MMUSIC A. Begen
Internet-Draft Cisco Systems
Obsoletes: 4756 October 19, 2009
(if approved)
Updates: 3388bis, 5576
(if approved)
Intended status: Standards Track
Expires: April 22, 2010
Forward Error Correction Grouping Semantics in Session Description
Protocol
draft-ietf-mmusic-rfc4756bis-05
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Abstract
The Session Description Protocol (SDP) supports grouping media lines.
SDP also has semantics defined for grouping the associated source and
Forward Error Correction (FEC)-based repair flows. However, the
semantics that was defined in RFC 4756 generally fail to provide the
specific grouping relationships between the source and repair flows
when there are more than one source and/or repair flows in the same
group. Furthermore, the existing semantics does not support
describing additive repair flows. This document addresses these
issues by introducing new FEC grouping semantics. SSRC-level
grouping semantics is also introduced in this document for Real-time
Transport Protocol (RTP) streams using SSRC multiplexing.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 5
3. Requirements and Changes from RFC 4756 . . . . . . . . . . . . 5
3.1. Source and Repair Flow Association . . . . . . . . . . . . 5
3.2. Support for Additivity . . . . . . . . . . . . . . . . . . 6
4. FEC Grouping . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. New Grouping Semantics . . . . . . . . . . . . . . . . . . 6
4.2. SDP Example . . . . . . . . . . . . . . . . . . . . . . . 7
4.3. Grouping for SSRC-Multiplexed RTP Streams . . . . . . . . 8
4.4. SDP Offer-Answer Model and Backward Compatibility
Considerations . . . . . . . . . . . . . . . . . . . . . . 9
4.5. ABNF Syntax . . . . . . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
Any application that needs a reliable transmission over an unreliable
packet network has to cope with packet losses. Forward Error
Correction (FEC) is an effective approach that provides reliable
transmission particularly in multicast and broadcast applications
where the feedback from the receiver(s) is potentially limited.
In a nutshell, FEC groups source packets into blocks and applies
protection to generate a desired number of repair packets. These
repair packets may be sent on demand or independently of any receiver
feedback. The choice depends on the FEC scheme, the packet loss
characteristics of the underlying network, the transport scheme
(e.g., unicast, multicast and broadcast) and the application. At the
receiver side, lost packets can be recovered by erasure decoding
provided that a sufficient number of source and repair packets have
been received.
For example, one of the most basic FEC schemes is the parity codes,
where an exclusive OR (XOR) operation is applied to a group of
packets (i.e., source block) to generate a single repair packet. At
the receiver side, this scheme provides a full recovery if only one
packet is lost within the source block and the repair packet is
received. There are various other ways of generating repair packets,
possibly with different loss-recovery capabilities.
The FEC Framework [I-D.ietf-fecframe-framework] outlines a general
framework for using FEC codes in multimedia applications that stream
audio, video or other types of multimedia content. The FEC Framework
specification states that source and repair packets MUST be carried
in different streams, which are referred to as the source and repair
flows, respectively. At the receiver side, the receivers should know
which flows are the source flows and which flows are the repair
flows. The receivers should also know the exact association of the
source and repair flows so that they can use the correct data to
repair the original content in case there is a packet loss.
Currently, SDP [RFC4566] uses [RFC3388] and [RFC4756] for this
purpose.
In order to provide applications more flexibility, the FEC Framework
[I-D.ietf-fecframe-framework] allows a source flow to be protected by
multiple FEC schemes, each of which requires an instance of the FEC
Framework. Thus, multiple instances of the FEC Framework MAY exist
at the sender and the receiver(s). Furthermore, within a single FEC
Framework instance, multiple source flows MAY be grouped and
protected by one or more repair flows.
It should be noted that the FEC Framework requires the source and
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repair packets to be carried in different streams. When Real-time
Transport Protocol (RTP) [RFC3550] is used to carry the source and
repair streams, the FEC Framework recommends that each stream is
carried in its own RTP session. This provides flexibility in using
FEC in a backward-compatible manner. However, in some scenarios, a
single RTP session may be desired to carry multiple RTP streams via
SSRC multiplexing in order to reduce the port usage. For such
scenarios, an appropriate grouping semantics is also required.
A basic example scenario is shown in Figure 1. Here, source flow S1
is protected by repair flow R1. Also, source flows S1 and S2 are
grouped and protected together by repair flow R2.
