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.
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Provisions Relating to IETF Documents
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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].
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.
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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.
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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.
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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
Rosenberg Standards Track [Page 6]
