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Versions: (draft-wenger-avt-rtp-svc) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 RFC 6190

Network Working Group                                         S. Wenger
Internet-Draft                                               Y.-K. Wang
Intended status: Standards Track                                  Nokia
Expires: August 24, 2008                                     T. Schierl
                                                         Fraunhofer HHI
                                                      February 25, 2008


                   RTP Payload Format for SVC Video
                     draft-ietf-avt-rtp-svc-08.txt


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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   This Internet-Draft will expire on August 24, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   This memo describes an RTP payload format for scalable video coding
   (SVC) defined in_Annex G of the ITU-T Recommendation H.264 video
   codec which is technically identical to Amendment 3 of ISO/IEC
   International Standard 14496-10.  The RTP payload format allows for



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   packetization of one or more Network Abstraction Layer (NAL) units,
   produced by the video encoder, in each RTP packet payload.  The
   payload format has wide applicability, such as low bit-rate
   conversational, Internet video streaming, or high bit-rate
   entertainment quality video.












































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Table of Contents

   RTP Payload Format for SVC Video...................................1
   1.   Introduction .................................................5
   2.   Conventions ..................................................5
   3.   The SVC Codec ................................................5
   3.1. Overview .....................................................5
   3.2. Parameter Set Concept ........................................7
   3.3. Network Abstraction Layer Unit Header ........................8
   4.   Scope .......................................................11
   5.   Definitions and Abbreviations ...............................13
   5.1. Definitions .................................................13
   5.1.1.Definitions per SVC specification...........................13
   5.1.2.Definitions local to this memo..............................14
   5.2. Abbreviations ...............................................17
   6.   RTP Payload Format ..........................................17
   6.1. Design Principles ...........................................18
   6.2. RTP Header Usage ............................................18
   6.3. Common Structure of the RTP Payload Format ..................18
   6.4. NAL Unit Header Usage .......................................18
   6.5. Packetization Modes .........................................20
   6.6. Decoding Order Number (DON) .................................20
   6.7. Aggregation Packets .........................................20
   6.8. Fragmentation Units (FUs) ...................................20
   6.9. Payload Content Scalability Information (PACSI) NAL Unit ....20
   7.   Packetization Rules .........................................26
   7.1. Packetization Rules for Layered Multicast ...................27
   8.   De-Packetization Process (Informative) ......................28
   8.1. De-Packetization Process for NAL Units Conveyed using Session
   Multiplexing......................................................29
   8.1.1.The Classical RTP Decoding Order Recovery Mode..............29
   8.1.2.The CL-DON Decoding Order Recovery Mode.....................32
   9.   Payload Format Parameters ...................................34
   9.1. Media Type Registration .....................................34
   9.2. SDP Parameters ..............................................51
   9.2.1.Mapping of Payload Type Parameters to SDP...................52
   9.2.2.Usage with the SDP Offer/Answer Model.......................52
   9.2.3.Usage with Session Multiplexing.............................57
   9.2.4.Usage in Declarative Session Descriptions...................57
   9.3. Examples ....................................................58
   9.3.1.Example for offering a single SVC session...................58
   9.3.2.Example for offering session multiplexing...................59
   9.4. Parameter Set Considerations ................................60
   10.  Security Considerations .....................................60
   11.  Congestion Control ..........................................60
   12.  IANA Consideration ..........................................61
   13.  Informative Appendix: Application Examples ..................61


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   13.1.Introduction ................................................61
   13.2.Layered Multicast ...........................................62
   13.3.Streaming of an SVC scalable stream .........................62
   13.4.Multicast to MANE, SVC scalable stream to endpoint ..........63
   13.5.Scenarios currently not considered for being unaligned with I
   philosophy........................................................65
   13.6.SSRC Multiplexing ...........................................66
   14.  References ..................................................67
   14.1.Normative References ........................................67
   14.2.Informative References ......................................67
   15.  Author's Addresses ..........................................68
   16.  Copyright Statement .........................................69
   17.  Disclaimer of Validity ......................................69
   18.  Intellectual Property Statement .............................69
   19.  Acknowledgement .............................................70
   20.  RFC Editor Considerations ...................................70
   21.  Open Issues .................................................70
   22.  Changes Log .................................................71































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1. Introduction

   This memo specifies an RTP [RFC3550] payload format for the Scalable
   Video Coding (SVC) extension of the H.264/AVC video coding standard.
   Formally, SVC takes the form of Amendment 3 to ISO/IEC 14496 Part 10
   [MPEG4-10], and Annex G of ITU-T Rec. H.264/AVC [H.264]. The
   specification of SVC is available in [SVC].

   SVC covers the whole application ranges of H.264/AVC, starting with
   low bit-rate Internet streaming applications to HDTV broadcast and
   Digital Cinema with nearly lossless coding and requiring dozens or
   hundreds of MBit/s.

   This memo defines a backwards compatible enhancement to the H264/AVC
   payload format [RFC3984], in which the specific features introduced
   by SVC are taken into account. [Edt. Note (AE): Review backwards
   compatibility assertion, and qualify, when memo is completed.]
   Specifically, it documents the enhancements relevant from an RTP
   transport viewpoint, and defines signaling support for SVC,
   including a new media subtype name.

2. Conventions

   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].

   This specification uses the notion of setting and clearing a bit
   when bit fields are handled.  Setting a bit is the same as assigning
   that bit the value of 1 (On).  Clearing a bit is the same as
   assigning that bit the value of 0 (Off).

3. The SVC Codec

3.1. Overview

   SVC defines a coded video representation in which a given bitstream
   offers representations of the source material at different levels of
   fidelity (hence the term "scalable").  Scalable video coding
   bitstreams, or scalable bitstreams, are constructed in a pyramidal
   fashion: the coding process creates bitstream components that
   improve the fidelity of hierarchically lower components.

   The fidelity dimensions offered by SVC are spatial (picture size),
   quality (or Signal-to-Noise Ratio - SNR), as well as temporal


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   (pictures per second).  Bitstream components associated with a given
   level of spatial, quality, and temporal fidelity are identified
   using corresponding parameters in the bitstream: dependency_id,
   quality_id, and temporal_id (see also Section 3.3).  The fidelity
   identifiers have integer values, where higher values designate
   components that are higher in the hierarchy.  It is noted that SVC
   offers significant flexibility in terms of how an encoder may choose
   to structure the dependencies between the various components.
   Decoding of a particular component requires the availability of all
   the components it depends upon, either directly, or indirectly.  An
   operation point of an SVC bitstream consists of the bistream
   components required to be able to decode a particular dependency_id,
   quality_id, and temporal_id combination.

   SVC maintains the bitstream organization introduced in H.264/AVC.
   Specifically, all bitstream components are encapsulated in Network
   Abstraction Layer (NAL) units which are organized as Access Units
   (AU).  An AU is associated with a single sampling instance in time.
   A subset of the NAL unit types correspond to the Video Coding Layer
   (VCL), and contain the coded picture data associated with the source
   content.  Coded picture data at the various fidelity dimensions are
   organized in slices.  Within one AU, a coded picture of an operation
   point consists of all the coded slices required for decoding up to
   the particular combination of dependency_id and quality_id values
   at the time instance corresponding to the AU.  The NAL encapsulates
   each slice generated by the VCL into one or more NAL units.  Please
   consult RFC 3984 for a more in-depth discussion of the NAL unit
   concept.  SVC specifies the decoding order of NAL units.

   It is noted that the concept of temporal scalability is already
   present in H.264/AVC as profiles defined in Annex A of [H.264]
   already support it.  Specifically, in [H.264] sub-sequences have
   been introduced in order to allow optional use of temporal layers.
   SVC extends this approach by exposing the temporal scalability
   information using the temporal_id parameter, alongside the
   dependency_id and quality_id values that are used for spatial and
   quality scalability.  For coded picture data defined in Annex G of
   [SVC] this is accomplished by using a new type of NAL unit where the
   fidelity parameters are part of its header.  For coded picture data
   that follow H.264/AVC, and to ensure compatibility with existing
   H.264/AVC receivers, a new type of "prefix" NAL unit has been
   defined to carry this header information.  This prefix NAL unit type
   is among those ignored by H.264/AVC receivers as explained in
   [RFC3984].

   Within an AU, the VCL NAL units associated with a given
   dependency_id and quality_id are referred to as a "layer


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   representation".  The layer representation corresponding to the
   lowest values of dependency_id and quality_id (i.e., zero) is the
   base layer representation and is compliant by design to H.264/AVC.
   The set of VCL and associated non-VCL NAL units across all AUs in a
   bitstream associated with a particular combination of values of
   dependency_id and quality_id, and regardless of the value of
   temporal_id, is conceptually a scalable layer.  Due to the backwards
   compatibility with H.264/AVC, it is important to differentiate
   however whether or not SVC-specific NAL units are present in a given
   bitstream or not.  This is particularly important for the lowest
   fidelity values in terms of dependency_id and quality_id (zero for
   both), as the corresponding VCL data are compliant to H.264/AVC, and
   may or may not be accompanied by associated prefix NAL units.  This
   memo therefore uses the term "AVC base layer" to designate the layer
   that contains only H.264/AVC VCL NAL units, and "SVC base layer" to
   designate the same layer but with the addition of the associated SVC
   prefix NAL units.  Note that the SVC specification [SVC] uses the
   term "base layer" for what in this memo will be referred to as "AVC
   base layer".  Similarly, it is also important to be able to
   differentiate, within a layer, the temporal fidelity components it
   contains.  This memo uses the term "T0" to indicate, within a
   particular layer, the subset that contains the NAL units associated
   with temporal_id equal to 0.

   The term "layer" is used in various contexts in this memo.  For
   example, in the terms "Video Coding Layer" and "Network Abstraction
   Layer" it refers to conceptual organization levels.  When referring
   to bitstream syntax elements such as block layer or macroblock
   layer, it refers to hierarchical bitstream structure levels.  When
   used in the context of bitstream scalability, e.g., "AVC base
   layer", it refers to a level of representation fidelity of the
   source signal with a specific set of NAL units included.  The
   correct interpretation is supported by providing the appropriate
   context.

   SNR scalability in SVC is offered in two different ways.  In what is
   called Coarse-Grained Scalability (CGS), scalability is provided by
   including or excluding a complete layer when decoding a particular
   bitstream.  In contrast, in Medium-Grained Scalability (MGS),
   scalability is provided by selectively omitting the decoding of
   specific NAL units belonging to MGS layers.  The selection of the
   NAL units to omit can be based on fixed length fields in the NAL
   unit header.

3.2. Parameter Set Concept




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   The parameter set concept is inherited from [H.264].  Please refer
   to section 1.2 of RFC 3984 for more details.

   SVC introduced a new type of sequence parameter set, referred to as
   a subset sequence parameter set.  Subset sequence parameter sets
   have NAL unit type equal to 15, which is different from the NAL unit
   type value (7) of sequence parameter set.  VCL NAL units of NAL unit
   type 1 to 5 must only (indirectly) refer to sequence parameter sets,
   while VCL NAL units of NAL unit type 20 must only (indirectly) refer
   to subset sequence parameter sets.  Subset sequence parameter sets
   use a separate identifier value space than sequence parameter sets.

