MMUSIC Working Group Adam Li INTERNET-DRAFT HyerVision Expires: December 25, 2006 June 25, 2006 Forward Error Correction Grouping Semantics in Session Description Protocol Status of this memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This document is an individual submission to the IETF. Comments should be directed to the authors. Abstract This document defines the semantics that allows for grouping of forward error correction (FEC) streams with the protected payload streams in Session Description Protocol (SDP). The semantics defined in this document is to be used with Grouping of Media Lines in the Session Description Protocol (RFC 3388) to group together "m" lines in the same session. Adam Li [Page 1] INTERNET-DRAFT FEC Grouping Semantics in SDP June 25, 2006 Table of Contents 1. Introduction.....................................................2 2. Terminology......................................................2 3. Forward Error Correction (FEC)...................................2 4. FEC Grouping.....................................................3 4.1. FEC Group...................................................3 4.2. Offer / Answer Consideration................................3 4.3. Example of FEC Grouping.....................................4 5. Security Consideration...........................................4 6. IANA Considerations..............................................5 7. Acknowledgments..................................................5 8. Author's Address.................................................5 9. References.......................................................5 9.1. Normative References........................................5 9.2. Informative References......................................5 1. 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 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 extending RFC 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 in RFC 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. Compare to other error correction technique (such as re-transmission), FEC can achieve much lower transmission delay, and does not have the problem of implosion from retransmission requests in various multicast scenarios. Adam Li [Page 2] INTERNET-DRAFT FEC Grouping Semantics in SDP June 25, 2006 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 is indicated as specified in RTP Payload for Redundant Audio Data (RFC 2198) [4]. As an example, such relationship can be indicated as in the generic RFC payload format for FEC [5]. 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. 4. FEC Grouping 4.1. FEC Group Each "a=group" line is used to indicate an association relationship between the FEC streams and the payload stream. The streams included in one "a=group" line are called a "FEC Group". Each FEC group MAY have one or more than one FEC streams, and one or more than one payload streams. For example, it is possible to have one payload streams protected by more than one FEC streams, 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 (RFC YYYY) [5] to indicate the association relationship between the FEC stream and the payload stream. The detailed protection level and length information for the 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 in RFC 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 which ignores the grouping attribute, or (2) with a refusal to the request (e.g., 488 Not acceptable here or 606 Not Acceptable in SIP). Adam Li [Page 3] INTERNET-DRAFT FEC Grouping Semantics in SDP June 25, 2006 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 multicast group and has the same 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) forms 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:4 5. Security Consideration 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 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 to only trust SDP from trusted sources. Adam Li [Page 4] INTERNET-DRAFT FEC Grouping Semantics in SDP June 25, 2006 6. IANA Considerations This document defines the semantics to be used with grouping of media lines in SDP as defined in RFC 3388. The semantics defined in this document are to be registered by the IANA when they are published in standard track RFCs. The following semantics need to be registered with IANA in Semantic for the "group" SDP Attribute under SDP Parameters. Semantics Token Reference ------------------------ ----- ---------- Forward Error Correction FEC [RFC XXXX] 7. Acknowledgments The author would like to thank Magnus Westerlund, Colin Perkins, Joerg Ott, and Cullen Jennings for their feedback on this document. 8. Author's Address Adam Li HyerVision 10194 Wateridge Circle #152 San Diego, CA 92121 U.S.A. Tel: +1 858 622 9038 Email: adamli@hyervision.com 9. References 9.1. Normative References [1] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [2] G. Camarillo, J. Holler, and H. Schulzrinne, "Grouping of Media Lines in the Session Description Protocol (SDP)", RFC 3388, December 2002. 9.2. Informative References [3] M. Handley, V. Jacobson, and C. Perkins, "SDP: Session Description Protocol", IETF work in progress, January 2006. [4] C. Perkins, I. Kouvelas, O. Hodson, V. Hardman, M. Handley, J.C. Bolot, A. Vega-Garcia, and S. Fosse-Parisis, "RTP Payload for Redundant Audio Data", RFC 2198, September 1997. [5] A. Li, "An RFC Payload Format for Generic FEC", IETF work in progress, March 2006. Adam Li [Page 5] INTERNET-DRAFT FEC Grouping Semantics in SDP June 25, 2006 Copyright Statement Copyright (C) The Internet Society (2006). 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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Adam Li [Page 6] INTERNET-DRAFT FEC Grouping Semantics in SDP June 25, 2006 RFC Editor Considerations The RFC-editor is kindly requested to perform the following modifications upon the publication of this specification: - Replace all occurrences of RFC XXXX with the RFC number this specification receives when being published. - Replace reference [5] and all occurrences of RFC YYYY with the corresponding title and RFC number of that ID when it is published. - Remove this Section. This Internet-Draft expires December 25, 2006. Adam Li [Page 7]