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

一个开源的sip源代码

Internet Engineering Task Force                          Greg Herlein
Internet Draft                                        Jean-Marc Valin
draft-herlein-avt-rtp-speex-00.txt                       Simon Morlat
March 3, 2004                                          Roger Hardiman
Expires: September 3, 2004                                  Phil Kerr


                 RTP Payload Format for the Speex Codec

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
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Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.


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) and a preliminary method of
   using Speex within H.323 applications.


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 [5].

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2. Overview of the Speex Codec

   Speex is based on the CELP [12] 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    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


3.1 RTP Header

      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 [8] and [9] for details). For 
   Speex the following values are used.

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   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.  P is set if the total packet size is less than 
      the MTU.  

   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 [8], 

   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 is 
      detailed further in section 5 below.  In normal usage this bit 
      is set if the packet contains comfort noise.

   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.

   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].  



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3.2 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 [11], 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 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 byte 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.


3.2.1 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              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+


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


3.4 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 [11] are able to detect the the bitrate from the
   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |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..                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


4. MIME registration of Speex

   Full definition of the MIME type for Speex will be part of the Ogg
   Vorbis MIME type definition application [10].  

   MIME media type name: audio

   MIME subtype: speex

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   Optional parameters:

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

   Encoding considerations:

   Security Considerations:
         See Section 6 of RFC 3047.

   Interoperability considerations: none

   Published specification:  

   Applications which use this media type:

   Additional information: none

   Person & email address to contact for further information:
	 Greg Herlein <[e-mail]>