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Network Working Group T. Schierl
Request for Comments: 5583 Fraunhofer HHI
Category: Standards Track S. Wenger
Independent
July 2009
Signaling Media Decoding Dependency in
the Session Description Protocol (SDP)
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 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 media format
description(s).
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 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 in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
Schierl & Wenger Standards Track [Page 1]
RFC 5583 Signaling Media Decoding Dependency in SDP July 2009
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................4
3. Definitions .....................................................4
4. Motivation, Use Cases, and Architecture .........................5
4.1. Motivation .................................................5
4.2. Use Cases ..................................................7
5. Signaling Media Dependencies ....................................7
5.1. Design Principles ..........................................7
5.2. Semantics ..................................................8
5.2.1. SDP Grouping Semantics for Decoding Dependency ......8
5.2.2. "depend" Attribute for Dependency Signaling
per Media-Stream ....................................8
6. Usage of New Semantics in SDP ..................................10
6.1. Usage with the SDP Offer/Answer Model .....................10
6.2. Declarative usage .........................................12
6.3. Usage with AVP and SAVP RTP Profiles ......................12
6.4. Usage with Capability Negotiation .........................12
6.5. Examples ..................................................12
7. Security Considerations ........................................15
8. IANA Considerations ............................................15
9. Informative Note on "The SDP (Session Description Protocol)
Grouping Framework" ............................................16
10. References ....................................................16
10.1. Normative References .....................................16
10.2. Informative References ...................................17
Appendix A. Acknowledgements .....................................18
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RFC 5583 Signaling Media Decoding Dependency in SDP July 2009
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. 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. This document defines an SDP extension to
indicate such a decoding dependency.
The trigger for the present memo has been the standardization process
of the RTP payload format for the Scalable Video Coding (SVC)
extension to ITU-T Rec. H.264 / MPEG-4 AVC [AVT-RTP-SVC]. When
drafting [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. Likely, the second technology utilizing the
mechanisms of this memo will be Multi-View video coding. In Multi-
View Coding (MVC) [AVT-RTP-MVC], layered dependencies between views
are used to increase the coding efficiency, and, therefore, the
properties of MVC with respect to the SDP signaling are comparable to
those of SVC.
The mechanisms defined herein are media transport protocol dependent,
and applicable only in conjunction with the use of RTP [RFC3550].
The SDP grouping of Media Lines of different media types is out of
scope of this memo.
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RFC 5583 Signaling Media Decoding Dependency in SDP July 2009
2. Terminology
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: layered coding
and Multiple Description Coding.
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.
Multiple 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
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certain point of quality, error resilience, or another property, and
that 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 Multiple
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 the like.
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.
SDP [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
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current form, does not allow for the signaling of these complex
relationships. Therefore, receivers cannot make an informed
decision on which layers to subscribe (in case of layered
multicast).
Layered decoding dependencies may also exist in a Multi-View
Coding environment. Views may be coded using inter-view
dependencies to increase coding efficiency. This results in Media
Bitstreams, that 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 Multiple 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 the 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.
o Other decoding dependency relationships:
At the time of writing, no decoding dependency relationships
beyond the two mentioned above have been identified that would
warrant standardization. However, the mechanisms of this memo
could be extended by introducing new codepoints for new decoding
dependency types. If such an extension becomes necessary, as
formally required in Section 5.2.2, the new decoding dependency
type MUST be documented in an IETF Standards-Track document.
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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:
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 (QoS)
(guaranteed quality / best effort), or other factors.
Layered and MDC coding allows matching of 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]. All media descriptions
grouped according to this specification MUST have the same media
type. Other dependencies relations expressed by SDP grouping have to
be addressed in other specifications. A media description MUST NOT
be part of more than one group of the grouping type defined in this
specification.
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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 media format description (RTP payload type number) within a
media description. In a DDP group, all media streams MUST have the
same type of decoding dependency (as signaled by the attribute
defined in Section 5.2.2). All media streams MUST contain at least
one Operation Point. The DDP group type informs a receiver about the
requirement for handling the media streams of the group according to
the new media level attribute "depend", as defined in Section 5.2.2.
When using multiple codecs, e.g., for the Offer/Answer model, the
media streams MUST have the same dependency structure, regardless of
which media format description (RTP payload type number) is used.
