RFC 5583

Network Working Group                                         T. Schierl
Request for Comments: 5583                                Fraunhofer HHI
Category: Standards Track                                      S. Wenger
                                                               July 2009

                Signaling Media Decoding Dependency in
                 the Session Description Protocol (SDP)


   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

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

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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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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.  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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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.

   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

      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

      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

   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

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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].

   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

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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


   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

   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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   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"

   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

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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   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

   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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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

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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      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

      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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         o=svcsrv 289083124 289083124 IN IP4 host.example.com
         t=0 0
         c=IN IP4
         a=group:DDP L1 L2 L3
         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 L1:96,97; 99 lay L1:97
         m=video 40004 RTP/AVP 100 101
         a=rtpmap:100 H264-SVC/90000
         a=rtpmap:101 H264-SVC/90000
         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

         o=mdcsrv 289083124 289083124 IN IP4 host.example.com
         t=0 0
         c=IN IP4
         a=group:DDP M1 M2 M3
         m=video 40000 RTP/AVP 104
         a=depend:104 mdc M2:105 M3:106
         m=video 40002 RTP/AVP 105
         a=depend:105 mdc M1:104 M3:106
         m=video 40004 RTP/AVP 106
         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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RFC 5583       Signaling Media Decoding Dependency in SDP      July 2009

   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.

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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

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.

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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

   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

   Phone: +49-30-31002-227
   EMail: ts@thomas-schierl.de

   Stephan Wenger
   2400 Skyfarm Dr.
   Hillsborough, CA 94010

   Phone: +1-415-713-5473
   EMail: stewe@stewe.org

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