   In SVC, coded picture data from different layers may use the same or
   different sequence and picture parameter sets.  At any time instant
   during the decoding process there may be one active sequence
   parameter set (for the layer representation with the highest value
   of (dependency_id * 16 + quality_id)) and one or more active layer
   SVC sequence parameter set(s) (for layer representations with lower
   values of (dependency_id * 16 + quality_id)).  The active sequence
   parameter set or an active layer SVC sequence parameter set remains
   unchanged throughout a coded video sequence in the scalable layer in
   which the active sequence parameter set or active layer SVC sequence
   parameter set is referred to.  This means that the referred sequence
   parameter set or subset sequence parameter set can only change at
   IDR access units for any layer.  At any time instant during the
   decoding process there may be one active picture parameter set (for
   the layer representation with the highest value of (dependency_id *
   16 + quality_id)) and one or more active layer picture parameter
   set(s) (for layer representations with lower values of
   (dependency_id * 16 + quality_id)).  The active picture parameter
   set or an active layer picture parameter set remains unchanged
   throughout a layer representation in which the active picture
   parameter set or active layer picture parameter set is referred to,
   but may change from one AU to the next.

3.3. Network Abstraction Layer Unit Header

   SVC NAL units of type 20 encapsulate VCL data as defined in Annex G
   of [SVC].  A special type of an SVC NAL unit is the prefix NAL unit
   (type 14) that includes descriptive information of the associated
   H.264/AVC VCL NAL unit (type 1 or 5) that immediately follows the
   prefix NAL unit.

   SVC extends the one-byte H.264/AVC NAL unit header by three
   additional octets.  The header indicates the type of the NAL unit,
   the (potential) presence of bit errors or syntax violations in the
   NAL unit payload, information regarding the relative importance of


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   the NAL unit for the decoding process, the layer identification
   information, and other fields as discussed below.

   The syntax and semantics of the NAL unit header are formally
   specified in [SVC], but the essential properties of the NAL unit
   header are summarized below.

   The first byte of the NAL unit header has the following format (the
   bit fields are the same as defined for the one-byte H.264/AVC NAL
   unit header, while the semantics of some fields have changed
   slightly, in a backward compatible way):

         +---------------+
         |0|1|2|3|4|5|6|7|
         +-+-+-+-+-+-+-+-+
         |F|NRI|  Type   |
         +---------------+

   F: 1 bit
   forbidden_zero_bit.  H.264/AVC declares a value of 1 as a syntax
   violation.

   NRI: 2 bits
   nal_ref_idc.  A value of '00' (in binary form) indicates that the
   content of the NAL unit is not used to reconstruct reference
   pictures for future prediction.  Such NAL units can be discarded
   without risking the integrity of the reference pictures in the same
   Layer.  A value greater than '00' indicates that the decoding of the
   NAL unit is required to maintain the integrity of reference pictures
   in the same Layer, or that the NAL unit contains parameter sets.

   Type: 5 bits
   nal_unit_type.  This component specifies the NAL unit type as
   defined in table 7-1 of [SVC], and later within this memo.  For a
   reference of all currently defined NAL unit types and their
   semantics, please refer to section 7.4.1 in [SVC].

   In H.264/AVC, NAL unit types 14, 15 and 20 are reserved for future
   extensions.  SVC uses these three NAL unit types.  NAL unit type 14
   is used for prefix NAL unit, NAL unit type 15 is used for subset
   sequence parameter set and NAL unit type 20 is used for coded slice
   in scalable extension (see section 7.4.1 in [SVC]).  NAL unit types
   14 and 20 indicate the presence of three additional octets in the
   NAL unit header, as shown below.

            +---------------+---------------+---------------+
            |0|1|2|3|4|5|6|7|0|1|2|3|4|5|6|7|0|1|2|3|4|5|6|7|


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            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |R|I|   PRID    |N| DID |  QID  | TID |U|D|O| RR|
            +---------------+---------------+---------------+

   R: 1 bit
   reserved_one_bit.  Reserved bit for future extension.  R MUST be
   equal to 1.  Receivers SHOULD ignore the value of R.

   I: 1 bit
   idr_flag.  This component specifies whether the layer representation
   is an instantaneous decoding refresh (IDR) layer representation
   (when equal to 1) or not (when equal to 0).

   PRID: 6 bits
   priority_id.  This flag specifies a priority identifier for the NAL
   unit.  A lower value of PRID indicates a higher priority.

   N: 1 bit
   no_inter_layer_pred_flag.  This flag specifies, when present in a
   coded slice NAL unit, whether inter-layer prediction may be used for
   decoding the coded slice (when equal to 1) or not (when equal to 0).

   DID: 3 bits
   dependency_id.  This component indicates the inter-layer coding
   dependency level of a layer representation.  At any access unit, a
   layer representation with a given dependency_id may be used for
   inter-layer prediction for coding of a layer representation with a
   higher dependency_id, while a layer representation with a given
   dependency_id shall not be used for inter-layer prediction for
   coding of a layer representation with a lower dependency_id.

   QID: 4 bits
   quality_id.  This component indicates the quality level of an MGS
   layer representation.  At any access unit and for identical
   dependency_id values, a layer representation with quality_id equal
   to ql uses a layer representation with quality_id equal to ql-1 for
   inter-layer prediction.

   TID: 3 bits
   temporal_id.  This component indicates the temporal level of a layer
   representation.  The temporal_id is associated with the frame rate,
   with lower values of _temporal_id corresponding to lower frame
   rates.  A layer representation at a given temporal_id typically
   depends on layer representations with lower temporal_id values, but
   it never depends on layer representations with higher temporal_id
   values.



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   U: 1 bit
   use_ref_base_pic_flag.  A value of 1 indicates that only reference
   base pictures are used during the inter prediction process.  A value
   of 0 indicates that the reference base pictures are not used during
   the inter prediction process.

   D: 1 bit
   discardable_flag.  A value of 1 indicates that the current NAL unit
   is not used for decoding NAL units with values of dependency_id
   higher than the one of the current NAL unit, in the current and all
   subsequent access units.  Such NAL units can be discarded without
   risking the integrity of layers with higher dependency_id values.
   discardable_flag equal to 0 indicates that the decoding of the NAL
   unit is required to maintain the integrity of layers with higher
   dependency_id.

   O: 1 bit
   output_flag: Affects the decoded picture output process as defined
   in Annex C of [SVC].

   RR: 2 bits
   reserved_three_2bits.  Reserved bits for future extension.  RR MUST
   be equal to '11' (in binary form).  Receivers SHOULD ignore the
   value of RR.

   This memo reuses the same additional NAL unit types introduced in
   RFC 3984, which are presented in section 6.3.  In addition, this
   memo introduces one OPTIONAL NAL unit type, 30, as specified in
   section 6.9. These NAL unit types are marked as unspecified in [SVC]
   and intentionally reserved for use in systems specifications like
   this memo.  Moreover, this specification extends the semantics of F,
   NRI, I, PRID, DID, QID, TID, U, and D as described in section 6.4.

4. Scope

   This payload specification can only be used to carry the "naked" NAL
   unit stream over RTP, and not the byte stream format according to
   Annex B of [SVC].  The likely applications of this specification
   will be in the IP based multimedia communications fields including
   conversational multimedia, video telephony or video conferencing,
   Internet streaming and TV over IP.

   This specification allows, in a given RTP stream, to encapsulate NAL
   units belonging to
     o the T0 AVC base layer or the T0 SVC base layer only, as detailed
       in [RFC3984], or
     o one or more enhancement layers, or


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     o the T0 SVC base layer, and one or more enhancement layers

   Session multiplexing SHOULD be used when different receivers in the
   multicast session may request different operation points of the
   scalable bitstream.  In session multiplexing, layers are carried in
   multiple RTP sessions, and each RTP session is associated with one
   RTP stream.  The RTP stream in each RTP session MAY carry one or
   more layers, which can be any of the above three.  When each
   operation point corresponding to a layer may be required by some
   receivers, then each Layer SHOULD be carried in its own RTP stream
   and its own RTP session.  When fewer operation points are required
   by the receivers, then multiple layers MAY be encapsulated within
   one RTP stream in one RTP session.

      Informative note: Layered multicast is a term commonly used to
      describe the application where multicast is used to transmit data
      that has been encapsulated into more than one RTP session using
      session multiplexing.  This application allows different
      receivers in the multicast session to receive different operation
      points of the scalable bitstream.  Layered multicast, among other
      application examples, is discussed in more detail in the
      informative Section 13.2.

   When session multiplexing is not used, the following applies.

     o When an H.264/AVC compatible subset of the SVC base layer is
        transmitted, the subset SHOULD be carried in one RTP stream
        that MUST be encapsulated according to RFC 3984.  This way, a
        legacy RFC 3984 receiver will be able to receive the H.264/AVC
        compatible bitstream subset.
     o When a set of layers including one or more SVC enhancement
        layers is transmitted, the set SHOULD be carried in one RTP
        stream that SHOULD be encapsulated according to this memo.

   This RTP payload specification is designed to be unaware of the
   octet string in the NAL unit payload defined in [SVC].  The NAL unit
   header defined in [SVC] co-serves as the payload header of this RTP
   payload format, when single NAL unit packetization is used, i.e. one
   NAL unit per RTP packet.  In this case, the payload of a NAL unit
   follows immediately.  Additionally to [RFC3984], this memo locally
   defines a NAL unit type in the unspecified NAL unit type space of
   [SVC].  If other than the single NAL unit packetization mode is used
   as defined in [RFC3984] or this memo, locally defined NAL unit types
   may be additionally present in the RTP packets, together with one or
   more NAL unit types as specified in [SVC].




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5. Definitions and Abbreviations

5.1. Definitions

5.1.1.    Definitions per SVC specification

   This document uses the definitions of [SVC].  The following terms,
   defined in [SVC], are summed up for convenience:

   access unit:  A set of NAL units always containing exactly one
   primary coded picture.  In addition to the primary coded picture, an
   access unit may also contain one or more redundant coded pictures,
   one auxiliary coded picture, or other NAL units not containing
   slices or slice data partitions of a coded picture.  The decoding of
   an access unit always results in a decoded picture.

   base layer: A bitstream subset that contains all the NAL units with
   the nal_unit_type syntax element equal to 1 and 5 of the bitstream
   and does not contain any NAL unit with the nal_unit_type syntax
   element equal to 14, 15, or 20 and conforms to one or more of the
   profiles specified in Annex A of [H.264].

   base quality layer representation: The layer representation of the
   target dependency representation of an access unit that is
   associated with the quality_id syntax element equal to 0.

   coded video sequence:  A sequence of access units that consists, in
   decoding order, of an IDR access unit followed by zero or more non-
   IDR access units including all subsequent access units up to but not
   including any subsequent IDR access unit.

   dependency representation: A subset of VCL NAL units within an
   access unit that are associated with the same value of the
   dependency_id syntax element, which is provided as part of the NAL
   unit header or by an associated prefix NAL unit.  A dependency
   representation consist of one or more layer representations.

   IDR access unit:  An access unit in which the primary coded picture
   is an IDR picture.

   IDR picture:  A coded picture in which all slices of the target
   dependency representation within the access unit are I or EI slices
   that causes the decoding process to mark all reference pictures as
   "unused for reference" immediately after decoding the IDR picture.
   After the decoding of an IDR picture all following coded pictures in
   decoding order can be decoded without inter prediction from any



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   picture decoded prior to the IDR picture.  The first picture of each
   coded video sequence is an IDR picture.

   layer representation:  A subset of VCL NAL units within an access
   unit that are associated with the same values of the dependency_id
   and quality_id syntax elements, which are provided as part of the
   VCL NAL unit header or by an associated prefix NAL unit.  One or
   more layer representations represent a dependency representation.

   prefix NAL unit:  A NAL unit with nal_unit_type equal to 14 that
   immediately precedes in decoding order a NAL unit with nal_unit_type
   equal to 1, 5, or 12.  The NAL unit that immediately succeeds in
   decoding order the prefix NAL unit is referred to as the associated
   NAL unit.  The prefix NAL unit contains data associated with the
   associated NAL unit, which are considered to be part of the
   associated NAL unit.

   reference base picture: A reference picture that is obtained by
   decoding a base quality layer representation with the nal_ref_idc
   syntax element not equal to 0 and the store_ref_base_pic_flag syntax
   element equal to 1 of an access unit and all layer representations
   of the access unit that are referred to by inter-layer prediction of
   the base quality layer representation.  A reference base picture is
   not an output of the decoding process, but the samples of a
   reference base picture may be used for inter prediction in the
   decoding process of subsequent pictures in decoding order.
   Reference base picture is a collective term for a reference base
   field or a reference base frame.

   scalable bitstream:  A bitstream with the property that one or more
   bitstream subsets that are not identical to the scalable bitstream
   form another bitstream that conforms to the SVC specification[SVC].

   target dependency representation: The dependency representation of
   an access unit that is associated with the largest value of the
   dependency_id syntax element for all dependency representations of
   the access unit.

   target layer representation:  The layer representation of the target
   dependency representation of an access unit that is associated with
   the largest value of the quality_id syntax element for all layer
   representations of the target dependency representation of the
   access unit.

