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Network Working Group                                         A. SollaudRequest for Comments: 4749                                France TelecomCategory: Standards Track                                   October 2006RTP Payload Format for the G.729.1 Audio CodecStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2006).Abstract   This document specifies a Real-time Transport Protocol (RTP) payload   format to be used for the International Telecommunication Union   (ITU-T) G.729.1 audio codec.  A media type registration is included   for this payload format.Table of Contents1. Introduction ....................................................22. Background ......................................................23. Embedded Bit Rates Considerations ...............................34. RTP Header Usage ................................................35. Payload Format ..................................................45.1. Payload Structure ..........................................45.2. Payload Header: MBS Field ..................................45.3. Payload Header: FT Field ...................................65.4. Audio Data .................................................66. Payload Format Parameters .......................................76.1. Media Type Registration ....................................76.2. Mapping to SDP Parameters ..................................86.2.1. Offer-Answer Model Considerations ...................96.2.2. Declarative SDP Considerations .....................117. Congestion Control .............................................118. Security Considerations ........................................119. IANA Considerations ............................................1210. References ....................................................1210.1. Normative References .....................................1210.2. Informative References ...................................12Sollaud                     Standards Track                     [Page 1]

RFC 4749             RTP Payload Format for G.729.1         October 20061.  Introduction   The International Telecommunication Union (ITU-T) recommendation   G.729.1 [1] is a scalable and wideband extension of the   recommendation G.729 [9] audio codec.  This document specifies the   payload format for packetization of G.729.1 encoded audio signals   into the Real-time Transport Protocol (RTP).   The payload format itself is described inSection 5.  A media type   registration and the details for the use of G.729.1 with SDP are   given inSection 6.   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 inRFC 2119 [2].2.  Background   G.729.1 is an 8-32 kbps scalable wideband (50-7000 Hz) speech and   audio coding algorithm interoperable with G.729, G.729 Annex A, and   G.729 Annex B.  It provides a standardized solution for packetized   voice applications that allows a smooth transition from narrowband to   wideband telephony.   The most important services addressed are IP telephony and   videoconferencing, either for enterprise corporate networks or for   mass market (like Public Switched Telephone Network (PSTN) emulation   over DSL or wireless access).  Target devices can be IP phones or   other VoIP handsets, home gateways, media gateways, IP Private Branch   Exchange (IPBX), trunking equipment, voice messaging servers, etc.   For all those applications, the scalability feature allows tuning the   bit rate versus quality trade-off, possibly in a dynamic way during a   session, taking into account service requirements and network   transport constraints.   The G.729.1 coder produces an embedded bitstream structured in 12   layers corresponding to 12 available bit rates between 8 and 32 kbps.   The first layer, at 8 kbps, is called the core layer and is bitstream   compatible with the ITU-T G.729/G.729A coder.  At 12 kbps, a second   layer improves the narrowband quality.  Upper layers provide wideband   audio (50-7000 Hz) between 14 and 32 kbps, with a 2 kbps granularity   allowing graceful quality improvements.  Only the core layer is   mandatory to decode understandable speech; upper layers provide   quality enhancement and wideband enlargement.Sollaud                     Standards Track                     [Page 2]

