data Transfer Data ................................72
           4.6.16. Data Transfer Mode ................................73
           4.6.17. Decryption Error Report ...........................73
           4.6.18. Decryption Error Report Period ....................74
           4.6.19. Delete MAC ACL Entry ..............................74
           4.6.20. Delete Station ....................................75
           4.6.21. Discovery Type ....................................75
           4.6.22. Duplicate IPv4 Address ............................76
           4.6.23. Duplicate IPv6 Address ............................77
           4.6.24. Idle Timeout ......................................78
           4.6.25. ECN Support .......................................78
           4.6.26. Image Data ........................................79
           4.6.27. Image Identifier ..................................79
           4.6.28. Image Information .................................80
           4.6.29. Initiate Download .................................81
           4.6.30. Location Data .....................................81
           4.6.31. Maximum Message Length ............................81
           4.6.32. MTU Discovery Padding .............................82
           4.6.33. Radio Administrative State ........................82
           4.6.34. Radio Operational State ...........................83
           4.6.35. Result Code .......................................84
           4.6.36. Returned Message Element ..........................85
           4.6.37. Session ID ........................................86
           4.6.38. Statistics Timer ..................................87
           4.6.39. Vendor Specific Payload ...........................87
           4.6.40. WTP Board Data ....................................88
           4.6.41. WTP Descriptor ....................................89
           4.6.42. WTP Fallback ......................................92
           4.6.43. WTP Frame Tunnel Mode .............................92
           4.6.44. WTP MAC Type ......................................93
           4.6.45. WTP Name ..........................................94
           4.6.46. WTP Radio Statistics ..............................94
           4.6.47. WTP Reboot Statistics .............................96
           4.6.48. WTP Static IP Address Information .................97
      4.7. CAPWAP Protocol Timers ....................................98



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           4.7.1. ChangeStatePendingTimer ............................98
           4.7.2. DataChannelKeepAlive ...............................98
           4.7.3. DataChannelDeadInterval ............................99
           4.7.4. DataCheckTimer .....................................99
           4.7.5. DiscoveryInterval ..................................99
           4.7.6. DTLSSessionDelete ..................................99
           4.7.7. EchoInterval .......................................99
           4.7.8. IdleTimeout ........................................99
           4.7.9. ImageDataStartTimer ...............................100
           4.7.10. MaxDiscoveryInterval .............................100
           4.7.11. ReportInterval ...................................100
           4.7.12. RetransmitInterval ...............................100
           4.7.13. SilentInterval ...................................100
           4.7.14. StatisticsTimer ..................................100
           4.7.15. WaitDTLS .........................................101
           4.7.16. WaitJoin .........................................101
      4.8. CAPWAP Protocol Variables ................................101
           4.8.1. AdminState ........................................101
           4.8.2. DiscoveryCount ....................................101
           4.8.3. FailedDTLSAuthFailCount ...........................101
           4.8.4. FailedDTLSSessionCount ............................101
           4.8.5. MaxDiscoveries ....................................102
           4.8.6. MaxFailedDTLSSessionRetry .........................102
           4.8.7. MaxRetransmit .....................................102
           4.8.8. RetransmitCount ...................................102
           4.8.9. WTPFallBack .......................................102
      4.9. WTP Saved Variables ......................................102
           4.9.1. AdminRebootCount ..................................102
           4.9.2. FrameEncapType ....................................102
           4.9.3. LastRebootReason ..................................103
           4.9.4. MacType ...........................................103
           4.9.5. PreferredACs ......................................103
           4.9.6. RebootCount .......................................103
           4.9.7. Static IP Address .................................103
           4.9.8. WTPLinkFailureCount ...............................103
           4.9.9. WTPLocation .......................................103
           4.9.10. WTPName ..........................................103
   5. CAPWAP Discovery Operations ...................................103
      5.1. Discovery Request Message ................................103
      5.2. Discovery Response Message ...............................105
      5.3. Primary Discovery Request Message ........................106
      5.4. Primary Discovery Response ...............................107
   6. CAPWAP Join Operations ........................................108
      6.1. Join Request .............................................108
      6.2. Join Response ............................................110
   7. Control Channel Management ....................................111
      7.1. Echo Request .............................................111
      7.2. Echo Response ............................................112



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   8. WTP Configuration Management ..................................112
      8.1. Configuration Consistency ................................112
           8.1.1. Configuration Flexibility .........................113
      8.2. Configuration Status Request .............................114
      8.3. Configuration Status Response ............................115
      8.4. Configuration Update Request .............................116
      8.5. Configuration Update Response ............................117
      8.6. Change State Event Request ...............................117
      8.7. Change State Event Response ..............................118
      8.8. Clear Configuration Request ..............................119
      8.9. Clear Configuration Response .............................119
   9. Device Management Operations ..................................120
      9.1. Firmware Management ......................................120
           9.1.1. Image Data Request ................................124
           9.1.2. Image Data Response ...............................125
      9.2. Reset Request ............................................126
      9.3. Reset Response ...........................................127
      9.4. WTP Event Request ........................................127
      9.5. WTP Event Response .......................................128
      9.6. Data Transfer ............................................128
           9.6.1. Data Transfer Request .............................130
           9.6.2. Data Transfer Response ............................131
   10. Station Session Management ...................................131
      10.1. Station Configuration Request ...........................131
      10.2. Station Configuration Response ..........................132
   11. NAT Considerations ...........................................132
   12. Security Considerations ......................................134
      12.1. CAPWAP Security .........................................134
           12.1.1. Converting Protected Data into Unprotected Data ..135
           12.1.2. Converting Unprotected Data into
                   Protected Data (Insertion) .......................135
           12.1.3. Deletion of Protected Records ....................135
           12.1.4. Insertion of Unprotected Records .................135
           12.1.5. Use of MD5 .......................................136
           12.1.6. CAPWAP Fragmentation .............................136
      12.2. Session ID Security .....................................136
      12.3. Discovery or DTLS Setup Attacks .........................137
      12.4. Interference with a DTLS Session ........................137
      12.5. CAPWAP Pre-Provisioning .................................138
      12.6. Use of Pre-Shared Keys in CAPWAP ........................139
      12.7. Use of Certificates in CAPWAP ...........................140
      12.8. Use of MAC Address in CN Field ..........................140
      12.9. AAA Security ............................................141
      12.10. WTP Firmware ...........................................141
   13. Operational Considerations ...................................141
   14. Transport Considerations .....................................142
   15. IANA Considerations ..........................................143
      15.1. IPv4 Multicast Address ..................................143



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      15.2. IPv6 Multicast Address ..................................144
      15.3. UDP Port ................................................144
      15.4. CAPWAP Message Types ....................................144
      15.5. CAPWAP Header Flags .....................................144
      15.6. CAPWAP Control Message Flags ............................145
      15.7. CAPWAP Message Element Type .............................145
      15.8. CAPWAP Wireless Binding Identifiers .....................145
      15.9. AC Security Types .......................................146
      15.10. AC DTLS Policy .........................................146
      15.11. AC Information Type ....................................146
      15.12. CAPWAP Transport Protocol Types ........................146
      15.13. Data Transfer Type .....................................147
      15.14. Data Transfer Mode .....................................147
      15.15. Discovery Types ........................................147
      15.16. ECN Support ............................................148
      15.17. Radio Admin State ......................................148
      15.18. Radio Operational State ................................148
      15.19. Radio Failure Causes ...................................148
      15.20. Result Code ............................................149
      15.21. Returned Message Element Reason ........................149
      15.22. WTP Board Data Type ....................................149
      15.23. WTP Descriptor Type ....................................149
      15.24. WTP Fallback Mode ......................................150
      15.25. WTP Frame Tunnel Mode ..................................150
      15.26. WTP MAC Type ...........................................150
      15.27. WTP Radio Stats Failure Type ...........................151
      15.28. WTP Reboot Stats Failure Type ..........................151
   16. Acknowledgments ..............................................151
   17. References ...................................................151
      17.1. Normative References ....................................151
      17.2. Informative References ..................................153




















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1.  Introduction

   This document describes the CAPWAP protocol, a standard,
   interoperable protocol that enables an Access Controller (AC) to
   manage a collection of Wireless Termination Points (WTPs).  The
   CAPWAP protocol is defined to be independent of Layer 2 (L2)
   technology, and meets the objectives in "Objectives for Control and
   Provisioning of Wireless Access Points (CAPWAP)" [RFC4564].

   The emergence of centralized IEEE 802.11 Wireless Local Area Network
   (WLAN) architectures, in which simple IEEE 802.11 WTPs are managed by
   an Access Controller (AC), suggested that a standards-based,
   interoperable protocol could radically simplify the deployment and
   management of wireless networks.  WTPs require a set of dynamic
   management and control functions related to their primary task of
   connecting the wireless and wired mediums.  Traditional protocols for
   managing WTPs are either manual static configuration via HTTP,
   proprietary Layer 2-specific or non-existent (if the WTPs are self-
   contained).  An IEEE 802.11 binding is defined in [RFC5416] to
   support use of the CAPWAP protocol with IEEE 802.11 WLAN networks.

   CAPWAP assumes a network configuration consisting of multiple WTPs
   communicating via the Internet Protocol (IP) to an AC.  WTPs are
   viewed as remote radio frequency (RF) interfaces controlled by the
   AC.  The CAPWAP protocol supports two modes of operation: Split and
   Local MAC (medium access control).  In Split MAC mode, all L2
   wireless data and management frames are encapsulated via the CAPWAP
   protocol and exchanged between the AC and the WTP.  As shown in
   Figure 1, the wireless frames received from a mobile device, which is
   referred to in this specification as a Station (STA), are directly
   encapsulated by the WTP and forwarded to the AC.

              +-+         wireless frames        +-+
              | |--------------------------------| |
              | |              +-+               | |
              | |--------------| |---------------| |
              | |wireless PHY/ | |     CAPWAP    | |
              | | MAC sublayer | |               | |
              +-+              +-+               +-+
              STA              WTP                AC

        Figure 1: Representative CAPWAP Architecture for Split MAC

   The Local MAC mode of operation allows for the data frames to be
   either locally bridged or tunneled as 802.3 frames.  The latter
   implies that the WTP performs the 802.11 Integration function.  In
   either case, the L2 wireless management frames are processed locally




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   by the WTP and then forwarded to the AC.  Figure 2 shows the Local
   MAC mode, in which a station transmits a wireless frame that is
   encapsulated in an 802.3 frame and forwarded to the AC.

              +-+wireless frames +-+ 802.3 frames +-+
              | |----------------| |--------------| |
              | |                | |              | |
              | |----------------| |--------------| |
              | |wireless PHY/   | |     CAPWAP   | |
              | | MAC sublayer   | |              | |
              +-+                +-+              +-+
              STA                WTP               AC

        Figure 2: Representative CAPWAP Architecture for Local MAC

   Provisioning WTPs with security credentials and managing which WTPs
   are authorized to provide service are traditionally handled by
   proprietary solutions.  Allowing these functions to be performed from
   a centralized AC in an interoperable fashion increases manageability
   and allows network operators to more tightly control their wireless
   network infrastructure.

1.1.  Goals

   The goals for the CAPWAP protocol are listed below:

   1. To centralize the authentication and policy enforcement functions
      for a wireless network.  The AC may also provide centralized
      bridging, forwarding, and encryption of user traffic.
      Centralization of these functions will enable reduced cost and
      higher efficiency by applying the capabilities of network
      processing silicon to the wireless network, as in wired LANs.

   2. To enable shifting of the higher-level protocol processing from
      the WTP.  This leaves the time-critical applications of wireless
      control and access in the WTP, making efficient use of the
      computing power available in WTPs, which are subject to severe
      cost pressure.

   3. To provide an extensible protocol that is not bound to a specific
      wireless technology.  Extensibility is provided via a generic
      encapsulation and transport mechanism, enabling the CAPWAP
      protocol to be applied to many access point types in the future,
      via a specific wireless binding.

   The CAPWAP protocol concerns itself solely with the interface between
   the WTP and the AC.  Inter-AC and station-to-AC communication are
   strictly outside the scope of this document.



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1.2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.3.  Contributing Authors

   This section lists and acknowledges the authors of significant text
   and concepts included in this specification.

   The CAPWAP Working Group selected the Lightweight Access Point
   Protocol (LWAPP) [LWAPP] to be used as the basis of the CAPWAP
   protocol specification.  The following people are authors of the
   LWAPP document:

      Bob O'Hara
      Email: bob.ohara@computer.org

      Pat Calhoun, Cisco Systems, Inc.
      170 West Tasman Drive, San Jose, CA  95134
      Phone: +1 408-902-3240, Email: pcalhoun@cisco.com

      Rohit Suri, Cisco Systems, Inc.
      170 West Tasman Drive, San Jose, CA  95134
      Phone: +1 408-853-5548, Email: rsuri@cisco.com

      Nancy Cam Winget, Cisco Systems, Inc.
      170 West Tasman Drive, San Jose, CA  95134
      Phone: +1 408-853-0532, Email: ncamwing@cisco.com

      Scott Kelly, Aruba Networks
      1322 Crossman Ave, Sunnyvale, CA 94089
      Phone: +1  408-754-8408, Email: skelly@arubanetworks.com

      Michael Glenn Williams, Nokia, Inc.
      313 Fairchild Drive, Mountain View, CA  94043
      Phone: +1 650-714-7758, Email: Michael.G.Williams@Nokia.com

      Sue Hares, Green Hills Software
      825 Victors Way, Suite 100, Ann Arbor, MI  48108
      Phone: +1 734 222 1610, Email: shares@ndzh.com

   Datagram Transport Layer Security (DTLS) [RFC4347] is used as the
   security solution for the CAPWAP protocol.  The following people are
   authors of significant DTLS-related text included in this document:





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      Scott Kelly, Aruba Networks
      1322 Crossman Ave, Sunnyvale, CA 94089
      Phone: +1  408-754-8408
      Email: skelly@arubanetworks.com

      Eric Rescorla, Network Resonance
      2483 El Camino Real, #212,Palo Alto CA, 94303
      Email: ekr@networkresonance.com

   The concept of using DTLS to secure the CAPWAP protocol was part of
   the Secure Light Access Point Protocol (SLAPP) proposal [SLAPP].  The
   following people are authors of the SLAPP proposal:

      Partha Narasimhan, Aruba Networks
      1322 Crossman Ave, Sunnyvale, CA  94089
      Phone: +1 408-480-4716
      Email: partha@arubanetworks.com

      Dan Harkins
      Trapeze Networks
      5753 W. Las Positas Blvd, Pleasanton, CA  94588
      Phone: +1-925-474-2212
      EMail: dharkins@trpz.com

      Subbu Ponnuswamy, Aruba Networks
      1322 Crossman Ave, Sunnyvale, CA  94089
      Phone: +1 408-754-1213
      Email: subbu@arubanetworks.com

   The following individuals contributed significant security-related
   text to the document [RFC5418]:

      T. Charles Clancy, Laboratory for Telecommunications Sciences,
      8080 Greenmead Drive, College Park, MD 20740
      Phone: +1 240-373-5069, Email: clancy@ltsnet.net

      Scott Kelly, Aruba Networks
      1322 Crossman Ave, Sunnyvale, CA 94089
      Phone: +1  408-754-8408, Email: scott@hyperthought.com

1.4.  Terminology

   Access Controller (AC): The network entity that provides WTP access
   to the network infrastructure in the data plane, control plane,
   management plane, or a combination therein.






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   CAPWAP Control Channel: A bi-directional flow defined by the AC IP
   Address, WTP IP Address, AC control port, WTP control port, and the
   transport-layer protocol (UDP or UDP-Lite) over which CAPWAP Control
   packets are sent and received.

   CAPWAP Data Channel: A bi-directional flow defined by the AC IP
   Address, WTP IP Address, AC data port, WTP data port, and the
   transport-layer protocol (UDP or UDP-Lite) over which CAPWAP Data
   packets are sent and received.

   Station (STA): A device that contains an interface to a wireless
   medium (WM).

   Wireless Termination Point (WTP): The physical or network entity that
   contains an RF antenna and wireless Physical Layer (PHY) to transmit
   and receive station traffic for wireless access networks.

   This document uses additional terminology defined in [RFC3753].

2.  Protocol Overview

   The CAPWAP protocol is a generic protocol defining AC and WTP control
   and data plane communication via a CAPWAP protocol transport
   mechanism.  CAPWAP Control messages, and optionally CAPWAP Data
   messages, are secured using Datagram Transport Layer Security (DTLS)
   [RFC4347].  DTLS is a standards-track IETF protocol based upon TLS.
   The underlying security-related protocol mechanisms of TLS have been
   successfully deployed for many years.

   The CAPWAP protocol transport layer carries two types of payload,
   CAPWAP Data messages and CAPWAP Control messages.  CAPWAP Data
   messages encapsulate forwarded wireless frames.  CAPWAP protocol
   Control messages are management messages exchanged between a WTP and
   an AC.  The CAPWAP Data and Control packets are sent over separate
   UDP ports.  Since both data and control packets can exceed the
   Maximum Transmission Unit (MTU) length, the payload of a CAPWAP Data
   or Control message can be fragmented.  The fragmentation behavior is
   defined in Section 3.

   The CAPWAP Protocol begins with a Discovery phase.  The WTPs send a
   Discovery Request message, causing any Access Controller (AC)
   receiving the message to respond with a Discovery Response message.
   From the Discovery Response messages received, a WTP selects an AC
   with which to establish a secure DTLS session.  In order to establish
   the secure DTLS connection, the WTP will need some amount of pre-
   provisioning, which is specified in Section 12.5.  CAPWAP protocol
   messages will be fragmented to the maximum length discovered to be
   supported by the network.



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   Once the WTP and the AC have completed DTLS session establishment, a
   configuration exchange occurs in which both devices agree on version
   information.  During this exchange, the WTP may receive provisioning
   settings.  The WTP is then enabled for operation.

   When the WTP and AC have completed the version and provision exchange
   and the WTP is enabled, the CAPWAP protocol is used to encapsulate
   the wireless data frames sent between the WTP and AC.  The CAPWAP
   protocol will fragment the L2 frames if the size of the encapsulated
   wireless user data (Data) or protocol control (Management) frames
   causes the resulting CAPWAP protocol packet to exceed the MTU
   supported between the WTP and AC.  Fragmented CAPWAP packets are
   reassembled to reconstitute the original encapsulated payload.  MTU
   Discovery and Fragmentation are described in Section 3.

   The CAPWAP protocol provides for the delivery of commands from the AC
   to the WTP for the management of stations that are communicating with
   the WTP.  This may include the creation of local data structures in
   the WTP for the stations and the collection of statistical
   information about the communication between the WTP and the stations.
   The CAPWAP protocol provides a mechanism for the AC to obtain
   statistical information collected by the WTP.

   The CAPWAP protocol provides for a keep-alive feature that preserves
   the communication channel between the WTP and AC.  If the AC fails to
   appear alive, the WTP will try to discover a new AC.

2.1.  Wireless Binding Definition

   The CAPWAP protocol is independent of a specific WTP radio
   technology, as well its associated wireless link layer protocol.
   Elements of the CAPWAP protocol are designed to accommodate the
   specific needs of each wireless technology in a standard way.
   Implementation of the CAPWAP protocol for a particular wireless
   technology MUST follow the binding requirements defined for that
   technology.

   When defining a binding for wireless technologies, the authors MUST
   include any necessary definitions for technology-specific messages
   and all technology-specific message elements for those messages.  At
   a minimum, a binding MUST provide:

   1. The definition for a binding-specific Statistics message element,
      carried in the WTP Event Request message.

   2. A message element carried in the Station Configuration Request
      message to configure station information on the WTP.




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   3. A WTP Radio Information message element carried in the Discovery,
      Primary Discovery, and Join Request and Response messages,
      indicating the binding-specific radio types supported at the WTP
      and AC.

   If technology-specific message elements are required for any of the
   existing CAPWAP messages defined in this specification, they MUST
   also be defined in the technology binding document.

   The naming of binding-specific message elements MUST begin with the
   name of the technology type, e.g., the binding for IEEE 802.11,
   provided in [RFC5416], begins with "IEEE 802.11".

   The CAPWAP binding concept MUST also be used in any future
   specifications that add functionality to either the base CAPWAP
   protocol specification, or any published CAPWAP binding
   specification.  A separate WTP Radio Information message element MUST
   be created to properly advertise support for the specification.  This
   mechanism allows for future protocol extensibility, while providing
   the necessary capabilities advertisement, through the WTP Radio
   Information message element, to ensure WTP/AC interoperability.

2.2.  CAPWAP Session Establishment Overview

   This section describes the session establishment process message
   exchanges between a CAPWAP WTP and AC.  The annotated ladder diagram
   shows the AC on the right, the WTP on the left, and assumes the use
   of certificates for DTLS authentication.  The CAPWAP protocol state
   machine is described in detail in Section 2.3.  Note that DTLS allows
   certain messages to be aggregated into a single frame, which is
   denoted via an asterisk in Figure 3.

           ============                         ============
               WTP                                   AC
           ============                         ============
            [----------- begin optional discovery ------------]

                           Discover Request
                 ------------------------------------>
                           Discover Response
                 <------------------------------------

            [----------- end optional discovery ------------]

                      (-- begin DTLS handshake --)

                             ClientHello
                 ------------------------------------>



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                      HelloVerifyRequest (with cookie)
                 <------------------------------------


                        ClientHello (with cookie)
                 ------------------------------------>
                                ServerHello,
                                Certificate,
                                ServerHelloDone*
                 <------------------------------------

                (-- WTP callout for AC authorization --)

                        Certificate (optional),
                         ClientKeyExchange,
                     CertificateVerify (optional),
                         ChangeCipherSpec,
                             Finished*
                 ------------------------------------>

                (-- AC callout for WTP authorization --)

                         ChangeCipherSpec,
                             Finished*
                 <------------------------------------

                (-- DTLS session is established now --)

                              Join Request
                 ------------------------------------>
                              Join Response
                 <------------------------------------
                      [-- Join State Complete --]

                   (-- assume image is up to date --)

                      Configuration Status Request
                 ------------------------------------>
                      Configuration Status Response
                 <------------------------------------
                    [-- Configure State Complete --]

                       Change State Event Request
                 ------------------------------------>
                       Change State Event Response
                 <------------------------------------
                   [-- Data Check State Complete --]




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                        (-- enter RUN state --)

                                   :
                                   :

                              Echo Request
                 ------------------------------------>
                             Echo Response
                 <------------------------------------

                                   :
                                   :

                              Event Request
                 ------------------------------------>
                             Event Response
                 <------------------------------------

                                   :
                                   :

                Figure 3: CAPWAP Control Protocol Exchange

   At the end of the illustrated CAPWAP message exchange, the AC and WTP
   are securely exchanging CAPWAP Control messages.  This illustration
   is provided to clarify protocol operation, and does not include any
   possible error conditions.  Section 2.3 provides a detailed
   description of the corresponding state machine.

2.3.  CAPWAP State Machine Definition

   The following state diagram represents the lifecycle of a WTP-AC
   session.  Use of DTLS by the CAPWAP protocol results in the
   juxtaposition of two nominally separate yet tightly bound state
   machines.  The DTLS and CAPWAP state machines are coupled through an
   API consisting of commands (see Section 2.3.2.1) and notifications
   (see Section 2.3.2.2).  Certain transitions in the DTLS state machine
   are triggered by commands from the CAPWAP state machine, while
   certain transitions in the CAPWAP state machine are triggered by
   notifications from the DTLS state machine.











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                            /-------------------------------------\
                            |          /-------------------------\|
                            |         p|                         ||
                            |    q+----------+ r +------------+  ||
                            |     |   Run    |-->|   Reset    |-\||
                            |     +----------+   +------------+ |||
                           n|  o      ^           ^     ^      s|||
                +------------+--------/           |     |       |||
                | Data Check |             /-------/    |       |||
                +------------+<-------\   |             |       |||
                                      |   |             |       |||
                       /------------------+--------\    |       |||
                      f|             m|  h|    j   v   k|       |||
               +--------+     +-----------+     +--------------+|||
               |  Join  |---->| Configure |     |  Image Data  ||||
               +--------+  n  +-----------+     +--------------+|||
                ^   |g                 i|                    l| |||
                |   |                   \-------------------\ | |||
                |   \--------------------------------------\| | |||
                \------------------------\                 || | |||
         /--------------<----------------+---------------\ || | |||
         | /------------<----------------+-------------\ | || | |||
         | |  4                          |d           t| | vv v vvv
         | |   +----------------+   +--------------+   +-----------+
         | |   |   DTLS Setup   |   | DTLS Connect |-->|  DTLS TD  |
       /-|-|---+----------------+   +--------------+ e +-----------+
       | | |    |$  ^  ^   |5  ^6         ^              ^  |w
       v v v    |   |  |   |   \-------\  |              |  |
       | | |    |   |  |   \---------\ |  |  /-----------/  |
       | | |    |   |  \--\          | |  |  |              |
       | | |    |   |     |          | |  |  |              |
       | | |    v  3|  1  |%     #   v |  |a |b             v
       | | \->+------+-->+------+   +-----------+    +--------+
       | |    | Idle |   | Disc |   | Authorize |    |  Dead  |
       | |    +------+<--+------+   +-----------+    +--------+
       | |     ^   0^  2      |!
       | |     |    |         |   +-------+
      *| |u    |    \---------+---| Start |
       | |     |@             |   +-------+
       | \->+---------+<------/
       \--->| Sulking |
            +---------+&

                 Figure 4: CAPWAP Integrated State Machine

   The CAPWAP protocol state machine, depicted above, is used by both
   the AC and the WTP.  In cases where states are not shared (i.e., not
   implemented in one or the other of the AC or WTP), this is explicitly



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   called out in the transition descriptions below.  For every state
   defined, only certain messages are permitted to be sent and received.
   The CAPWAP Control message definitions specify the state(s) in which
   each message is valid.

   Since the WTP only communicates with a single AC, it only has a
   single instance of the CAPWAP state machine.  The state machine works
   differently on the AC since it communicates with many WTPs.  The AC
   uses the concept of three threads.  Note that the term thread used
   here does not necessarily imply that implementers must use threads,
   but it is one possible way of implementing the AC's state machine.

   Listener Thread:   The AC's Listener thread handles inbound DTLS
      session establishment requests, through the DTLSListen command.
      Upon creation, the Listener thread starts in the DTLS Setup state.
      Once a DTLS session has been validated, which occurs when the
      state machine enters the "Authorize" state, the Listener thread
      creates a WTP session-specific Service thread and state context.
      The state machine transitions in Figure 4 are represented by
      numerals.  It is necessary for the AC to protect itself against
      various attacks that exist with non-authenticated frames.  See
      Section 12 for more information.

   Discovery Thread:   The AC's Discovery thread is responsible for
      receiving, and responding to, Discovery Request messages.  The
      state machine transitions in Figure 4 are represented by numerals.
      Note that the Discovery thread does not maintain any per-WTP-
      specific context information, and a single state context exists.
      It is necessary for the AC to protect itself against various
      attacks that exist with non-authenticated frames.  See Section 12
      for more information.

   Service Thread:   The AC's Service thread handles the per-WTP states,
      and one such thread exists per-WTP connection.  This thread is
      created by the Listener thread when the Authorize state is
      reached.  When created, the Service thread inherits a copy of the
      state machine context from the Listener thread.  When
      communication with the WTP is complete, the Service thread is
      terminated and all associated resources are released.  The state
      machine transitions in Figure 4 are represented by alphabetic and
      punctuation characters.

2.3.1.  CAPWAP Protocol State Transitions

   This section describes the various state transitions, and the events
   that cause them.  This section does not discuss interactions between
   DTLS- and CAPWAP-specific states.  Those interactions, and DTLS-
   specific states and transitions, are discussed in Section 2.3.2.



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   Start to Idle (0):  This transition occurs once device initialization
      is complete.

      WTP:  This state transition is used to start the WTP's CAPWAP
            state machine.

      AC:   The AC creates the Discovery and Listener threads and starts
            the CAPWAP state machine.

   Idle to Discovery (1):  This transition occurs to support the CAPWAP
      discovery process.

      WTP:  The WTP enters the Discovery state prior to transmitting the
            first Discovery Request message (see Section 5.1).  Upon
            entering this state, the WTP sets the DiscoveryInterval
            timer (see Section 4.7).  The WTP resets the DiscoveryCount
            counter to zero (0) (see Section 4.8).  The WTP also clears
            all information from ACs it may have received during a
            previous Discovery phase.

      AC:   This state transition is executed by the AC's Discovery
            thread, and occurs when a Discovery Request message is
            received.  The AC SHOULD respond with a Discovery Response
            message (see Section 5.2).

   Discovery to Discovery (#):  In the Discovery state, the WTP
      determines to which AC to connect.

      WTP:  This transition occurs when the DiscoveryInterval timer
            expires.  If the WTP is configured with a list of ACs, it
            transmits a Discovery Request message to every AC from which
            it has not received a Discovery Response message.  For every
            transition to this event, the WTP increments the
            DiscoveryCount counter.  See Section 5.1 for more
            information on how the WTP knows the ACs to which it should
            transmit the Discovery Request messages.  The WTP restarts
            the DiscoveryInterval timer whenever it transmits Discovery
            Request messages.

      AC:   This is an invalid state transition for the AC.

   Discovery to Idle (2):  This transition occurs on the AC's Discovery
      thread when the Discovery processing is complete.

      WTP:  This is an invalid state transition for the WTP.






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      AC:   This state transition is executed by the AC's Discovery
            thread when it has transmitted the Discovery Response, in
            response to a Discovery Request.

   Discovery to Sulking (!):  This transition occurs on a WTP when AC
      Discovery fails.

      WTP:  The WTP enters this state when the DiscoveryInterval timer
            expires and the DiscoveryCount variable is equal to the
            MaxDiscoveries variable (see Section 4.8).  Upon entering
            this state, the WTP MUST start the SilentInterval timer.
            While in the Sulking state, all received CAPWAP protocol
            messages MUST be ignored.

      AC:   This is an invalid state transition for the AC.

   Sulking to Idle (@):  This transition occurs on a WTP when it must
      restart the Discovery phase.

      WTP:  The WTP enters this state when the SilentInterval timer (see
            Section 4.7) expires.  The FailedDTLSSessionCount,
            DiscoveryCount, and FailedDTLSAuthFailCount counters are
            reset to zero.

      AC:   This is an invalid state transition for the AC.

   Sulking to Sulking (&):  The Sulking state provides the silent
      period, minimizing the possibility for Denial-of-Service (DoS)
      attacks.

      WTP:  All packets received from the AC while in the sulking state
            are ignored.

      AC:   This is an invalid state transition for the AC.

   Idle to DTLS Setup (3):  This transition occurs to establish a secure
      DTLS session with the peer.