5.1.2.    Definitions local to this memo




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   anchor layer representation:  An anchor layer representation is such
   a layer representation that, if decoding of the operation point
   corresponding to the layer starts from the access unit containing
   this layer representation, all the following layer representations
   of the layer, in output order, can be correctly decoded.  An anchor
   layer representation is a random access point to the layer the
   anchor layer representation belongs to.  However, some layer
   representations, succeeding an anchor layer representation in
   decoding order but preceding the anchor layer representation in
   output order, may refer to earlier layer representations for inter
   prediction, and hence may not be correctly decoded if random access
   is performed at the anchor layer representation.

   AVC base layer: The subset of the SVC base layer in which all prefix
   NAL units (type 14) are removed. Note that this is equivalent to the
   term "base layer" as defined in Annex G of [SVC].

   base RTP session:  The RTP session, among all the RTP sessions using
   session multiplexing, that carries the RTP stream containing the T0
   AVC base layer or the T0 SVC base layer, and zero or more
   enhancement layers.  This RTP session does not depend on any other
   RTP session as indicated by mechanisms defined in [I-D.ietf-mmusic-
   decoding-dependency].  The base RTP session may carry NAL units of
   NAL unit type equal to 14 and 15.

   enhancement RTP session:  An RTP session, among all the RTP sessions
   using session multiplexing, that is not the base RTP session.  An
   enhancement RTP session typically contains an RTP stream that
   depends on at least one other RTP session as indicated by mechanisms
   defined in [I-D.ietf-mmusic-decoding-dependency].  A lower RTP
   session to an enhancement RTP session is an RTP session which the
   enhancement RTP session depends on.

   cross-layer decoding order number (CL-DON): A derived variable
   indicating NAL unit decoding order number over all NAL units within
   all layers of an SVC bitstream.

   enhancement layer:  A layer in which at least one of the values of
   dependency_id or quality_id is higher than 0, or a layer in which
   none of the NAL units is associated with the value of temporal_id
   equal to 0.  An operation point constructed using the maximum
   temporal_id, dependency_id, and quality_id values associated with an
   enhancement layer may or may not conform to one or more of the
   profiles specified in Annex A of [H.264].

   H.264/AVC compatible:  A biststream subset that conforms to one or
   more of the profiles specified in Annex A of [H.264].


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   intra layer representation:  A layer representation that contains
   only slices that use intra prediction, and hence do not refer to any
   earlier layer representation in decoding order in the same layer.
   Note that in [SVC] intra prediction includes intra-layer intra
   prediction as well as inter-layer intra prediction.

   layer:  A bistream subset in which all NAL units of type 1, 5, 12,
   14, or 20 have the same values of dependency_id and quality_id,
   either directly through their NAL unit header (for NAL units of type
   14 or 20) or through association to a prefix (type 14) NAL unit (for
   NAL unit types 1, 5, or 12) whether these prefix NAL units are
   present or not.  A layer may contain NAL units associated with more
   than one values of temporal_id.

   operation point:  An operation point is identified by a set of
   values of temporal_id, dependency_id, and quality_id and is a
   bitstream subset constructed by removing all NAL units associated
   with a higher value of dependency_id, and all NAL units associated
   with the same value of dependency_id but higher values of quality_id
   or temporal_id. An operation point conforms to at least one of the
   profiles defined in Annex A or Annex G of [SVC], and offers a
   representation of the original video signal at a certain fidelity.
   [Edt. Note (YkW): The definition implies that all the non-VCL NAL
   units that are not directly associated with temporal_id,
   dependency_id, and quality_id are included any operation point.
   Let's see whether this is always OK or any improvement is needed.]

   operation point representation: The set of all NAL units of an
   operation point within the same access unit.

   RTP packet stream: A sequence of RTP packets with increasing
   sequence numbers (except for wrap-around), identical PT and
   identical SSRC (Synchronization Source), carried in one RTP session.
   Within the scope of this memo, one RTP packet stream is utilized to
   transport one or more layers.

   session multiplexing:  The scalable SVC bitstream is distributed
   onto different RTP sessions, whereby each RTP session carries a
   single RTP packet stream.  Each RTP session requires a separate
   signaling and has a separate Timestamp, Sequence Number, and SSRC
   space.  Timestamps for the RTP sessions SHALL be derived from the
   same clock instance.  Dependency between sessions MUST be signaled
   according to [I-D.ietf-mmusic-decoding-dependency] and this memo.

   SVC base layer: The layer that includes all NAL units associated
   with dependency_id and quality_id values both equal to 0.


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   SVC enhancement layer:  A layer in which at least one of the values
   of dependency_id or quality_id is higher than 0.  An operation point
   constructed using the maximum temporal_id, dependency_id, and
   quality_id values associated with an SVC enhancement layer does not
   conform to any of the profiles specified in Annex A of [H.264].

   SVC NAL unit:  A NAL unit of NAL unit type 14, 15, or 20 as
   specified in Annex G of [SVC].

   SVC NAL unit header: A four-byte header resulting from the addition
   of a three-byte SVC-specific header extension added in NAL unit
   types 14 and 20.

   SVC RTP session: Either the base RTP session or an enhancement RTP
   session.  The lowest SVC RTP session is the base RTP session, and
   the highest RTP session is the enhancement RTP session which no
   other RTP session depends on, or the base RTP session if no
   enhancement RTP session exists.
   [Edt. Note (YkW): There may be multiple RTP sessions that no other
   RTP session depends on.  We may limit the scope of lower or higher
   RTP sessions to be within a given receiver, which either receive one
   session or multiple sessions with at most one session no other
   session depends on. However, in that case, the lowest session may
   not be the base session.]

   T0 AVC base layer: A subset of the AVC base layer constructed by
   removing all VCL NAL units associated with temporal_id values higher
   than 0.

   T0 SVC base layer: A subset of the SVC base layer constructed by
   removing all VCL NAL units associated with temporal_id values higher
   than 0 as well as their associated prefix NAL units.

5.2. Abbreviations

   In addition to the abbreviations defined in [RFC3984], the following
   ones are defined.

   CGS:       Coarse-Grain Scalability
   CL-DON:    Cross-Layer Decoding Order Number
   MGS:       Medium-Grain Scalability
   PACSI:     Payload Content Scalability Information
   SVC:       Scalable Video Coding

6. RTP Payload Format



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6.1. Design Principles

   The following design principles have been observed:

   o Backward compatibility with [RFC3984] wherever possible.

   o The SVC base layer or any H.264/AVC compatible subset containing
   the T0 SVC base layer and one or more temporal enhancement layers,
   when transmitted in its own session, MUST be
     encapsulated using [RFC3984].  Requiring this has the desirable
     side effect that it can be used by [RFC3984] legacy devices.

   o MANEs are signaling aware and rely on signaling information.
     MANEs have state.

   o MANEs can aggregate multiple RTP streams, possibly from multiple
   RTP sessions.

   o MANEs can perform media-aware stream thinning.  By using the
   payload
     header information identifying Layers within an RTP session,
     MANEs are able to remove packets from the incoming RTP packet
   stream.  This implies rewriting
     the RTP headers of the outgoing packet stream and rewriting of
     RTCP Receiver Reports.

6.2. RTP Header Usage

   Please see section 5.1 of [RFC3984].

6.3. Common Structure of the RTP Payload Format

   Please see section 5.2 of [RFC3984].

6.4. NAL Unit Header Usage

   The structure and semantics of the NAL unit header were introduced
   in section 3.3.  This section specifies the semantics of F, NRI, I,
   PRID, DID, QID, TID, U, and D according to this specification.

   The semantics of F specified in section 5.3 of [RFC3984] also
   applies herein.

   For NRI, for the bitstream conforming to one of the profiles defined
   in Annex A of [H.264] and transported using [RFC3984], the semantics
   specified in section 5.3 of [RFC3984] are applicable, i.e., NRI also
   indicates the relative importance of NAL units.  In an SVC context,


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   in addition to the semantics specified in Annex G of [SVC], NRI also
   indicates the relative importance of NAL units within a layer.

   [Edt. Note (YkW): "SVC context" to be clearly specified.]

   For I, in addition to the semantics specified in Annex G of [SVC],
   according to this memo, MANEs MAY use this information to protect
   NAL units with I equal to 1 better than NAL units with I equal to 0.
   MANEs MAY also utilize information of NAL units with I equal to 1 to
   decide when to forward more packets for an RTP packet stream. For
   example, when it is sensed that spatial layer switching has happened
   such that the operation point has changed to a higher value of DID,
   MANEs MAY start to forward NAL units with the higher value of DID
   only after forwarding a NAL unit with I equal to 1 with the higher
   value of DID.

   Note that, in the context of this section, "protecting a NAL unit"
   means any RTP or network transport mechanism that could improve the
   probability of success delivery of the packet conveying the NAL
   unit, including applying a QoS-enabled network, Forward Error
   Correction (FEC), retransmissions, and advanced scheduling behavior,
   whenever possible.

   For PRID, the semantics specified in Annex G of [SVC] applies.
   Note, that MANEs implementing unequal error protection MAY use this
   information to protect NAL units with smaller PRID values better
   than those with larger PRID values, for example by including only
   the more important NAL units in an FEC protection mechanism.  The
   importance for the decoding process decreases as the PRID value
   increases.

   For DID, QID, TID, in addition to the semantics specified in Annex G
   of [SVC], according to this memo, values of DID, QID, or TID
   indicate the relative importance in their respective dimension.  A
   lower value of DID, QID, or TID indicates a higher importance if the
   other two components are identical.  MANEs MAY use this information
   to protect more important NAL units better than less important NAL
   units.

   For U, in addition to the semantics specified in Annex G of [SVC],
   according to this memo, MANEs MAY use this information to protect
   NAL units with U equal to 1 better than NAL units with U equal to 0.

   For D, in addition to the semantics specified in Annex G of [SVC],
   according to this memo, MANEs MAY use this information to determine
   whether a given NAL unit is required for successfully decoding a



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   certain Operation Point of the SVC bitstream, hence to decide
   whether to forward the NAL unit.

6.5. Packetization Modes

   Please see section 5.4 of [RFC3984].

6.6. Decoding Order Number (DON)

   Please see section 5.5 of [RFC3984].  The following applies in
   addition.

   If different layers of a SVC bitstream are transported in more than
   one RTP session, the DON values derived according to RFC 3984 of all
   the NAL units in the RTP sessions using interleaved mode MUST
   indicate CL-DON values.

