


MMUSIC Working Group                                            M. Saito
Internet-Draft                                        NTT Communications
Intended status: Informational                                   D. Wing
Expires: September 25, 2010                                Cisco Systems
                                                               M. Toyama
                                                         NTT Corporation
                                                          March 24, 2010


  Media Description for IKE in the Session Description Protocol (SDP)
                     draft-saito-mmusic-sdp-ike-07

Abstract

   This document specifies how to establish a media session which
   represents a virtual private network using Session Initiation
   Protocol for the purpose of on-demand media/application sharing
   between peers.  It extends the protocol identifier of Session
   Description Protocol (SDP) so that it can negotiate the use of
   Internet Key Exchange Protocol (IKE) for media sessions in the SDP
   offer/answer model.  It also specifies a method to boot up IKE and
   generate IPsec security associations using a self-signed certificate
   under the mechanism of connection-oriented media transport over the
   Transport Layer Security in the SDP (comedia-tls).  This document
   extends RFC 4572.  In addition, it defines a new attribute "udp-
   setup", which is similar to the "setup" attribute defined in RFC
   4145, to enable endpoints to negotiate their roles in an IKE session.
   To use pre-shared keys for authentication in IKE, a new attribute
   "psk-fingerprint" is also defined.






















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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 [RFC2119].

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   This Internet-Draft will expire on September 25, 2010.

Copyright Notice

   Copyright (c) 2010 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
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   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.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  4
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Problem Statement  . . . . . . . . . . . . . . . . . . . .  5
     2.2.  Approach to Solution . . . . . . . . . . . . . . . . . . .  5
     2.3.  Alternative Solution under Prior Relationship between
           Two Nodes  . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.4.  Authorization Model  . . . . . . . . . . . . . . . . . . .  7
   3.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Protocol Identifiers . . . . . . . . . . . . . . . . . . . . . 10
   5.  Example of SDP Offer and Answer Exchange without IPsec
       NAT-Traversal  . . . . . . . . . . . . . . . . . . . . . . . . 12
   6.  Example of SDP Offer and Answer Exchange with IPsec
       NAT-Traversal  . . . . . . . . . . . . . . . . . . . . . . . . 14
     6.1.  Port Usage . . . . . . . . . . . . . . . . . . . . . . . . 14
     6.2.  Offer and Answer Exchange with ICE . . . . . . . . . . . . 14
     6.3.  Multiplex of UDP Messages  . . . . . . . . . . . . . . . . 16
   7.  Application to IKE . . . . . . . . . . . . . . . . . . . . . . 18
   8.  Specifications Assuming Prior Relationship between Two
       Nodes  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     8.1.  Certificates Signed by Trusted Third Party . . . . . . . . 19
     8.2.  Configured Pre-Shared Key  . . . . . . . . . . . . . . . . 19
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 25
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 26
     12.2. Informative References . . . . . . . . . . . . . . . . . . 27
   Appendix A.  Changes since draft-saito-mmusic-sdp-ike-05 . . . . . 28
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29




















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1.  Applicability Statement

   This document proposes to use Session Initiation Protocol (SIP)
   [RFC3261] as a name resolution and authentication mechanism to
   initiate an Internet Key Exchange Protocol (IKE) [RFC4306] session.
   The purpose of this document is to establish an on-demand virtual
   private network (VPN) to a home router that does not have a fixed IP
   address using self-signed certificates.  It extends comedia-tls
   [RFC4572] and is only applicable under the condition that the
   integrity of Session Description Protocol (SDP) [RFC4566] is assured.
   The method to ensure this integrity of SDP is outside the scope of
   this document.  This document specifies the process in which a pair
   of SIP user agents resolve each other's names, exchange the
   fingerprints of their self-signed certificates securely, and agree to
   establish an IPsec [RFC4301] based VPN.  However, it does not make
   any modifications to the specifications of IPsec/IKE.  Despite the
   limitations of the conditions under which this document can be
   applied, there are sufficient use cases in which this specification
   is helpful as follows.

   o  Sharing media using a framework developed by Digital Living
      Network Alliance (DLNA) or similar protocols over VPN between two
      user devices.

   o  Remote desktop applications over VPN initiated by SIP call.  As an
      additional function of click-to-call, a customer service agent can
      access a customer's PC remotely to troubleshoot the problem while
      talking with the customer over the phone.

   o  Accessing and controlling medical equipment (medical robotics)
      remotely to monitor the elderly in a rural area (remote care
      services).

   o  Local area network (LAN)-based gaming protocol based on peer-to-
      peer rather than via a gaming server.
















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2.  Introduction

   This section describes the problem in accessing home networks and
   provides an overview of the proposed solution.

