





Network Working Group                                      T. Melia, Ed.
Request for Comments: 5677                                Alcatel-Lucent
Category: Standards Track                                       G. Bajko
                                                                   Nokia
                                                                  S. Das
                                             Telcordia Technologies Inc.
                                                               N. Golmie
                                                                    NIST
                                                              JC. Zuniga
                                        InterDigital Communications, LLC
                                                           December 2009


         IEEE 802.21 Mobility Services Framework Design (MSFD)

Abstract

   This document describes a mobility services framework design (MSFD)
   for the IEEE 802.21 Media Independent Handover (MIH) protocol that
   addresses identified issues associated with the transport of MIH
   messages.  The document also describes mechanisms for Mobility
   Services (MoS) discovery and transport-layer mechanisms for the
   reliable delivery of MIH messages.  This document does not provide
   mechanisms for securing the communication between a mobile node (MN)
   and the Mobility Server.  Instead, it is assumed that either lower-
   layer (e.g., link-layer) security mechanisms or overall system-
   specific proprietary security solutions are used.

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.

IESG Note

   As described later in this specification, this protocol does not
   provide security mechanisms.  In some deployment situations lower-
   layer security services may be sufficient.  Other situations require
   proprietary mechanisms or as yet incomplete standard mechanisms, such
   as the ones currently considered by IEEE.  For these reasons, the
   specification recommends careful analysis before considering any
   deployment.






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   The IESG emphasizes the importance of these recommendations.  The
   IESG also notes that this specification deviates from the traditional
   IETF requirement that support for security in the open Internet
   environment is a mandatory part of any Standards Track protocol
   specification.  An exception has been made for this specification,
   but this should not be taken to mean that other future specifications
   are free from this requirement.

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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the BSD License.





























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Table of Contents

   1. Introduction ....................................................4
   2. Terminology .....................................................4
      2.1. Requirements Language ......................................7
   3. Deployment Scenarios ............................................7
      3.1. Scenario S1: Home Network MoS ..............................8
      3.2. Scenario S2: Visited Network MoS ...........................8
      3.3. Scenario S3: Third-Party MoS ...............................9
      3.4. Scenario S4: Roaming MoS ...................................9
   4. Solution Overview ..............................................10
      4.1. Architecture ..............................................11
      4.2. MIHF Identifiers (FQDN, NAI) ..............................12
   5. MoS Discovery ..................................................12
      5.1. MoS Discovery When MN and MoSh Are in the Home
           Network (Scenario S1) .....................................13
      5.2. MoS Discovery When MN and MoSv Both Are in Visited
           Network (Scenario S2) .....................................14
      5.3. MoS Discovery When MIH Services Are in a
           Third-Party Remote Network (Scenario S3) ..................14
      5.4. MoS Discovery When the MN Is in a Visited Network
           and Services Are at the Home Network (Scenario S4) ........15
   6. MIH Transport Options ..........................................15
      6.1. MIH Message Size ..........................................16
      6.2. MIH Message Rate ..........................................17
      6.3. Retransmission ............................................17
      6.4. NAT Traversal .............................................18
      6.5. General Guidelines ........................................18
   7. Operation Flows ................................................19
   8. Security Considerations ........................................21
      8.1. Security Considerations for MoS Discovery .................21
      8.2. Security Considerations for MIH Transport .................21
   9. IANA Considerations ............................................22
   10. Acknowledgements ..............................................23
   11. References ....................................................23
      11.1. Normative References .....................................23
      11.2. Informative References ...................................23














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1.  Introduction

   This document proposes a solution to the issues identified in the
   problem statement document [RFC5164] for the layer 3 transport of
   IEEE 802.21 MIH protocols.

   The MIH Layer 3 transport problem is divided into two main parts: the
   discovery of a node that supports specific Mobility Services (MoS)
   and the transport of the information between a mobile node (MN) and
   the discovered node.  The discovery process is required for the MN to
   obtain the information needed for MIH protocol communication with a
   peer node.  The information includes the transport address (e.g., the
   IP address) of the peer node and the types of MoS provided by the
   peer node.

   This document lists the major MoS deployment scenarios.  It describes
   the solution architecture, including the MSFD reference model and
   MIHF identifiers.  MoS discovery procedures explain how the MN
   discovers Mobility Servers in its home network, in a visited network
   or in a third-party network.  The remainder of this document
   describes the MIH transport architecture, example message flows for
   several signaling scenarios, and security issues.

   This document does not provide mechanisms for securing the
   communication between a mobile node and the Mobility Server.
   Instead, it is assumed that either lower layer (e.g., link layer)
   security mechanisms, or overall system-specific proprietary security
   solutions, are used.  The details of such lower layer and/or
   proprietary mechanisms are beyond the scope of this document.  It is
   RECOMMENDED against using this protocol without careful analysis that
   these mechanisms meet the desired requirements, and encourages future
   standardization work in this area.  The IEEE 802.21a Task Group has
   recently started work on MIH security issues that may provide some
   solution in this area.  For further information, please refer to
   Section 8.

2.  Terminology

   The following acronyms and terminology are used in this document:

   Media Independent Handover (MIH): the handover support architecture
      defined by the IEEE 802.21 working group that consists of the MIH
      Function (MIHF), MIH Network Entities, and MIH protocol messages.








