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Internet Engineering Task Force (IETF)                        M. PerumalRequest for Comments: 7261                                 Cisco SystemsCategory: Standards Track                                   P. RavindranISSN: 2070-1721                                                      NSN                                                                May 2014Offer/Answer Considerations for G723 Annex A and G729 Annex BAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7261.Copyright Notice   Copyright (c) 2014 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   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   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.Perumal & Ravindran          Standards Track                    [Page 1]

RFC 7261        Offer/Answer G723 AnnexA and G729 AnnexB        May 2014Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .33.  Offer/Answer Considerations . . . . . . . . . . . . . . . . .33.1.  Considerations for Use of Comfort Noise Frames  . . . . .33.2.  Offer/Answer Considerations for G723 Annex A  . . . . . .3     3.3.  Offer/Answer Considerations for G729 Annex B, G729D Annex           B, and G729E Annex B  . . . . . . . . . . . . . . . . . .44.  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  . . . . . . . . . . . . . . . . . . . . .65.  Security Considerations . . . . . . . . . . . . . . . . . . .66.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .77.  Normative References  . . . . . . . . . . . . . . . . . . . .71.  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   parameters in offer/answer.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.  Offer/Answer Considerations   The general objective of the offer/answer considerations is to make   sure that if the offerer or answerer indicates it does not support   Annex A or Annex B, then Annex A or Annex B is not used.3.1.  Considerations for Use of Comfort Noise Frames   [RFC3551] states that:      Receivers MUST accept comfort noise frames if restriction of their      use has not been signaled.  The MIME registration for G729 inRFC3555 specifies a parameter that MAY be used with MIME or SDP to      restrict the use of comfort noise frames.   Hence, if the SDP offer or answer indicates that comfort noise is not   supported, comfort noise frames MUST NOT be used.3.2.  Offer/Answer Considerations for G723 Annex A   When the offer or answer has G723 and the annexa parameter is absent,   the offerer or answerer knows that it has implied the default   "annexa=yes".  This is because the annexa attribute is part of the   original registration of audio/G723 [RFC4856].  All implementations   that support G723 understand the annexa attribute.  Hence, this case   MUST be considered as if the offer or answer has G723 with   annexa=yes.   When the offer has G723 with annexa=yes and the answer has G723 with   annexa=no, the offerer and answerer MUST proceed as if G723 is   negotiated with annexa=no.Perumal & Ravindran          Standards Track                    [Page 3]

RFC 7261        Offer/Answer G723 AnnexA and G729 AnnexB        May 2014   When the offer has G723 with annexa=no, after the offer/answer   completion the offerer and answerer MUST proceed as if G723 is   negotiated with annexa=no.      When the offer has G723 with annexa=no, the reason for not      mandating that the answer MUST have annexa=no for G723 is that      there are implementations that omit the annexa parameter in      answer.  In such cases, the offerer and answerer proceed as if      G723 is negotiated with annexa=no.   When the offer has G723 with no annexa parameter and the answer has   G723 with annexa=yes, the offerer and answerer MUST proceed as if   G723 is negotiated with annexa=yes.3.3.  Offer/Answer Considerations for G729 Annex B, G729D Annex B, and      G729E Annex B   In this section, G729 represents any of G729 or G729D or G729E.   When the offer or answer has G729 and the annexb parameter is absent,   the offerer or answerer knows that it has implied the default   "annexb=yes".  This is because the annexb attribute is part of the   original registration of audio/G729 [RFC4856].  All implementations   that support G729 understand the annexb attribute.  Hence, this case   MUST be considered as if the offer or answer has G729 with   annexb=yes.   When the offer has G729 with annexb=yes and the answer has G729 with   annexb=no, the offerer and answerer MUST proceed as if G729 is   negotiated with annexb=no.   When the offer has G729 with annexb=no, after the offer/answer   completion the offerer and answerer MUST proceed as if G729 is   negotiated with annexb=no.      When the offer has G729 with annexb=no, the reason for not      mandating that the answer MUST have annexb=no for G729 is that      there are implementations that omit the annexb parameter in the      answer.  In such cases, the offerer and answerer proceed as if      G729 is negotiated with annexb=no.   When the offer has G729 with no annexb parameter and the answer has   G729 with annexb=yes, the offerer and answerer MUST proceed as if   G729 is negotiated with annexb=yes.Perumal & Ravindran          Standards Track                    [Page 4]