2.1.  Problem Statement

   Home servers and network-capable consumer electronic devices have
   been widely deployed.  People using such devices are willing to share
   content and applications and are therefore seeking ways to establish
   multiple communication channels with each other.  However, there are
   several obstacles to be overcome in the case of remote home access.

   It is often not possible for a device outside the home network to
   connect to another device inside the home network because the home
   device is behind a network address translation (NAT) or firewall that
   allows outgoing connections but blocks incoming connections.  One
   effective solution for this problem is VPN remote access to the NAT
   device, which is usually a home router.  With this approach, once the
   external device participates in the home network securely,
   establishing connections with all the devices inside the home will
   become easy because popular LAN-based communication methods such as
   DLNA can be used transparently.  However, there are more difficult
   cases in which a home router itself is located behind the NAT.  In
   such cases, it is also necessary to consider NAT traversal of the
   remote access to the home router.  In many cases, because the global
   IP address of the home router is not always fixed, it is necessary to
   make use of an effective name resolution mechanism.

   In addition, there is the problem of how a remote client and a home
   router authenticate each other over IKE that establishes IPsec for
   remote access.  It is not always possible for the two devices to
   exchange a pre-shared key securely in advance.  Administrative costs
   can make it impractical to distribute authentication certificates
   signed by well-known root certification authority (CA) to all the
   devices.  In addition, it is inefficient to publish a temporary
   certificate to a device that does not have a fixed IP address or
   hostname.  To resolve these authentication issues, this document
   proposes a mechanism that enables the devices to authenticate each
   other using self-signed certificates.

2.2.  Approach to Solution

   This document proposes the use of SIP as a name resolution and
   authentication mechanism because there are three main advantages as
   follows.





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   o  Delegation of Authentication to Third Party
      Devices can be free from managing their signed certificates and
      whitelists by taking advantage of authentication and authorization
      mechanisms supported by SIP.

   o  UDP Hole Punching for IKE/IPsec
      SIP has a cross-NAT rendezvous mechanism and ICE
      [I-D.ietf-mmusic-ice] has a function to open ports through the
      NAT.  The combination of these effective functions can be used for
      general applications as well as real-time media.  It is difficult
      to setup a session between devices without SIP if the devices are
      behind various types of NAT.

   o  Reuse of Existing SIP Infrastructure
      SIP servers are widely distributed as a scalable infrastructure,
      and it is quite practical to reuse them without any modifications.

   Today, SIP is applied to not only VoIP but also various applications
   and is recognized as a general protocol for session initiation.
   Therefore, it can also be used to initiate IKE/IPsec sessions.

   However, there is also a specification that uses a self-signed
   certificate for authentication in the SIP/SDP framework.  Comedia-tls
   specifies a method to exchange the fingerprint of a self-signed
   certificate to establish a Transport Layer Security (TLS) [RFC5246]
   connection.  This specification defines a mechanism by which self-
   signed certificates can be used securely, provided that the integrity
   of the SDP description is assured.  Because a certificate itself is
   used for authentication not only in TLS but also in IKE, this
   mechanism will be applied to the establishment of IPsec SA by
   extending the protocol identifier of SDP so that it can specify IKE.

   One easy method to protect the integrity of the SDP description,
   which is the premise of this specification, is to use the SIP
   identity [RFC4474] mechanism.  This approach is also referred to in
   [I-D.ietf-sip-dtls-srtp-framework].  Because the SIP identity
   mechanism can protect the integrity of a body part as well as the
   value of the From header in a SIP request by using a valid Identity
   header, the receiver of the request can establish secure IPsec
   connections with the sender by confirming that the hash value of the
   certificate sent during IKE negotiation matches the fingerprint in
   the SDP.  Although SIP identity does not protect the identity of the
   receiver of the SIP request, SIP-connected identity [RFC4916] does.
   Note that the possible deficiencies discussed in
   [I-D.rosenberg-sip-rfc4474-concerns] could affect this specification
   if SIP identity is used for the security mechanism.

   Considering the above background, this document defines new media



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   formats "ike-esp" and "ike-esp-udpencap", which can be used when the
   protocol identifier is "udp", to enable the negotiation of using IKE
   for media sessions over SDP exchange on the condition that the
   integrity of the SDP description is assured.  It also specifies the
   method to setup an IPsec SA by exchanging fingerprints of self-signed
   certificates based on comedia-tls, and it notes the example of SDP
   offer/answer [RFC3264] and the points that should be taken care of by
   implementation.  Because there is a chance that devices are behind
   NAT, it also covers the method to combine IKE/IPsec NAT-Traversal
   [RFC3947][RFC3948] with ICE.  In addition, it defines an attribute
   "udp-setup" for UDP media sessions, similar to the "setup" attribute
   for TCP-based media transport defined in RFC 4145 [RFC4145].  It is
   used to negotiate the role of each endpoint in the IKE session.