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   Media Independent Handover Function (MIHF): a switching function that
      provides handover services including the Event Service (ES),
      Information Service (IS), and Command Service (CS), through
      service access points (SAPs) defined by the IEEE 802.21 working
      group [IEEE80221].

   MIHF User: An entity that uses the MIH SAPs to access MIHF services,
      and which is responsible for initiating and terminating MIH
      signaling.

   Media Independent Handover Function Identifier (MIHFID): an
      identifier required to uniquely identify the MIHF endpoints for
      delivering mobility services (MoS); it is implemented as either a
      FQDN or NAI.

   Mobility Services (MoS): composed of Information Service, Command
      Service, and Event Service provided by the network to mobile nodes
      to facilitate handover preparation and handover decision, as
      described in [IEEE80221] and [RFC5164].

   MoSh:  Mobility Services provided by the mobile node's Home Network.

   MoSv:  Mobility Services provided by the Visited Network.

   MoS3: Mobility Services provided by a third-party network, which is a
      network that is neither the Home Network nor the current Visited
      Network.

   Mobile Node (MN): an Internet device whose location changes, along
      with its point of connection to the network.

   Mobility Services Transport Protocol (MSTP): a protocol that is used
      to deliver MIH protocol messages from an MIHF to other MIH-aware
      nodes in a network.

   Information Service (IS): a MoS that originates at the lower or upper
      layers of the protocol stack and sends information to the local or
      remote upper or lower layers of the protocol stack.  The purpose
      of IS is to exchange information elements (IEs) relating to
      various neighboring network information.

   Event Service (ES): a MoS that originates at a remote MIHF or the
      lower layers of the local protocol stack and sends information to
      the local MIHF or local higher layers.  The purpose of the ES is
      to report changes in link status (e.g., Link Going Down messages)
      and various lower layer events.





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   Command Service (CS): a MoS that sends commands from the remote MIHF
      or local upper layers to the remote or local lower layers of the
      protocol stack to switch links or to get link status.

   Fully Qualified Domain Name (FQDN): a complete domain name for a host
      on the Internet, showing (in reverse order) the full delegation
      path from the DNS root and top-level domain down to the host name
      (e.g., myexample.example.org).

   Network Access Identifier (NAI): the user ID that a user submits
      during network access authentication [RFC4282].  For mobile users,
      the NAI identifies the user and helps to route the authentication
      request message.

   Network Address Translator (NAT): a device that implements the
      Network Address Translation function described in [RFC3022], in
      which local or private network layer addresses are mapped to
      routable (outside the NAT domain) network addresses and port
      numbers.

   Dynamic Host Configuration Protocol (DHCP): protocols described in
      [RFC2131] and [RFC3315] that allow Internet devices to obtain
      respectively IPv4 and IPv6 addresses, subnet masks, default
      gateway addresses, and other IP configuration information from
      DHCP servers.

   Domain Name System (DNS): a protocol described in [RFC1035] that
      translates domain names to IP addresses.

   Authentication, Authorization, and Accounting (AAA): a set of network
      management services that respectively determine the validity of a
      user's ID, determine whether a user is allowed to use network
      resources, and track users' use of network resources.

   Home AAA (AAAh): an AAA server located on the MN's home network.

   Visited AAA (AAAv): an AAA server located in a visited network that
      is not the MN's home network.

   MIH Acknowledgement (MIH ACK): an MIH signaling message that an MIHF
      sends in response to an MIH message from a sending MIHF.

   Point of Service (PoS): a network-side MIHF instance that exchanges
      MIH messages with an MN-based MIHF.







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   Network Access Server (NAS): a server to which an MN initially
      connects when it is trying to gain a connection to a network and
      that determines whether the MN is allowed to connect to the NAS's
      network.

   User Datagram Protocol (UDP): a connectionless transport-layer
      protocol used to send datagrams between a source and a destination
      at a given port, defined in RFC 768.

   Transmission Control Protocol (TCP): a stream-oriented transport-
      layer protocol that provides a reliable delivery service with
      congestion control, defined in RFC 793.

   Round-Trip Time (RTT): an estimation of the time required for a
      segment to travel from a source to a destination and an
      acknowledgement to return to the source that is used by TCP in
      connection with timer expirations to determine when a segment is
      considered lost and should be resent.

   Maximum Transmission Unit (MTU): the largest size of an IP packet
      that can be sent on a network segment without requiring
      fragmentation [RFC1191].

   Path MTU (PMTU): the largest size of an IP packet that can be sent on
      an end-to-end network path without requiring IP fragmentation.

   Transport Layer Security Protocol (TLS): an application layer
      protocol that primarily assures privacy and data integrity between
      two communicating network entities [RFC5246].

   Sender Maximum Segment Size (SMSS): size of the largest segment that
      the sender can transmit as per [RFC5681].

2.1.  Requirements Language

      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].

3.  Deployment Scenarios

   This section describes the various possible deployment scenarios for
   the MN and the Mobility Server.  The relative positioning of the MN
   and Mobility Server affects MoS discovery as well as the performance
   of the MIH signaling service.  This document addresses the scenarios
   listed in [RFC5164] and specifies transport options to carry the MIH
   protocol over IP.



