


AVT Working Group                                             G. Herlein
Internet-Draft                                                 S. Morlat
Expires: April 15, 2006                                     J. Jean-Marc
                                                             R. Hardiman
                                                                 P. Kerr
                                                        October 12, 2005


                      draft-ietf-avt-rtp-speex-00
                 RTP Payload Format for the Speex Codec

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   Speex is an open-source voice codec suitable for use in Voice over IP
   (VoIP) type applications.  This document describes the payload format
   for Speex generated bit streams within an RTP packet.  Also included
   here are the necessary details for the use of Speex with the Session
   Description Protocol (SDP).




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Editors Note

   All references to RFC XXXX are to be replaced by references to the
   RFC number of this memo, when published.

Table of Contents

   1.   Conventions used in this document  . . . . . . . . . . . . .   3
   2.   Overview of the Speex Codec  . . . . . . . . . . . . . . . .   3
   3.   RTP payload format for Speex . . . . . . . . . . . . . . . .   3
   4.   RTP Header . . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.   Speex payload  . . . . . . . . . . . . . . . . . . . . . . .   5
   6.   Example Speex packet . . . . . . . . . . . . . . . . . . . .   6
   7.   Multiple Speex frames in a RTP packet  . . . . . . . . . . .   6
   8.   MIME registration of Speex . . . . . . . . . . . . . . . . .   7
   9.   SDP usage of Speex . . . . . . . . . . . . . . . . . . . . .   8
   10.  ITU H.323 Use of Speex . . . . . . . . . . . . . . . . . . .  10
   11.  Security Considerations  . . . . . . . . . . . . . . . . . .  10
   12.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  11
   13.  References . . . . . . . . . . . . . . . . . . . . . . . . .  11
     13.1   Normative References . . . . . . . . . . . . . . . . . .  11
     13.2   Informative References . . . . . . . . . . . . . . . . .  12
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  12
        Intellectual Property and Copyright Statements . . . . . . .  14



























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1.  Conventions used in this document

   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 [1].

2.  Overview of the Speex Codec

   Speex is based on the CELP [8] encoding technique with support for
   either narrowband (nominal 8kHz), wideband (nominal 16kHz) or ultra-
   wideband (nominal 32kHz), and (non-optimal) rates up to 48 kHz
   sampling also available.  The main characteristics can be summarized
   as follows:

   o  Free software/open-source
   o  Integration of wideband and narrowband in the same bit-stream
   o  Wide range of bit-rates available
   o  Dynamic bit-rate switching and variable bit-rate (VBR)
   o  Voice Activity Detection (VAD, integrated with VBR)
   o  Variable complexity

3.  RTP payload format for Speex

   For RTP based transportation of Speex encoded audio the standard RTP
   header [2] is followed by one or more payload data blocks.  An
   optional padding terminator may also be used.

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         RTP Header                            |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
       |                 one or more frames of Speex ....              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |        one or more frames of Speex ....       |    padding    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


4.  RTP Header












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        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |V=2|P|X|  CC   |M|     PT      |       sequence number         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           timestamp                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           synchronization source (SSRC) identifier            |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
       |            contributing source (CSRC) identifiers             |
       |                              ...                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The RTP header begins with an octet of fields (V, P, X, and CC) to
   support specialized RTP uses (see [2] and [5] for details).  For
   Speex the following values are used.

   Version (V): 2 bits

   This field identifies the version of RTP.  The version used by this
   specification is two [2].

   Padding (P): 1 bit

   If the padding bit is set, the packet contains one or more additional
   padding octets at the end which are not part of the payload.

   Extension (X): 1 bit

   If the extension, X, bit is set, the fixed header MUST be followed by
   exactly one header extension, with a format defined in Section 5.3.1.
   of [2].

   CSRC count (CC): 4 bits

   The CSRC count contains the number of CSRC identifiers.

   Marker (M): 1 bit

   The M bit indicates if the packet contains comfort noise.  This field
   is used in conjunction with the cng SDP attribute and conforms to
   Section 4.1. of [5].