2.3.  Alternative Solution under Prior Relationship between Two Nodes

   Under quite limited conditions, certificates signed by trusted third
   parties or pre-shared keys between endpoints could be used for
   authentication in IKE, with use of SIP servers only for name
   resolution and authorization of session initiation.  We address such
   limited cases in chapter 8.

2.4.  Authorization Model

   In this document, SIP servers are used for authorization of each SIP
   call.  The actual media sessions of IPsec/IKE are not authorized by
   SIP servers but by the remote client and the home router based on the
   information in SIP/SDP.  For example, the home router recognizes the
   remote client with its SIP-URI and IP address in the SDP.  If it
   decides to accept the remote client as the peer of a VPN session, it
   will accept the following IKE session.  And then during the IKE
   negotiation the certificate fingerprint in the SDP is compared with
   the certificate exchanged in the IKE session.  If they match, IKE
   negotiation continues and only a successful IKE negotiation
   establishes an IPsec session with the remote peer.
















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3.  Protocol Overview

   As shown in Figure 1, for example, there is a case of VPN remote
   access from a device outside the home to a home router whose IP
   address is not fixed.  In this case, the external device, a remote
   client, recognizes the Address of Record of the home router, but does
   not have any information about its contact address and certificate.
   Generally, establishing IPsec SA dynamically and securely in this
   situation is difficult.  However, as specified in comedia-tls, if the
   integrity of SDP session descriptions is assured, it is possible for
   the home router and the remote client to have a prior relationship
   with each other by exchanging certificate fingerprints, i.e., secure
   one-way hashes of the distinguished encoding rules (DER) form of the
   certificates.

                 REGISTRATION                REGISTRATION
                    (1)       +----------+      (1)
               +------------->|          |<---------+
               |    INVITE(2) |          |          |
               | +----------->|   SIP    |--------+ |
               | |  200 OK(2) |   Proxy  |        | |
               | | +----------|          |<-----+ | |
               | | |          |          |      | | |  _________
               | | V          +----------+      | V | /         \
            +----------+  IKE(Media Session) +---------+         \
            | Remote   |<---------(3)------->| Home    |  Home    \
            | Client   |                     | Router  | Network   |
            |         ============(4)====================          |
            |(SIP UAC) |     VPN (IPsec SA)  |(SIP UAS)|          /
            +----------+                     +---------+         /
                                                      \_________/

                  Figure 1: Remote Access to Home Network

   (1)  Both Remote Client and Home Router generate secure signaling
      channels.  They may REGISTER to SIP Proxy using TLS.

   (2)  Remote Client sends an offer SDP with an INVITE request to Home
      Router and Home Router returns an answer SDP with a reliable
      response (e.g., 200 OK).  Both exchange the fingerprints of their
      self-signed certificates in SDP during this transaction.  Remote
      Client MUST NOT accept an answer SDP with an unreliable response
      as the final response.

   (3)  After SDP exchange, Remote Client, which has the active role,
      initiates IKE with Home Router, which has the passive role, to
      establish IPsec SA.  Both validate that the certificate presented
      in the IKE exchange has a fingerprint that matches the fingerprint



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      from SDP.  If they match, IKE negotiation proceeds as normal.

   (4)  Remote Client joins the Home Network.

   By this method, the self-signed certificates of both parties are used
   for authentication in IKE, but SDP itself is not concerned with all
   the negotiations related to key-exchange, such as those of encryption
   and authentication algorithms.  These negotiations are up to IKE.  In
   many cases where IPsec is used for remote access, a remote client
   needs to dynamically obtain a private address inside the home network
   while initiating the remote access.  Therefore, the IPsec security
   policy also needs to be set dynamically at the same time.  However,
   such a management function of the security policy is the
   responsibility of the high-level application.  SDP is not concerned
   with it.  The roles of SDP here are to determine the IP addresses of
   both parties used for IKE connection with c-line in SDP and to
   exchange the fingerprints of the certificates used for authentication
   in IKE with the fingerprint attribute in SDP.

   If the high-level application recognizes a VPN session as the media
   session, it MAY discard the IPsec SA and terminate IKE when that
   media session is terminated by BYE request.  Therefore, the
   application MUST NOT send a BYE request as long as it needs the IPsec
   SA.  On the other hand, if the high-level application detects that a
   VPN session is terminated, it MAY terminate the media associated with
   the VPN or the entire SIP session.  Session timers in SIP [RFC4028]
   MAY be used for the session maintenance of the SIP call, but this
   does not necessarily ensure that the VPN session is alive.  If the
   VPN session needs session maintenance such as keep-alive and
   rekeying, it MUST be done by its own maintenance mechanisms.  SIP re-
   INVITE MUST NOT be used for this purpose.  Note that each party can
   cache the certificate of the other party as described in the Security
   Consideration of comedia-tls.

