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Errata Exist
Internet Engineering Task Force (IETF)                      J. Rosenberg
Request for Comments: 5688                                         Skype
Category: Standards Track                                   January 2010
ISSN: 2070-1721

     A Session Initiation Protocol (SIP) Media Feature Tag for MIME
                          Application Subtypes


   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,
   and how to provide feedback on it may be obtained at

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

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RFC 5688                Application Subtype Tag             January 2010

   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.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
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   material may not have granted the IETF Trust the right to allow
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   Without obtaining an adequate license from the person(s) controlling
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   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  sip.app-subtype Media Feature Tag . . . . . . . . . . . . . . . 3
   4.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     7.2.  Informative References  . . . . . . . . . . . . . . . . . . 7

1.  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),

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RFC 5688                Application Subtype Tag             January 2010

   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-

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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

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RFC 5688                Application Subtype Tag             January 2010

   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.

4.  Example

   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".

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RFC 5688                Application Subtype Tag             January 2010

   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: <sip:Y1@pc.example.com>

   Such a registration indicates that an INVITE of the following form:

   INVITE sip:Y@example.com SIP/2.0
   To: sip:Y@example.com
   Content-Type: application/sdp
   Content-Length: ...

   o=jdoe 2890844526 2890842807 IN IP4
   c=IN IP4
   t=0 0
   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].

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RFC 5688                Application Subtype Tag             January 2010

   Media feature tag name:  sip.app-subtype

   ASN.1 Identifier:

   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.  References

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,
              June 2002.

   [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.

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RFC 5688                Application Subtype Tag             January 2010

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,
              June 2002.

   [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.

Author's Address

   Jonathan Rosenberg
   Monmouth, NJ

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

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