draft-ietf-avt-rtp-speex-00.txt

一个开源的sip源代码

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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