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Network Working Group                                           P. JonesRequest for Comments: 4102                           Cisco Systems, Inc.Category: Standards Track                                      June 2005Registration of the text/red MIME Sub-TypeStatus of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2005).Abstract   This document defines the text/red MIME sub-type.  "Red" is short for   redundant.  The actual RTP packetization for this MIME type is   specified inRFC 2198.1.  Introduction   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   experience.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 4103 [8] specifies one usage ofRFC 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.Jones                       Standards Track                     [Page 1]

RFC 4102                 text/red MIME Sub-Type                June 20052.  Conventions Used in This Document   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described inRFC 2119 [3].3.  IANA Considerations   One new MIME sub-type has been registered by the IANA, 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 withRFC2198.  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      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.  Because it is      part of the list, which must itself be a quoted-string, the      quotation marks MUST be quoted with backslash quoting as specified      inRFC 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 (these attributes are originally      defined inRFC 2327 [5] andRFC 3267 [6], respectively)   Restrictions on Usage:      This type is defined only for transfer via RTP.      It shall not be defined for a storage format.Jones                       Standards Track                     [Page 2]

RFC 4102                 text/red MIME Sub-Type                June 2005   Encoding considerations:      See restrictions on Usage above; this section is included per      the requirements inRFC 3555 [7].   Security considerations: Refer tosection 5 of RFC 4102.   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:      Paul E. Jones      paulej@packetizer.com   Change Controller:      AVT Working Group delegated from the IESG4.  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 theRFC 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.   -  The parameters "ptime" and "maxptime" go in the SDP "a=ptime" and      "a=maxptime" attributes, respectively.Jones                       Standards Track                     [Page 3]

RFC 4102                 text/red MIME Sub-Type                June 2005   -  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 are not present in the      actual message encoding, but are 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 inRFC 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 [9].Jones                       Standards Track                     [Page 4]

RFC 4102                 text/red MIME Sub-Type                June 20056.  Normative References   [1] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., Handley, M.,       Bolot, J., Vega-Garcia, A., and S. Fosse-Parisis, "RTP Payload       for Redundant Audio Data",RFC 2198, September 1997.   [2] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,       "RTP: A Transport Protocol for Real-Time Applications", STD 64,RFC 3550, July 2003.   [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement       Levels",BCP 14,RFC 2119, March 1997.   [4] Freed, N. and N. Borenstein, "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] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie, "Real-       Time Transport Protocol (RTP) Payload Format and File Storage       Format for the Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate       Wideband (AMR-WB) Audio Codecs",RFC 3267, June 2002.   [7] Casner, S. and P. Hoschka, "MIME Type Registration of RTP Payload       Formats",RFC 3555, July 2003.7.  Informative References   [8] Hellstrom, G. and P. Jones, "RTP Payload for Text Conversation",RFC 4103, June 2005.   [9] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.       Norrman, "The Secure Real-time Transport Protocol (SRTP)",RFC3711, March 2004.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.comJones                       Standards Track                     [Page 5]

RFC 4102                 text/red MIME Sub-Type                June 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM 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.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Jones                       Standards Track                     [Page 6]