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Versions: 00 01 02 RFC 5768

SIP                                                         J. Rosenberg
Internet-Draft                                             Cisco Systems
Expires: July 7, 2007                                    January 3, 2007


 Indicating Support for Interactive Connectivity Establishment (ICE) in
                 the Session Initiation Protocol (SIP)
                    draft-ietf-sip-ice-option-tag-00

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   This Internet-Draft will expire on July 7, 2007.

Copyright Notice

   Copyright (C) The Internet Society (2007).

Abstract

   This specification defines a media feature tag and an option tag for
   use with the Session Initiation Protocol (SIP).  The media feature
   tag allows a UA to communicate to its registrar that it supports ICE.
   The option tag allows a User Agent (UA) to require support for ICE in
   order for a call to proceed.






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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . . . 3
     3.1.  Gateways  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     3.2.  Mandating Support for ICE . . . . . . . . . . . . . . . . . 4
   4.  Media Feature Tag Definition  . . . . . . . . . . . . . . . . . 4
   5.  Option Tag Definition . . . . . . . . . . . . . . . . . . . . . 4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
     7.1.  Option Tag  . . . . . . . . . . . . . . . . . . . . . . . . 5
     7.2.  Media Feature Tag . . . . . . . . . . . . . . . . . . . . . 5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8
   Intellectual Property and Copyright Statements  . . . . . . . . . . 9

































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1.  Introduction

   RFC 3264 [3] defines a two-phase exchange of Session Description
   Protocol (SDP) messages [5] for the purposes of establishment of
   multimedia sessions.  This offer/answer mechanism is used by
   protocols such as the Session Initiation Protocol (SIP) [2].

   Protocols using offer/answer are difficult to operate through Network
   Address Translators (NAT).  Because their purpose is to establish a
   flow of media packets, they tend to carry IP addresses within their
   messages, which is known to be problematic through NAT [7].  To
   remedy this, an extension to SDP, called Interactive Connectivity
   Establishment (ICE) has been defined [6].  ICE defines procedures by
   which agents gather a multiplicity of addresses, include all of them
   in an SDP offer or answer, and then use peer-to-peer Simple Traversal
   Underneath NAT (STUN) [8] connectivity checks to determine a valid
   address.

   This specification defines a media feature tag, "sip.ice", and a SIP
   option tag, "ice", that can be used by SIP user agents that make use
   of ICE.  Section 3 motivates the need for the media feature tag and
   option tag, and Section 4 and Section 5 formally define them.


2.  Terminology

   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 [1].


3.  Motivation

   There are two primary motivations for defining an option tag and a
   media feature tag.  They are support for gateways, and requiring ICE
   for a call.

3.1.  Gateways

   Unfortunately, ICE requires both endpoints to support it in order for
   it to be used.  Within a domain, there will typically be user agents
   that do and do not support ICE.  In order to facilitate deployment of
   ICE, it is anticipated that domains will make use of gateways which
   act as ICE agents on one side, an non-ICE agents on the other side.
   This would allow a call from domain A into domain B to make use of
   ICE, even if the device in domain B does not itself yet support ICE.
   However, when domain B receives a call, it will need to know whether
   the call needs to pass through such a gateway, or whether it can go



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   to the terminating UA directly.

   In order to make such a determination, this specification defines a
   media feature tag, sip.ice, which can be included in the Contact
   header field of a REGISTER request [4].  This allows the registrar to
   track whether a UA supports ICE or not.  This information can be
   accessed by a proxy in order to determine whether a call needs to
   route through a gateway or not.

3.2.  Mandating Support for ICE

   Although ICE provides a built in fall back to non-ICE operation when
   the answerer doesn't support it, there are cases where the offerer
   would rather abort the call rather than proceed without ICE.
   Typically, this is because they would like to choose a different m/c-
   line address for a non-ICE peer than they would for an ICE capable
   peer.