5.2.2. "depend" Attribute for Dependency Signaling per Media-Stream
This memo defines a new media-level attribute, "depend", with the
following ABNF [RFC5234]. The identification-tag is defined in
[RFC3388]. In the following ABNF, fmt, token, SP, and CRLF are used
as defined in [RFC4566].
<CODE BEGINS>
Copyright (c) 2009 IETF Trust and the persons identified as authors
of the code. All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
- Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
- Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the
distribution.
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RFC 5583 Signaling Media Decoding Dependency in SDP July 2009
- Neither the name of Internet Society, IETF or IETF Trust, nor the
names of specific contributors, may be used to endorse or promote
products derived from this software without specific prior written
permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
depend-attribute =
"a=depend:" dependent-fmt SP dependency-tag
*(";" SP dependent-fmt SP dependency-tag) CRLF
dependency-tag =
dependency-type *1( SP identification-tag ":"
fmt-dependency *("," fmt-dependency ))
dependency-type = "lay"
/ "mdc"
/ token
dependent-fmt = fmt
fmt-dependency = fmt
<CODE ENDS>
dependency-tag indicates one or more dependencies of one dependent-
fmt in the media description. These dependencies are signaled as
fmt-dependency values, which indicate fmt values of other media
descriptions. These other media descriptions are identified by their
identification-tag values in the depend-attribute. There MUST be
exactly one dependency-tag indicated per dependent-fmt.
dependent-fmt indicates the media format description, as defined in
[RFC4566], that depends on one or more media format descriptions in
the media description indicated by the value of the identification-
tag within the dependency-tag.
fmt-dependency indicates the media format description in the media
description identified by the identification-tag within the
dependency-tag, on which the dependent-fmt of the dependent media
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RFC 5583 Signaling Media Decoding Dependency in SDP July 2009
description depends. In case a list of fmt-dependency values is
given, any element of the list is sufficient to satisfy the
dependency, at the choice of the decoding entity.
The depend-attribute describes the decoding dependency. The depend-
attribute MUST be followed by a sequence of dependent-fmt and the
corresponding dependency-tag fields, which identify all related media
format descriptions in all related media descriptions of the
dependent-fmt. The attribute MAY be used with multicast as well as
with unicast transport addresses. The following dependency-type
values are defined in this memo:
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 media streams required
for decoding the Operation Point MUST be identified by
identification-tag and fmt-dependency following the "lay"
string.
o mdc: Multi-descriptive decoding dependency -- signals that the
described media stream is part of a set of a MDC Media
Bitstream. By 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 fmt-
dependency following the "mdc" string.
Further, dependency types MUST be defined in a Standards-Track
document.
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 Section 8.4 of [RFC3388], as summarized below.
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).
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RFC 5583 Signaling Media Decoding Dependency in SDP July 2009
In case (1), if the original sender of the offer still wishes to
establish communications, it SHOULD generate a new offer with 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 case
(2), if the sender of the original offer has identified that the
refusal to the request is caused by the use of DDP grouping, and if
the sender of the offer still wishes to establish the session, it
SHOULD retry the request with an offer including only a single media
stream.
If the answerer understands the DDP semantics, it is necessary to
take the "depend" attribute into consideration in the Offer/Answer
procedure. The main rule for the "depend" attribute is that the
offerer decides the number of media streams and the dependency
between them. The answerer cannot change the dependency relations.
For unicast sessions where the answerer receives media, i.e., for
offers including media streams that have a directionality indicated
by "sendonly", "sendrecv", or have no directionality indicated, the
answerer MAY remove media Operation Points. The answerer MUST use
the dependency relations provided in the offer when sending media.
The answerer MAY send according to all of the Operation Points
present in the offer, even if the answerer has removed some of those
Operation Points. Thus, an answerer can limit the number of
Operation Points being delivered to the answerer while the answerer
can still send media to the offerer using all of the Operation Points
indicated in the offer.
For multicast sessions, the answerer MUST accept all Operation Points
and their related decoding dependencies or MUST remove non-accepted
Operation Points completely. Due to the nature of multicast, the
receiver can select which Operation Points it actually receives and
processes. For multicast sessions that allow the answerer to also
send data, the answerer MAY send all of the offered Operation Points.
In any case, if the answerer cannot accept one or more offered
Operation Points and/or the media stream's dependencies, the answerer
MAY re-invite with an offer including acceptable Operation Points
and/or dependencies.
Note: Applications may limit the possibility of performing a re-
invite. The previous offer is also a good hint to the capabilities
of the other agent.
