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rfc:rfc5768

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
 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 Simplified BSD License.

Rosenberg Standards Track [Page 1] RFC 5768 ICE Support April 2010

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].

Rosenberg Standards Track [Page 2] RFC 5768 ICE Support April 2010

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.

Rosenberg Standards Track [Page 3] RFC 5768 ICE Support April 2010

 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.

Rosenberg Standards Track [Page 4] RFC 5768 ICE Support April 2010

 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.

Rosenberg Standards Track [Page 5] RFC 5768 ICE Support April 2010

 [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

Rosenberg Standards Track [Page 6]

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