      WTP:  The WTP initiates this transition by invoking the DTLSStart
            command (see Section 2.3.2.1), which starts the DTLS session
            establishment with the chosen AC and the WaitDTLS timer is
            started (see Section 4.7).  When the Discovery phase is
            bypassed, it is assumed the WTP has locally configured ACs.








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      AC:   Upon entering the Idle state from the Start state, the newly
            created Listener thread automatically transitions to the
            DTLS Setup and invokes the DTLSListen command (see
            Section 2.3.2.1), and the WaitDTLS timer is started (see
            Section 4.7).

   Discovery to DTLS Setup (%):  This transition occurs to establish a
      secure DTLS session with the peer.

      WTP:  The WTP initiates this transition by invoking the DTLSStart
            command (see Section 2.3.2.1), which starts the DTLS session
            establishment with the chosen AC.  The decision of to which
            AC to connect is the result of the Discovery phase, which is
            described in Section 3.3.

      AC:   This is an invalid state transition for the AC.

   DTLS Setup to Idle ($):  This transition occurs when the DTLS
      connection setup fails.

      WTP:  The WTP initiates this state transition when it receives a
            DTLSEstablishFail notification from DTLS (see
            Section 2.3.2.2), and the FailedDTLSSessionCount or the
            FailedDTLSAuthFailCount counter have not reached the value
            of the MaxFailedDTLSSessionRetry variable (see Section 4.8).
            This error notification aborts the secure DTLS session
            establishment.  When this notification is received, the
            FailedDTLSSessionCount counter is incremented.  This state
            transition also occurs if the WaitDTLS timer has expired.

      AC:   This is an invalid state transition for the AC.

   DTLS Setup to Sulking (*):  This transition occurs when repeated
      attempts to set up the DTLS connection have failed.

      WTP:  The WTP enters this state when the FailedDTLSSessionCount or
            the FailedDTLSAuthFailCount counter reaches the value of the
            MaxFailedDTLSSessionRetry variable (see Section 4.8).  Upon
            entering this state, the WTP MUST start the SilentInterval
            timer.  While in the Sulking state, all received CAPWAP and
            DTLS protocol messages received MUST be ignored.

      AC:   This is an invalid state transition for the AC.

   DTLS Setup to DTLS Setup (4):  This transition occurs when the DTLS
      Session failed to be established.

      WTP:  This is an invalid state transition for the WTP.



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      AC:   The AC's Listener initiates this state transition when it
            receives a DTLSEstablishFail notification from DTLS (see
            Section 2.3.2.2).  This error notification aborts the secure
            DTLS session establishment.  When this notification is
            received, the FailedDTLSSessionCount counter is incremented.
            The Listener thread then invokes the DTLSListen command (see
            Section 2.3.2.1).

   DTLS Setup to Authorize (5):  This transition occurs when an incoming
      DTLS session is being established, and the DTLS stack needs
      authorization to proceed with the session establishment.

      WTP:  This state transition occurs when the WTP receives the
            DTLSPeerAuthorize notification (see Section 2.3.2.2).  Upon
            entering this state, the WTP performs an authorization check
            against the AC credentials.  See Section 2.4.4 for more
            information on AC authorization.

      AC:   This state transition is handled by the AC's Listener thread
            when the DTLS module initiates the DTLSPeerAuthorize
            notification (see Section 2.3.2.2).  The Listener thread
            forks an instance of the Service thread, along with a copy
            of the state context.  Once created, the Service thread
            performs an authorization check against the WTP credentials.
            See Section 2.4.4 for more information on WTP authorization.

   Authorize to DTLS Setup (6):  This transition is executed by the
      Listener thread to enable it to listen for new incoming sessions.

      WTP:  This is an invalid state transition for the WTP.

      AC:   This state transition occurs when the AC's Listener thread
            has created the WTP context and the Service thread.  The
            Listener thread then invokes the DTLSListen command (see
            Section 2.3.2.1).

   Authorize to DTLS Connect (a):  This transition occurs to notify the
      DTLS stack that the session should be established.

      WTP:  This state transition occurs when the WTP has successfully
            authorized the AC's credentials (see Section 2.4.4).  This
            is done by invoking the DTLSAccept DTLS command (see
            Section 2.3.2.1).

      AC:   This state transition occurs when the AC has successfully
            authorized the WTP's credentials (see Section 2.4.4).  This
            is done by invoking the DTLSAccept DTLS command (see
            Section 2.3.2.1).



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   Authorize to DTLS Teardown (b):  This transition occurs to notify the
      DTLS stack that the session should be aborted.

      WTP:  This state transition occurs when the WTP has been unable to
            authorize the AC, using the AC credentials.  The WTP then
            aborts the DTLS session by invoking the DTLSAbortSession
            command (see Section 2.3.2.1).  This state transition also
            occurs if the WaitDTLS timer has expired.  The WTP starts
            the DTLSSessionDelete timer (see Section 4.7.6).

      AC:   This state transition occurs when the AC has been unable to
            authorize the WTP, using the WTP credentials.  The AC then
            aborts the DTLS session by invoking the DTLSAbortSession
            command (see Section 2.3.2.1).  This state transition also
            occurs if the WaitDTLS timer has expired.  The AC starts the
            DTLSSessionDelete timer (see Section 4.7.6).

   DTLS Connect to DTLS Teardown (c):  This transition occurs when the
      DTLS Session failed to be established.

      WTP:  This state transition occurs when the WTP receives either a
            DTLSAborted or DTLSAuthenticateFail notification (see
            Section 2.3.2.2), indicating that the DTLS session was not
            successfully established.  When this transition occurs due
            to the DTLSAuthenticateFail notification, the
            FailedDTLSAuthFailCount is incremented; otherwise, the
            FailedDTLSSessionCount counter is incremented.  This state
            transition also occurs if the WaitDTLS timer has expired.
            The WTP starts the DTLSSessionDelete timer (see
            Section 4.7.6).

      AC:   This state transition occurs when the AC receives either a
            DTLSAborted or DTLSAuthenticateFail notification (see
            Section 2.3.2.2), indicating that the DTLS session was not
            successfully established, and both of the
            FailedDTLSAuthFailCount and FailedDTLSSessionCount counters
            have not reached the value of the MaxFailedDTLSSessionRetry
            variable (see Section 4.8).  This state transition also
            occurs if the WaitDTLS timer has expired.  The AC starts the
            DTLSSessionDelete timer (see Section 4.7.6).

   DTLS Connect to Join (d):  This transition occurs when the DTLS
      Session is successfully established.

      WTP:  This state transition occurs when the WTP receives the
            DTLSEstablished notification (see Section 2.3.2.2),
            indicating that the DTLS session was successfully
            established.  When this notification is received, the



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            FailedDTLSSessionCount counter is set to zero.  The WTP
            enters the Join state by transmitting the Join Request to
            the AC.  The WTP stops the WaitDTLS timer.

      AC:   This state transition occurs when the AC receives the
            DTLSEstablished notification (see Section 2.3.2.2),
            indicating that the DTLS session was successfully
            established.  When this notification is received, the
            FailedDTLSSessionCount counter is set to zero.  The AC stops
            the WaitDTLS timer, and starts the WaitJoin timer.

   Join to DTLS Teardown (e):  This transition occurs when the join
      process has failed.

      WTP:  This state transition occurs when the WTP receives a Join
            Response message with a Result Code message element
            containing an error, or if the Image Identifier provided by
            the AC in the Join Response message differs from the WTP's
            currently running firmware version and the WTP has the
            requested image in its non-volatile memory.  This causes the
            WTP to initiate the DTLSShutdown command (see
            Section 2.3.2.1).  This transition also occurs if the WTP
            receives one of the following DTLS notifications:
            DTLSAborted, DTLSReassemblyFailure, or DTLSPeerDisconnect.
            The WTP starts the DTLSSessionDelete timer (see
            Section 4.7.6).

      AC:   This state transition occurs either if the WaitJoin timer
            expires or if the AC transmits a Join Response message with
            a Result Code message element containing an error.  This
            causes the AC to initiate the DTLSShutdown command (see
            Section 2.3.2.1).  This transition also occurs if the AC
            receives one of the following DTLS notifications:
            DTLSAborted, DTLSReassemblyFailure, or DTLSPeerDisconnect.
            The AC starts the DTLSSessionDelete timer (see
            Section 4.7.6).

   Join to Image Data (f):  This state transition is used by the WTP and
      the AC to download executable firmware.

      WTP:  The WTP enters the Image Data state when it receives a
            successful Join Response message and determines that the
            software version in the Image Identifier message element is
            not the same as its currently running image.  The WTP also
            detects that the requested image version is not currently
            available in the WTP's non-volatile storage (see Section 9.1
            for a full description of the firmware download process).
            The WTP initializes the EchoInterval timer (see



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            Section 4.7), and transmits the Image Data Request message
            (see Section 9.1.1) requesting the start of the firmware
            download.

      AC:   This state transition occurs when the AC receives the Image
            Data Request message from the WTP, after having sent its
            Join Response to the WTP.  The AC stops the WaitJoin timer.
            The AC MUST transmit an Image Data Response message (see
            Section 9.1.2) to the WTP, which includes a portion of the
            firmware.

   Join to Configure (g):  This state transition is used by the WTP and
      the AC to exchange configuration information.

      WTP:  The WTP enters the Configure state when it receives a
            successful Join Response message, and determines that the
            included Image Identifier message element is the same as its
            currently running image.  The WTP transmits the
            Configuration Status Request message (see Section 8.2) to
            the AC with message elements describing its current
            configuration.

      AC:   This state transition occurs when it receives the
            Configuration Status Request message from the WTP (see
            Section 8.2), which MAY include specific message elements to
            override the WTP's configuration.  The AC stops the WaitJoin
            timer.  The AC transmits the Configuration Status Response
            message (see Section 8.3) and starts the
            ChangeStatePendingTimer timer (see Section 4.7).

   Configure to Reset (h):  This state transition is used to reset the
      connection either due to an error during the configuration phase,
      or when the WTP determines it needs to reset in order for the new
      configuration to take effect.  The CAPWAP Reset command is used to
      indicate to the peer that it will initiate a DTLS teardown.

      WTP:  The WTP enters the Reset state when it receives a
            Configuration Status Response message indicating an error or
            when it determines that a reset of the WTP is required, due
            to the characteristics of a new configuration.

      AC:   The AC transitions to the Reset state when it receives a
            Change State Event message from the WTP that contains an
            error for which AC policy does not permit the WTP to provide
            service.  This state transition also occurs when the AC
            ChangeStatePendingTimer timer expires.





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   Configure to DTLS Teardown (i):  This transition occurs when the
      configuration process aborts due to a DTLS error.

      WTP:  The WTP enters this state when it receives one of the
            following DTLS notifications: DTLSAborted,
            DTLSReassemblyFailure, or DTLSPeerDisconnect (see
            Section 2.3.2.2).  The WTP MAY tear down the DTLS session if
            it receives frequent DTLSDecapFailure notifications.  The
            WTP starts the DTLSSessionDelete timer (see Section 4.7.6).

      AC:   The AC enters this state when it receives one of the
            following DTLS notifications: DTLSAborted,
            DTLSReassemblyFailure, or DTLSPeerDisconnect (see
            Section 2.3.2.2).  The AC MAY tear down the DTLS session if
            it receives frequent DTLSDecapFailure notifications.  The AC
            starts the DTLSSessionDelete timer (see Section 4.7.6).

   Image Data to Image Data (j):  The Image Data state is used by the
      WTP and the AC during the firmware download phase.

      WTP:  The WTP enters the Image Data state when it receives an
            Image Data Response message indicating that the AC has more
            data to send.  This state transition also occurs when the
            WTP receives the subsequent Image Data Requests, at which
            time it resets the ImageDataStartTimer time to ensure it
            receives the next expected Image Data Request from the AC.
            This state transition can also occur when the WTP's
            EchoInterval timer (see Section 4.7.7) expires, in which
            case the WTP transmits an Echo Request message (see
            Section 7.1), and resets its EchoInterval timer.  The state
            transition also occurs when the WTP receives an Echo
            Response from the AC (see Section 7.2).

      AC:   This state transition occurs when the AC receives the Image
            Data Response message from the WTP while already in the
            Image Data state.  This state transition also occurs when
            the AC receives an Echo Request (see Section 7.1) from the
            WTP, in which case it responds with an Echo Response (see
            Section 7.2), and resets its EchoInterval timer (see
            Section 4.7.7).











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   Image Data to Reset (k):  This state transition is used to reset the
      DTLS connection prior to restarting the WTP after an image
      download.

      WTP:  When an image download completes, or if the
            ImageDataStartTimer timer expires, the WTP enters the Reset
            state.  The WTP MAY also transition to this state upon
            receiving an Image Data Response message from the AC (see
            Section 9.1.2) indicating a failure.

      AC:   The AC enters the Reset state either when the image transfer
            has successfully completed or an error occurs during the
            image download process.

   Image Data to DTLS Teardown (l):  This transition occurs when the
      firmware download process aborts due to a DTLS error.

      WTP:  The WTP enters this state when it receives one of the
            following DTLS notifications: DTLSAborted,
            DTLSReassemblyFailure, or DTLSPeerDisconnect (see
            Section 2.3.2.2).  The WTP MAY tear down the DTLS session if
            it receives frequent DTLSDecapFailure notifications.  The
            WTP starts the DTLSSessionDelete timer (see Section 4.7.6).

      AC:   The AC enters this state when it receives one of the
            following DTLS notifications: DTLSAborted,
            DTLSReassemblyFailure, or DTLSPeerDisconnect (see
            Section 2.3.2.2).  The AC MAY tear down the DTLS session if
            it receives frequent DTLSDecapFailure notifications.  The AC
            starts the DTLSSessionDelete timer (see Section 4.7.6).

   Configure to Data Check (m):  This state transition occurs when the
      WTP and AC confirm the configuration.

      WTP:  The WTP enters this state when it receives a successful
            Configuration Status Response message from the AC.  The WTP
            transmits the Change State Event Request message (see
            Section 8.6).

      AC:   This state transition occurs when the AC receives the Change
            State Event Request message (see Section 8.6) from the WTP.
            The AC responds with a Change State Event Response message
            (see Section 8.7).  The AC MUST start the DataCheckTimer
            timer and stops the ChangeStatePendingTimer timer (see
            Section 4.7).

   Data Check to DTLS Teardown (n):  This transition occurs when the WTP
      does not complete the Data Check exchange.



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      WTP:  This state transition occurs if the WTP does not receive the
            Change State Event Response message before a CAPWAP
            retransmission timeout occurs.  The WTP also transitions to
            this state if the underlying reliable transport's
            RetransmitCount counter has reached the MaxRetransmit
            variable (see Section 4.7).  The WTP starts the
            DTLSSessionDelete timer (see Section 4.7.6).

      AC:   The AC enters this state when the DataCheckTimer timer
            expires (see Section 4.7).  The AC starts the
            DTLSSessionDelete timer (see Section 4.7.6).

   Data Check to Run (o):  This state transition occurs when the linkage
      between the control and data channels is established, causing the
      WTP and AC to enter their normal state of operation.

      WTP:  The WTP enters this state when it receives a successful
            Change State Event Response message from the AC.  The WTP
            initiates the data channel, which MAY require the
            establishment of a DTLS session, starts the
            DataChannelKeepAlive timer (see Section 4.7.2) and transmits
            a Data Channel Keep-Alive packet (see Section 4.4.1).  The
            WTP then starts the EchoInterval timer and
            DataChannelDeadInterval timer (see Section 4.7).

      AC:   This state transition occurs when the AC receives the Data
            Channel Keep-Alive packet (see Section 4.4.1), with a
            Session ID message element matching that included by the WTP
            in the Join Request message.  The AC disables the
            DataCheckTimer timer.  Note that if AC policy is to require
            the data channel to be encrypted, this process would also
            require the establishment of a data channel DTLS session.
            Upon receiving the Data Channel Keep-Alive packet, the AC
            transmits its own Data Channel Keep Alive packet.

   Run to DTLS Teardown (p):  This state transition occurs when an error
      has occurred in the DTLS stack, causing the DTLS session to be
      torn down.

      WTP:  The WTP enters this state when it receives one of the
            following DTLS notifications: DTLSAborted,
            DTLSReassemblyFailure, or DTLSPeerDisconnect (see
            Section 2.3.2.2).  The WTP MAY tear down the DTLS session if
            it receives frequent DTLSDecapFailure notifications.  The
            WTP also transitions to this state if the underlying
            reliable transport's RetransmitCount counter has reached the
            MaxRetransmit variable (see Section 4.7).  The WTP starts
            the DTLSSessionDelete timer (see Section 4.7.6).



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      AC:   The AC enters this state when it receives one of the
            following DTLS notifications: DTLSAborted,
            DTLSReassemblyFailure, or DTLSPeerDisconnect (see
            Section 2.3.2.2).  The AC MAY tear down the DTLS session if
            it receives frequent DTLSDecapFailure notifications.  The AC
            transitions to this state if the underlying reliable
            transport's RetransmitCount counter has reached the
            MaxRetransmit variable (see Section 4.7).  This state
            transition also occurs when the AC's EchoInterval timer (see
            Section 4.7.7) expires.  The AC starts the DTLSSessionDelete
            timer (see Section 4.7.6).

   Run to Run (q):  This is the normal state of operation.

      WTP:  This is the WTP's normal state of operation.  The WTP resets
            its EchoInterval timer whenever it transmits a request to
            the AC.  There are many events that result in this state
            transition:

            Configuration Update:  The WTP receives a Configuration
                  Update Request message (see Section 8.4).  The WTP
                  MUST respond with a Configuration Update Response
                  message (see Section 8.5).

            Change State Event:  The WTP receives a Change State Event
                  Response message, or determines that it must initiate
                  a Change State Event Request message, as a result of a
                  failure or change in the state of a radio.

            Echo Request:  The WTP sends an Echo Request message
                  (Section 7.1) or receives the corresponding Echo
                  Response message, (see Section 7.2) from the AC.  When
                  the WTP receives the Echo Response, it resets its
                  EchoInterval timer (see Section 4.7.7).

            Clear Config Request:  The WTP receives a Clear
                  Configuration Request message (see Section 8.8) and
                  MUST generate a corresponding Clear Configuration
                  Response message (see Section 8.9).  The WTP MUST
                  reset its configuration back to manufacturer defaults.

            WTP Event:  The WTP sends a WTP Event Request message,
                  delivering information to the AC (see Section 9.4).
                  The WTP receives a WTP Event Response message from the
                  AC (see Section 9.5).






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            Data Transfer:  The WTP sends a Data Transfer Request or
                  Data Transfer Response message to the AC (see
                  Section 9.6).  The WTP receives a Data Transfer
                  Request or Data Transfer Response message from the AC
                  (see Section 9.6).  Upon receipt of a Data Transfer
                  Request, the WTP transmits a Data Transfer Response to
                  the AC.

            Station Configuration Request:  The WTP receives a Station
                  Configuration Request message (see Section 10.1), to
                  which it MUST respond with a Station Configuration
                  Response message (see Section 10.2).

      AC:   This is the AC's normal state of operation.  Note that the
            receipt of any Request from the WTP causes the AC to reset
            its EchoInterval timer (see Section 4.7.7).

            Configuration Update:  The AC sends a Configuration Update
                  Request message (see Section 8.4) to the WTP to update
                  its configuration.  The AC receives a Configuration
                  Update Response message (see Section 8.5) from the
                  WTP.

            Change State Event:  The AC receives a Change State Event
                  Request message (see Section 8.6), to which it MUST
                  respond with the Change State Event Response message
                  (see Section 8.7).

            Echo Request:  The AC receives an Echo Request message (see
                  Section 7.1), to which it MUST respond with an Echo
                  Response message (see Section 7.2).

            Clear Config Response:  The AC sends a Clear Configuration
                  Request message (see Section 8.8) to the WTP to clear
                  its configuration.  The AC receives a Clear
                  Configuration Response message from the WTP (see
                  Section 8.9).

            WTP Event:  The AC receives a WTP Event Request message from
                  the WTP (see Section 9.4) and MUST generate a
                  corresponding WTP Event Response message (see
                  Section 9.5).

            Data Transfer:  The AC sends a Data Transfer Request or Data
                  Transfer Response message to the WTP (see
                  Section 9.6).  The AC receives a Data Transfer Request





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                  or Data Transfer Response message from the WTP (see
                  Section 9.6).  Upon receipt of a Data Transfer
                  Request, the AC transmits a Data Transfer Response to
                  the WTP.

            Station Configuration Request:  The AC sends a Station
                  Configuration Request message (see Section 10.1) or
                  receives the corresponding Station Configuration
                  Response message (see Section 10.2) from the WTP.

   Run to Reset (r):  This state transition is used when either the AC
      or WTP tears down the connection.  This may occur as part of
      normal operation, or due to error conditions.

      WTP:  The WTP enters the Reset state when it receives a Reset
            Request message from the AC.

      AC:   The AC enters the Reset state when it transmits a Reset
            Request message to the WTP.

   Reset to DTLS Teardown (s):  This transition occurs when the CAPWAP
      reset is complete to terminate the DTLS session.

      WTP:  This state transition occurs when the WTP transmits a Reset
            Response message.  The WTP does not invoke the DTLSShutdown
            command (see Section 2.3.2.1).  The WTP starts the
            DTLSSessionDelete timer (see Section 4.7.6).

      AC:   This state transition occurs when the AC receives a Reset
            Response message.  This causes the AC to initiate the
            DTLSShutdown command (see Section 2.3.2.1).  The AC starts
            the DTLSSessionDelete timer (see Section 4.7.6).

   DTLS Teardown to Idle (t):  This transition occurs when the DTLS
      session has been shut down.

      WTP:  This state transition occurs when the WTP has successfully
            cleaned up all resources associated with the control plane
            DTLS session, or if the DTLSSessionDelete timer (see
            Section 4.7.6) expires.  The data plane DTLS session is also
            shut down, and all resources released, if a DTLS session was
            established for the data plane.  Any timers set for the
            current instance of the state machine are also cleared.

      AC:   This is an invalid state transition for the AC.






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   DTLS Teardown to Sulking (u):  This transition occurs when repeated
      attempts to setup the DTLS connection have failed.

      WTP:  The WTP enters this state when the FailedDTLSSessionCount or
            the FailedDTLSAuthFailCount counter reaches the value of the
            MaxFailedDTLSSessionRetry variable (see Section 4.8).  Upon
            entering this state, the WTP MUST start the SilentInterval
            timer.  While in the Sulking state, all received CAPWAP and
            DTLS protocol messages received MUST be ignored.

      AC:   This is an invalid state transition for the AC.

   DTLS Teardown to Dead (w):  This transition occurs when the DTLS
      session has been shut down.

      WTP:  This is an invalid state transition for the WTP.

      AC:   This state transition occurs when the AC has successfully
            cleaned up all resources associated with the control plane
            DTLS session , or if the DTLSSessionDelete timer (see
            Section 4.7.6) expires.  The data plane DTLS session is also
            shut down, and all resources released, if a DTLS session was
            established for the data plane.  Any timers set for the
            current instance of the state machine are also cleared.  The
            AC's Service thread is terminated.

2.3.2.  CAPWAP/DTLS Interface

   This section describes the DTLS Commands used by CAPWAP, and the
   notifications received from DTLS to the CAPWAP protocol stack.

2.3.2.1.  CAPWAP to DTLS Commands

   Six commands are defined for the CAPWAP to DTLS API.  These
   "commands" are conceptual, and may be implemented as one or more
   function calls.  This API definition is provided to clarify
   interactions between the DTLS and CAPWAP components of the integrated
   CAPWAP state machine.

   Below is a list of the minimal command APIs:

   o  DTLSStart is sent to the DTLS component to cause a DTLS session to
      be established.  Upon invoking the DTLSStart command, the WaitDTLS
      timer is started.  The WTP initiates this DTLS command, as the AC
      does not initiate DTLS sessions.

   o  DTLSListen is sent to the DTLS component to allow the DTLS
      component to listen for incoming DTLS session requests.



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   o  DTLSAccept is sent to the DTLS component to allow the DTLS session
      establishment to continue successfully.

   o  DTLSAbortSession is sent to the DTLS component to cause the
      session that is in the process of being established to be aborted.
      This command is also sent when the WaitDTLS timer expires.  When
      this command is executed, the FailedDTLSSessionCount counter is
      incremented.

   o  DTLSShutdown is sent to the DTLS component to cause session
      teardown.

   o  DTLSMtuUpdate is sent by the CAPWAP component to modify the MTU
      size used by the DTLS component.  See Section 3.5 for more
      information on MTU Discovery.  The default size is 1468 bytes.

2.3.2.2.  DTLS to CAPWAP Notifications

   DTLS notifications are defined for the DTLS to CAPWAP API.  These
   "notifications" are conceptual and may be implemented in numerous
   ways (e.g., as function return values).  This API definition is
   provided to clarify interactions between the DTLS and CAPWAP
   components of the integrated CAPWAP state machine.  It is important
   to note that the notifications listed below MAY cause the CAPWAP
   state machine to jump from one state to another using a state
   transition not listed in Section 2.3.1.  When a notification listed
   below occurs, the target CAPWAP state shown in Figure 4 becomes the
   current state.

   Below is a list of the API notifications:

   o  DTLSPeerAuthorize is sent to the CAPWAP component during DTLS
      session establishment once the peer's identity has been received.
      This notification MAY be used by the CAPWAP component to authorize
      the session, based on the peer's identity.  The authorization
      process will lead to the CAPWAP component initiating either the
      DTLSAccept or DTLSAbortSession commands.

   o  DTLSEstablished is sent to the CAPWAP component to indicate that a
      secure channel now exists, using the parameters provided during
      the DTLS initialization process.  When this notification is
      received, the FailedDTLSSessionCount counter is reset to zero.
      When this notification is received, the WaitDTLS timer is stopped.

   o  DTLSEstablishFail is sent when the DTLS session establishment has
      failed, either due to a local error or due to the peer rejecting
      the session establishment.  When this notification is received,
      the FailedDTLSSessionCount counter is incremented.



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   o  DTLSAuthenticateFail is sent when DTLS session establishment has
      failed due to an authentication error.  When this notification is
      received, the FailedDTLSAuthFailCount counter is incremented.

   o  DTLSAborted is sent to the CAPWAP component to indicate that
      session abort (as requested by CAPWAP) is complete; this occurs to
      confirm a DTLS session abort or when the WaitDTLS timer expires.
      When this notification is received, the WaitDTLS timer is stopped.

   o  DTLSReassemblyFailure MAY be sent to the CAPWAP component to
      indicate DTLS fragment reassembly failure.

   o  DTLSDecapFailure MAY be sent to the CAPWAP module to indicate a
      decapsulation failure.  DTLSDecapFailure MAY be sent to the CAPWAP
      module to indicate an encryption/authentication failure.  This
      notification is intended for informative purposes only, and is not
      intended to cause a change in the CAPWAP state machine (see
      Section 12.4).

   o  DTLSPeerDisconnect is sent to the CAPWAP component to indicate the
      DTLS session has been torn down.  Note that this notification is
      only received if the DTLS session has been established.

2.4.  Use of DTLS in the CAPWAP Protocol

   DTLS is used as a tightly integrated, secure wrapper for the CAPWAP
   protocol.  In this document, DTLS and CAPWAP are discussed as
   nominally distinct entities; however, they are very closely coupled,
   and may even be implemented inseparably.  Since there are DTLS
   library implementations currently available, and since security
   protocols (e.g., IPsec, TLS) are often implemented in widely
   available acceleration hardware, it is both convenient and forward-
   looking to maintain a modular distinction in this document.

   This section describes a detailed walk-through of the interactions
   between the DTLS module and the CAPWAP module, via 'commands' (CAPWAP
   to DTLS) and 'notifications' (DTLS to CAPWAP) as they would be
   encountered during the normal course of operation.

2.4.1.  DTLS Handshake Processing

   Details of the DTLS handshake process are specified in [RFC4347].
   This section describes the interactions between the DTLS session
   establishment process and the CAPWAP protocol.  Note that the
   conceptual DTLS state is shown below to help understand the point at
   which the DTLS states transition.  In the normal case, the DTLS
   handshake will proceed as shown in Figure 5.  (NOTE: this example
   uses certificates, but pre-shared keys are also supported.)



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           ============                         ============
               WTP                                   AC
           ============                         ============
           ClientHello           ------>
                                 <------       HelloVerifyRequest
                                                   (with cookie)

           ClientHello           ------>
           (with cookie)
                                 <------       ServerHello
                                 <------       Certificate
                                 <------       ServerHelloDone

           (WTP callout for AC authorization
                    occurs in CAPWAP Auth state)

           Certificate*
           ClientKeyExchange
           CertificateVerify*
           ChangeCipherSpec
           Finished              ------>

                                (AC callout for WTP authorization
                                 occurs in CAPWAP Auth state)

                                               ChangeCipherSpec
                                 <------       Finished

                         Figure 5: DTLS Handshake

   DTLS, as specified, provides its own retransmit timers with an
   exponential back-off.  [RFC4347] does not specify how long
   retransmissions should continue.  Consequently, timing out incomplete
   DTLS handshakes is entirely the responsibility of the CAPWAP module.

   The DTLS implementation used by CAPWAP MUST support TLS Session
   Resumption.  Session resumption is typically used to establish the
   DTLS session used for the data channel.  Since the data channel uses
   different port numbers than the control channel, the DTLS
   implementation on the WTP MUST provide an interface that allows the
   CAPWAP module to request session resumption despite the use of the
   different port numbers (TLS implementations usually attempt session
   resumption only when connecting to the same IP address and port
   number).  Note that session resumption is not guaranteed to occur,
   and a full DTLS handshake may occur instead.






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   The DTLS implementation used by CAPWAP MUST use replay detection, per
   Section 3.3 of [RFC4347].  Since the CAPWAP protocol handles
   retransmissions by re-encrypting lost frames, any duplicate DTLS
   frames are either unintentional or malicious and should be silently
   discarded.

2.4.2.  DTLS Session Establishment

   The WTP, either through the Discovery process or through pre-
   configuration, determines to which AC to connect.  The WTP uses the
   DTLSStart command to request that a secure connection be established
   to the selected AC.  Prior to initiation of the DTLS handshake, the
   WTP sets the WaitDTLS timer.  Upon invoking the DTLSStart or
   DTLSListen commands, the WTP and AC, respectively, set the WaitDTLS
   timer.  If the DTLSEstablished notification is not received prior to
   timer expiration, the DTLS session is aborted by issuing the
   DTLSAbortSession DTLS command.  This notification causes the CAPWAP
   module to transition to the Idle state.  Upon receiving a
   DTLSEstablished notification, the WaitDTLS timer is deactivated.

