Network Working Group A. Li
Request for Comments: 4756 Hyervision
Category: Standards Track November 2006
Forward Error Correction Grouping Semantics
in Session Description Protocol
Status 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 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 ....................................3
5. Security Considerations .........................................4
6. IANA Considerations .............................................4
7. Acknowledgments .................................................5
8. References ......................................................5
8.1. Normative References .......................................5
8.2. Informative References .....................................5
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RFC 4756 FEC Grouping Semantics in SDP November 2006
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 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 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. 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.
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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 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 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 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 multicast
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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:4
5. 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 in RFC 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 FEC RFC 4756
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7. Acknowledgments
The author would like to thank Magnus Westerlund, Colin Perkins,
Joerg Ott, and Cullen Jennings for their feedback on this document.
8. References
8.1. 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.com
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RFC 4756 FEC Grouping Semantics in SDP November 2006
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