   When the CL-DON decoding order recovery mode is used with session
   multiplexing as described in section 7.1 and at least one STAP-A
   packet is present in any of the RTP sessions, the following applies.
   - A PACSI NAL unit MUST be present in each STAP-A packet.
   - A DONC field MUST be present in the PACSI NAL unit included in
     each STAP-A.
   - The DON values for the NAL units in each STAP-A packet MUST be
     derived as follows and MUST indicate CL-DON values.
        o The DONC field in the PACSI NAL unit specifies the value of
          DON for the first NAL unit in the STAP-A in transmission
          order.  For each successive NAL unit in appearance order in
          the STAP-A, the value of DON is equal to (the value of DON
          of the previous NAL unit in the STAP-A + 1) % 65536, wherein
          '%' stands for modulo operation.

6.7. Aggregation Packets

   Please see section 5.7 of [RFC3984].

6.8. Fragmentation Units (FUs)

   Please see section 5.8 of [RFC3984].

6.9. Payload Content Scalability Information (PACSI) NAL Unit

   A new NAL unit type is specified in this memo, and referred to as
   payload content scalability information (PACSI) NAL unit.  The
   OPTIONAL PACSI NAL unit, if present, MUST be the first NAL unit in
   an aggregation packet, and it MUST NOT be present in other types of
   packets.  The PACSI NAL unit indicates scalability information and


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   other characteristics that are common for all the remaining NAL
   units in the payload of the aggregation packet. Furthermore, a PACSI
   NAL unit MAY contain zero or more SEI NAL units.  PACSI NAL unit
   makes it easier for MANEs to decide whether to
   forward/process/discard the aggregation packet containing the PACSI
   NAL unit.  Other reasons to use PACSI NAL units are explained later
   when specifying the semantics of the fields.  Senders MAY create
   PACSI NAL units and receivers MAY ignore them, or use them as hints
   to enable efficient aggregation packet processing.  Note that the
   NAL unit type for the PACSI NAL unit is selected among those values
   that are unspecified in [SVC] and [RFC3984].

   When the first aggregation unit of an aggregation packet contains a
   PACSI NAL unit, there MUST be at least one additional aggregation
   unit present in the same packet.  The RTP header and payload header
   fields of the aggregation packet are set according to the remaining
   NAL units in the aggregation packet.

   When a PACSI NAL unit is included in a multi-time aggregation packet
   (MTAP), the decoding order number (DON) for the PACSI NAL unit MUST
   be set to indicate that the PACSI NAL unit has an identical DON to
   the first NAL unit in decoding order among the remaining NAL units
   in the aggregation packet.

   The structure of a PACSI NAL unit is as follows.  The first four
   octets are exactly the same as the four-byte SVC NAL unit header as
   discussed in section 3.3.  They are followed by one always present
   octet, five optional octets, and zero or more SEI NAL units, each
   SEI NAL unit preceded by a 16-bit unsigned size field (in network
   byte order) that indicates the size of the following NAL unit in
   bytes (excluding these two octets, but including the NAL unit type
   octet of the SEI NAL unit).  Figure 1 illustrates the PACSI NAL unit
   structure and an example of a PACSI NAL unit containing two SEI NAL
   units.

   The bits A, P, C, S, and E are specified only if the bit X is equal
   to 1.  The fields TL0PICIDX and IDRPICID are present only if the bit
   Y is equal to 1.  The fields TL0PICIDX and IDRPICID MUST NOT be
   present if the bit Y is equal to 0.  The field DONC is present only
   if the bit T is equal to 1.  The field T MUST be equal to 0 if the
   aggregation packet containing the PACSI NAL unit is not an STAP-A
   packet.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |F|NRI|  Type   |R|I|   PRID    |N| DID |  QID  | TID |U|D|O| RR|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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      |X|Y|T|A|P|C|S|E| TL0PICIDX (o.)|        IDRPICID (o.)          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          DONC (o.)            |        NAL unit size 1        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                 SEI NAL unit 1                                |
      |                                                               |
      |                         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         |        NAL unit size 2        |     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     |
      |                                                               |
      |            SEI NAL unit 2                                     |
      |                                           +-+-+-+-+-+-+-+-+-+-+
      |                                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 1.  PACSI NAL unit structure.  Fields suffixed by
                    "(o.)" are OPTIONAL.

   The values of the fields in PACSI NAL unit MUST be set as follows.
   The term "target NAL units" are used in the semantics of some
   fields.  The target NAL units are such NAL units contained in the
   aggregation packet, but not included in the PACSI NAL unit, that are
   within the access unit to which the first NAL unit following the
   PACSI NAL unit in the aggregation packet belongs.

   o The F bit MUST be set to 1 if the F bit in at least one of the
     remaining NAL units in the payload of the aggregation packet is
     equal to 1.  Otherwise, the F bit MUST be set to 0.

   o The NRI field MUST be set to the highest value of NRI field among
     all the remaining NAL units in the payload of the aggregation
     packet.

   o The Type field MUST be set to 30.

   o The R bit MUST be set to 1.  Receivers SHOULD ignore the value of
   R.

   o The I bit MUST be set to 1 if the I bit of at least one of the
     remaining NAL units in the payload of the aggregation packet is
   equal to 1.  Otherwise, the I bit MUST be set to 0.

   o The PRID field MUST be set to the lowest value of the PRID values
     of all the remaining NAL units in the payload of the aggregation
     packet.





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   o The N bit MUST be set to 1 if the N bit of all the remaining NAL
     units in the payload is equal to 1.  Otherwise, the N bit MUST be
     set to 0.

   o The DID field MUST be set to the lowest value of the DID values
     of all the remaining NAL units in the payload of the aggregation
   packet.

   o The QID field MUST be set to the lowest value of the QID values
     of all the remaining NAL units with the lowest value of DID in the
     payload.

   o The TID field MUST be set to the lowest value of the TID values
     of all the remaining NAL units with the lowest value of DID in the
     payload.

   o The U bit MUST be set to 1 if the U bit of at least one of the
     remaining NAL units in the payload of the aggregation packet is
   equal to 1.  Otherwise, the
     U bit MUST be set to 0.

   o The D bit MUST be set to 1 if the D value of all the remaining NAL
     unit in the payload is equal to 1.  Otherwise, the D bit MUST be
     set to 0.

   o The O bit MUST be set to 1 if the O bit of at least one of the
     remaining NAL units in the payload of the aggregation packet is
   equal to 1.  Otherwise, the
     O bit MUST be set to 0.

   o The RR field MUST be set to '11' (in binary form).  Receivers
   SHOULD ignore the value of RR.

   o If the X bit is equal to 1, the bits A, P, C, S, and E are
   specified as in below. Otherwise, the bits A, P, C, S, and E are
   unspecified, and receivers MUST ignore these bits.  The X bit SHOULD
   be identical for all the PACSI NAL units in all the RTP sessions
   conveying an SVC bitstream.

   o If the Y bit is equal to 1, the OPTIONAL fields TL0PICIDX and
   IDRPICID MUST be present and specified as in below. Otherwise, the
   fields TL0PICIDX and IDRPICID MUST NOT be present.  The Y bit SHOULD
   be identical for all the PACSI NAL units involved in all the RTP
   sessions conveying an SVC bitstream.






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   o If the T bit is equal to 1, the OPTIONAL field DONC MUST be
   present and specified as below. Otherwise, the field DONC MUST NOT
   be present.

   o The A bit MUST be set to 1 if all the target NAL units belong to
   anchor layer representations.  Otherwise, the A bit MUST be set to
   0.  The A bit SHOULD be identical for all the PACSI NAL units for
   which the target NAL units belong to the same access unit.

      Informative note: The A bit indicates whether CGS or spatial
      layer switching at a non-IDR layer representation (a layer
      representation with nal_unit_type not equal to 5 and idr_flag not
      equal to 1) can be performed.  When the coded pattern like IBBP
      is in use, non-IDR intra layer representation can be used for
      random access.  Compared to using only IDR layer representations,
      higher coding efficiency can be achieved.  The H.264/AVC or SVC
      solution to indicate the random accessibility of a non-IDR intra
      layer representation is using recovery point SEI message.
      However, with this A bit it is much easier to parse than to parse
      the recovery point SEI message, which may even be buried deeply
      in an SEI NAL unit.  Furthermore, the SEI message may not be
      present in the bitstream.

   o The P bit MUST be set to 1 if all the remaining NAL units in the
   payload of the aggregation packet are with redundant_pic_cnt greater
   than 0, i.e. the slices are redundant slices.  Otherwise, the P bit
   MUST be set to 0.

      Informative note: The P bit indicates whether the packet can be
      discarded because it contains only redundant slice NAL units.
      Without this bit, the corresponding information can be concluded
      from the syntax element redundant_pic_cnt, which is buried in the
      variable-length coded slice header.

   o The C bit MUST be set to 1 if the target NAL units (as defined
   above) belong to an access unit for which the layer representation
   having the greatest value of dependency_id among all the layer
   representations containing the target NAL units is an intra layer
   representation.  Otherwise, the C bit MUST be set to 0.  The C bit
   SHOULD be identical for all the PACSI NAL units for which the target
   NAL units belong to the same access unit.

      Informative note: The C bit indicates whether the packet contains
      intra slices which may be the only packets to be forwarded for a
      fast forward playback, e.g. when the network condition is
      extremely bad.



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   o The S bit MUST be set to 1, if the first VCL NAL unit, in decoding
   order, of the layer representation containing the first NAL unit
   following the PACSI NAL unit in the aggregation packet is present in
   the payload.  Otherwise, the S bit MUST be set to 0.

   o The E bit MUST be set to 1, if the last VCL NAL unit, in decoding
   order, of the layer representation containing the first NAL unit
   following the PACSI NAL unit in the aggregation packet is present in
   the payload.  Otherwise, the E field MUST be set to 0.

      Informative note: The S or E bit indicates whether the first or
      last slice, in decoding order, of a layer representation is in
      the packet, to enable a MANE to detect slice loss and take proper
      action such as requesting a retransmission as soon as possible,
      as well as to allow an efficient playout buffer handling
      similarly as the M bit in the RTP header.  The M bit in the RTP
      header still indicates the end of an access unit, not the end of
      a layer representation.

   o When present, the TL0PICIDX field MUST be set to equal to
   tl0_dep_rep_idx as specified in Annex G of [SVC] for the layer
   representation containing the first NAL unit following the PACSI NAL
   unit in the aggregation packet.

   o When present, the IDRPICID field MUST be set to equal to
   effective_idr_pic_id as specified in Annex G of [SVC] for the layer
   representation containing the first NAL unit following the PACSI NAL
   unit in the aggregation packet.

      Informative note: The TL0PICIDX and IDRPICID fields enable the
      detection of the loss of layer representations in the most
      important temporal layer, by receivers as well as MANEs.  SVC
      includes a solution by using SEI messages, which are harder to
      parse and may not be present in the bitstream at all.

   o When present, the field DONC indicates the cross-layer decoding
   order number for the first NAL unit in the STAP-A in transmission
   order.

   The PACSI NAL unit SHALL include a subset (zero to all) of the SEI
   NAL units associated with the access unit to which the target NAL
   units belong, and SHALL NOT contain SEI NAL units associated with
   any other access unit.

      Informative note: Senders may repeat such SEI NAL units in the
      PACSI NAL unit the presence of which in more than one packet is
      essential for packet loss robustness.  Receivers may use the


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      repeated SEI messages in place of missing SEI messages.  In
      H.264/AVC and SVC, within each access unit, SEI NAL units must
      appear before any VCL NAL unit in decoding order.  Therefore,
      without using PACSI NAL units, SEI messages are typically only
      conveyed in the first packet of those packets conveying an access
      unit.

   An SEI message SHOULD NOT be included in a PACSI NAL unit and
   included in one of the remaining NAL units contained in the same
   aggregation packet.