2.4.3.  DTLS Error Handling

   If the AC or WTP does not respond to any DTLS handshake messages sent
   by its peer, the DTLS specification calls for the message to be
   retransmitted.  Note that during the handshake, when both the AC and
   the WTP are expecting additional handshake messages, they both
   retransmit if an expected message has not been received (note that
   retransmissions for CAPWAP Control messages work differently: all
   CAPWAP Control messages are either requests or responses, and the
   peer who sent the request is responsible for retransmissions).

   If the WTP or the AC does not receive an expected DTLS handshake
   message despite of retransmissions, the WaitDTLS timer will
   eventually expire, and the session will be terminated.  This can
   happen if communication between the peers has completely failed, or
   if one of the peers sent a DTLS Alert message that was lost in
   transit (DTLS does not retransmit Alert messages).

   If a cookie fails to validate, this could represent a WTP error, or
   it could represent a DoS attack.  Hence, AC resource utilization
   SHOULD be minimized.  The AC MAY log a message indicating the
   failure, and SHOULD treat the message as though no cookie were
   present.

   Since DTLS Handshake messages are potentially larger than the maximum
   record size, DTLS supports fragmenting of Handshake messages across
   multiple records.  There are several potential causes of re-assembly




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   errors, including overlapping and/or lost fragments.  The DTLS
   component MUST send a DTLSReassemblyFailure notification to the
   CAPWAP component.  Whether precise information is given along with
   notification is an implementation issue, and hence is beyond the
   scope of this document.  Upon receipt of such an error, the CAPWAP
   component SHOULD log an appropriate error message.  Whether
   processing continues or the DTLS session is terminated is
   implementation dependent.

   DTLS decapsulation errors consist of three types: decryption errors,
   authentication errors, and malformed DTLS record headers.  Since DTLS
   authenticates the data prior to encapsulation, if decryption fails,
   it is difficult to detect this without first attempting to
   authenticate the packet.  If authentication fails, a decryption error
   is also likely, but not guaranteed.  Rather than attempt to derive
   (and require the implementation of) algorithms for detecting
   decryption failures, decryption failures are reported as
   authentication failures.  The DTLS component MUST provide a
   DTLSDecapFailure notification to the CAPWAP component when such
   errors occur.  If a malformed DTLS record header is detected, the
   packets SHOULD be silently discarded, and the receiver MAY log an
   error message.

   There is currently only one encapsulation error defined: MTU
   exceeded.  As part of DTLS session establishment, the CAPWAP
   component informs the DTLS component of the MTU size.  This may be
   dynamically modified at any time when the CAPWAP component sends the
   DTLSMtuUpdate command to the DTLS component (see Section 2.3.2.1).
   The value provided to the DTLS stack is the result of the MTU
   Discovery process, which is described in Section 3.5.  The DTLS
   component returns this notification to the CAPWAP component whenever
   a transmission request will result in a packet that exceeds the MTU.

2.4.4.  DTLS Endpoint Authentication and Authorization

   DTLS supports endpoint authentication with certificates or pre-shared
   keys.  The TLS algorithm suites for each endpoint authentication
   method are described below.

2.4.4.1.  Authenticating with Certificates

   CAPWAP implementations only use cipher suites that are recommended
   for use with DTLS, see [DTLS-DESIGN].  At present, the following
   algorithms MUST be supported when using certificates for CAPWAP
   authentication:

   o  TLS_RSA_WITH_AES_128_CBC_SHA [RFC5246]




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   The following algorithms SHOULD be supported when using certificates:

   o  TLS_DHE_RSA_WITH_AES_128_CBC_SHA [RFC5246]

   The following algorithms MAY be supported when using certificates:

   o  TLS_RSA_WITH_AES_256_CBC_SHA [RFC5246]

   o  TLS_DHE_RSA_WITH_AES_256_CBC_SHA [RFC5246]

   Additional ciphers MAY be defined in subsequent CAPWAP
   specifications.

2.4.4.2.  Authenticating with Pre-Shared Keys

   Pre-shared keys present significant challenges from a security
   perspective, and for that reason, their use is strongly discouraged.
   Several methods for authenticating with pre-shared keys are defined
   [RFC4279], and we focus on the following two:

   o  Pre-Shared Key (PSK) key exchange algorithm - simplest method,
      ciphersuites use only symmetric key algorithms.

   o  DHE_PSK key exchange algorithm - use a PSK to authenticate a
      Diffie-Hellman exchange.  These ciphersuites give some additional
      protection against dictionary attacks and also provide Perfect
      Forward Secrecy (PFS).

   The first approach (plain PSK) is susceptible to passive dictionary
   attacks; hence, while this algorithm MUST be supported, special care
   should be taken when choosing that method.  In particular, user-
   readable passphrases SHOULD NOT be used, and use of short PSKs SHOULD
   be strongly discouraged.

   The following cryptographic algorithms MUST be supported when using
   pre-shared keys:

   o  TLS_PSK_WITH_AES_128_CBC_SHA [RFC5246]

   o  TLS_DHE_PSK_WITH_AES_128_CBC_SHA [RFC5246]

   The following algorithms MAY be supported when using pre-shared keys:

   o  TLS_PSK_WITH_AES_256_CBC_SHA [RFC5246]

   o  TLS_DHE_PSK_WITH_AES_256_CBC_SHA [RFC5246]

   Additional ciphers MAY be defined in following CAPWAP specifications.



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2.4.4.3.  Certificate Usage

   Certificate authorization by the AC and WTP is required so that only
   an AC may perform the functions of an AC and that only a WTP may
   perform the functions of a WTP.  This restriction of functions to the
   AC or WTP requires that the certificates used by the AC MUST be
   distinguishable from the certificate used by the WTP.  To accomplish
   this differentiation, the x.509 certificates MUST include the
   Extended Key Usage (EKU) certificate extension [RFC5280].

   The EKU field indicates one or more purposes for which a certificate
   may be used.  It is an essential part in authorization.  Its syntax
   is described in [RFC5280] and [ISO.9834-1.1993] and is as follows:

         ExtKeyUsageSyntax  ::=  SEQUENCE SIZE (1..MAX) OF KeyPurposeId

         KeyPurposeId  ::=  OBJECT IDENTIFIER

   Here we define two KeyPurposeId values, one for the WTP and one for
   the AC.  Inclusion of one of these two values indicates a certificate
   is authorized for use by a WTP or AC, respectively.  These values are
   formatted as id-kp fields.

             id-kp  OBJECT IDENTIFIER  ::=
                 { iso(1) identified-organization(3) dod(6) internet(1)
                   security(5) mechanisms(5) pkix(7) 3 }

              id-kp-capwapAC   OBJECT IDENTIFIER  ::=  { id-kp 18 }

              id-kp-capwapWTP  OBJECT IDENTIFIER  ::=  { id-kp 19 }

   All capwap devices MUST support the ExtendedKeyUsage certificate
   extension if it is present in a certificate.  If the extension is
   present, then the certificate MUST have either the id-kp-capwapAC or
   the id-kp-anyExtendedKeyUsage keyPurposeID to act as an AC.
   Similarly, if the extension is present, a device MUST have the id-kp-
   capwapWTP or id-kp-anyExtendedKeyUsage keyPurposeID to act as a WTP.

   Part of the CAPWAP certificate validation process includes ensuring
   that the proper EKU is included and allowing the CAPWAP session to be
   established only if the extension properly represents the device.
   For instance, an AC SHOULD NOT accept a connection request from
   another AC, and therefore MUST verify that the id-kp-capwapWTP EKU is
   present in the certificate.

   CAPWAP implementations MUST support certificates where the common
   name (CN) for both the WTP and AC is the MAC address of that device.




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   The MAC address MUST be encoded in the PrintableString format, using
   the well-recognized MAC address format of 01:23:45:67:89:ab.  The CN
   field MAY contain either of the EUI-48 [EUI-48] or EUI-64 [EUI-64]
   MAC Address formats.  This seemingly unconventional use of the CN
   field is consistent with other standards that rely on device
   certificates that are provisioned during the manufacturing process,
   such as Packet Cable [PacketCable], Cable Labs [CableLabs], and WiMAX
   [WiMAX].  See Section 12.8 for more information on the use of the MAC
   address in the CN field.

   ACs and WTPs MUST authorize (e.g., through access control lists)
   certificates of devices to which they are connecting, e.g., based on
   the issuer, MAC address, or organizational information specified in
   the certificate.  The identities specified in the certificates bind a
   particular DTLS session to a specific pair of mutually authenticated
   and authorized MAC addresses.  The particulars of authorization
   filter construction are implementation details which are, for the
   most part, not within the scope of this specification.  However, at
   minimum, all devices MUST verify that the appropriate EKU bit is set
   according to the role of the peer device (AC versus WTP), and that
   the issuer of the certificate is appropriate for the domain in
   question.

2.4.4.4.  PSK Usage

   When DTLS uses PSK Ciphersuites, the ServerKeyExchange message MUST
   contain the "PSK identity hint" field and the ClientKeyExchange
   message MUST contain the "PSK identity" field.  These fields are used
   to help the WTP select the appropriate PSK for use with the AC, and
   then indicate to the AC which key is being used.  When PSKs are
   provisioned to WTPs and ACs, both the PSK Hint and PSK Identity for
   the key MUST be specified.

   The PSK Hint SHOULD uniquely identify the AC and the PSK Identity
   SHOULD uniquely identify the WTP.  It is RECOMMENDED that these hints
   and identities be the ASCII HEX-formatted MAC addresses of the
   respective devices, since each pairwise combination of WTP and AC
   SHOULD have a unique PSK.  The PSK Hint and Identity SHOULD be
   sufficient to perform authorization, as simply having knowledge of a
   PSK does not necessarily imply authorization.

   If a single PSK is being used for multiple devices on a CAPWAP
   network, which is NOT RECOMMENDED, the PSK Hint and Identity can no
   longer be a MAC address, so appropriate hints and identities SHOULD
   be selected to identify the group of devices to which the PSK is
   provisioned.





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3.  CAPWAP Transport

   Communication between a WTP and an AC is established using the
   standard UDP client/server model.  The CAPWAP protocol supports both
   UDP and UDP-Lite [RFC3828] transport protocols.  When run over IPv4,
   UDP is used for the CAPWAP Control and Data channels.

   When run over IPv6, the CAPWAP Control channel always uses UDP, while
   the CAPWAP Data channel may use either UDP or UDP-Lite.  UDP-Lite is
   the default transport protocol for the CAPWAP Data channel.  However,
   if a middlebox or IPv4 to IPv6 gateway has been discovered, UDP is
   used for the CAPWAP Data channel.

   This section describes how the CAPWAP protocol is carried over IP and
   UDP/UDP-Lite transport protocols.  The CAPWAP Transport Protocol
   message element, Section 4.6.14, describes the rules to use in
   determining which transport protocol is to be used.

   In order for CAPWAP to be compatible with potential middleboxes in
   the network, CAPWAP implementations MUST send return traffic from the
   same port on which they received traffic from a given peer.  Further,
   any unsolicited requests generated by a CAPWAP node MUST be sent on
   the same port.

3.1.  UDP Transport

   One of the CAPWAP protocol requirements is to allow a WTP to reside
   behind a middlebox, firewall, and/or Network Address Translation
   (NAT) device.  Since a CAPWAP session is initiated by the WTP
   (client) to the well-known UDP port of the AC (server), the use of
   UDP is a logical choice.  When CAPWAP is run over IPv4, the UDP
   checksum field in CAPWAP packets MUST be set to zero.

   CAPWAP protocol control packets sent from the WTP to the AC use the
   CAPWAP Control channel, as defined in Section 1.4.  The CAPWAP
   control port at the AC is the well-known UDP port 5246.  The CAPWAP
   control port at the WTP can be any port selected by the WTP.

   CAPWAP protocol data packets sent from the WTP to the AC use the
   CAPWAP Data channel, as defined in Section 1.4.  The CAPWAP data port
   at the AC is the well-known UDP port 5247.  If an AC permits the
   administrator to change the CAPWAP control port, the CAPWAP data port
   MUST be the next consecutive port number.  The CAPWAP data port at
   the WTP can be any port selected by the WTP.







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3.2.  UDP-Lite Transport

   When CAPWAP is run over IPv6, UDP-Lite is the default transport
   protocol, which reduces the checksum processing required for each
   packet (compared to the use of UDP over IPv6 [RFC2460]).  When UDP-
   Lite is used, the checksum field MUST have a coverage of 8 [RFC3828].

   UDP-Lite uses the same port assignments as UDP.

3.3.  AC Discovery

   The AC Discovery phase allows the WTP to determine which ACs are
   available and choose the best AC with which to establish a CAPWAP
   session.  The Discovery phase occurs when the WTP enters the optional
   Discovery state.  A WTP does not need to complete the AC Discovery
   phase if it uses a pre-configured AC.  This section details the
   mechanism used by a WTP to dynamically discover candidate ACs.

   A WTP and an AC will frequently not reside in the same IP subnet
   (broadcast domain).  When this occurs, the WTP must be capable of
   discovering the AC, without requiring that multicast services are
   enabled in the network.

   When the WTP attempts to establish communication with an AC, it sends
   the Discovery Request message and receives the Discovery Response
   message from the AC(s).  The WTP MUST send the Discovery Request
   message to either the limited broadcast IP address (255.255.255.255),
   the well-known CAPWAP multicast address (224.0.1.140), or to the
   unicast IP address of the AC.  For IPv6 networks, since broadcast
   does not exist, the use of "All ACs multicast address" (FF0X:0:0:0:0:
   0:0:18C) is used instead.  Upon receipt of the Discovery Request
   message, the AC sends a Discovery Response message to the unicast IP
   address of the WTP, regardless of whether the Discovery Request
   message was sent as a broadcast, multicast, or unicast message.

   WTP use of a limited IP broadcast, multicast, or unicast IP address
   is implementation dependent.  ACs, on the other hand, MUST support
   broadcast, multicast, and unicast discovery.

   When a WTP transmits a Discovery Request message to a unicast
   address, the WTP must first obtain the IP address of the AC.  Any
   static configuration of an AC's IP address on the WTP non-volatile
   storage is implementation dependent.  However, additional dynamic
   schemes are possible, for example:







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   DHCP:  See [RFC5417] for more information on the use of DHCP to
      discover AC IP addresses.

   DNS:  The WTP MAY support use of DNS Service Records (SRVs) [RFC2782]
      to discover the AC address(es).  In this case, the WTP first
      obtains (e.g., from local configuration) the correct domain name
      suffix (e.g., "example.com") and performs an SRV lookup with
      Service name "capwap-control" and Proto "udp".  Thus, the name
      resolved in DNS would be, e.g., "_capwap-
      control._udp.example.com".  Note that the SRV record MAY specify a
      non-default port number for the control channel; the port number
      for the data channel is the next port number (control channel port
      + 1).

   An AC MAY also communicate alternative ACs to the WTP within the
   Discovery Response message through the AC IPv4 List (see
   Section 4.6.2) and AC IPv6 List (see Section 4.6.2).  The addresses
   provided in these two message elements are intended to help the WTP
   discover additional ACs through means other than those listed above.

   The AC Name with Priority message element (see Section 4.6.5) is used
   to communicate a list of preferred ACs to the WTP.  The WTP SHOULD
   attempt to utilize the ACs listed in the order provided by the AC.
   The Name-to-IP Address mapping is handled via the Discovery message
   exchange, in which the ACs provide their identity in the AC Name (see
   Section 4.6.4) message element in the Discovery Response message.

   Once the WTP has received Discovery Response messages from the
   candidate ACs, it MAY use other factors to determine the preferred
   AC.  For instance, each binding defines a WTP Radio Information
   message element (see Section 2.1), which the AC includes in Discovery
   Response messages.  The presence of one or more of these message
   elements is used to identify the CAPWAP bindings supported by the AC.
   A WTP MAY connect to an AC based on the supported bindings
   advertised.

3.4.  Fragmentation/Reassembly

   While fragmentation and reassembly services are provided by IP, the
   CAPWAP protocol also provides such services.  Environments where the
   CAPWAP protocol is used involve firewall, NAT, and "middlebox"
   devices, which tend to drop IP fragments to minimize possible DoS
   attacks.  By providing fragmentation and reassembly at the
   application layer, any fragmentation required due to the tunneling
   component of the CAPWAP protocol becomes transparent to these
   intermediate devices.  Consequently, the CAPWAP protocol can be used
   in any network topology including firewall, NAT, and middlebox
   devices.



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   It is important to note that the fragmentation mechanism employed by
   CAPWAP has known limitations and deficiencies, which are similar to
   those described in [RFC4963].  The limited size of the Fragment ID
   field (see Section 4.3) can cause wrapping of the field, and hence
   cause fragments from different datagrams to be incorrectly spliced
   together (known as "mis-associated").  For example, a 100Mpbs link
   with an MTU of 1500 (causing fragmentation at 1450 bytes) would cause
   the Fragment ID field wrap in 8 seconds.  Consequently, CAPWAP
   implementers are warned to properly size their buffers for reassembly
   purposes based on the expected wireless technology throughput.

   CAPWAP implementations SHOULD perform MTU Discovery (see
   Section 3.5), which can avoid the need for fragmentation.  At the
   time of writing of this specification, most enterprise switching and
   routing infrastructure were capable of supporting "mini-jumbo" frames
   (1800 bytes), which eliminates the need for fragmentation (assuming
   the station's MTU is 1500 bytes).  The need for fragmentation
   typically continues to exist when the WTP communicates with the AC
   over a Wide Area Network (WAN).  Therefore, future versions of the
   CAPWAP protocol SHOULD consider either increasing the size of the
   Fragment ID field or providing alternative extensions.

3.5.  MTU Discovery

   Once a WTP has discovered the AC with which it wishes to establish a
   CAPWAP session, it SHOULD perform a Path MTU (PMTU) discovery.  One
   recommendation for performing PMTU discovery is to have the WTP
   transmit Discovery Request (see Section 5.1) messages, and include
   the MTU Discovery Padding message element (see Section 4.6.32).  The
   actual procedures used for PMTU discovery are described in [RFC1191]
   for IPv4; for IPv6, [RFC1981] SHOULD be used.  Alternatively,
   implementers MAY use the procedures defined in [RFC4821].  The WTP
   SHOULD also periodically re-evaluate the PMTU using the guidelines
   provided in these two RFCs, using the Primary Discovery Request (see
   Section 5.3) along with the MTU Discovery Padding message element
   (see Section 4.6.32).  When the MTU is initially known, or updated in
   the case where an existing session already exists, the discovered
   PMTU is used to configure the DTLS component (see Section 2.3.2.1),
   while non-DTLS frames need to be fragmented to fit the MTU, defined
   in Section 3.4.

4.  CAPWAP Packet Formats

   This section contains the CAPWAP protocol packet formats.  A CAPWAP
   protocol packet consists of one or more CAPWAP Transport Layer packet
   headers followed by a CAPWAP message.  The CAPWAP message can be
   either of type Control or Data, where Control packets carry




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   signaling, and Data packets carry user payloads.  The CAPWAP frame
   formats for CAPWAP Data packets, and for DTLS encapsulated CAPWAP
   Data and Control packets are defined below.

   The CAPWAP Control protocol includes two messages that are never
   protected by DTLS: the Discovery Request message and the Discovery
   Response message.  These messages need to be in the clear to allow
   the CAPWAP protocol to properly identify and process them.  The
   format of these packets are as follows:

       CAPWAP Control Packet (Discovery Request/Response):
       +-------------------------------------------+
       | IP  | UDP | CAPWAP | Control | Message    |
       | Hdr | Hdr | Header | Header  | Element(s) |
       +-------------------------------------------+

   All other CAPWAP Control protocol messages MUST be protected via the
   DTLS protocol, which ensures that the packets are both authenticated
   and encrypted.  These packets include the CAPWAP DTLS Header, which
   is described in Section 4.2.  The format of these packets is as
   follows:

    CAPWAP Control Packet (DTLS Security Required):
    +------------------------------------------------------------------+
    | IP  | UDP | CAPWAP   | DTLS | CAPWAP | Control| Message   | DTLS |
    | Hdr | Hdr | DTLS Hdr | Hdr  | Header | Header | Element(s)| Trlr |
    +------------------------------------------------------------------+
                           \---------- authenticated -----------/
                                  \------------- encrypted ------------/

   The CAPWAP protocol allows optional protection of data packets, using
   DTLS.  Use of data packet protection is determined by AC policy.
   When DTLS is utilized, the optional CAPWAP DTLS Header is present,
   which is described in Section 4.2.  The format of CAPWAP Data packets
   is shown below:
















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       CAPWAP Plain Text Data Packet :
       +-------------------------------+
       | IP  | UDP | CAPWAP | Wireless |
       | Hdr | Hdr | Header | Payload  |
       +-------------------------------+

       DTLS Secured CAPWAP Data Packet:
       +--------------------------------------------------------+
       | IP  | UDP |  CAPWAP  | DTLS | CAPWAP | Wireless | DTLS |
       | Hdr | Hdr | DTLS Hdr | Hdr  |  Hdr   | Payload  | Trlr |
       +--------------------------------------------------------+
                              \------ authenticated -----/
                                     \------- encrypted --------/

   UDP Header:  All CAPWAP packets are encapsulated within either UDP,
      or UDP-Lite when used over IPv6.  Section 3 defines the specific
      UDP or UDP-Lite usage.

   CAPWAP DTLS Header:  All DTLS encrypted CAPWAP protocol packets are
      prefixed with the CAPWAP DTLS Header (see Section 4.2).

   DTLS Header:  The DTLS Header provides authentication and encryption
      services to the CAPWAP payload it encapsulates.  This protocol is
      defined in [RFC4347].

   CAPWAP Header:  All CAPWAP protocol packets use a common header that
      immediately follows the CAPWAP preamble or DTLS Header.  The
      CAPWAP Header is defined in Section 4.3.

   Wireless Payload:  A CAPWAP protocol packet that contains a wireless
      payload is a CAPWAP Data packet.  The CAPWAP protocol does not
      specify the format of the wireless payload, which is defined by
      the appropriate wireless standard.  Additional information is in
      Section 4.4.

   Control Header:  The CAPWAP protocol includes a signaling component,
      known as the CAPWAP Control protocol.  All CAPWAP Control packets
      include a Control Header, which is defined in Section 4.5.1.
      CAPWAP Data packets do not contain a Control Header field.

   Message Elements:  A CAPWAP Control packet includes one or more
      message elements, which are found immediately following the
      Control Header.  These message elements are in a Type/Length/Value
      style header, defined in Section 4.6.

   A CAPWAP implementation MUST be capable of receiving a reassembled
   CAPWAP message of length 4096 bytes.  A CAPWAP implementation MAY
   indicate that it supports a higher maximum message length, by



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   including the Maximum Message Length message element, see
   Section 4.6.31, in the Join Request message or the Join Response
   message.

4.1.  CAPWAP Preamble

   The CAPWAP preamble is common to all CAPWAP transport headers and is
   used to identify the header type that immediately follows.  The
   reason for this preamble is to avoid needing to perform byte
   comparisons in order to guess whether or not the frame is DTLS
   encrypted.  It also provides an extensibility framework that can be
   used to support additional transport types.  The format of the
   preamble is as follows:

         0
         0 1 2 3 4 5 6 7
        +-+-+-+-+-+-+-+-+
        |Version| Type  |
        +-+-+-+-+-+-+-+-+

   Version:  A 4-bit field that contains the version of CAPWAP used in
      this packet.  The value for this specification is zero (0).

   Type:  A 4-bit field that specifies the payload type that follows the
      UDP header.  The following values are supported:

      0 -   CAPWAP Header.  The CAPWAP Header (see Section 4.3)
            immediately follows the UDP header.  If the packet is
            received on the CAPWAP Data channel, the CAPWAP stack MUST
            treat the packet as a clear text CAPWAP Data packet.  If
            received on the CAPWAP Control channel, the CAPWAP stack
            MUST treat the packet as a clear text CAPWAP Control packet.
            If the control packet is not a Discovery Request or
            Discovery Response packet, the packet MUST be dropped.

      1 -   CAPWAP DTLS Header.  The CAPWAP DTLS Header (and DTLS
            packet) immediately follows the UDP header (see
            Section 4.2).

4.2.  CAPWAP DTLS Header

   The CAPWAP DTLS Header is used to identify the packet as a DTLS
   encrypted packet.  The first eight bits include the common CAPWAP
   Preamble.  The remaining 24 bits are padding to ensure 4-byte
   alignment, and MAY be used in a future version of the protocol.  The
   DTLS packet [RFC4347] always immediately follows this header.  The
   format of the CAPWAP DTLS Header is as follows:




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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |CAPWAP Preamble|                    Reserved                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   CAPWAP Preamble:  The CAPWAP Preamble is defined in Section 4.1.  The
      CAPWAP Preamble's Payload Type field MUST be set to one (1).

   Reserved:  The 24-bit field is reserved for future use.  All
      implementations complying with this protocol MUST set to zero any
      bits that are reserved in the version of the protocol supported by
      that implementation.  Receivers MUST ignore all bits not defined
      for the version of the protocol they support.

4.3.  CAPWAP Header

   All CAPWAP protocol messages are encapsulated using a common header
   format, regardless of the CAPWAP Control or CAPWAP Data transport
   used to carry the messages.  However, certain flags are not
   applicable for a given transport.  Refer to the specific transport
   section in order to determine which flags are valid.

   Note that the optional fields defined in this section MUST be present
   in the precise order shown below.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |CAPWAP Preamble|  HLEN   |   RID   | WBID    |T|F|L|W|M|K|Flags|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Fragment ID          |     Frag Offset         |Rsvd |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 (optional) Radio MAC Address                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            (optional) Wireless Specific Information           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Payload ....                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   CAPWAP Preamble:  The CAPWAP Preamble is defined in Section 4.1.  The
      CAPWAP Preamble's Payload Type field MUST be set to zero (0).  If
      the CAPWAP DTLS Header is present, the version number in both
      CAPWAP Preambles MUST match.  The reason for this duplicate field
      is to avoid any possible tampering of the version field in the
      preamble that is not encrypted or authenticated.





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   HLEN:  A 5-bit field containing the length of the CAPWAP transport
      header in 4-byte words (similar to IP header length).  This length
      includes the optional headers.

   RID:  A 5-bit field that contains the Radio ID number for this
      packet, whose value is between one (1) and 31.  Given that MAC
      Addresses are not necessarily unique across physical radios in a
      WTP, the Radio Identifier (RID) field is used to indicate with
      which physical radio the message is associated.

   WBID:  A 5-bit field that is the wireless binding identifier.  The
      identifier will indicate the type of wireless packet associated
      with the radio.  The following values are defined:

      0 -  Reserved

      1 -  IEEE 802.11

      2 -  Reserved

      3 -  EPCGlobal [EPCGlobal]

   T: The Type 'T' bit indicates the format of the frame being
      transported in the payload.  When this bit is set to one (1), the
      payload has the native frame format indicated by the WBID field.
      When this bit is zero (0), the payload is an IEEE 802.3 frame.

   F: The Fragment 'F' bit indicates whether this packet is a fragment.
      When this bit is one (1), the packet is a fragment and MUST be
      combined with the other corresponding fragments to reassemble the
      complete information exchanged between the WTP and AC.

   L: The Last 'L' bit is valid only if the 'F' bit is set and indicates
      whether the packet contains the last fragment of a fragmented
      exchange between WTP and AC.  When this bit is one (1), the packet
      is the last fragment.  When this bit is (zero) 0, the packet is
      not the last fragment.

   W: The Wireless 'W' bit is used to specify whether the optional
      Wireless Specific Information field is present in the header.  A
      value of one (1) is used to represent the fact that the optional
      header is present.

   M: The Radio MAC 'M' bit is used to indicate that the Radio MAC
      Address optional header is present.  This is used to communicate
      the MAC address of the receiving radio.





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   K: The Keep-Alive 'K' bit indicates the packet is a Data Channel
      Keep-Alive packet.  This packet is used to map the data channel to
      the control channel for the specified Session ID and to maintain
      freshness of the data channel.  The 'K' bit MUST NOT be set for
      data packets containing user data.

   Flags:  A set of reserved bits for future flags in the CAPWAP Header.
      All implementations complying with this protocol MUST set to zero
      any bits that are reserved in the version of the protocol
      supported by that implementation.  Receivers MUST ignore all bits
      not defined for the version of the protocol they support.

   Fragment ID:  A 16-bit field whose value is assigned to each group of
      fragments making up a complete set.  The Fragment ID space is
      managed individually for each direction for every WTP/AC pair.
      The value of Fragment ID is incremented with each new set of
      fragments.  The Fragment ID wraps to zero after the maximum value
      has been used to identify a set of fragments.

   Fragment Offset:  A 13-bit field that indicates where in the payload
      this fragment belongs during re-assembly.  This field is valid
      when the 'F' bit is set to 1.  The fragment offset is measured in
      units of 8 octets (64 bits).  The first fragment has offset zero.
      Note that the CAPWAP protocol does not allow for overlapping
      fragments.

   Reserved:  The 3-bit field is reserved for future use.  All
      implementations complying with this protocol MUST set to zero any
      bits that are reserved in the version of the protocol supported by
      that implementation.  Receivers MUST ignore all bits not defined
      for the version of the protocol they support.

   Radio MAC Address:  This optional field contains the MAC address of
      the radio receiving the packet.  Because the native wireless frame
      format to IEEE 802.3 format causes the MAC address of the WTP's
      radio to be lost, this field allows the address to be communicated
      to the AC.  This field is only present if the 'M' bit is set.  The
      HLEN field assumes 4-byte alignment, and this field MUST be padded
      with zeroes (0x00) if it is not 4-byte aligned.












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      The field contains the basic format:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Length    |                  MAC Address
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Length:  The length of the MAC address field.  The formats and
         lengths specified in [EUI-48] and [EUI-64] are supported.

      MAC Address:  The MAC address of the receiving radio.

   Wireless Specific Information:  This optional field contains
      technology-specific information that may be used to carry per-
      packet wireless information.  This field is only present if the
      'W' bit is set.  The WBID field in the CAPWAP Header is used to
      identify the format of the Wireless-Specific Information optional
      field.  The HLEN field assumes 4-byte alignment, and this field
      MUST be padded with zeroes (0x00) if it is not 4-byte aligned.

      The Wireless-Specific Information field uses the following format:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Length     |                Data...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Length:  The 8-bit field contains the length of the data field,
         with a maximum size of 255.

      Data:  Wireless-specific information, defined by the wireless-
         specific binding specified in the CAPWAP Header's WBID field.

   Payload:  This field contains the header for a CAPWAP Data Message or
      CAPWAP Control Message, followed by the data contained in the
      message.

4.4.  CAPWAP Data Messages

   There are two different types of CAPWAP Data packets: CAPWAP Data
   Channel Keep-Alive packets and Data Payload packets.  The first is
   used by the WTP to synchronize the control and data channels and to
   maintain freshness of the data channel.  The second is used to
   transmit user payloads between the AC and WTP.  This section
   describes both types of CAPWAP Data packet formats.




