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Network Working Group                                              A. LiRequest for Comments: 4756                                    HyervisionCategory: Standards Track                                  November 2006Forward Error Correction Grouping Semanticsin Session Description ProtocolStatus 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 IETF Trust (2006).Abstract   This document defines the semantics that allow for grouping of   Forward Error Correction (FEC) streams with the protected payload   streams in Session Description Protocol (SDP).  The semantics defined   in this document are to be used with "Grouping of Media Lines in the   Session Description Protocol" (RFC 3388) to group together "m" lines   in the same session.Table of Contents1. Introduction ....................................................22. Terminology .....................................................23. Forward Error Correction (FEC) ..................................24. FEC Grouping ....................................................34.1. FEC Group ..................................................34.2. Offer / Answer Consideration ...............................34.3. Example of FEC Grouping ....................................35. Security Considerations .........................................46. IANA Considerations .............................................47. Acknowledgments .................................................58. References ......................................................58.1. Normative References .......................................58.2. Informative References .....................................5Li                          Standards Track                     [Page 1]

RFC 4756             FEC Grouping Semantics in SDP         November 20061.  Introduction   The media lines in an SDP [3] session may be associated with each   other in various ways.  SDP itself does not provide methods to convey   the relationships between the media lines.  Such relationships are   indicated by the extension to SDP as defined in "Grouping of Media   Lines in the Session Description Protocol" (RFC 3388) [2].RFC 3388   defines two types of semantics: Lip Synchronization and Flow   Identification.   Forward Error Correction (FEC) is a common technique to achieve   robust communication in error-prone environments.  In this document,   we define the semantics that allows for grouping of FEC streams with   the protected payload streams in SDP by further extendingRFC 3388.2.  Terminology   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 [1].3.  Forward Error Correction (FEC)   Forward Error Correction (FEC) is a common technique to achieve   robust communication in error-prone environments.  In FEC,   communication uses a bandwidth that is more than payload to send   redundantly coded payload information.  The receivers can readily   recover the original payload even when some communication is lost in   the transmission.  Compared to other error correction techniques   (such as retransmission), FEC can achieve much lower transmission   delay, and it does not have the problem of implosion from   retransmission requests in various multicast scenarios.   In general, the FEC data can be sent in two different ways: (1)   multiplexed together with the original payload stream or (2) as a   separate stream.  It is thus necessary to define mechanisms to   indicate the association relationship between the FEC data and the   payload data they protect.   When FEC data are multiplexed with the original payload stream, the   association relationship may, for example, be indicated as specified   in "An RTP Payload for Redundant Audio Data" (RFC 2198) [4].  The   generic RTP payload format for FEC [5] uses that method.   When FEC data are sent as a separate stream from the payload data,   the association relationship can be indicated in various ways.  This   document on the FEC media line grouping specifies a mechanism for   indicating such relationships.Li                          Standards Track                     [Page 2]

RFC 4756             FEC Grouping Semantics in SDP         November 20064.  FEC Grouping4.1.  FEC Group   Each "a=group" line is used to indicate an association relationship   between the FEC streams and the payload streams.  The streams   included in one "a=group" line are called a "FEC Group".   Each FEC group MAY have one or more than one FEC stream, and one or   more than one payload stream.  For example, it is possible to have   one payload stream protected by more than one FEC stream , or   multiple payload streams sharing one FEC stream.   Grouping streams in a FEC group only indicates the association   relationship between streams.  The detailed FEC protection   scheme/parameters are conveyed through the mechanism of the   particular FEC algorithm used.  For example, the FEC grouping is used   for generic RTP payload for FEC [5] to indicate the association   relationship between the FEC stream and the payload stream.  The   detailed protection level and length information for the Unequal Loss   Protection (ULP) algorithm is communicated in band within the FEC   stream.4.2.  Offer / Answer Consideration   The backward compatibility in offer / answer is generally handled as   specified inRFC 3388 [2].   Depending on the implementation, a node that does not understand FEC   grouping (either does not understand line grouping at all, or just   does not understand the FEC semantics) SHOULD respond to an offer   containing FEC grouping either (1) with an answer that ignores the   grouping attribute or (2) with a refusal to the request (e.g., 488   Not acceptable here or 606 Not acceptable in SIP).   In the first case, the original sender of the offer MUST establish   the connection without FEC.  In the second case, if the sender of the   offer still wishes to establish the session, it SHOULD re-try the   request with an offer without FEC.4.3.  Example of FEC Grouping   The following example shows a session description of a multicast   conference.  The first media stream (mid:1) contains the audio   stream.  The second media stream (mid:2) contains the Generic FEC [5]   protection for the audio stream.  These two streams form an FEC   group.  The relationship between the two streams is indicated by the   "a=group:FEC 1 2" line.  The FEC stream is sent to the same multicastLi                          Standards Track                     [Page 3]

