draft-ietf-mmusic-rfc4756bis-08.txt   draft-ietf-mmusic-rfc4756bis-09.txt 
MMUSIC A. Begen MMUSIC A. Begen
Internet-Draft Cisco Internet-Draft Cisco
Obsoletes: 4756 April 28, 2010 Obsoletes: 4756 May 10, 2010
(if approved) (if approved)
Intended status: Standards Track Intended status: Standards Track
Expires: October 30, 2010 Expires: November 11, 2010
Forward Error Correction Grouping Semantics in Session Description Forward Error Correction Grouping Semantics in Session Description
Protocol Protocol
draft-ietf-mmusic-rfc4756bis-08 draft-ietf-mmusic-rfc4756bis-09
Abstract Abstract
The Session Description Protocol (SDP) supports grouping media lines. This document defines the semantics for grouping the associated
SDP also has semantics defined for grouping the associated source and source and Forward Error Correction (FEC)-based repair flows in the
Forward Error Correction (FEC)-based repair flows. However, the Session Description Protocol (SDP). The semantics defined in this
semantics that were defined in RFC 4756 generally fail to provide the document are to be used with the SDP Grouping Framework (RFC
specific grouping relationships between the source and repair flows 3388bis). These semantics allow the description of grouping
when there is more than one source and/or repair flow in the same relationships between the source and repair flows when one or more
group. Furthermore, the existing semantics do not support describing source and/or repair flow are associated in the same group, and they
additive repair flows. This document addresses these issues by provide support for additive repair flows. Synchronization Source
introducing new FEC grouping semantics. Synchronization Source (SSRC)-level grouping semantics are also defined in this document for
(SSRC)-level grouping semantics are also introduced in this document Real-time Transport Protocol (RTP) streams using SSRC multiplexing.
for Real-time Transport Protocol (RTP) streams using SSRC
multiplexing.
Status of this Memo Status of this Memo
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provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 30, 2010. This Internet-Draft will expire on November 11, 2010.
Copyright Notice Copyright Notice
Copyright (c) 2010 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.
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Provisions Relating to IETF Documents Provisions Relating to IETF Documents
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publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 20 skipping to change at page 2, line 18
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
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 Associations . . . . . . . . . . . 5 3.1. FEC Grouping Requirements . . . . . . . . . . . . . . . . 5
3.2. Support for Additivity . . . . . . . . . . . . . . . . . . 6 3.2. Source and Repair Flow Associations . . . . . . . . . . . 6
4. FEC Grouping . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.3. Support for Additivity . . . . . . . . . . . . . . . . . . 6
4.1. Old "FEC" Grouping Semantics . . . . . . . . . . . . . . . 6 4. FEC Grouping . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. New "FEC-FR" Grouping Semantics . . . . . . . . . . . . . 7 4.1. "FEC-FR" Grouping Semantics . . . . . . . . . . . . . . . 7
4.3. SDP Example . . . . . . . . . . . . . . . . . . . . . . . 7 4.2. SDP Example . . . . . . . . . . . . . . . . . . . . . . . 7
4.4. Grouping for SSRC-Multiplexed RTP Streams . . . . . . . . 8 4.3. FEC Grouping for SSRC-Multiplexed RTP Streams . . . . . . 9
4.5. SDP Offer/Answer Model and Backward-Compatibility 4.4. "FEC" Grouping Semantics . . . . . . . . . . . . . . . . . 10
Considerations . . . . . . . . . . . . . . . . . . . . . . 10 4.5. SDP Offer/Answer Model and RFC 4756
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11 Backward-Compatibility Considerations . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Normative References . . . . . . . . . . . . . . . . . . . 12 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.2. Informative References . . . . . . . . . . . . . . . . . . 13 8.1. Normative References . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13 8.2. Informative References . . . . . . . . . . . . . . . . . . 14
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 improves the Correction (FEC) is an effective approach that improves the
reliabilty of the transmission particularly in multicast and reliability of the transmission particularly in multicast and
broadcast applications where the feedback from the receiver(s) is broadcast applications where the feedback from the receiver(s) is
potentially limited. potentially limited.
