Network Working Group A. Sollaud Internet-Draft France Telecom Updates: 4749 (if approved) February 8, 2008 Intended status: Standards Track Expires: August 11, 2008 G.729.1 RTP Payload Format update: DTX support draft-ietf-avt-rfc4749-dtx-update-00 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 11, 2008. Copyright Notice Copyright (C) The IETF Trust (2008). Abstract This document updates the Real-time Transport Protocol (RTP) payload format to be used for the International Telecommunication Union (ITU-T) Recommendation G.729.1 audio codec. It adds Discontinuous Transmission (DTX) support to the RFC 4749 specification, in a backward-compatible way. An updated media type registration is included for this payload format. Sollaud Expires August 11, 2008 [Page 1]
Internet-Draft G.729.1 DTX support in RTP February 2008 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. RTP Header Usage . . . . . . . . . . . . . . . . . . . . . . . 3 4. Payload Format . . . . . . . . . . . . . . . . . . . . . . . . 4 5. Payload Format Parameters . . . . . . . . . . . . . . . . . . . 5 5.1. Media Type Registration Update . . . . . . . . . . . . . . 5 5.2. Mapping to SDP Parameters . . . . . . . . . . . . . . . . . 5 5.2.1. DTX Offer-Answer Model Considerations . . . . . . . . . 6 5.2.2. DTX Declarative SDP Considerations . . . . . . . . . . 6 6. Congestion Control . . . . . . . . . . . . . . . . . . . . . . 6 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 9.1. Normative References . . . . . . . . . . . . . . . . . . . 6 9.2. Informative References . . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7 Intellectual Property and Copyright Statements . . . . . . . . . . 8 Sollaud Expires August 11, 2008 [Page 2]
Internet-Draft G.729.1 DTX support in RTP February 2008 1. Introduction The International Telecommunication Union (ITU-T) Recommendation G.729.1 [1] is a scalable and wideband extension of the Recommendation G.729 [6] audio codec. RFC 4749 [3] specifies the payload format for packetization of G.729.1 encoded audio signals into the Real-time Transport Protocol (RTP) [4]. The ITU-T is about to release an Appendix, to add Discontinuous Transmission (DTX) support to G.729.1. This document updates the RTP payload format to allow usage of this Appendix. Only changes or additions to RFC 4749 [3] will be described in the following sections. 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. Background G.729.1 supports Discontinuous Transmission (DTX), a.k.a. silence suppression. It means that the coder includes a Voice Activity Detection (VAD) algorithm, to determine if an audio frame contains silence or actual audio. During silence periods, the coder may significantly decrease the transmitted bit rate by sending a small frame called Silence Insertion Descriptor (SID), and then stop transmission. The receiver's decoder will generate comfort noise according to the parameters contained in the SID. G.729.1 SID has an embedded structure. The core SID is the same as the legacy G.729 SID [7]. A first enhancement layer adds some parameters for narrowband comfort noise, while a second enhancement layer adds wideband information. In the current state of the specification, SID can be 2, 3, or 6 octets long. 3. RTP Header Usage The fields of the RTP header must be used as described in RFC 4749, except for the Marker (M) bit. If DTX is used, the first packet of a talkspurt, that is, the first packet after a silence period during which packets have not been transmitted contiguously, SHOULD be distinguished by setting the M bit in the RTP data header to one. The M bit in all other packets MUST be set to zero. The beginning of a talkspurt MAY be used to Sollaud Expires August 11, 2008 [Page 3]
Internet-Draft G.729.1 DTX support in RTP February 2008 adjust the playout delay to reflect changing network delays. If DTX is not used, the M bit MUST be set to zero in all packets. 4. Payload Format The payload format is the same as in RFC 4749, with the option to add a SID at the end. So the complete payload consists of a payload header of 1 octet (MBS and FT fields), followed by zero or more consecutive audio frames at the same bit rate, followed by zero or one SID. Note that it is consistent with the payload format of G.729 described in section 4.5.6 of RFC 3551 [8]. To be able to transport a SID alone, that is, without actual audio frames, we assign the FT value 14 to SID. The actual SID size (2, 3, or 6 octets) is inferred from the payload size. When a SID is appended to actual audio frames, the FT value remains the one describing the encoding rate of the audio frames. Since the SID is much smaller than any other frame, it can be easily detected at the receiver side, and it will not hinder the calculation of the number of frames. The full FT table is given for convenience: +-------+---------------+-------------------+ | FT | encoding rate | frame size | +-------+---------------+-------------------+ | 0 | 8 kbps | 20 octets | | 1 | 12 kbps | 30 octets | | 2 | 14 kbps | 35 octets | | 3 | 16 kbps | 40 octets | | 4 | 18 kbps | 45 octets | | 5 | 20 kbps | 50 octets | | 6 | 22 kbps | 55 octets | | 7 | 24 kbps | 60 octets | | 8 | 26 kbps | 65 octets | | 9 | 28 kbps | 70 octets | | 10 | 30 kbps | 75 octets | | 11 | 32 kbps | 80 octets | | 12-13 | (reserved) | - | | 14 | SID | 2, 3, or 6 octets | | 15 | NO_DATA | 0 | +-------+---------------+-------------------+ Sollaud Expires August 11, 2008 [Page 4]