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   Both CAPWAP Data messages are transmitted on the CAPWAP Data channel.

4.4.1.  CAPWAP Data Channel Keep-Alive

   The CAPWAP Data Channel Keep-Alive packet is used to bind the CAPWAP
   control channel with the data channel, and to maintain freshness of
   the data channel, ensuring that the channel is still functioning.
   The CAPWAP Data Channel Keep-Alive packet is transmitted by the WTP
   when the DataChannelKeepAlive timer expires (see Section 4.7.2).
   When the CAPWAP Data Channel Keep-Alive packet is transmitted, the
   WTP sets the DataChannelDeadInterval timer.

   In the CAPWAP Data Channel Keep-Alive packet, all of the fields in
   the CAPWAP Header, except the HLEN field and the 'K' bit, are set to
   zero upon transmission.  Upon receiving a CAPWAP Data Channel Keep-
   Alive packet, the AC transmits a CAPWAP Data Channel Keep-Alive
   packet back to the WTP.  The contents of the transmitted packet are
   identical to the contents of the received packet.

   Upon receiving a CAPWAP Data Channel Keep-Alive packet, the WTP
   cancels the DataChannelDeadInterval timer and resets the
   DataChannelKeepAlive timer.  The CAPWAP Data Channel Keep-Alive
   packet is retransmitted by the WTP in the same manner as the CAPWAP
   Control messages.  If the DataChannelDeadInterval timer expires, the
   WTP tears down the control DTLS session, and the data DTLS session if
   one existed.

   The CAPWAP Data Channel Keep-Alive packet contains the following
   payload immediately following the CAPWAP Header (see Section 4.3).

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Message Element Length     |  Message Element [0..N] ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Message Element Length:   The 16-bit Length field indicates the
      number of bytes following the CAPWAP Header, with a maximum size
      of 65535.

   Message Element[0..N]:   The message element(s) carry the information
      pertinent to each of the CAPWAP Data Channel Keep-Alive message.
      The following message elements MUST be present in this CAPWAP
      message:

         Session ID, see Section 4.6.37.





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4.4.2.  Data Payload

   A CAPWAP protocol Data Payload packet encapsulates a forwarded
   wireless frame.  The CAPWAP protocol defines two different modes of
   encapsulation: IEEE 802.3 and native wireless.  IEEE 802.3
   encapsulation requires that for 802.11 frames, the 802.11
   *Integration* function be performed in the WTP.  An IEEE 802.3-
   encapsulated user payload frame has the following format:

       +------------------------------------------------------+
       | IP Header | UDP Header | CAPWAP Header | 802.3 Frame |
       +------------------------------------------------------+

   The CAPWAP protocol also defines the native wireless encapsulation
   mode.  The format of the encapsulated CAPWAP Data frame is subject to
   the rules defined by the specific wireless technology binding.  Each
   wireless technology binding MUST contain a section entitled "Payload
   Encapsulation", which defines the format of the wireless payload that
   is encapsulated within CAPWAP Data packets.

   For 802.3 payload frames, the 802.3 frame is encapsulated (excluding
   the IEEE 802.3 Preamble, Start Frame Delimiter (SFD), and Frame Check
   Sequence (FCS) fields).  If the encapsulated frame would exceed the
   transport layer's MTU, the sender is responsible for the
   fragmentation of the frame, as specified in Section 3.4.  The CAPWAP
   protocol can support IEEE 802.3 frames whose length is defined in the
   IEEE 802.3as specification [FRAME-EXT].

4.4.3.  Establishment of a DTLS Data Channel

   If the AC and WTP are configured to tunnel the data channel over
   DTLS, the proper DTLS session must be initiated.  To avoid having to
   reauthenticate and reauthorize an AC and WTP, the DTLS data channel
   SHOULD be initiated using the TLS session resumption feature
   [RFC5246].

   The AC DTLS implementation MUST NOT initiate a data channel session
   for a DTLS session for which there is no active control channel
   session.

4.5.  CAPWAP Control Messages

   The CAPWAP Control protocol provides a control channel between the
   WTP and the AC.  Control messages are divided into the following
   message types:

   Discovery:  CAPWAP Discovery messages are used to identify potential
      ACs, their load and capabilities.



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   Join:  CAPWAP Join messages are used by a WTP to request service from
      an AC, and for the AC to respond to the WTP.

   Control Channel Management:  CAPWAP Control channel management
      messages are used to maintain the control channel.

   WTP Configuration Management:  The WTP Configuration messages are
      used by the AC to deliver a specific configuration to the WTP.
      Messages that retrieve statistics from a WTP are also included in
      WTP Configuration Management.

   Station Session Management:  Station Session Management messages are
      used by the AC to deliver specific station policies to the WTP.

   Device Management Operations:  Device management operations are used
      to request and deliver a firmware image to the WTP.

   Binding-Specific CAPWAP Management Messages:  Messages in this
      category are used by the AC and the WTP to exchange protocol-
      specific CAPWAP management messages.  These messages may or may
      not be used to change the link state of a station.

   Discovery, Join, Control Channel Management, WTP Configuration
   Management, and Station Session Management CAPWAP Control messages
   MUST be implemented.  Device Management Operations messages MAY be
   implemented.

   CAPWAP Control messages sent from the WTP to the AC indicate that the
   WTP is operational, providing an implicit keep-alive mechanism for
   the WTP.  The Control Channel Management Echo Request and Echo
   Response messages provide an explicit keep-alive mechanism when other
   CAPWAP Control messages are not exchanged.

4.5.1.  Control Message Format

   All CAPWAP Control messages are sent encapsulated within the CAPWAP
   Header (see Section 4.3).  Immediately following the CAPWAP Header is
   the control header, which has the following format:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Message Type                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Seq Num    |        Msg Element Length     |     Flags     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Msg Element [0..N] ...
     +-+-+-+-+-+-+-+-+-+-+-+-+



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4.5.1.1.  Message Type

   The Message Type field identifies the function of the CAPWAP Control
   message.  To provide extensibility, the Message Type field is
   comprised of an IANA Enterprise Number [RFC3232] and an enterprise-
   specific message type number.  The first three octets contain the
   IANA Enterprise Number in network byte order, with zero used for
   CAPWAP base protocol (this specification) defined message types.  The
   last octet is the enterprise-specific message type number, which has
   a range from 0 to 255.

   The Message Type field is defined as:

         Message Type =
                 IANA Enterprise Number * 256 +
                     Enterprise Specific Message Type Number

   The CAPWAP protocol reliability mechanism requires that messages be
   defined in pairs, consisting of both a Request and a Response
   message.  The Response message MUST acknowledge the Request message.
   The assignment of CAPWAP Control Message Type Values always occurs in
   pairs.  All Request messages have odd numbered Message Type Values,
   and all Response messages have even numbered Message Type Values.
   The Request value MUST be assigned first.  As an example, assigning a
   Message Type Value of 3 for a Request message and 4 for a Response
   message is valid, while assigning a Message Type Value of 4 for a
   Response message and 5 for the corresponding Request message is
   invalid.

   When a WTP or AC receives a message with a Message Type Value field
   that is not recognized and is an odd number, the number in the
   Message Type Value Field is incremented by one, and a Response
   message with a Message Type Value field containing the incremented
   value and containing the Result Code message element with the value
   (Unrecognized Request) is returned to the sender of the received
   message.  If the unknown message type is even, the message is
   ignored.














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   The valid values for CAPWAP Control Message Types are specified in
   the table below:

           CAPWAP Control Message           Message Type
                                              Value
           Discovery Request                    1
           Discovery Response                   2
           Join Request                         3
           Join Response                        4
           Configuration Status Request         5
           Configuration Status Response        6
           Configuration Update Request         7
           Configuration Update Response        8
           WTP Event Request                    9
           WTP Event Response                  10
           Change State Event Request          11
           Change State Event Response         12
           Echo Request                        13
           Echo Response                       14
           Image Data Request                  15
           Image Data Response                 16
           Reset Request                       17
           Reset Response                      18
           Primary Discovery Request           19
           Primary Discovery Response          20
           Data Transfer Request               21
           Data Transfer Response              22
           Clear Configuration Request         23
           Clear Configuration Response        24
           Station Configuration Request       25
           Station Configuration Response      26

4.5.1.2.  Sequence Number

   The Sequence Number field is an identifier value used to match
   Request and Response packets.  When a CAPWAP packet with a Request
   Message Type Value is received, the value of the Sequence Number
   field is copied into the corresponding Response message.

   When a CAPWAP Control message is sent, the sender's internal sequence
   number counter is monotonically incremented, ensuring that no two
   pending Request messages have the same sequence number.  The Sequence
   Number field wraps back to zero.

4.5.1.3.  Message Element Length

   The Length field indicates the number of bytes following the Sequence
   Number field.



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4.5.1.4.  Flags

   The Flags field MUST be set to zero.

4.5.1.5.  Message Element [0..N]

   The message element(s) carry the information pertinent to each of the
   control message types.  Every control message in this specification
   specifies which message elements are permitted.

   When a WTP or AC receives a CAPWAP message without a message element
   that is specified as mandatory for the CAPWAP message, then the
   CAPWAP message is discarded.  If the received message was a Request
   message for which the corresponding Response message carries message
   elements, then a corresponding Response message with a Result Code
   message element indicating "Failure - Missing Mandatory Message
   Element" is returned to the sender.

   When a WTP or AC receives a CAPWAP message with a message element
   that the WTP or AC does not recognize, the CAPWAP message is
   discarded.  If the received message was a Request message for which
   the corresponding Response message carries message elements, then a
   corresponding Response message with a Result Code message element
   indicating "Failure - Unrecognized Message Element" and one or more
   Returned Message Element message elements is included, containing the
   unrecognized message element(s).

4.5.2.  Quality of Service

   The CAPWAP base protocol does not provide any Quality of Service
   (QoS) recommendations for use with the CAPWAP Data messages.  Any
   wireless-specific CAPWAP binding specification that has QoS
   requirements MUST define the application of QoS to the CAPWAP Data
   messages.

   The IP header also includes the Explicit Congestion Notification
   (ECN) bits [RFC3168].  Section 9.1.1 of [RFC3168] describes two
   levels of ECN functionality: full functionality and limited
   functionality.  CAPWAP ACs and WTPs SHALL implement the limited
   functionality and are RECOMMENDED to implement the full functionality
   described in [RFC3168].










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4.5.2.1.  Applying QoS to CAPWAP Control Message

   It is recommended that CAPWAP Control messages be sent by both the AC
   and the WTP with an appropriate Quality-of-Service precedence value,
   ensuring that congestion in the network minimizes occurrences of
   CAPWAP Control channel disconnects.  Therefore, a QoS-enabled CAPWAP
   device SHOULD use the following values:

   802.1Q:   The priority tag of 7 SHOULD be used.

   DSCP:   The CS6 per-hop behavior Service Class SHOULD be used, which
      is described in [RFC2474]).

4.5.3.  Retransmissions

   The CAPWAP Control protocol operates as a reliable transport.  For
   each Request message, a Response message is defined, which is used to
   acknowledge receipt of the Request message.  In addition, the control
   header Sequence Number field is used to pair the Request and Response
   messages (see Section 4.5.1).

   Response messages are not explicitly acknowledged; therefore, if a
   Response message is not received, the original Request message is
   retransmitted.

   Implementations MUST keep track of the sequence number of the last
   received Request message, and MUST cache the corresponding Response
   message.  If a retransmission with the same sequence number is
   received, the cached Response message MUST be retransmitted without
   re-processing the Request.  If an older Request message is received,
   meaning one where the sequence number is smaller, it MUST be ignored.
   A newer Request message, meaning one whose sequence number is larger,
   is processed as usual.

   Note: A sequence number is considered "smaller" when s1 is smaller
   than s2 modulo 256 if and only if (s1<s2 and (s2-s1)<128) or
   (s1>s2 and (s1-s2)>128).

   Both the WTP and the AC can only have a single request outstanding at
   any given time.  Retransmitted Request messages MUST NOT be altered
   by the sender.

   After transmitting a Request message, the RetransmitInterval (see
   Section 4.7) timer and MaxRetransmit (see Section 4.8) variable are
   used to determine if the original Request message needs to be
   retransmitted.  The RetransmitInterval timer is used the first time
   the Request is retransmitted.  The timer is then doubled every




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   subsequent time the same Request message is retransmitted, up to
   MaxRetransmit but no more than half the EchoInterval timer (see
   Section 4.7.7).  Response messages are not subject to these timers.

   If the sender stops retransmitting a Request message before reaching
   MaxRetransmit retransmissions (which leads to transition to DTLS
   Teardown, as described in Section 2.3.1), it cannot know whether the
   recipient received and processed the Request or not.  In most
   situations, the sender SHOULD NOT do this, and instead continue
   retransmitting until a Response message is received, or transition to
   DTLS Teardown occurs.  However, if the sender does decide to continue
   the connection with a new or modified Request message, the new
   message MUST have a new sequence number, and be treated as a new
   Request message by the receiver.  Note that there is a high chance
   that both the WTP and the AC's sequence numbers will become out of
   sync.

   When a Request message is retransmitted, it MUST be re-encrypted via
   the DTLS stack.  If the peer had received the Request message, and
   the corresponding Response message was lost, it is necessary to
   ensure that retransmitted Request messages are not identified as
   replays by the DTLS stack.  Similarly, any cached Response messages
   that are retransmitted as a result of receiving a retransmitted
   Request message MUST be re-encrypted via DTLS.

   Duplicate Response messages, identified by the Sequence Number field
   in the CAPWAP Control message header, SHOULD be discarded upon
   receipt.

4.6.  CAPWAP Protocol Message Elements

   This section defines the CAPWAP Protocol message elements that are
   included in CAPWAP protocol control messages.

   Message elements are used to carry information needed in control
   messages.  Every message element is identified by the Type Value
   field, defined below.  The total length of the message elements is
   indicated in the message element's length field.

   All of the message element definitions in this document use a diagram
   similar to the one below in order to depict its format.  Note that to
   simplify this specification, these diagrams do not include the header
   fields (Type and Length).  The header field values are defined in the
   message element descriptions.







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   Unless otherwise specified, a control message that lists a set of
   supported (or expected) message elements MUST NOT expect the message
   elements to be in any specific order.  The sender MAY include the
   message elements in any order.  Unless otherwise noted, one message
   element of each type is present in a given control message.

   Unless otherwise specified, any configuration information sent by the
   AC to the WTP MAY be saved to non-volatile storage (see Section 8.1)
   for more information).

   Additional message elements may be defined in separate IETF
   documents.

   The format of a message element uses the TLV format shown here:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Type             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Value ...   |
     +-+-+-+-+-+-+-+-+

   The 16-bit Type field identifies the information carried in the Value
   field and Length (16 bits) indicates the number of bytes in the Value
   field.  The value of zero (0) is reserved and MUST NOT be used.  The
   rest of the Type field values are allocated as follows:

              Usage                              Type Values

   CAPWAP Protocol Message Elements                   1 - 1023
   IEEE 802.11 Message Elements                    1024 - 2047
   Reserved for Future Use                         2048 - 3071
   EPCGlobal Message Elements                      3072 - 4095
   Reserved for Future Use                         4096 - 65535

   The table below lists the CAPWAP protocol Message Elements and their
   Type values.













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   CAPWAP Message Element                            Type Value

   AC Descriptor                                         1
   AC IPv4 List                                          2
   AC IPv6 List                                          3
   AC Name                                               4
   AC Name with Priority                                 5
   AC Timestamp                                          6
   Add MAC ACL Entry                                     7
   Add Station                                           8
   Reserved                                              9
   CAPWAP Control IPV4 Address                          10
   CAPWAP Control IPV6 Address                          11
   CAPWAP Local IPV4 Address                            30
   CAPWAP Local IPV6 Address                            50
   CAPWAP Timers                                        12
   CAPWAP Transport Protocol                            51
   Data Transfer Data                                   13
   Data Transfer Mode                                   14
   Decryption Error Report                              15
   Decryption Error Report Period                       16
   Delete MAC ACL Entry                                 17
   Delete Station                                       18
   Reserved                                             19
   Discovery Type                                       20
   Duplicate IPv4 Address                               21
   Duplicate IPv6 Address                               22
   ECN Support                                          53
   Idle Timeout                                         23
   Image Data                                           24
   Image Identifier                                     25
   Image Information                                    26
   Initiate Download                                    27
   Location Data                                        28
   Maximum Message Length                               29
   MTU Discovery Padding                                52
   Radio Administrative State                           31
   Radio Operational State                              32
   Result Code                                          33
   Returned Message Element                             34
   Session ID                                           35
   Statistics Timer                                     36
   Vendor Specific Payload                              37
   WTP Board Data                                       38
   WTP Descriptor                                       39
   WTP Fallback                                         40
   WTP Frame Tunnel Mode                                41
   Reserved                                             42



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   Reserved                                             43
   WTP MAC Type                                         44
   WTP Name                                             45
   Unused/Reserved                                      46
   WTP Radio Statistics                                 47
   WTP Reboot Statistics                                48
   WTP Static IP Address Information                    49

4.6.1.  AC Descriptor

   The AC Descriptor message element is used by the AC to communicate
   its current state.  The value contains the following fields.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Stations           |             Limit             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Active WTPs          |            Max WTPs           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Security   |  R-MAC Field  |   Reserved1   |  DTLS Policy  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  AC Information Sub-Element...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   1 for AC Descriptor

   Length:   >= 12

   Stations:   The number of stations currently served by the AC

   Limit:   The maximum number of stations supported by the AC

   Active WTPs:   The number of WTPs currently attached to the AC

   Max WTPs:   The maximum number of WTPs supported by the AC

   Security:   An 8-bit mask specifying the authentication credential
      type supported by the AC (see Section 2.4.4).  The field has the
      following format:

         0 1 2 3 4 5 6 7
        +-+-+-+-+-+-+-+-+
        |Reserved |S|X|R|
        +-+-+-+-+-+-+-+-+






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      Reserved:  A set of reserved bits for future use.  All
         implementations complying with this protocol MUST set to zero
         any bits that are reserved in the version of the protocol
         supported by that implementation.  Receivers MUST ignore all
         bits not defined for the version of the protocol they support.

      S:    The AC supports the pre-shared secret authentication, as
            described in Section 12.6.

      X:    The AC supports X.509 Certificate authentication, as
            described in Section 12.7.

      R:    A reserved bit for future use.  All implementations
            complying with this protocol MUST set to zero any bits that
            are reserved in the version of the protocol supported by
            that implementation.  Receivers MUST ignore all bits not
            defined for the version of the protocol they support.

   R-MAC Field:   The AC supports the optional Radio MAC Address field
      in the CAPWAP transport header (see Section 4.3).  The following
      enumerated values are supported:

      0 -  Reserved

      1 -  Supported

      2 -  Not Supported

   Reserved:  A set of reserved bits for future use.  All
      implementations complying with this protocol MUST set to zero any
      bits that are reserved in the version of the protocol supported by
      that implementation.  Receivers MUST ignore all bits not defined
      for the version of the protocol they support.

   DTLS Policy:   The AC communicates its policy on the use of DTLS for
      the CAPWAP data channel.  The AC MAY communicate more than one
      supported option, represented by the bit field below.  The WTP
      MUST abide by one of the options communicated by AC.  The field
      has the following format:

         0 1 2 3 4 5 6 7
        +-+-+-+-+-+-+-+-+
        |Reserved |D|C|R|
        +-+-+-+-+-+-+-+-+







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      Reserved:  A set of reserved bits for future use.  All
         implementations complying with this protocol MUST set to zero
         any bits that are reserved in the version of the protocol
         supported by that implementation.  Receivers MUST ignore all
         bits not defined for the version of the protocol they support.

      D:    DTLS-Enabled Data Channel Supported

      C:    Clear Text Data Channel Supported

      R:    A reserved bit for future use.  All implementations
            complying with this protocol MUST set to zero any bits that
            are reserved in the version of the protocol supported by
            that implementation.  Receivers MUST ignore all bits not
            defined for the version of the protocol they support.

   AC Information Sub-Element:   The AC Descriptor message element
      contains multiple AC Information sub-elements, and defines two
      sub-types, each of which MUST be present.  The AC Information sub-
      element has the following format:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                AC Information Vendor Identifier               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      AC Information Type      |     AC Information Length     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     AC Information Data...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      AC Information Vendor Identifier:   A 32-bit value containing the
         IANA-assigned "Structure of Management Information (SMI)
         Network Management Private Enterprise Codes".

      AC Information Type:   Vendor-specific encoding of AC information
         in the UTF-8 format [RFC3629].  The following enumerated values
         are supported.  Both the Hardware and Software Version sub-
         elements MUST be included in the AC Descriptor message element.
         The values listed below are used in conjunction with the AC
         Information Vendor Identifier field, whose value MUST be set to
         zero (0).  This field, combined with the AC Information Vendor
         Identifier set to a non-zero (0) value, allows vendors to use a
         private namespace.







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         4 -   Hardware Version: The AC's hardware version number.

         5 -   Software Version: The AC's Software (firmware) version
               number.

      AC Information Length:   Length of vendor-specific encoding of AC
         information, with a maximum size of 1024.

      AC Information Data:   Vendor-specific encoding of AC information.

4.6.2.  AC IPv4 List

   The AC IPv4 List message element is used to configure a WTP with the
   latest list of ACs available for the WTP to join.


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       AC IP Address[]                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   2 for AC IPv4 List

   Length:   >= 4

   AC IP Address:   An array of 32-bit integers containing AC IPv4
      Addresses, containing no more than 1024 addresses.

4.6.3.  AC IPv6 List

   The AC IPv6 List message element is used to configure a WTP with the
   latest list of ACs available for the WTP to join.


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       AC IP Address[]                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       AC IP Address[]                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       AC IP Address[]                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       AC IP Address[]                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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   Type:   3 for AC IPV6 List

   Length:   >= 16

   AC IP Address:   An array of 128-bit integers containing AC IPv6
      Addresses, containing no more than 1024 addresses.

4.6.4.  AC Name

   The AC Name message element contains an UTF-8 [RFC3629]
   representation of the AC identity.  The value is a variable-length
   byte string.  The string is NOT zero terminated.

      0
      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |   Name ...
     +-+-+-+-+-+-+-+-+

   Type:   4 for AC Name

   Length:   >= 1

   Name:   A variable-length UTF-8 encoded string [RFC3629] containing
      the AC's name, whose maximum size MUST NOT exceed 512 bytes.

4.6.5.  AC Name with Priority

   The AC Name with Priority message element is sent by the AC to the
   WTP to configure preferred ACs.  The number of instances of this
   message element is equal to the number of ACs configured on the WTP.
   The WTP also uses this message element to send its configuration to
   the AC.

      0                   1
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Priority  |   AC Name...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   5 for AC Name with Priority

   Length:   >= 2

   Priority:   A value between 1 and 255 specifying the priority order
      of the preferred AC.  For instance, the value of one (1) is used
      to set the primary AC, the value of two (2) is used to set the
      secondary, etc.



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   AC Name:   A variable-length UTF-8 encoded string [RFC3629]
      containing the AC name, whose maximum size MUST NOT exceed 512
      bytes.

4.6.6.  AC Timestamp

   The AC Timestamp message element is sent by the AC to synchronize the
   WTP clock.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Timestamp                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   6 for AC Timestamp

   Length:   4

   Timestamp:   The AC's current time, allowing all of the WTPs to be
      time synchronized in the format defined by Network Time Protocol
      (NTP) in RFC 1305 [RFC1305].  Only the most significant 32 bits of
      the NTP time are included in this field.

4.6.7.  Add MAC ACL Entry

   The Add MAC Access Control List (ACL) Entry message element is used
   by an AC to add a MAC ACL list entry on a WTP, ensuring that the WTP
   no longer provides service to the MAC addresses provided in the
   message.  The MAC addresses provided in this message element are not
   expected to be saved in non-volatile memory on the WTP.  The MAC ACL
   table on the WTP is cleared every time the WTP establishes a new
   session with an AC.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Num of Entries|    Length     |         MAC Address ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   7 for Add MAC ACL Entry

   Length:   >= 8

   Num of Entries:   The number of instances of the Length/MAC Address
      fields in the array.  This value MUST NOT exceed 255.





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   Length:  The length of the MAC Address field.  The formats and
      lengths specified in [EUI-48] and [EUI-64] are supported.

   MAC Address:   MAC addresses to add to the ACL.

4.6.8.  Add Station

   The Add Station message element is used by the AC to inform a WTP
   that it should forward traffic for a station.  The Add Station
   message element is accompanied by technology-specific binding
   information element(s), which may include security parameters.
   Consequently, the security parameters MUST be applied by the WTP for
   the station.

   After station policy has been delivered to the WTP through the Add
   Station message element, an AC MAY change any policies by sending a
   modified Add Station message element.  When a WTP receives an Add
   Station message element for an existing station, it MUST override any
   existing state for the station.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Radio ID   |     Length    |          MAC Address ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  VLAN Name...
     +-+-+-+-+-+-+-+-+

   Type:   8 for Add Station

   Length:   >= 8

   Radio ID:   An 8-bit value representing the radio, whose value is
      between one (1) and 31.

   Length:  The length of the MAC Address field.  The formats and
      lengths specified in [EUI-48] and [EUI-64] are supported.

   MAC Address:   The station's MAC address.

   VLAN Name:   An optional variable-length UTF-8 encoded string
      [RFC3629], with a maximum length of 512 octets, containing the
      VLAN Name on which the WTP is to locally bridge user data.  Note
      this field is only valid with WTPs configured in Local MAC mode.







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4.6.9.  CAPWAP Control IPv4 Address

   The CAPWAP Control IPv4 Address message element is sent by the AC to
   the WTP during the Discovery process and is used by the AC to provide
   the interfaces available on the AC, and the current number of WTPs
   connected.  When multiple CAPWAP Control IPV4 Address message
   elements are returned, the WTP SHOULD perform load balancing across
   the multiple interfaces (see Section 6.1).

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           IP Address                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           WTP Count           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   10 for CAPWAP Control IPv4 Address

   Length:   6

   IP Address:   The IP address of an interface.

   WTP Count:   The number of WTPs currently connected to the interface,
      with a maximum value of 65535.

4.6.10.  CAPWAP Control IPv6 Address

   The CAPWAP Control IPv6 Address message element is sent by the AC to
   the WTP during the Discovery process and is used by the AC to provide
   the interfaces available on the AC, and the current number of WTPs
   connected.  This message element is useful for the WTP to perform
   load balancing across multiple interfaces (see Section 6.1).

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           IP Address                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           IP Address                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           IP Address                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           IP Address                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           WTP Count           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   Type:   11 for CAPWAP Control IPv6 Address

   Length:   18

   IP Address:   The IP address of an interface.

   WTP Count:   The number of WTPs currently connected to the interface,
      with a maximum value of 65535.

4.6.11.  CAPWAP Local IPv4 Address

   The CAPWAP Local IPv4 Address message element is sent by either the
   WTP, in the Join Request, or by the AC, in the Join Response.  The
   CAPWAP Local IPv4 Address message element is used to communicate the
   IP Address of the transmitter.  The receiver uses this to determine
   whether a middlebox exists between the two peers, by comparing the
   source IP address of the packet against the value of the message
   element.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           IP Address                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   30 for CAPWAP Local IPv4 Address

   Length:   4

   IP Address:   The IP address of the sender.

4.6.12.  CAPWAP Local IPv6 Address

   The CAPWAP Local IPv6 Address message element is sent by either the
   WTP, in the Join Request, or by the AC, in the Join Response.  The
   CAPWAP Local IPv6 Address message element is used to communicate the
   IP Address of the transmitter.  The receiver uses this to determine
   whether a middlebox exists between the two peers, by comparing the
   source IP address of the packet against the value of the message
   element.











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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           IP Address                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           IP Address                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           IP Address                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           IP Address                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   50 for CAPWAP Local IPv6 Address

   Length:   16

   IP Address:   The IP address of the sender.

4.6.13.  CAPWAP Timers

   The CAPWAP Timers message element is used by an AC to configure
   CAPWAP timers on a WTP.

      0                   1
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Discovery   | Echo Request  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   12 for CAPWAP Timers

   Length:   2

   Discovery:   The number of seconds between CAPWAP Discovery messages,
      when the WTP is in the Discovery phase.  This value is used to
      configure the MaxDiscoveryInterval timer (see Section 4.7.10).

   Echo Request:   The number of seconds between WTP Echo Request CAPWAP
      messages.  This value is used to configure the EchoInterval timer
      (see Section 4.7.7).  The AC sets its EchoInterval timer to this
      value, plus the maximum retransmission time as described in
      Section 4.5.3.









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4.6.14.  CAPWAP Transport Protocol

   When CAPWAP is run over IPv6, the UDP-Lite or UDP transports MAY be
   used (see Section 3).  The CAPWAP IPv6 Transport Protocol message
   element is used by either the WTP or the AC to signal which transport
   protocol is to be used for the CAPWAP data channel.

   Upon receiving the Join Request, the AC MAY set the CAPWAP Transport
   Protocol to UDP-Lite in the Join Response message if the CAPWAP
   message was received over IPv6, and the CAPWAP Local IPv6 Address
   message element (see Section 4.6.12) is present and no middlebox was
   detected (see Section 11).

   Upon receiving the Join Response, the WTP MAY set the CAPWAP
   Transport Protocol to UDP-Lite in the Configuration Status Request or
   Image Data Request message if the AC advertised support for UDP-Lite,
   the message was received over IPv6, the CAPWAP Local IPv6 Address
   message element (see Section 4.6.12) and no middlebox was detected
   (see Section 11).  Upon receiving either the Configuration Status
   Request or the Image Data Request, the AC MUST observe the preference
   indicated by the WTP in the CAPWAP Transport Protocol, as long as it
   is consistent with what the AC advertised in the Join Response.

   For any other condition, the CAPWAP Transport Protocol MUST be set to
   UDP.

      0
      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |   Transport   |
     +-+-+-+-+-+-+-+-+

   Type:   51 for CAPWAP Transport Protocol

   Length:   1

   Transport:   The transport to use for the CAPWAP Data channel.  The
      following enumerated values are supported:

      1 -   UDP-Lite: The UDP-Lite transport protocol is to be used for
            the CAPWAP Data channel.  Note that this option MUST NOT be
            used if the CAPWAP Control channel is being used over IPv4.

      2 -   UDP: The UDP transport protocol is to be used for the CAPWAP
            Data channel.






