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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   described in the Simplified BSD License.

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