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RFC 5583 Signaling Media Decoding Dependency in SDP July 2009
6.2. Declarative usage
If a Real Time Streaming Protocol (RTSP) receiver understands
signaling according to this memo, it SHALL set up 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 types included in the
SDP can be processed, then obviously no setup can occur.
6.3. Usage with AVP and SAVP RTP Profiles
The signaling mechanisms defined in this document MUST NOT be used to
negotiate between using the attribute-value pair (AVP) [RFC3551] and
SAVP [RFC3711] profile for RTP. However, both profiles MAY be used
separately or jointly with the signaling mechanism defined in this
document.
6.4. Usage with Capability Negotiation
This memo does not cover the interaction with Capability Negotiation
[MMUSIC]. This issue is for further study and will be addressed in a
different memo.
6.5. Examples
a.) Example for signaling layered decoding dependency:
The example below shows a session description with three media
descriptions, all of type video and with layered decoding
dependency ("lay"). Each of the media descriptions includes two
possible media format descriptions with different encoding
parameters as, e.g., "packetization-mode" (not shown in the
example) for the media subtypes "H264" and "H264-SVC" given by the
"a=rtpmap:"-line. The first media description includes two H264
payload types as media format descriptions, "96" and "97", as
defined in [RFC3984] and represents the base layer Operation Point
(identified by "mid:L1"). The two other media descriptions
(identified by "mid:L2" and "mid:L3") include H264-SVC payload
types as defined in [AVT-RTP-SVC], which contain enhancements to
the base layer Operation Point or the first enhancement layer
Operation Point (media description identified by "mid:L2").
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RFC 5583 Signaling Media Decoding Dependency in SDP July 2009
The example shows the dependencies of the media format
descriptions of the different media descriptions indicated by
"DDP" grouping, "mid", and "depend" attributes. The "depend"
attribute is used with the decoding dependency type "lay"
indicating layered decoding dependency. For example, the third
media description ("m=video 40004...") identified by "mid:L3" has
different dependencies on the media format descriptions of the two
other media descriptions: Media format description "100" depends
on media format description "96" or "97" of the media description
indentified by "mid:L1". This is an exclusive-OR, i.e., payload
type "100" may be used with payload type "96" or with "97", but
one of the two combinations is required for decoding payload type
"100".
For media format description "101", it is different. This one
depends on two of the other media descriptions at the same time,
i.e., it depends on media format description "97" of the media
description indentified by "mid:L1" and it also depends on media
format description "99" of the media description indentified by
"mid:L2". For decoding media format description "101", both media
format description "97" and media format description "99" are
required by definition.
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RFC 5583 Signaling Media Decoding Dependency in SDP July 2009
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 L1 L2 L3
m=video 40000 RTP/AVP 96 97
b=AS:90
a=framerate:15
a=rtpmap:96 H264/90000
a=rtpmap:97 H264/90000
a=mid:L1
m=video 40002 RTP/AVP 98 99
b=AS:64
a=framerate:15
a=rtpmap:98 H264-SVC/90000
a=rtpmap:99 H264-SVC/90000
a=mid:L2
a=depend:98 lay L1:96,97; 99 lay L1:97
m=video 40004 RTP/AVP 100 101
b=AS:128
a=framerate:30
a=rtpmap:100 H264-SVC/90000
a=rtpmap:101 H264-SVC/90000
a=mid:L3
a=depend:100 lay L1:96,97; 101 lay L1:97 L2:99
b.) Example for signaling of multi-descriptive decoding dependency:
The example shows a session description with three media
descriptions, all of type video and with multi-descriptive
decoding dependency. Each of the media descriptions includes one
media format description. The example shows the dependencies of
the media format descriptions of the different media descriptions
indicated by "DDP" grouping, "mid", and "depend" attributes. The
"depend" attribute is used with the decoding dependency type "mdc"
indicating layered decoding dependency. For example, media format
description "104" in the media description ("m=video 40000...")
with "mid:M1" depends on the two other media descriptions. It
depends on media format description "105" of media description
with "mid:M2", and it also depends on media format description
"106" of media description with "mid:M3". In case of the multi-
descriptive decoding dependency, media format description "105"
and "106" can be used by definition to enhance the decoding
process of media format description "104", but they are not
required for decoding.
