draft-ietf-mmusic-rfc4756bis-05.txt   draft-ietf-mmusic-rfc4756bis-06.txt 
MMUSIC A. Begen MMUSIC A. Begen
Internet-Draft Cisco Systems Internet-Draft Cisco
Obsoletes: 4756 October 19, 2009 Updates: 4756, 3388bis, 5576 February 11, 2010
(if approved)
Updates: 3388bis, 5576
(if approved) (if approved)
Intended status: Standards Track Intended status: Standards Track
Expires: April 22, 2010 Expires: August 15, 2010
Forward Error Correction Grouping Semantics in Session Description Forward Error Correction Grouping Semantics in Session Description
Protocol Protocol
draft-ietf-mmusic-rfc4756bis-05 draft-ietf-mmusic-rfc4756bis-06
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.
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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Drafts. Drafts.
skipping to change at page 1, line 37 skipping to change at page 1, line 49
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This Internet-Draft will expire on April 22, 2010. This Internet-Draft will expire on August 15, 2010.
Copyright Notice Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of Provisions Relating to IETF Documents
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Please review these documents carefully, as they describe your rights publication of this document. Please review these documents
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Abstract include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
The Session Description Protocol (SDP) supports grouping media lines. described in the BSD License.
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 Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 5 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 5
3. Requirements and Changes from RFC 4756 . . . . . . . . . . . . 5 3. Requirements and Changes from RFC 4756 . . . . . . . . . . . . 5
3.1. Source and Repair Flow Association . . . . . . . . . . . . 5 3.1. Source and Repair Flow Association . . . . . . . . . . . . 5
3.2. Support for Additivity . . . . . . . . . . . . . . . . . . 6 3.2. Support for Additivity . . . . . . . . . . . . . . . . . . 6
4. FEC Grouping . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. FEC Grouping . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. New Grouping Semantics . . . . . . . . . . . . . . . . . . 6 4.1. New Grouping Semantics . . . . . . . . . . . . . . . . . . 6
4.2. SDP Example . . . . . . . . . . . . . . . . . . . . . . . 7 4.2. SDP Example . . . . . . . . . . . . . . . . . . . . . . . 7
4.3. Grouping for SSRC-Multiplexed RTP Streams . . . . . . . . 8 4.3. Grouping for SSRC-Multiplexed RTP Streams . . . . . . . . 8
4.4. SDP Offer-Answer Model and Backward Compatibility 4.4. SDP Offer-Answer Model and Backward Compatibility
Considerations . . . . . . . . . . . . . . . . . . . . . . 9 Considerations . . . . . . . . . . . . . . . . . . . . . . 10
4.5. ABNF Syntax . . . . . . . . . . . . . . . . . . . . . . . 10 4.5. ABNF Syntax . . . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11 5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Normative References . . . . . . . . . . . . . . . . . . . 12 8.1. Normative References . . . . . . . . . . . . . . . . . . . 13
8.2. Informative References . . . . . . . . . . . . . . . . . . 12 8.2. Informative References . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction 1. Introduction
Any application that needs a reliable transmission over an unreliable Any application that needs a reliable transmission over an unreliable
packet network has to cope with packet losses. Forward Error packet network has to cope with packet losses. Forward Error
Correction (FEC) is an effective approach that provides reliable Correction (FEC) is an effective approach that provides reliable
transmission particularly in multicast and broadcast applications transmission particularly in multicast and broadcast applications
where the feedback from the receiver(s) is potentially limited. where the feedback from the receiver(s) is potentially limited.
In a nutshell, FEC groups source packets into blocks and applies In a nutshell, FEC groups source packets into blocks and applies
skipping to change at page 3, line 34 skipping to change at page 3, line 34
where an exclusive OR (XOR) operation is applied to a group of where an exclusive OR (XOR) operation is applied to a group of
packets (i.e., source block) to generate a single repair packet. At packets (i.e., source block) to generate a single repair packet. At
the receiver side, this scheme provides a full recovery if only one the receiver side, this scheme provides a full recovery if only one
packet is lost within the source block and the repair packet is packet is lost within the source block and the repair packet is
received. There are various other ways of generating repair packets, received. There are various other ways of generating repair packets,
possibly with different loss-recovery capabilities. possibly with different loss-recovery capabilities.