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3.1.  Scenario S1: Home Network MoS

   In this scenario, the MN and the services are located in the home
   network.  We refer to this set of services as MoSh as shown in Figure
   1.  The MoSh can be located at the access network the MN uses to
   connect to the home network, or it can be located elsewhere.

                         +--------------+  +====+
                         | HOME NETWORK |  |MoSh|
                         +--------------+  +====+
                                       /\
                                       ||
                                       \/
                                +--------+
                                |   MN   |
                                +--------+

                     Figure 1: MoS in the Home Network

3.2.  Scenario S2: Visited Network MoS

   In this scenario, the MN is in the visited network and mobility
   services are provided by the visited network.  We refer to this as
   MoSv as shown in Figure 2.

                                  +--------------+
                                  | HOME NETWORK |
                                  +--------------+
                                            /\
                                            ||
                                            \/
                        +====+ +-----------------+
                        |MoSv| | VISITED NETWORK |
                        +====+ +-----------------+
                                            /\
                                            ||
                                            \/
                                      +--------+
                                      |   MN   |
                                      +--------+

                   Figure 2: MoSv in the Visited Network









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3.3.  Scenario S3: Third-Party MoS

   In this scenario, the MN is in its home network or in a visited
   network and services are provided by a third-party network.  We refer
   to this situation as MoS3 as shown in Figure 3.  (Note that MoS can
   exist both in home and in visited networks.)

                                            +--------------+
                                            | HOME NETWORK |
         +====+    +--------------+         +--------------+
         |MoS3|    | THIRD PARTY  |  <===>        /\
         +====+    +--------------+               ||
                                                  \/
                                          +-----------------+
                                          | VISITED NETWORK |
                                          +-----------------+
                                                  /\
                                                  ||
                                                  \/
                                              +--------+
                                              |   MN   |
                                              +--------+

               Figure 3: MoS from a Third Party

3.4.  Scenario S4: Roaming MoS

   In this scenario, the MN is located in the visited network and all
   MIH services are provided by the home network, as shown in Figure 4.

                    +====+   +--------------+
                    |MoSh|   | HOME NETWORK |
                    +====+   +--------------+
                                   /\
                                   ||
                                   \/
                          +-----------------+
                          | VISITED NETWORK |
                          +-----------------+
                                   /\
                                   ||
                                   \/
                               +--------+
                               |   MN   |
                               +--------+

            Figure 4: MoS Provided by the Home While in Visited




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   Different types of MoS can be provided independently of other types
   and there is no strict relationship between ES, CS, and IS, nor is
   there a requirement that the entities that provide these services
   should be co-located.  However, while IS tends to involve a large
   amount of static information, ES and CS are dynamic services and some
   relationships between them can be expected, e.g., a handover command
   (CS) could be issued upon reception of a link event (ES).  This
   document does not make any assumption on the location of the MoS
   (although there might be some preferred configurations), and aims at
   flexible MSFD to discover different services in different locations
   to optimize handover performance.  MoS discovery is discussed in more
   detail in Section 5.

4.  Solution Overview

   As mentioned in Section 1, the solution space is being divided into
   two functional domains: discovery and transport.  The following
   assumptions have been made:

   o  The solution is primarily aimed at supporting IEEE 802.21 MIH
      services -- namely, Information Service (IS), Event Service (ES),
      and Command Service (CS).

   o  If the MIHFID is available, FQDN or NAI's realm is used for
      mobility service discovery.

   o  The solutions are chosen to cover all possible deployment
      scenarios as described in Section 3.

   o  MoS discovery can be performed during initial network attachment
      or at any time thereafter.

   The MN may know the realm of the Mobility Server to be discovered.
   The MN may also be pre-configured with the address of the Mobility
   Server to be used.  In case the MN does not know what realm /
   Mobility Server to query, dynamic assignment methods are described in
   Section 5.

   The discovery of the Mobility Server (and the related configuration
   at MIHF level) is required to bind two MIHF peers (e.g., MN and
   Mobility Server) with their respective IP addresses.  Discovery MUST
   be executed in the following conditions:

   o  Bootstrapping: upon successful Layer 2 network attachment, the MN
      MAY be required to use DHCP for address configuration.  These
      procedures can carry the required information for MoS
      configuration in specific DHCP options.




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   o  If the MN does not receive MoS information during network
      attachment and the MN does not have a pre-configured Mobility
      Server, it MUST run a discovery procedure upon initial IP address
      configuration.

   o  If the MN changes its IP address (e.g., upon handover), it MUST
      refresh MIHF peer bindings (i.e., MIHF registration process).  In
      case the Mobility Server used is not suitable anymore (e.g., too
      large RTT experienced), the MN MAY need to perform a new discovery
      procedure.

   o  If the MN is a multi-homed device and it communicates with the
      same Mobility Server via different IP addresses, it MAY run
      discovery procedures if one of the IP addresses changes.

   Once the MIHF peer has been discovered, MIH information can be
   exchanged between MIH peers over a transport protocol such as UDP or
   TCP.  The usage of transport protocols is described in Section 6 and
   packing of the MIH messages does not require extra framing since the
   MIH protocol defined in [IEEE80221] already contains a length field.