RFC 4749             RTP Payload Format for G.729.1         October 2006   The codec operates on 20-ms frames, and the default sampling rate is   16 kHz.  Input and output at 8 kHz are also supported, at all bit   rates.3.  Embedded Bit Rates Considerations   The embedded property of G.729.1 streams provides a mechanism to   adjust the bandwidth demand.  At any time, a sender can change its   sending bit rate without external signalling, and the receiver will   be able to properly decode the frames.  It may help to control   congestion, since the bandwidth can be adjusted by selecting another   bit rate.   The ability to adjust the bandwidth may also help when having a fixed   bandwidth link dedicated to voice calls, for example in a residential   or trunking gateway.  In that case, the system can change the bit   rates depending on the number of simultaneous calls.  This will only   impact the sending bandwidth.  In order to adjust the receiving   bandwidth as well, we introduce an in-band signalling to request the   other party to change its own sending bit rate.  This in-band request   is called MBS, for Maximum Bit rate Supported.  It is described inSection 5.2.  Note that it is only useful for two-way unicast G.729.1   traffic, because when A sends an in-band MBS to B in order to request   that B modify its sending bit rate, it concerns the stream from B to   A.  If there is no G.729.1 stream in the reverse direction, the MBS   will have no effect.4.  RTP Header Usage   The format of the RTP header is specified inRFC 3550 [3].  This   payload format uses the fields of the header in a manner consistent   with that specification.   The RTP timestamp clock frequency is the same as the default sampling   frequency: 16 kHz.   G.729.1 has also the capability to operate with 8 kHz sampled input/   output signals at all bit rates.  It does not affect the bitstream,   and the decoder does not require a priori knowledge about the   sampling rate of the original signal at the input of the encoder.   Therefore, depending on the implementation and the audio acoustic   capabilities of the devices, the input of the encoder and/or the   output of the decoder can be configured at 8 kHz; however, a 16 kHz   RTP clock rate MUST always be used.   The duration of one frame is 20 ms, corresponding to 320 samples at   16 kHz.  Thus the timestamp is increased by 320 for each consecutive   frame.Sollaud                     Standards Track                     [Page 3]

RFC 4749             RTP Payload Format for G.729.1         October 2006   The M bit MUST be set to zero in all packets.   The assignment of an RTP payload type for this packet format is   outside the scope of the document, and will not be specified here.   It is expected that the RTP profile under which this payload format   is being used will assign a payload type for this codec or specify   that the payload type is to be bound dynamically (seeSection 6.2).5.  Payload Format5.1.  Payload Structure   The complete payload consists of a payload header of 1 octet,   followed by zero or more consecutive audio frames at the same bit   rate.   The payload header consists of two fields: MBS (seeSection 5.2) and   FT (seeSection 5.3).      0                   1                   2                   3      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |  MBS  |   FT  |                                               |     +-+-+-+-+-+-+-+-+                                               +     :                zero or more frames at the same bit rate       :     :                                                               :     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+5.2.  Payload Header: MBS Field   MBS (4 bits): maximum bit rate supported.  Indicates a maximum bit   rate to the encoder at the site of the receiver of this payload.  The   value of the MBS field is set according to the following table:Sollaud                     Standards Track                     [Page 4]

RFC 4749             RTP Payload Format for G.729.1         October 2006                         +-------+--------------+                         |  MBS  | max bit rate |                         +-------+--------------+                         |   0   |    8 kbps    |                         |   1   |    12 kbps   |                         |   2   |    14 kbps   |                         |   3   |    16 kbps   |                         |   4   |    18 kbps   |                         |   5   |    20 kbps   |                         |   6   |    22 kbps   |                         |   7   |    24 kbps   |                         |   8   |    26 kbps   |                         |   9   |    28 kbps   |                         |   10  |    30 kbps   |                         |   11  |    32 kbps   |                         | 12-14 |  (reserved)  |                         |   15  |    NO_MBS    |                         +-------+--------------+   The MBS is used to tell the other party the maximum bit rate one can   receive.  The encoder MUST NOT exceed the sending rate indicated by   the received MBS.  Note that, due to the embedded property of the   coding scheme, the encoder can send frames at the MBS rate or any   lower rate.  As long as it does not exceed the MBS, the encoder can   change its bit rate at any time without previous notice.   Note that the MBS is a codec bit rate; the actual network bit rate is   higher and depends on the overhead of the underlying protocols.   The MBS received is valid until the next MBS is received, i.e., a   newly received MBS value overrides the previous one.   If a payload with a reserved MBS value is received, the MBS MUST be   ignored.   The MBS field MUST be set to 15 for packets sent to a multicast group   and MUST be ignored on packets received from a multicast group.   The MBS field MUST be set to 15 in all packets when the actual MBS   value is sent through non-RTP means.  This is out of the scope of   this specification.   See Sections3 and7 for more details on the use of MBS for   congestion control.Sollaud                     Standards Track                     [Page 5]