   Payload Type (PT): 7 bits

   An RTP profile for a class of applications is expected to assign a
   payload type for this format, or a dynamically allocated payload type
   SHOULD be chosen which designates the payload as Speex.



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   Sequence number: 16 bits

   The sequence number increments by one for each RTP data packet sent,
   and may be used by the receiver to detect packet loss and to restore
   packet sequence.  This field is detailed further in [2].

   Timestamp: 32 bits

   A timestamp representing the sampling time of the first sample of the
   first Speex packet in the RTP packet.  The clock frequency MUST be
   set to the sample rate of the encoded audio data.  Speex uses 20 msec
   frames and a variable sampling rate clock.  The RTP timestamp MUST be
   in units of 1/X of a second where X is the sample rate used.  Speex
   uses a nominal 8kHz sampling rate for narrowband use, a nominal 16kHz
   sampling rate for wideband use, and a nominal 32kHz sampling rate for
   ultra-wideband use.

   SSRC/CSRC identifiers:

   These two fields, 32 bits each with one SSRC field and a maximum of
   16 CSRC fields, are as defined in [2].

5.  Speex payload

   For the purposes of packetizing the bit stream in RTP, it is only
   necessary to consider the sequence of bits as output by the Speex
   encoder [7], and present the same sequence to the decoder.  The
   payload format described here maintains this sequence.

   A typical Speex frame, encoded at the maximum bitrate, is approx. 110
   octets and the total number of Speex frames SHOULD be kept less than
   the path MTU to prevent fragmentation.  Speex frames MUST NOT be
   fragmented across multiple RTP packets,

   An RTP packet MAY contain Speex frames of the same bit rate or of
   varying bit rates, since the bit-rate for a frame is conveyed in band
   with the signal.

   The encoding and decoding algorithm can change the bit rate at any 20
   msec frame boundary, with the bit rate change notification provided
   in-band with the bit stream.  Each frame contains both "mode"
   (narrowband, wideband or ultra-wideband) and "sub-mode" (bit-rate)
   information in the bit stream.  No out-of-band notification is
   required for the decoder to process changes in the bit rate sent by
   the encoder.

   It is RECOMMENDED that values of 8000, 16000 and 32000 be used for
   normal internet telephony applications, though the sample rate is



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   supported at rates as low as 6000 Hz and as high as 48 kHz.

   The RTP payload MUST be padded to provide an integer number of octets
   as the payload length.  These padding bits are LSB aligned in network
   octet order and consist of a 0 followed by all ones (until the end of
   the octet).  This padding is only required for the last frame in the
   packet, and only to ensure the packet contents ends on an octet
   boundary.

6.  Example Speex packet

   In the example below we have a single Speex frame with 5 bits of
   padding to ensure the packet size falls on an octet boundary.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |V=2|P|X|  CC   |M|     PT      |       sequence number         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           timestamp                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         synchronization source (SSRC) identifier              |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

       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
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |         contributing source (CSRC) identifiers                |
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        ..speex data..                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        ..speex data..               |0 1 1 1 1|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


7.  Multiple Speex frames in a RTP packet

   Below is an example of two Speex frames contained within one RTP
   packet.  The Speex frame length in this example fall on an octet
   boundary so there is no padding.

   Speex codecs [7] are able to detect the bitrate from the payload and
   are responsible for detecting the 20 msec boundaries between each
   frame.





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       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |V=2|P|X|  CC   |M|     PT      |       sequence number         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           timestamp                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         synchronization source (SSRC) identifier              |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |         contributing source (CSRC) identifiers                |
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        ..speex data..                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        ..speex data..         |        ..speex data..         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        ..speex data..                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


8.  MIME registration of Speex

   Full definition of the MIME [3] type for Speex will be part of the
   Ogg Vorbis MIME type definition application [6].

   MIME media type name: audio

   MIME subtype: speex

   Optional parameters:

   Required parameters: to be included in the Ogg MIME specification.

   Encoding considerations:

   This type is only defined for transfer via HTTP as specified in RFC
   XXXX.