4.1.  Architecture

   Figure 5 depicts the MSFD reference model and its components within a
   node.  The topmost layer is the MIHF user.  This set of applications
   consists of one or more MIH clients that are responsible for
   operations such as generating query and response, processing Layer 2
   triggers as part of the ES, and initiating and carrying out handover
   operations as part of the CS.  Beneath the MIHF user is the MIHF
   itself.  This function is responsible for MoS discovery, as well as
   creating, maintaining, modifying, and destroying MIH signaling
   associations with other MIHFs located in MIH peer nodes.  Below the
   MIHF are various transport-layer protocols as well as address
   discovery functions.

















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                    +--------------------------+
                    |       MIHF User          |
                    +--------------------------+
                                 ||
                    +--------------------------+
                    |           MIHF           |
                    +--------------------------+
                        ||         ||       ||
                        ||      +------+ +-----+
                        ||      | DHCP | | DNS |
                        ||      +------+ +-----+
                        ||         ||       ||
                    +--------------------------+
                    |         TCP/UDP          |
                    +--------------------------+

                         Figure 5: MN Stack

   The MIHF relies on the services provided by TCP and UDP for
   transporting MIH messages, and relies on DHCP and DNS for peer
   discovery.  In cases where the peer MIHF IP address is not pre-
   configured, the source MIHF needs to discover it either via DHCP or
   DNS as described in Section 5.  Once the peer MIHF is discovered, the
   MIHF must exchange messages with its peer over either UDP or TCP.
   Specific recommendations regarding the choice of transport protocols
   are provided in Section 6.

   There are no security features currently defined as part of the MIH
   protocol level.  However, security can be provided either at the
   transport or IP layer where it is necessary.  Section 8 provides
   guidelines and recommendations for security.

4.2.  MIHF Identifiers (FQDN, NAI)

   MIHFID is required to uniquely identify the MIHF end points for
   delivering the mobility services (MoS).  Thus an MIHF identifier
   needs to be unique within a domain where mobility services are
   provided and independent of the configured IP address(es).  An MIHFID
   MUST be represented either in the form of an FQDN [RFC2181] or NAI
   [RFC4282].  An MIHFID can be pre-configured or discovered through the
   discovery methods described in Section 5.

5.  MoS Discovery

   The MoS discovery method depends on whether the MN attempts to
   discover a Mobility Server in the home network, in the visited
   network, or in a third-party remote network that is neither the home
   network nor the visited network.  In the case where the MN already



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   has a Mobility Server address pre-configured, it is not necessary to
   run the discovery procedure.  If the MN does not have pre-configured
   Mobility Server, the following procedure applies.

   In the case where a Mobility Server is provided locally (scenarios S1
   and S2), the discovery techniques described in [RFC5678] and
   [RFC5679] are both applicable as described in Sections 5.1 and 5.2.

   In the case where a Mobility Server is located in the home network
   while the MN is in the visited network (scenario S4), the DNS-based
   discovery described in [RFC5679] is applicable.

   In the case where a Mobility Server is located in a third-party
   network that is different from the current visited network (scenario
   S3), only the DNS-based discovery method described in [RFC5679] is
   applicable.

   It should be noted that authorization of an MN to use a specific
   Mobility Server is neither in scope of this document nor is currently
   specified in [IEEE80221].  We further assume all devices can access
   discovered MoS.  In case future deployments will implement
   authorization policies, the mobile nodes should fall back to other
   learned MoS if authorization is denied.

5.1.  MoS Discovery When MN and MoSh Are in the Home Network (Scenario
      S1)

   To discover a Mobility Server in the home network, the MN SHOULD use
   the DNS-based MoS discovery method described in [RFC5679].  In order
   to use that mechanism, the MN MUST have its home domain pre-
   configured (i.e., subscription is tied to a network).  The DNS query
   option is shown in Figure 6a.  Alternatively, the MN MAY use the DHCP
   options for MoS discovery [RFC5678] as shown in Figure 6b (in some
   deployments, a DHCP relay may not be present).

















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            (a)                       +-------+
                       +----+         |Domain |
                       | MN |-------->|Name   |
                       +----+         |Server |
                     MN@example.org   +-------+

            (b)
                                    +-----+      +------+
                       +----+       |     |      |DHCP  |
                       | MN |<----->| DHCP|<---->|Server|
                       +----+       |Relay|      |      |
                                    +-----+      +------+

   Figure 6: MOS Discovery (a) Using DNS Query, (b) Using DHCP Option

5.2.  MoS Discovery When MN and MoSv Both Are in Visited Network
      (Scenario S2)

   To discover a Mobility Server in the visited network, the MN SHOULD
   attempt to use the DHCP options for MoS discovery [RFC5678] as shown
   in Figure 7.

                            +-----+      +------+
               +----+       |     |      |DHCP  |
               | MN |<----->| DHCP|<---->|Server|
               +----+       |Relay|      |      |
                            +-----+      +------+

                Figure 7: MoS Discovery Using DHCP Options

5.3.  MoS Discovery When MIH Services Are in a Third-Party Remote
      Network (Scenario S3)

   To discover a Mobility Server in a remote network other than home
   network, the MN MUST use the DNS-based MoS discovery method described
   in [RFC5679].  The MN MUST first learn the domain name of the network
   containing the MoS it is searching for.  The MN can query its current
   Mobility Server to find out the domain name of a specific network or
   the domain name of a network at a specific location (as in Figure
   8a).  IEEE 802.21 defines information elements such as OPERATOR ID
   and SERVICE PROVIDER ID that can be a domain name.  An IS query can
   provide this information, see [IEEE80221].