The FEC Framework [I-D.ietf-fecframe-framework] outlines a general The FEC Framework [I-D.ietf-fecframe-framework] outlines a general
framework for using FEC codes in multimedia applications that stream framework for using FEC codes in multimedia applications that stream
audio, video or other types of multimedia content. The FEC Framework audio, video or other types of multimedia content. The FEC Framework
specification states that source and repair packets MUST be carried specification states that source and repair packets must be carried
in different streams, which are referred to as the source and repair in different streams, which are referred to as the source and repair
flows, respectively. At the receiver side, the receivers should know flows, respectively. At the receiver side, the receivers should know
which flows are the source flows and which flows are the repair which flows are the source flows and which flows are the repair
flows. The receivers should also know the exact association of the flows. The receivers should also know the exact association of the
source and repair flows so that they can use the correct data to source and repair flows so that they can use the correct data to
repair the original content in case there is a packet loss. repair the original content in case there is a packet loss.
Currently, SDP [RFC4566] uses [RFC3388] and [RFC4756] for this Currently, SDP [RFC4566] uses [RFC3388] and [RFC4756] for this
purpose. purpose.
In order to provide applications more flexibility, the FEC Framework In order to provide applications more flexibility, the FEC Framework
[I-D.ietf-fecframe-framework] allows a source flow to be protected by [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 multiple FEC schemes, each of which requires an instance of the FEC
Framework. Thus, multiple instances of the FEC Framework MAY exist Framework. Thus, multiple instances of the FEC Framework may exist
at the sender and the receiver(s). Furthermore, within a single FEC at the sender and the receiver(s). Furthermore, within a single FEC
Framework instance, multiple source flows MAY be grouped and Framework instance, multiple source flows may be grouped and
protected by one or more repair flows. protected by one or more repair flows.
It should be noted that the FEC Framework requires the source and It should be noted that the FEC Framework requires the source and
repair packets to be carried in different streams. When Real-time repair packets to be carried in different streams. When Real-time
Transport Protocol (RTP) [RFC3550] is used to carry the source and Transport Protocol (RTP) [RFC3550] is used to carry the source and
repair streams, the FEC Framework recommends that each stream is repair streams, the FEC Framework recommends that each stream is
carried in its own RTP session. This provides flexibility in using carried in its own RTP session. This provides flexibility in using
FEC in a backward-compatible manner. However, in some scenarios, a FEC in a backward-compatible manner. However, in some scenarios, a
single RTP session may be desired to carry multiple RTP streams via single RTP session may be desired to carry multiple RTP streams via
SSRC multiplexing in order to reduce the port usage. For such SSRC multiplexing in order to reduce the port usage. For such
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SOURCE FLOWS | FEC FRAMEWORK INSTANCE #1 SOURCE FLOWS | FEC FRAMEWORK INSTANCE #1
S3: Source Flow |---------| R3: Repair Flow S3: Source Flow |---------| R3: Repair Flow
| |
|---------| FEC FRAMEWORK INSTANCE #2 |---------| FEC FRAMEWORK INSTANCE #2
| R4: Repair Flow | R4: Repair Flow
Figure 2: Example scenario with two FEC Framework instances, each Figure 2: Example scenario with two FEC Framework instances, each
with a single repair flow protecting the same source flow S3 with a single repair flow protecting the same source flow S3
In summary, based on the FEC Framework [I-D.ietf-fecframe-framework], In summary, based on the FEC Framework [I-D.ietf-fecframe-framework],
the SDP grouping semantics for FEC MUST support the ability to the SDP grouping semantics for FEC must support the ability to
indicate that: indicate that:
1. A given source flow is protected by multiple different FEC 1. A given source flow is protected by multiple different FEC
schemes. schemes.