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4.6.15.  Data Transfer Data

   The Data Transfer Data message element is used by the WTP to provide
   information to the AC for debugging purposes.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Data Type   |   Data Mode   |         Data Length           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Data ....
     +-+-+-+-+-+-+-+-+

   Type:   13 for Data Transfer Data

   Length:   >= 5

   Data Type:   An 8-bit value representing the transfer Data Type.  The
      following enumerated values are supported:

      1 -  Transfer data is included.

      2 -  Last Transfer Data Block is included (End of File (EOF)).

      5 -  An error occurred.  Transfer is aborted.

   Data Mode:   An 8-bit value describing the type of information being
      transmitted.  The following enumerated values are supported:

      0 -  Reserved

      1 -  WTP Crash Data

      2 -  WTP Memory Dump

   Data Length:   Length of data field, with a maximum size of 65535.

   Data:   Data being transferred from the WTP to the AC, whose type is
      identified via the Data Mode field.












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4.6.16.  Data Transfer Mode

   The Data Transfer Mode message element is used by the WTP to indicate
   the type of data transfer information it is sending to the AC for
   debugging purposes.

      0
      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |   Data Mode   |
     +-+-+-+-+-+-+-+-+

   Type:   14 for Data Transfer Mode

   Length:   1

   Data Mode:   An 8-bit value describing the type of information being
      requested.  The following enumerated values are supported:

      0 -  Reserved

      1 -  WTP Crash Data

      2 -  WTP Memory Dump

4.6.17.  Decryption Error Report

   The Decryption Error Report message element value is used by the WTP
   to inform the AC of decryption errors that have occurred since the
   last report.  Note that this error reporting mechanism is not used if
   encryption and decryption services are provided in the AC.

      0                   1                   2
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Radio ID    |Num Of Entries |     Length    | MAC Address...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   15 for Decryption Error Report

   Length:   >= 9

   Radio ID:   The Radio Identifier refers to an interface index on the
      WTP, whose value is between one (1) and 31.

   Num of Entries:   The number of instances of the Length/MAC Address
      fields in the array.  This field MUST NOT exceed the value of 255.




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   Length:  The length of the MAC Address field.  The formats and
      lengths specified in [EUI-48] and [EUI-64] are supported.

   MAC Address:   MAC address of the station that has caused decryption
      errors.

4.6.18.  Decryption Error Report Period

   The Decryption Error Report Period message element value is used by
   the AC to inform the WTP how frequently it should send decryption
   error report messages.  Note that this error reporting mechanism is
   not used if encryption and decryption services are provided in the
   AC.

      0                   1                   2
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Radio ID    |        Report Interval        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   16 for Decryption Error Report Period

   Length:   3

   Radio ID:   The Radio Identifier refers to an interface index on the
      WTP, whose value is between one (1) and 31.

   Report Interval:   A 16-bit unsigned integer indicating the time, in
      seconds.  The default value for this message element can be found
      in Section 4.7.11.

4.6.19.  Delete MAC ACL Entry

   The Delete MAC ACL Entry message element is used by an AC to delete a
   MAC ACL entry on a WTP, ensuring that the WTP provides service to the
   MAC addresses provided in the message.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Num of Entries|     Length    |          MAC Address ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   17 for Delete MAC ACL Entry

   Length:   >= 8





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   Num of Entries:   The number of instances of the Length/MAC Address
      fields in the array.  This field MUST NOT exceed the value of 255.

   Length:  The length of the MAC Address field.  The formats and
      lengths specified in [EUI-48] and [EUI-64] are supported.

   MAC Address:   An array of MAC addresses to delete from the ACL.

4.6.20.  Delete Station

   The Delete Station message element is used by the AC to inform a WTP
   that it should no longer provide service to a particular station.
   The WTP MUST terminate service to the station immediately upon
   receiving this message element.

   The transmission of a Delete Station message element could occur for
   various reasons, including for administrative reasons, or if the
   station has roamed to another WTP.

   The Delete Station message element MAY be sent by the WTP, in the WTP
   Event Request message, to inform the AC that a particular station is
   no longer being provided service.  This could occur as a result of an
   Idle Timeout (see section 4.4.43), due to internal resource shortages
   or for some other reason.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Radio ID   |     Length    |        MAC Address...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   18 for Delete Station

   Length:   >= 8

   Radio ID:   An 8-bit value representing the radio, whose value is
      between one (1) and 31.

   Length:  The length of the MAC Address field.  The formats and
      lengths specified in [EUI-48] and [EUI-64] are supported.

   MAC Address:   The station's MAC address.

4.6.21.  Discovery Type

   The Discovery Type message element is used by the WTP to indicate how
   it has come to know about the existence of the AC to which it is
   sending the Discovery Request message.



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      0
      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     | Discovery Type|
     +-+-+-+-+-+-+-+-+

   Type:   20 for Discovery Type

   Length:   1

   Discovery Type:   An 8-bit value indicating how the WTP discovered
      the AC.  The following enumerated values are supported:

      0 -   Unknown

      1 -   Static Configuration

      2 -   DHCP

      3 -   DNS

      4 -   AC Referral (used when the AC was configured either through
            the AC IPv4 List or AC IPv6 List message element)

4.6.22.  Duplicate IPv4 Address

   The Duplicate IPv4 Address message element is used by a WTP to inform
   an AC that it has detected another IP device using the same IP
   address that the WTP is currently using.

   The WTP MUST transmit this message element with the status set to 1
   after it has detected a duplicate IP address.  When the WTP detects
   that the duplicate IP address has been cleared, it MUST send this
   message element with the status set to 0.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          IP Address                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Status    |     Length    |          MAC Address ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   21 for Duplicate IPv4 Address

   Length:   >= 12

   IP Address:   The IP address currently used by the WTP.



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   Status:   The status of the duplicate IP address.  The value MUST be
      set to 1 when a duplicate address is detected, and 0 when the
      duplicate address has been cleared.

   Length:  The length of the MAC Address field.  The formats and
      lengths specified in [EUI-48] and [EUI-64] are supported.

   MAC Address:   The MAC address of the offending device.

4.6.23.  Duplicate IPv6 Address

   The Duplicate IPv6 Address message element is used by a WTP to inform
   an AC that it has detected another host using the same IP address
   that the WTP is currently using.

   The WTP MUST transmit this message element with the status set to 1
   after it has detected a duplicate IP address.  When the WTP detects
   that the duplicate IP address has been cleared, it MUST send this
   message element with the status set to 0.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          IP Address                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          IP Address                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          IP Address                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          IP Address                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Status    |     Length    |         MAC Address ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   22 for Duplicate IPv6 Address

   Length:   >= 24

   IP Address:   The IP address currently used by the WTP.

   Status:   The status of the duplicate IP address.  The value MUST be
      set to 1 when a duplicate address is detected, and 0 when the
      duplicate address has been cleared.

   Length:  The length of the MAC Address field.  The formats and
      lengths specified in [EUI-48] and [EUI-64] are supported.

   MAC Address:   The MAC address of the offending device.



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4.6.24.  Idle Timeout

   The Idle Timeout message element is sent by the AC to the WTP to
   provide the Idle Timeout value that the WTP SHOULD enforce for its
   active stations.  The value applies to all radios on the WTP.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Timeout                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   23 for Idle Timeout

   Length:   4

   Timeout:   The current Idle Timeout, in seconds, to be enforced by
      the WTP.  The default value for this message element is specified
      in Section 4.7.8.

4.6.25.  ECN Support

   The ECN Support message element is sent by both the WTP and the AC to
   indicate their support for the Explicit Congestion Notification (ECN)
   bits, as defined in [RFC3168].

      0
      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |  ECN Support  |
     +-+-+-+-+-+-+-+-+

   Type:   53 for ECN Support

   Length:   1

   ECN Support:   An 8-bit value representing the sender's support for
      ECN, as defined in [RFC3168].  All CAPWAP Implementations MUST
      support the Limited ECN Support mode.  Full ECN Support is used if
      both the WTP and AC advertise the capability for "Full and Limited
      ECN" Support; otherwise, Limited ECN Support is used.

      0 -  Limited ECN Support

      1 -  Full and Limited ECN Support






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4.6.26.  Image Data

   The Image Data message element is present in the Image Data Request
   message sent by the AC and contains the following fields.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Data Type   |                    Data ....
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   24 for Image Data

   Length:   >= 1

   Data Type:   An 8-bit value representing the image Data Type.  The
      following enumerated values are supported:

      1 -  Image data is included.

      2 -  Last Image Data Block is included (EOF).

      5 -  An error occurred.  Transfer is aborted.

   Data:   The Image Data field contains up to 1024 characters, and its
      length is inferred from this message element's length field.  If
      the block being sent is the last one, the Data Type field is set
      to 2.  The AC MAY opt to abort the data transfer by setting the
      Data Type field to 5.  When the Data Type field is 5, the Value
      field has a zero length.

4.6.27.  Image Identifier

   The Image Identifier message element is sent by the AC to the WTP to
   indicate the expected active software version that is to be run on
   the WTP.  The WTP sends the Image Identifier message element in order
   to request a specific software version from the AC.  The actual
   download process is defined in Section 9.1.  The value is a variable-
   length UTF-8 encoded string [RFC3629], which is NOT zero terminated.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Vendor Identifier                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                             Data...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   Type:   25 for Image Identifier

   Length:   >= 5

   Vendor Identifier:   A 32-bit value containing the IANA-assigned "SMI
      Network Management Private Enterprise Codes".

   Data:   A variable-length UTF-8 encoded string [RFC3629] containing
      the firmware identifier to be run on the WTP, whose length MUST
      NOT exceed 1024 octets.  The length of this field is inferred from
      this message element's length field.

4.6.28.  Image Information

   The Image Information message element is present in the Image Data
   Response message sent by the AC to the WTP and contains the following
   fields.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           File Size                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              Hash                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              Hash                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              Hash                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              Hash                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   26 for Image Information

   Length:   20

   File Size:   A 32-bit value containing the size of the file, in
      bytes, that will be transferred by the AC to the WTP.

   Hash:   A 16-octet MD5 hash of the image using the procedures defined
      in [RFC1321].










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4.6.29.  Initiate Download

   The Initiate Download message element is used by the WTP to inform
   the AC that the AC SHOULD initiate a firmware upgrade.  The AC
   subsequently transmits an Image Data Request message, which includes
   the Image Data message element.  This message element does not
   contain any data.

   Type:   27 for Initiate Download

   Length:   0

4.6.30.  Location Data

   The Location Data message element is a variable-length byte UTF-8
   encoded string [RFC3629] containing user-defined location information
   (e.g., "Next to Fridge").  This information is configurable by the
   network administrator, and allows the WTP location to be determined.
   The string is not zero terminated.

      0
      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+-
     | Location ...
     +-+-+-+-+-+-+-+-+-

   Type:   28 for Location Data

   Length:   >= 1

   Location:   A non-zero-terminated UTF-8 encoded string [RFC3629]
      containing the WTP location, whose maximum size MUST NOT exceed
      1024.

4.6.31.  Maximum Message Length

   The Maximum Message Length message element is included in the Join
   Request message by the WTP to indicate the maximum CAPWAP message
   length that it supports to the AC.  The Maximum Message Length
   message element is optionally included in Join Response message by
   the AC to indicate the maximum CAPWAP message length that it supports
   to the WTP.

         0              1
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |    Maximum Message Length     |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   Type:   29 for Maximum Message Length

   Length:   2

   Maximum Message Length  A 16-bit unsigned integer indicating the
      maximum message length.

4.6.32.  MTU Discovery Padding

   The MTU Discovery Padding message element is used as padding to
   perform MTU discovery, and MUST contain octets of value 0xFF, of any
   length.

      0
      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |  Padding...
     +-+-+-+-+-+-+-+-


   Type:   52 for MTU Discovery Padding

   Length:   Variable

   Pad:   A variable-length pad, filled with the value 0xFF.

4.6.33.  Radio Administrative State

   The Radio Administrative State message element is used to communicate
   the state of a particular radio.  The Radio Administrative State
   message element is sent by the AC to change the state of the WTP.
   The WTP saves the value, to ensure that it remains across WTP resets.
   The WTP communicates this message element during the configuration
   phase, in the Configuration Status Request message, to ensure that
   the AC has the WTP radio current administrative state settings.  The
   message element contains the following fields:

         0                   1
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Radio ID    |  Admin State  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   31 for Radio Administrative State

   Length:   2





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   Radio ID:   An 8-bit value representing the radio to configure, whose
      value is between one (1) and 31.  The Radio ID field MAY also
      include the value of 0xff, which is used to identify the WTP.  If
      an AC wishes to change the administrative state of a WTP, it
      includes 0xff in the Radio ID field.

   Admin State:   An 8-bit value representing the administrative state
      of the radio.  The default value for the Admin State field is
      listed in Section 4.8.1.  The following enumerated values are
      supported:

      0 -  Reserved

      1 -  Enabled

      2 -  Disabled

4.6.34.  Radio Operational State

   The Radio Operational State message element is sent by the WTP to the
   AC to communicate a radio's operational state.  This message element
   is included in the Configuration Update Response message by the WTP
   if it was requested to change the state of its radio, via the Radio
   Administrative State message element, but was unable to comply to the
   request.  This message element is included in the Change State Event
   message when a WTP radio state was changed unexpectedly.  This could
   occur due to a hardware failure.  Note that the operational state
   setting is not saved on the WTP, and therefore does not remain across
   WTP resets.  The value contains three fields, as shown below.

      0                   1                   2
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Radio ID    |     State     |     Cause     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   32 for Radio Operational State

   Length:   3

   Radio ID:   The Radio Identifier refers to an interface index on the
      WTP, whose value is between one (1) and 31.  A value of 0xFF is
      invalid, as it is not possible to change the WTP's operational
      state.

   State:   An 8-bit Boolean value representing the state of the radio.
      The following enumerated values are supported:




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      0 -  Reserved

      1 -  Enabled

      2 -  Disabled

   Cause:   When a radio is inoperable, the cause field contains the
      reason the radio is out of service.  The following enumerated
      values are supported:

      0 -  Normal

      1 -  Radio Failure

      2 -  Software Failure

      3 -  Administratively Set

4.6.35.  Result Code

   The Result Code message element value is a 32-bit integer value,
   indicating the result of the Request message corresponding to the
   sequence number included in the Response message.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Result Code                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   33 for Result Code

   Length:   4

   Result Code:   The following enumerated values are defined:

      0  Success

      1  Failure (AC List Message Element MUST Be Present)

      2  Success (NAT Detected)

      3  Join Failure (Unspecified)

      4  Join Failure (Resource Depletion)

      5  Join Failure (Unknown Source)




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      6  Join Failure (Incorrect Data)

      7  Join Failure (Session ID Already in Use)

      8  Join Failure (WTP Hardware Not Supported)

      9  Join Failure (Binding Not Supported)

      10 Reset Failure (Unable to Reset)

      11 Reset Failure (Firmware Write Error)

      12 Configuration Failure (Unable to Apply Requested Configuration
         - Service Provided Anyhow)

      13 Configuration Failure (Unable to Apply Requested Configuration
         - Service Not Provided)

      14 Image Data Error (Invalid Checksum)

      15 Image Data Error (Invalid Data Length)

      16 Image Data Error (Other Error)

      17 Image Data Error (Image Already Present)

      18 Message Unexpected (Invalid in Current State)

      19 Message Unexpected (Unrecognized Request)

      20 Failure - Missing Mandatory Message Element

      21 Failure - Unrecognized Message Element

      22 Data Transfer Error (No Information to Transfer)

4.6.36.  Returned Message Element

   The Returned Message Element is sent by the WTP in the Change State
   Event Request message to communicate to the AC which message elements
   in the Configuration Status Response it was unable to apply locally.
   The Returned Message Element message element contains a result code
   indicating the reason that the configuration could not be applied,
   and encapsulates the failed message element.







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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Reason     |    Length     |       Message Element...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   34 for Returned Message Element

   Length:   >= 6

   Reason:   The reason the configuration in the offending message
      element could not be applied by the WTP.  The following enumerated
      values are supported:

      0 -  Reserved

      1 -  Unknown Message Element

      2 -  Unsupported Message Element

      3 -  Unknown Message Element Value

      4 -  Unsupported Message Element Value

   Length:   The length of the Message Element field, which MUST NOT
      exceed 255 octets.

   Message Element:   The Message Element field encapsulates the message
      element sent by the AC in the Configuration Status Response
      message that caused the error.

4.6.37.  Session ID

   The Session ID message element value contains a randomly generated
   unsigned 128-bit integer.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Session ID                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Session ID                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Session ID                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Session ID                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   Type:   35 for Session ID

   Length:   16

   Session ID:   A 128-bit unsigned integer used as a random session
      identifier

4.6.38.  Statistics Timer

   The Statistics Timer message element value is used by the AC to
   inform the WTP of the frequency with which it expects to receive
   updated statistics.

      0                   1
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Statistics Timer       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   36 for Statistics Timer

   Length:   2

   Statistics Timer:   A 16-bit unsigned integer indicating the time, in
      seconds.  The default value for this timer is specified in
      Section 4.7.14.

4.6.39.  Vendor Specific Payload

   The Vendor Specific Payload message element is used to communicate
   vendor-specific information between the WTP and the AC.  The Vendor
   Specific Payload message element MAY be present in any CAPWAP
   message.  The exchange of vendor-specific data between the MUST NOT
   modify the behavior of the base CAPWAP protocol and state machine.
   The message element uses the following format:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Vendor Identifier                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Element ID           |    Data...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   37 for Vendor Specific Payload

   Length:   >= 7




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   Vendor Identifier:   A 32-bit value containing the IANA-assigned "SMI
      Network Management Private Enterprise Codes" [RFC3232].

   Element ID:   A 16-bit Element Identifier that is managed by the
      vendor.

   Data:   Variable-length vendor-specific information, whose contents
      and format are proprietary and understood based on the Element ID
      field.  This field MUST NOT exceed 2048 octets.

4.6.40.  WTP Board Data

   The WTP Board Data message element is sent by the WTP to the AC and
   contains information about the hardware present.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Vendor Identifier                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Board Data Sub-Element...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   38 for WTP Board Data

   Length:   >=14

   Vendor Identifier:   A 32-bit value containing the IANA-assigned "SMI
      Network Management Private Enterprise Codes", identifying the WTP
      hardware manufacturer.  The Vendor Identifier field MUST NOT be
      set to zero.

   Board Data Sub-Element:   The WTP Board Data message element contains
      multiple Board Data sub-elements, some of which are mandatory and
      some are optional, as described below.  The Board Data Type values
      are not extensible by vendors, and are therefore not coupled along
      with the Vendor Identifier field.  The Board Data sub-element has
      the following format:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Board Data Type        |       Board Data Length       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Board Data Value...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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      Board Data Type:   The Board Data Type field identifies the data
         being encoded.  The CAPWAP protocol defines the following
         values, and each of these types identify whether their presence
         is mandatory or optional:

      0 -   WTP Model Number: The WTP Model Number MUST be included in
            the WTP Board Data message element.

      1 -   WTP Serial Number: The WTP Serial Number MUST be included in
            the WTP Board Data message element.

      2 -   Board ID: A hardware identifier, which MAY be included in
            the WTP Board Data message element.

      3 -   Board Revision: A revision number of the board, which MAY be
            included in the WTP Board Data message element.

      4 -   Base MAC Address: The WTP's Base MAC address, which MAY be
            assigned to the primary Ethernet interface.

   Board Data Length:   The length of the data in the Board Data Value
      field, whose length MUST NOT exceed 1024 octets.

   Board Data Value:   The data associated with the Board Data Type
      field for this Board Data sub-element.

4.6.41.  WTP Descriptor

   The WTP Descriptor message element is used by a WTP to communicate
   its current hardware and software (firmware) configuration.  The
   value contains the following fields:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Max Radios  | Radios in use |  Num Encrypt  |Encryp Sub-Elmt|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Encryption Sub-Element    |    Descriptor Sub-Element...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   39 for WTP Descriptor

   Length:   >= 33








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   Max Radios:   An 8-bit value representing the number of radios (where
      each radio is identified via the Radio ID field) supported by the
      WTP.

   Radios in use:   An 8-bit value representing the number of radios in
      use in the WTP.

   Num Encrypt:   The number of 3-byte Encryption sub-elements that
      follow this field.  The value of the Num Encrypt field MUST be
      between one (1) and 255.

   Encryption Sub-Element:   The WTP Descriptor message element MUST
      contain at least one Encryption sub-element.  One sub-element is
      present for each binding supported by the WTP.  The Encryption
      sub-element has the following format:

      0                   1                   2
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Resvd|  WBID   |  Encryption Capabilities      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Resvd:  The 3-bit field is reserved for future use.  All
         implementations complying with this protocol MUST set to zero
         any bits that are reserved in the version of the protocol
         supported by that implementation.  Receivers MUST ignore all
         bits not defined for the version of the protocol they support.

      WBID:   A 5-bit field that is the wireless binding identifier.
         The identifier will indicate the type of wireless packet
         associated with the radio.  The WBIDs defined in this
         specification can be found in Section 4.3.

      Encryption Capabilities:   This 16-bit field is used by the WTP to
         communicate its capabilities to the AC.  A WTP that does not
         have any encryption capabilities sets this field to zero (0).
         Refer to the specific wireless binding for further
         specification of the Encryption Capabilities field.

   Descriptor Sub-Element:   The WTP Descriptor message element contains
      multiple Descriptor sub-elements, some of which are mandatory and
      some are optional, as described below.  The Descriptor sub-element
      has the following format:








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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  Descriptor Vendor Identifier                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Descriptor Type        |       Descriptor Length       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Descriptor Data...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Descriptor Vendor Identifier:   A 32-bit value containing the
         IANA-assigned "SMI Network Management Private Enterprise
         Codes".

      Descriptor Type:   The Descriptor Type field identifies the data
         being encoded.  The format of the data is vendor-specific
         encoded in the UTF-8 format [RFC3629].  The CAPWAP protocol
         defines the following values, and each of these types identify
         whether their presence is mandatory or optional.  The values
         listed below are used in conjunction with the Descriptor Vendor
         Identifier field, whose value MUST be set to zero (0).  This
         field, combined with the Descriptor Vendor Identifier set to a
         non-zero (0) value, allows vendors to use a private namespace.

         0 -   Hardware Version: The WTP hardware version number MUST be
               present.

         1 -   Active Software Version: The WTP running software version
               number MUST be present.

         2 -   Boot Version: The WTP boot loader version number MUST be
               present.

         3 -   Other Software Version: The WTP non-running software
               (firmware) version number MAY be present.  This type is
               used to communicate alternate software versions that are
               available on the WTP's non-volatile storage.

      Descriptor Length:   Length of the vendor-specific encoding of the
         Descriptor Data field, whose length MUST NOT exceed 1024
         octets.

      Descriptor Data:   Vendor-specific data of WTP information encoded
         in the UTF-8 format [RFC3629].







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4.6.42.  WTP Fallback

   The WTP Fallback message element is sent by the AC to the WTP to
   enable or disable automatic CAPWAP fallback in the event that a WTP
   detects its preferred AC to which it is not currently connected.

      0
      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |     Mode      |
     +-+-+-+-+-+-+-+-+

   Type:   40 for WTP Fallback

   Length:   1

   Mode:   The 8-bit value indicates the status of automatic CAPWAP
      fallback on the WTP.  When enabled, if the WTP detects that its
      primary AC is available, and that the WTP is not connected to the
      primary AC, the WTP SHOULD automatically disconnect from its
      current AC and reconnect to its primary AC.  If disabled, the WTP
      will only reconnect to its primary AC through manual intervention
      (e.g., through the Reset Request message).  The default value for
      this field is specified in Section 4.8.9.  The following
      enumerated values are supported:

      0 -  Reserved

      1 -  Enabled

      2 -  Disabled

4.6.43.  WTP Frame Tunnel Mode

   The WTP Frame Tunnel Mode message element allows the WTP to
   communicate the tunneling modes of operation that it supports to the
   AC.  A WTP that advertises support for all types allows the AC to
   select which type will be used, based on its local policy.

      0
      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |Reservd|N|E|L|U|
     +-+-+-+-+-+-+-+-+







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   Type:   41 for WTP Frame Tunnel Mode

   Length:   1

   Reservd:   A set of reserved bits for future use.  All
      implementations complying with this protocol MUST set to zero any
      bits that are reserved in the version of the protocol supported by
      that implementation.  Receivers MUST ignore all bits not defined
      for the version of the protocol they support.

   N:    Native Frame Tunnel mode requires the WTP and AC to encapsulate
         all user payloads as native wireless frames, as defined by the
         wireless binding (see for example Section 4.4)

   E:    The 802.3 Frame Tunnel Mode requires the WTP and AC to
         encapsulate all user payload as native IEEE 802.3 frames (see
         Section 4.4).  All user traffic is tunneled to the AC.  This
         value MUST NOT be used when the WTP MAC Type is set to Split
         MAC.

   L:    When Local Bridging is used, the WTP does not tunnel user
         traffic to the AC; all user traffic is locally bridged.  This
         value MUST NOT be used when the WTP MAC Type is set to Split
         MAC.

   R:    A reserved bit for future use.  All implementations complying
         with this protocol MUST set to zero any bits that are reserved
         in the version of the protocol supported by that
         implementation.  Receivers MUST ignore all bits not defined for
         the version of the protocol they support.

4.6.44.  WTP MAC Type

   The WTP MAC-Type message element allows the WTP to communicate its
   mode of operation to the AC.  A WTP that advertises support for both
   modes allows the AC to select the mode to use, based on local policy.

      0
      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |   MAC Type    |
     +-+-+-+-+-+-+-+-+

   Type:   44 for WTP MAC Type







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   Length:   1

   MAC Type:   The MAC mode of operation supported by the WTP.  The
      following enumerated values are supported:

      0 -   Local MAC: Local MAC is the default mode that MUST be
            supported by all WTPs.  When tunneling is enabled (see
            Section 4.6.43), the encapsulated frames MUST be in the
            802.3 format (see Section 4.4.2), unless a wireless
            management or control frame which MAY be in its native
            format.  Any CAPWAP binding needs to specify the format of
            management and control wireless frames.

      1 -   Split MAC: Split MAC support is optional, and allows the AC
            to receive and process native wireless frames.

      2 -   Both: WTP is capable of supporting both Local MAC and Split
            MAC.

4.6.45.  WTP Name

   The WTP Name message element is a variable-length byte UTF-8 encoded
   string [RFC3629].  The string is not zero terminated.

      0
      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+-
     |  WTP Name ...
     +-+-+-+-+-+-+-+-+-

   Type:   45 for WTP Name

   Length:   >= 1

   WTP Name:   A non-zero-terminated UTF-8 encoded string [RFC3629]
      containing the WTP name, whose maximum size MUST NOT exceed 512
      bytes.

4.6.46.  WTP Radio Statistics

   The WTP Radio Statistics message element is sent by the WTP to the AC
   to communicate statistics on radio behavior and reasons why the WTP
   radio has been reset.  These counters are never reset on the WTP, and
   will therefore roll over to zero when the maximum size has been
   reached.






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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Radio ID    | Last Fail Type|          Reset Count          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       SW Failure Count        |        HW Failure Count       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Other  Failure Count      |     Unknown Failure Count     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Config Update Count      |     Channel Change Count      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       Band Change Count       |      Current Noise Floor      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   47 for WTP Radio Statistics

   Length:   20

   Radio ID:   The radio ID of the radio to which the statistics apply,
      whose value is between one (1) and 31.

   Last Failure Type:   The last WTP failure.  The following enumerated
      values are supported:

      0 -  Statistic Not Supported

      1 -  Software Failure

      2 -  Hardware Failure

      3 -  Other Failure

      255 -  Unknown (e.g., WTP doesn't keep track of info)

   Reset Count:   The number of times that the radio has been reset.

   SW Failure Count:   The number of times that the radio has failed due
      to software-related reasons.

   HW Failure Count:   The number of times that the radio has failed due
      to hardware-related reasons.

   Other Failure Count:   The number of times that the radio has failed
      due to known reasons, other than software or hardware failure.







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   Unknown Failure Count:   The number of times that the radio has
      failed for unknown reasons.

   Config Update Count:   The number of times that the radio
      configuration has been updated.

   Channel Change Count:   The number of times that the radio channel
      has been changed.

   Band Change Count:   The number of times that the radio has changed
      frequency bands.

   Current Noise Floor:   A signed integer that indicates the noise
      floor of the radio receiver in units of dBm.

4.6.47.  WTP Reboot Statistics

   The WTP Reboot Statistics message element is sent by the WTP to the
   AC to communicate reasons why WTP reboots have occurred.  These
   counters are never reset on the WTP, and will therefore roll over to
   zero when the maximum size has been reached.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Reboot Count          |      AC Initiated Count       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Link Failure Count       |       SW Failure Count        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       HW Failure Count        |      Other Failure Count      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Unknown Failure Count     |Last Failure Type|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   48 for WTP Reboot Statistics

   Length:   15

   Reboot Count:   The number of reboots that have occurred due to a WTP
      crash.  A value of 65535 implies that this information is not
      available on the WTP.

   AC Initiated Count:   The number of reboots that have occurred at the
      request of a CAPWAP protocol message, such as a change in
      configuration that required a reboot or an explicit CAPWAP
      protocol reset request.  A value of 65535 implies that this
      information is not available on the WTP.




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   Link Failure Count:   The number of times that a CAPWAP protocol
      connection with an AC has failed due to link failure.

   SW Failure Count:   The number of times that a CAPWAP protocol
      connection with an AC has failed due to software-related reasons.

   HW Failure Count:   The number of times that a CAPWAP protocol
      connection with an AC has failed due to hardware-related reasons.

   Other Failure Count:   The number of times that a CAPWAP protocol
      connection with an AC has failed due to known reasons, other than
      AC initiated, link, SW or HW failure.

   Unknown Failure Count:   The number of times that a CAPWAP protocol
      connection with an AC has failed for unknown reasons.

   Last Failure Type:   The failure type of the most recent WTP failure.
      The following enumerated values are supported:

      0 -  Not Supported

      1 -  AC Initiated (see Section 9.2)

      2 -  Link Failure

      3 -  Software Failure

      4 -  Hardware Failure

      5 -  Other Failure

      255 -  Unknown (e.g., WTP doesn't keep track of info)

4.6.48.  WTP Static IP Address Information

   The WTP Static IP Address Information message element is used by an
   AC to configure or clear a previously configured static IP address on
   a WTP.  IPv6 WTPs are expected to use dynamic addresses.













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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          IP Address                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Netmask                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Gateway                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Static     |
     +-+-+-+-+-+-+-+-+

   Type:   49 for WTP Static IP Address Information

   Length:   13

   IP Address:   The IP address to assign to the WTP.  This field is
      only valid if the static field is set to one.

   Netmask:   The IP Netmask.  This field is only valid if the static
      field is set to one.

   Gateway:   The IP address of the gateway.  This field is only valid
      if the static field is set to one.

