Internet DRAFT - draft-ietf-mmusic-rfc4756bis
draft-ietf-mmusic-rfc4756bis
MMUSIC A. Begen
Internet-Draft Cisco
Obsoletes: 4756 June 9, 2010
(if approved)
Intended status: Standards Track
Expires: December 11, 2010
Forward Error Correction Grouping Semantics in Session Description
Protocol
draft-ietf-mmusic-rfc4756bis-10
Abstract
This document defines the semantics for grouping the associated
source and Forward Error Correction (FEC)-based repair flows in the
Session Description Protocol (SDP). The semantics defined in this
document are to be used with the SDP Grouping Framework (RFC
3388bis). These semantics allow the description of grouping
relationships between the source and repair flows when one or more
source and/or repair flow are associated in the same group, and they
provide support for additive repair flows. Synchronization Source
(SSRC)-level grouping semantics are also defined in this document for
Real-time Transport Protocol (RTP) streams using SSRC multiplexing.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on December 11, 2010.
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
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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
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 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 [I-D.ietf-fecframe-framework] outlines a general
framework for using FEC codes in multimedia applications that stream
audio, video or other types of multimedia content. The FEC Framework
specification states that source and repair packets must be carried
in different streams, which are referred to as the source and repair
flows, respectively. At the receiver side, the receivers should know
which flows are the source flows and which 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
[I-D.ietf-mmusic-rfc3388bis] and this RFC for this purpose.
In order to provide applications more flexibility, the FEC Framework
[I-D.ietf-fecframe-framework] allows a source flow to be protected by
multiple FEC schemes, each of which requires an instance of the FEC
Framework. Thus, multiple instances of the FEC Framework may exist
at the sender and the receiver(s). Furthermore, within a single FEC
Framework instance, multiple source flows may be grouped and
protected by one or more repair flows.
The FEC Framework requires the source and repair packets to be
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carried in different streams. When Real-time Transport Protocol
(RTP) [RFC3550] is used to carry the source and repair streams, the
FEC Framework recommends that each stream is carried in its own RTP
session. This provides flexibility in using FEC in a backward-
compatible manner. However, in some scenarios, a single RTP session
may be desired to carry multiple RTP streams via 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 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
decode. Alternatively, different instances (whether they use the
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same FEC scheme or not) 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
[I-D.ietf-fecframe-framework], the SDP grouping semantics for FEC
must support the ability to indicate that:
1. A given source flow is protected by multiple different FEC
schemes.
2. Multiple repair flows are associated with a given FEC scheme.
3. Multiple source flows are grouped prior to applying FEC
protection.
4. One or more repair flow(s) 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 [I-D.ietf-mmusic-rfc3388bis] 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 [I-D.ietf-mmusic-rfc3388bis] 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 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 Section 4.1 and Section 4.3. In addition to
complying with the requirements defined in Section 4.1 and
Section 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 [I-D.ietf-fecframe-framework] 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.
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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
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, they MUST be considered to be
additive. Repair flows that are not additive MUST be indicated in
separate FEC groups. However, if two (or more) repair flows are
additive in an FEC group, it does not necessarily mean that these
repair flows will also be additive in any other FEC group.
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 [I-D.ietf-mmusic-rfc3388bis], 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]. Thi 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 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
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:fec@example.com
a=ssrc:1010 cname:fec@example.com
a=ssrc:2110 cname:fec@example.com
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
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
[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 [I-D.ietf-mmusic-rfc3388bis], the node responds to the offer
either:
o With an answer that ignores the grouping attribute.
In this case, if the original sender of the offer
* does support the "FEC" semantics described in Section 4.4, it
MUST first check whether using the "FEC" semantics will create
any ambiguity or not. 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 With 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
* does support the "FEC" semantics and still wish to establish
the session, it MUST first check whether using the "FEC"
semantics will create any ambiguity or not. 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 [I-D.ietf-mmusic-rfc3388bis], if the node does not
understand the "FEC" semantics, it responds to the offer, either (1)
with an answer that ignores the grouping attribute, or (2) with a
refusal to the request. In the first case, the sender must send a
new offer without FEC. In the second case, if the sender 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 mishandle
other media payload streams. The receiver SHOULD do an integrity
check on SDP and follow the security considerations of SDP [RFC4566]
to only trust SDP from trusted sources.
6. IANA Considerations
This document registers the following semantics with IANA in
Semantics for the 'group' SDP Attribute under SDP Parameters:
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Note to the RFC Editor: In the following registrations, please
replace "XXXX" with the number of this document prior to publication
as an RFC.
Note to IANA: Please note the change in the reference for the "FEC"
semantics.
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
Semantics for the 'ssrc-group' SDP Attribute under SDP Parameters:
Semantics Token Reference
--------------------------- ------ ---------
Forward Error Correction FR FEC-FR [RFCXXXX]
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.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[I-D.ietf-mmusic-rfc3388bis]
Camarillo, G. and H. Schulzrinne, "The SDP (Session
Description Protocol) Grouping Framework",
draft-ietf-mmusic-rfc3388bis-04 (work in progress),
November 2009.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
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[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009.
8.2. Informative References
[I-D.ietf-fecframe-framework]
Watson, M., "Forward Error Correction (FEC) Framework",
draft-ietf-fecframe-framework-08 (work in progress),
June 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: abegen@cisco.com
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