Internet Engineering Task Force (IETF) A. Begen

Internet Engineering Task Force (IETF) A. Begen
Request for Comments: 5956 Cisco
Obsoletes: 4756 September 2010
Category: Standards Track
ISSN: 2070-1721

Forward Error Correction Grouping Semantics
in the Session Description Protocol

Abstract

This document defines the semantics for grouping the associated
source and FEC-based (Forward Error Correction) repair flows in the
Session Description Protocol (SDP). The semantics defined in this
document are to be used with the SDP Grouping Framework (RFC 5888).
These semantics allow the description of grouping relationships
between the source and repair flows when one or more source and/or
repair flows are associated in the same group, and they provide
support for additive repair flows. SSRC-level (Synchronization
Source) grouping semantics are also defined in this document for
Real-time Transport Protocol (RTP) streams using SSRC multiplexing.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5956.

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Copyright Notice

Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction ....................................................3
2. Requirements Notation ...........................................5
3. Requirements and Changes from RFC 4756 ..........................5
3.1. FEC Grouping Requirements ..................................5
3.2. Source and Repair Flow Associations ........................6
3.3. Support for Additivity .....................................6
4. FEC Grouping ....................................................7
4.1. "FEC-FR" Grouping Semantics ................................7
4.2. SDP Example ................................................7
4.3. FEC Grouping for SSRC-Multiplexed RTP Streams ..............9
4.4. "FEC" Grouping Semantics ..................................10
4.5. SDP Offer/Answer Model and RFC 4756
Backward-Compatibility Considerations .....................11
5. Security Considerations ........................................12
6. IANA Considerations ............................................12
7. Acknowledgments ................................................13
8. References .....................................................13
8.1. Normative References ......................................13
8.2. Informative References ....................................14

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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 improves the
reliability of the 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 [FEC-FRAMEWK] 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 ones 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. SDP [RFC4566] uses
[RFC5888] and this RFC for this purpose.

In order to provide applications more flexibility, the FEC Framework
[FEC-FRAMEWK] 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.

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The FEC Framework requires the source and repair packets to be
carried in different streams. When the Real-time Transport Protocol
(RTP) [RFC3550] is used to carry the source and repair streams, the
FEC Framework recommends that each stream be carried in its own RTP
session. This provides flexibility in using FEC in a backward-
compatible manner. However, in some scenarios, it may be desirable
for a single RTP session to carry multiple RTP streams via
Synchronization Source (SSRC) multiplexing in order to reduce the
port usage. For such scenarios, appropriate grouping semantics are
also required.

A basic example scenario is shown in Figure 1. Here, the source flow
S1 is protected by the repair flow R1. Also, the source flows S1 and
S2 are grouped and protected together by the 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. The
repair flow R2 protects the combination of the base and enhancement
layers for the receivers that receive both layers, whereas the repair
flow R1 protects the base layer only, for the receivers that want the
base layer only or that receive both layers but prefer FEC protection
for the base layer only due to a bandwidth or any other limitation.

The grouping semantics defined in this document offer the flexibility
to determine how source streams are grouped together prior to
applying FEC protection. However, not all FEC schemes may 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 to
receive the appropriate repair flow(s) that they can support and

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decode. Alternatively, different instances (whether or not they use
the same FEC scheme) may use larger and smaller source block 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

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. FEC Grouping Requirements

As illustrated in the introduction and based on the FEC Framework
[FEC-FRAMEWK], 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.

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3.2. Source and Repair Flow Associations

The FEC grouping semantics defined in this document and the SDP
"group" attribute defined in [RFC5888] are used to associate source
and repair flows. This document also specifies how the "group"
attribute is used to group multiple repair flows with one or more
source flows.

Note that [RFC5888] obsoleted [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 the
definitions contained in this document of the FEC grouping semantics,
a sender can 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 defines the FEC grouping semantics and obsoletes
[RFC4756]. Implementations compliant with this document MUST use the
semantics introduced in Sections 4.1 and 4.3. In addition to
complying with the requirements defined in Sections 4.1 and 4.3,
implementations are RECOMMENDED to support the "FEC" semantics
specified in Section 4.4 for backward-compatibility reasons in
scenarios described in Section 4.5.

3.3. Support for Additivity

The FEC Framework [FEC-FRAMEWK] describes support for 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, the 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

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This document defines the mechanisms to support additive repair flows
that were not included in [RFC4756].

4. FEC Grouping

4.1. "FEC-FR" 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 is more than one
repair flow included in an FEC group, these repair flows 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. 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.

Using the framework in [RFC5888], this document defines "FEC-FR" as
the grouping semantics to indicate support for the FEC Framework
features.

The "a=group:FEC-FR" semantics MUST 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
(see Section 4.3). Note that additivity is not necessarily a
transitive relationship. Thus, each set of additive repair flows
MUST be stated explicitly in SDP, as illustrated in the example
below.

