Internet Engineering Task Force (IETF) J. Rosenberg
Request for Comments: 5688 Skype
Category: Standards Track January 2010
A Session Initiation Protocol (SIP) Media Feature Tag for MIME
The caller preferences specification for the Session Initiation
Protocol (SIP) allows a caller to express preferences that the call
be routed to a User Agent (UA) with particular capabilities.
Similarly, a specification exists to allow a UA to indicate its
capabilities in a registration. Amongst those capabilities are the
type of media streams the agent supports, described as top-level MIME
types. The 'application' MIME type is used to describe a broad range
of stream types, and it provides insufficient granularity as a
capability. This specification allows a UA to indicate which
application subtypes the agent supports.
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,
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 22. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 33. sip.app-subtype Media Feature Tag . . . . . . . . . . . . . . . 34. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45. Security Considerations . . . . . . . . . . . . . . . . . . . . 56. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 57. References . . . . . . . . . . . . . . . . . . . . . . . . . . 67.1. Normative References . . . . . . . . . . . . . . . . . . . 67.2. Informative References . . . . . . . . . . . . . . . . . . 71. Introduction
The caller preferences specification [RFC3841] for the Session
Initiation Protocol (SIP) [RFC3261] allows a user to express
preferences for the routing of SIP requests. These preferences are
expressed as a set of desired capabilities and characteristics of a
receiving agent. When a user agent registers to the SIP network, it
includes, as part of its registration, its own capabilities and
characteristics [RFC3840]. These capabilities are stored as part of
the registration, and then made available to the proxy in the
network. When a request arrives at the proxy with caller
preferences, the preferences in the request are compared with the
supported characteristics and capabilities stored in the
registrations, and the result is used to select the target user
agents for the request.
RFC 3840 makes use of media feature tags [RFC2506]. Each tag has a
name and a type. The tags defined in RFC 3840 describe some of the
basic characteristics of user agents, including whether or not they
are automata (the sip.automata tag), their class (the sip.class tag),
whether they support media in one or both directions (the
sip.duplex), and whether they are a conference focus (sip.isfocus).
These tags also include SIP capabilities, including the schemes
supported by the agent (sip.schemes), the methods (sip.methods), and
the event packages (sip.events) [RFC3265].
RFC 3840 also defines media feature tags for multimedia stream types.
There is a media feature tag defined for each top-level media type --
sip.audio for audio streams, sip.video for video streams, and so on.
The primary use case for this is to correctly deliver multimedia
sessions to the user agent that supports that media type. Consider a
caller on a videophone that wants to have a video call with another
user. That user has two devices -- a mobile phone that only supports
audio and a videophone. We'd like to deliver the videophone call to
the videophone as a first priority, and only 'ring' the mobile device
for an audio-only call if the user is not present on the videophone.
RFC 3840 defines media feature tags for each and every top-level
media type, including 'application'. This media type covers an
extremely broad range of subtypes -- multiplayer games of all sorts,
shared whiteboards and application sharing, and so on. With audio
and video, where there is often a common codec supported by agents
(i.e., a common subtype). Consequently, if a caller wants an audio
session, routing the request to any user agent that supports audio is
likely to result in successful communications. However, with
application streams, just routing a request to an agent that supports
*some* application stream isn't useful; application streams for
different applications are wildly different. Consequently, the
application media feature tag does not provide sufficient granularity
for call preferences. The specific application subtype needs to be
indicated as well.
To remedy this, this specification defines a new media feature tag
that indicates which application subtypes are supported by the agent
for streaming. The name of this media feature tag is 'sip.app-
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].
3. sip.app-subtype Media Feature Tag
The 'sip.app-subtype' media feature tag is of type token with a case-
insensitive equality relationship. Its value can be any registered
or private MIME application subtype compliant to the subtype-name
grammar defined in [RFC4288]. When included in the Contact header
field of a REGISTER request, an agent SHOULD include all application
subtypes that it can support as streaming formats. An application
subtype is supported if the user agent would be capable of processing
a Session Description Protocol (SDP) [RFC4566] offer [RFC3264] that
contained that subtype as a format in the m-line of the SDP.
When included in the Accept-Contact or Reject-Contact header field,
it indicates a desire on the part of a User Agent Client (UAC) to be
connected to a User Agent Server (UAS) that can support or cannot
support, respectively, streaming using that application subtype.
It is important to note that this media feature tag is only
indicating the streaming media types that a user agent is capable of
supporting. It says nothing about the functionality provided by the
user agent itself or the MIME types that the agent can send or
receive in SIP messages or emails. For example, let us assume that a
SIP user agent is capable of supporting a chess game. The game is
played by each user sending chess moves as binary objects over UDP
between a pair of user agents. Those objects have a MIME type of
"application/example". When a UA includes the sip.app-subtype media
feature tag in a Contact header field with a value of "example", it
means that the UA can handle a SIP INVITE that contained an SDP with
an application media line and format of "example". It does not mean
that the SIP user agent is a chess application, or that the user
agent can accept SIP requests that include bodies of type
"application/example". To indicate that a user agent can accept SIP
requests that include bodies of type "application/example", the agent
would utilize the "type" media feature tag as defined in [RFC3840].
A consequence of this is that, as new streaming media type formats
are defined (such as game stream formats, whiteboard session formats,
and so on), they SHOULD be defined using the SDP application stream
and utilize a MIME application subtype.
The following is an example SIP REGISTER message fragment indicating
usage of this media feature tag. The REGISTER indicates that the UA
can participate in application media sessions utilizing exchange of
objects of type "application/example".
REGISTER sip:example.com SIP/2.0
Such a registration indicates that an INVITE of the following form:
INVITE sip:Y@example.com SIP/2.0
o=jdoe 2890844526 2890842807 IN IP4 10.47.16.5
c=IN IP4 184.108.40.206
m=audio 49170 RTP/AVP 0
m=application 8493 udp example
would be accepted by the UA. The SDP in the INVITE indicates an
audio session and an application session that runs over UDP and
exchanges "application/example" object formats.
5. Security Considerations
When present in a REGISTER request, this media feature tag gives
information on the set of supported application media streams. It is
possible that this information is sensitive, providing insight into
the capabilities of a product. These considerations are already
discussed in RFC 3840, and those considerations apply here as well.
Applications that utilize this media feature tag SHOULD provide a
means for ensuring its integrity. Similarly, the media feature tag
should only be trusted as valid when it comes from the user or user
agent described by the feature tag. As a result, mechanisms for
conveying the feature tag SHOULD provide a mechanism for guaranteeing
6. IANA Considerations
This specification adds a new media feature tag to the SIP Media
Feature Tag Registration Tree defined in RFC 3840 [RFC3840].
Media feature tag name: sip.app-subtype
ASN.1 Identifier: 220.127.116.11.8.4.24
Summary of the media feature indicated by this tag: This feature tag
indicates the MIME application subtypes supported by the agent for
purposes of streaming media.
Values appropriate for use with this feature tag: Token (equality
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
Examples of typical use: Routing a call to a phone that can support
a multiplayer game.
Related standards or documents: RFC 5688
Security Considerations: Security considerations for this media
feature tag are discussed in Section 5 of RFC 5688.
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
[RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
"Indicating User Agent Capabilities in the Session
Initiation Protocol (SIP)", RFC 3840, August 2004.
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December 2005.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
7.2. Informative References
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC3841] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
Preferences for the Session Initiation Protocol (SIP)",
RFC 3841, August 2004.
[RFC2506] Holtman, K., Mutz, A., and T. Hardie, "Media Feature Tag
Registration Procedure", BCP 31, RFC 2506, March 1999.