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Versions: 00 01 02 03 04 RFC 5688

SIP                                                         J. Rosenberg
Internet-Draft                                                     Cisco
Intended status: Standards Track                       November 12, 2007
Expires: May 15, 2008

     A Session Initiation Protocol (SIP) Media Feature Tag for MIME
                         Application Sub-Types

Status of this Memo

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   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

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   of stream types, and provides insufficient granularity as a
   capability.  This specification allows a UA to indicate which
   application sub-types the agent supports.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   3.  sip.app-subtype Media Feature Tag . . . . . . . . . . . . . . . 4
   4.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     7.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 7
   Intellectual Property and Copyright Statements  . . . . . . . . . . 9

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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 they are
   automata or not (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 protocol capabilities, including the
   schemes supported by the agent (sip.schemes), the methods
   (sip.methods), and the event packages [RFC3265] (sip.events).

   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

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   application media feature tag does not provide sufficient granularity
   for call preferences.  The specific application sub-type needs to be
   indicated as well.

   To remedy this, this specification defines a new media feature tag
   that indicates which application sub-types 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-
   sensitive equality relationship.  Its value can be any registered or
   private MIME application sub-type.  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 sub-type is supported if the user agent would be capable
   of processing an Session Description Protocol (SDP) [RFC4566] offer
   [RFC3264] that contained that sub-type 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 UAC to be connected to a UAS
   which can support, or cannot support respectively, streaming using
   that application sub-type.

   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

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   "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 sub-type.

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 particiate in application media sessions utilizing exchange of
   objects of type "application/example".

        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

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   would be accepted by the UA.  The SDP in the INVITE indicates an
   audio session and an application session which 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 which 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:

   Summary of the media feature indicated by this tag:  This feature tag
      indicates the MIME application sub-types supported by the agent
      for purposes of streaming media.

   Values appropriate for use with this feature tag:  Token.

   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
      a multiplayer game.

   Related standards or documents:  RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]

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   Security Considerations:  Security considerations for this media
      feature tag are discussed in Section 5 of RFC XXXX .  [[Note to
      IANA: Please replace XXXX with the RFC number of this

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.

   [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,
              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.

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Author's Address

   Jonathan Rosenberg
   Edison, NJ

   Email: jdrosen@cisco.com
   URI:   http://www.jdrosen.net

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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an

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