7. Packetization Rules

   Please see section 6 of [RFC3984].  The following rules apply in
   addition.

   All receivers MUST support the single NAL unit packetization mode to
   provide backward compatibility to endpoints supporting only the
   single NAL unit mode of RFC 3984. However, the use of single NAL
   unit packetization mode (packetization-mode equal to 0) SHOULD be
   avoided whenever possible, because encapsulating NAL units of small
   sizes, e.g. small NAL units containing parameter sets or SEI
   messages, in their own packets is typically less efficient because
   of the relatively big overhead.

   All receivers MUST support the non-interleaved mode of [RFC3984].

     Informative note: The non-interleaved mode allows an application
     to encapsulate a single NAL unit in a single RTP packet.
     Historically, the single NAL unit mode has been included into
     [RFC3984] only for compatibility with ITU-T Rec. H.241 Annex A
     [H.241].  There is no point in carrying this historic ballast
     towards a new application space such as the one provided with SVC.
     More technically speaking, the implementation complexity increase
     for providing the additional mechanisms of the non-interleaved
     mode (namely STAP-A and FU-A) is minor, and the benefits are
     great, that STAP-A and FU-A implementation is required.

   A NAL unit of small size SHOULD be encapsulated in an aggregation
   packet together with one or more other NAL units. For example, non-
   VCL NAL units such as access unit delimiter, parameter set, or SEI
   NAL unit are typically small.

   A prefix NAL unit and the NAL unit with which it is associated, and
   which follows the prefix NAL unit in decoding order, SHOULD be



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   included in the same aggregation packet whenever an aggregation
   packet is used for the associated NAL unit.

      Informative note: When either the prefix NAL unit or the
      associated NAL unit containing an H.264/AVC coded slice is lost,
      the remaining one would be hardly useful in SVC context, wherein
      the prefix NAL unit must be available for decoded picture buffer
      management operations of the decoding process.

   When the first aggregation unit of an aggregation packet contains a
   PACSI NAL unit, there MUST be at least one additional aggregation
   unit present in the same packet.

7.1. Packetization Rules for session multiplexing

   When session multiplexing is used, decoding order recovery for NAL
   units carried in all the RTP sessions is needed.  Two alternative
   decoding order recovery modes are provided for session multiplexing.
   The first is referred to as the classical RTP decoding order
   recovery mode, where CL-DON MUST NOT be used.  The second is
   referred to as the CL-DON decoding order recovery mode, where CL-DON
   MUST be used.

   [Edt. Note (TS): Definition of the CL-DON as in section 6.6 in this
   draft allows for INTERLEAVED transmission of NAL units using the
   non-INTERLEAVED packetization mode.  This is really dangerous for
   interoperability and backward compatibility.  Thus the whole
   specification of the (CL-)DON basically allows for the INTERLEAVING
   feature, the only way out may be to connect the use of CL-DON to the
   interleaved mode (packetization-mode equal to 2).]

   If the classical RTP decoding order recovery mode is in use, either
   the single NAL unit packetization mode, the non-interleaved or the
   interleaved packetization mode can be used.  Different RTP sessions
   MAY still use different packetization modes.

   If the CL-DON decoding order recovery mode is in use, either the
   non-interleaved packetization mode, restricted to STAP-A packets
   only, i.e. FU-A and Single NAL unit packets MUST NOT be used, or the
   interleaved packetization mode MAY be used.  As CL-DON MUST be used,
   the CL-DON value must be derivable from the payload structure for
   this decoding order recovery mode.  Different RTP sessions MAY still
   use different packetization modes.

   The respective packetization rules for the two decoding order
   recovery modes in session multiplexing are as follows.



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  I.     The classical RTP decoding order recovery mode

    a. If an access unit of sampling time instance X is present in RTP
      session A, this access unit MUST be also present in any RTP
      session, which depends on RTP session A.

    b. When a PACSI NAL unit is present, the T bit MUST be equal to 0,
      i.e. the DONC field MUST NOT be present.

    c. The sprop-cl-don parameter MUST NOT be present in the session
      description.

        Informative note:  Restriction a. may be achieved for pre-
        encoded content by inserting filler data NAL units (NAL unit
        type 12) or filler payload SEI messages (NAL unit type 6, SEI
        message payload type equal to 3) as defined in [SVC].  This
        insertion can be achieved by encoders, servers, as well as by
        MANEs.


   II. The CL-DON decoding order recovery mode

     a. For each RTP session, the non-interleaved packetization mode or
       the interleaved packetization mode MUST be used.

     b. For any RTP session that uses the interleaved packetization
       mode, the DON values derived as specified in RFC 3984 MUST
       indicate CL-DON.

     c. For any RTP session that uses the non-interleaved packetization
       mode, the following applies.

              i. STAP-A MUST be used, and any other type of packets
                 allowed (i.e. single NAL unit packet or FU-A packets)
                 MUST NOT be used.
             ii. Each STAP-A MUST contain a PACSI NAL unit and the DONC
                 field MUST be present in the PACSI NAL unit.

     d. The sprop-cl-don parameter MUST be present in the session
       description.


8. De-Packetization Process (Informative)

   For a single RTP session, the de-packetization process specified in
   section 7 of [RFC3984] applies [Edt. Note: with some fixes to



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   section 7 of RFC 3984 and some changes/additions to section 7.3
   (Additional De-Packetization Guidelines) of RFC 3984 - TDB].

   For receiving more than one of multiple RTP sessions conveying a
   scalable bitstream, the de-packetization process is specified in
   section 8.1.

8.1. De-Packetization Process for NAL Units Conveyed using Session
Multiplexing

   As for a single RTP session, the general concept behind these de-
   packetization rules is to reorder NAL units from transmission order
   to the NAL unit decoding order.

   In this section, "the RTP sessions" refer to the RTP sessions for
   which the NAL units are de-packetized.

   The sessions to be received SHALL be identified by mechanisms
   specified in [I-D.ietf-mmusic-decoding-dependency].

   For each of the RTP sessions, the RTP reception process as specified
   in RFC 3550 is applied, such that the received packets are passed in
   increasing order of timestamp, and, for those RTP packets with
   identical RTP timestamp, in increasing order of sequence number,
   into the payload de-packetization to NAL units as defined in this
   memo.

   The decoding order of the NAL units carried in all the RTP sessions
   is then recovered by applying section 8.1.1 or 8.1.2, depending on
   the presence of the parameter sprop-cl-don in the session
   description.

8.1.1.    The Classical RTP Decoding Order Recovery Mode

   This process SHALL be used when the parameter sprop-cl-don is not
   present in the session description.

   In this section, the NAL unit decoding order recovery process is
   described for the constraints in section 7 using the non-
   interleaved, interleaved mode or Single NAL unit mode for all RTP
   sessions, i.e. CL-DON SHALL NOT be present in any of these sessions.
   The process is based on RTP session dependency signaling, RTP
   sequence numbers, and timestamps.


   1.  Within each RTP stream, the decoding order of NAL units SHALL be
     recovered according to the following rules:


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     o When using the single NAL unit mode (packetization-mode equal
     to 0) or the non-interleaved mode (packetizaton mode equal to 1),
     the RTP header sequence number SHALL give the decoding order as
     specified in [RFC3984].

     o When the interleaved mode (packetization-mode equal to 2) is
     used, the Decoding Order Number SHALL give the decoding order as
     specified in [RFC3984].

          Informative note:
          The decoding order recovery process cannot rely on timestamp
          increase as indicator for decoding order.

   2. The decoding order of NAL units from multiple RTP streams in
     multiple RTP sessions SHALL be recovered into a single sequence of
     NAL units, grouped into access units, by performing the following
     rules:

     o NAL units with the same (RTP or NTP) timestamp are grouped in
     decoding order to operation point representations in each RTP
     stream.

     o Operation point representations with the same (NTP) timestamp
     SHALL be grouped to access units in order of the SVC RTP session
     dependency, from lowest to highest.

          Note: There may be operation point representations which do
          not have any corresponding operation point representations in
          RTP streams of lower SVC RTP sessions.

     o [Edt. Note (TS): When SEI messages are not part of the base RTP
     session, they are not transported in decoding order. Only in that
     case this paragraph is required. See open issue section.]
     SEI messages (NAL unit type equal to 6), when present in any RTP
     session not equal to the base RTP session, SHALL be re-ordered so
     that the resulting access unit order conforms to [SVC], i.e. NAL
     units with nal_unit_type equal to 6 present in any of the RTP
     streams shall be grouped and precede directly any NAL units of
     type 1 , 5, 14, 15 and 20 in the access unit.

     o The access units SHALL be passed in order of decoding order
     appearance of timestamps in the highest SVC RTP session to the
     decoder.


   Informative example:


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   The example shown in Figure 2. refers to three SVC RTP sessions A, B
   and C.  In the example, the dependency signaling as described in
   9.2.3, indicates, that Session A does not depend on any other of the
   sessions; B depends on A; C depends on A and B as restricted in
   section 7.  Session A has the lowest frame rate and Session B and C
   have the same, but a higher frame rate.  Figure 2. shows an example
   for buffering with different jitters in the sessions, i.e. at
   buffering startup not all packets of the same time instance are
   available.

   The process first proceeds to TS [8] and remove/ignore all preceding
   NAL units in each of the buffers of RTP session A,B, and C.  Then
   starting from session C, the first timestamp available in decoding
   order (TS [1]) is selected and all operation point representations
   in lower RTP sessions A and B are moved in order of the RTP session
   dependency (in the example form session A -> B -> C) into the
   decoder.  Then the next timestamp in the highest RTP session C is
   selected and the process described above is repeated.  Note, that
   there may be no matching operation point representations at the
   lowest RTP session A, e.g. for TS[5].

   In case of "real" packet loss at TS[4] and TS[2], a similar
   processing as described above may be applied to get synchronized
   with the timestamp order as given by the highest RTP session (in the
   example - session "C").  During an interval where losses are present
   in the highest RTP session, reordering may be in some cases only
   possible up to the highest loss-free received RTP session.  Such
   cases may occur, when multiple losses in the highest RTP session are
   present and the correct order of timestamps over all the sessions
   cannot be recovered by any other than the highest RTP session.


   Decoding order and dependency of NAL units per received RTP session
   with different jitters in sessions at buffering startup time:

   C: ------------(1,2)-(3,4)--(5)---(6)---(7,8)(9,10)-(11)--(12)----
        |     |     |     |     |     |      |    |     |      |
   B: -(1,2)-(3,4)-(5)---(6)--(7,8)-(9,10)-(11)-(12)--(13,14)(15,15)-
        |     |                 |     |                 |      |
   A: -------(1)---------------(2)---(3)---------------(4)----(5)----
   ------------------------------------------------------------------->
   TS: [4]   [2]   [1]   [3]   [8]   [6]   [5]   [7]   [12]   [10]


   Key:
   A, B, C                - RTP sessions


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   Integer values in '()' - NAL unit decoding order per RTP session
   '( )'                  - groups the NAL units of an operation point
                            representation in a RTP session
   '|'                    - indicates corresponding operation point
                            representations in the RTP sessions
   Integer values in '[]' - media Timestamp (TS), sampling time

   Figure 2. Example for session multiplexing with different jitters in
   sessions at startup


8.1.2.    The CL-DON Decoding Order Recovery Mode

   This process SHALL used when the parameter sprop-cl-don is present
   in the session description.

   In this mode, for each NAL unit a CL-DON value can be derived.  This
   enables NAL unit decoding order recovery for all the RTP sessions
   without requiring separate decoding order recovery for each RTP
   session beforehand.

   The RTP packets output from the RTP-level reception processing for
   each session are placed into the de-session-multiplexing buffer.