   Static:   An 8-bit Boolean stating whether or not the WTP should use
      a static IP address.  A value of zero disables the static IP
      address, while a value of one enables it.

4.7.  CAPWAP Protocol Timers

   This section contains the definition of the CAPWAP timers.

4.7.1.  ChangeStatePendingTimer

   The maximum time, in seconds, the AC will wait for the Change State
   Event Request from the WTP after having transmitted a successful
   Configuration Status Response message.

   Default: 25 seconds

4.7.2.  DataChannelKeepAlive

   The DataChannelKeepAlive timer is used by the WTP to determine the
   next opportunity when it must transmit the Data Channel Keep-Alive,
   in seconds.

   Default: 30 seconds



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4.7.3.  DataChannelDeadInterval

   The minimum time, in seconds, a WTP MUST wait without having received
   a Data Channel Keep-Alive packet before the destination for the Data
   Channel Keep-Alive packets may be considered dead.  The value of this
   timer MUST be no less than 2*DataChannelKeepAlive seconds and no
   greater that 240 seconds.

   Default: 60

4.7.4.  DataCheckTimer

   The number of seconds the AC will wait for the Data Channel Keep
   Alive, which is required by the CAPWAP state machine's Data Check
   state.  The AC resets the state machine if this timer expires prior
   to transitioning to the next state.

   Default: 30

4.7.5.  DiscoveryInterval

   The minimum time, in seconds, that a WTP MUST wait after receiving a
   Discovery Response message, before initiating a DTLS handshake.

   Default: 5

4.7.6.  DTLSSessionDelete

   The minimum time, in seconds, a WTP MUST wait for DTLS session
   deletion.

   Default: 5

4.7.7.  EchoInterval

   The minimum time, in seconds, between sending Echo Request messages
   to the AC with which the WTP has joined.

   Default: 30

4.7.8.  IdleTimeout

   The default Idle Timeout is 300 seconds.








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4.7.9.  ImageDataStartTimer

   The number of seconds the WTP will wait for its peer to transmit the
   Image Data Request.

   Default: 30

4.7.10.  MaxDiscoveryInterval

   The maximum time allowed between sending Discovery Request messages,
   in seconds.  This value MUST be no less than 2 seconds and no greater
   than 180 seconds.

   Default: 20 seconds.

4.7.11.  ReportInterval

   The ReportInterval is used by the WTP to determine the interval the
   WTP uses between sending the Decryption Error message elements to
   inform the AC of decryption errors, in seconds.

   The default Report Interval is 120 seconds.

4.7.12.  RetransmitInterval

   The minimum time, in seconds, in which a non-acknowledged CAPWAP
   packet will be retransmitted.

   Default: 3

4.7.13.  SilentInterval

   For a WTP, this is the minimum time, in seconds, a WTP MUST wait
   before it MAY again send Discovery Request messages or attempt to
   establish a DTLS session.  For an AC, this is the minimum time, in
   seconds, during which the AC SHOULD ignore all CAPWAP and DTLS
   packets received from the WTP that is in the Sulking state.

   Default: 30 seconds

4.7.14.  StatisticsTimer

   The StatisticsTimer is used by the WTP to determine the interval the
   WTP uses between the WTP Events Requests it transmits to the AC to
   communicate its statistics, in seconds.

   Default: 120 seconds




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4.7.15.  WaitDTLS

   The maximum time, in seconds, a WTP MUST wait without having received
   a DTLS Handshake message from an AC.  This timer MUST be greater than
   30 seconds.

   Default: 60

4.7.16.  WaitJoin

   The maximum time, in seconds, an AC will wait after the DTLS session
   has been established until it receives the Join Request from the WTP.
   This timer MUST be greater than 20 seconds.

   Default: 60

4.8.  CAPWAP Protocol Variables

   This section defines the CAPWAP protocol variables, which are used
   for various protocol functions.  Some of these variables are
   configurable, while others are counters or have a fixed value.  For
   non-counter-related variables, default values are specified.
   However, when a WTP's variable configuration is explicitly overridden
   by an AC, the WTP MUST save the new value.

4.8.1.  AdminState

   The default Administrative State value is enabled (1).

4.8.2.  DiscoveryCount

   The number of Discovery Request messages transmitted by a WTP to a
   single AC.  This is a monotonically increasing counter.

4.8.3.  FailedDTLSAuthFailCount

   The number of failed DTLS session establishment attempts due to
   authentication failures.

4.8.4.  FailedDTLSSessionCount

   The number of failed DTLS session establishment attempts.









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4.8.5.  MaxDiscoveries

   The maximum number of Discovery Request messages that will be sent
   after a WTP boots.

   Default: 10

4.8.6.  MaxFailedDTLSSessionRetry

   The maximum number of failed DTLS session establishment attempts
   before the CAPWAP device enters a silent period.

   Default: 3

4.8.7.  MaxRetransmit

   The maximum number of retransmissions for a given CAPWAP packet
   before the link layer considers the peer dead.

   Default: 5

4.8.8.  RetransmitCount

   The number of retransmissions for a given CAPWAP packet.  This is a
   monotonically increasing counter.

4.8.9.  WTPFallBack

   The default WTP Fallback value is enabled (1).

4.9.  WTP Saved Variables

   In addition to the values defined in Section 4.8, the following
   values SHOULD be saved on the WTP in non-volatile memory.  CAPWAP
   wireless bindings MAY define additional values that SHOULD be stored
   on the WTP.

4.9.1.  AdminRebootCount

   The number of times the WTP has rebooted administratively, defined in
   Section 4.6.47.

4.9.2.  FrameEncapType

   For WTPs that support multiple Frame Encapsulation Types, it is
   useful to save the value configured by the AC.  The Frame
   Encapsulation Type is defined in Section 4.6.43.




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4.9.3.  LastRebootReason

   The reason why the WTP last rebooted, defined in Section 4.6.47.

4.9.4.  MacType

   For WTPs that support multiple MAC-Types, it is useful to save the
   value configured by the AC.  The MAC-Type is defined in
   Section 4.6.44.

4.9.5.  PreferredACs

   The preferred ACs, with the index, defined in Section 4.6.5.

4.9.6.  RebootCount

   The number of times the WTP has rebooted, defined in Section 4.6.47.

4.9.7.  Static IP Address

   The static IP address assigned to the WTP, as configured by the WTP
   Static IP address Information message element (see Section 4.6.48).

4.9.8.  WTPLinkFailureCount

   The number of times the link to the AC has failed, see
   Section 4.6.47.

4.9.9.  WTPLocation

   The WTP Location, defined in Section 4.6.30.

4.9.10.  WTPName

   The WTP Name, defined in Section 4.6.45.

5.  CAPWAP Discovery Operations

   The Discovery messages are used by a WTP to determine which ACs are
   available to provide service, and the capabilities and load of the
   ACs.

5.1.  Discovery Request Message

   The Discovery Request message is used by the WTP to automatically
   discover potential ACs available in the network.  The Discovery
   Request message provides ACs with the primary capabilities of the




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   WTP.  A WTP must exchange this information to ensure subsequent
   exchanges with the ACs are consistent with the WTP's functional
   characteristics.

   Discovery Request messages MUST be sent by a WTP in the Discover
   state after waiting for a random delay less than
   MaxDiscoveryInterval, after a WTP first comes up or is
   (re)initialized.  A WTP MUST send no more than the maximum of
   MaxDiscoveries Discovery Request messages, waiting for a random delay
   less than MaxDiscoveryInterval between each successive message.

   This is to prevent an explosion of WTP Discovery Request messages.
   An example of this occurring is when many WTPs are powered on at the
   same time.

   If a Discovery Response message is not received after sending the
   maximum number of Discovery Request messages, the WTP enters the
   Sulking state and MUST wait for an interval equal to SilentInterval
   before sending further Discovery Request messages.

   Upon receiving a Discovery Request message, the AC will respond with
   a Discovery Response message sent to the address in the source
   address of the received Discovery Request message.  Once a Discovery
   Response has been received, if the WTP decides to establish a session
   with the responding AC, it SHOULD perform an MTU discovery, using the
   process described in Section 3.5.

   It is possible for the AC to receive a clear text Discovery Request
   message while a DTLS session is already active with the WTP.  This is
   most likely the case if the WTP has rebooted, perhaps due to a
   software or power failure, but could also be caused by a DoS attack.
   In such cases, any WTP state, including the state machine instance,
   MUST NOT be cleared until another DTLS session has been successfully
   established, communicated via the DTLSSessionEstablished DTLS
   notification (see Section 2.3.2.2).

   The binding specific WTP Radio Information message element (see
   Section 2.1) is included in the Discovery Request message to
   advertise WTP support for one or more CAPWAP bindings.

   The Discovery Request message is sent by the WTP when in the
   Discovery state.  The AC does not transmit this message.

   The following message elements MUST be included in the Discovery
   Request message:

   o  Discovery Type, see Section 4.6.21




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   o  WTP Board Data, see Section 4.6.40

   o  WTP Descriptor, see Section 4.6.41

   o  WTP Frame Tunnel Mode, see Section 4.6.43

   o  WTP MAC Type, see Section 4.6.44

   o  WTP Radio Information message element(s) that the WTP supports;
      These are defined by the individual link layer CAPWAP Binding
      Protocols (see Section 2.1).

   The following message elements MAY be included in the Discovery
   Request message:

   o  MTU Discovery Padding, see Section 4.6.32

   o  Vendor Specific Payload, see Section 4.6.39

5.2.  Discovery Response Message

   The Discovery Response message provides a mechanism for an AC to
   advertise its services to requesting WTPs.

   When a WTP receives a Discovery Response message, it MUST wait for an
   interval not less than DiscoveryInterval for receipt of additional
   Discovery Response messages.  After the DiscoveryInterval elapses,
   the WTP enters the DTLS-Init state and selects one of the ACs that
   sent a Discovery Response message and send a DTLS Handshake to that
   AC.

   One or more binding-specific WTP Radio Information message elements
   (see Section 2.1) are included in the Discovery Request message to
   advertise AC support for the CAPWAP bindings.  The AC MAY include
   only the bindings it shares in common with the WTP, known through the
   WTP Radio Information message elements received in the Discovery
   Request message, or it MAY include all of the bindings supported.
   The WTP MAY use the supported bindings in its AC decision process.
   Note that if the WTP joins an AC that does not support a specific
   CAPWAP binding, service for that binding MUST NOT be provided by the
   WTP.

   The Discovery Response message is sent by the AC when in the Idle
   state.  The WTP does not transmit this message.

   The following message elements MUST be included in the Discovery
   Response Message:




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   o  AC Descriptor, see Section 4.6.1

   o  AC Name, see Section 4.6.4

   o  WTP Radio Information message element(s) that the AC supports;
      these are defined by the individual link layer CAPWAP Binding
      Protocols (see Section 2.1 for more information).

   o  One of the following message elements MUST be included in the
      Discovery Response Message:

      *  CAPWAP Control IPv4 Address, see Section 4.6.9

      *  CAPWAP Control IPv6 Address, see Section 4.6.10

   The following message elements MAY be included in the Discovery
   Response message:

   o  Vendor Specific Payload, see Section 4.6.39

5.3.  Primary Discovery Request Message

   The Primary Discovery Request message is sent by the WTP to:

   o  determine whether its preferred (or primary) AC is available, or

   o  perform a Path MTU Discovery (see Section 3.5).

   A Primary Discovery Request message is sent by a WTP when it has a
   primary AC configured, and is connected to another AC.  This
   generally occurs as a result of a failover, and is used by the WTP as
   a means to discover when its primary AC becomes available.  Since the
   WTP only has a single instance of the CAPWAP state machine, the
   Primary Discovery Request is sent by the WTP when in the Run state.
   The AC does not transmit this message.

   The frequency of the Primary Discovery Request messages should be no
   more often than the sending of the Echo Request message.

   Upon receipt of a Primary Discovery Request message, the AC responds
   with a Primary Discovery Response message sent to the address in the
   source address of the received Primary Discovery Request message.

   The following message elements MUST be included in the Primary
   Discovery Request message.

   o  Discovery Type, see Section 4.6.21




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   o  WTP Board Data, see Section 4.6.40

   o  WTP Descriptor, see Section 4.6.41

   o  WTP Frame Tunnel Mode, see Section 4.6.43

   o  WTP MAC Type, see Section 4.6.44

   o  WTP Radio Information message element(s) that the WTP supports;
      these are defined by the individual link layer CAPWAP Binding
      Protocols (see Section 2.1 for more information).

   The following message elements MAY be included in the Primary
   Discovery Request message:

   o  MTU Discovery Padding, see Section 4.6.32

   o  Vendor Specific Payload, see Section 4.6.39

5.4.  Primary Discovery Response

   The Primary Discovery Response message enables an AC to advertise its
   availability and services to requesting WTPs that are configured to
   have the AC as its primary AC.

   The Primary Discovery Response message is sent by an AC after
   receiving a Primary Discovery Request message.

   When a WTP receives a Primary Discovery Response message, it may
   establish a CAPWAP protocol connection to its primary AC, based on
   the configuration of the WTP Fallback Status message element on the
   WTP.

   The Primary Discovery Response message is sent by the AC when in the
   Idle state.  The WTP does not transmit this message.

   The following message elements MUST be included in the Primary
   Discovery Response message.

   o  AC Descriptor, see Section 4.6.1

   o  AC Name, see Section 4.6.4

   o  WTP Radio Information message element(s) that the AC supports;
      These are defined by the individual link layer CAPWAP Binding
      Protocols (see Section 2.1 for more information).





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   One of the following message elements MUST be included in the
   Discovery Response Message:

   o  CAPWAP Control IPv4 Address, see Section 4.6.9

   o  CAPWAP Control IPv6 Address, see Section 4.6.10

   The following message elements MAY be included in the Primary
   Discovery Response message:

   o  Vendor Specific Payload, see Section 4.6.39

6.  CAPWAP Join Operations

   The Join Request message is used by a WTP to request service from an
   AC after a DTLS connection is established to that AC.  The Join
   Response message is used by the AC to indicate that it will or will
   not provide service.

6.1.  Join Request

   The Join Request message is used by a WTP to request service through
   the AC.  If the WTP is performing the optional AC Discovery process
   (see Section 3.3), the join process occurs after the WTP has received
   one or more Discovery Response messages.  During the Discovery
   process, an AC MAY return more than one CAPWAP Control IPv4 Address
   or CAPWAP Control IPv6 Address message elements.  When more than one
   such message element is returned, the WTP SHOULD perform "load
   balancing" by choosing the interface that is servicing the least
   number of WTPs (known through the WTP Count field of the message
   element).  Note, however, that other load balancing algorithms are
   also permitted.  Once the WTP has determined its preferred AC, and
   its associated interface, to which to connect, it establishes the
   DTLS session, and transmits the Join Request over the secured control
   channel.  When an AC receives a Join Request message it responds with
   a Join Response message.

   Upon completion of the DTLS handshake and receipt of the
   DTLSEstablished notification, the WTP sends the Join Request message
   to the AC.  When the AC is notified of the DTLS session
   establishment, it does not clear the WaitDTLS timer until it has
   received the Join Request message, at which time it sends a Join
   Response message to the WTP, indicating success or failure.

   One or more WTP Radio Information message elements (see Section 2.1)
   are included in the Join Request to request service for the CAPWAP
   bindings by the AC.  Including a binding that is unsupported by the
   AC will result in a failed Join Response.



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   If the AC rejects the Join Request, it sends a Join Response message
   with a failure indication and initiates an abort of the DTLS session
   via the DTLSAbort command.

   If an invalid (i.e., malformed) Join Request message is received, the
   message MUST be silently discarded by the AC.  No response is sent to
   the WTP.  The AC SHOULD log this event.

   The Join Request is sent by the WTP when in the Join State.  The AC
   does not transmit this message.

   The following message elements MUST be included in the Join Request
   message.

   o  Location Data, see Section 4.6.30

   o  WTP Board Data, see Section 4.6.40

   o  WTP Descriptor, see Section 4.6.41

   o  WTP Name, see Section 4.6.45

   o  Session ID, see Section 4.6.37

   o  WTP Frame Tunnel Mode, see Section 4.6.43

   o  WTP MAC Type, see Section 4.6.44

   o  WTP Radio Information message element(s) that the WTP supports;
      these are defined by the individual link layer CAPWAP Binding
      Protocols (see Section 2.1 for more information).

   o  ECN Support, see Section 4.6.25

   At least one of the following message element MUST be included in the
   Join Request message.

   o  CAPWAP Local IPv4 Address, see Section 4.6.11

   o  CAPWAP Local IPv6 Address, see Section 4.6.12

   The following message element MAY be included in the Join Request
   message.

   o  CAPWAP Transport Protocol, see Section 4.6.14

   o  Maximum Message Length, see Section 4.6.31




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   o  WTP Reboot Statistics, see Section 4.6.47

   o  Vendor Specific Payload, see Section 4.6.39

6.2.  Join Response

   The Join Response message is sent by the AC to indicate to a WTP that
   it is capable and willing to provide service to the WTP.

   The WTP, receiving a Join Response message, checks for success or
   failure.  If the message indicates success, the WTP clears the
   WaitDTLS timer for the session and proceeds to the Configure state.

   If the WaitDTLS Timer expires prior to reception of the Join Response
   message, the WTP MUST terminate the handshake, deallocate session
   state and initiate the DTLSAbort command.

   If an invalid (malformed) Join Response message is received, the WTP
   SHOULD log an informative message detailing the error.  This error
   MUST be treated in the same manner as AC non-responsiveness.  The
   WaitDTLS timer will eventually expire, and the WTP MAY (if it is so
   configured) attempt to join a new AC.

   If one of the WTP Radio Information message elements (see
   Section 2.1) in the Join Request message requested support for a
   CAPWAP binding that the AC does not support, the AC sets the Result
   Code message element to "Binding Not Supported".

   The AC includes the Image Identifier message element to indicate the
   software version it expects the WTP to run.  This information is used
   to determine whether the WTP MUST change its currently running
   firmware image or download a new version (see Section 9.1.1).

   The Join Response message is sent by the AC when in the Join State.
   The WTP does not transmit this message.

   The following message elements MUST be included in the Join Response
   message.

   o  Result Code, see Section 4.6.35

   o  AC Descriptor, see Section 4.6.1

   o  AC Name, see Section 4.6.4

   o  WTP Radio Information message element(s) that the AC supports;
      these are defined by the individual link layer CAPWAP Binding
      Protocols (see Section 2.1).



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   o  ECN Support, see Section 4.6.25

   One of the following message elements MUST be included in the Join
   Response Message:

   o  CAPWAP Control IPv4 Address, see Section 4.6.9

   o  CAPWAP Control IPv6 Address, see Section 4.6.10

   One of the following message elements MUST be included in the Join
   Response Message:

   o  CAPWAP Local IPv4 Address, see Section 4.6.11

   o  CAPWAP Local IPv6 Address, see Section 4.6.12

   The following message elements MAY be included in the Join Response
   message.

   o  AC IPv4 List, see Section 4.6.2

   o  AC IPv6 List, see Section 4.6.3

   o  CAPWAP Transport Protocol, see Section 4.6.14

   o  Image Identifier, see Section 4.6.27

   o  Maximum Message Length, see Section 4.6.31

   o  Vendor Specific Payload, see Section 4.6.39

7.  Control Channel Management

   The Control Channel Management messages are used by the WTP and AC to
   maintain a control communication channel.  CAPWAP Control messages,
   such as the WTP Event Request message sent from the WTP to the AC
   indicate to the AC that the WTP is operational.  When such control
   messages are not being sent, the Echo Request and Echo Response
   messages are used to maintain the control communication channel.

7.1.  Echo Request

   The Echo Request message is a keep-alive mechanism for CAPWAP control
   messages.







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   Echo Request messages are sent periodically by a WTP in the Image
   Data or Run state (see Section 2.3) to determine the state of the
   control connection between the WTP and the AC.  The Echo Request
   message is sent by the WTP when the EchoInterval timer expires.

   The Echo Request message is sent by the WTP when in the Run state.
   The AC does not transmit this message.

   The following message elements MAY be included in the Echo Request
   message:

   o  Vendor Specific Payload, see Section 4.6.39

   When an AC receives an Echo Request message it responds with an Echo
   Response message.

7.2.  Echo Response

   The Echo Response message acknowledges the Echo Request message.

   An Echo Response message is sent by an AC after receiving an Echo
   Request message.  After transmitting the Echo Response message, the
   AC SHOULD reset its EchoInterval timer (see Section 4.7.7).  If
   another Echo Request message or other control message is not received
   by the AC when the timer expires, the AC SHOULD consider the WTP to
   be no longer reachable.

   The Echo Response message is sent by the AC when in the Run state.
   The WTP does not transmit this message.

   The following message elements MAY be included in the Echo Response
   message:

   o  Vendor Specific Payload, see Section 4.6.39

   When a WTP receives an Echo Response message it initializes the
   EchoInterval to the configured value.

8.  WTP Configuration Management

   WTP Configuration messages are used to exchange configuration
   information between the AC and the WTP.

8.1.  Configuration Consistency

   The CAPWAP protocol provides flexibility in how WTP configuration is
   managed.  A WTP can behave in one of two ways, which is
   implementation specific:



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   1. The WTP retains no configuration and accepts the configuration
      provided by the AC.

   2. The WTP saves the configuration of parameters provided by the AC
      that are non-default values into local non-volatile memory, and
      are enforced during the WTP's power up initialization phase.

   If the WTP opts to save configuration locally, the CAPWAP protocol
   state machine defines the Configure state, which allows for
   configuration exchange.  In the Configure state, the WTP sends its
   current configuration overrides to the AC via the Configuration
   Status Request message.  A configuration override is a non-default
   parameter.  As an example, in the CAPWAP protocol, the default
   antenna configuration is internal omni antenna.  A WTP that either
   has no internal antennas, or has been explicitly configured by the AC
   to use external antennas, sends its antenna configuration during the
   configure phase, allowing the AC to become aware of the WTP's current
   configuration.

   Once the WTP has provided its configuration to the AC, the AC sends
   its configuration to the WTP.  This allows the WTP to receive
   configuration and policies from the AC.

   The AC maintains a copy of each active WTP configuration.  There is
   no need for versioning or other means to identify configuration
   changes.  If a WTP becomes inactive, the AC MAY delete the inactive
   WTP configuration.  If a WTP fails, and connects to a new AC, the WTP
   provides its overridden configuration parameters, allowing the new AC
   to be aware of the WTP configuration.

   This model allows for resiliency in case of an AC failure, ensuring
   another AC can provide service to the WTP.  A new AC would be
   automatically updated with WTP configuration changes, eliminating the
   need for inter-AC communication and the need for all ACs to be aware
   of the configuration of all WTPs in the network.

   Once the CAPWAP protocol enters the Run state, the WTPs begin to
   provide service.  It is common for administrators to require that
   configuration changes be made while the network is operational.
   Therefore, the Configuration Update Request is sent by the AC to the
   WTP to make these changes at run-time.

8.1.1.  Configuration Flexibility

   The CAPWAP protocol provides the flexibility to configure and manage
   WTPs of varying design and functional characteristics.  When a WTP
   first discovers an AC, it provides primary functional information




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   relating to its type of MAC and to the nature of frames to be
   exchanged.  The AC configures the WTP appropriately.  The AC also
   establishes corresponding internal state for the WTP.

8.2.  Configuration Status Request

   The Configuration Status Request message is sent by a WTP to deliver
   its current configuration to the AC.

   The Configuration Status Request message carries binding-specific
   message elements.  Refer to the appropriate binding for the
   definition of this structure.

   When an AC receives a Configuration Status Request message, it acts
   upon the content of the message and responds to the WTP with a
   Configuration Status Response message.

   The Configuration Status Request message includes multiple Radio
   Administrative State message elements, one for the WTP, and one for
   each radio in the WTP.

   The Configuration Status Request message is sent by the WTP when in
   the Configure State.  The AC does not transmit this message.

   The following message elements MUST be included in the Configuration
   Status Request message.

   o  AC Name, see Section 4.6.4

   o  Radio Administrative State, see Section 4.6.33

   o  Statistics Timer, see Section 4.6.38

   o  WTP Reboot Statistics, see Section 4.6.47

   The following message elements MAY be included in the Configuration
   Status Request message.

   o  AC Name with Priority, see Section 4.6.5

   o  CAPWAP Transport Protocol, see Section 4.6.14

   o  WTP Static IP Address Information, see Section 4.6.48

   o  Vendor Specific Payload, see Section 4.6.39






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8.3.  Configuration Status Response

   The Configuration Status Response message is sent by an AC and
   provides a mechanism for the AC to override a WTP's requested
   configuration.

   A Configuration Status Response message is sent by an AC after
   receiving a Configuration Status Request message.

   The Configuration Status Response message carries binding-specific
   message elements.  Refer to the appropriate binding for the
   definition of this structure.

   When a WTP receives a Configuration Status Response message, it acts
   upon the content of the message, as appropriate.  If the
   Configuration Status Response message includes a Radio Operational
   State message element that causes a change in the operational state
   of one of the radios, the WTP transmits a Change State Event to the
   AC, as an acknowledgement of the change in state.

   The Configuration Status Response message is sent by the AC when in
   the Configure state.  The WTP does not transmit this message.

   The following message elements MUST be included in the Configuration
   Status Response message.

   o  CAPWAP Timers, see Section 4.6.13

   o  Decryption Error Report Period, see Section 4.6.18

   o  Idle Timeout, see Section 4.6.24

   o  WTP Fallback, see Section 4.6.42

   One or both of the following message elements MUST be included in the
   Configuration Status Response message:

   o  AC IPv4 List, see Section 4.6.2

   o  AC IPv6 List, see Section 4.6.3

   The following message element MAY be included in the Configuration
   Status Response message.

   o  WTP Static IP Address Information, see Section 4.6.48

   o  Vendor Specific Payload, see Section 4.6.39




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8.4.  Configuration Update Request

   Configuration Update Request messages are sent by the AC to provision
   the WTP while in the Run state.  This is used to modify the
   configuration of the WTP while it is operational.

   When a WTP receives a Configuration Update Request message, it
   responds with a Configuration Update Response message, with a Result
   Code message element indicating the result of the configuration
   request.

   The AC includes the Image Identifier message element (see
   Section 4.6.27) to force the WTP to update its firmware while in the
   Run state.  The WTP MAY proceed to download the requested firmware if
   it determines the version specified in the Image Identifier message
   element is not in its non-volatile storage by transmitting an Image
   Data Request (see Section 9.1.1) that includes the Initiate Download
   message element (see Section 4.6.29).

   The Configuration Update Request is sent by the AC when in the Run
   state.  The WTP does not transmit this message.

   One or more of the following message elements MAY be included in the
   Configuration Update message:

   o  AC Name with Priority, see Section 4.6.5

   o  AC Timestamp, see Section 4.6.6

   o  Add MAC ACL Entry, see Section 4.6.7

   o  CAPWAP Timers, see Section 4.6.13

   o  Decryption Error Report Period, see Section 4.6.18

   o  Delete MAC ACL Entry, see Section 4.6.19

   o  Idle Timeout, see Section 4.6.24

   o  Location Data, see Section 4.6.30

   o  Radio Administrative State, see Section 4.6.33

   o  Statistics Timer, see Section 4.6.38

   o  WTP Fallback, see Section 4.6.42

   o  WTP Name, see Section 4.6.45



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   o  WTP Static IP Address Information, see Section 4.6.48

   o  Image Identifier, see Section 4.6.27

   o  Vendor Specific Payload, see Section 4.6.39

8.5.  Configuration Update Response

   The Configuration Update Response message is the acknowledgement
   message for the Configuration Update Request message.

   The Configuration Update Response message is sent by a WTP after
   receiving a Configuration Update Request message.

   When an AC receives a Configuration Update Response message, the
   result code indicates if the WTP successfully accepted the
   configuration.

   The Configuration Update Response message is sent by the WTP when in
   the Run state.  The AC does not transmit this message.

   The following message element MUST be present in the Configuration
   Update message.

   Result Code, see Section 4.6.35

   The following message elements MAY be present in the Configuration
   Update Response message.

   o  Radio Operational State, see Section 4.6.34

   o  Vendor Specific Payload, see Section 4.6.39

8.6.  Change State Event Request

   The Change State Event Request message is used by the WTP for two
   main purposes:

   o  When sent by the WTP following the reception of a Configuration
      Status Response message from the AC, the WTP uses the Change State
      Event Request message to provide an update on the WTP radio's
      operational state and to confirm that the configuration provided
      by the AC was successfully applied.

   o  When sent during the Run state, the WTP uses the Change State
      Event Request message to notify the AC of an unexpected change in
      the WTP's radio operational state.




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   When an AC receives a Change State Event Request message it responds
   with a Change State Event Response message and modifies its data
   structures for the WTP as needed.  The AC MAY decide not to provide
   service to the WTP if it receives an error, based on local policy,
   and to transition to the Reset state.

   The Change State Event Request message is sent by a WTP to
   acknowledge or report an error condition to the AC for a requested
   configuration in the Configuration Status Response message.  The
   Change State Event Request message includes the Result Code message
   element, which indicates whether the configuration was successfully
   applied.  If the WTP is unable to apply a specific configuration
   request, it indicates the failure by including one or more Returned
   Message Element message elements (see Section 4.6.36).

   The Change State Event Request message is sent by the WTP in the
   Configure or Run state.  The AC does not transmit this message.

   The WTP MAY save its configuration to persistent storage prior to
   transmitting the response.  However, this is implementation specific
   and is not required.

   The following message elements MUST be present in the Change State
   Event Request message.

   o  Radio Operational State, see Section 4.6.34

   o  Result Code, see Section 4.6.35

   One or more of the following message elements MAY be present in the
   Change State Event Request message:

   o  Returned Message Element(s), see Section 4.6.36

   o  Vendor Specific Payload, see Section 4.6.39

8.7.  Change State Event Response

   The Change State Event Response message acknowledges the Change State
   Event Request message.

   A Change State Event Response message is sent by an AC in response to
   a Change State Event Request message.

   The Change State Event Response message is sent by the AC when in the
   Configure or Run state.  The WTP does not transmit this message.





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   The following message element MAY be included in the Change State
   Event Response message:

   o  Vendor Specific Payload, see Section 4.6.39

   The WTP does not take any action upon receipt of the Change State
   Event Response message.

8.8.  Clear Configuration Request

   The Clear Configuration Request message is used to reset the WTP
   configuration.

   The Clear Configuration Request message is sent by an AC to request
   that a WTP reset its configuration to the manufacturing default
   configuration.  The Clear Config Request message is sent while in the
   Run state.

   The Clear Configuration Request is sent by the AC when in the Run
   state.  The WTP does not transmit this message.