In a nutshell, FEC groups source packets into blocks and applies In a nutshell, FEC groups source packets into blocks and applies
protection to generate a desired number of repair packets. These protection to generate a desired number of repair packets. These
repair packets may be sent on demand or independently of any receiver repair packets may be sent on demand or independently of any receiver
feedback. The choice depends on the FEC scheme, the packet loss feedback. The choice depends on the FEC scheme, the packet loss
characteristics of the underlying network, the transport scheme characteristics of the underlying network, the transport scheme
(e.g., unicast, multicast and broadcast) and the application. At the (e.g., unicast, multicast and broadcast) and the application. At the
receiver side, lost packets can be recovered by erasure decoding receiver side, lost packets can be recovered by erasure decoding
skipping to change at page 3, line 38 skipping to change at page 3, line 38
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 ones are the repair flows.
flows. The receivers should also know the exact association of the The receivers should also know the exact association of the source
source and repair flows so that they can use the correct data to and repair flows so that they can use the correct data to repair the
repair the original content in case there is a packet loss. original content in case there is a packet loss. SDP [RFC4566] uses
Currently, SDP [RFC4566] uses [RFC3388] and [RFC4756] for this [I-D.ietf-mmusic-rfc3388bis] and this RFC 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.
The FEC Framework requires the source and repair packets to be The FEC Framework requires the source and repair packets to be
carried in different streams. When Real-time Transport Protocol carried in different streams. When Real-time Transport Protocol
(RTP) [RFC3550] is used to carry the source and repair streams, the (RTP) [RFC3550] is used to carry the source and repair streams, the
FEC Framework recommends that each stream is carried in its own RTP FEC Framework recommends that each stream is carried in its own RTP
session. This provides flexibility in using FEC in a backward- session. This provides flexibility in using FEC in a backward-
compatible manner. However, in some scenarios, a single RTP session compatible manner. However, in some scenarios, a single RTP session
may be desired to carry multiple RTP streams via Synchronization may be desired to carry multiple RTP streams via Synchronization
Source (SSRC) multiplexing in order to reduce the port usage. For Source (SSRC) multiplexing in order to reduce the port usage. For
such scenarios, appropriate grouping semantics are also required. such scenarios, appropriate grouping semantics are also required.
A basic example scenario is shown in Figure 1. Here, source flow S1 A basic example scenario is shown in Figure 1. Here, the source flow
is protected by repair flow R1. Also, source flows S1 and S2 are S1 is protected by the repair flow R1. Also, the source flows S1 and
grouped and protected together by repair flow R2. S2 are grouped and protected together by the repair flow R2.
SOURCE FLOWS | FEC FRAMEWORK INSTANCE #1 SOURCE FLOWS | FEC FRAMEWORK INSTANCE #1
| S1: Source Flow |--------| R1: Repair Flow | S1: Source Flow |--------| R1: Repair Flow
+---| +---|
| | S2: Source Flow | | S2: Source Flow
| |
+______________________________| FEC FRAMEWORK INSTANCE #2 +______________________________| FEC FRAMEWORK INSTANCE #2
| R2: Repair Flow | R2: Repair Flow
Figure 1: Example scenario with two FEC Framework instances where R1 Figure 1: Example scenario with two FEC Framework instances where R1
protects S1, and R2 protects the group of S1 and S2 protects S1, and R2 protects the group of S1 and S2
Grouping source flows before applying FEC protection may allow us to Grouping source flows before applying FEC protection may allow us to
achieve a better coding performance. As a typical scenario, suppose achieve a better coding performance. As a typical scenario, suppose
that source flows S1 and S2 in Figure 1 correspond to the base and that source flows S1 and S2 in Figure 1 correspond to the base and
enhancement layers in a layered video content, respectively. Repair enhancement layers in a layered video content, respectively. The
flow R2 protects the combination of the base and enhancement layers repair flow R2 protects the combination of the base and enhancement
for the receivers who receive both layers, and repair flow R1 layers for the receivers that receive both layers, whereas the repair
protects the base layer only, for the receivers who want the base flow R1 protects the base layer only, for the receivers that want the
layer only, or who receive both layers but prefer FEC protection for base layer only, or receive both layers but prefer FEC protection for
the base layer only due to a bandwidth and/or any other limitation. the base layer only due to a bandwidth or any other limitation.