   Forking to multiple registered instances is outside the scope in this
   use case.  At least, it is assumed that a UAC establishes a session
   with only one UAS.  If a UAC encounters with forked answers, it
   should be treated as an illegal process.  The above example is for
   tunnel mode IPsec used for remote access, but the actual usage of
   negotiated IPsec is not limited.  For example, IKE can negotiate
   transport mode IPsec to encrypt multiple media sessions between two
   parties with only a pair of IPsec security associations.  The only
   thing that the SDP offer/answer model is responsible for is to
   exchange the fingerprints of certificates used for IKE; therefore, it
   does not take care of the security policy.






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4.  Protocol Identifiers

   This document defines two SDP media formats for the "udp" protocol
   under the "application" media type: "ike-esp" and "ike-esp-udpencap".
   The format "ike-esp" indicates that the media described is IKE for
   the establishment of an IPsec security association as described in
   IPsec ESP [RFC4303].  In contrast, "ike-esp-udpencap" indicates that
   the media described is IKE which is capable of NAT traversal for the
   establishment of UDP encapsulation of IPsec packets through NAT boxes
   as specified in RFC3947 and RFC3948.  Even if offerer and answerer
   exchange "ike-esp-udpencap", IKE conforming to RFC3947 and RFC3948
   can end up with establishing normal IPsec tunnel when there is no
   need to use UDP encapsulation of IPsec.  Both offerer and answerer
   can negotiate IKE by specifying "udp" in the "proto" field and "ike-
   esp" or "ike-esp-udpencap" in the "fmt" field in SDP.

   In addition, this document defines a new attribute "udp-setup", which
   can be used when the protocol identifier is "udp" and the "fmt" field
   is "ike-esp" or "ike-esp-udpencap", in order to describe how
   endpoints should perform the IKE session setup procedure.  The "udp-
   setup" attribute indicates which of the end points should initiate
   the establishment of an IKE session.  The "udp-setup" attribute is
   charset-independent and can be a session- or media-level attribute.
   The following is the ABNF of the "udp-setup" attribute.

      udp-setup-attr = "a=udp-setup:" role
      role           = "active" / "passive" / "actpass"

      'active':   The endpoint will initiate an outgoing session.
      'passive':  The endpoint will accept an incoming session.
      'actpass':  The endpoint is willing to accept an incoming
                  session or to initiate an outgoing session.

   Both endpoints use the SDP offer/answer model to negotiate the value
   of "udp-setup", following the procedures determined for the "setup"
   attribute defined in 4.1 of RFC 4145.  However, "holdconn" defined in
   RFC 4145 is not defined for the "udp-setup" attribute because UDP
   does not establish a connection.

      Offer       Answer
      ----------------------------
      active      passive
      passive     active
      actpass     active / passive

   The semantics for the "udp-setup" attribute values of "active",
   "passive", and "actpass" in the offer/answer exchange are the same as
   those described for the "setup" attribute in 4.1 of RFC 4145, except



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   that "udp-setup" applies to an IKE session instead of a TCP
   connection.  The default value of the "udp-setup" attribute is
   "active" in the offer and "passive" in the answer.
















































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5.  Example of SDP Offer and Answer Exchange without IPsec NAT-Traversal

   If IPsec NAT-Traversal is not necessary, SDP negotiation to setup IKE
   is quite simple.  An example of SDP exchange is as follows.

   (Note: Due to RFC formatting conventions, this document splits SDP
   across lines whose content would exceed 72 characters.  A backslash
   character marks where this line folding has taken place.  This
   backslash and its trailing CRLF and whitespace would not appear in
   actual SDP content.)

   offer SDP
      ...
      m=application 500 udp ike-esp
      c=IN IP4 192.0.2.10
      a=udp-setup:active
      a=fingerprint:SHA-1 \
      4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
      ...

   answer SDP
      ...
      m=application 500 udp ike-esp
      c=IN IP4 192.0.2.20
      a=udp-setup:passive
      a=fingerprint:SHA-1 \
      D2:9F:6F:1E:CD:D3:09:E8:70:65:1A:51:7C:9D:30:4F:21:E4:4A:8E
      ...

   Following comedia-tls specification, the fingerprint attribute may be
   either a session- or a media-level SDP attribute.  If it is a
   session-level attribute, it applies to all IKE sessions and TLS
   sessions for which no media-level fingerprint attribute is defined.

