draft-herlein-speex-rtp-profile-02.txt

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   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.

      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



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   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.

      penh:	use of perceptual enhancement.  1 indicates to the decoder
      that perceptual enhancement is recommended, 0 indicates that it is
      not.  Defaults to on (1).


   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 3.

   The offerer may suggest to the remote decoder to activate its
   perceptual enhancement filter like this:

      m=audio 8088 RTP/AVP 97
      a=rtmap:97 speex/8000
      a=fmtp:97 penh=1

   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;penh=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/H.245 Use of Speex

   Application is underway to make Speex a standard ITU codec.  However,
   until that is finalized, Speex MAY be used in H.323 [5] by using a
   non-standard codec block definition in the H.245 [6] codec capability
   negotiations.

11.  NonStandardMessage format

   For Speex use in H.245 [6] based systems, the fields in the
   NonStandardMessage should be:





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      t35CountryCode   = Hex: B5
      t35Extension     = Hex: 00
      manufacturerCode = Hex: 0026
      [Length of the Binary Sequence (8 bit number)]
      [Binary Sequence consisting of an ASCII string, no NULL
      terminator]

   The binary sequence is an ascii string merely for ease of use.  The
   string is not null terminated.  The format of this string is

      speex [optional variables]

   The optional variables are identical to those used for the SDP a=fmtp
   strings discussed in section 5 above.  The string is built to be all
   on one line, each key-value pair separated by a semi-colon.  The
   optional variables MAY be omitted, which causes the default values to
   be assumed.  They are:

      ebw=narrow;mode=3;vbr=off;cng=off;ptime=20;sr=8000;penh=no;

   The fifth octet of the block is the length of the binary sequence.

   NOTE:  this method can result in the advertising of a large number of
   Speex 'codecs' based on the number of variables possible.  For most
   VoIP applications, use of the default binary sequence of 'speex' is
   RECOMMENDED to be used in addition to all other options.  This
   maximizes the chances that two H.323 based applications that support
   Speex can find a mutual codec.

12.  RTP Payload Types

   Dynamic payload type codes MUST be negotiated 'out-of-band' for the
   assignment of a dynamic payload type from the range of 96-127.  H.323
   applications MUST use the H.245 H2250LogicalChannelParameters
   encoding to accomplish this.

13.  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
   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



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   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.

14.  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.

15.  References

15.1  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]  "Packet-based Multimedia Communications Systems", ITU-T
        Recommendation H.323.

   [6]  "Control of communications between Visual Telephone Systems and
        Terminal Equipment", ITU-T Recommendation H.245.

   [7]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video



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        Conferences with Minimal Control.", RFC 3551.

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

15.2  Informative References

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

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


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]


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

   EMail: [e-mail]




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   Phil Kerr
   England

   EMail: [e-mail]















































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