   Security Considerations:

   See Section 6 of RFC 3047.

   Interoperability considerations: none

   Published specification:

   Applications which use this media type:



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   Additional information: none

   Person & email address to contact for further information:

      Greg Herlein <gherlein@herlein.com>
      Jean-Marc Valin <jean-marc.valin@usherbrooke.ca>

   Intended usage: COMMON

   Author/Change controller:

      Author:  Greg Herlein <gherlein@herlein.com>
      Change controller: Greg Herlein <gherlein@herlein.com>
      Change controller: IETF AVT Working Group

   This transport type signifies that the content is to be interpreted
   according to this document if the contents are transmitted over RTP.
   Should this transport type appear over a lossless streaming protocol
   such as TCP, the content encapsulation should be interpreted as an
   Ogg Stream in accordance with [6], with the exception that the
   content of the Ogg Stream may be assumed to be Speex audio and Speex
   audio only.

9.  SDP usage of Speex

   When conveying information by SDP [4], the encoding name MUST be set
   to "speex".  An example of the media representation in SDP for
   offering a single channel of Speex at 8000 samples per second might
   be:

      m=audio 8088 RTP/AVP 97
      a=rtpmap:97 speex/8000

   Note that the RTP payload type code of 97 is defined in this media
   definition to be 'mapped' to the speex codec at an 8kHz sampling
   frequency using the 'a=rtpmap' line.  Any number from 96 to 127 could
   have been chosen (the allowed range for dynamic types).

   The value of the sampling frequency is typically 8000 for narrow band
   operation, 16000 for wide band operation, and 32000 for ultra-wide
   band operation.

   If for some reason the offerer has bandwidth limitations, the client
   may use the "b=" header, as explained in SDP [4].  The following
   example illustrates the case where the offerer cannot receive more
   than 10 kbit/s.





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      m=audio 8088 RTP/AVP 97
      b=AS:10
      a=rtmap:97 speex/8000

   In this case, if the remote part agrees, it should configure its
   Speex encoder so that it does not use modes that produce more than 10
   kbit/s.  Note that the "b=" constraint also applies on all payload
   types that may be proposed in the media line ("m=").

   An other way to make recommendations to the remote Speex encoder is
   to use its specific parameters via the a=fmtp: directive.  The
   following parameters are defined for use in this way:

      ptime: duration of each packet in milliseconds.

      sr:    actual sample rate in Hz.

      ebw:   encoding bandwidth - either 'narrow' or 'wide' or 'ultra'
      (corresponds to nominal 8000, 16000, and 32000 Hz sampling rates).

      vbr:   variable bit rate  - either 'on' 'off' or 'vad' (defaults
      to off).  If on, variable bit rate is enabled.  If off, disabled.
      If set to 'vad' then constant bit rate is used but silence will be
      encoded with special short frames to indicate a lack of voice for
      that period.

      cng:   comfort noise generation - either 'on' or 'off'.  If off
      then silence frames will be silent; if 'on' then those frames will
      be filled with comfort noise.

      mode:  Speex encoding mode.  Can be {1,2,3,4,5,6,any} defaults to
      3 in narrowband, 6 in wide and ultra-wide.


   Examples:

      m=audio 8008 RTP/AVP 97
      a=rtpmap:97 speex/8000
      a=fmtp:97 mode=4

   This examples illustrate an offerer that wishes to receive a Speex
   stream at 8000Hz, but only using speex mode 4.

   Several Speex specific parameters can be given in a single a=fmtp
   line provided that they are separated by a semi-colon:






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      a=fmtp:97 mode=any;mode=1

   The offerer may indicate that it wishes to send variable bit rate
   frames with comfort noise:

      m=audio 8088 RTP/AVP 97
      a=rtmap:97 speex/8000
      a=fmtp:97 vbr=on;cng=on

   The "ptime" attribute is used to denote the packetization interval
   (ie, how many milliseconds of audio is encoded in a single RTP
   packet).  Since Speex uses 20 msec frames, ptime values of multiples
   of 20 denote multiple Speex frames per packet.  Values of ptime which
   are not multiples of 20 MUST be ignored and clients MUST use the
   default value of 20 instead.