   Note that it is possible for an offerer to become the IKE responder
   and an answerer to become the IKE initiator.  For example, when an
   RAS server sends an INVITE to a RAS client, the server may expect the
   client to become an IKE initiator.  In this case, the server sends an
   offer SDP with udp-setup:passive and the client returns an answer SDP
   with udp-setup:active as follows.

   offer SDP
      ...
      m=application 500 udp ike-esp
      c=IN IP4 192.0.2.10
      a=udp-setup:passive
      a=fingerprint:SHA-1 \
      4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB



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      ...

   answer SDP
      ...
      m=application 500 udp ike-esp
      c=IN IP4 192.0.2.20
      a=udp-setup:active
      a=fingerprint:SHA-1 \
      D2:9F:6F:1E:CD:D3:09:E8:70:65:1A:51:7C:9D:30:4F:21:E4:4A:8E
      ...

   The offer MAY contain media lines for media other than "ike-esp"
   (e.g., for normal audio).  If that occurs, the negotiation described
   in this draft occurs only for the "ike-esp" media lines; other media
   lines are negotiated and set up normally.  If the home router
   determines it will refuse the IKE session without beginning the IKE
   negotiation (e.g., the From: address is not on the permitted list),
   it SHOULD reject the "ike-esp" media line in the normal manner by
   setting the port number in the SDP answer to 0 and SHOULD process the
   other media lines normally.































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6.  Example of SDP Offer and Answer Exchange with IPsec NAT-Traversal

   If either of the endpoints that negotiate IKE is behind the NAT, the
   endpoints need to transmit both IKE and IPsec packets over the NAT.
   That mechanism is specified in RFC3947 and RFC3948: both endpoints
   encapsulate IPsec-ESP packets with a UDP header and multiplex them
   into the UDP path that IKE generates.  However, they also need to
   decide their transport addresses (combination of IP address and port)
   before starting IKE, making use of the ICE framework.  Because UDP-
   encapsulated ESP packets and IKE packets go through the same UDP hole
   of a NAT, IPsec NAT-Traversal works if ICE reserves simply one UDP
   path through the NAT.  However, those UDP packets need to be
   multiplexed with STUN [RFC5389] packets.  In this chapter, a method
   to coordinate IPsec NAT-Traversal and ICE is described.

6.1.  Port Usage

   IKE generally uses local UDP port 500, but the IPsec NAT-Traversal
   specification requires a port transition to UDP port 4500 during IKE
   negotiation because the problem that IPsec-aware NAT may multiplex
   IKE sessions using port 500 without changing the port number may
   occur.  This port transition of IKE means ICE has to generate an
   additional UDP path for port 4500, and this would be an inefficient
   overhead.  However, IPsec NAT-Traversal allows an IKE session to use
   local UDP port 4500 from the beginning without using port 500.
   Therefore, the endpoints SHOULD use their local UDP port 4500 for an
   IKE session from the beginning and ICE will only need to generate a
   UDP path of port 4500.

   When using ICE, a responder's IKE port observed by an initiator is
   not necessarily 500 or 4500.  Therefore, an IKE initiator MUST allow
   any destination ports in addition to 500 and 4500 for the IKE packets
   that it itself sends.  An IKE initiator just initiates an IKE session
   to the port number decided by SDP offer/answer or ICE.

6.2.  Offer and Answer Exchange with ICE

   We consider the following scenario here.













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                         +---------------------+
                         |                     |
                         |      Internet       |
                         |                     |
                         +---------------------+
                           |                |
                           |                |(192.0.2.20:45664)
                           |           +---------+
                           |           |   NAT   |
                           |           +---------+
                           |                |
          (192.0.2.10:4500)|                |(192.0.2.100:4500)
                      +---------+      +----------+
                      | offerer |      | answerer |
                      +---------+      +----------+

                     Figure 2: NAT-Traversal Scenario

   As shown above, an offerer is on the Internet but an answerer is
   behind the NAT.  The offerer cannot initiate an IKE session unless
   the answerer prepares a global routable transport address that
   accepts IKE packets.  In this case, the following offer/answer
   exchange will take place.

   offer SDP
      ...
      a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
      a=ice-ufrag:9uB6
      m=application 4500 udp ike-esp-udpencap
      c=IN IP4 192.0.2.10
      a=udp-setup:active
      a=fingerprint:SHA-1 \
      4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
      a=candidate:1 1 udp 2130706431 192.0.2.10 4500 typ host
      ...

   answer SDP
      ...
      a=ice-pwd:asd88fgpdd777uzjYhagZg
      a=ice-ufrag:8hhY
      m=application 45664 udp ike-esp-udpencap
      c=IN IP4 192.0.2.20
      a=udp-setup:passive
      a=fingerprint:SHA-1 \
      D2:9F:6F:1E:CD:D3:09:E8:70:65:1A:51:7C:9D:30:4F:21:E4:4A:8E
      a=candidate:1 1 udp 2130706431 192.0.2.100 4500 typ host
      a=candidate:2 1 udp 1694498815 192.0.2.20 45664 typ srflx \
      raddr 192.0.2.100 rport 4500



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      ...

