MMUSIC Working Group Adam Li
INTERNET-DRAFT HyerVision
Expires: December 25, 2006 June 25, 2006
Forward Error Correction Grouping Semantics
in Session Description Protocol
<draft-ietf-mmusic-fec-grouping-05.txt>
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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.
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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.
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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).
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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.
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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.
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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.
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