Network Working Group T. Schierl
Internet-Draft Fraunhofer HHI
Intended status: Standards Track S. Wenger
Expires: August 24, 2008 Nokia
February 25, 2008
Signaling media decoding dependency in Session Description Protocol
(SDP)
draft-ietf-mmusic-decoding-dependency-01
Status of this Memo
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
This memo defines semantics that allow for signaling the decoding
dependency of different media descriptions with the same media type in
the Session Description Protocol (SDP). This is required, for example,
if media data is separated and transported in different network streams
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as a result of the use of a layered or multiple descriptive media coding
process.
A new grouping type "DDP" -- decoding dependency -- is defined, to be
used in conjunction with RFC 3388 entitled "Grouping of Media Lines in
the Session Description Protocol". In addition, an attribute is
specified describing the relationship of the media streams in a "DDP"
group indicated by media identification attribute(s) and RTP payload
type(s).
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Table of Contents
1. Introduction .................................................. 4
2. Terminology ................................................... 4
3. Definitions ................................................... 5
4. Motivation, Use Cases, and Architecture ....................... 6
4.1. Motivation .................................................. 6
4.2. Use cases ................................................... 7
5. Signaling Media Dependencies .................................. 8
5.1. Design Principles ........................................... 8
5.2. Semantics ................................................... 8
5.2.1. SDP grouping semantics for decoding dependency............. 8
5.2.2. Attribute for dependency signaling per media-stream........ 9
6. Usage of new semantics in SDP ................................ 10
6.1. Usage with the SDP Offer/Answer Model ...................... 10
6.2. Declarative usage .......................................... 10
6.3. Usage with Capability Negotiation .......................... 10
6.4. Examples ................................................... 11
7. Security Considerations ...................................... 12
8. IANA Considerations .......................................... 12
9. Open Issues .................................................. 13
10. References ................................................... 13
10.1. Normative References ...................................... 13
10.2. Informative References .................................... 13
Appendix A. Changes From Earlier Versions........................ 14
Authors' Addresses................................................ 15
Full Copyright Statement.......................................... 15
Intellectual Property Statement................................... 15
Acknowledgements.................................................. 16
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1. Introduction
An SDP session description may contain one or more media
descriptions, each identifying a single media stream. A media
description is identified by one "m=" line. Today, if more than one
"m=" lines exist indicating the same media type, a receiver cannot
identify a specific relationship between those media.
A Multiple Description Coding (MDC) or layered Media Bitstream
contains, by definition, one or more Media Partitions that are
conveyed in their own media stream. In Multi View Coding (MVC) [I-
D.wang-avt-rtp-mvc] layered dependencies between views are used to
increase the coding efficiency. The cases we are interested in are
layered and MDC Bitstreams with two or more Media Partitions.
Carrying more than one Media Partition in its own session is one of
the key use cases for employing layered or MDC coded media Senders,
network elements, or receivers can suppress
sending/forwarding/subscribing/decoding individual Media Partitions
and still preserve perhaps suboptimal, but still useful media
quality.
One property of all Media Bitstreams relevant to this memo is that
their Media Partitions have a well-defined usage relationship. For
example, in layered coding, "higher" Media Partitions are useless
without "lower" ones. In MDC coding, Media Partitions are
complementary -- the more Media Partitions one receives, the better a
reproduced quality may be. At present, SDP and its supporting
infrastructure of RFCs lack the means to express such a usage
relationship.
Trigger for the present memo has been the standardization process of
the RTP payload format for the Scalable Video Coding extension to
ITU-T Rec. H.264 / MPEG-4 AVC [I-D.ietf-avt-rtp-svc]. When drafting
[I-D.ietf-avt-rtp-svc] , it was observed that the aforementioned lack
in signaling support is one that is not specific to SVC, but applies
to all layered or MDC codecs. Therefore, this memo presents a
generic solution.
The mechanisms defined herein are media transport protocol dependent,
i.e. applicable to the use of RTP [RFC3550] only.
The SDP grouping of Media Lines of different media types is out of
scope of this memo.
2. Terminology
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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 BCP 14, RFC 2119
[RFC2119].
