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

一个开源的sip源代码

   Additional information: none

   Person & email address to contact for further information:

      Greg Herlein <[e-mail]>
      Jean-Marc Valin <[e-mail]>

   Intended usage: COMMON

   Author/Change controller:

      Author:  Greg Herlein <[e-mail]>
      Change controller: Greg Herlein <[e-mail]>
      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
   <[e-mail]> 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: [e-mail]


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

   Email: [e-mail]


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

   Email: [e-mail]











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

   Email: [e-mail]


   Phil Kerr
   England

   Email: [e-mail]







































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