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

一个开源的sip源代码

	 Jean-Marc Valin <[e-mail]>

   Intended usage: COMMON

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

   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 RFC 3534, with the exception that the
   content of the Ogg Stream may be assumed to be Speex audio and 
   Speex audio only.


5. 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).  



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   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 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).






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   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:
   
   	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. 



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   Care must be taken when setting the value of ptime so that the 
   RTP packet size does not exceed the path MTU. 


6. 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 [6] by
   using a non-standard codec block definition in the H.245 [7] codec
   capability negotiations.  


6.1 NonStandardMessage format

   For Speex use in H.245 [7] based systems, the fields in the
   NonStandardMessage should be:

   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 byte 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.   


6.2 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.  

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7. 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
   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.  


8. Normative References

   1.  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
       9, RFC 2026, October 1996.

   2.  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:
       A Transport Protocol for real-time applications", RFC 1889,
       January 1996.

   3.  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
       Extensions (MIME) Part One: Format of Internet Message Bodies",
       RFC 2045, November 1996.

   4.  Handley, M. and V. Jacobson, "SDP: Session Description 
       Protocol", RFC 2327, April 1998.

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

   6.  ITU-T Recommendation H.323.  "Packet-based Multimedia 
       Communications Systems," 1998.

   7.  ITU-T Recommendation H.245 (1998), "Control of communications
       between Visual Telephone Systems and Terminal Equipment".

   8.  RTP: A transport protocol for real-time applications. Work   
       in progress, draft-ietf-avt-rtp-new-12.txt.

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   9.  RTP Profile for Audio and Video Conferences with Minimal  
       Control.  Work in progress, draft-ietf-avt-profile-new-13.txt.

   10. L. Walleij, "The application/ogg Media Type", RFC 3534, May 
       2003.


8.1 Informative References

   11. Speexenc/speexdec, reference command-line encoder/decoder, 
       Speex website, http://www.speex.org/
 
   12. CELP, U.S. Federal Standard 1016.  National Technical 
       Information Service (NTIS) website, http://www.ntis.gov/ 


9. 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.


10. Author's Address

   Greg Herlein <[e-mail]>
   2034 Filbert Street
   San Francisco, CA 
   United States 94123
   

   Jean-Marc Valin <[e-mail]>
   Department of Electrical and Computer Engineering
   University of Sherbrooke
   2500 blvd Universit