   Conforming to ICE, they start a STUN connectivity check after SDP
   exchange.  Then the offerer initiates the IKE session making use of
   the UDP path generated by STUN packets.  In addition, UDP-
   encapsulated ESP packets are multiplexed into the same UDP path as
   IKE.  Thus, it is necessary to multiplex the three different packets,
   STUN, IKE, and UDP-encapsulated ESP, into the same UDP path.

   Similar to the case in chapter 5, The offer MAY contain the media
   lines for media other than "ike-esp-udpencap" (e.g., for normal
   audio) and they are negotiated and set up normally.

6.3.  Multiplex of UDP Messages

   As described above, STUN, IKE, and UDP-encapsulated ESP packets are
   multiplexed into the same UDP path.  This section describes how to
   demultiplex these three packets.

   At the first step, the endpoint that received a UDP packet at the
   multiplexed port MUST check the first 32 bits (bits 0-31) of the UDP
   payload.  If they are all 0, which is defined as a non-ESP marker,
   that packet MUST be treated as an IKE packet.

   Otherwise, it is judged as an ESP packet in the IPsec NAT-Traversal
   specification.  It is furthermore necessary to distinguish STUN from
   ESP.  Therefore, the bits 32-63 from the beginning of the UDP payload
   MUST be checked.  If the bits do not match the magic cookie of STUN
   0x2112A442 (most packets do not match), the packet is treated as an
   ESP packet because it is no longer a STUN packet.

   If the bits do, however, match the magic cookie, an additional test
   is necessary to determine if the packet is STUN or ESP.  The magic
   cookie field of STUN overlaps the sequence number field of ESP, so a
   possibility still remains that the sequence number of ESP coincides
   with 0x2112A442.  In this additional test, the validity of the
   fingerprint attribute of the STUN message MUST be checked.  If there
   is a valid fingerprint in the message, it is judged as a STUN packet;
   otherwise, it is an ESP packet.

   The above logic is expressed as follows.










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      if SPI-field-is-all-zeros
           { packet is IKE }
        else
           {
           if bits-32-through-63 == stun-magic-cookie-value and
              bits-0-through-1 == 0 and
              bits-2-through-15 == a STUN message type and
              bits-16-through-31 == length of this UDP packet
              {
               fingerprint_found == parse_for_stun_fingerprint();
               if fingerprint_found == 1
                  { packet is STUN }
               else
                  { packet is ESP }
              }
           else
              { packet is ESP }
           }

































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7.  Application to IKE

   After the fingerprints of both parties are securely shared over the
   SDP exchange, the IKE initiator MAY start the IKE session to the
   other party.  To follow this specification, a digital signature MUST
   be chosen as an authentication method in IKE phase 1.  In this
   process, a certificate whose hashed value matches the fingerprint
   exchanged over SDP MUST be used.  If the certificate used in IKE does
   not match the original fingerprint, the endpoint MUST terminate the
   IKE session by detecting an authentication failure.

   In addition, each party MUST present a certificate and be
   authenticated by each other.






































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8.  Specifications Assuming Prior Relationship between Two Nodes

   This section describes the specification for the limited cases in
   which certificates signed by trusted third parties or pre-shared keys
   between endpoints can be used for authentication in IKE.  Because the
   endpoints already have a prior relationship in this case, they use
   SIP servers for only name resolution and authorization.  However,
   even in this case, the integrity of the SDP description MUST be
   assured.

8.1.  Certificates Signed by Trusted Third Party

   The protocol overview in this case is the same as in chapter 3.  The
   SDP offer/answer procedure is also the same as in chapters 5 and 6.
   Both endpoints have a prior relationship through the trusted third
   parties, and SIP servers are used for name resolution and
   authorization of session initiation.  Even so, they MAY exchange
   fingerprints in the SDP because one device can have several
   certificates and it would be necessary to specify in advance which
   certificate will be used for the following IKE authentication.  This
   process also ensures that the certificate offered in the IKE process
   is the same as that owned by the peer that has been authorized at the
   SIP/SDP layer.  By this process, authorization in SIP and
   authentication in IKE become consistent with each other.