The grouping semantics defined in this document offer the flexibility The grouping semantics defined in this document offer the flexibility
to determine how source streams are grouped together prior to to determine how source streams are grouped together prior to
applying FEC protection. However, not all FEC schemes may support applying FEC protection. However, not all FEC schemes may support
the full range of the possible scenarios (e.g., when the source the full range of the possible scenarios (e.g., when the source
streams carry different top-level media types such as audio and streams carry different top-level media types such as audio and
video). video).
Using multiple FEC Framework instances for a single source flow Using multiple FEC Framework instances for a single source flow
provides flexibility to the receivers. An example scenario is provides flexibility to the receivers. An example scenario is
skipping to change at page 5, line 21 skipping to change at page 5, line 21
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], 2. Requirements Notation
the SDP grouping semantics for FEC must support the ability to
indicate that: 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. FEC Grouping Requirements
As illustrated in the introduction and 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 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.
4. One or more repair flows protect a group of source flows. 4. One or more repair flow(s) 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 Associations 3.2. Source and Repair Flow Associations
The FEC grouping semantics and 'group' attribute defined in this The FEC grouping semantics defined in this document and the SDP
document and [I-D.ietf-mmusic-rfc3388bis], respectively, are used to 'group' attribute defined in [I-D.ietf-mmusic-rfc3388bis] are used to
associate source and repair flows. This document also specifies how associate source and repair flows. This document also specifies how
the 'group' attribute [I-D.ietf-mmusic-rfc3388bis] in SDP is used to the 'group' attribute is used to group multiple repair flows with one
group multiple repair flows with one or more source flows. or more source flows.
[I-D.ietf-mmusic-rfc3388bis] obsoletes [RFC3388] to allow an "m" line Note that [I-D.ietf-mmusic-rfc3388bis] obsoleted [RFC3388] to allow
identified by its 'mid' attribute to appear in more than one an "m" line identified by its 'mid' attribute to appear in more than
"a=group" line using the same semantics. With this change and other one "a=group" line using the same semantics. With this change and
required changes in the grouping semantics for FEC, a sender is the definitions contained in this document the FEC grouping
allowed to indicate the specific associations between the source and semantics, a sender can indicate the specific associations between
repair flows, and a receiver can determine which repair flow(s) the source and repair flows, and a receiver can determine which
protect which source flow(s). repair flow(s) protect which source flow(s).
This document introduces the changes required in the FEC grouping This document defines the FEC grouping semantics and obsoletes
semantics and obsoletes [RFC4756]. New implementations SHOULD use [RFC4756]. Implementations compliant with this document MUST use the
the new semantics introduced in Section 4.2 and Section 4.4 of this semantics introduced in Section 4.1 and Section 4.3. In addition to
document whenever possible, but they may need to use the obsoleted complying with the requirements defined in Section 4.1 and
semantics given in Section 4.1 when backward compatibility is Section 4.3, implementations are RECOMMENDED to support the "FEC"
desired, as described in Section 4.5. semantics specified in Section 4.4 for backward compatibility reasons
in scenarios described in Section 4.5.
3.2. Support for Additivity 3.3. Support for Additivity
The FEC Framework also supports additive repair flows. Additivity The FEC Framework [I-D.ietf-fecframe-framework] describes support for
among the repair flows means that multiple repair flows may be additive repair flows. Additivity among the repair flows means that
decoded jointly to improve the recovery chances of the missing multiple repair flows may be decoded jointly to improve the recovery
packets in a single or the same set of source flows. Additive repair chances of the missing packets in a single or the same set of source
flows can be generated by the same FEC scheme or different FEC flows. Additive repair flows can be generated by the same FEC scheme
schemes. or different FEC schemes.
For example, in Figure 3, repair flows R5 and R6 may be additive For example, in Figure 3, the repair flows R5 and R6 may be additive
within the FEC Framework instance #1. Alternatively, all three within the FEC Framework instance #1. Alternatively, all three
repair flows R5, R6 and R7 could be additive, too. repair flows R5, R6 and R7 could be additive, too.