2. Multiple repair flows are associated with a given FEC scheme. 2. Multiple repair flows are associated with a given FEC scheme.
3. Multiple source flows are grouped prior to applying FEC 3. Multiple source flows are grouped prior to applying FEC
protection. protection.
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[I-D.ietf-mmusic-rfc3388bis] updates [RFC3388] to allow an "m" line [I-D.ietf-mmusic-rfc3388bis] updates [RFC3388] to allow an "m" line
identified by its 'mid' attribute to appear in more than one identified by its 'mid' attribute to appear in more than one
"a=group" line using the same semantics. With this change and other "a=group" line using the same semantics. With this change and other
required changes in the grouping semantics for FEC, a sender is required changes in the grouping semantics for FEC, a sender is
allowed to indicate the specific associations between the source and allowed to indicate the specific associations between the source and
repair flows, and a receiver can determine which repair flow(s) repair flows, and a receiver can determine which repair flow(s)
protect which source flow(s). protect which source flow(s).
This document introduces the changes required in the FEC grouping This document introduces the changes required in the FEC grouping
semantics and obsoletes [RFC4756]. semantics and updates [RFC4756]. New implementations SHOULD use the
new semantics introduced in this document whenever possible, but they
may need to use [RFC4756] semantics when backward compatibility is
desired, as described in Section 4.4.
3.2. Support for Additivity 3.2. Support for Additivity
The FEC Framework also supports additive repair flows. Additivity The FEC Framework also supports additive repair flows. Additivity
among the repair flows means that multiple repair flows may be among the repair flows means that multiple repair flows may be
decoded jointly to improve the recovery chances of the missing decoded jointly to improve the recovery chances of the missing
packets in a single or the same set of source flows. Additive repair 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 flows can be generated by the same FEC scheme or different FEC
schemes. schemes.
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4.1. New Grouping Semantics 4.1. New Grouping Semantics
Each "a=group" line is used to indicate an association relationship Each "a=group" line is used to indicate an association relationship
between the source and repair flows. The flows included in one 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 "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 repair flows included in an FEC group, they MUST be considered to be
additive. Repair flows that are not additive MUST be indicated in additive. Repair flows that are not additive MUST be indicated in
separate FEC groups. However, if two (or more) repair flows are separate FEC groups. However, if two (or more) repair flows are
additive in an FEC group, it does not necessarily mean that these additive in an FEC group, it does not necessarily mean that these
repair flows will also be additive in any other FEC group. repair flows will also be additive in any other FEC group.
Generally, in order to express multiple relations between the source Generally, in order to express multiple relations between the source
and repair flows, each source and repair flow MAY appear in more than and repair flows, each source and repair flow MAY appear in more than
one FEC group. one FEC group.
By extending [I-D.ietf-mmusic-rfc3388bis] we define "FEC-XR" as the By extending [I-D.ietf-mmusic-rfc3388bis] we define "FEC-FR" as the
new grouping semantics that can support the features of the FEC new grouping semantics that can support the features of the FEC
Framework. Framework.
The "a=group:FEC-XR" semantics MUST always be used to associate the The "a=group:FEC-FR" semantics MUST always be used to associate the
source and repair flows except when the source and repair flows are 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. specified in the same media description, i.e., in the same "m" line.
Note that additivity is not necessarily a transitive relation. Thus,
each set of additive repair flows MUST be stated explicitly.