RFC 7261        Offer/Answer G723 AnnexA and G729 AnnexB        May 20144.  Examples4.1.  Offer with G729 annexb=yes and Answer with G729 annexb=no           [Offer with G729 annexb=yes]           v=0           o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com           s=           c=IN IP4 host.atlanta.example.com           t=0 0           m=audio 49170 RTP/AVP 18           a=rtpmap:18 G729/8000           a=fmtp:18 annexb=yes           [Answer with G729 annexb=no]           v=0           o=bob 1890844326 1890844326 IN IP4 host.bangalore.example.com           s=           c=IN IP4 host.bangalore.example.com           t=0 0           m=audio 19140 RTP/AVP 18           a=rtpmap:18 G729/8000           a=fmtp:18 annexb=no   In the above example, the offerer and answerer proceed as if G729 is   negotiated with annexb=no.4.2.  Offer with G729 annexb=yes and Answer with G729 and No annexb      Parameter           [Offer with G729 annexb=yes]           v=0           o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com           s=           c=IN IP4 host.atlanta.example.com           t=0 0           m=audio 49170 RTP/AVP 18           a=rtpmap:18 G729/8000           a=fmtp:18 annexb=yesPerumal & Ravindran          Standards Track                    [Page 5]

RFC 7261        Offer/Answer G723 AnnexA and G729 AnnexB        May 2014           [Answer with G729 and no annexb parameter]           v=0           o=bob 1890844326 1890844326 IN IP4 host.bangalore.example.com           s=           c=IN IP4 host.bangalore.example.com           t=0 0           m=audio 19140 RTP/AVP 18           a=rtpmap:18 G729/8000   In the above example, the offerer and answerer proceed as if G729 is   negotiated with annexb=yes.4.3.  Offer with G729 and No annexb Parameter and Answer with G729      annexb=no           [Offer with G729 and no annexb parameter]           v=0           o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com           s=           c=IN IP4 host.atlanta.example.com           t=0 0           m=audio 49170 RTP/AVP 18           a=rtpmap:18 G729/8000           [Answer with G729 annexb=no]           v=0           o=bob 1890844326 1890844326 IN IP4 host.bangalore.example.com           s=           c=IN IP4 host.bangalore.example.com           t=0 0           m=audio 19140 RTP/AVP 18           a=rtpmap:18 G729/8000           a=fmtp:18 annexb=no   In the above example, the offerer and answerer proceed as if G729 is   negotiated with annexb=no.5.  Security Considerations   The guidelines described in [RFC6562] are to be followed for Use of   Voice Activity Detection (i.e., Annex A or Annex B) with the Secure   Real-time Transport Protocol (SRTP).Perumal & Ravindran          Standards Track                    [Page 6]

RFC 7261        Offer/Answer G723 AnnexA and G729 AnnexB        May 20146.  Acknowledgments   Thanks to Flemming Andreasen (Cisco), Miguel A. Garcia (Ericsson),   Ali C. Begen (Cisco), Paul Kyzivat (Huawei), Roni Even (Huawei),   Kevin Riley (Sonus), Ashish Sharma (Sonus), Kevin P. Fleming   (Digium), Dale Worley (Avaya), Cullen Jennings (Cisco), Ari Keranen   (Ericsson), Harprit S. Chhatwal (InnoMedia), Aurelien Sollaud   (Orange), SM, Stephen Casner, Keith Drage (Alcatel-Lucent), Stephen   Farrell, Barry Leiba, and Ted Lemon for their valuable input and   comments.  Martin Dolly (ATT) and Hadriel Kaplan (Acme Packet)   provided useful suggestions at the mic at IETF 83.7.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model              with Session Description Protocol (SDP)",RFC 3264, June              2002.   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and              Video Conferences with Minimal Control", STD 65,RFC 3551,              July 2003.   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session              Description Protocol",RFC 4566, July 2006.   [RFC4856]  Casner, S., "Media Type Registration of Payload Formats in              the RTP Profile for Audio and Video Conferences",RFC4856, February 2007.   [RFC6562]  Perkins, C. and JM. Valin, "Guidelines for the Use of              Variable Bit Rate Audio with Secure RTP",RFC 6562, March              2012.Perumal & Ravindran          Standards Track                    [Page 7]

RFC 7261        Offer/Answer G723 AnnexA and G729 AnnexB        May 2014Authors' Addresses   Muthu Arul Mozhi Perumal   Cisco Systems   Cessna Business Park   Sarjapur-Marathahalli Outer Ring Road   Bangalore, Karnataka  560103   India   EMail: mperumal@cisco.com   Parthasarathi Ravindran   NSN   Manyata Embassy Business park   Bangalore, Karnataka  560045   India   EMail: partha@parthasarathi.co.inPerumal & Ravindran          Standards Track                    [Page 8]