8.2.  Configured Pre-Shared Key

   If a pre-shared key for IKE authentication is installed in both
   endpoints in advance, they need not exchange the fingerprints of
   their certificates.  However, they may still need to specify which
   pre-shared key they will use in the following IKE authentication in
   SDP because they may have several pre-shared keys.  Therefore, a new
   attribute "psk-fingerprint" is defined to exchange the fingerprint of
   a pre-shared key over SDP.  It also has a role of making
   authorization in SIP consistent with authentication in IKE.
   Attribute "psk-fingerprint" is applied to pre-shared keys as the
   "fingerprint" defined in RFC4572 is applied to certificates.  The
   following is the ABNF of the "psk-fingerprint" attribute.  The use of
   "psk-fingerprint" is OPTIONAL.












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   attribute                 =/ psk-fingerprint-attribute

   psk-fingerprint-attribute = "psk-fingerprint" ":" hash-func SP
                               psk-fingerprint

   hash-func                 = "sha-1" / "sha-224" / "sha-256" /
                               "sha-384" / "sha-512" / token
                               ; Additional hash functions can only come
                               ; from updates to RFC 3279

   psk-fingerprint           = 2UHEX *(":" 2UHEX)
                               ; Each byte in upper-case hex, separated
                               ; by colons.

   UHEX                      = DIGIT / %x41-46 ; A-F uppercase

   An example of SDP negotiation for IKE with pre-shared key
   authentication without IPsec NAT-Traversal is as follows.

   offer SDP
      ...
      m=application 500 udp ike-esp
      c=IN IP4 192.0.2.10
      a=udp-setup:active
      a=psk-fingerprint:SHA-1 \
      12:DF:3E:5D:49:6B:19:E5:7C:AB:4A:AD:B9:B1:3F:82:18:3B:54:02
      ...

   answer SDP
      ...
      m=application 500 udp ike-esp
      c=IN IP4 192.0.2.20
      a=udp-setup:passive
      a=psk-fingerprint:SHA-1 \
      1A:51:7C:9D:30:4F:21:E4:4A:8E:D2:9F:6F:1E:CD:D3:09:E8:70:65
      ...















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9.  Security Considerations

   This entire document concerns security, but the security
   considerations applicable to SDP in general are described in the SDP
   specification.  The security issues that should be considered in
   using comedia-tls are described in Section 7 in its specification.
   This section mainly describes the security considerations specific to
   the negotiation of IKE using comedia-tls.

   Offering IKE in SDP (or agreeing to one in the SDP offer/answer
   model) does not create an obligation for an endpoint to accept any
   IKE session with the given fingerprint.  However, the endpoint must
   engage in the standard IKE negotiation procedure to ensure that the
   chosen IPsec security associations (including encryption and
   authentication algorithms) meet the security requirements of the
   higher-level application.  When IKE has finished negotiating, the
   decision to conclude IKE and establish an IPsec security association
   with the remote peer is entirely the decision of each endpoint.  This
   procedure is similar to how VPNs are typically established in the
   absence of SIP.

   In the general authentication process in IKE, subject DN or
   subjectAltName is recognized as the identity of the remote party.
   However, by using SIP identity and SIP-connected identity mechanisms
   in this spec, certificates are used simply as carriers for the public
   keys of the peers and there is no need for the information about who
   is the signer of the certificate and who is indicated by subject DN.

   In this document, the purpose of using IKE is to launch the IPsec SA;
   it is not for the security mechanism of RTP and RTCP [RFC3550]
   packets.  In fact, this mechanism cannot provide end-to-end security
   inside the VPN as long as the VPN uses tunnel mode IPsec.  Therefore,
   other security methods such as SRTP [RFC3711] must be used to secure
   the packets.

   When using the spec of this document, it needs to be considered that
   under following circumstances, security based on SIP authentication
   provided by SIP proxy may be breached.

   o  If a legitimate user's terminal is used by another person, it may
      be able to establish a VPN with the legitimate identity
      information.  This issue also applies to the general VPN cases
      based on the shared secret key.  Furthermore, in SIP we have a
      similar problem when file transfer, IM or Comedia where non-voice/
      video is used as a communication.

   o  If the proxy is hijacked by malicious users, he or she can use
      whatever credential on the ACL to gain access to the home network.



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   For the countermeasures to these issues, it is recommended to use
   unique information such as password which only a legitimate user
   knows for VPN establishment.  Checking the originating user by voice
   or video before establishing VPN would be another method.















































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10.  IANA Considerations

   The IANA is hereby requested to register the following new SDP
   attributes and media formats as follows.















































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   Attribute name:      udp-setup
   Long form name:      UDP setup extensions
   Type of attribute:   Session-level and media-level
   Subject to charset:  No
   Purpose:             Attribute to indicate initiator and responder of
                        UDP-based media session
   Appropriate values:  See Section 4 of RFCXXXX
                        -- Note to RFC editor:
                        -- replace RFCXXXX with this RFC number
   Contact name:        Makoto Saito, ma.saito@nttv6.jp