   Alternatively, the MN MAY query a Mobility Server previously known to
   learn the domain name of the desired network.  Finally, the MN MUST
   use DNS-based discovery mechanisms to find a Mobility Server in the





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   remote network as in Figure 8b.  It should be noted that step b can
   only be performed upon obtaining the domain name of the remote
   network.

            (a)
                                      +------------+
                       +----+         |            |
                       |    |         |Information |
                       | MN |-------->| Server     |
                       |    |         |(previously |
                       +----+         |discovered) |
                                      +------------+

            (b)
                                      +-------+
                       +----+         |Domain |
                       | MN |-------->|Name   |
                       +----+         |Server |
                    MN@example.org    +-------+

   Figure 8: MOS Discovery Using (a) IS Query to a Known IS Server,
                                 (b) DNS Query

5.4.  MoS Discovery When the MN Is in a Visited Network and Services Are
      at the Home Network (Scenario S4)

   To discover a Mobility Server in the visited network when MIH
   services are provided by the home network, the DNS-based discovery
   method described in [RFC5679] is applicable.  To discover the
   Mobility Server at home while in a visited network using DNS, the MN
   SHOULD use the procedures described in Section 5.1.

6.  MIH Transport Options

   Once the MoS have been discovered, MIH peers run a capability
   discovery and subscription procedure as specified in [IEEE80221].
   MIH peers MAY exchange information over TCP, UDP, or any other
   transport supported by both the server and the client.  The client
   MAY use the DNS discovery mechanism to discover which transport
   protocols are supported by the server in addition to TCP and UDP that
   are recommended in this document.  While either protocol can provide
   the basic transport functionality required, there are performance
   trade-offs and unique characteristics associated with each that need
   to be considered in the context of the MIH services for different
   network loss and congestion conditions.  The objectives of this
   section are to discuss these trade-offs for different MIH settings
   such as the MIH message size and rate, and the retransmission
   parameters.  In addition, factors such as NAT traversal are also



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   discussed.  Given the reliability requirements for the MIH transport,
   it is assumed in this discussion that the MIH ACK mechanism is to be
   used in conjunction with UDP, while it MUST NOT be used with TCP
   since TCP includes acknowledgement and retransmission functionality.

6.1.  MIH Message Size

   Although the MIH message size varies widely from about 30 bytes (for
   a capability discovery request) to around 65000 bytes (for an IS
   MIH_Get_Information response primitive), a typical MIH message size
   for the ES or CS ranges between 50 to 100 bytes [IEEE80221].  Thus,
   considering the effects of the MIH message size on the performance of
   the transport protocol brings us to discussing two main issues,
   related to fragmentation of long messages in the context of UDP and
   the concatenation of short messages in the context of TCP.

   Since transporting long MIH messages may require fragmentation that
   is not available in UDP, if MIH is using UDP a limit MUST be set on
   the size of the MIH message based on the path MTU to destination (or
   the Minimum MTU where PMTU is not implemented).  The Minimum MTU
   depends on the IP version used for transmission, and is the lesser of
   the first hop MTU, and 576 or 1280 bytes for IPv4 [RFC1122] or for
   IPv6 [RFC2460], respectively, although applications may reduce these
   values to guard against the presence of tunnels.

   According to [IEEE80221], when an MIH message is sent using an L3 or
   higher-layer transport, L3 takes care of any fragmentation issue and
   the MIH protocol does not handle fragmentation in such cases.  Thus,
   MIH layer fragmentation MUST NOT be used together with IP layer
   fragmentation and MUST not be used when MIH packets are carried over
   TCP.

   The loss of an IP fragment leads to the retransmission of an entire
   MIH message, which in turn leads to poor end-to-end delay performance
   in addition to wasted bandwidth.  Additional recommendations in
   [RFC5405] apply for limiting the size of the MIH message when using
   UDP and assuming IP layer fragmentation.  In terms of dealing with
   short messages, TCP has the capability to concatenate very short
   messages in order to reduce the overall bandwidth overhead.  However,
   this reduced overhead comes at the cost of additional delay to
   complete an MIH transaction, which may not be acceptable for CS and
   ES.  Note also that TCP is a stream-oriented protocol and measures
   data flow in terms of bytes, not messages.  Thus, it is possible to
   split messages across multiple TCP segments if they are long enough.
   Even short messages can be split across two segments.  This can also
   cause unacceptable delays, especially if the link quality is severely
   degraded as is likely to happen when the MN is exiting a wireless
   access coverage area.  The use of the TCP_NODELAY option can



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   alleviate this problem by triggering transmission of a segment less
   than the SMSS.  (It should be noted that [RFC4960] addresses both of
   these problems, but discussion of SCTP is omitted here, as it is
   generally not used for the mobility services discussed in this
   document.)