RFC 4749             RTP Payload Format for G.729.1         October 20065.3.  Payload Header: FT Field   FT (4 bits): Frame type of the frame(s) in this packet, as per the   following table:                  +-------+---------------+------------+                  |   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 |                  | 12-14 |   (reserved)  |            |                  |   15  |    NO_DATA    |      0     |                  +-------+---------------+------------+   The FT value 15 (NO_DATA) indicates that there is no audio data in   the payload.  This MAY be used to update the MBS value when there is   no audio frame to transmit.  The payload will then be reduced to the   payload header.   If a payload with a reserved FT value is received, the whole payload   MUST be ignored.5.4.  Audio Data   Audio data of a payload contains one or more consecutive audio frames   at the same bit rate.  The audio frames are packed in order of time,   that is, oldest first.   The size of one frame is given by the FT field, as per the table inSection 5.3, and the actual number of frames is easy to infer from   the size of the audio data part:      nb_frames = (size_of_audio_data) / (size_of_one_frame).   Only full frames must be considered.  So if there is a remainder to   the division above, the corresponding remaining bytes in the received   payload MUST be ignored.Sollaud                     Standards Track                     [Page 6]

RFC 4749             RTP Payload Format for G.729.1         October 2006   Note that if FT=15, there will be no audio frame in the payload.6.  Payload Format Parameters   This section defines the parameters that may be used to configure   optional features in the G.729.1 RTP transmission.   The parameters are defined here as part of the media subtype   registration for the G.729.1 codec.  A mapping of the parameters into   the Session Description Protocol (SDP) [5] is also provided for those   applications that use SDP.  In control protocols that do not use MIME   or SDP, the media type parameters must be mapped to the appropriate   format used with that control protocol.6.1.  Media Type Registration   This registration is done using the template defined inRFC 4288 [6]   and followingRFC 3555 [7].   Type name: audio   Subtype name: G7291   Required parameters: none   Optional parameters:   maxbitrate:  the absolute maximum codec bit rate for the session, in      bits per second.  Permissible values are 8000, 12000, 14000,      16000, 18000, 20000, 22000, 24000, 26000, 28000, 30000, and 32000.      32000 is implied if this parameter is omitted.  The maxbitrate      restricts the range of bit rates which can be used.  The bit rates      indicated by FT and MBS fields in the RTP packets MUST NOT exceed      maxbitrate.   mbs:  the current maximum codec bit rate supported as a receiver, in      bits per second.  Permissible values are in the same set as for      the maxbitrate parameter, with the constraint that mbs MUST be      lower or equal to maxbitrate.  If the mbs parameter is omitted, it      is set to the maxbitrate value.  So if both mbs and maxbitrate are      omitted, they are both set to 32000.  The mbs parameter      corresponds to a MBS value in the RTP packets as per table inSection 5.2 of RFC 4749.  Note that this parameter may be      dynamically updated by the MBS field of the RTP packets sent; it      is not an absolute value for the session.   ptime:  the recommended length of time (in milliseconds) represented      by the media in a packet.  SeeSection 6 of RFC 4566 [5].Sollaud                     Standards Track                     [Page 7]