   To do this, the "ice" SIP option tag can be included in the Require
   header field of an INVITE request.


4.  Media Feature Tag Definition

   The sip.ice media feature tag indicates support for ICE.  An agent
   supports ICE if it is capable of acting in either full or passive-
   only mode, and consequently, is capable of including candidate
   attributes in an SDP offer or answer for at least one transport
   protocol.  An agent that supports ICE SHOULD include this media
   feature tag in the Contact header field of its REGISTER requests and
   OPTION responses.  An agent MAY include them in the Contact header
   field of an INVITE or INVITE response; however, doing so is redundant
   with ICE attributes in the SDP which indicate the same thing.


5.  Option Tag Definition

   This "ice" OPTION tag SHOULD NOT be used in conjunction with the
   Supported header field.  The media feature tag is used as the one and
   only mechanism for indicating support for ICE.  The option tag is
   meant to be used only with the Require header field.  When placed in
   the Require header field of an INVITE request, it indicates that the
   UAS must support ICE in order to process the call.  An agent supports
   ICE if it is capable of acting in either full or passive-only mode,
   and consequently, is capable of including candidate attributes in an
   SDP offer or answer for at least one transport protocol.





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6.  Security Considerations

   A malicious intermediary might attempt to modify a SIP message by
   inserting a Require header field containing the "ice" option tag.  If
   ICE were not supported on the UAS, this would cause the call to fail
   when it would otherwise succeed.  Of course, this attack is not
   specific to ICE, and can be done using any option tag.  This attack
   is prevented by usage of the SIPS mechanism as defined in RFC 3261.

   Similarly, an intermediary might attempt to remove the media feature
   tag from a REGISTER request or OPTIONS request, which might cause a
   call to skip ICE processing when it otherwise might make use of it.
   This attack is also prevented using the SIP mechanism.


7.  IANA Considerations

   This specification defines a new media feature tag and SIP option
   tag.

7.1.  Option Tag

   This section defines a new SIP option tag per the guidelines in
   Section 27.1 of RFC 3261.

   Name: ice

   Description: This option tag is used to identify the Interactive
      Connectivity Establishment (ICE) extension.  When present in a
      Supported header field, it indicates that ICE is supported by an
      agent.

7.2.  Media Feature Tag

   This section registers a new media feature tag in the SIP tree,
   defined in Section 12.1 of RFC 3840 [4].

   Media feature tag name: sip.ice

   ASN.1 Identifier: 1.3.6.1.8.4.22

   Summary of the media feature indicated by this tag: This feature tag
      indicates that the device supports Interactive Connectivity
      Establishment (ICE).







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   Values appropriate for use with this feature tag: Boolean.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Routing a call to a phone that can support
      ICE.

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]

   Security Considerations: Security considerations for this media
      feature tag are discussed in Section 6 of RFC XXXX .  [[Note to
      IANA: Please replace XXXX with the RFC number of this
      specification.]]


8.  References

8.1.  Normative References

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

   [2]  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.

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

   [4]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating User
        Agent Capabilities in the Session Initiation Protocol (SIP)",
        RFC 3840, August 2004.

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

   [6]  Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
        Methodology for Network  Address Translator (NAT) Traversal for
        Offer/Answer Protocols", draft-ietf-mmusic-ice-11 (work in
        progress), October 2006.







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8.2.  Informative References

   [7]  Senie, D., "Network Address Translator (NAT)-Friendly
        Application Design Guidelines", RFC 3235, January 2002.

   [8]  Rosenberg, J., "Simple Traversal Underneath Network Address
        Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-04 (work
        in progress), July 2006.











































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Author's Address

   Jonathan Rosenberg
   Cisco Systems
   600 Lanidex Plaza
   Parsippany, NJ  07054
   US

   Phone: +1 973 952-5000
   Email: jdrosen@cisco.com
   URI:   http://www.jdrosen.net








































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   Internet Society.




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