RFC 4756             FEC Grouping Semantics in SDP         November 2006   group and has the same Time to Live (TTL) as the audio, but on a port   number two higher.  Likewise, the video stream (mid:3) and its   Generic FEC protection stream (mid:4) form another FEC group.  The   relationship between the two streams is indicated by the "a=group:FEC   3 4" line.  The FEC stream is sent to a different multicast address,   but has the same port number (30004) as the payload video stream.       v=0       o=adam 289083124 289083124 IN IP4 host.example.com       s=ULP FEC Seminar       t=0 0       c=IN IP4 224.2.17.12/127       a=group:FEC 1 2       a=group:FEC 3 4       m=audio 30000 RTP/AVP 0       a=mid:1       m=audio 30002 RTP/AVP 100       a=rtpmap:100 ulpfec/8000       a=mid:2       m=video 30004 RTP/AVP 31       a=mid:3       m=video 30004 RTP/AVP 101       c=IN IP4 224.2.17.13/127       a=rtpmap:101 ulpfec/8000       a=mid:45.  Security Considerations   There is a weak threat for the receiver that the FEC grouping can be   modified to indicate FEC relationships that do not exist.  Such   attacks may result in failure of FEC to protect, and/or mishandling   of other media payload streams.  It is recommended that the receiver   SHOULD do integrity check on SDP and follow the security   considerations of SDP [3] to only trust SDP from trusted sources.6.  IANA Considerations   This document defines the semantics to be used with grouping of media   lines in SDP as defined inRFC 3388.  The semantics defined in this   document are to be registered by the IANA when they are published in   standards track RFCs.   The following semantics have been registered by IANA in Semantics for   the "group" SDP Attribute under SDP Parameters.   Semantics                  Token   Reference   ------------------------   -----   ----------   Forward Error Correction   FECRFC 4756Li                          Standards Track                     [Page 4]

RFC 4756             FEC Grouping Semantics in SDP         November 20067.  Acknowledgments   The author would like to thank Magnus Westerlund, Colin Perkins,   Joerg Ott, and Cullen Jennings for their feedback on this document.8.  References8.1.  Normative References   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement        Levels",BCP 14,RFC 2119, March 1997.   [2]  Camarillo, G., Eriksson, G., Holler, J., and H. Schulzrinne,        "Grouping of Media Lines in the Session Description Protocol        (SDP)",RFC 3388, December 2002.   [3]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session        Description Protocol",RFC 4566, July 2006.8.2.  Informative References   [4]  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.   [5]  Li, A.,"An RFC Payload Format for Generic FEC", Work in        Progress.Author's Address   Adam H. Li   HyerVision   10194 Wateridge Circle #152   San Diego, CA 92121   U.S.A.   Tel:    +1 858 622 9038   EMail:  adamli@hyervision.comLi                          Standards Track                     [Page 5]

RFC 4756             FEC Grouping Semantics in SDP         November 2006Full Copyright Statement   Copyright (C) The IETF Trust (2006).   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, THE IETF TRUST,   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.Li                          Standards Track                     [Page 6]