RFC 4749             RTP Payload Format for G.729.1         October 2006   maxptime:  the maximum length of time (in milliseconds) that can be      encapsulated in a packet.  SeeSection 6 of RFC 4566 [5]   Encoding considerations: This media type is framed and contains      binary data; seeSection 4.8 of RFC 4288 [6].   Security considerations: SeeSection 8 of RFC 4749   Interoperability considerations: none   Published specification:RFC 4749   Applications which use this media type: Audio and video conferencing      tools.   Additional information: none   Person & email address to contact for further information:      Aurelien Sollaud, aurelien.sollaud@orange-ftgroup.com   Intended usage: COMMON   Restrictions on usage: This media type depends on RTP framing, and      hence is only defined for transfer via RTP [3].   Author: Aurelien Sollaud   Change controller: IETF Audio/Video Transport working group delegated      from the IESG6.2.  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.  When SDP is   used to specify sessions employing the G.729.1 codec, the mapping is   as follows:   o  The media type ("audio") goes in SDP "m=" as the media name.   o  The media subtype ("G7291") goes in SDP "a=rtpmap" as the encoding      name.  The RTP clock rate in "a=rtpmap" MUST be 16000 for G.729.1.   o  The parameters "ptime" and "maxptime" go in the SDP "a=ptime" and      "a=maxptime" attributes, respectively.Sollaud                     Standards Track                     [Page 8]

RFC 4749             RTP Payload Format for G.729.1         October 2006   o  Any remaining parameters go in the SDP "a=fmtp" attribute by      copying them directly from the media type string as a semicolon      separated list of parameter=value pairs.   Some example SDP session descriptions utilizing G.729.1 encodings   follow.   Example 1: default parameters      m=audio 53146 RTP/AVP 98      a=rtpmap:98 G7291/16000   Example 2: recommended packet duration of 40 ms (=2 frames), maximum   bit rate is 12 kbps, and initial MBS set to 8 kbps.  It could be a   loaded PSTN gateway which can operate at 12 kbps but asks to   initially reduce the bit rate to 8 kbps.      m=audio 51258 RTP/AVP 99      a=rtpmap:99 G7291/16000      a=fmtp:99 maxbitrate=12000; mbs=8000      a=ptime:406.2.1.  Offer-Answer Model Considerations   The following considerations apply when using SDP offer-answer   procedures [8] to negotiate the use of G.729.1 payload in RTP:   o  Since G.729.1 is an extension of G.729, the offerer SHOULD      announce G.729 support in its "m=audio" line, with G.729.1      preferred.  This will allow interoperability with both G.729.1 and      G.729-only capable parties.      Below is an example of such an offer:         m=audio 55954 RTP/AVP 98 18         a=rtpmap:98 G7291/16000         a=rtpmap:18 G729/8000      If the answerer supports G.729.1, it will keep the payload type 98      in its answer, and the conversation will be done using G.729.1.      Else, if the answerer supports only G.729, it will leave only the      payload type 18 in its answer, and the conversation will be done      using G.729 (the payload format for G.729 is defined inSection4.5.6 of RFC 3551 [4]).Sollaud                     Standards Track                     [Page 9]

RFC 4749             RTP Payload Format for G.729.1         October 2006      Note that when used at 8 kbps in G.729-compatible mode, the      G.729.1 decoder supports G.729 Annex B.  Therefore, Annex B can be      advertised (by default, annexb=yes for G729 media type; seeSection 4.1.9 of RFC 3555 [7]).   o  The "maxbitrate" parameter is bi-directional.  If the offerer sets      a maxbitrate value, the answerer MUST reply with a smaller or      equal value.  The actual maximum bit rate for the session will be      the minimum.   o  If the received value for "maxbitrate" is between 8000 and 32000      but not in the permissible values set, it SHOULD be read as the      closest lower valid value.  If the received value is lower than      8000 or greater than 32000, the session MUST be rejected.   o  The "mbs" parameter is not symmetric.  Values in the offer and the      answer are independent and take into account local constraints.      One party MUST NOT start sending frames at a bit rate higher than      the "mbs" of the other party.  The parameter allows announcing      this value, prior to the sending of any packet, to prevent the      remote sender from exceeding the MBS at the beginning of the      session.   o  If the received value for "mbs" is greater or equal to 8000 but      not in the permissible values set, it SHOULD be read as the      closest lower valid value.  If the received value is lower than      8000, the session MUST be rejected.   o  The parameters "ptime" and "maxptime" will in most cases not      affect interoperability.  The SDP offer-answer handling of the      "ptime" parameter is described inRFC 3264 [8].  The "maxptime"      parameter MUST be handled in the same way.   o  Any unknown parameter in an offer MUST be ignored by the receiver      and MUST NOT be included in the answer.   Some special rules apply for mono-directional traffic:   o  For sendonly streams, the "mbs" parameter is useless and SHOULD      NOT be used.   o  For recvonly streams, the "mbs" parameter is the only way to      communicate the MBS to the sender, since there is no RTP stream      towards it.  So to request a bit rate change, the receiver will      need to use an out-of-band mechanism, like a SIP RE-INVITE.Sollaud                     Standards Track                    [Page 10]