4.2. SDP Example 4.2. SDP Example
For the scenario sketched in Figure 1, we MUST write the following For the scenario sketched in Figure 1, we need to write the following
SDP: SDP:
v=0 v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com o=ali 1122334455 1122334466 IN IP4 fec.example.com
s=New FEC Grouping Semantics s=New FEC Grouping Semantics
t=0 0 t=0 0
a=group:FEC-XR S1 R1 a=group:FEC-FR S1 R1
a=group:FEC-XR S1 S2 R2 a=group:FEC-FR S1 S2 R2
m=video 30000 RTP/AVP 100 m=video 30000 RTP/AVP 100
c=IN IP4 233.252.0.1/127 c=IN IP4 233.252.0.1/127
a=rtpmap:100 MP2T/90000 a=rtpmap:100 MP2T/90000
a=mid:S1 a=mid:S1
m=video 30000 RTP/AVP 101 m=video 30000 RTP/AVP 101
c=IN IP4 233.252.0.2/127 c=IN IP4 233.252.0.2/127
a=rtpmap:101 MP2T/90000 a=rtpmap:101 MP2T/90000
a=mid:S2 a=mid:S2
m=application 30000 RTP/AVP 110 m=application 30000 RTP/AVP 110
c=IN IP4 233.252.0.3/127 c=IN IP4 233.252.0.3/127
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a=fmtp:110 L=5; D=10; repair-window=200000 a=fmtp:110 L=5; D=10; repair-window=200000
a=mid:R1 a=mid:R1
m=application 30000 RTP/AVP 111 m=application 30000 RTP/AVP 111
c=IN IP4 233.252.0.4/127 c=IN IP4 233.252.0.4/127
a=rtpmap:111 1d-interleaved-parityfec/90000 a=rtpmap:111 1d-interleaved-parityfec/90000
a=fmtp:111 L=10; D=10; repair-window=400000 a=fmtp:111 L=10; D=10; repair-window=400000
a=mid:R2 a=mid:R2
In this example, the source and repair flows are carried in their own In this example, the source and repair flows are carried in their own
RTP sessions and the grouping is achieved through the "a=group: RTP sessions and the grouping is achieved through the "a=group:
FEC-XR" lines. FEC-FR" lines.
For the additivity issues, let us consider the scenario sketched in For the additivity issues, let us consider the scenario sketched in
Figure 3. Suppose that repair flows R5 and R6 are additive but Figure 3. Suppose that repair flows R5 and R6 are additive but
repair flow R7 is not additive with any of the other repair flows. repair flow R7 is not additive with any of the other repair flows.
In this case, we MUST write 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-FR S4 R5 R6
a=group:FEC-FR S4 R7
a=group:FEC-XR S4 R5 If none of the repair flows are additive, we must write
a=group:FEC-XR S4 R6
a=group:FEC-XR S4 R7
Note that additivity is not necessarily a transitive relation. Thus, a=group:FEC-FR S4 R5
each set of additive repair flows MUST be stated explicitly. a=group:FEC-FR S4 R6
a=group:FEC-FR S4 R7
4.3. Grouping for SSRC-Multiplexed RTP Streams 4.3. Grouping for SSRC-Multiplexed RTP Streams
[RFC5576] defines a grouping attribute, called 'ssrc-group', for the [RFC5576] defines a grouping attribute, called 'ssrc-group', for the
RTP streams that are SSRC multiplexed and carried in the same RTP RTP streams that are SSRC multiplexed and carried in the same RTP
session. The grouping is based on the Synchronization Source (SSRC) session. The grouping is based on the Synchronization Source (SSRC)
identifiers. Since SSRC-multiplexed RTP streams are defined in the identifiers. Since SSRC-multiplexed RTP streams are defined in the
same "m" line, the 'group' attribute cannot be used. same "m" line, the 'group' attribute cannot be used.
This document extends [RFC5576] in two ways. First, we define how This document extends [RFC5576] in two ways. First, we define how
FEC is applied to source and repair flows for SSRC-multiplexed FEC is applied to source and repair flows for SSRC-multiplexed
streams using the 'ssrc-group' attribute. We then specify how the streams using the 'ssrc-group' attribute. We then specify how the
additivity of the repair flows is expressed for the SSRC-multiplexed additivity of the repair flows is expressed for the SSRC-multiplexed
streams. streams.
