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Versions: 00 01 02 03 04 05 06 07 08 RFC 5583

Network Working Group                                        T. Schierl
Internet-Draft                                           Fraunhofer HHI
Intended status: Standards Track                              S. Wenger
Expires: November 25, 2008                                        Nokia
                                                           May 25, 2008

  Signaling media decoding dependency in Session Description Protocol

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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Copyright Notice

   Copyright (C) The IETF Trust (2008).


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
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

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Table of Contents

   1.  Introduction .................................................. 4
   2.  Terminology ................................................... 5
   3.  Definitions ................................................... 5
   4.  Motivation, Use Cases, and Architecture ....................... 6
   4.1.  Motivation .................................................. 6
   4.2.  Use cases ................................................... 8
   5.  Signaling Media Dependencies .................................. 8
   5.1.  Design Principles ........................................... 8
   5.2.  Semantics ................................................... 9
   5.2.1.  SDP grouping semantics for decoding dependency ............ 9
   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 .......................................... 11
   6.3.  Usage with Capability Negotiation .......................... 11
   6.4.  Examples ................................................... 12
   7.  Security Considerations ...................................... 13
   8.  IANA Considerations........................................... 14
   9.  References ................................................... 15
   9.1.  Normative References ....................................... 15
   9.2.  Informative References ..................................... 15
   Appendix A.  Changes From Earlier Versions ....................... 16
   Authors' Addresses ............................................... 17
   Full Copyright Statement ......................................... 17
   Intellectual Property Statement .................................. 17
   Acknowledgements ................................................. 18

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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.  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

   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 extensions 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.  Likely, the second technology utilizing the
   mechanisms of this memo will be Multi-View video coding.  In Multi
   View Coding (MVC) [I-D.wang-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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2.   Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, RFC 2119

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

   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.

   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.

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   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
   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.

     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.

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     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, 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,
     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

   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.

   o Other decoding dependency relationships:

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     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 a new decoding
     dependency types, if a need could be shown.  If such an extension
     were becoming necessary, as formally required in section 5.2.2, the
     new decoding dependency type MUST be documented in an IETF
     standard's track document.

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

   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
     (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

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   attribute ("mid"), as defined in [RFC3388].  All "m=" lines grouped
   according to this specification MUST have the same media type.

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
   "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
   handling the media streams of the group according to the new media
   level attribute "depend", as defined in 5.2.2.

   When using multiple codecs, e.g. for Offer/Answer model, the media
  streams MUST have the same dependency structure, regardless which
  media format description (RTP 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 [RFC5234].  The identification-tag is defined in
   [RFC3388].  In the following ABNF, fmt, token, SP, and CRLF are used
   as defined in [RFC4566].

     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

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     fmt-dependency = fmt

   dependent-fmt, indicates the media format description, as defined in
   [RFC4566], that depends on one or more media format description in
   the "m="-line indicated by the value of identification-tag within the

   fmt-dependency, indicates the media format description in the "m="-
   line identified by the identification-tag within the dependency-tag,
   which the dependent-fmt 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 fields
   which identify all related media format description in all related
   "m="-lines.  The attribute MAY be used with multicast as well as with
   unicast transport addresses.  The following dependency-types 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 coding 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

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], section 8.4, 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

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   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.

   Edt. Note: we received a comment from Joerg Ott as follows [JO: Maybe
   double-check that the RTSP Transport: header is sufficiently explicit
   to allow this kind of choice.  SAVPF has some discussion on what do
   with RTSP.].  Neither of us feels competent in following up this
   point.  Input of the MMUSIC WG is requested.  If the WG feels that
   there is no need to address this comment, perhaps we can remove this
   whole paragraph?  An alternative would be to use language comparable
   to section 6.3 below, and leave the issue for further study.  We DO
   NOT want to see a normative dependency to RTSP v2 in this draft due
   to timing and deployment considerations.

6.3.  Usage with Capability Negotiation

   This memo does not cover the interaction with Capability Negotiation
   [I-D.ietf-mmusic-sdp-capability-negotiation].  This issue is for
   further study and will be addressed in a different memo.

