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Versions: (draft-jones-avt-text-red) 00 01 02 03 04 05 RFC 4102

Network Working Group
Internet Draft                                                 P. Jones
<draft-ietf-avt-text-red-04.txt>                    Cisco Systems, Inc.
Expires: October 2004                                        April 2004

                Registration of the text/red MIME Sub-Type

Status of this Memo

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   This document defines the text/red MIME sub-type.  The actual RTP
   packetization for this MIME type is specified in RFC 2198.

   [Note to RFC Editor: All references to RFC XXXX are to be replaced by
   references to the RFC number of this memo when published.]

1.   Introduction

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   Text is an important component of any multimedia communication
   system.  Like audio, the transport of text can benefit from the use
   of redundancy in order to improve reliability and end-user

   RFC 2198 [1] defines an RTP [2] payload format for redundant audio
   data.  The format defined in that document is quite suitable for
   providing redundancy for text, as well as audio.

   RFC 2793 [7] specifies one usage of RFC 2198 and the text/red MIME
   type for the transport of redundant text data.

   This memo provides the MIME sub-type registration information for
   text/red.  While this document focuses on the use of this MIME sub-
   type in SDP [5], the application of this MIME sub-type is not
   restricted to SDP.

2.   Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [3].

3.   IANA Considerations

   One new MIME sub-type is to be registered, as described below:

      MIME media type name: text

      MIME subtype name: RED

      Required parameters:
         rate: the RTP clock rate of the payload carried within the RTP
         packet.  Typically, this rate is 1000, but other rates MAY be
         specified.  This parameter MUST be set equal to the clock rate
         of the text payload format carried as the primary encoding.

         pt: a comma-separated ordered list of RTP payload types
         enumerating the primary, secondary, etc., in accordance with
         RFC 2198.  Because comma is a special character, the list MUST
         be a quoted-string (enclosed in double quotes).  For static
         payload types, each list element is simply the type number.
         For dynamic payload types, each list element is a mapping of
         the dynamic payload type number to an embedded MIME content-
         type specification for the payload format corresponding to the
         dynamic payload type. The format of the mapping is:

              dynamic-payload-type "=" content-type

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         If the content-type string includes a comma, then the content-
         type string MUST be a quoted-string.  If the content-type
         string does not include a comma, it MAY still be quoted. Since
         it is part of the list which must itself be a quoted-string,
         that means the quotation marks MUST be quoted with backslash
         quoting as specified in RFC 2045 [4].  If the content-type
         string itself contains a quoted-string, then the requirement
         for backslash quoting is recursively applied.

      Optional parameters: ptime, maxptime

      Encoding considerations:
         This type is only defined for transfer via RTP.

      Security considerations: Refer to section 5 of RFC XXXX.

      Interoperability considerations: none

      Published specification: RFC 2198

      Applications which use this media type:
         Text streaming and conferencing tools.

      Additional information: none

      Person & email address to contact for further information:
         Paul E. Jones
         E-mail: paulej@packetizer.com

      Intended usage: COMMON

      Author:                       / Change controller:
        Paul E. Jones               | IETF avt WG
        paulej@packetizer.com       |

4.   Mapping to SDP Parameters

   The information carried in the MIME media type specification has a
   specific mapping to fields in the Session Description Protocol (SDP)
   [5], which is commonly used to describe RTP sessions.  When SDP is
   used to specify sessions employing the RFC 2198 in a text session,
   the mapping is as follows:

   -  The MIME type ("text") goes in SDP "m=" as the media name.

   -  The value of the parameter "rate" goes in SDP "a=rtpmap".

   -  The MIME subtype (RED) goes in SDP "a=rtpmap"
      as the encoding name.

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   -  The parameters "ptime" and "maxptime" go in the SDP "a=ptime"
      and "a=maxptime" attributes, respectively.

   -  The pt parameter is mapped to an a=fmtp attribute by eliminating
      the parameter name (pt) and changing the commas to slashes.  For
      example, 'pt="101,102"' maps to 'a=fmtp:99 101/102', where '99' is
      the payload type of the redundancy frames.  Note that the single
      quote marks (') used in this example is not present in the
      actual message encoding, but is present here only for readability.
      The level of redundancy is shown by the number of elements in the
      payload type list.

   Any dynamic payload type in the list MUST be represented by its
   payload type number and not by its content-type. The mapping of
   payload types to the content-type is done using the normal SDP
   procedures with "a=rtpmap".

   An example of SDP is:

       m=text 11000 RTP/AVP 98 100
       a=rtpmap:98 t140/1000
       a=rtpmap:100 red/1000
       a=fmtp:100 98/98

   For each redundancy payload type defined, the ordering of the primary
   and redundancy encoding(s) is fixed. If more than one combination of
   primary and redundancy encoding(s) is desired, multiple redundancy
   payload types needs to be defined.

5.   Security Considerations

   The security considerations listed in RFC 2198 apply.  Further, it
   should be understood that text data, perhaps even more so than audio
   data, is susceptible to unwanted modification that may lead to
   undesired results.  To prevent modification of the primary, secondary
   or header information, payload integrity protection over at least the
   complete RTP packet is RECOMMENDED, for example using SRTP [8].

6.   Normative References

   [1] Perkins, C., et al., "RTP Payload for Redundant Audio Data", RFC
       2198, September 1997.

   [2] Schulzrinne, et al., "RTP: A Transport Protocol for Real-Time
       Applications", RFC 3550, July 2003.

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

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

   [5] Handley, M., Jackson, V., "SDP: Session Description Protocol",
       RFC 2327, April 1998.

   [6] Casner, S., Hoschka, P., "MIME Type Registration of RTP Payload
       Formats", RFC 3555, July 2003.

7.   Informative References

   [7] Hellstrom, G., "RTP Payload for Text Conversation", RFC 2793,
       May 2000.

   [8] Baugher, et al., "The Secure Real-time Transport Protocol", RFC
       3711, March 2004.

   [9] Rosenberg, J., Schulzrinne, H., "An Offer/Answer Model with the
       Session Description Protocol (SDP)", RFC 3264, June 2002.

8.   Author's Address

   Paul E. Jones
   Cisco Systems, Inc.
   7025 Kit Creek Rd.
   Research Triangle Park, NC 27709, USA
   Phone: +1 919 392 6948
   Email: paulej@packetizer.com

9.   Full Copyright Statement

   Copyright (C) The Internet Society (2004).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.


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