Per [RFC3550], the SSRC identifiers for the RTP streams that are Per [RFC3550], the SSRC identifiers for the RTP streams that are
carried in the same RTP session MUST be unique. However, the SSRC carried in the same RTP session MUST be unique. However, the SSRC
identifiers are not guaranteed to be unique among different RTP identifiers are not guaranteed to be unique among different RTP
sessions. Thus, the 'ssrc-group' attribute MUST only be used at the sessions. Thus, the 'ssrc-group' attribute MUST only be used at the
media level [RFC5576]. The semantics of "FEC-XR" for the 'ssrc- media level [RFC5576]. The semantics of "FEC-FR" for the 'ssrc-
group' attribute is exactly the same as the one defined for the group' attribute is exactly the same as the one defined for the
'group' attribute. 'group' attribute.
Let us consider the following scenario where there are two source 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 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 protects only one of the source flows (e.g., video). Suppose that
all these flows are separate RTP streams that are SSRC multiplexed in all these flows are separate RTP streams that are SSRC multiplexed in
the same RTP session. the same RTP session.
SOURCE FLOWS | FEC FRAMEWORK INSTANCE #1 SOURCE FLOWS | FEC FRAMEWORK INSTANCE #1
skipping to change at page 9, line 27 skipping to change at page 10, line 18
t=0 0 t=0 0
m=video 30000 RTP/AVP 100 101 110 m=video 30000 RTP/AVP 100 101 110
c=IN IP4 233.252.0.1/127 c=IN IP4 233.252.0.1/127
a=rtpmap:100 JPEG/90000 a=rtpmap:100 JPEG/90000
a=rtpmap:101 L16/32000/2 a=rtpmap:101 L16/32000/2
a=rtpmap:110 1d-interleaved-parityfec/90000 a=rtpmap:110 1d-interleaved-parityfec/90000
a=fmtp:110 L=5; D=10; repair-window=200000 a=fmtp:110 L=5; D=10; repair-window=200000
a=ssrc:1000 cname:fec@example.com a=ssrc:1000 cname:fec@example.com
a=ssrc:1010 cname:fec@example.com a=ssrc:1010 cname:fec@example.com
a=ssrc:2110 cname:fec@example.com a=ssrc:2110 cname:fec@example.com
a=ssrc-group:FEC-XR 1000 2110 a=ssrc-group:FEC-FR 1000 2110
a=mid:Group1 a=mid:Group1
Note that in actual use, SSRC values, which are random 32-bit Note that in actual use, SSRC values, which are random 32-bit
numbers, may be much larger than the ones shown in this example. numbers, may be much larger than the ones shown in this example.
Also note that before receiving an RTP packet for each stream, the Also note that before receiving an RTP packet for each stream, the
receiver cannot know which SSRC identifier is associated with which receiver cannot know which SSRC identifier is associated with which
payload type. payload type.
The additivity of the repair flows is handled in the same way as 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 described in Section 4.2. In other words, the repair flows that are
skipping to change at page 9, line 49 skipping to change at page 10, line 40
that are not additive MUST be indicated in separate "a=ssrc-group" that are not additive MUST be indicated in separate "a=ssrc-group"
lines. lines.
4.4. SDP Offer-Answer Model and Backward Compatibility Considerations 4.4. SDP Offer-Answer Model and Backward Compatibility Considerations
When offering FEC grouping using SDP in an Offer/Answer model When offering FEC grouping using SDP in an Offer/Answer model
[RFC3264], the following considerations apply. [RFC3264], the following considerations apply.
A node that is receiving an offer from a sender may or may not A node that is receiving an offer from a sender may or may not
understand line grouping. It is also possible that the node understand line grouping. It is also possible that the node
understands line grouping but it does not understand the "FEC-XR" understands line grouping but it does not understand the "FEC-FR"
semantics. From the viewpoint of the sender of the offer, these semantics. From the viewpoint of the sender of the offer, these
cases are indistinguishable. cases are indistinguishable.