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6.4. Examples

   a.)  Example for signaling layered decoding dependency:

        The example shows a session description with three "m"-lines,
        all of type video.  Each of the "m"-lines include two possible
        media format descriptions, in the example: RTP payload types.
        The first "m"-line includes a H264 payload type as defined by
        [RFC3984] and the other "m"-lines include H264-SVC payload
        types.  The example shows the dependencies of the RTP payload
        types of the different "m"-lines indicated by "DDP" grouping,
        "mid" and "depend" attributes.

          o=svcsrv 289083124 289083124 IN IP4 host.example.com
          t=0 0

          c=IN IP4
          a=group:DDP 1 2 3 4

          m=video 40000 RTP/AVP 96 97
          a=rtpmap:96 H264/90000
          a=rtpmap:97 H264/90000

          m=video 40002 RTP/AVP 98 99
          a=rtpmap:98 H264-SVC/90000
          a=rtpmap:99 H264-SVC/90000
          a=depend:98 lay 1:96,97; 99 lay 1:97

          m=video 40004 RTP/AVP 100 101
          a=rtpmap:100 H264-SVC/90000
          a=rtpmap:101 H264-SVC/90000
          a=depend:100 lay 1:96 2:98; 101 lay 1:97 2:99

          m=video 40006 RTP/SAVP 102 103
          c=IN IP4

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          a=rtpmap:102 H264-SVC/90000
          a=rtpmap:103 H264-SVC/90000
          a=depend:102 lay 1:96,97 2:98; 103 lay 1:97 2:99

   b.)  Example for signaling of multi descriptive coding dependency:

        The example shows a session description with three "m"-lines,
        all of type video.  Each of the "m"-lines includes one media
        format description, in the example: RTP payload type, with multi
        descriptive decoding dependency.  The example shows the
        dependencies of the RTP payload types of the different "m"-lines
        indicated by "DDP" grouping, "mid" and "depend" attributes.

          o=mdcsrv 289083124 289083124 IN IP4 host.example.com
          t=0 0

          c=IN IP4
          a=group:DDP 1 2 3

          m=video 40000 RTP/AVP 104
          a=depend:104 mdc 2:105 3:106

          m=video 40002 RTP/AVP 105
          a=depend:105 mdc 1:104 3:106

          m=video 40004 RTP/AVP 106
          c=IN IP4
          a=depend:106 mdc 1:104 2:105

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 media bitstream out

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   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

   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 XXXX

   The SDP media level attribute "depend" has been registered by IANA in
   Semantics for "att-field (media level only)".

   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 XXXX
   Reference:          RFC XXXX
   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:

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   Semantics                              Token     Reference
   ----------------------------           -----     ---------
   Layered decoding dependency            lay       RFC XXXX
   multi descriptive coding dependency    mdc       RFC XXXX

9.   References

9.1. 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.

9.2. Informative References

             Wenger, S., Wang Y.-K.,T. Schierl and A. Eleftheriadis,
             "RTP Payload Format for SVC Video",
             draft-ietf-avt-rtp-svc-09 (work in progress), May 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
             Wang, Y.-K. and T. Schierl, "RTP Payload Format
             for MVC Video", draft-wang-avt-rtp-mvc-01 (work in
             progress), February 2008.
             Andreasen, F., "SDP Capability Negotiation",
             draft-ietf-mmusic-sdp-capability-negotiation-08, (work in
             progress), December 2007
[RFC3984]    Wenger, S., Hannuksela, M., Stockhammer, T., Westerlund,M.,
             and Singer, D., "RTP Payload Format for H.264 Video", RFC
             3984, February 2005.

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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
   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

   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
   Different codecs may be present within the same DDP group.

A.3  Changes from draft-ietf-mmusic-decoding-dependency-01 to

   19-Mar08: Fixed PT# in example, removed unused references, updated
   ABNF reference in text, IANA section updates, require std. track doc
   for new dependencies, editorial changes
   23-May08: Replacing payload-type with media format description/fmt,
   renaming of dependent-payload-type to dependent-fmt, renaming
   payload-type-dependency to fmt-dependency, editorial changes.

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Authors' Addresses

   Thomas Schierl
   Fraunhofer HHI
   Einsteinufer 37
   D-10587 Berlin

   Phone: +49-30-31002-227
   Email: schierl@hhi.fhg.de

   Stephan Wenger
   955 Page Mill Road
   Palo Alto, CA, 94304

   Phone: +1-650-862-7368
   Email: stewe@stewe.org

Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

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   on the procedures with respect to rights in RFC documents can be

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Internet-Draft   draft-ietf-mmusic-decoding-dependency-02      May 2008

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   Funding for the RFC Editor function is currently provided by the
   Internet Society.  Further, 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
   and Joerg Ott for their valuable and constructive comments to this

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