RFC 4749             RTP Payload Format for G.729.1         October 2006   Some special rules apply for multicast:   o  The "mbs" parameter MUST NOT be used.   o  The "maxbitrate" 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.6.2.2.  Declarative SDP Considerations   For declarative use of SDP such as in SAP [10] and RTSP [11], the   following considerations apply:   o  The "mbs" parameter MUST NOT be used.   o  The "maxbitrate" parameter is declarative and provides the      parameter that SHALL be used when receiving and/or sending the      configured stream.7.  Congestion Control   Congestion control for RTP SHALL be used in accordance withRFC 3550   [3] and any appropriate profile (for example,RFC 3551 [4]).  The   embedded and variable bit rates capability of G.729.1 provides a   mechanism that may help to control congestion; seeSection 3 for more   details.   The number of frames encapsulated in each RTP payload influences the   overall bandwidth of the RTP stream, due to the header overhead.   Packing more frames in each RTP payload can reduce the number of   packets sent and hence the header overhead, at the expense of   increased delay and reduced error robustness.8.  Security Considerations   RTP packets using the payload format defined in this specification   are subject to the general security considerations discussed in the   RTP specification [3] and any appropriate profile (for example,RFC3551 [4]).   As this format transports encoded speech/audio, the main security   issues include confidentiality, integrity protection, and   authentication of the speech/audio itself.  The payload format itself   does not have any built-in security mechanisms.  Any suitable   external mechanisms, such as SRTP [12], MAY be used.Sollaud                     Standards Track                    [Page 11]

RFC 4749             RTP Payload Format for G.729.1         October 2006   This payload format and the G.729.1 encoding do not exhibit any   significant non-uniformity in the receiver-end computational load and   thus are unlikely to pose a denial-of-service threat due to the   receipt of pathological datagrams.9.  IANA Considerations   IANA has registered audio/G7291 as a media subtype; seeSection 6.1.10.  References10.1.  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        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]  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,        "RTP: A Transport Protocol for Real-Time Applications", STD 64,RFC 3550, July 2003.   [4]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video        Conferences with Minimal Control", STD 65,RFC 3551, July 2003.   [5]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session        Description Protocol",RFC 4566, July 2006.   [6]  Freed, N. and J. Klensin, "Media Type Specifications and        Registration Procedures",BCP 13,RFC 4288, December 2005.   [7]  Casner, S. and P. Hoschka, "MIME Type Registration of RTP        Payload Formats",RFC 3555, July 2003.   [8]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with        Session Description Protocol (SDP)",RFC 3264, June 2002.10.2.  Informative References   [9]  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.   [10] Handley, M., Perkins, C., and E. Whelan, "Session Announcement        Protocol",RFC 2974, October 2000.Sollaud                     Standards Track                    [Page 12]

RFC 4749             RTP Payload Format for G.729.1         October 2006   [11] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming        Protocol (RTSP)",RFC 2326, April 1998.   [12] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.        Norrman, "The Secure Real-time Transport Protocol (SRTP)",RFC 3711, March 2004.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.comSollaud                     Standards Track                    [Page 13]

RFC 4749             RTP Payload Format for G.729.1         October 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 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 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 inBCP 78 andBCP 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 athttp://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.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Sollaud                     Standards Track                    [Page 14]