3. Definitions
Media stream:
As per [RFC4566].
Media Bitstream:
A valid, decodable stream, containing all media partitions generated
by the encoder. A Media Bitstream normally conforms to a media
coding standard.
Media Partition:
A subset of a Media Bitstream intended for independent
transportation. An integer number of Media Partitions forms a Media
Bitstream. In layered coding, a Media Partition represents one or
more layers that are handled as a unit. In MDC coding, a Media
Partition represents one or more descriptions that are handled as a
unit.
Decoding dependency:
The class of relationships media partitions have to each other. At
present, this memo defines two decoding dependencies: layering and
multiple description.
Layered coding dependency:
Each Media Partition is only useful (i.e. can be decoded) when all of
the Media Partitions it depends on are available. The dependencies
between the Media Partitions therefore create a directed graph.
Note: normally, in layered coding, the more Media Partitions are
employed (following the rule above), the better a reproduced quality
is possible.
Multi description coding (MDC) dependency:
N of M Media Partitions are required to form a Media Bitstream, but
there is no hierarchy between these Media Partitions. Most MDC
schemes aim at an increase of reproduced media quality when more
media partitions are decoded. Some MDC schemes require more than one
Media Partition to form an Operation point.
Operation point:
In layered coding, a subset of a layered Media Bitstream that
includes all Media Partitions required for reconstruction at a
certain point of quality, error resilience, or another property, and
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does not include any other Media Partitions. In MDC coding, a subset
of an MDC Media Bitstream that is compliant with the MDC coding
standard in question.
4. Motivation, Use Cases, and Architecture
4.1. Motivation
This memo is concerned with two types of decoding dependencies:
layered, and multi-description. The transport of layered and multi
description coding share as key motivators the desire for media
adaptation to network conditions, i.e. related to bandwidth, error
rates, connectivity of endpoints in multicast or broadcast scenarios,
and similar.
o Layered decoding dependency:
In layered coding, the partitions of a Media Bitstream are known as
media layers or simply layers. One or more layers may be transported
in different media streams in the sense of [RFC4566]. A classic use
case is known as receiver-driven layered multicast, in which a
receiver selects a combination of media streams in response to
quality or bit-rate requirements.
Back in the mid 1990s, the then available layered media formats and
codecs envisioned primarily (or even exclusively) a one-dimensional
hierarchy of layers. That is, each so-called enhancement layer
referred to exactly one layer "below". The single exception has been
the base layer, which is self-contained. Therefore, the
identification of one enhancement layer fully specifies the operation
point of a layered coding scheme, including knowledge about all the
other layers that need to be decoded.
[RFC4566] contains rudimentary support for exactly this use case and
media formats, in that it allows for signaling a range of transport
addresses in a certain media description. By definition, a higher
transport address identifies a higher layer in the one-dimensional
hierarchy. A receiver needs only to decode data conveyed over this
transport address and lower transport addresses to decode this
Operation Point.
Newer media formats depart from this simple one-dimensional
hierarchy, in that highly complex (at least tree-shaped) dependency
hierarchies can be implemented. Compelling use cases for these
complex hierarchies have been identified by industry. Support for it
is therefore desirable. However, SDP, in its current form, does not
allow for the signaling of these complex relationships. Therefore,
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receivers cannot make an informed decision on which layers to
subscribe (in case of layered multicast).
Layered decoding dependency may also exit in a Multi View Coding
environment. Views may be coded using inter-view dependencies to
increase coding efficiency. This results in Media Bitstreams, which
logically may be separated into Media Partitions representing
different views of the reconstructed video signal. These Media
Partitions cannot be decoded independently, and, therefore, other
Media Partitions are required for reconstruction. To express this
relationship, the signaling needs to express the dependencies of the
views, which in turn are Media Partitions in the sense of this
document.
o Multi descriptive decoding dependency:
In the most basic form of MDC, each Media Partition forms an
independent representation of the media. That is, decoding of any of
the Media Partitions yields useful reproduced media data. When more
than one Media Partition is available, then a decoder can process
them jointly, and the resulting media quality increases. The highest
reproduced quality is available if all original Media Partitions are
available for decoding.