6.2.  MIH Message Rate

   The frequency of MIH messages varies according to the MIH service
   type.  It is expected that CS/ES messages arrive at a rate of one in
   hundreds of milliseconds in order to capture quick changes in the
   environment and/or process handover commands.  On the other hand, IS
   messages are exchanged mainly every time a new network is visited,
   which may be in order of hours or days.  Therefore, a burst of either
   short CS/ES messages or long IS message exchanges (in the case where
   multiple MIH nodes request information) may lead to network
   congestion.  While the built-in rate-limiting controls available in
   TCP may be well suited for dealing with these congestion conditions,
   this may result in large transmission delays that may be unacceptable
   for the timely delivery of ES or CS messages.  On the other hand, if
   UDP is used, a rate-limiting effect similar to the one obtained with
   TCP SHOULD be obtained by adequately adjusting the parameters of a
   token bucket regulator as defined in the MIH specifications
   [IEEE80221].  Recommendations for token bucket parameter settings are
   as follows:

   o If the MIHF knows the RTT (e.g., based on the request/response MIH
      protocol exchange between two MIH peers), the rate can be based
      upon this as specified in [IEEE80221].

   o  If not, then on average it SHOULD NOT send more than one UDP
      message every 3 seconds.

6.3.  Retransmission

   For TCP, the retransmission timeout is adjusted according to the
   measured RTT.  However due to the exponential backoff mechanism, the
   delay associated with retransmission timeouts may increase
   significantly with increased packet loss.

   If UDP is being used to carry MIH messages, MIH MUST use MIH ACKs.
   An MIH message is retransmitted if its corresponding MIH ACK is not
   received by the generating node within a timeout interval set by the
   MIHF.  The maximum number of retransmissions is configurable and the
   value of the retransmission timer is computed according to the
   algorithm defined in [RFC2988].  The default maximum number of





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   retransmissions is set to 2 and the initial retransmission timer
   (TMO) is set to 3s when RTT is not known.  The maximum TMO is set to
   30s.

6.4.  NAT Traversal

   There are no known issues for NAT traversal when using TCP.  The
   default connection timeout of 2 hours 4 minutes [RFC5382] (assuming a
   2-hour TCP keep-alive) is considered adequate for MIH transport
   purposes.  However, issues with NAT traversal using UDP are
   documented in [RFC5405].  Communication failures are experienced when
   middleboxes destroy the per-flow state associated with an application
   session during periods when the application does not exchange any UDP
   traffic.  Hence, communication between the MN and the Mobility Server
   SHOULD be able to gracefully handle such failures and implement
   mechanisms to re-establish their UDP sessions.  In addition and in
   order to avoid such failures, MIH messages MAY be sent periodically,
   similarly to keep-alive messages, in an attempt to refresh middlebox
   state.  As [RFC4787] requires a minimum state timeout of 2 minutes or
   more, MIH messages using UDP as transport SHOULD be sent once every 2
   minutes.  Re-registration or event indication messages as defined in
   [IEEE80221] MAY be used for this purpose.

6.5.  General Guidelines

   The ES and CS messages are small in nature and have tight latency
   requirements.  On the other hand, IS messages are more resilient in
   terms of latency constraints, and some long IS messages could exceed
   the MTU of the path to the destination.  TCP SHOULD be used as the
   default transport for all messages.  However, UDP in combination with
   MIH acknowledgement SHOULD be used for transporting ES and CS
   messages that are shorter than or equal to the path MTU as described
   in Section 6.1.

   For both UDP and TCP cases, if a port number is not explicitly
   assigned (e.g., by the DNS SRV), MIH messages sent over UDP, TCP, or
   other supported transport MUST use the default port number defined in
   Section 9 for that particular transport.

   A Mobility Server MUST support both UDP and TCP for MIH transport and
   the MN MUST support TCP.  Additionally, the server and MN MAY support
   additional transport mechanisms.  The MN MAY use the procedures
   defined in [RFC5679] to discover additional transport protocols
   supported by the server (e.g., SCTP).







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7.  Operation Flows

   Figure 9 gives an example operation flow between MIHF peers when an
   MIH user requests an IS and both the MN and the Mobility Server are
   in the MN's home network.  DHCP is used for Mobility Services (MoS)
   discovery, and TCP is used for establishing a transport connection to
   carry the IS messages.  When the Mobility Server is not pre-
   configured, the MIH user needs to discover the IP address of the
   Mobility Server to communicate with the remote MIHF.  Therefore, the
   MIH user sends a discovery request message to the local MIHF as
   defined in [IEEE80221].

   In this example (one could draw similar mechanisms with DHCPv6), we
   assume that MoS discovery is performed before a transport connection
   is established with the remote MIHF, and the DHCP client process is
   invoked via some internal APIs.  The DHCP client sends a DHCP INFORM
   message according to standard DHCP and with the MoS option as defined
   in [RFC5678].  The DHCP server replies via a DHCP ACK message with
   the IP address of the Mobility Server.  The Mobility Server address
   is then passed to the MIHF locally via some internal APIs.  The MIHF
   generates the discovery response message and passes it on to the
   corresponding MIH user.  The MIH user generates an IS query addressed
   to the remote Mobility Server.  The MIHF invokes the underlying TCP
   client, which establishes a transport connection with the remote
   peer.  Once the transport connection is established, the MIHF sends
   the IS query via an MIH protocol REQUEST message.  The message and
   query arrive at the destination MIHF and MIH user, respectively.  The
   Mobility Server MIH user responds to the corresponding IS query and
   the Mobility Server MIHF sends the IS response via an MIH protocol
   RESPONSE message.  The message arrives at the source MIHF, which
   passes the IS response on to the corresponding MIH user.




