   The NAL unit decoding order recovery process as described below is
   then exactly the same as the single session decoding order recovery
   process for the interleaved packetization mode as specified in
   subsection 7.2 of RFC 3984, with deinterleaving buffer replaced by
   de-session-multiplexing buffer and DON replaced by CL-DON.

   It is RECOMMENDED to set the size of the de-session-multiplexing
   buffer, in terms of number of bytes, equal to or greater than the
   value of the sprop-deint-buf-req media type parameter of the RTP
   session conveying the SVC Layer for which the decoding requires the
   presence of the SVC Layers conveyed in all the other RTP sessions,
   referred to the highest RTP session.

   There are two buffering states in the receiver: initial buffering
   and buffering while playing.  Initial buffering occurs when the RTP
   sessions are initialized.  After initial buffering, decoding and
   playback are started, and the buffering-while-playing mode is used.

   Regardless of the buffering state, the receiver stores incoming NAL
   units.  The CL-DON value is calculated and stored for each NAL unit.

   The receiver operation is described below with the help of the
   following functions and constants:


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      o  Function AbsDON is specified in section 9.1 of this
   specification.

      o  Function don_diff is specified in section 5.5 of RFC 3984.

      o  Constant N is the value of the OPTIONAL sprop-interleaving-
   depth media type parameter of the highest RTP session incremented by
   1.

   Initial buffering lasts until one of the following conditions is
   fulfilled:

      o  There are N or more VCL NAL units in the de-session-
   multiplexing buffer.

      o  If sprop-max-don-diff of the highest SVC RTP session is
   present, don_diff(m,n) is greater than the value of sprop-max-don-
   diff of the highest RTP session, in which n corresponds to the NAL
   unit having the greatest value of AbsDON among the received NAL
   units and m corresponds to the NAL unit having the smallest value of
   AbsDON among the received NAL units.

      o  Initial buffering has lasted for the duration equal to or
   greater than the value of the OPTIONAL sprop-init-buf-time media
   type parameter of the highest SVC RTP session.

   The NAL units to be removed from the de-session-multiplexing buffer
   are determined as follows:

      o  If the de-session-multiplexing buffer contains at least N VCL
   NAL units, NAL units are removed from the de-session-multiplexing
   buffer and passed to the decoder in the order specified below until
   the buffer contains N-1 VCL NAL units.

      o  If sprop-max-don-diff of the highest SVC RTP session is
   present, all NAL units m for which don_diff(m,n) is greater than
   sprop-max-don-diff of the highest RTP session are removed from the
   de-session-multiplexing buffer and passed to the decoder in the
   order specified below.  Herein, n corresponds to the NAL unit having
   the greatest value of AbsDON among the NAL units in the de-session-
   multiplexing buffer.

   The order in which NAL units are passed to the decoder is specified
   as follows:




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      o  Let PDON be a variable that is initialized to 0 at the
   beginning of the RTP sessions.

      o  For each NAL unit associated with a value of CL-DON, a CL-DON
   distance is calculated as follows.  If the value of CL-DON of the
   NAL unit is larger than the value of PDON, the CL-DON distance is
   equal to CL-DON - PDON.  Otherwise, the CL-DON distance is equal to
   65535 - PDON + CL-DON + 1.

      o  NAL units are delivered to the decoder in ascending order of
   CL-DON distance.  If several NAL units share the same value of CL-
   DON distance, they can be passed to the decoder in any order.

      o  When a desired number of NAL units have been passed to the
   decoder, the value of PDON is set to the value of CL-DON for the
   last NAL unit passed to the decoder.

9. Payload Format Parameters

   This section specifies the parameters that MAY be used to select
   optional features of the payload format and certain features of the
   bitstream.  The parameters are specified here as part of the media
   type registration for the SVC codec.  A mapping of the parameters
   into the Session Description Protocol (SDP) [RFC4566] is also
   provided for applications that use SDP.  Equivalent parameters could
   be defined elsewhere for use with control protocols that do not use
   SDP.

   Some parameters provide a receiver with the properties of the stream
   that will be sent.  The names of all these parameters start with
   "sprop" for stream properties.  Some of these "sprop" parameters are
   limited by other payload or codec configuration parameters.  For
   example, the sprop-parameter-sets parameter is constrained by the
   profile-level-id parameter.  The media sender selects all "sprop"
   parameters rather than the receiver.  This uncommon characteristic
   of the "sprop" parameters may not be compatible with some signaling
   protocol concepts, in which case the use of these parameters SHOULD
   be avoided.

9.1. Media Type Registration

   The media subtype for the SVC codec is allocated from the IETF tree.

   The receiver MUST ignore any unspecified parameter.

     Informative note: Requiring that the receiver ignores unspecified
     parameters allows for backward compatibility of future extensions.


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     For example, if a future specification that is backward compatible
     to this specification specifies some new parameters, then a
     receiver according to this specification is capable of receiving
     data per the new payload but ignoring those parameters newly
     specified in the new payload specification.  This provision is
     also present in RFC 3984.

   Media Type name:     video

   Media subtype name:  H264-SVC or H264

   The media subtype "H264" MUST be used for RTP streams using RFC
   3984, i.e. not using any of the new features introduced by this
   specification compared to RFC 3984.  [Edt. Note: The new features
   are to be listed herein.]  For RTP streams using any of the new
   features introduced by this specification compared to RFC 3984, the
   media subtype "H264-SVC" SHOULD be used, and the media subtype
   "H264" MAY be used.  Use of the media subtype "H264" for RTP streams
   using the new features allows for RFC 3984 receivers to negotiate
   and receive H.264/AVC or SVC streams packetized according to this
   specification, but to ignore media parameters and NAL unit types it
   does not recognize.

   Required parameters: none

   OPTIONAL parameters:

       profile-level-id:
                     A base16 [RFC3548] (hexadecimal) representation of
                     the following three bytes in the sequence
                     parameter set NAL unit specified in [SVC]: 1)
                     profile_idc, 2) a byte herein referred to as
                     profile-iop, composed of the values of
                     constraint_set0_flag, constraint_set1_flag,
                     constraint_set2_flag, constraint_set3_flag, and
                     reserved_zero_4bits
                     in bit-significance order, starting from the
                     most significant bit, and 3) level_idc.  Note
                     that reserved_zero_4bits is required to be
                     equal to 0 in [SVC], but other values for it may
                     be specified in the future by ITU-T or ISO/IEC.

                     If the profile-level-id parameter is used to
                     indicate properties of a NAL unit stream, it
                     indicates the profile and level that a decoder
                     has to support in order to comply with [SVC] when
                     it decodes the NAL unit stream.  The profile-iop
                     byte indicates whether the NAL unit stream also


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                     obeys all the constraints as specified in
                     subsection 7.4.2.1.1 of [SVC].  Herein the NAL
                     unit stream refers to the one consisting of all
                     NAL units conveyed in the current RTP session,
                     and all NAL units conveyed in other RTP sessions,
                     if present, the current RTP session depends on.
                     The current RTP session MAY depend on other RTP
                     sessions when a scalable bitstream is transported
                     with more than one RTP session and the current
                     session is not an independent RTP session.

                     If the profile-level-id parameter is used for
                     capability exchange or session setup procedure,
                     it indicates the profile that the codec
                     supports and the highest level
                     supported for the signaled profile.  The
                     profile-iop byte indicates whether the codec
                     has additional limitations whereby only the
                     common subset of the algorithmic features and
                     limitations signaled with the
                     profile-iop byte is supported by the codec.  For
                     example, if a codec supports only the common
                     subset of the coding tools of the Baseline
                     profile and the Main profile at level 2.1 and
                     below, the profile-level-id becomes 42E015, in
                     which 42 stands for the Baseline profile, E0
                     indicates that only the common subset for all
                     profiles is supported, and 15 indicates level
                     2.1.

                     Informative note: Capability exchange and
                     session setup procedures should provide
                     means to list the capabilities for each
                     supported codec profile separately.  For
                     example, the one-of-N codec selection
                     procedure of the SDP Offer/Answer model can
                     be used (section 10.2 of [RFC4566]).

                     If no profile-level-id is present, the Baseline
                     Profile without additional constraints at Level
                     1 MUST be implied.

       max-mbps, max-fs, max-cpb, max-dpb, and max-br:
                     These parameters MAY be used to signal the
                     capabilities of a receiver or a sender
                     implementation.
                     These parameters MUST NOT be used for any other
                     purpose.  The profile-level-id parameter MUST
                     be present in the same receiver capability
                     description that contains any of these
                     parameters.  The level conveyed in the value of

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                     the profile-level-id parameter MUST be such
                     that the receiver is fully capable of
                     supporting.  max-mbps, max-fs, max-cpb, max-
                     dpb, and max-br MAY be used to indicate
                     capabilities of the receiver that extend the
                     required capabilities of the signaled level, as
                     specified below.

                     When more than one parameter from the set (max-
                     mbps, max-fs, max-cpb, max-dpb, max-br) is
                     present, the receiver MUST support all signaled
                     capabilities simultaneously.  For example, if
                     both max-mbps and max-br are present, the
                     signaled level with the extension of both the
                     frame rate and bit rate is supported.  That is,
                     the receiver is able to decode NAL unit
                     streams in which the macroblock processing rate
                     is up to max-mbps (inclusive), the bit rate is
                     up to max-br (inclusive), the coded picture
                     buffer size is derived as specified in the
                     semantics of the max-br parameter below, and
                     other properties comply with the level
                     specified in the value of the profile-level-id
                     parameter.

                     A receiver MUST NOT signal values of max-
                     mbps, max-fs, max-cpb, max-dpb, and max-br that
                     meet the requirements of a higher level,
                     referred to as level A herein, compared to the
                     level specified in the value of the profile-
                     level-id parameter, if the receiver can support
                     all the properties of level A.

                         Informative note: When the OPTIONAL media
                         type parameters are used to signal the
                         properties of a NAL unit stream, max-mbps,
                         max-fs, max-cpb, max-dpb, and max-br are
                         not present, and the value of profile-
                         level-id must always be such that the NAL
                         unit stream complies fully with the
                         specified profile and level.

       max-mbps:     The value of max-mbps is an integer indicating
                     the maximum macroblock processing rate in units
                     of macroblocks per second.  The max-mbps
                     parameter signals that the receiver is capable
                     of decoding video at a higher rate than is
                     required by the signaled level conveyed in the
                     value of the profile-level-id parameter.  When
                     max-mbps is signaled, the receiver MUST be able
                     to decode NAL unit streams that conform to the

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                     signaled level, with the exception that the
                     MaxMBPS value in Table A-1 or Table G-n of [SVC]
                     for the
                     signaled level is replaced with the value of
                     max-mbps.  The value of max-mbps MUST be
                     greater than or equal to the value of MaxMBPS
                     for the level given in Table A-1 or Table G-n of
                     [SVC].
                     Senders MAY use this knowledge to send pictures
                     of a given size at a higher picture rate than
                     is indicated in the signaled level.

       max-fs:       The value of max-fs is an integer indicating
                     the maximum frame size in units of macroblocks.
                     The max-fs parameter signals that the receiver
                     is capable of decoding larger picture sizes
                     than are required by the signaled level conveyed
                     in the value of the profile-level-id parameter.
                     When max-fs is signaled, the receiver MUST be
                     able to decode NAL unit streams that conform to
                     the signaled level, with the exception that the
                     MaxFS value in Table A-1 or Table G-n of [SVC]
                     for the
                     signaled level is replaced with the value of
                     max-fs.  The value of max-fs MUST be greater
                     than or equal to the value of MaxFS for the
                     level given in Table A-1 or Table G-n of [SVC].
                     Senders MAY
                     use this knowledge to send larger pictures at a
                     proportionally lower frame rate than is
                     indicated in the signaled level.

       max-cpb       The value of max-cpb is an integer indicating
                     the maximum coded picture buffer size in units
                     of 1000 bits for the VCL HRD parameters (see
                     A.3.1 item i or G.n item m of [SVC]) and in units
                     of 1200 bits
                     for the NAL HRD parameters (see A.3.1 item j or
                     G.n item m of
                     [SVC]).  The max-cpb parameter signals that the
                     receiver has more memory than the minimum
                     amount of coded picture buffer memory required
                     by the signaled level conveyed in the value of
                     the profile-level-id parameter.  When max-cpb
                     is signaled, the receiver MUST be able to
                     decode NAL unit streams that conform to the
                     signaled level, with the exception that the
                     MaxCPB value in Table A-1 or Table G-n of [SVC]
                     for the
                     signaled level is replaced with the value of
                     max-cpb.  The value of max-cpb MUST be greater

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                     than or equal to the value of MaxCPB for the
                     level given in Table A-1 or Table G-n of [SVC].
                     Senders MAY
                     use this knowledge to construct coded video
                     streams with greater variation of bit rate
                     than can be achieved with the
                     MaxCPB value in Table A-1 or Table G-n of [SVC].