SOURCE FLOWS | FEC FRAMEWORK INSTANCE #1
| S1: Source Flow |--------| R1: Repair Flow
+---|
| | S2: Source Flow
|
+______________________________| FEC FRAMEWORK INSTANCE #2
| R2: Repair Flow
Figure 1: Example scenario with two FEC Framework instances where R1
protects S1, and R2 protects the group of S1 and S2
Grouping source flows before applying FEC protection may allow us to
achieve a better coding performance. As a typical scenario, suppose
that source flows S1 and S2 in Figure 1 correspond to the base and
enhancement layers in a layered video content, respectively. Repair
flow R2 protects the combination of the base and enhancement layers
for the receivers who receive both layers, and repair flow R1
protects the base layer only, for the receivers who want the base
layer only, or who receive both layers but prefer FEC protection for
the base layer only due to a bandwidth and/or any other limitation.
It should be noted that the grouping semantics defined in this
document offers the flexibility to determine how source streams are
grouped together prior to applying FEC protection. However, not all
FEC schemes support the full range of the possible scenarios (e.g.,
when the source streams carry different top-level media types such as
audio and video).
Using multiple FEC Framework instances for a single source flow
provides flexibility to the receivers. An example scenario is
sketched in Figure 2. Different instances may offer repair flows
that are generated by different FEC schemes, and receivers choose
receiving the appropriate repair flow(s) that they can support and
decode. Alternatively, different instances (whether they use the
same FEC scheme or not) may use larger and smaller source block
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sizes, which accommodate the receivers that have looser and tighter
latency requirements, respectively. In addition, different instances
may also provide FEC protection at different redundancy levels. This
is particularly useful in multicast scenarios where different
receivers may experience different packet loss rates and each
receiver can choose the repair flow that is tailored to its needs.
SOURCE FLOWS | FEC FRAMEWORK INSTANCE #1
S3: Source Flow |---------| R3: Repair Flow
|
|---------| FEC FRAMEWORK INSTANCE #2
| R4: Repair Flow
Figure 2: Example scenario with two FEC Framework instances, each
with a single repair flow protecting the same source flow S3
In summary, based on the FEC Framework [I-D.ietf-fecframe-framework],
the SDP grouping semantics for FEC MUST support the ability to
indicate that:
1. A given source flow is protected by multiple different FEC
schemes.
2. Multiple repair flows are associated with a given FEC scheme.
3. Multiple source flows are grouped prior to applying FEC
protection.
4. One or more repair flows protect a group of source flows.
2. Requirements Notation
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 [RFC2119].
3. Requirements and Changes from RFC 4756
3.1. Source and Repair Flow Association
The FEC grouping semantics and 'group' attribute defined in this
document and [I-D.ietf-mmusic-rfc3388bis], respectively, are used to
associate source and repair flows together. This document also
specifies how the 'group' attribute in SDP is used to group multiple
repair flows with one or more source flows.
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[I-D.ietf-mmusic-rfc3388bis] updates [RFC3388] to allow an "m" line
identified by its 'mid' attribute to appear in more than one
"a=group" line using the same semantics. With this change and other
required changes in the grouping semantics for FEC, a sender is
allowed to indicate the specific associations between the source and
repair flows, and a receiver can determine which repair flow(s)
protect which source flow(s).
This document introduces the changes required in the FEC grouping
semantics and obsoletes [RFC4756].
3.2. Support for Additivity
The FEC Framework also supports additive repair flows. Additivity
among the repair flows means that multiple repair flows may be
decoded jointly to improve the recovery chances of the missing
packets in a single or the same set of source flows. Additive repair
flows can be generated by the same FEC scheme or different FEC
schemes.
For example, in Figure 3, repair flows R5 and R6 may be additive
within the FEC Framework instance #1. Alternatively, all three
repair flows R5, R6 and R7 could be additive, too.
SOURCE FLOWS | FEC FRAMEWORK INSTANCE #1
S4: Source Flow |---------| R5: Repair Flow
| | R6: Repair Flow
|
|---------| FEC FRAMEWORK INSTANCE #2
| R7: Repair Flow
Figure 3: Example scenario with two FEC Framework instances, where
two repair flows in the first instance and a single repair flow in
the second instance protect the same source flow S4
4. FEC Grouping
4.1. New Grouping Semantics
Each "a=group" line is used to indicate an association relationship
between the source and repair flows. The flows included in one
"a=group" line are called an FEC Group. If there are more than one
repair flows included in an FEC group, they MUST be considered to be
additive. Repair flows that are not additive MUST be indicated in
separate FEC groups. However, if two (or more) repair flows are
additive in an FEC group, it does not necessarily mean that these
repair flows will also be additive in any other FEC group.
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Generally, in order to express multiple relations between the source
and repair flows, each source and repair flow MAY appear in more than
one FEC group.