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                MN                                         MoS
|===================================|    |======| |===================|
+ ---------+                                                +---------+
| MIH USER |       +------+  +------+    +------+  +------+ | MIH USER|
| +------+ |       | TCP  |  |DHCP  |    |DHCP  |  | TCP  | | +------+|
| | MIHF | |       |Client|  |Client|    |Server|  |Server| | | MIHF ||
+----------+       +------+  +------+    +------+  +------++----------+
    |                 |         |           |         |          |
  MIH Discovery       |         |           |         |          |
  Request             |         |           |         |          |
    |                 |         |           |         |          |
    |Invoke DHCP Client         |           |         |          |
    |(Internal process with MoS)|DHCP INFORM|         |          |
    |==========================>|==========>|         |          |
    |                 |         |           |         |          |
    |  Inform Mobility Server   |  DHCP ACK |         |          |
    |         Address           |<==========|         |          |
    |<==========================|           |         |          |
    |    (internal process)     |           |         |          |
    |                 |         |           |         |          |
  MIH Discovery       |         |           |         |          |
  Response            |         |           |         |          |
    |                 |         |           |         |          |
  IS Query            |         |           |         |          |
  MIH User-> MIHF     |         |           |         |          |
    |                 |         |           |         |          |
    |Invoke TCP Client|         |           |         |          |
    |================>|  TCP connection established   |          |
  Internal process    |<=============================>|          |
    |                 |         |           |         |          |
    |                 IS  QUERY REQUEST (via MIH protocol)       |
    |===========================================================>|
    |                 |         |           |         |  IS QUERY|
    |                 |         |           |         |   REQUEST|
    |                 |         |           |    MIHF-> MIH User |
    |                 |         |           |         |     QUERY|
    |                 |         |           |         |  RESPONSE|
    |                 |         |           |   MIHF <-MIH User  |
    |                 |         |           |         |          |
    |                 | IS QUERY RESPONSE (via MIH protocol)     |
    |<===========================================================|
    |                 |         |           |         |          |
    IS RESPONSE       |         |           |         |          |
    MIH User <-MIHF   |         |           |         |          |
    |                 |         |           |         |          |

          Figure 9: Example Flow of Operation Involving MIH User




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8.  Security Considerations

   There are two components to the security considerations: MoS
   discovery and MIH transport.  For MoS discovery, DHCP and DNS
   recommendations are hereby provided per IETF guidelines.  For MIH
   transport, we describe the security threats and expect that the
   system deployment will have means to mitigate such threats when
   sensitive information is being exchanged between the mobile node and
   Mobility Server.  Since IEEE 802.21 base specification does not
   provide MIH protocol level security, it is assumed that either lower
   layer security (e.g., link layer) or overall system-specific (e.g.,
   proprietary) security solutions are available.  The present document
   does not provide any guidelines in this regard.  It is stressed that
   the IEEE 802.21a Task Group has recently started work on MIH security
   issues that may provide some solution in this area.  Finally,
   authorization of an MN to use a specific Mobility Server, as stated
   in Section 5, is neither in scope of this document nor is currently
   specified in [IEEE80221].

8.1.  Security Considerations for MoS Discovery

   There are a number of security issues that need to be taken into
   account during node discovery.  In the case where DHCP is used for
   node discovery and authentication of the source and content of DHCP
   messages is required, network administrators SHOULD use the DHCP
   authentication option described in [RFC3118], where available, or
   rely upon link layer security.  [RFC3118] provides mechanisms for
   both entity authentication and message authentication.  In the case
   where the DHCP authentication mechanism is not available,
   administrators may need to rely upon the underlying link layer
   security.  In such cases, the link between the DHCP client and Layer
   2 termination point may be protected, but the DHCP message source and
   its messages cannot be authenticated or the integrity of the latter
   checked unless there exits a security binding between link layer and
   DHCP layer.

   In the case where DNS is used for discovering MoS, fake DNS requests
   and responses may cause denial of service (DoS) and the inability of
   the MN to perform a proper handover, respectively.  Where networks
   are exposed to such DoS, it is RECOMMENDED that DNS service providers
   use the Domain Name System Security Extensions (DNSSEC) as described
   in [RFC4033].  Readers may also refer to [RFC4641] to consider the
   aspects of DNSSEC operational practices.

8.2.  Security Considerations for MIH Transport

   The communication between an MN and a Mobility Server is exposed to a
   number of security threats:



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   o  Mobility Server identity spoofing.  A fake Mobility Server could
      provide the MNs with bogus data and force them to select the wrong
      network or to make a wrong handover decision.

   o  Tampering.  Tampering with the information provided by a Mobility
      Server may result in the MN making wrong network selection or
      handover decisions.

   o  Replay attack.  Since Mobility Services as defined in [IEEE80221]
      support a 'PUSH model', they can send large amounts of data to the
      MNs whenever the Mobility Server thinks that the data is relevant
      for the MN.  An attacker may intercept the data sent by the
      Mobility Server to the MNs and replay it at a later time, causing
      the MNs to make network selection or handover decisions that are
      not valid at that point in time.

   o  Eavesdropping.  By snooping the communication between an MN and a
      Mobility Server, an attacker may be able to trace a user's
      movement between networks or cells, or predict future movements,
      by inspecting handover service messages.

