Offer/Answer Considerations for G723 Annex A and G729 Annex B
RFC 7261
Internet Engineering Task Force (IETF) M. Perumal
Request for Comments: 7261 Cisco Systems
Category: Standards Track P. Ravindran
ISSN: 2070-1721 NSN
May 2014
Offer/Answer Considerations for G723 Annex A and G729 Annex B
Abstract
This document provides the offer/answer considerations for the annexa
parameter of G723 and the annexb parameter of G729, G729D, and G729E
when the value of the annexa or annexb parameter does not match in
the Session Description Protocol (SDP) offer and answer.
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/rfc7261.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Perumal & Ravindran Standards Track [Page 1]
RFC 7261 Offer/Answer G723 AnnexA and G729 AnnexB May 2014
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Offer/Answer Considerations . . . . . . . . . . . . . . . . . 3
3.1. Considerations for Use of Comfort Noise Frames . . . . . 3
3.2. Offer/Answer Considerations for G723 Annex A . . . . . . 3
3.3. Offer/Answer Considerations for G729 Annex B, G729D Annex
B, and G729E Annex B . . . . . . . . . . . . . . . . . . 4
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Offer with G729 annexb=yes and Answer with G729 annexb=no 5
4.2. Offer with G729 annexb=yes and Answer with G729 and No
annexb Parameter . . . . . . . . . . . . . . . . . . . . 5
4.3. Offer with G729 and No annexb Parameter and Answer with
G729 annexb=no . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
7. Normative References . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
[RFC4856] describes the annexa parameter for G723 as follows:
annexa: indicates that Annex A, voice activity detection, is used
or preferred. Permissible values are "yes" and "no" (without the
quotes); "yes" is implied if this parameter is omitted.
Also, [RFC4856] describes the annexb parameter for G729, G729D, and
G729E as follows:
annexb: indicates that Annex B, voice activity detection, is used
or preferred. Permissible values are "yes" and "no" (without the
quotes); "yes" is implied if this parameter is omitted.
However, a problem arises when the value of the annexa or annexb
parameter does not match in the SDP [RFC4566] offer and answer.
For example, if the offer has G729 with annexb=yes and the answer has
G729 with annexb=no, it can be interpreted in two different ways:
o The offerer and answerer proceed as if G729 is negotiated with
annexb=yes, or
o The offerer and answerer proceed as if G729 is negotiated with
annexb=no.
Perumal & Ravindran Standards Track [Page 2]
RFC 7261 Offer/Answer G723 AnnexA and G729 AnnexB May 2014
Since this is not clear in the existing specifications, various
implementations have interpreted the offer/answer in their own ways,
resulting in a different codec being chosen to call failure, when the
parameter value does not match in the offer and answer.
[RFC3264] requires SDP extensions that define new fmtp parameters to
specify their proper interpretation in offer/answer. This document
specifies the proper interpretation for the annexa and annexb
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