SOURCE FLOWS | FEC FRAMEWORK INSTANCE #1 SOURCE FLOWS | FEC FRAMEWORK INSTANCE #1
S4: Source Flow |---------| R5: Repair Flow S4: Source Flow |---------| R5: Repair Flow
| | R6: Repair Flow | | R6: Repair Flow
| |
|---------| FEC FRAMEWORK INSTANCE #2 |---------| FEC FRAMEWORK INSTANCE #2
| R7: Repair Flow | R7: Repair Flow
Figure 3: Example scenario with two FEC Framework instances, where Figure 3: Example scenario with two FEC Framework instances, where
two repair flows in the first instance and a single repair flow in two repair flows in the first instance and a single repair flow in
the second instance protect the same source flow S4 the second instance protect the same source flow S4
4. FEC Grouping This document defines the mechanisms to support additive repair flows
that were not included in [RFC4756].
4.1. Old "FEC" Grouping Semantics
The old "FEC" grouping semantics had been introduced in [RFC4756],
and were based on [RFC3388]. The "FEC" semantics used the "a=group"
line to form an FEC Group to indicate the association relationship
between the source and repair flows.
In the "FEC" semantics, a source or repair flow could only appear in
a single "a=group:FEC" line. Thus, all the source and repair flows
that are somehow related to each other have to be listed in the same
"a=group:FEC" line. For example, for the scenario sketched in
Figure 1, we must write "a=group:FEC S1 S2 R1 R2" regardless of which
repair flows protect which particular source flows. Similarly, for
the scenario sketched in Figure 3, we must write "a=group:FEC S4 R5
R6 R7" regardless of which repair flows are additive.
In certain simple scenarios such as where there is one source flow or 4. FEC Grouping
one repair flow, these limitations will not be a concern. In
scenarios where using the "FEC" grouping semantics will provide an
exact association among the source and repair flows and will not
create any ambiguity, the "FEC" semantics can be safely used when
backward compatibility is essential.
4.2. New "FEC-FR" Grouping Semantics 4.1. "FEC-FR" 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 is more than one "a=group" line are called an FEC Group. If there is more than one
repair flow included in an FEC group, they MUST be considered to be repair flow 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-FR" as the Using the framework in [I-D.ietf-mmusic-rfc3388bis], this document
new grouping semantics that can support the features of the FEC defines "FEC-FR" as the grouping semantics to indicate support for
Framework. the FEC Framework features.
The "a=group:FEC-FR" semantics MUST always be used to associate the The "a=group:FEC-FR" semantics MUST be used to associate the source
source and repair flows except when the source and repair flows are 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 relationship. (See Section 4.3). Note that additivity is not necessarily a
Thus, each set of additive repair flows MUST be stated explicitly. transitive relationship. Thus, each set of additive repair flows
MUST be stated explicitly in SDP as illustrated in the example below.
4.3. SDP Example 4.2. SDP Example
For the scenario sketched in Figure 1, we need to 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=FEC Grouping Semantics
t=0 0 t=0 0
a=group:FEC-FR S1 R1 a=group:FEC-FR S1 R1
a=group:FEC-FR 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
skipping to change at page 8, line 34 skipping to change at page 8, line 34
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-FR" lines. FEC-FR" lines.
For the additivity issues, let us consider the scenario sketched in For the additivity example, 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-FR S4 R5 R6 a=group:FEC-FR S4 R5 R6
a=group:FEC-FR S4 R7 a=group:FEC-FR S4 R7
If none of the repair flows is additive, we must write If none of the repair flows is additive, we must write
a=group:FEC-FR S4 R5 a=group:FEC-FR S4 R5
a=group:FEC-FR S4 R6 a=group:FEC-FR S4 R6
a=group:FEC-FR S4 R7 a=group:FEC-FR S4 R7
4.4. Grouping for SSRC-Multiplexed RTP Streams 4.3. FEC Grouping for SSRC-Multiplexed RTP Streams
[RFC5576] defines a grouping attribute, called 'ssrc-group', for the [RFC5576] defines an SDP media-level attribute, called 'ssrc-group',
RTP streams that are SSRC multiplexed and carried in the same RTP for grouping the RTP streams that are SSRC multiplexed and carried in
session. The grouping is based on the Synchronization Source (SSRC) the same RTP session. The grouping is based on the Synchronization
identifiers. Since SSRC-multiplexed RTP streams are defined in the Source (SSRC) identifiers. Since SSRC-multiplexed RTP streams are
same "m" line, the 'group' attribute cannot be used. defined in the same "m" line, the 'group' attribute cannot be used.