   There are many deployment-specific system security solutions
   available, which can be used to countermeasure the above mentioned
   threats.  For example, for the MoSh and MoSv scenarios (including
   roaming scenarios), link layer security may be sufficient to protect
   the communication between the MN and Mobility Server.  This is a
   typical mobile operator environment where link layer security
   provides authentication, data confidentiality, and integrity.  In
   other scenarios, such as the third-party MoS, link layer security
   solutions may not be sufficient to protect the communication path
   between the MN and the Mobility Server.  The communication channel
   between MN and Mobility Server needs to be secured by other means.

   The present document does not provide any specific guidelines about
   the way these security solutions should be deployed.  However, if in
   the future the IEEE 802.21 Working Group amends the specification
   with MIH protocol level security or recommends the deployment
   scenarios, IETF may revisit the security considerations and recommend
   specific transport-layer security as appropriate.

9.  IANA Considerations

   This document registers the following TCP and UDP ports with IANA:

    Keyword    Decimal             Description
    --------   ---------------     ------------
    ieee-mih   4551/tcp            MIH Services
    ieee-mih   4551/udp            MIH Services



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10.  Acknowledgements

   The authors would like to thank Yoshihiro Ohba, David Griffith, Kevin
   Noll, Vijay Devarapalli, Patrick Stupar, and Sam Xia for their
   valuable comments, reviews, and fruitful discussions.

11.  References

11.1.  Normative References

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2181]   Elz, R. and R. Bush, "Clarifications to the DNS
               Specification", RFC 2181, July 1997.

   [RFC3118]   Droms, R., Ed., and W. Arbaugh, Ed., "Authentication for
               DHCP Messages", RFC 3118, June 2001.

   [RFC3315]   Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
               C., and M. Carney, "Dynamic Host Configuration Protocol
               for IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC4033]   Arends, R., Austein, R., Larson, M., Massey, D., and S.
               Rose, "DNS Security Introduction and Requirements", RFC
               4033, March 2005.

   [RFC4282]   Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
               Network Access Identifier", RFC 4282, December 2005.

   [RFC5678]   Bajko, G. and S. Das, "Dynamic Host Configuration
               Protocol (DHCPv4 and DHCPv6) Options for IEEE 802.21
               Mobility Services (MoS) Discovery", RFC 5678, December
               2009.

   [RFC5679]   Bajko, G., "Locating IEEE 802.21 Mobility Services Using
               DNS", RFC 5679, December 2009.

11.2.  Informative References

   [IEEE80221] "IEEE Standard for Local and Metropolitan Area Networks -
               Part 21: Media Independent Handover Services", IEEE
               LAN/MAN Std 802.21-2008, January 2009,
               http://www.ieee802.org/21/private/Published%20Spec/
               802.21-2008.pdf (access to the document requires
               membership).





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   [RFC1035]   Mockapetris, P., "Domain names - implementation and
               specification", STD 13, RFC 1035, November 1987.

   [RFC1122]   Braden, R., "Requirements for Internet Hosts -
               Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC1191]   Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
               November 1990.

   [RFC2131]   Droms, R., "Dynamic Host Configuration Protocol", RFC
               2131, March 1997.

   [RFC2460]   Deering, S. and R. Hinden, "Internet Protocol, Version 6
               (IPv6) Specification", RFC 2460, December 1998.

   [RFC2988]   Paxson, V. and M. Allman, "Computing TCP's Retransmission
               Timer", RFC 2988, November 2000.

   [RFC3022]   Srisuresh, P. and K. Egevang, "Traditional IP Network
               Address Translator (Traditional NAT)", RFC 3022, January
               2001.

   [RFC4641]   Kolkman, O. and R. Gieben, "DNSSEC Operational
               Practices", RFC 4641, September 2006.

   [RFC4787]   Audet, F., Ed., and C. Jennings, "Network Address
               Translation (NAT) Behavioral Requirements for Unicast
               UDP", BCP 127, RFC 4787, January 2007.

   [RFC4960]   Stewart, R., Ed., "Stream Control Transmission Protocol",
               RFC 4960, September 2007.

   [RFC5164]   Melia, T., Ed., "Mobility Services Transport: Problem
               Statement", RFC 5164, March 2008.

   [RFC5246]   Dierks, T. and E. Rescorla, "The Transport Layer Security
               (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5382]   Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and
               P. Srisuresh, "NAT Behavioral Requirements for TCP", BCP
               142, RFC 5382, October 2008.

   [RFC5405]   Eggert, L. and G. Fairhurst, "Unicast UDP Usage
               Guidelines for Application Designers", BCP 145, RFC 5405,
               November 2008.

   [RFC5681]   Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
               Control", RFC 5681, September 2009.



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Authors' Addresses

   Telemaco Melia (editor)
   Alcatel-Lucent
   Route de Villejust
   Nozay  91620
   France

   EMail: telemaco.melia@alcatel-lucent.com


   Gabor Bajko
   Nokia

   EMail: Gabor.Bajko@nokia.com


   Subir Das
   Telcordia Technologies Inc.

   EMail: subir@research.telcordia.com


   Nada Golmie
   NIST

   EMail: nada.golmie@nist.gov


   Juan Carlos Zuniga
   InterDigital Communications, LLC

   EMail: j.c.zuniga@ieee.org


















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