   The following message element MAY be included in the Clear
   Configuration Request message:

   o  Vendor Specific Payload, see Section 4.6.39

   When a WTP receives a Clear Configuration Request message, it resets
   its configuration to the manufacturing default configuration.

8.9.  Clear Configuration Response

   The Clear Configuration Response message is sent by the WTP after
   receiving a Clear Configuration Request message and resetting its
   configuration parameters to the manufacturing default values.

   The Clear Configuration Response is sent by the WTP when in the Run
   state.  The AC does not transmit this message.

   The Clear Configuration Response message MUST include the following
   message element:

   o  Result Code, see Section 4.6.35

   The following message element MAY be included in the Clear
   Configuration Request message:

   o  Vendor Specific Payload, see Section 4.6.39




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9.  Device Management Operations

   This section defines CAPWAP operations responsible for debugging,
   gathering statistics, logging, and firmware management.  The
   management operations defined in this section are used by the AC to
   either push/pull information to/from the WTP, or request that the WTP
   reboot.  This section does not deal with the management of the AC per
   se, and assumes that the AC is operational and configured.

9.1.  Firmware Management

   This section describes the firmware download procedures used by the
   CAPWAP protocol.  Firmware download can occur during the Image Data
   or Run state.  The former allows the download to occur at boot time,
   while the latter is used to trigger the download while an active
   CAPWAP session exists.  It is important to note that the CAPWAP
   protocol does not provide the ability for the AC to identify whether
   the firmware information provided by the WTP is correct or whether
   the WTP is properly storing the firmware (see Section 12.10 for more
   information).

   Figure 6 provides an example of a WTP that performs a firmware
   upgrade while in the Image Data state.  In this example, the WTP does
   not already have the requested firmware (Image Identifier = x), and
   downloads the image from the AC.


























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             WTP                                               AC

                                Join Request
         -------------------------------------------------------->

                     Join Response (Image Identifier = x)
         <------------------------------------------------------

              Image Data Request (Image Identifier = x,
                                  Initiate Download)
         -------------------------------------------------------->

           Image Data Response (Result Code = Success,
                                Image Information = {size,hash})
         <------------------------------------------------------

                Image Data Request (Image Data = Data)
         <------------------------------------------------------

                Image Data Response (Result Code = Success)
         -------------------------------------------------------->

                                  .....

                Image Data Request (Image Data = EOF)
         <------------------------------------------------------

                Image Data Response (Result Code = Success)
         -------------------------------------------------------->

                     (WTP enters the Reset State)

                  Figure 6: WTP Firmware Download Case 1

   Figure 7 provides an example in which the WTP has the image specified
   by the AC in its non-volatile storage, but is not its current running
   image.  In this case, the WTP opts to NOT download the firmware and
   immediately reset to the requested image.













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             WTP                                               AC

                                Join Request
         -------------------------------------------------------->

                     Join Response (Image Identifier = x)
         <------------------------------------------------------

                     (WTP enters the Reset State)

                  Figure 7: WTP Firmware Download Case 2

   Figure 8 provides an example of a WTP that performs a firmware
   upgrade while in the Run state.  This mode of firmware upgrade allows
   the WTP to download its image while continuing to provide service.
   The WTP will not automatically reset until it is notified by the AC,
   with a Reset Request message.


































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             WTP                                               AC

                Configuration Update Request (Image Identifier = x)
         <------------------------------------------------------

            Configuration Update Response (Result Code = Success)
         -------------------------------------------------------->


              Image Data Request (Image Identifier = x,
                                  Initiate Download)
         -------------------------------------------------------->

              Image Data Response (Result Code = Success,
                                   Image Information = {size,hash})
         <------------------------------------------------------

                Image Data Request (Image Data = Data)
         <------------------------------------------------------

                Image Data Response (Result Code = Success)
         -------------------------------------------------------->

                                  .....

                Image Data Request (Image Data = EOF)
         <------------------------------------------------------

                Image Data Response (Result Code = Success)
         -------------------------------------------------------->

                                  .....

                (administratively requested reboot request)
                   Reset Request (Image Identifier = x)
         <------------------------------------------------------

                  Reset Response (Result Code = Success)
         -------------------------------------------------------->

                  Figure 8: WTP Firmware Download Case 3

   Figure 9 provides another example of the firmware download while in
   the Run state.  In this example, the WTP already has the image
   specified by the AC in its non-volatile storage.  The WTP opts to NOT
   download the firmware.  The WTP resets upon receipt of a Reset
   Request message from the AC.




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             WTP                                               AC

             Configuration Update Request (Image Identifier = x)
         <------------------------------------------------------

      Configuration Update Response (Result Code = Already Have Image)
         -------------------------------------------------------->

                                  .....

                (administratively requested reboot request)
                   Reset Request (Image Identifier = x)
         <------------------------------------------------------

                  Reset Response (Result Code = Success)
         -------------------------------------------------------->

                  Figure 9: WTP Firmware Download Case 4

9.1.1.  Image Data Request

   The Image Data Request message is used to update firmware on the WTP.
   This message and its companion Response message are used by the AC to
   ensure that the image being run on each WTP is appropriate.

   Image Data Request messages are exchanged between the WTP and the AC
   to download a new firmware image to the WTP.  When a WTP or AC
   receives an Image Data Request message, it responds with an Image
   Data Response message.  The message elements contained within the
   Image Data Request message are required to determine the intent of
   the request.

   The decision that new firmware is to be downloaded to the WTP can
   occur in one of two ways:

      When the WTP joins the AC, the Join Response message includes the
      Image Identifier message element, which informs the WTP of the
      firmware it is expected to run.  If the WTP does not currently
      have the requested firmware version, it transmits an Image Data
      Request message, with the appropriate Image Identifier message
      element.  If the WTP already has the requested firmware in its
      non-volatile flash, but is not its currently running image, it
      simply resets to run the proper firmware.

      Once the WTP is in the Run state, it is possible for the AC to
      cause the WTP to initiate a firmware download by sending a
      Configuration Update Request message with the Image Identifier
      message elements.  This will cause the WTP to transmit an Image



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      Data Request with the Image Identifier and the Initiate Download
      message elements.  Note that when the firmware is downloaded in
      this way, the WTP does not automatically reset after the download
      is complete.  The WTP will only reset when it receives a Reset
      Request message from the AC.  If the WTP already had the requested
      firmware version in its non-volatile storage, the WTP does not
      transmit the Image Data Request message and responds with a
      Configuration Update Response message with the Result Code set to
      Image Already Present.

   Regardless of how the download was initiated, once the AC receives an
   Image Data Request message with the Image Identifier message element,
   it begins the transfer process by transmitting an Image Data Request
   message that includes the Image Data message element.  This continues
   until the firmware image has been transferred.

   The Image Data Request message is sent by the WTP or the AC when in
   the Image Data or Run state.

   The following message elements MAY be included in the Image Data
   Request message:

   o  CAPWAP Transport Protocol, see Section 4.6.14

   o  Image Data, see Section 4.6.26

   o  Vendor Specific Payload, see Section 4.6.39

   The following message elements MAY be included in the Image Data
   Request message when sent by the WTP:

   o  Image Identifier, see Section 4.6.27

   o  Initiate Download, see Section 4.6.29

9.1.2.  Image Data Response

   The Image Data Response message acknowledges the Image Data Request
   message.

   An Image Data Response message is sent in response to a received
   Image Data Request message.  Its purpose is to acknowledge the
   receipt of the Image Data Request message.  The Result Code is
   included to indicate whether a previously sent Image Data Request
   message was invalid.

   The Image Data Response message is sent by the WTP or the AC when in
   the Image Data or Run state.



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   The following message element MUST be included in the Image Data
   Response message:

   o  Result Code, see Section 4.6.35

   The following message element MAY be included in the Image Data
   Response message:

   o  Vendor Specific Payload, see Section 4.6.39

   The following message element MAY be included in the Image Data
   Response message when sent by the AC:

   o  Image Information, see Section 4.6.28

   Upon receiving an Image Data Response message indicating an error,
   the WTP MAY retransmit a previous Image Data Request message, or
   abandon the firmware download to the WTP by transitioning to the
   Reset state.

9.2.  Reset Request

   The Reset Request message is used to cause a WTP to reboot.

   A Reset Request message is sent by an AC to cause a WTP to
   reinitialize its operation.  If the AC includes the Image Identifier
   message element (see Section 4.6.27), it indicates to the WTP that it
   SHOULD use that version of software upon reboot.

   The Reset Request is sent by the AC when in the Run state.  The WTP
   does not transmit this message.

   The following message element MUST be included in the Reset Request
   message:

   o  Image Identifier, see Section 4.6.27

   The following message element MAY be included in the Reset Request
   message:

   o  Vendor Specific Payload, see Section 4.6.39

   When a WTP receives a Reset Request message, it responds with a Reset
   Response message indicating success and then reinitializes itself.
   If the WTP is unable to write to its non-volatile storage, to ensure
   that it runs the requested software version indicated in the Image
   Identifier message element, it MAY send the appropriate Result Code
   message element, but MUST reboot.  If the WTP is unable to reset,



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   including a hardware reset, it sends a Reset Response message to the
   AC with a Result Code message element indicating failure.  The AC no
   longer provides service to the WTP.

9.3.  Reset Response

   The Reset Response message acknowledges the Reset Request message.

   A Reset Response message is sent by the WTP after receiving a Reset
   Request message.

   The Reset Response is sent by the WTP when in the Run state.  The AC
   does not transmit this message.

   The following message elements MAY be included in the Reset Response
   message.

   o  Result Code, see Section 4.6.35

   o  Vendor Specific Payload, see Section 4.6.39

   When an AC receives a successful Reset Response message, it is
   notified that the WTP will reinitialize its operation.  An AC that
   receives a Reset Response message indicating failure may opt to no
   longer provide service to the WTP.

9.4.  WTP Event Request

   The WTP Event Request message is used by a WTP to send information to
   its AC.  The WTP Event Request message MAY be sent periodically, or
   sent in response to an asynchronous event on the WTP.  For example, a
   WTP MAY collect statistics and use the WTP Event Request message to
   transmit the statistics to the AC.

   When an AC receives a WTP Event Request message it will respond with
   a WTP Event Response message.

   The presence of the Delete Station message element is used by the WTP
   to inform the AC that it is no longer providing service to the
   station.  This could be the result of an Idle Timeout (see
   Section 4.6.24), due to resource shortages, or some other reason.

   The WTP Event Request message is sent by the WTP when in the Run
   state.  The AC does not transmit this message.







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   The WTP Event Request message MUST contain one of the message
   elements listed below, or a message element that is defined for a
   specific wireless technology.  More than one of each message element
   listed MAY be included in the WTP Event Request message.

   o  Decryption Error Report, see Section 4.6.17

   o  Duplicate IPv4 Address, see Section 4.6.22

   o  Duplicate IPv6 Address, see Section 4.6.23

   o  WTP Radio Statistics, see Section 4.6.46

   o  WTP Reboot Statistics, see Section 4.6.47

   o  Delete Station, see Section 4.6.20

   o  Vendor Specific Payload, see Section 4.6.39

9.5.  WTP Event Response

   The WTP Event Response message acknowledges receipt of the WTP Event
   Request message.

   A WTP Event Response message is sent by an AC after receiving a WTP
   Event Request message.

   The WTP Event Response message is sent by the AC when in the Run
   state.  The WTP does not transmit this message.

   The following message element MAY be included in the WTP Event
   Response message:

   o  Vendor Specific Payload, see Section 4.6.39

9.6.  Data Transfer

   This section describes the data transfer procedures used by the
   CAPWAP protocol.  The data transfer mechanism is used to upload
   information available at the WTP to the AC, such as crash or debug
   information.  The data transfer messages can only be exchanged while
   in the Run state.

   Figure 10 provides an example of an AC that requests that the WTP
   transfer its latest crash file.  Once the WTP acknowledges that it
   has information to send, via the Data Transfer Response, it transmits
   its own Data Transfer Request.  Upon receipt, the AC responds with a




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   Data Transfer Response, and the exchange continues until the WTP
   transmits a Data Transfer Data message element that indicates an End
   of File (EOF).

             WTP                                               AC

           Data Transfer Request (Data Transfer Mode = Crash Data)
         <------------------------------------------------------

              Data Transfer Response (Result Code = Success)
         -------------------------------------------------------->

              Data Transfer Request (Data Transfer Data = Data)
         -------------------------------------------------------->

              Data Transfer Response (Result Code = Success)
         <------------------------------------------------------

                                  .....

                Data Transfer Request (Data Transfer Data = EOF)
         -------------------------------------------------------->

              Data Transfer Response (Result Code = Success)
         <------------------------------------------------------


                    Figure 10: WTP Data Transfer Case 1

   Figure 11 provides an example of an AC that requests that the WTP
   transfer its latest crash file.  However, in this example, the WTP
   does not have any crash information to send, and therefore sends a
   Data Transfer Response with a Result Code indicating the error.

            WTP                                               AC

          Data Transfer Request (Data Transfer Mode = Crash Data)
        <------------------------------------------------------

             Data Transfer Response (Result Code = Data Transfer
                                     Error (No Information to Transfer))
        -------------------------------------------------------->


                    Figure 11: WTP Data Transfer Case 2






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9.6.1.  Data Transfer Request

   The Data Transfer Request message is used to deliver debug
   information from the WTP to the AC.

   The Data Transfer Request messages can be sent either by the AC or
   the WTP.  When sent by the AC, it is used to request that data be
   transmitted from the WTP to the AC, and includes the Data Transfer
   Mode message element, which specifies the information desired by the
   AC.  The Data Transfer Request is sent by the WTP in order to
   transfer actual data to the AC, through the Data Transfer Data
   message element.

   Given that the CAPWAP protocol minimizes the need for WTPs to be
   directly managed, the Data Transfer Request is an important
   troubleshooting tool used by the AC to retrieve information that may
   be available on the WTP.  For instance, some WTP implementations may
   store crash information to help manufacturers identify software
   faults.  The Data Transfer Request message can be used to send such
   information from the WTP to the AC.  Another possible use would be to
   allow a remote debugger function in the WTP to use the Data Transfer
   Request message to send console output to the AC for debugging
   purposes.

   When the WTP or AC receives a Data Transfer Request message, it
   responds to the WTP with a Data Transfer Response message.  The AC
   MAY log the information received through the Data Transfer Data
   message element.

   The Data Transfer Request message is sent by the WTP or AC when in
   the Run state.

   When sent by the AC, the Data Transfer Request message MUST contain
   the following message element:

   o  Data Transfer Mode, see Section 4.6.16

   When sent by the WTP, the Data Transfer Request message MUST contain
   the following message element:

   o  Data Transfer Data, see Section 4.6.15

   Regardless of whether the Data Transfer Request is sent by the AC or
   WTP, the following message element MAY be included in the Data
   Transfer Request message:

   o  Vendor Specific Payload, see Section 4.6.39




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9.6.2.  Data Transfer Response

   The Data Transfer Response message acknowledges the Data Transfer
   Request message.

   A Data Transfer Response message is sent in response to a received
   Data Transfer Request message.  Its purpose is to acknowledge receipt
   of the Data Transfer Request message.  When sent by the WTP, the
   Result Code message element is used to indicate whether the data
   transfer requested by the AC can be completed.  When sent by the AC,
   the Result Code message element is used to indicate receipt of the
   data transferred in the Data Transfer Request message.

   The Data Transfer Response message is sent by the WTP or AC when in
   the Run state.

   The following message element MUST be included in the Data Transfer
   Response message:

   o  Result Code, see Section 4.6.35

   The following message element MAY be included in the Data Transfer
   Response message:

   o  Vendor Specific Payload, see Section 4.6.39

   Upon receipt of a Data Transfer Response message, the WTP transmits
   more information, if more information is available.

10.  Station Session Management

   Messages in this section are used by the AC to create, modify, or
   delete station session state on the WTPs.

10.1.  Station Configuration Request

   The Station Configuration Request message is used to create, modify,
   or delete station session state on a WTP.  The message is sent by the
   AC to the WTP, and MAY contain one or more message elements.  The
   message elements for this CAPWAP Control message include information
   that is generally highly technology specific.  Refer to the
   appropriate binding document for definitions of the messages elements
   that are included in this control message.

   The Station Configuration Request message is sent by the AC when in
   the Run state.  The WTP does not transmit this message.





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   The following CAPWAP Control message elements MAY be included in the
   Station Configuration Request message.  More than one of each message
   element listed MAY be included in the Station Configuration Request
   message:

   o  Add Station, see Section 4.6.8

   o  Delete Station, see Section 4.6.20

   o  Vendor Specific Payload, see Section 4.6.39

10.2.  Station Configuration Response

   The Station Configuration Response message is used to acknowledge a
   previously received Station Configuration Request message.

   The Station Configuration Response message is sent by the WTP when in
   the Run state.  The AC does not transmit this message.

   The following message element MUST be present in the Station
   Configuration Response message:

   o  Result Code, see Section 4.6.35

   The following message element MAY be included in the Station
   Configuration Response message:

   o  Vendor Specific Payload, see Section 4.6.39

   The Result Code message element indicates that the requested
   configuration was successfully applied, or that an error related to
   processing of the Station Configuration Request message occurred on
   the WTP.

11.  NAT Considerations

   There are three specific situations in which a NAT deployment may be
   used in conjunction with a CAPWAP-enabled deployment.  The first
   consists of a configuration in which a single WTP is behind a NAT
   system.  Since all communication is initiated by the WTP, and all
   communication is performed over IP using two UDP ports, the protocol
   easily traverses NAT systems in this configuration.

   In the second case, two or more WTPs are deployed behind the same NAT
   system.  Here, the AC would receive multiple connection requests from
   the same IP address, and therefore cannot use the WTP's IP address
   alone to bind the CAPWAP Control and Data channel.  The CAPWAP Data
   Check state, which establishes the data plane connection and



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   communicates the CAPWAP Data Channel Keep-Alive, includes the Session
   Identifier message element, which is used to bind the control and
   data plane.  Use of the Session Identifier message element enables
   the AC to match the control and data plane flows from multiple WTPs
   behind the same NAT system (multiple WTPs sharing the same IP
   address).  CAPWAP implementations MUST also use DTLS session
   information on any encrypted CAPWAP channel to validate the source of
   both the control and data plane, as described in Section 12.2.

   In the third configuration, the AC is deployed behind a NAT.  In this
   case, the AC is not reachable by the WTP unless a specific rule has
   been configured on the NAT to translate the address and redirect
   CAPWAP messages to the AC.  This deployment presents two issues.
   First, an AC communicates its interfaces and corresponding WTP load
   using the CAPWAP Control IPv4 Address and CAPWAP Control IPv6 Address
   message elements.  This message element is mandatory, but contains IP
   addresses that are only valid in the private address space used by
   the AC, which is not reachable by the WTP.  The WTP MUST NOT utilize
   the information in these message elements if it detects a NAT (as
   described in the CAPWAP Transport Protocol message element in
   Section 4.6.14).  Second, since the addresses cannot be used by the
   WTP, this effectively disables the load-balancing capabilities (see
   Section 6.1) of the CAPWAP protocol.  Alternatively, the AC could
   have a configured NAT'ed address, which it would include in either of
   the two control address message elements, and the NAT would need to
   be configured accordingly.

   In order for a CAPWAP WTP or AC to detect whether a middlebox is
   present, both the Join Request (see Section 6.1) and the Join
   Response (see Section 6.2) include either the CAPWAP Local IPv4
   Address (see Section 4.6.11) or the CAPWAP Local IPv6 Address (see
   Section 4.6.12) message element.  Upon receiving one of these
   messages, if the packet's source IP address differs from the address
   found in either one of these message elements, it indicates that a
   middlebox is present.

   In order for CAPWAP to be compatible with potential middleboxes in
   the network, CAPWAP implementations MUST send return traffic from the
   same port on which it received traffic from a given peer.  Further,
   any unsolicited requests generated by a CAPWAP node MUST be sent on
   the same port.

   Note that this middlebox detection technique is not foolproof.  If
   the public IP address assigned to the NAT is identical to the private
   IP address used by the AC, detection by the WTP would fail.  This
   failure can lead to various protocol errors, so it is therefore
   necessary for deployments to ensure that the NAT's IP address is not
   the same as the ACs.



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   The CAPWAP protocol allows for all of the AC identities supporting a
   group of WTPs to be communicated through the AC List message element.
   This feature MUST be ignored by the WTP when it detects the AC is
   behind a middlebox.

   The CAPWAP protocol allows an AC to configure a static IP address on
   a WTP using the WTP Static IP Address Information message element.
   This message element SHOULD NOT be used in NAT'ed environments,
   unless the administrator is familiar with the internal IP addressing
   scheme within the WTP's private network, and does not rely on the
   public address seen by the AC.

   When a WTP detects the duplicate address condition, it generates a
   message to the AC, which includes the Duplicate IP Address message
   element.  The IP address embedded within this message element is
   different from the public IP address seen by the AC.

12.  Security Considerations

   This section describes security considerations for the CAPWAP
   protocol.  It also provides security recommendations for protocols
   used in conjunction with CAPWAP.

12.1.  CAPWAP Security

   As it is currently specified, the CAPWAP protocol sits between the
   security mechanisms specified by the wireless link layer protocol
   (e.g., IEEE 802.11i) and Authentication, Authorization, and
   Accounting (AAA).  One goal of CAPWAP is to bootstrap trust between
   the STA and WTP using a series of preestablished trust relationships:

         STA            WTP           AC            AAA
         ==============================================

                            DTLS Cred     AAA Cred
                         <------------><------------->

                         EAP Credential
          <------------------------------------------>

           wireless link layer
           (e.g., 802.11 PTK)
          <--------------> or
          <--------------------------->
              (derived)

                       Figure 12: STA Session Setup




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   Within CAPWAP, DTLS is used to secure the link between the WTP and
   AC.  In addition to securing control messages, it's also a link in
   this chain of trust for establishing link layer keys.  Consequently,
   much rests on the security of DTLS.

   In some CAPWAP deployment scenarios, there are two channels between
   the WTP and AC: the control channel, carrying CAPWAP Control
   messages, and the data channel, over which client data packets are
   tunneled between the AC and WTP.  Typically, the control channel is
   secured by DTLS, while the data channel is not.

   The use of parallel protected and unprotected channels deserves
   special consideration, but does not create a threat.  There are two
   potential concerns: attempting to convert protected data into
   unprotected data and attempting to convert un-protected data into
   protected data.  These concerns are addressed below.

12.1.1.  Converting Protected Data into Unprotected Data

   Since CAPWAP does not support authentication-only ciphers (i.e., all
   supported ciphersuites include encryption and authentication), it is
   not possible to convert protected data into unprotected data.  Since
   encrypted data is (ideally) indistinguishable from random data, the
   probability of an encrypted packet passing for a well-formed packet
   is effectively zero.

12.1.2.  Converting Unprotected Data into Protected Data (Insertion)

   The use of message authentication makes it impossible for the
   attacker to forge protected records.  This makes conversion of
   unprotected records to protected records impossible.

12.1.3.  Deletion of Protected Records

   An attacker could remove protected records from the stream, though
   not undetectably so, due the built-in reliability of the underlying
   CAPWAP protocol.  In the worst case, the attacker would remove the
   same record repeatedly, resulting in a CAPWAP session timeout and
   restart.  This is effectively a DoS attack, and could be accomplished
   by a man in the middle regardless of the CAPWAP protocol security
   mechanisms chosen.

12.1.4.   Insertion of Unprotected Records

   An attacker could inject packets into the unprotected channel, but
   this may become evident if sequence number desynchronization occurs
   as a result.  Only if the attacker is a man in the middle (MITM) can




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   packets be inserted undetectably.  This is a consequence of that
   channel's lack of protection, and not a new threat resulting from the
   CAPWAP security mechanism.

12.1.5.  Use of MD5

   The Image Information message element (Section 4.6.28) makes use of
   MD5 to compute the hash field.  The authenticity and integrity of the
   image file is protected by DTLS, and in this context, MD5 is not used
   as a cryptographically secure hash, but just as a basic checksum.
   Therefore, the use of MD5 is not considered a security vulnerability,
   and no mechanisms for algorithm agility are provided.

12.1.6.  CAPWAP Fragmentation

   RFC 4963 [RFC4963] describes a possible security vulnerability where
   a malicious entity can "corrupt" a flow by injecting fragments.  By
   sending "high" fragments (those with offset greater than zero) with a
   forged source address, the attacker can deliberately cause
   corruption.  The use of DTLS on the CAPWAP Data channel can be used
   to avoid this possible vulnerability.

12.2.  Session ID Security

   Since DTLS does not export a unique session identifier, there can be
   no explicit protocol binding between the DTLS layer and CAPWAP layer.
   As a result, implementations MUST provide a mechanism for performing
   this binding.  For example, an AC MUST NOT associate decrypted DTLS
   control packets with a particular WTP session based solely on the
   Session ID in the packet header.  Instead, identification should be
   done based on which DTLS session decrypted the packet.  Otherwise,
   one authenticated WTP could spoof another authenticated WTP by
   altering the Session ID in the encrypted CAPWAP Header.

   It should be noted that when the CAPWAP Data channel is unencrypted,
   the WTP Session ID is exposed and possibly known to adversaries and
   other WTPs.  This would allow the forgery of the source of data-
   channel traffic.  This, however, should not be a surprise for
   unencrypted data channels.  When the data channel is encrypted, the
   Session ID is not exposed, and therefore can safely be used to
   associate a data and control channel.  The 128-bit length of the
   Session ID mitigates online guessing attacks where an adversarial,
   authenticated WTP tries to correlate his own data channel with
   another WTP's control channel.  Note that for encrypted data
   channels, the Session ID should only be used for correlation for the
   first packet immediately after the initial DTLS handshake.  Future
   correlation should instead be done via identification of a packet's
   DTLS session.



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12.3.  Discovery or DTLS Setup Attacks

   Since the Discovery Request messages are sent in the clear, it is
   important that AC implementations NOT assume that receiving a
   Discovery Request message from a WTP implies that the WTP has
   rebooted, and consequently tear down any active DTLS sessions.
   Discovery Request messages can easily be spoofed by malicious
   devices, so it is important that the AC maintain two separate sets of
   states for the WTP until the DTLSSessionEstablished notification is
   received, indicating that the WTP was authenticated.  Once a new DTLS
   session is successfully established, any state referring to the old
   session can be cleared.

   Similarly, when the AC is entering the DTLS Setup phase, it SHOULD
   NOT assume that the WTP has reset, and therefore should not discard
   active state until the DTLS session has been successfully
   established.  While the HelloVerifyRequest provides some protection
   against denial-of-service (DoS) attacks on the AC, an adversary
   capable of receiving packets at a valid address (or a malfunctioning
   or misconfigured WTP) may repeatedly attempt DTLS handshakes with the
   AC, potentially creating a resource shortage.  If either the
   FailedDTLSSessionCount or the FailedDTLSAuthFailCount counter reaches
   the value of MaxFailedDTLSSessionRetry variable (see Section 4.8),
   implementations MAY choose to rate-limit new DTLS handshakes for some
   period of time.  It is RECOMMENDED that implementations choosing to
   implement rate-limiting use a random discard technique, rather than
   mimicking the WTP's sulking behavior.  This will ensure that messages
   from valid WTPs will have some probability of eliciting a response,
   even in the face of a significant DoS attack.

   Some CAPWAP implementations may wish to restrict the DTLS setup
   process to only those peers that have been configured in the access
   control list, authorizing only those clients to initiate a DTLS
   handshake.  Note that the impact of this on mitigating denial-of-
   service attacks against the DTLS layer is minimal, because DTLS
   already uses client-side cookies to minimize processor consumption
   attacks.

12.4.  Interference with a DTLS Session

   If a WTP or AC repeatedly receives packets that fail DTLS
   authentication or decryption, this could indicate a DTLS
   desynchronization between the AC and WTP, a link prone to
   undetectable bit errors, or an attacker trying to disrupt a DTLS
   session.






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   In the state machine (section 2.3), transitions to the DTLS Tear Down
   (TD) state can be triggered by frequently receiving DTLS packets with
   authentication or decryption errors.  The threshold or technique for
   deciding when to move to the tear down state should be chosen
   carefully.  Being able to easily transition to DTLS TD allows easy
   detection of malfunctioning devices, but allows for denial-of-service
   attacks.  Making it difficult to transition to DTLS TD prevents
   denial-of-service attacks, but makes it more difficult to detect and
   reset a malfunctioning session.  Implementers should set this policy
   with care.

12.5.  CAPWAP Pre-Provisioning

   In order for CAPWAP to establish a secure communication with a peer,
   some level of pre-provisioning on both the WTP and AC is necessary.
   This section will detail the minimal number of configuration
   parameters.

   When using pre-shared keys, it is necessary to configure the pre-
   shared key for each possible peer with which a DTLS session may be
   established.  To support this mode of operation, one or more entries
   of the following table may be configured on either the AC or WTP:

   o  Identity: The identity of the peering AC or WTP.  This format MAY
      be in the form of either an IP address or host name (the latter of
      which needs to be resolved to an IP address using DNS).

   o  Key: The pre-shared key for use with the peer when establishing
      the DTLS session (see Section 12.6 for more information).

   o  PSK Identity: Identity hint associated with the provisioned key
      (see Section 2.4.4.4 for more information).

   When using certificates, the following items need to be pre-
   provisioned:

   o  Device Certificate: The local device's certificate (see
      Section 12.7 for more information).

   o  Trust Anchor: Trusted root certificate chain used to validate any
      certificate received from CAPWAP peers.  Note that one or more
      root certificates MAY be configured on a given device.

   Regardless of the authentication method, the following item needs to
   be pre-provisioned:






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   o  Access Control List: The access control list table contains the
      identities of one or more CAPWAP peers, along with a rule.  The
      rule is used to determine whether communication with the peer is
      permitted (see Section 2.4.4.3 for more information).

12.6.  Use of Pre-Shared Keys in CAPWAP

   While use of pre-shared keys may provide deployment and provisioning
   advantages not found in public-key-based deployments, it also
   introduces a number of operational and security concerns.  In
   particular, because the keys must typically be entered manually, it
   is common for people to base them on memorable words or phrases.
   These are referred to as "low entropy passwords/passphrases".

   Use of low-entropy pre-shared keys, coupled with the fact that the
   keys are often not frequently updated, tends to significantly
   increase exposure.  For these reasons, the following recommendations
   are made:

   o  When DTLS is used with a pre-shared key (PSK) ciphersuite, each
      WTP SHOULD have a unique PSK.  Since WTPs will likely be widely
      deployed, their physical security is not guaranteed.  If PSKs are
      not unique for each WTP, key reuse would allow the compromise of
      one WTP to result in the compromise of others.

   o  Generating PSKs from low entropy passwords is NOT RECOMMENDED.

   o  It is RECOMMENDED that implementations that allow the
      administrator to manually configure the PSK also provide a
      capability for generation of new random PSKs, taking RFC 4086
      [RFC4086] into account.

   o  Pre-shared keys SHOULD be periodically updated.  Implementations
      MAY facilitate this by providing an administrative interface for
      automatic key generation and periodic update, or it MAY be
      accomplished manually instead.

