Session Description Protocol (SDP) Simple Capability Declaration
RFC 3407

Document Type RFC - Proposed Standard (October 2002; Errata)
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Network Working Group                                       F. Andreasen
Request for Comments: 3407                                 Cisco Systems
Category: Standards Track                                   October 2002

   Session Description Protocol (SDP) Simple Capability Declaration

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document defines a set of Session Description Protocol (SDP)
   attributes that enables SDP to provide a minimal and backwards
   compatible capability declaration mechanism.  Such capability
   declarations can be used as input to a subsequent session
   negotiation, which is done by means outside the scope of this
   document.  This provides a simple and limited solution to the general
   capability negotiation problem being addressed by the next generation
   of SDP, also known as SDPng.

1. Conventions Used in this Document

   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 [2].

2. Introduction

   The Session Description Protocol (SDP) [3] describes multimedia
   sessions for the purposes of session announcement, session
   invitation, and other forms of multimedia session initiation.  SDP
   was not intended to provide capability negotiation.  However, as the
   need for this has become increasingly important, work has begun on a
   "next generation SDP" (SDPng) [4,5] that supports both session
   description and capability negotiation.  SDPng is not anticipated to
   be backwards compatible with SDP and work on SDPng is currently in
   the early stages.  However, several other protocols, e.g. SIP [6] and
   Media Gateway Control Protocol (MGCP) [7], use SDP and are likely to

Andreasen                   Standards Track                     [Page 1]
RFC 3407           SDP Simple Capability Declaration        October 2002

   continue doing so for the foreseeable future.  Nevertheless, in many
   cases these signaling protocols have an urgent need for some limited
   form of capability negotiation.

   For example, an endpoint may support G.711 audio (over RTP) as well
   as T.38 fax relay (over UDP or TCP).  Unless the endpoint is willing
   to support two media streams at the same time, this cannot currently
   be expressed in SDP.  Another example involves support for multiple
   codecs.  An endpoint indicates this by including all the codecs in
   the "m=" line in the session description.  However, the endpoint
   thereby also commits to simultaneous support for each of these
   codecs.  In practice, Digital Signal Processor (DSP) memory and
   processing power limitations may not make this feasible.

   As noted in [4], the problem with SDP is that media descriptions are
   used to describe session parameters as well as capabilities without a
   clear distinction between the two.

   In this document, we define a minimal and backwards compatible
   capability declaration feature in SDP by defining a set of new SDP
   attributes.  Together, these attributes define a capability set,
   which consists of a capability set sequence number followed by one or
   more capability descriptions.  Each capability description in the set
   contains information about supported media formats, but the endpoint
   is not committing to use any of these.  In order to actually use a
   declared capability, session negotiation will have to be done by
   means outside the scope of this document, e.g., using the
   offer/answer model [8].

   It should be noted that the mechanism is not intended to solve the
   general capability negotiation problem targeted by SDPng.  It is
   merely intended as a simple and limited solution to the most urgent
   problems facing current users of SDP.

3. Simple Capability Declaration Attributes

   The SDP Simple Capability Declaration (simcap) is defined by a set of
   SDP attributes.  Together, these attributes form a capability set
   which describes the complete media capabilities of the endpoint.  Any
   previous capability sets issued by the endpoint for the session in
   question no longer apply.  The capability set consists of a sequence
   number and one or more capability descriptions.  Each such capability
   description describes the media type and media formats supported and
   may include one or more capability parameters to further define the
   capability.  A session description MUST NOT contain more than one
   capability set, however the capability set can describe capabilities
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