This document extends [RFC5576] in two ways. First, we define how This section specifies how FEC is applied to source and repair flows
FEC is applied to source and repair flows for SSRC-multiplexed for SSRC-multiplexed streams using the 'ssrc-group' attribute
streams using the 'ssrc-group' attribute. We then specify how the [RFC5576]. Thi section also specifies how the additivity of the
additivity of the repair flows is expressed for the SSRC-multiplexed repair flows is expressed for the SSRC-multiplexed streams.
streams.
Per [RFC3550], the SSRC identifiers for the RTP streams that are The semantics of "FEC-FR" for the 'ssrc-group' attribute are the same
carried in the same RTP session MUST be unique. However, the SSRC as the one defined for the 'group' attribute except that the SSRC
identifiers are used to designate the FEC grouping associations:
a=ssrc-group:FEC-FR *(SP ssrc-id) [RFC5576].
The SSRC identifiers for the RTP streams that are carried in the same
RTP session MUST be unique per [RFC3550]. 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-FR" for the 'ssrc- media level [RFC5576].
group' attribute are exactly the same as the one defined for the
'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
S5: Source Flow |--------| R8: Repair Flow S5: Source Flow |--------| R8: Repair Flow
S6: Source Flow S6: Source Flow
Figure 4: Example scenario with one FEC Framework instance, where a Figure 4: Example scenario with one FEC Framework instance, where a
single repair flow protects only one of the source flows single repair flow protects only one of the source flows
The following SDP describes the scenario sketched in Figure 4. The following SDP describes the scenario sketched in Figure 4.
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 for SSRC Multiplexing s=FEC Grouping Semantics for SSRC Multiplexing
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-FR 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 4.3. In other words, the repair flows that are 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 included in an "a=ssrc-group" line MUST be additive. Repair flows
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.
4.5. SDP Offer/Answer Model and Backward-Compatibility Considerations 4.4. "FEC" Grouping Semantics
This document deprecates the usage of the "FEC" semantics: "FEC-FR"
must be used instead. However, it is RECOMMENDED to implement the
"FEC" semantics as defined in this section for backward compatibility
reasons.
The "FEC" grouping semantics had been originally introduced in
[RFC4756]. The "FEC" semantics used the "a=group" line from
[RFC3388] to form an FEC Group to indicate the association
relationship between the source and repair flows.
In the "FEC" semantics, a source or repair flow can only appear in a
single "a=group:FEC" line. Thus, all the source and repair flows
that are somehow related to each other have to be listed in the same
"a=group:FEC" line. For example, for the scenario sketched in
Figure 1, we have to write "a=group:FEC S1 S2 R1 R2" regardless of
which repair flows protect which particular source flows. Similarly,
for the scenario sketched in Figure 3, we have to write "a=group:FEC
S4 R5 R6 R7" regardless of which repair flows are additive. However,
the interpretation of these lines would be ambiguous.
In certain simple scenarios such as where there is one source flow
and one repair flow, these limitations may not be a concern. In
Offer/Answer model scenarios, when the "FEC-FR" semantics are not
understood by the answerer, the "FEC" semantics may be offered
provided that the "FEC" semantics provide an exact association among
the source and repair flows and do not create any ambiguity. See
Section 4.5 for details.
4.5. SDP Offer/Answer Model and RFC 4756 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-FR" 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.
Implementations are RECOMMENDED to support the "FEC" semantics
specified in Section 4.4 for backward compatibility reasons. If the
sender of the offer supports the "FEC" semantics, it SHOULD fall back
to using the "FEC" semantics when the "FEC-FR" semantics are not
understood by the node.
