rfc2793.txt

RFC 的详细文档！


RFC 2793           RTP Payload for Text Conversation            May 2000


3.4 Transmission during "silent periods" when redundancy is used.

   When using the redundancy transmission scheme, and there is nothing
   more to transmit from T.140, the latest T140block has a risk of
   getting old before it is transmitted as redundant data. The result is
   less useful protection against packet loss at the end of a text input
   sequence. For cases where this should be avoided, a zero-length
   primary T140block MAY be transmitted with the redundant data.

   Any zero-length T140blocks that are sent as primary data MUST be
   included as redundant T140blocks on subsequent packets just as normal
   text T140blocks would be so that sequence number inference for the
   redundant T140blocks will be correct, as explained in section 2.3.

   Redundancy for the last T140block SHOULD NOT be implemented by
   repeatedly transmitting the same packet (with the same sequence
   number) because this will cause the packet loss count, as reported in
   RTCP, to decrement.

































Hellstrom                   Standards Track                     [Page 6]

RFC 2793           RTP Payload for Text Conversation            May 2000


4. Examples

   This is an example of a T140 RTP packet without redundancy.
    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=0  |M|   T140 PT   |       sequence number         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      timestamp (1000Hz)                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           synchronization source (SSRC) identifier            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +                      T.140 encoded data                       +
   |                                                               |
   +                                               +---------------+
   |                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   This is an example of an RTP packet with one redundant T140block.
    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=0  |M|  "RED" PT   |   sequence number of primary  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              timestamp  of primary encoding "P"               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           synchronization source (SSRC) identifier            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|   T140 PT   |  timestamp offset of "R"  | "R" block length  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|   T140 PT   |                                               |
   +-+-+-+-+-+-+-+-+                                               +
   |                                                               |
   +               "R" T.140 encoded redundant data                +
   |                                                               |
   +                                               +---------------+
   |                                               |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
   |                "P" T.140 encoded primary data                 |
   +                                                               +
   +                                               +---------------+
   |                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure: Examples of RTP text packets.






Hellstrom                   Standards Track                     [Page 7]

RFC 2793           RTP Payload for Text Conversation            May 2000


5.  Security Considerations

   Since the intention of the described payload format is to carry text
   in a text conversation, security measures in the form of encryption
   are of importance. The amount of data in a text conversation session
   is low and therefore any encryption method MAY be selected and
   applied to T.140 session contents or to the whole RTP packets. When
   redundant data is included, the same security considerations as for
   RFC 2198 apply.

6.  MIME Media Type Registrations

   This document defines a new RTP payload name and associated MIME
   type, T140 (text/t140).

6.1  Registration of MIME media type text/t140

   MIME media type name: text

   MIME subtype name: t140

   Required parameters: None

   Optional parameters: None

   Encoding considerations: T140 text can be transmitted with RTP as
   specified in RFC 2793.

   Security considerations: None

   Interoperability considerations: None

   Published specification: ITU-T T.140 Recommendation.
                            RFC 2793.

   Applications which use this media type:
     Text communication terminals and text conferencing tools.

   Additional information: None

     Magic number(s): None
     File extension(s): None
     Macintosh File Type Code(s): None

   Person & email address to contact for further information:
     Gunnar Hellstrom
     e-mail: [e-mail]




Hellstrom                   Standards Track                     [Page 8]

RFC 2793           RTP Payload for Text Conversation            May 2000


   Intended usage: COMMON
     Author                      / Change controller:
     Gunnar Hellstrom            | IETF avt WG
     [e-mail] | c/o Steve Casner [e-mail]

7. Author's Address

   Gunnar Hellstrom
   Omnitor AB
   Alsnogatan 7, 4 tr
   SE-116 41 Stockholm
   Sweden

   Phone: [phone]
   Fax:   [phone]
   EMail: [e-mail]

8. Acknowledgements

   The author wants to thank Stephen Casner and Colin Perkins for
   valuable support with reviews and advice on creation of this
   document, to Mickey Nasiri at Ericsson Mobile Communication for
   providing the development environment, and Michele Mizarro for
   verification of the usability of the payload format for its intended
   purpose.

9. References

   [1]  ITU-T Recommendation T.140 (1998) - Text conversation protocol
        for multimedia application, with amendment 1, (2000).

   [2]  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
        "RTP: A Transport Protocol for Real-Time Applications", RFC
        1889, January 1996.

   [3]  Perkins, C., Kouvelas, I., Hardman, V., Handley, M. and J.
        Bolot, "RTP Payload for Redundant Audio Data", RFC 2198,
        September 1997.

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

   [5]  ISO/IEC 10646-1: (1993), Universal Multiple Octet Coded
        Character Set.

   [6]  Yergeau, F., "UTF-8, a transformation format of ISO 10646",  RFC
        2279, January 1998.




Hellstrom                   Standards Track                     [Page 9]

RFC 2793           RTP Payload for Text Conversation            May 2000


10. Full Copyright Statement

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















Hellstrom                   Standards Track                    [Page 10]