   Media format name:      ike-esp
   Long form name:         IKE followed by IPsec ESP
   Associated media:       application
   Associated proto:       udp
   Subject to charset:     No
   Purpose:                Media format that indicates IKE and IPsec ESP
                           as a VPN session
   Reference to the spec:  See Section 5 of RFCXXXX
                           -- Note to RFC editor:
                           -- replace RFCXXXX with this RFC number
   Contact name:           Makoto Saito, ma.saito@nttv6.jp

   Media format name:      ike-esp-udpencap
   Long form name:         IKE followed by IPsec ESP or UDP encapsulated
                           IPsec ESP
   Associated media:       application
   Associated proto:       udp
   Subject to charset:     No
   Purpose:                Media format that indicates IKE which
                           supports NAT-Traversal and IPsec ESP or UDP
                           encapsulation of IPsec ESP packets as a VPN
                           session
   Reference to the spec:  See Section 6.2. of RFCXXXX
                           -- Note to RFC editor:
                           -- replace RFCXXXX with this RFC number
   Contact name:           Makoto Saito, ma.saito@nttv6.jp

   Attribute name:       psk-fingerprint
   Long form name:       Fingerprint of pre-shared key extensions
   Type of attribute:    Session-level and media-level
   Subject to charset:   No
   Purpose:              Attribute to indicate a pre-shared key that
                         will be used in the following media session
   Appropriate values:   See Section 8.2. of RFCXXXX
                         -- Note to RFC editor:
                         -- replace RFCXXXX with this RFC number
   Contact name:         Makoto Saito, ma.saito@nttv6.jp



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11.  Acknowledgments

   We would like to thank Remi Denis-Courmont, Dale Worley, Richard
   Barnes, David Hancock, Stuart Hoggan, and Jean-Francois Mule for
   providing comments and suggestions contributing to this document.
   Eric Rescorla especially gave insightful comments from security point
   of view.  Shintaro Mizuno also contributed a lot of effort to
   improving this document.











































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12.  References

12.1.  Normative References

   [I-D.ietf-mmusic-ice]
              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-19 (work in progress), October 2007.

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

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [RFC3947]  Kivinen, T., Swander, B., Huttunen, A., and V. Volpe,
              "Negotiation of NAT-Traversal in the IKE", RFC 3947,
              January 2005.

   [RFC3948]  Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
              Stenberg, "UDP Encapsulation of IPsec ESP Packets",
              RFC 3948, January 2005.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.

   [RFC4306]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
              RFC 4306, December 2005.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC4572]  Lennox, J., "Connection-Oriented Media Transport over the
              Transport Layer Security (TLS) Protocol in the Session
              Description Protocol (SDP)", RFC 4572, July 2006.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,



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              October 2008.

12.2.  Informative References

   [I-D.ietf-sip-dtls-srtp-framework]
              Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
              for Establishing an SRTP Security Context using DTLS",
              draft-ietf-sip-dtls-srtp-framework-07 (work in progress),
              March 2009.

   [I-D.rosenberg-sip-rfc4474-concerns]
              Rosenberg, J., "Concerns around the Applicability of RFC
              4474", draft-rosenberg-sip-rfc4474-concerns-00 (work in
              progress), February 2008.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC4028]  Donovan, S. and J. Rosenberg, "Session Timers in the
              Session Initiation Protocol (SIP)", RFC 4028, April 2005.

   [RFC4145]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in
              the Session Description Protocol (SDP)", RFC 4145,
              September 2005.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

   [RFC4916]  Elwell, J., "Connected Identity in the Session Initiation
              Protocol (SIP)", RFC 4916, June 2007.

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












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Appendix A.  Changes since draft-saito-mmusic-sdp-ike-05

   Instruction to RFC Editor: please remove this section prior to
   publication as an RFC

   o  Added a description that a UAC is assumed to establish a session
      with only one UAS and it treats forked messages as an illegal
      process.

   o  Modified ike-esp-udpencap to indicate IKE which is capable of NAT
      traversal.

   o  Deleted md2 and md5 from hash functions.

   o  Added a security note about possible vulnerability on SIP
      authentication provided by SIP proxy in Security Considertations.

   o  Minor grammatical edits.

































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Authors' Addresses

   Makoto Saito
   NTT Communications
   1-1-6 Uchisaiwai-Cho, Chiyoda-ku
   Tokyo  100-8019
   Japan

   Email: ma.saito@nttv6.jp


   Dan Wing
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134
   United States

   Email: dwing@cisco.com


   Masashi Toyama
   NTT Corporation
   9-11 Midori-Cho 3-Chome, Musashino-Shi
   Tokyo  180-8585
   Japan

   Email: toyama.masashi@lab.ntt.co.jp
























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