   In the example below the ptime value is set to 40, indicating that
   there are 2 frames in each packet.

      m=audio 8008 RTP/AVP 97
      a=rtpmap:97 speex/8000
      a=ptime:40

   Note that the ptime parameter applies to all payloads listed in the
   media line and is not used as part of an a=fmtp directive.

   Values of ptime not multiple of 20 msec are meaningless, so the
   receiver of such ptime values MUST ignore them.  If during the life
   of an RTP session the ptime value changes, when there are multiple
   Speex frames for example, the SDP value must also reflect the new
   value.

   Care must be taken when setting the value of ptime so that the RTP
   packet size does not exceed the path MTU.

10.  ITU H.323 Use of Speex

   It is outside the scope of this document to cover the use of Speex
   and H.323, more details may be found on the Speex website [9].

11.  Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [2], and any appropriate RTP profile.  This implies
   that confidentiality of the media streams is achieved by encryption.
   Because the data compression used with this payload format is applied
   end-to-end, encryption may be performed after compression so there is



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   no conflict between the two operations.

   A potential denial-of-service threat exists for data encodings using
   compression techniques that have non-uniform receiver-end
   computational load.  The attacker can inject pathological datagrams
   into the stream which are complex to decode and cause the receiver to
   be overloaded.  However, this encoding does not exhibit any
   significant non-uniformity.

   As with any IP-based protocol, in some circumstances a receiver may
   be overloaded simply by the receipt of too many packets, either
   desired or undesired.  Network-layer authentication may be used to
   discard packets from undesired sources, but the processing cost of
   the authentication itself may be too high.

12.  Acknowledgments

   The authors would like to thank Equivalence Pty Ltd of Australia for
   their assistance in attempting to standardize the use of Speex in
   H.323 applications, and for implementing Speex in their open source
   OpenH323 stack.  The authors would also like to thank Brian C. Wiles
   <brian@streamcomm.com> of StreamComm for his assistance in developing
   the proposed standard for Speex use in H.323 applications.

   The authors would also like to thank the following members of the
   Speex and AVT communities for their input:  Ross Finlayson, Federico
   Montesino Pouzols, Henning Schulzrinne, Magnus Westerlund.

13.  References

13.1  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", RFC 2119.

   [2]  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
        "RTP: A Transport Protocol for real-time applications",
        RFC 3550.

   [3]  "Multipurpose Internet Mail Extensions (MIME) Part One: Format
        of Internet Message Bodies", RFC 2045.

   [4]  Jacobson, V. and M. Handley, "SDP: Session Description
        Protocol", RFC 2327.

   [5]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
        Conferences with Minimal Control.", RFC 3551.




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   [6]  Walleij, L., "The application/ogg Media Type", RFC 3534.

13.2  Informative References

   [7]  "Speexenc/speexdec, reference command-line encoder/decoder",
        Speex website http://www.speex.org/.

   [8]  "CELP, U.S. Federal Standard 1016.", National Technical
        Information Service (NTIS) website http://www.ntis.gov/.

   [9]  "ITU H.323/H.245 Use of Speex", Speex
        website http://www.speex.org/itu/.


Authors' Addresses

   Greg Herlein
   2034 Filbert Street
   San Francisco, California  94123
   United States

   Email: gherlein@herlein.com


   Simon Morlat
   35, av de Vizille App 42
   Grenoble  38000
   France

   Email: simon.morlat@linphone.org


   Jean-Marc Valin
   Department of Electrical and Computer Engineering
   University of Sherbrooke
   2500 blvd Universite
   Sherbrooke, Quebec  J1K 2R1
   Canada

   Email: jean-marc.valin@usherbrooke.ca











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   Roger Hardiman
   49 Nettleton Road
   Cheltenham, Gloucestershire  GL51 6NR
   England

   Email: roger@freebsd.org


   Phil Kerr
   England

   Email: phil@plus24.com







































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   Internet Society.




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