More complex forms of multiple description coding can also be
envisioned, i.e. where, as a minimum, N out of M total Media
Partitions need to be available to allow meaningful decoding.
MDC has not yet been embraced heavily by the media standardization
community, though it is subject of a lot of academic research. As an
example, we refer to [MDC].
In this memo, we cover MDC because we a) envision that MDC media
formats will come into practical use within the lifetime of this
memo, and b) the solution for its signaling is very similar to the
one of layered coding.
4.2. Use cases
o Receiver driven layered multicast
This technology is discussed in [RFC3550] and references therein. We
refrain from elaborating further; the subject is well known and
understood.
o Multiple end-to-end transmission with different properties
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Assume a unicast and point-to-point topology, wherein one endpoint
sends media to another. Assume further that different forms of media
transmission are available. The difference may lie in the cost of
the transmission (free, charged), in the available protection
(unprotected/secure), in the quality of service (guaranteed quality /
best effort), or other factors.
Layered and MDC coding allow to match the media characteristics to
the available transmission path(s). For example, in layered coding
it makes sense to convey the base layer over high QoS. Enhancement
layers, on the other hand, can be conveyed over best effort, as they
are "optional" in their characteristic -- nice to have, but non-
essential for media consumption. In a different scenario, the base
layer may be offered in a non-encrypted session as a free preview.
An encrypted enhancement layer references this base layer and allows
optimal quality play-back; however, it is only accessible to users
who have the key, which may have been distributed by a conditional
access mechanism.
5. Signaling Media Dependencies
5.1. Design Principles
The dependency signaling is only feasible between media descriptions
described with an "m="-line and with an assigned media identification
attribute ("mid"), as defined in [RFC3388].
5.2. Semantics
5.2.1. SDP grouping semantics for decoding dependency
This specification defines a new grouping semantic
Decoding Dependency "DDP":
DDP associates a media stream, identified by its mid attribute, with
a DDP group. Each media stream MUST be composed of an integer number
of Media Partitions. A media stream is identified by a session-
unique RTP payload type number within a "m="-line. In a DDP group,
all media streams MUST have the same type of decoding dependency (as
signaled by the attribute defined in 5.2.2). All media streams MUST
contain at least one operation point. The DDP group type informs a
receiver about the requirement for treating the media streams of the
group according to the new media level attribute "depend", as defined
in 5.2.2.
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When using multiple codecs, e.g. for Offer/Answer model, the media
streams MUST have the same dependency structure, regardless which
payload type number is used.
5.2.2. Attribute for dependency signaling per media-stream
This memo defines a new media-level attribute, "depend", with the
following ABNF [RFC4234]. The "identification-tag" is defined in
[RFC3388]:
depend-attribute = "a" "=" "depend" ":"
( dependent-payload-type dependency-tag ";" )
*( SP dependent-payload-type dependency-tag ";" )
CRLF
dependency-tag = dependency-type
*1( SP identification-tag ":"
payload-type-dependency
*( "," payload-type-dependency )
)
dependency-type = "lay" / "mdc"
"dependent-payload-type", indicates the payload type number, as
defined in [RFC4566], that depends on a "payload-type-dependency" in
the "m="-line indicated by the value of "identification-tag" within
the "dependency-tag".
"payload-type-dependency", indicates the payload type number in the
"m="-line identified by the "identification-tag" within the
"dependency-tag", which the "dependent-payload-type" number of the
dependent "m="-line depends on.
The "depend"-attribute describes the decoding dependency. The
"depend"-attribute MAY be followed by a sequence of "dependency-
tag"(s) which identify all related RTP payload types in all related
"m="-lines. The attribute MAY be used with multicast as well as with
unicast transport addresses. The following types of dependencies are
defined:
o lay: Layered decoding dependency -- identifies the described media
stream as one or more Media Partitions of a layered Media Bitstream.