                         Informative note: The coded picture buffer
                         is used in the hypothetical reference
                         decoder (Annex C) of SVC.  The use of the
                         hypothetical reference decoder is
                         recommended in SVC encoders to verify
                         that the produced bitstream conforms to the
                         standard and to control the output bitrate.
                         Thus, the coded picture buffer is
                         conceptually independent of any other
                         potential buffers in the receiver,
                         including de-interleaving and de-jitter
                         buffers.  The coded picture buffer need not
                         be implemented in decoders as specified in
                         Annex C of SVC, but rather standard-
                         compliant decoders can have any buffering
                         arrangements provided that they can decode
                         standard-compliant bitstreams.  Thus, in
                         practice, the input buffer for video
                         decoder can be integrated with de-
                         interleaving and de-jitter buffers of the
                         receiver.

       max-dpb:      The value of max-dpb is an integer indicating
                     the maximum decoded picture buffer size in
                     units of 1024 bytes.  The max-dpb parameter
                     signals that the receiver has more memory than
                     the minimum amount of decoded picture buffer
                     memory required by the signaled level conveyed
                     in the value of the profile-level-id parameter.
                     When max-dpb is signaled, the receiver MUST be
                     able to decode NAL unit streams that conform to
                     the signaled level, with the exception that the
                     MaxDPB value in Table A-1 or Table G-n of [SVC]
                     for the
                     signaled level is replaced with the value of
                     max-dpb.  Consequently, a receiver that signals
                     max-dpb MUST be capable of storing the
                     following number of decoded frames,
                     complementary field pairs, and non-paired
                     fields in its decoded picture buffer:

                     Min(1024 * max-dpb / ( PicWidthInMbs *
                     FrameHeightInMbs * 256 * ChromaFormatFactor ),

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                     16)

                     PicWidthInMbs, FrameHeightInMbs, and
                     ChromaFormatFactor are defined in [SVC].

                     The value of max-dpb MUST be greater than or
                     equal to the value of MaxDPB for the level
                     given in Table A-1 or Table G-n of [SVC].
                     Senders MAY use
                     this knowledge to construct coded video streams
                     with improved compression.

                         Informative note: This parameter was added
                         primarily to complement a similar codepoint
                         in the ITU-T Recommendation H.245, so as to
                         facilitate signaling gateway designs.  The
                         decoded picture buffer stores reconstructed
                         samples.  There is no relationship
                         between the size of the decoded picture
                         buffer and the buffers used in RTP,
                         especially de-interleaving and de-jitter
                         buffers.

       max-br:       The value of max-br is an integer indicating
                     the maximum video bit rate in units of 1000
                     bits per second for the VCL HRD parameters (see
                     A.3.1 item i or G.n item m of [SVC]) and in units
                     of 1200 bits
                     per second for the NAL HRD parameters (see
                     A.3.1 item j or G.n item m of [SVC]).

                     The max-br parameter signals that the video
                     decoder of the receiver is capable of decoding
                     video at a higher bit rate than is required by
                     the signaled level conveyed in the value of the
                     profile-level-id parameter.

                     When max-br is signaled, the video codec of the
                     receiver MUST be able to decode NAL unit
                     streams that conform to the signaled level,
                     conveyed in the profile-level-id parameter,
                     with the following exceptions in the limits
                     specified by the level:
                     o The value of max-br replaces the MaxBR value
                       of the signaled level (in Table A-1 of or Table
                       G-n of [SVC]).
                     o When the max-cpb parameter is not present,
                       the result of the following formula replaces
                       the value of MaxCPB in Table A-1 or Table G-n
                       of [SVC]:
                       (MaxCPB of the signaled level) * max-br /

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                       (MaxBR of the signaled level).

                     For example, if a receiver signals capability
                     for Level 1.2 with max-br equal to 1550, this
                     indicates a maximum video bitrate of 1550
                     kbits/sec for VCL HRD parameters, a maximum
                     video bitrate of 1860 kbits/sec for NAL HRD
                     parameters, and a CPB size of 4036458 bits
                     (1550000 / 384000 * 1000 * 1000).

                     The value of max-br MUST be greater than or
                     equal to the value MaxBR for the signaled level
                     given in Table A-1 or Table G-n of [SVC].

                     Senders MAY use this knowledge to send higher
                     bitrate video as allowed in the level
                     definition of SVC, to achieve
                     improved video quality.

                         Informative note: This parameter was added
                         primarily to complement a similar codepoint
                         in the ITU-T Recommendation H.245, so as to
                         facilitate signaling gateway designs.  No
                         assumption can be made from the value of
                         this parameter that the network is capable
                         of handling such bit rates at any given
                         time.  In particular, no conclusion can be
                         drawn that the signaled bit rate is
                         possible under congestion control
                         constraints.

      redundant-pic-cap:
                     This parameter signals the capabilities of a
                     receiver implementation.  When equal to 0, the
                     parameter indicates that the receiver makes no
                     attempt to use redundant coded pictures to
                     correct incorrectly decoded primary coded
                     pictures.  When equal to 0, the receiver is not
                     capable of using redundant slices; therefore, a
                     sender SHOULD avoid sending redundant slices to
                     save bandwidth.  When equal to 1, the receiver
                     is capable of decoding any such redundant slice
                     that covers a corrupted area in a primary
                     decoded picture (at least partly), and therefore
                     a sender MAY send redundant slices.  When the
                     parameter is not present, then a value of 0
                     MUST be used for redundant-pic-cap.  When
                     present, the value of redundant-pic-cap MUST be
                     either 0 or 1.

                     When the profile-level-id parameter is present

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                     in the same capability signaling as the
                     redundant-pic-cap parameter, and the profile
                     indicated in profile-level-id is such that it
                     disallows the use of redundant coded pictures
                     (e.g., Main Profile), the value of redundant-
                     pic-cap MUST be equal to 0.  When a receiver
                     indicates redundant-pic-cap equal to 0, the
                     received stream SHOULD NOT contain redundant
                     coded pictures.

                         Informative note: Even if redundant-pic-cap
                         is equal to 0, the decoder is able to
                         ignore redundant codec pictures provided
                         that the decoder supports such a profile
                         (Baseline, Extended) in which redundant
                         coded pictures are allowed.

                         Informative note: Even if redundant-pic-cap
                         is equal to 1, the receiver may also choose
                         other error concealment strategies to
                         replace or complement decoding of redundant
                         slices.

       sprop-parameter-sets:
                     This parameter MAY be used to convey
                     any sequence and picture parameter set NAL
                     units (herein referred to as the initial
                     parameter set NAL units) that MUST be placed in
                     the NAL unit stream to precede any
                     other NAL units in decoding order by the receiver.
                     The parameter MUST NOT be used to indicate codec
                     capability in any capability exchange
                     procedure.  The value of the parameter is the
                     base64 [RFC3548] representation of the initial
                     parameter set NAL units as specified in
                     sections 7.3.2.1, 7.3.2.2 and G.7.3.2.1.3 of
                     [SVC].  The
                     parameter sets are conveyed in decoding order,
                     and no framing of the parameter set NAL units
                     takes place.  A comma is used to separate any
                     pair of parameter sets in the list.  Note that
                     the number of bytes in a parameter set NAL unit
                     is typically less than 10, but a picture
                     parameter set NAL unit can contain several
                     hundreds of bytes.

                        Informative note: When several payload
                        types are offered in the SDP Offer/Answer
                        model, each with its own sprop-parameter-
                        sets parameter, then the receiver cannot
                        assume that those parameter sets do not use

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                        conflicting storage locations (i.e.,
                        identical values of parameter set
                        identifiers).  Therefore, a receiver should
                        double-buffer all sprop-parameter-sets and
                        make them available to the decoder instance
                        that decodes a certain payload type.

       parameter-add:
                     This parameter MAY be used to signal whether
                     the receiver of this parameter is allowed to
                     add parameter sets in its signaling response
                     using the sprop-parameter-sets media parameter.
                     The value of this parameter is either 0 or 1.
                     0 is equal to false; i.e., it is not allowed to
                     add parameter sets.  1 is equal to true; i.e.,
                     it is allowed to add parameter sets.  If the
                     parameter is not present, its value MUST be 1.

       packetization-mode:
                     This parameter signals the properties of an
                     RTP payload type or the capabilities of a
                     receiver implementation.  Only a single
                     configuration point can be indicated; thus,
                     when capabilities to support more than one
                     packetization-mode are declared, multiple
                     configuration points (RTP payload types) must
                     be used.

                     When the value of packetization-mode is equal
                     to 0 or packetization-mode is not present, the
                     single NAL mode, as defined in section 6.2 of
                     RFC 3984, MUST be used.  This mode is in use in
                     standards using ITU-T Recommendation H.241
                     [H.241] (see section 12.1 of RFC 3984).  When the
                    value of
                    packetization-mode is equal to 1, the non-
                    interleaved mode, as defined in section 6.3 of
                    RFC 3984, MUST be used.  When the value of
                    packetization-mode is equal to 2, the
                    interleaved mode, as defined in section 6.4 of
                    RFC 3984, MUST be used.  The value of
                    packetization mode MUST be an integer in the
                    range of 0 to 2, inclusive.

       sprop-interleaving-depth:
                     This parameter MUST NOT be present when the
                     current RTP session does not depend on any other
                     RTP session, and packetization-mode is not present
                     or the value of packetization-mode is equal to 0
                     or 1.  This parameter MUST be present when
                     sprop-cl-don value is present or the value of

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                     packetization-mode is equal to 2.

                     This parameter signals the properties of a NAL
                     unit stream.  It specifies the maximum number
                     of VCL NAL units that precede any VCL NAL unit
                     in the NAL unit stream in transmission order
                     and follow the VCL NAL unit in decoding order.
                     Consequently, it is guaranteed that receivers
                     can reconstruct NAL unit decoding order when
                     the buffer size for NAL unit decoding order
                     recovery is at least the value of sprop-
                     interleaving-depth + 1 in terms of VCL NAL
                     units.  Herein the NAL
                     unit stream refers to the one consisting of all
                     NAL units conveyed in the current RTP session,
                     and all NAL units conveyed in other RTP sessions,
                     if present, the current RTP session depends on.

