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PROPOSED STANDARD

Internet Engineering Task Force (IETF)                      J. Rosenberg
Request for Comments: 5768                                   jdrosen.net
Category: Standards Track                                     April 2010
ISSN: 2070-1721


  Indicating Support for Interactive Connectivity Establishment (ICE)
                in the Session Initiation Protocol (SIP)

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 User Agent (UA) to communicate to its registrar that it
   supports ICE.  The option tag allows a UA to require support for ICE
   in order for a call to proceed.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5768.

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
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   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 Simplified BSD License.






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Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................2
   3. Motivation ......................................................3
      3.1. Gateways ...................................................3
      3.2. Mandating Support for ICE ..................................3
   4. Media Feature Tag Definition ....................................3
   5. Option Tag Definition ...........................................4
   6. Security Considerations .........................................4
   7. IANA Considerations .............................................4
      7.1. Option Tag .................................................4
      7.2. Media Feature Tag ..........................................5
   8. References ......................................................5
      8.1. Normative References .......................................5
      8.2. Informative References .....................................6

1.  Introduction

   RFC 3264 [RFC3264] defines a two-phase exchange of Session
   Description Protocol (SDP) [RFC4566] messages for the purposes of
   establishment of multimedia sessions.  This offer/answer mechanism is
   used by protocols such as the Session Initiation Protocol (SIP)
   [RFC3261].

   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 [RFC3235].  To
   remedy this, an extension to SDP, called Interactive Connectivity
   Establishment (ICE) has been defined [RFC5245].  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 Session Traversal Utilities for NAT (STUN) [RFC5389]
   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 [RFC2119].





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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 that
   act as ICE agents on one side, and 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
   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 [RFC3840].  This allows the
   registrar to track whether or not a UA supports ICE.  This
   information can be accessed by a proxy in order to determine whether
   or not a call needs to route through a gateway.

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 either a lite or full implementation, 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.





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   An agent MAY include the media feature tag in the Contact header
   field of an INVITE or INVITE response; however, doing so is redundant
   with ICE attributes in the SDP that indicate the same thing.  In
   cases where an INVITE omits an offer, the lack or presence of the
   media feature tag in the Contact header field cannot be used by the
   callee (which will be the offerer) to determine whether the caller
   supports ICE.  In cases of third-party call control [RFC3725], the
   caller may be a controller that does (or doesn't) support ICE, while
   the answerer may be an agent that does (or doesn't) support ICE.

5.  Option Tag Definition

   This "ice" OPTION tag SHOULD NOT be used in conjunction with the
   Supported header field (this SHOULD NOT include responses to OPTION
   requests).  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 User
   Agent Server (UAS) must support ICE in order to process the call.  An
   agent supports ICE if it is either a full or lite implementation, and
   consequently, is capable of including candidate attributes in an SDP
   offer or answer for at least one transport protocol.

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 SIPS 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.





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   Name:  ice

   Description:  This option tag is used to identify the Interactive
      Connectivity Establishment (ICE) extension.  When present in a
      Require header field, it indicates that ICE is required 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 [RFC3840].

   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).

   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 5768

   Security Considerations:  Security considerations for this media
      feature tag are discussed in Section 6 of this document.

8.  References

8.1.  Normative References

   [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.




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

   [RFC3840]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
              "Indicating User Agent Capabilities in the Session
              Initiation Protocol (SIP)", RFC 3840, August 2004.

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

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245, April
              2010.

8.2.  Informative References

   [RFC3235]  Senie, D., "Network Address Translator (NAT)-Friendly
              Application Design Guidelines", RFC 3235, January 2002.

   [RFC3725]  Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
              Camarillo, "Best Current Practices for Third Party Call
              Control (3pcc) in the Session Initiation Protocol (SIP)",
              BCP 85, RFC 3725, April 2004.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

Author's Address

   Jonathan Rosenberg
   jdrosen.net
   Monmouth, NJ
   US

   EMail: jdrosen@jdrosen.net
   URI:   http://www.jdrosen.net












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