By extending [I-D.ietf-mmusic-rfc3388bis] we define "FEC-XR" as the
new grouping semantics that can support the features of the FEC
Framework.
The "a=group:FEC-XR" semantics MUST always be used to associate the
source and repair flows except when the source and repair flows are
specified in the same media description, i.e., in the same "m" line.
4.2. SDP Example
For the scenario sketched in Figure 1, we MUST write the following
SDP:
v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com
s=New FEC Grouping Semantics
t=0 0
a=group:FEC-XR S1 R1
a=group:FEC-XR S1 S2 R2
m=video 30000 RTP/AVP 100
c=IN IP4 233.252.0.1/127
a=rtpmap:100 MP2T/90000
a=mid:S1
m=video 30000 RTP/AVP 101
c=IN IP4 233.252.0.2/127
a=rtpmap:101 MP2T/90000
a=mid:S2
m=application 30000 RTP/AVP 110
c=IN IP4 233.252.0.3/127
a=rtpmap:110 1d-interleaved-parityfec/90000
a=fmtp:110 L=5; D=10; repair-window=200000
a=mid:R1
m=application 30000 RTP/AVP 111
c=IN IP4 233.252.0.4/127
a=rtpmap:111 1d-interleaved-parityfec/90000
a=fmtp:111 L=10; D=10; repair-window=400000
a=mid:R2
In this example, the source and repair flows are carried in their own
RTP sessions and the grouping is achieved through the "a=group:
FEC-XR" lines.
For the additivity issues, let us consider the scenario sketched in
Figure 3. Suppose that repair flows R5 and R6 are additive but
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repair flow R7 is not additive with any of the other repair flows.
In this case, we MUST write
a=group:FEC-XR S4 R5 R6
a=group:FEC-XR S4 R7
If none of the repair flows are additive, we MUST write
a=group:FEC-XR S4 R5
a=group:FEC-XR S4 R6
a=group:FEC-XR S4 R7
Note that additivity is not necessarily a transitive relation. Thus,
each set of additive repair flows MUST be stated explicitly.
4.3. Grouping for SSRC-Multiplexed RTP Streams
[RFC5576] defines a grouping attribute, called 'ssrc-group', for the
RTP streams that are SSRC multiplexed and carried in the same RTP
session. The grouping is based on the Synchronization Source (SSRC)
identifiers. Since SSRC-multiplexed RTP streams are defined in the
same "m" line, the 'group' attribute cannot be used.
This document extends [RFC5576] in two ways. First, we define how
FEC is applied to source and repair flows for SSRC-multiplexed
streams using the 'ssrc-group' attribute. We then specify how the
additivity of the repair flows is expressed for the SSRC-multiplexed
streams.
Per [RFC3550], the SSRC identifiers for the RTP streams that are
carried in the same RTP session MUST be unique. However, the SSRC
identifiers are not guaranteed to be unique among different RTP
sessions. Thus, the 'ssrc-group' attribute MUST only be used at the
media level [RFC5576]. The semantics of "FEC-XR" for the 'ssrc-
group' attribute is exactly the same as the one defined for the
'group' attribute.
Let us consider the following scenario where there are two source
flows (e.g., one video and one audio) and a single repair flow that
protects only one of the source flows (e.g., video). Suppose that
all these flows are separate RTP streams that are SSRC multiplexed in
the same RTP session.
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SOURCE FLOWS | FEC FRAMEWORK INSTANCE #1
S5: Source Flow |--------| R8: Repair Flow
S6: Source Flow
Figure 4: Example scenario with one FEC Framework instance, where a
single repair flow protects only one of the source flows
The following SDP describes the scenario sketched in Figure 4.
v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com
s=New FEC Grouping Semantics for SSRC Multiplexing
t=0 0
m=video 30000 RTP/AVP 100 101 110
c=IN IP4 233.252.0.1/127
a=rtpmap:100 JPEG/90000
a=rtpmap:101 L16/32000/2
a=rtpmap:110 1d-interleaved-parityfec/90000
a=fmtp:110 L=5; D=10; repair-window=200000
a=ssrc:1000 cname:fec@example.com
a=ssrc:1010 cname:fec@example.com
a=ssrc:2110 cname:fec@example.com
a=ssrc-group:FEC-XR 1000 2110
a=mid:Group1
Note that in actual use, SSRC values, which are random 32-bit
numbers, may be much larger than the ones shown in this example.
Also note that before receiving an RTP packet for each stream, the
receiver cannot know which SSRC identifier is associated with which
payload type.