   Every pairwise combination of WTP and AC on the network SHOULD have a
   unique PSK.  This prevents the domino effect (see "Guidance for
   Authentication, Authorization, and Accounting (AAA) Key Management"
   [RFC4962]).  If PSKs are tied to specific WTPs, then knowledge of the
   PSK implies a binding to a specified identity that can be authorized.

   If PSKs are shared, this binding between device and identity is no
   longer possible.  Compromise of one WTP can yield compromise of
   another WTP, violating the CAPWAP security hierarchy.  Consequently,
   sharing keys between WTPs is NOT RECOMMENDED.




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12.7.  Use of Certificates in CAPWAP

   For public-key-based DTLS deployments, each device SHOULD have unique
   credentials, with an extended key usage authorizing the device to act
   as either a WTP or AC.  If devices do not have unique credentials, it
   is possible that by compromising one device, any other device using
   the same credential may also be considered to be compromised.

   Certificate validation involves checking a large variety of things.
   Since the necessary things to validate are often environment-
   specific, many are beyond the scope of this document.  In this
   section, we provide some basic guidance on certificate validation.

   Each device is responsible for authenticating and authorizing devices
   with which they communicate.  Authentication entails validation of
   the chain of trust leading to the peer certificate, followed by the
   peer certificate itself.  Implementations SHOULD also provide a
   secure method for verifying that the credential in question has not
   been revoked.

   Note that if the WTP relies on the AC for network connectivity (e.g.,
   the AC is a Layer 2 switch to which the WTP is directly connected),
   the WTP may not be able to contact an Online Certificate Status
   Protocol (OCSP) server or otherwise obtain an up-to-date Certificate
   Revocation List (CRL) if a compromised AC doesn't explicitly permit
   this.  This cannot be avoided, except through effective physical
   security and monitoring measures at the AC.

   Proper validation of certificates typically requires checking to
   ensure the certificate has not yet expired.  If devices have a real-
   time clock, they SHOULD verify the certificate validity dates.  If no
   real-time clock is available, the device SHOULD make a best-effort
   attempt to validate the certificate validity dates through other
   means.  Failure to check a certificate's temporal validity can make a
   device vulnerable to man-in-the-middle attacks launched using
   compromised, expired certificates, and therefore devices should make
   every effort to perform this validation.

12.8.  Use of MAC Address in CN Field

   The CAPWAP protocol is an evolution of an existing protocol [LWAPP],
   which is implemented on a large number of already deployed ACs and
   WTPs.  Every one of these devices has an existing X.509 certificate,
   which is provisioned at the time of manufacturing.  These X.509
   certificates use the device's MAC address in the Common Name (CN)
   field.  It is well understood that encoding the MAC address in the CN
   field is less than optimal, and using the SubjectAltName field would
   be preferable.  However, at the time of publication, there is no URN



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   specification that allows for the MAC address to be used in the
   SubjectAltName field.  As such a specification is published by the
   IETF, future versions of the CAPWAP protocol MAY require support for
   the new URN scheme.

12.9.  AAA Security

   The AAA protocol is used to distribute Extensible Authentication
   Protocol (EAP) keys to the ACs, and consequently its security is
   important to the overall system security.  When used with Transport
   Layer Security (TLS) or IPsec, security guidelines specified in RFC
   3539 [RFC3539] SHOULD be followed.

   In general, the link between the AC and AAA server SHOULD be secured
   using a strong ciphersuite keyed with mutually authenticated session
   keys.  Implementations SHOULD NOT rely solely on Basic RADIUS shared
   secret authentication as it is often vulnerable to dictionary
   attacks, but rather SHOULD use stronger underlying security
   mechanisms.

12.10.  WTP Firmware

   The CAPWAP protocol defines a mechanism by which the AC downloads new
   firmware to the WTP.  During the session establishment process, the
   WTP provides information about its current firmware to the AC.  The
   AC then decides whether the WTP's firmware needs to be updated.  It
   is important to note that the CAPWAP specification makes the explicit
   assumption that the WTP is providing the correct firmware version to
   the AC, and is therefore not lying.  Further, during the firmware
   download process, the CAPWAP protocol does not provide any mechanisms
   to recognize whether the WTP is actually storing the firmware for
   future use.

13.  Operational Considerations

   The CAPWAP protocol assumes that it is the only configuration
   interface to the WTP to configure parameters that are specified in
   the CAPWAP specifications.  While the use of a separate management
   protocol MAY be used for the purposes of monitoring the WTP directly,
   configuring the WTP through a separate management interface is not
   recommended.  Configuring the WTP through a separate protocol, such
   as via a command line interface (CLI) or Simple Network Management
   Protocol (SNMP), could lead to the AC state being out of sync with
   the WTP.







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   The CAPWAP protocol does not deal with the management of the ACs.
   The AC is assumed to be configured through some separate management
   interface, which could be via a proprietary CLI, SNMP, Network
   Configuration Protocol (NETCONF), or some other management protocol.

   The CAPWAP protocol's control channel is fairly lightweight from a
   traffic perspective.  Once the WTP has been configured, the WTP sends
   periodic statistics.  Further, the specification calls for a keep-
   alive packet to be sent on the protocol's data channel to make sure
   that any possible middleboxes (e.g., NAT) maintain their UDP state.
   The overhead associated with the control and data channel is not
   expected to impact network traffic.  That said, the CAPWAP protocol
   does allow for the frequency of these packets to be modified through
   the DataChannelKeepAlive and StatisticsTimer (see Section 4.7.2 and
   Section 4.7.14, respectively).

14.  Transport Considerations

   The CAPWAP WG carefully considered the congestion control
   requirements of the CAPWAP protocol, both for the CAPWAP Control and
   Data channels.

   CAPWAP specifies a single-threaded command/response protocol to be
   used on the control channel, and we have specified that an
   exponential back-off algorithm should be used when commands are
   retransmitted.  When CAPWAP runs in its default mode (Local MAC), the
   control channel is the only CAPWAP channel.

   However, CAPWAP can also be run in Split MAC mode, in which case
   there will be a DTLS-encrypted data channel between each WTP and the
   AC.  The WG discussed various options for providing congestion
   control on this channel.  However, due to performance problems with
   TCP when it is run over another congestion control mechanism and the
   fact that the vast majority of traffic run over the CAPWAP Data
   channel is likely to be congestion-controlled IP traffic, the CAPWAP
   WG felt that specifying a congestion control mechanism for the CAPWAP
   Data channel would be more likely to cause problems than to resolve
   any.

   Because there is no congestion control mechanism specified for the
   CAPWAP Data channel, it is RECOMMENDED that non-congestion-controlled
   traffic not be tunneled over CAPWAP.  When a significant amount of
   non-congestion-controlled traffic is expected to be present on a
   WLAN, the CAPWAP connection between the AC and the WTP for that LAN
   should be configured to remain in Local MAC mode with Distribution
   function at the WTP.





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   The lock step nature of the CAPWAP protocol's control channel can
   cause the firmware download process to take some time, depending upon
   the round-trip time (RTT).  This is not expected to be a problem
   since the CAPWAP protocol allows firmware to be downloaded while the
   WTP provides service to wireless clients/devices.

   It is necessary for the WTP and AC to configure their MTU based on
   the capabilities of the path.  See Section 3.5 for more information.

   The CAPWAP protocol mandates support of the Explicit Congestion
   Notification (ECN) through a mode of operation named "limited
   functionality option", detailed in section 9.1.1 of [RFC3168].
   Future versions of the CAPWAP protocol should consider mandating
   support for the "full functionality option".

15.  IANA Considerations

   This section details the actions that IANA has taken in preparation
   for publication of the specification.  Numerous registries have been
   created, and the contents, document action (see [RFC5226], and
   registry format are all included below.  Note that in cases where bit
   fields are referred to, the bit numbering is left to right, where the
   leftmost bit is labeled as bit zero (0).

   For future registration requests where an Expert Review is required,
   a Designated Expert should be consulted, which is appointed by the
   responsible IESG Area Director.  The intention is that any allocation
   will be accompanied by a published RFC, but given that other SDOs may
   want to create standards built on top of CAPWAP, a document the
   Designated Expert can review is also acceptable.  IANA should allow
   for allocation of values prior to documents being approved for
   publication, so the Designated Expert can approve allocations once it
   seems clear that publication will occur.  The Designated Expert will
   post a request to the CAPWAP WG mailing list (or a successor
   designated by the Area Director) for comment and review.  Before a
   period of 30 days has passed, the Designated Expert will either
   approve or deny the registration request and publish a notice of the
   decision to the CAPWAP WG mailing list or its successor, as well as
   informing IANA.  A denial notice must be justified by an explanation,
   and in the cases where it is possible, concrete suggestions on how
   the request can be modified so as to become acceptable should be
   provided.

15.1.  IPv4 Multicast Address

   IANA has registered a new IPv4 multicast address called "capwap-ac"
   from the Internetwork Control Block IPv4 multicast address registry;
   see Section 3.3.



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15.2.  IPv6 Multicast Address

   IANA has registered a new organization local multicast address called
   the "All ACs multicast address" in the Variable Scope IPv6 multicast
   address registry; see Section 3.3.

15.3.  UDP Port

   IANA registered two new UDP Ports, which are organization-local
   multicast addresses, in the registered port numbers registry; see
   Section 3.1.  The following values have been registered:

   Keyword         Decimal    Description                  References
   -------         -------    -----------                  ----------
   capwap-control  5246/udp   CAPWAP Control Protocol      This Document
   capwap-data     5247/udp   CAPWAP Data Protocol         This Document


15.4.  CAPWAP Message Types

   The Message Type field in the CAPWAP Header (see Section 4.5.1.1) is
   used to identify the operation performed by the message.  There are
   multiple namespaces, which are identified via the first three octets
   of the field containing the IANA Enterprise Number [RFC5226].

   IANA maintains the CAPWAP Message Types registry for all message
   types whose Enterprise Number is set to zero (0).  The namespace is 8
   bits (0-255), where the value of zero (0) is reserved and must not be
   assigned.  The values one (1) through 26 are allocated in this
   specification, and can be found in Section 4.5.1.1.  Any new
   assignments of a CAPWAP Message Type whose Enterprise Number is set
   to zero (0) requires an Expert Review.  The registry maintained by
   IANA has the following format:

           CAPWAP Control Message           Message Type     Reference
                                              Value

15.5.  CAPWAP Header Flags

   The Flags field in the CAPWAP Header (see Section 4.3) is 9 bits in
   length and is used to identify any special treatment related to the
   message.  This specification defines bits zero (0) through five (5),
   while bits six (6) through eight (8) are reserved.  There are
   currently three unused, reserved bits that are managed by IANA and
   whose assignment require an Expert Review.  IANA created the CAPWAP
   Header Flags registry, whose format is:

           Flag Field Name                   Bit Position    Reference



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15.6.  CAPWAP Control Message Flags

   The Flags field in the CAPWAP Control Message header (see
   Section 4.5.1.4) is used to identify any special treatment related to
   the control message.  There are currently eight (8) unused, reserved
   bits.  The assignment of these bits is managed by IANA and requires
   an Expert Review.  IANA created the CAPWAP Control Message Flags
   registry, whose format is:

           Flag Field Name                   Bit Position    Reference

15.7.  CAPWAP Message Element Type

   The Type field in the CAPWAP Message Element header (see Section 4.6)
   is used to identify the data being transported.  The namespace is 16
   bits (0-65535), where the value of zero (0) is reserved and must not
   be assigned.  The values one (1) through 53 are allocated in this
   specification, and can be found in Section 4.5.1.1.

   The 16-bit namespace is further divided into blocks of addresses that
   are reserved for specific CAPWAP wireless bindings.  The following
   blocks are reserved:

         CAPWAP Protocol Message Elements                   1 - 1023
         IEEE 802.11 Message Elements                    1024 - 2047
         EPCGlobal Message Elements                      3072 - 4095

   This namespace is managed by IANA and assignments require an Expert
   Review.  IANA created the CAPWAP Message Element Type registry, whose
   format is:

           CAPWAP Message Element           Type Value       Reference

15.8.  CAPWAP Wireless Binding Identifiers

   The Wireless Binding Identifier (WBID) field in the CAPWAP Header
   (see Section 4.3) is used to identify the wireless technology
   associated with the packet.  This specification allocates the values
   one (1) and three (3).  Due to the limited address space available, a
   new WBID request requires Expert Review.  IANA created the CAPWAP
   Wireless Binding Identifier registry, whose format is:

           CAPWAP Wireless Binding Identifier  Type Value      Reference








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15.9.  AC Security Types

   The Security field in the AC Descriptor message element (see
   Section 4.6.1) is 8 bits in length and is used to identify the
   authentication methods available on the AC.  This specification
   defines bits five (5) and six (6), while bits zero (0) through four
   (4) as well as bit seven (7) are reserved and unused.  These reserved
   bits are managed by IANA and assignment requires Standards Action.
   IANA created the AC Security Types registry, whose format is:

           AC Security Type                  Bit Position    Reference

15.10.  AC DTLS Policy

   The DTLS Policy field in the AC Descriptor message element (see
   Section 4.6.1) is 8 bits in length and is used to identify whether
   the CAPWAP Data Channel is to be secured.  This specification defines
   bits five (5) and six (6), while bits zero (0) through four (4) as
   well as bit seven (7) are reserved and unused.  These reserved bits
   are managed by IANA and assignment requires Standards Action.  IANA
   created the AC DTLS Policy registry, whose format is:

           AC DTLS Policy                    Bit Position    Reference

15.11.  AC Information Type

   The Information Type field in the AC Descriptor message element (see
   Section 4.6.1) is used to represent information about the AC.  The
   namespace is 16 bits (0-65535), where the value of zero (0) is
   reserved and must not be assigned.  This field, combined with the AC
   Information Vendor ID, allows vendors to use a private namespace.
   This specification defines the AC Information Type namespace when the
   AC Information Vendor ID is set to zero (0), for which the values
   four (4) and five (5) are allocated in this specification, and can be
   found in Section 4.6.1.  This namespace is managed by IANA and
   assignments require an Expert Review.  IANA created the AC
   Information Type registry, whose format is:

           AC Information Type              Type Value       Reference

15.12.  CAPWAP Transport Protocol Types

   The Transport field in the CAPWAP Transport Protocol message element
   (see Section 4.6.14) is used to identify the transport to use for the
   CAPWAP Data Channel.  The namespace is 8 bits (0-255), where the
   value of zero (0) is reserved and must not be assigned.  The values
   one (1) and two (2) are allocated in this specification, and can be




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   found in Section 4.6.14.  This namespace is managed by IANA and
   assignments require an Expert Review.  IANA created the CAPWAP
   Transport Protocol Types registry, whose format is:

           CAPWAP Transport Protocol Type   Type Value       Reference

15.13.  Data Transfer Type

   The Data Type field in the Data Transfer Data message element (see
   Section 4.6.15) and Image Data message element (see Section 4.6.26)
   is used to provide information about the data being carried.  The
   namespace is 8 bits (0-255), where the value of zero (0) is reserved
   and must not be assigned.  The values one (1), two (2), and five (5)
   are allocated in this specification, and can be found in
   Section 4.6.15.  This namespace is managed by IANA and assignments
   require an Expert Review.  IANA created the Data Transfer Type
   registry, whose format is:

           Data Transfer Type               Type Value       Reference

15.14.  Data Transfer Mode

   The Data Mode field in the Data Transfer Data message element (see
   Section 4.6.15) and Data Transfer Mode message element (see
   Section 15.14) is used to provide information about the data being
   carried.  The namespace is 8 bits (0-255), where the value of zero
   (0) is reserved and must not be assigned.  The values one (1) and two
   (2) are allocated in this specification, and can be found in
   Section 15.14.  This namespace is managed by IANA and assignments
   require an Expert Review.  IANA created the Data Transfer Mode
   registry, whose format is:

           Data Transfer Mode               Type Value       Reference

15.15.  Discovery Types

   The Discovery Type field in the Discovery Type message element (see
   Section 4.6.21) is used by the WTP to indicate to the AC how it was
   discovered.  The namespace is 8 bits (0-255).  The values zero (0)
   through four (4) are allocated in this specification and can be found
   in Section 4.6.21.  This namespace is managed by IANA and assignments
   require an Expert Review.  IANA created the Discovery Types registry,
   whose format is:

           Discovery Types                  Type Value       Reference






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15.16.  ECN Support

   The ECN Support field in the ECN Support message element (see
   Section 4.6.25) is used by the WTP to represent its ECN Support.  The
   namespace is 8 bits (0-255).  The values zero (0) and one (1) are
   allocated in this specification, and can be found in Section 4.6.25.
   This namespace is managed by IANA and assignments require an Expert
   Review.  IANA created the ECN Support registry, whose format is:

           ECN Support                      Type Value       Reference

15.17.  Radio Admin State

   The Radio Admin field in the Radio Administrative State message
   element (see Section 4.6.33) is used by the WTP to represent the
   state of its radios.  The namespace is 8 bits (0-255), where the
   value of zero (0) is reserved and must not be assigned.  The values
   one (1) and two (2) are allocated in this specification, and can be
   found in Section 4.6.33.  This namespace is managed by IANA and
   assignments require an Expert Review.  IANA created the Radio Admin
   State registry, whose format is:

           Radio Admin State                Type Value       Reference

15.18.  Radio Operational State

   The State field in the Radio Operational State message element (see
   Section 4.6.34) is used by the WTP to represent the operational state
   of its radios.  The namespace is 8 bits (0-255), where the value of
   zero (0) is reserved and must not be assigned.  The values one (1)
   and two (2) are allocated in this specification, and can be found in
   Section 4.6.34.  This namespace is managed by IANA and assignments
   require an Expert Review.  IANA created the Radio Operational State
   registry, whose format is:

           Radio Operational State          Type Value       Reference

15.19.  Radio Failure Causes

   The Cause field in the Radio Operational State message element (see
   Section 4.6.34) is used by the WTP to represent the reason a radio
   may have failed.  The namespace is 8 bits (0-255), where the value of
   zero (0) through three (3) are allocated in this specification, and
   can be found in Section 4.6.34.  This namespace is managed by IANA
   and assignments require an Expert Review.  IANA created the Radio
   Failure Causes registry, whose format is:

           Radio Failure Causes             Type Value       Reference



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15.20.  Result Code

   The Result Code field in the Result Code message element (see
   Section 4.6.35) is used to indicate the success or failure of a
   CAPWAP Control message.  The namespace is 32 bits (0-4294967295),
   where the value of zero (0) through 22 are allocated in this
   specification, and can be found in Section 4.6.35.  This namespace is
   managed by IANA and assignments require an Expert Review.  IANA
   created the Result Code registry, whose format is:

           Result Code                      Type Value       Reference

15.21.  Returned Message Element Reason

   The Reason field in the Returned Message Element message element (see
   Section 4.6.36) is used to indicate the reason why a message element
   was not processed successfully.  The namespace is 8 bits (0-255),
   where the value of zero (0) is reserved and must not be assigned.
   The values one (1) through four (4) are allocated in this
   specification, and can be found in Section 4.6.36.  This namespace is
   managed by IANA and assignments require an Expert Review.  IANA
   created the Returned Message Element Reason registry, whose format
   is:

           Returned Message Element Reason  Type Value       Reference

15.22.  WTP Board Data Type

   The Board Data Type field in the WTP Board Data message element (see
   Section 4.6.40) is used to represent information about the WTP
   hardware.  The namespace is 16 bits (0-65535).  The WTP Board Data
   Type values zero (0) through four (4) are allocated in this
   specification, and can be found in Section 4.6.40.  This namespace is
   managed by IANA and assignments require an Expert Review.  IANA
   created the WTP Board Data Type registry, whose format is:

           WTP Board Data Type              Type Value       Reference

15.23.  WTP Descriptor Type

   The Descriptor Type field in the WTP Descriptor message element (see
   Section 4.6.41) is used to represent information about the WTP
   software.  The namespace is 16 bits (0-65535).  This field, combined
   with the Descriptor Vendor ID, allows vendors to use a private
   namespace.  This specification defines the WTP Descriptor Type
   namespace when the Descriptor Vendor ID is set to zero (0), for which
   the values zero (0) through three (3) are allocated in this




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   specification, and can be found in Section 4.6.41.  This namespace is
   managed by IANA and assignments require an Expert Review.  IANA
   created the WTP Board Data Type registry, whose format is:

           WTP Descriptor Type              Type Value       Reference

15.24.  WTP Fallback Mode

   The Mode field in the WTP Fallback message element (see
   Section 4.6.42) is used to indicate the type of AC fallback mechanism
   the WTP should employ.  The namespace is 8 bits (0-255), where the
   value of zero (0) is reserved and must not be assigned.  The values
   one (1) and two (2) are allocated in this specification, and can be
   found in Section 4.6.42.  This namespace is managed by IANA and
   assignments require an Expert Review.  IANA created the WTP Fallback
   Mode registry, whose format is:

           WTP Fallback Mode                Type Value       Reference

15.25.  WTP Frame Tunnel Mode

   The Tunnel Type field in the WTP Frame Tunnel Mode message element
   (see Section 4.6.43) is 8 bits and is used to indicate the type of
   tunneling to use between the WTP and the AC.  This specification
   defines bits four (4) through six (6), while bits zero (0) through
   three (3) as well as bit seven (7) are reserved and unused.  These
   reserved bits are managed by IANA and assignment requires an Expert
   Review.  IANA created the WTP Frame Tunnel Mode registry, whose
   format is:

           WTP Frame Tunnel Mode             Bit Position    Reference

15.26.  WTP MAC Type

   The MAC Type field in the WTP MAC Type message element (see
   Section 4.6.44) is used to indicate the type of MAC to use in
   tunneled frames between the WTP and the AC.  The namespace is 8 bits
   (0-255), where the value of zero (0) through two (2) are allocated in
   this specification, and can be found in Section 4.6.44.  This
   namespace is managed by IANA and assignments require an Expert
   Review.  IANA created the WTP MAC Type registry, whose format is:

           WTP MAC Type                     Type Value       Reference








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15.27.  WTP Radio Stats Failure Type

   The Last Failure Type field in the WTP Radio Statistics message
   element (see Section 4.6.46) is used to indicate the last WTP
   failure.  The namespace is 8 bits (0-255), where the value of zero
   (0) through three (3) as well as the value 255 are allocated in this
   specification, and can be found in Section 4.6.46.  This namespace is
   managed by IANA and assignments require an Expert Review.  IANA
   created the WTP Radio Stats Failure Type registry, whose format is:

           WTP Radio Stats Failure Type     Type Value       Reference

15.28.  WTP Reboot Stats Failure Type

   The Last Failure Type field in the WTP Reboot Statistics message
   element (see Section 4.6.47) is used to indicate the last reboot
   reason.  The namespace is 8 bits (0-255), where the value of zero (0)
   through five (5) as well as the value 255 are allocated in this
   specification, and can be found in Section 4.6.47.  This namespace is
   managed by IANA and assignments require an Expert Review.  IANA
   created the WTP Reboot Stats Failure Type registry, whose format is:

           WTP Reboot Stats Failure Type    Type Value       Reference

16.  Acknowledgments

   The following individuals are acknowledged for their contributions to
   this protocol specification: Puneet Agarwal, Abhijit Choudhury, Pasi
   Eronen, Saravanan Govindan, Peter Nilsson, David Perkins, and Yong
   Zhang.

   Michael Vakulenko contributed text to describe how CAPWAP can be used
   over Layer 3 (IP/UDP) networks.

17.  References

17.1.  Normative References

   [RFC1191]          Mogul, J. and S. Deering, "Path MTU discovery",
                      RFC 1191, November 1990.

   [RFC1321]          Rivest, R., "The MD5 Message-Digest Algorithm",
                      RFC 1321, April 1992.

   [RFC1305]          Mills, D., "Network Time Protocol (Version 3)
                      Specification, Implementation", RFC 1305,
                      March 1992.




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   [RFC1981]          McCann, J., Deering, S., and J. Mogul, "Path MTU
                      Discovery for IP version 6", RFC 1981,
                      August 1996.

   [RFC2119]          Bradner, S., "Key words for use in RFCs to
                      Indicate Requirement Levels", BCP 14, RFC 2119,
                      March 1997.

   [RFC2460]          Deering, S. and R. Hinden, "Internet Protocol,
                      Version 6 (IPv6) Specification", RFC 2460,
                      December 1998.

   [RFC2474]          Nichols, K., Blake, S., Baker, F., and D. Black,
                      "Definition of the Differentiated Services Field
                      (DS Field) in the IPv4 and IPv6 Headers",
                      RFC 2474, December 1998.

   [RFC2782]          Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS
                      RR for specifying the location of services (DNS
                      SRV)", RFC 2782, February 2000.

   [RFC3168]          Ramakrishnan, K., Floyd, S., and D. Black, "The
                      Addition of Explicit Congestion Notification (ECN)
                      to IP", RFC 3168, September 2001.

   [RFC3539]          Aboba, B. and J. Wood, "Authentication,
                      Authorization and Accounting (AAA) Transport
                      Profile", RFC 3539, June 2003.

   [RFC3629]          Yergeau, F., "UTF-8, a transformation format of
                      ISO 10646", STD 63, RFC 3629, November 2003.

   [RFC3828]          Larzon, L-A., Degermark, M., Pink, S., Jonsson,
                      L-E., and G. Fairhurst, "The Lightweight User
                      Datagram Protocol (UDP-Lite)", RFC 3828,
                      July 2004.

   [RFC4086]          Eastlake, D., Schiller, J., and S. Crocker,
                      "Randomness Requirements for Security", BCP 106,
                      RFC 4086, June 2005.

   [RFC4279]          Eronen, P. and H. Tschofenig, "Pre-Shared Key
                      Ciphersuites for Transport Layer Security (TLS)",
                      RFC 4279, December 2005.

   [RFC5246]          Dierks, T. and E. Rescorla, "The Transport Layer
                      Security (TLS) Protocol Version 1.2", RFC 5246,
                      August 2008.



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   [RFC4347]          Rescorla, E. and N. Modadugu, "Datagram Transport
                      Layer Security", RFC 4347, April 2006.

   [RFC4821]          Mathis, M. and J. Heffner, "Packetization Layer
                      Path MTU Discovery", RFC 4821, March 2007.

   [RFC4963]          Heffner, J., Mathis, M., and B. Chandler, "IPv4
                      Reassembly Errors at High Data Rates", RFC 4963,
                      July 2007.

   [RFC5226]          Narten, T. and H. Alvestrand, "Guidelines for
                      Writing an IANA Considerations Section in RFCs",
                      BCP 26, RFC 5226, May 2008.

   [RFC5280]          Cooper, D., Santesson, S., Farrell, S., Boeyen,
                      S., Housley, R., and W. Polk, "Internet X.509
                      Public Key Infrastructure Certificate and
                      Certificate Revocation List (CRL) Profile",
                      RFC 5280, May 2008.

   [ISO.9834-1.1993]  International Organization for Standardization,
                      "Procedures for the operation of OSI registration
                      authorities - part 1: general procedures",
                      ISO Standard 9834-1, 1993.

   [RFC5416]          Calhoun, P., Ed., Montemurro, M., Ed., and D.
                      Stanley, Ed., "Control And Provisioning of
                      Wireless Access Points (CAPWAP) Protocol Binding
                      for IEEE 802.11", RFC 5416, March 2009.

   [RFC5417]          Calhoun, P., "Control And Provisioning of Wireless
                      Access Points (CAPWAP) Access Controller DHCP
                      Option", RFC 5417, March 2009.

   [FRAME-EXT]        IEEE, "IEEE Standard 802.3as-2006", 2005.

17.2.  Informative References

   [RFC3232]          Reynolds, J., "Assigned Numbers: RFC 1700 is
                      Replaced by an On-line Database", RFC 3232,
                      January 2002.

   [RFC3753]          Manner, J. and M. Kojo, "Mobility Related
                      Terminology", RFC 3753, June 2004.







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   [RFC4564]          Govindan, S., Cheng, H., Yao, ZH., Zhou, WH., and
                      L. Yang, "Objectives for Control and Provisioning
                      of Wireless Access Points (CAPWAP)", RFC 4564,
                      July 2006.

   [RFC4962]          Housley, R. and B. Aboba, "Guidance for
                      Authentication, Authorization, and Accounting
                      (AAA) Key Management", BCP 132, RFC 4962,
                      July 2007.

   [LWAPP]            Calhoun, P., O'Hara, B., Suri, R., Cam Winget, N.,
                      Kelly, S., Williams, M., and S. Hares,
                      "Lightweight Access Point Protocol", Work in
                      Progress, March 2007.

   [SLAPP]            Narasimhan, P., Harkins, D., and S. Ponnuswamy,
                      "SLAPP: Secure Light Access Point Protocol", Work
                      in Progress, May 2005.

   [DTLS-DESIGN]      Modadugu, et al., N., "The Design and
                      Implementation of Datagram TLS", Feb 2004.

   [EUI-48]           IEEE, "Guidelines for use of a 48-bit Extended
                      Unique Identifier", Dec 2005.

   [EUI-64]           IEEE, "GUIDELINES FOR 64-BIT GLOBAL IDENTIFIER
                      (EUI-64) REGISTRATION AUTHORITY".

   [EPCGlobal]        "See http://www.epcglobalinc.org/home".

   [PacketCable]      "PacketCable Security Specification PKT-SP-SEC-
                      I12-050812", August 2005, <PacketCable>.

   [CableLabs]        "OpenCable System Security Specification OC-SP-
                      SEC-I07-061031", October 2006, <CableLabs>.

   [WiMAX]            "WiMAX Forum X.509 Device Certificate Profile
                      Approved Specification V1.0.1", April 2008,
                      <WiMAX>.

   [RFC5418]          Kelly, S. and C. Clancy, "Control And Provisioning
                      for Wireless Access Points (CAPWAP) Threat
                      Analysis for IEEE 802.11 Deployments", RFC 5418,
                      March 2009.







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Editors' Addresses

   Pat R. Calhoun (editor)
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134

   Phone: +1 408-902-3240
   EMail: pcalhoun@cisco.com

   Michael P. Montemurro (editor)
   Research In Motion
   5090 Commerce Blvd
   Mississauga, ON  L4W 5M4
   Canada

   Phone: +1 905-629-4746 x4999
   EMail: mmontemurro@rim.com


   Dorothy Stanley (editor)
   Aruba Networks
   1322 Crossman Ave
   Sunnyvale, CA  94089

   Phone: +1 630-363-1389
   EMail: dstanley@arubanetworks.com
























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