Internet-Draft G.729.1 DTX support in RTP February 2008 DTX has no impact on the MBS definition and use. 5. Payload Format Parameters Parameters defined in RFC 4749 are not modified. We add a new optional parameter to configure DTX. 5.1. Media Type Registration Update We add a new optional parameter to the audio/G7291 media subtype: dtx: indicates that discontinuous transmission (DTX) is used or preferred. Permissible values are 0 and 1. 0 means no DTX. 1 means DTX support, as described in Appendix ZZZ of ITU-T Recommendation G.729.1. 0 is implied if this parameter is omitted. When DTX is turned off, the RTP payload MUST NOT contain SID, and the FT value 14 MUST NOT be used. 5.2. Mapping to SDP Parameters The information carried in the media type specification has a specific mapping to fields in the Session Description Protocol (SDP) [5], which is commonly used to describe RTP sessions. The mapping described in RFC 4749 remains unchanged. The "dtx" parameter goes in the SDP "a=fmtp" attribute. Some example partial SDP session descriptions utilizing G.729.1 encodings follow. Example 1: default parameters (DTX off) m=audio 57586 RTP/AVP 96 a=rtpmap:96 G7291/16000 Example 2: recommended packet duration of 40 ms (=2 frames), maximum bit rate is 20 kbps, DTX supported and preferred. m=audio 49987 RTP/AVP 97 a=rtpmap:97 G7291/16000 a=fmtp:97 maxbitrate=20000; dtx=1 a=ptime:40 Sollaud Expires August 11, 2008 [Page 5]
Internet-Draft G.729.1 DTX support in RTP February 2008 5.2.1. DTX Offer-Answer Model Considerations The offer-answer model considerations of RFC 4749 fully apply. In this section we only define the management of the "dtx" parameter. The "dtx" parameter concerns both sending and receiving, so both sides of a bi-directional session MUST have the same DTX setting. If one party indicates it does not support DTX, DTX must be deactivated both ways. In other words, DTX is actually activated if, and only if, "dtx=1" in the offer and in the answer. A special rule apply for multicast: the "dtx" parameter becomes declarative and MUST NOT be negotiated. This parameter is fixed, and a participant MUST use the configuration that is provided for the session. 5.2.2. DTX Declarative SDP Considerations The "dtx" parameter is declarative and provides the parameter that SHALL be used when receiving and/or sending the configured stream. 6. Congestion Control The congestion control considerations of RFC 4749 apply. The use of DTX can help congestion control by reducing the number of transmitted RTP packets and the average bandwidth of audio streams. 7. Security Considerations DTX introduces no new security issue. 8. IANA Considerations It is requested that one new parameter for media subtype (audio/ G7291) is registered by IANA, see Section 5.1. 9. References 9.1. Normative References [1] International Telecommunications Union, "G.729 based Embedded Variable bit-rate coder: An 8-32 kbit/s scalable wideband coder bitstream interoperable with G.729", ITU-T Recommendation Sollaud Expires August 11, 2008 [Page 6]
Internet-Draft G.729.1 DTX support in RTP February 2008 G.729.1, May 2006. [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [3] Sollaud, A., "RTP Payload Format for the G.729.1 Audio Codec", RFC 4749, October 2006. [4] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [5] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. 9.2. Informative References [6] International Telecommunications Union, "Coding of speech at 8 kbit/s using conjugate-structure algebraic-code-excited linear- prediction (CS-ACELP)", ITU-T Recommendation G.729, March 1996. [7] International Telecommunications Union, "A silence compression scheme for G.729 optimized for terminals conforming to Recommendation V.70", ITU-T Recommendation G.729 Annex B, October 1996. [8] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", STD 65, RFC 3551, July 2003. Author's Address Aurelien Sollaud France Telecom 2 avenue Pierre Marzin Lannion Cedex 22307 France Phone: +33 2 96 05 15 06 Email: aurelien.sollaud@orange-ftgroup.com Sollaud Expires August 11, 2008 [Page 7]
Internet-Draft G.729.1 DTX support in RTP February 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Sollaud Expires August 11, 2008 [Page 8]