4.2. SDP Example

For the scenario sketched in Figure 1, we need to write the following
SDP:

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v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com
s=FEC Grouping Semantics
t=0 0
a=group:FEC-FR S1 R1
a=group:FEC-FR 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-FR" lines.

For the additivity example, let us consider the scenario sketched in
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.
In this case, we must write

a=group:FEC-FR S4 R5 R6
a=group:FEC-FR S4 R7

If none of the repair flows is additive, we must write

a=group:FEC-FR S4 R5
a=group:FEC-FR S4 R6
a=group:FEC-FR S4 R7

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4.3. FEC Grouping for SSRC-Multiplexed RTP Streams

[RFC5576] defines an SDP media-level attribute, called "ssrc-group",
for grouping 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 section specifies how FEC is applied to source and repair flows
for SSRC-multiplexed streams using the "ssrc-group" attribute
[RFC5576]. This section also specifies how the additivity of the
repair flows is expressed for the SSRC-multiplexed streams.

The semantics of "FEC-FR" for the "ssrc-group" attribute are the same
as those 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
sessions. Thus, the "ssrc-group" attribute MUST only be used at the
media level [RFC5576].

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.

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.

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v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com
s=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:[email protected]
a=ssrc:1010 cname:[email protected]
a=ssrc:2110 cname:[email protected]
a=ssrc-group:FEC-FR 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. "FEC" Grouping Semantics

This document deprecates the usage of the "FEC" semantics. Sessions
negotiated between two endpoints implementing this specification MUST
use the "FEC-FR" semantics and not the "FEC" semantics. Section 4.5
details how an implementation supporting this specification detects
peers that do not support this specification (based on their SDP
answer to the initial offer). When this occurs, the offering
implementation SHOULD initiate a new offer using the "FEC" semantics
as defined in this section.

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

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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 can be offered, as
long as 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
[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-FR"
semantics. From the viewpoint of the sender of the offer, these
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, but it does not support line grouping or the FEC grouping
semantics, as per [RFC5888], the node responds to the offer with one
of the following:

o An answer that ignores the grouping attribute.

In this case, if the original sender of the offer

* supports the "FEC" semantics described in Section 4.4, it MUST
first check whether or not using the "FEC" semantics will
create any ambiguity. If using the "FEC" semantics still
provides an exact association among the source and repair
flows, the sender SHOULD send a new offer using the "FEC"
semantics. However, if an exact association cannot be
described, it MUST send a new offer without FEC.

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* does not support the "FEC" semantics described in Section 4.4,
it MUST send a new offer without FEC.

o A refusal to the request (e.g., 488 Not Acceptable Here or 606 Not
Acceptable in SIP).

In this case, if the original sender of the offer

* supports the "FEC" semantics and still wishes to establish the
session, it MUST first check whether or not using the "FEC"
semantics will create any ambiguity. If using the "FEC"
semantics still provides an exact association among the source
and repair flows, the sender SHOULD send a new offer using the
"FEC" semantics. However, if an exact association cannot be
described, it SHOULD send a new offer without FEC.

* does not support the "FEC" semantics described in Section 4.4,
it SHOULD send a new offer without FEC.

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 the "FEC"
semantics will apply.

As specified in [RFC5888], if the node does not understand the "FEC"
semantics, it responds to the offer with either (1) an answer that
ignores the grouping attribute or (2) 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 still wishes to establish the session, it
should retry the request with an offer without FEC.

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 to mishandle,
other media payload streams. The receiver SHOULD do an integrity
check on SDP and follow the security considerations of SDP [RFC4566]
to trust only SDP from trusted sources.

6. IANA Considerations

This document registers the following semantics with IANA in the
"Semantics for the "group" SDP Attribute" registry under SDP
Parameters:

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Semantics Token Reference
------------------------------------- ------ ---------
Forward Error Correction (Deprecated) FEC [RFC5956]
Forward Error Correction FR FEC-FR [RFC5956]

This document also registers the following semantics with IANA in the
"Semantics for the "ssrc-group" SDP Attribute" registry under SDP
Parameters:

Token Semantics Reference
------- ----------------------------- ---------
FEC-FR Forward Error Correction FR [RFC5956]

7. Acknowledgments

Some parts of this document are based on [RFC4756]. Thus, the author
would like to thank those who contributed to [RFC4756]. Also, thanks
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

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[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.

[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP:
Session Description Protocol", RFC 4566, July 2006.

[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009.

[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session
Description Protocol (SDP) Grouping Framework",
RFC 5888, June 2010.

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8.2. Informative References

[FEC-FRAMEWK] Watson, M., "Forward Error Correction (FEC)
Framework", Work in Progress, September 2010.

[RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H.
Schulzrinne, "Grouping of Media Lines in the Session
Description Protocol (SDP)", RFC 3388, December 2002.

[RFC4756] Li, A., "Forward Error Correction Grouping Semantics
in Session Description Protocol", RFC 4756,
November 2006.

Author's Address

Ali Begen
Cisco
181 Bay Street
Toronto, ON M5J 2T3
Canada

EMail: [email protected]

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