When "lay" is used, all required media streams for the Operation
Point MUST be identified by "identification-tag" and "payload-type-
dependency" following the "lay" string.
o mdc: Multi descriptive coding dependency -- signals that the
described media stream is part of a set of a MDC Media Bitstream. By
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definition, at least N out of M media streams of the group need to be
available to from an Operation Point. The values of N and M depend on
the properties of the Media Bitstream and are not signaled within
this context. When "mdc" is used, all required media streams for the
Operation Point MUST be identified by "identification-tag" and
"payload-type-dependency" following the "mdc" string.
6. Usage of new semantics in SDP
6.1. Usage with the SDP Offer/Answer Model
The backward compatibility in offer / answer is generally handled as
specified in [RFC3388].
Depending on the implementation, a node that does not understand DDP
grouping (either does not understand line grouping at all, or just
does not understand the DDP semantics) SHOULD respond to an offer
containing DDP grouping either (1) with an answer that ignores the
grouping attribute or (2) with a refusal to the request (e.g., 488
Not acceptable here or 606 Not acceptable in SIP).
In the first case, the original sender of the offer MUST respond by
offering a single media stream that represents an Operation Point.
Note: in most cases, this will be the base layer of a layered Media
Bitstream, equally possible are Operation Points containing a set of
enhancement layers as long as all are part of a single media stream.
In the second case, if the sender of the offer still wishes to
establish the session, it SHOULD re-try the request with an offer
including only a single media stream.
6.2. Declarative usage
If an RTSP receiver understands signaling according to this memo, it
SHALL setup all media streams that are required to decode the
Operation Point of its choice.
If an RTSP receiver does not understand the signaling defined within
this memo, it falls back to normal SDP processing. Two likely cases
have to be distinguished: (1) if at least one of the media types
included in the SDP is within the receiver's capabilities, it selects
among those candidates according to implementation specific criteria
for setup, as usual. (2) If none of the media type included in the
SDP can be processed, then obviously no setup can occur.
6.3. Usage with Capability Negotiation
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This memo does not cover the interaction with Capability Negotiation
[I-D.ietf-mmusic-sdp-capability-negotiation]. This issue should be
addressed in a different memo.
6.4. Examples
a.) Example for signaling layered decoding dependency dependency:
v=0
o=svcsrv 289083124 289083124 IN IP4 host.example.com
s=LAYERED VIDEO SIGNALING Seminar
t=0 0
c=IN IP4 192.0.2.1/127
a=group:DDP 1 2 3 4
m=video 40000 RTP/AVP 94 194
b=AS:96
a=framerate:15
a=rtpmap:94 H264/90000
a=rtpmap: 194 H264/90000
a=mid:1
m=video 40002 RTP/AVP 95 195
b=AS:64
a=framerate:15
a=rtpmap:95 H264-SVC/90000
a=rtpmap:195 H264-SVC/90000
a=mid:2
a=depend:95 lay 1:94,194; 195 lay 1:194;
m=video 40004 RTP/AVP 96 196
b=AS:128
a=framerate:30
a=rtpmap:96 H264-SVC/90000
a=rtpmap:196 H264-SVC/90000
a=mid:3
a=depend:96 lay 1:94,194; 196 lay 1:194;
m=video 40004 RTP/SAVP 100 200
c=IN IP4 192.0.2.2/127
b=AS:512
k=uri:conditional-access-server.example.com
a=framerate:30
a=rtpmap:100 H264-SVC/90000
a=rtpmap:200 H264-SVC/90000
a=mid:4
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a=depend:100 lay 1:94,194 3:96; 200 lay 1:194 3:196;
b.) Example for signaling of multi descriptive coding dependency:
v=0
o=mdcsrv 289083124 289083124 IN IP4 host.example.com
s=MULTI DESCRIPTION VIDEO SIGNALING Seminar
t=0 0
c=IN IP4 192.0.2.1/127
a=group:DDP 1 2 3
m=video 40000 RTP/AVP 94
a=mid:1
a=depend:94 mdc 2:95 3:96;
m=video 40002 RTP/AVP 95
a=mid:2
a=depend:95 mdc 1:94 3:96;
m=video 40004 RTP/AVP 96
c=IN IP4 192.0.2.2/127
a=mid:3
a=depend:96 mdc 1:94 2:95;
7. Security Considerations
All security implications of SDP apply.