The additivity of the repair flows is handled in the same way as
described in Section 4.2. In other words, the repair flows that are
included in an "a=ssrc-group" line MUST be additive. Repair flows
that are not additive MUST be indicated in separate "a=ssrc-group"
lines.
4.4. SDP Offer-Answer Model and Backward Compatibility Considerations
When offering FEC grouping using SDP in an Offer/Answer model
[RFC3264], the following considerations apply.
A node that is receiving an offer from a sender may or may not
understand line grouping. It is also possible that the node
understands line grouping but it does not understand the "FEC-XR"
semantics. From the viewpoint of the sender of the offer, these
cases are indistinguishable.
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When a node is offered a session with the "FEC-XR" grouping semantics
but it does not support line grouping or the FEC grouping semantics,
the node SHOULD respond to the offer either:
o With an answer that ignores the grouping attribute.
In this case, the original sender of the offer MUST first check
whether using the "FEC" grouping semantics will create any
ambiguity or not, while keeping its limitations in mind. If using
the "FEC" semantics rather than the "FEC-XR" semantics still
provides an exact association among the source and repair flows,
the sender of the offer MUST send a new offer using the "FEC"
semantics. However, if an exact association cannot be described,
the sender MUST send a new offer without FEC.
o With a refusal to the request (e.g., 488 Not Acceptable Here or
606 Not Acceptable in SIP).
In this case, if the sender of the offer still wishes to establish
the session, it MUST first check whether using the "FEC" grouping
semantics will create any ambiguity or not, while keeping its
limitations in mind. If using the "FEC" semantics rather than the
"FEC-XR" semantics still provides an exact association among the
source and repair flows, the sender of the offer SHOULD send a new
offer using the "FEC" semantics. However, if an exact association
cannot be described, the sender SHOULD send a new offer without
FEC.
Note that in both cases described above, when the sender of the offer
sends a new offer with the "FEC" semantics, and the node understands
it, the session will be established and the rules pertaining to
[RFC4756] will be valid.
However, if the node does not understand the "FEC" semantics, it
SHOULD respond to the offer either (1) with an answer that ignores
the grouping attribute, or (2) with a refusal to the request. In the
first case, the sender MUST send a new offer without FEC. In the
second case, if the sender of the offer still wishes to establish the
session, it SHOULD retry the request with an offer without FEC.
4.5. ABNF Syntax
Note to the RFC Editor: In the following, please replace "XXXX" with
the number of this document prior to publication as an RFC.
A new semantics ("FEC-XR") is defined for the 'group' attribute
[I-D.ietf-mmusic-rfc3388bis]. Its formatting in SDP is described by
the following ABNF [RFC5234]:
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group-attribute = "a=group:" semantics
*(space identification-tag)
semantics = "LS" / "FID" /
"FEC" / ; from [RFC4756] for backward
; compatibility
"FEC-XR" ; from [RFCXXXX]
identification-tag = token
The identification tags MUST be unique within an SDP session
description.
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 [RFC4566] to only trust SDP from trusted
sources.
6. IANA Considerations
This document registers the following semantics with IANA in
Semantics for the 'group' SDP Attribute under SDP Parameters:
Note to the RFC Editor: In the following, please replace "XXXX" with
the number of this document prior to publication as an RFC.
Semantics Token Reference
--------------------------- ------ ---------
Forward Error Correction XR FEC-XR [RFCXXXX]
This document also registers the following semantics with IANA in
Semantics for the 'ssrc-group' SDP Attribute under SDP Parameters:
Semantics Token Reference
--------------------------- ------ ---------
Forward Error Correction XR FEC-XR [RFCXXXX]
7. Acknowledgments
Some parts of this document are based on [RFC4756]. Thus, the author
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would like to thank those who contributed to [RFC4756]. Also, thanks
to Jonathan Lennox who has contributed to Section 4.3.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[I-D.ietf-mmusic-rfc3388bis]
Camarillo, G., "The SDP (Session Description Protocol)
Grouping Framework", draft-ietf-mmusic-rfc3388bis-03 (work
in progress), July 2009.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
8.2. Informative References
[I-D.ietf-fecframe-framework]
Watson, M., "Forward Error Correction (FEC) Framework",
draft-ietf-fecframe-framework-05 (work in progress),
July 2009.
[RFC4756] Li, A., "Forward Error Correction Grouping Semantics in
Session Description Protocol", RFC 4756, November 2006.
[RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H.
Schulzrinne, "Grouping of Media Lines in the Session
Description Protocol (SDP)", RFC 3388, December 2002.
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Author's Address
Ali Begen
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
Email: abegen@cisco.com
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