A Session Initiation Protocol (SIP) Media Feature Tag for MIME Application Subtypes
RFC 5688

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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

Abstract

   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
   http://www.rfc-editor.org/info/rfc5688.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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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

Rosenberg                   Standards Track                     [Page 1]
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
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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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