When a node is offered a session with the "FEC-XR" grouping semantics When a node is offered a session with the "FEC-FR" grouping semantics
but it does not support line grouping or the FEC grouping semantics, but it does not support line grouping or the FEC grouping semantics,
the node SHOULD respond to the offer either: the node SHOULD respond to the offer either:
o With an answer that ignores the grouping attribute. o With an answer that ignores the grouping attribute.
In this case, the original sender of the offer MUST first check In this case, the original sender of the offer MUST first check
whether using the "FEC" grouping semantics will create any whether using the "FEC" grouping semantics will create any
ambiguity or not, while keeping its limitations in mind. If using ambiguity or not, while keeping its limitations in mind. If using
the "FEC" semantics rather than the "FEC-XR" semantics still the "FEC" semantics rather than the "FEC-FR" semantics still
provides an exact association among the source and repair flows, provides an exact association among the source and repair flows,
the sender of the offer MUST send a new offer using the "FEC" the sender of the offer MUST send a new offer using the "FEC"
semantics. However, if an exact association cannot be described, semantics. However, if an exact association cannot be described,
the sender MUST send a new offer without FEC. the sender MUST send a new offer without FEC.
o With a refusal to the request (e.g., 488 Not Acceptable Here or o With a refusal to the request (e.g., 488 Not Acceptable Here or
606 Not Acceptable in SIP). 606 Not Acceptable in SIP).
In this case, if the sender of the offer still wishes to establish In this case, if the sender of the offer still wishes to establish
the session, it MUST first check whether using the "FEC" grouping the session, it MUST first check whether using the "FEC" grouping
semantics will create any ambiguity or not, while keeping its semantics will create any ambiguity or not, while keeping its
limitations in mind. If using the "FEC" semantics rather than the limitations in mind. If using the "FEC" semantics rather than the
"FEC-XR" semantics still provides an exact association among the "FEC-FR" semantics still provides an exact association among the
source and repair flows, the sender of the offer SHOULD send a new source and repair flows, the sender of the offer SHOULD send a new
offer using the "FEC" semantics. However, if an exact association offer using the "FEC" semantics. However, if an exact association
cannot be described, the sender SHOULD send a new offer without cannot be described, the sender SHOULD send a new offer without
FEC. FEC.
Note that in both cases described above, when the sender of the offer 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 sends a new offer with the "FEC" semantics, and the node understands
it, the session will be established and the rules pertaining to it, the session will be established and the rules pertaining to
[RFC4756] will be valid. [RFC4756] will apply.
However, if the node does not understand the "FEC" semantics, it However, if the node does not understand the "FEC" semantics, it
SHOULD respond to the offer either (1) with an answer that ignores 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 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 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 second case, if the sender of the offer still wishes to establish the
session, it SHOULD retry the request with an offer without FEC. session, it SHOULD retry the request with an offer without FEC.
4.5. ABNF Syntax 4.5. ABNF Syntax
Note to the RFC Editor: In the following, please replace "XXXX" with Note to the RFC Editor: In the following, please replace "XXXX" with
the number of this document prior to publication as an RFC. the number of this document prior to publication as an RFC.
A new semantics ("FEC-XR") is defined for the 'group' attribute A new semantics ("FEC-FR") is defined for the 'group' attribute
[I-D.ietf-mmusic-rfc3388bis]. Its formatting in SDP is described by [I-D.ietf-mmusic-rfc3388bis]. Its formatting in SDP is described by
the following ABNF [RFC5234]: the following ABNF [RFC5234]:
group-attribute = "a=group:" semantics group-attribute = "a=group:" semantics
*(space identification-tag) *(space identification-tag)
semantics = "LS" / "FID" / semantics = "LS" / "FID" /
"FEC" / ; from [RFC4756] for backward "FEC" / ; from [RFC4756] for backward
; compatibility ; compatibility
"FEC-XR" ; from [RFCXXXX] "FEC-FR" ; from [RFCXXXX]
identification-tag = token identification-tag = token
The identification tags MUST be unique within an SDP session The identification tags MUST be unique within an SDP session
description. description.