When a node is offered a session with the "FEC-FR" 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 responds to the offer either: the node responds 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, if the original sender of the offer supports the
whether using the "FEC" grouping semantics from Section 4.1 will "FEC" semantics described in Section 4.4, it MUST first check
create any ambiguity or not, while keeping its limitations in whether using the "FEC" semantics will create any ambiguity or
mind. If using the "FEC" semantics rather than the "FEC-FR" not, while keeping its limitations in mind. If using the "FEC"
semantics still provides an exact association among the source and semantics rather than the "FEC-FR" semantics still provides an
repair flows, the sender of the offer MUST send a new offer using exact association among the source and repair flows, the sender
the "FEC" semantics. However, if an exact association cannot be SHOULD send a new offer using the "FEC" semantics. However, if an
described, the sender MUST send a new offer without FEC. exact association cannot be described or the sender does not
support the "FEC" semantics, it 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 and supports the "FEC" semantics, it MUST first check
semantics from Section 4.1 will create any ambiguity or not, while whether using the "FEC" grouping semantics from Section 4.4 will
keeping its limitations in mind. If using the "FEC" semantics create any ambiguity or not, while keeping its limitations in
rather than the "FEC-FR" semantics still provides an exact mind. If using the "FEC" semantics rather than the "FEC-FR"
association among the source and repair flows, the sender of the semantics still provides an exact association among the source and
offer SHOULD send a new offer using the "FEC" semantics. However, repair flows, the sender SHOULD send a new offer using the "FEC"
if an exact association cannot be described, the sender SHOULD semantics. However, if an exact association cannot be described
send a new offer without FEC. or the sender does not support the "FEC" semantics, it SHOULD send
a new offer without FEC.
This behaviour is as specified in [I-D.ietf-mmusic-rfc3388bis]. Note This behavior is as specified in [I-D.ietf-mmusic-rfc3388bis]. Note
that in both cases described above, when the sender of the offer 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 the it, the session will be established and the rules pertaining to the
"FEC" semantics will apply. "FEC" semantics will apply.
If the node does not understand the "FEC" semantics, it responds to If the node does not understand the "FEC" semantics, it responds to
the offer, as specified in [I-D.ietf-mmusic-rfc3388bis], either (1) the offer, as specified in [I-D.ietf-mmusic-rfc3388bis], either (1)
with an answer that ignores the grouping attribute, or (2) with a 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 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 new offer without FEC. In the second case, if the sender still
offer still wishes to establish the session, it SHOULD retry the wishes to establish the session, it should retry the request with an
request with an offer without FEC. offer without FEC.
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
attacks may result in failure of FEC to protect, and/or mishandle attacks may result in failure of FEC to protect, and/or mishandle
other media payload streams. The receiver SHOULD do an integrity other media payload streams. The receiver SHOULD do an integrity
check on SDP and follow the security considerations of SDP [RFC4566] check on SDP and follow the security considerations of SDP [RFC4566]
to only trust SDP from trusted sources. to only trust SDP from trusted sources.
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 registrations, please
the number of this document prior to publication as an RFC. replace "XXXX" with the number of this document prior to publication
as an RFC.
Semantics Token Reference Note to IANA: Please note the change in the reference for the "FEC"
--------------------------- ------ --------- semantics.
Forward Error Correction FR FEC-FR [RFCXXXX]
Semantics Token Reference
--------------------------- ------ ---------
Forward Error Correction (Deprecated) FEC [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 FR FEC-FR [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 4.4. to Jonathan Lennox who has contributed to Section 4.3 and Jean-
Francois Mule who has reviewed this document in great detail and
suggested text edits.
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.
skipping to change at page 13, line 24 skipping to change at page 14, line 24
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.
[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.
Author's Address Author's Address
Ali Begen Ali Begen
Cisco Cisco
170 West Tasman Drive 181 Bay Street
San Jose, CA 95134 Toronto, ON M5J 2T3
USA CANADA
Email: abegen@cisco.com Email: abegen@cisco.com
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