An Extension for Identification of Groups in the Session Description Protocol (SDP).
draft-roach-mmusic-groupid-00

Abstract

RFC 5888 defines a mechanism for semantically grouping media sections in the Session Description Protocol (SDP). One difficulty that has arisen in several applications of SDP is the need to uniquely identify these groups either in other protocols or elsewhere in the SDP itself.

This document proposes a simple, backwards-compatible mechanism that provides unambiguous identifiers for RFC 5888 groups.

This document updates RFC 5888.

Status of This Memo

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This Internet-Draft will expire on June 05, 2014.

Copyright Notice

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Table of Contents

• 1. Introduction
• 2. Terminology
• 3. Mechanism Description
• 4. Syntax
• 5. Security Considerations
• 6. IANA Considerations
• 7. Normative References
• Authors' Addresses

1. Introduction

[RFC5888] defines a mechanism for semantically grouping media sections in the Session Description Protocol (SDP) [RFC4566] for purposes such as lip sync and flow identification. That mechanism, however, defines anonymous groupings, which makes it impossible to reliably and unambiguously refer to such groups at a later time (e.g. elsewhere in the SDP, or in an application-layer protocol). This document defines a new attribute, "group-id", that can be used to attach identifiers to SDP groups.

2. Terminology

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 [RFC2119].

3. Mechanism Description

The mechanism used to assign identifiers to group is very simple and straightforward: implementations that wish to assign an identifier to groups include a single "group-id" attribute immediately before each "group" attribute in the session. Importantly, it retains backwards-compatible with existing [RFC5888] implementations. This "group-id" attribute contains a single token, unique within the session, that unambiguously identifies the group defined on the following line. Each media section included in that group additionally MUST contain an "in-group" attribute that includes the [RFC5888] semantic identifier and group-id. This "in-group" identifier is intended to allow media sections to be self-describing when they appear outside the context of a full session.

For clarity: if an implementation includes a group-id for any groups in a session, that implementation MUST include a group-id for every group in that session. Implementations MUST NOT include any attributes between a "group-id" attribute and the "group" attribute it identifies. Recipients of such SDP in which a "group-id" appears followed by anything other than the "group" attribute MUST ignore the errant "group-id" line.

A simple example of the new group-id syntax follows. This example defines a single lip-sync group, and identifies it with the group identifier "abc."

          v=0
          o=Laura 289083124 289083124 IN IP4 eight.example.com
          c=IN IP4 192.0.2.1
          t=0 0
          a=group-id:abc
          a=group:LS 1 2
          m=audio 30000 RTP/AVP 0
          a=mid:1
          a=in-group:LS abc
          m=audio 30000 RTP/AVP 8
          a=mid:2
          a=in-group:LS abc

4. Syntax

The new attributes introduced by this mechanism are defined by the following ABNF [RFC5234]:

           groupid-attribute = "a=group-id:" group-id

           group-id          = token ; token is defined in RFC 4566

           in-group-attr     = "a=in-group:" semantics SP group-id
                               ; semantics is defined in RFC 5888

5. Security Considerations

This mechanism does not introduce any security issues beyond those discussed in [RFC5888].

6. IANA Considerations

This document defines two SDP attributes: "group-id" and "in-group". They are to be registered by IANA in the "SDP Parameters" registry as follows:

   SDP Attribute ("att-field"):

     Attribute name:     group-id
     Long form:          Group ID
     Type of name:       att-field
     Type of attribute:  session level
     Subject to charset: no
     Purpose:            Identification of SDP groups
     Reference:          this document
     Values:             any token

   SDP Attribute ("att-field"):

     Attribute name:     in-group
     Long form:          Add media section to group
     Type of name:       att-field
     Type of attribute:  media level
     Subject to charset: no
     Purpose:            Associating media sections with groups
     Reference:          this document
     Values:             semantic type followed by group identifier

7. Normative References

Authors' Addresses