There may be a risk of manipulation the dependency signaling of a
session description by an attacker. This may mislead a receiver or
middle box, e.g. a receiver may try to compose a bitstream that does
not form an Operation Point, although the signaling made it believe
it would form a valid Operation Point, with potential fatal
consequences for the media decoding process. It is recommended that
the receiver SHOULD perform an integrity check on SDP and follow the
security considerations of SDP to only trust SDP from trusted
sources.
8. IANA Considerations
This document defines the "DDP" semantics to be used with grouping of
media lines in SDP as defined in RFC 3388. The "DDP" semantics
defined in this memo are to be registered by the IANA when it is
published in standard track RFCs.
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The attribute "depend" is to be registered by IANA as a new media-
level attribute. The purpose of this attribute is to express a
dependency, which may exist between "m"-lines of a media session.
9. Open Issues
- Requirement on media stream: With the new draft, different media
streams can be present in a DDP group, that is different codecs
may be used within the same DDP group?
- IANA registration for 'lay' and 'mdc'?
10. References
10.1. Normative References
[RFC4566] Handley, M., Jacobson, V, and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC3388] Camarillo, G., Holler, J., and H. Schulzrinne, "Grouping of
Media Lines in the Session Description Protocol (SDP)",
RFC 3388, December 2002.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 5234, January 2008.
[I-D.ietf-mmusic-sdp-capability-negotiation]
Andreasen, F., "SDP Capability Negotiation",
draft-ietf-mmusic-sdp-capability-negotiation-08, (work in
progress), December 2008
10.2. Informative References
[I-D.ietf-avt-rtp-svc]
Wenger, S., Wang Y.-K. and T. Schierl, "RTP Payload Format
for SVC Video", draft-ietf-avt-rtp-svc-07 (work in
progress), February 2008.
[MDC] Vitali, A., Borneo, A., Fumagalli, M., and R. Rinaldo,
"Video over IP using Standard-Compatible Multiple
Description Coding: an IETF proposal", Packet Video
Workshop, April 2006, Hangzhou, China
[I-D.wang-avt-rtp-mvc]
Wang, Y.-K. and T. Schierl, "RTP Payload Format
for MVC Video", draft-wang-avt-rtp-mvc-00 (work in
progress), November 2007.
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Appendix A. Changes From Earlier Versions
A.1 Changes from individual submission
19Dec06 / TS:
removed SSRC multiplexing and with that various information about RTP
draft title correction
corrected SDP reference
editorial modifications throughout the document
added Stephan Wenger to the list of authors
removed section "network elements not supporting dependency
signaling"
20-28Dec06 / TS, StW: Editorial improvements
3Mar07 / TS: adjustment for new I-D style, added Offer/Answer text,
corrected ABNF reference, added Security and IANA considerations,
added section Usage with existing entities not supporting new
signaling, added text for Declarative usage section, added Open
issues section.
21-Jun07: Numerous editorial changes and reworked section 6.
11-Nov07: Added Payload Type of media stream in question to
dependency signaling. Note on usage with Cap. Negotiation. Added
multi view coding (MVC) dependency as part of 'lay'-dependency. Added
ref. to MVC activity at ITU-T/MPEG.
A.2 Changes from draft-ietf-mmusic-decoding-dependency-00 to
draft-ietf-mmusic-decoding-dependency-01:
21-Feb08: Enhanced mechanism by multiple "payload-type-dependencies"
for the same "mid". Typically the case, when using different
packetization modes as defined in RFC3984.
25-Feb08: Modification throughout informative part of definition
section
Different codecs may be present within the same DDP group.
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Authors' Addresses
Thomas Schierl
Fraunhofer HHI
Einsteinufer 37
D-10587 Berlin
Germany
Phone: +49-30-31002-227
Email: schierl@hhi.fhg.de
Stephan Wenger
Nokia
955 Page Mill Road
Palo Alto, CA, 94304
USA
Phone: +1-650-862-7368
Email: stewe@stewe.org
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