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RFC 5583 Signaling Media Decoding Dependency in SDP July 2009
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 M1 M2 M3
m=video 40000 RTP/AVP 104
a=mid:M1
a=depend:104 mdc M2:105 M3:106
m=video 40002 RTP/AVP 105
a=mid:M2
a=depend:105 mdc M1:104 M3:106
m=video 40004 RTP/AVP 106
a=mid:M3
a=depend:106 mdc M1:104 M2:105
7. Security Considerations
All security implications of SDP apply.
There may be a risk of manipulation of 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 Media Bitstream out
of several RTP packet streams 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
The following contact information shall be used for all registrations
included here:
Contact: Thomas Schierl
email: ts@thomas-schierl.de
tel: +49-30-31002-227
The following semantics have been registered by IANA in Semantics for
the "group" SDP Attribute under SDP Parameters.
Semantics Token Reference
------------------- ----- ---------
Decoding Dependency DDP RFC 5583
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The SDP media-level attribute "depend" has been registered by IANA in
Semantics for "att-field (media level only)". The registration
procedure in Section 8.2.4 of [RFC4566] applies.
SDP Attribute ("att-field (media level only)"):
Attribute name: depend
Long form: decoding dependency
Type of name: att-field
Type of attribute: media level only
Subject to charset: no
Purpose: RFC 5583
Reference: RFC 5583
Values: see this document and registrations below.
The following semantics have been registered by IANA in Semantics for
the "depend" SDP Attribute under SDP Parameters:
Semantics of the "depend" SDP attribute:
Semantics Token Reference
---------------------------- ----- ---------
Layered decoding dependency lay RFC 5583
Multi-descriptive decoding dependency mdc RFC 5583
New registrations for semantics of the "depend" SDP attribute are
added by the "Specification Required" policy as defined in [RFC5226].
9. Informative Note on "The SDP (Session Description Protocol)
Grouping Framework"
Currently, there is ongoing work on [RFC3388bis]. In [RFC3388bis],
the grouping mechanism is extended in a way that a media description
can be part of more than one group of the same grouping type in the
same session description. However, media descriptions grouped by
this document must be at most part of one group of the type "DDP" in
the same session description.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H.
Schulzrinne, "Grouping of Media Lines in the Session
Description Protocol (SDP)", RFC 3388, December 2002.
Schierl & Wenger Standards Track [Page 16]
RFC 5583 Signaling Media Decoding Dependency in SDP July 2009
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio
and Video Conferences with Minimal Control", STD 65,
RFC 3551, July 2003.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and
K. Norrman, "The Secure Real-time Transport Protocol
(SRTP)", RFC 3711, March 2004.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP:
Session Description Protocol", RFC 4566, July 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC
5226, May 2008.
[RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234, January
2008.
10.2. Informative References
[AVT-RTP-SVC] Wenger, S., Wang Y.-K., Schierl, T. and A.
Eleftheriadis, "RTP Payload Format for SVC Video", Work
in Progress, March 2009.
[RFC3388bis] Camarillo, G "The SDP (Session Description Protocol)
Grouping Framework", Work in Progress, January 2009.
[MMUSIC] Andreasen, F., "SDP Capability Negotiation", Work in
Progress, May 2009.
[AVT-RTP-MVC] Wang, Y.-K. and T. Schierl, "RTP Payload Format for MVC
Video", Work in Progress, February 2009.
[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.
[RFC3984] Wenger, S., Hannuksela, M., Stockhammer, T.,
Westerlund, M., and D. Singer, "RTP Payload Format for
H.264 Video", RFC 3984, February 2005.
Schierl & Wenger Standards Track [Page 17]
RFC 5583 Signaling Media Decoding Dependency in SDP July 2009
Appendix A. Acknowledgements
The author Thomas Schierl of Fraunhofer HHI is sponsored by the
European Commission under the contract number FP7-ICT-214063, project
SEA.
We want to also thank Magnus Westerlund, Joerg Ott, Ali Begen, Dan
Wing, Helmut Burklin, and Jean-Francois Mule for their valuable and
constructive comments to this memo.
Authors' Addresses
Thomas Schierl
Fraunhofer HHI
Einsteinufer 37
D-10587 Berlin
Germany
Phone: +49-30-31002-227
EMail: ts@thomas-schierl.de
Stephan Wenger
2400 Skyfarm Dr.
Hillsborough, CA 94010
USA
Phone: +1-415-713-5473
EMail: stewe@stewe.org
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