5. Security Considerations 5. Security Considerations
There is a weak threat for the receiver that the FEC grouping can be There is a weak threat for the receiver that the FEC grouping can be
modified to indicate FEC relationships that do not exist. Such modified to indicate FEC relationships that do not exist. Such
skipping to change at page 11, line 37 skipping to change at page 12, line 37
6. IANA Considerations 6. IANA Considerations
This document registers the following semantics with IANA in This document registers the following semantics with IANA in
Semantics for the 'group' SDP Attribute under SDP Parameters: Semantics for the 'group' SDP Attribute under SDP Parameters:
Note to the RFC Editor: In the following, please replace "XXXX" with Note to the RFC Editor: In the following, please replace "XXXX" with
the number of this document prior to publication as an RFC. the number of this document prior to publication as an RFC.
Semantics Token Reference Semantics Token Reference
--------------------------- ------ --------- --------------------------- ------ ---------
Forward Error Correction XR FEC-XR [RFCXXXX] Forward Error Correction FR FEC-FR [RFCXXXX]
This document also registers the following semantics with IANA in This document also registers the following semantics with IANA in
Semantics for the 'ssrc-group' SDP Attribute under SDP Parameters: Semantics for the 'ssrc-group' SDP Attribute under SDP Parameters:
Semantics Token Reference Semantics Token Reference
--------------------------- ------ --------- --------------------------- ------ ---------
Forward Error Correction XR FEC-XR [RFCXXXX] Forward Error Correction FR FEC-FR [RFCXXXX]
7. Acknowledgments 7. Acknowledgments
Some parts of this document are based on [RFC4756]. Thus, the author Some parts of this document are based on [RFC4756]. Thus, the author
would like to thank those who contributed to [RFC4756]. Also, thanks would like to thank those who contributed to [RFC4756]. Also, thanks
to Jonathan Lennox who has contributed to Section 4.3. to Jonathan Lennox who has contributed to Section 4.3.
8. References 8. References
8.1. Normative References 8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006. Description Protocol", RFC 4566, July 2006.
[I-D.ietf-mmusic-rfc3388bis] [I-D.ietf-mmusic-rfc3388bis]
Camarillo, G., "The SDP (Session Description Protocol) Camarillo, G. and H. Schulzrinne, "The SDP (Session
Grouping Framework", draft-ietf-mmusic-rfc3388bis-03 (work Description Protocol) Grouping Framework",
in progress), July 2009. draft-ietf-mmusic-rfc3388bis-04 (work in progress),
November 2009.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, with Session Description Protocol (SDP)", RFC 3264,
June 2002. June 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003. Applications", STD 64, RFC 3550, July 2003.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific [RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
skipping to change at page 13, line 8 skipping to change at page 14, line 8
[RFC4756] Li, A., "Forward Error Correction Grouping Semantics in [RFC4756] Li, A., "Forward Error Correction Grouping Semantics in
Session Description Protocol", RFC 4756, November 2006. Session Description Protocol", RFC 4756, November 2006.
[RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H. [RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H.
Schulzrinne, "Grouping of Media Lines in the Session Schulzrinne, "Grouping of Media Lines in the Session
Description Protocol (SDP)", RFC 3388, December 2002. Description Protocol (SDP)", RFC 3388, December 2002.
Author's Address Author's Address
Ali Begen Ali Begen
Cisco Systems Cisco
170 West Tasman Drive 170 West Tasman Drive
San Jose, CA 95134 San Jose, CA 95134
USA USA
Email: abegen@cisco.com Email: abegen@cisco.com
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