


AVT                                                           G. Herlein
Internet-Draft
Intended status: Standards Track                                J. Valin
Expires: October 24, 2007                       University of Sherbrooke
                                                            A. Heggestad
                                                          April 22, 2007


                 RTP Payload Format for the Speex Codec
                      draft-ietf-avt-rtp-speex-01 (non-final)

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Copyright Notice

   Copyright (C) The Internet Society (2007).












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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.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  RTP usage for Speex  . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  RTP Speex Header Fields  . . . . . . . . . . . . . . . . .  6
     3.2.  RTP payload format for Speex . . . . . . . . . . . . . . .  6
     3.3.  Speex payload  . . . . . . . . . . . . . . . . . . . . . .  6
     3.4.  Example Speex packet . . . . . . . . . . . . . . . . . . .  7
     3.5.  Multiple Speex frames in a RTP packet  . . . . . . . . . .  7
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
     4.1.  Media Type Registration  . . . . . . . . . . . . . . . . .  9
       4.1.1.  Registration of media type audio/speex . . . . . . . .  9
   5.  SDP usage of Speex . . . . . . . . . . . . . . . . . . . . . . 11
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
   Intellectual Property and Copyright Statements . . . . . . . . . . 18
























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1.  Introduction

   Speex is based on the CELP [CELP] encoding technique with support for
   either narrowband (nominal 8kHz), wideband (nominal 16kHz) or ultra-
   wideband (nominal 32kHz).  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

   To be compliant with this specification, implementations MUST support
   8 kHz sampling rate (narrowband)" and SHOULD support 8 kbps bitrate.
   The sampling rate MUST be 8, 16 or 32 kHz.





























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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 [RFC2119] and
   indicate requirement levels for compliant RTP implementations.













































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3.  RTP usage for Speex

3.1.  RTP Speex Header Fields

   The RTP header is defined in the RTP specification [RFC3550].  This
   section defines how fields in the RTP header are used.

      Payload Type (PT): The assignment of an RTP payload type for this
      packet format is outside the scope of this document; it is
      specified by the RTP profile under which this payload format is
      used, or signaled dynamically out-of-band (e.g., using SDP).

      Marker (M) bit: The M bit is set to one to indicate that the RTP
      packet payload contains at least one complete frame

      Extension (X) bit: Defined by the RTP profile used.

      Timestamp: A 32-bit word that corresponds to the sampling instant
      for the first frame in the RTP packet.

3.2.  RTP payload format for Speex

   The RTP payload for Speex has the format shown in Figure 1.  No
   additional header fields specific to this payload format are
   required.  For RTP based transportation of Speex encoded audio the
   standard RTP header [RFC3550] 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    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 1: RTP payload for Speex

3.3.  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 [speexenc], 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



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   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.

   Sampling rate values of 8000, 16000 or 32000 Hz MUST be used.  Any
   other sampling rates MUST NOT be used.

   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.

3.4.  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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      RTP Header                               |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |                        ..speex data..                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        ..speex data..               |0 1 1 1 1|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.5.  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 [speexenc] are able to detect the bitrate from the



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   payload and are responsible for detecting the 20 msec boundaries
   between each frame.

       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                               |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |                     ..speex frame 1..                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       ..speex frame 1..       |      ..speex frame 2..        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      ..speex frame 2..                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





































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4.  IANA Considerations

   This document defines the Speex media type.

4.1.  Media Type Registration

   This section describes the media types and names associated with this
   payload format.  The section registers the media types, as per
   RFC4288 [RFC4288]

4.1.1.  Registration of media type audio/speex

   Media type name: audio

   Media subtype name: speex

   Required parameters:

   None

   Optional parameters:

      ptime: see RFC 4566.  SHOULD be a multiple of 20 msec.

      maxptime: see RFC 4566.  SHOULD be a multiple of 20 msec.

   Encoding considerations:

      This media type is framed and binary, see section 4.8 in
      [RFC4288].

   Security considerations: See Section 6

   Interoperability considerations:

      None.

   Published specification: RFC XXXX [This RFC].

   Applications which use this media type:

      Audio streaming and conferencing applications.

   Additional information: none

   Person and email address to contact for further information :





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      Alfred E. Heggestad: aeh@db.org

   Intended usage: COMMON

   Restrictions on usage:

      This media type depends on RTP framing, and hence is only defined
      for transfer via RTP [RFC3550].  Transport within other framing
      protocols is not defined at this time.

   Author: Alfred E. Heggestad

   Change controller:

      IETF Audio/Video Transport working group delegated from the IESG.




































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5.  SDP usage of Speex

   When conveying information by SDP [RFC4566], 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 [RFC4566].  The
   following example illustrates the case where the offerer cannot
   receive more than 10 kbit/s.

             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).






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      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:

      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.

   Implementations SHOULD support ptime of 20 msec (i.e. one frame per
   packet)

   In the example below the ptime value is set to 40, indicating that



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   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.


































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6.  Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [RFC3550], 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 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.





























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7.  Acknowledgements

   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.

   Thanks to former authors of this document; Simon Morlat, Roger
   Hardiman, Phil Kerr




































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8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

8.2.  Informative References

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

   [RFC4288]  Freed, N. and J. Klensin, "Media Type Specifications and
              Registration Procedures", BCP 13, RFC 4288, December 2005.

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


























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Authors' Addresses

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

   Email: gherlein@herlein.com


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

   Email: jean-marc.valin@usherbrooke.ca


   Alfred E. Heggestad
   Biskop J. Nilssonsgt. 20a
   Oslo  0659
   Norway

   Email: aeh@db.org
























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