                     The value of sprop-interleaving-depth MUST be
                     an integer in the range of 0 to 32767,
                     inclusive.

       sprop-deint-buf-req:
                     This parameter MUST NOT be present when the
                     current RTP session does not depend on any other
                     RTP session, and packetization-mode is not present
                     or the value of packetization-mode is equal to 0
                     or 1.  This parameter MUST be present when the
                     the sprop-cl-don value is present or the value of
                     packetization-mode is equal to 2.

                     sprop-deint-buf-req signals the required size
                     of the deinterleaving buffer for the NAL unit
                     stream.  The value of the parameter MUST be
                     greater than or equal to the maximum buffer
                     occupancy (in units of bytes) required in such
                     a deinterleaving buffer that is specified in
                     section 8 of this specification.  It is
                     guaranteed that receivers can perform the
                     deinterleaving of
                     interleaved NAL units into NAL unit decoding
                     order, when the deinterleaving buffer size is
                     at least the value of sprop-deint-buf-req in
                     terms of bytes.  Herein the NAL
                     unit stream refers to the one consisting of all
                     NAL units conveyed in the current RTP session,
                     and all NAL units conveyed in other RTP sessions,
                     if present, the current RTP session depends on.

                     The value of sprop-deint-buf-req MUST be an
                     integer in the range of 0 to 4294967295,

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                     inclusive.

                         Informative note: sprop-deint-buf-req
                         indicates the required size of the
                         deinterleaving buffer only.  When network
                         jitter can occur, an appropriately sized
                         jitter buffer has to be provisioned for
                         as well.  When a scalable bitstream is
                         conveyed in more than one RTP session, and
                         the sessions initiates at different time, the
                         session initiation variation has also to be
                         compensated by an appropriately sized buffer.

       deint-buf-cap:
                     This parameter signals the capabilities of a
                     receiver implementation and indicates the
                     amount of deinterleaving buffer space in units
                     of bytes that the receiver has available for
                     reconstructing the NAL unit decoding order, and
                     that the receiver is able to handle any stream
                     for which
                     the value of the sprop-deint-buf-req parameter
                     is smaller than or equal to this parameter.

                     If the parameter is not present, then a value
                     of 0 MUST be used for deint-buf-cap.  The value
                     of deint-buf-cap MUST be an integer in the
                     range of 0 to 4294967295, inclusive.

                         Informative note: deint-buf-cap indicates
                         the maximum possible size of the
                         deinterleaving buffer of the receiver only.
                         When network jitter can occur, an
                         appropriately sized jitter buffer has to
                         be provisioned for as well.

       sprop-init-buf-time:
                     This parameter MAY be used to signal the
                     properties of a NAL unit stream.  Herein
                     the NAL unit stream refers to the one consisting
                     of all NAL units conveyed in the current RTP
                     session, and all NAL units conveyed in other RTP
                     sessions, if present, the current RTP session
                     depends on and sprop-cl-don value is present.

                     The parameter signals the initial buffering
                     time for a receiver before
                     starting to recover the NAL unit
                     decoding order from the transmission order.
                     The parameter is the maximum value of
                     (transmission time of a NAL unit - decoding

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                     time of the NAL unit), assuming reliable and
                     instantaneous transmission, the same
                     timeline for transmission and decoding, and
                     that decoding starts when the first packet
                     arrives.

                     An example of specifying the value of sprop-
                     init-buf-time follows.  A NAL unit stream is
                     sent in the following interleaved order, in
                     which the value corresponds to the decoding
                     time and the transmission order is from left to
                     right:

                     0  2  1  3  5  4  6  8  7 ...

                     Assuming a steady transmission rate of NAL
                     units, the transmission times are:

                     0  1  2  3  4  5  6  7  8 ...

                     Subtracting the decoding time from the
                     transmission time column-wise results in the
                     following series:

                     0 -1  1  0 -1  1  0 -1  1 ...

                     Thus, in terms of intervals of NAL unit
                     transmission times, the value of
                     sprop-init-buf-time in this
                     example is 1.

                     The parameter is coded as a non-negative base10
                     integer representation in clock ticks of a 90-
                     kHz clock.  If the parameter is not present,
                     then no initial buffering time value is
                     defined.  Otherwise the value of sprop-init-
                     buf-time MUST be an integer in the range of 0
                     to 4294967295, inclusive.

                     In addition to the signaled sprop-init-buf-
                     time, receivers SHOULD take into account the
                     transmission delay jitter buffering, including
                     buffering for the delay jitter caused by
                     mixers, translators, gateways, proxies,
                     traffic-shapers, and other network elements.  Yet
                     another aspect receivers SHOULD take into account
                     is the session initiation variation when a
                     scalable bitstream is conveyed in more than one
                     session, including buffering the variation.

       sprop-max-don-diff:

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                     This parameter MAY be used to signal the
                     properties of a NAL unit stream.  It MUST NOT
                     be used to signal transmitter or receiver or
                     codec capabilities.  sprop-max-don-diff is an
                     integer in the range of 0 to 32767, inclusive.
                     If sprop-max-don-diff is not present, the value
                     of the parameter is unspecified.  Herein the NAL
                     unit stream refers to the one consisting of all
                     NAL units conveyed in the current RTP session,
                     and all NAL units conveyed in other RTP sessions,
                     if present, the current RTP session depends on.

                     sprop-max-don-diff is calculated as follows:

                     sprop-max-don-diff = max{AbsDON(i) -
                     AbsDON(j)},
                     for any i and any j>i,

                     where i and j indicate the index of the NAL
                     unit in the transmission order and AbsDON
                     denotes a decoding order number of the NAL
                     unit that does not wrap around to 0 after
                     65535.  In other words, AbsDON is calculated as
                     follows: Let m and n be consecutive NAL units
                     in transmission order.  For the very first NAL
                     unit in transmission order (whose index is 0),
                     AbsDON(0) = DON(0).  For other NAL units,
                     AbsDON is calculated as follows:

                     If DON(m) == DON(n), AbsDON(n) = AbsDON(m)

                     If (DON(m) < DON(n) and DON(n) - DON(m) <
                     32768),
                     AbsDON(n) = AbsDON(m) + DON(n) - DON(m)

                     If (DON(m) > DON(n) and DON(m) - DON(n) >=
                     32768),
                     AbsDON(n) = AbsDON(m) + 65536 - DON(m) + DON(n)

                     If (DON(m) < DON(n) and DON(n) - DON(m) >=
                     32768),

                     AbsDON(n) = AbsDON(m) - (DON(m) + 65536 -
                     DON(n))

                     If (DON(m) > DON(n) and DON(m) - DON(n) <
                     32768),
                     AbsDON(n) = AbsDON(m) - (DON(m) - DON(n))

                     where DON(i) is the decoding order number of
                     the NAL unit having index i in the transmission

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                     order.  The decoding order number is specified
                     in section 6.6 of this specification.

                         Informative note: Receivers may use sprop-
                         max-don-diff to trigger which NAL units in
                         the receiver buffer can be passed to the
                         decoder.

     max-rcmd-nalu-size:
                     This parameter MAY be used to signal the
                     capabilities of a receiver.  The parameter MUST
                     NOT be used for any other purposes.  The value
                     of the parameter indicates the largest NALU
                     size in bytes that the receiver can handle
                     efficiently.  The parameter value is a
                     recommendation, not a strict upper boundary.
                     The sender MAY create larger NALUs but must be
                     aware that the handling of these may come at a
                     higher cost than NALUs conforming to the
                     limitation.

                     The value of max-rcmd-nalu-size MUST be an
                     integer in the range of 0 to 4294967295,
                     inclusive.  If this parameter is not specified,
                     no known limitation to the NALU size exists.
                     Senders still have to consider the MTU size
                     available between the sender and the receiver
                     and SHOULD run MTU discovery for this purpose.

                     This parameter is motivated by, for example, an
                     IP to H.223 video telephony gateway, where
                     NALUs smaller than the H.223 transport data
                     unit will be more efficient.  A gateway may
                     terminate IP; thus, MTU discovery will normally
                     not work beyond the gateway.

                         Informative note: Setting this parameter to
                         a lower than necessary value may have a
                         negative impact.

   sprop-cl-don:
   When present in an RTP session description, the DONC field MUST be
   present in the PACSI NAL unit contained in any STAP-A packet in the
   current RTP session, and, if the interleaved packetization mode is
   in use, the DON values derived as specified in RFC 3984 MUST
   indicate CL-DON values.

   sprop-prebuf-size:
   This parameter MAY be present when the current RTP session depends
   on any other RTP session.  This parameter MUST NOT be present when

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   sprop-cl-don is present.  sprop-prebuf-size MAY signal the required
   size of the receiver buffer for the NAL unit stream per RTP session.
   This parameter may be useful to compensate the impact of inter-RTP
   session jitter, when the receiver buffer size is at least the value
   of sprop-prebuf-size in terms of bytes.  Herein the NAL unit stream
   refers to the one consisting of all NAL units conveyed in the
   current RTP session.

   The value of sprop-prebuf-size MUST be an integer in the range of 0
   to 4294967295, inclusive.

          Informative note: sprop-prebuf-size indicates the required
          size of the prebuffering receiver buffer only.  When network
          jitter can occur, an appropriately sized jitter buffer has to
          be provisioned for as well.  When a scalable bitstream is
          conveyed in more than one RTP session, and the sessions
          initiates at different time, the session initiation variation
          has also to be compensated by an appropriately sized buffer.

   sprop-prebuf-time:
   This parameter MAY be used to signal the properties of a NAL unit
   stream within a session multiplexing.  Herein the NAL unit stream
   refers to the one consisting of all NAL units conveyed in the
   current RTP session.  This parameter MUST NOT be present when sprop-
   cl-don is present.

   The parameter signals the initial buffering time is used for a
   receiver before starting to recover the NAL unit decoding order for
   more than one RTP session. The parameter is the maximum value of
   (transmission time of a NAL unit - decoding time of the NAL unit),
   assuming reliable and instantaneous transmission, the same timeline
   for transmission and decoding, and that decoding starts when the
   first packet arrives.

   The parameter is coded as a non-negative base10 integer
   representation in clock ticks of a 90-kHz clock.  If the parameter
   is not present, then no initial buffering time value is defined.
   Otherwise the value of sprop-prebuf-time MUST be an integer in the
   range of 0 to 4294967295, inclusive.

   In addition to the signaled sprop-prebuf-time, receivers SHOULD take
   into account the transmission delay jitter buffering, including
   buffering for the delay jitter caused by mixers, translators,
   gateways, proxies, traffic-shapers, and other network elements.  Yet
   another aspect receivers SHOULD take into account is the session
   initiation variation when a scalable bitstream is conveyed in more
   than one session, including buffering the variation.


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   sprop-scalability-info:
   This parameter MAY be used to convey the NAL unit containing the
   scalability information SEI message as specified in Annex G of
   [SVC].  This parameter MAY be used to signal the contained Layers of
   an SVC bitstream.  The parameter MUST NOT be used to indicate codec
   capability in any capability exchange procedure.  The value of the
   parameter is the base64 representation of the NAL unit containing
   the scalability information SEI message.  If present, the NAL unit
   MUST contain only a scalability information SEI message.

   This parameter MAY be used in an offering or declarative SDP message
   to indicate what Layers can be provided.  A receiver MAY indicate
   its choice of one Layer using the optional media type parameter
   scalable-layer-id.

   sprop-layer-range:
   This parameter MAY be used to signal two sets of the layer
   identification values of the lowest and highest operation points
   conveyed in the RTP session.  Each set is a base16 representation of
   a three-character value, with the first character representing DID,
   the second character representing QID, and the third character
   representing TID.  The two sets are comma separated.  Let DIDl and
   DIDh be the least DID value and the greatest DID value,
   respectively, among all the NAL units conveyed in the RTP session.
   Let QIDl and TIDl be the least QID value and the least TID value,
   respectively, among all the NAL units that are conveyed in the RTP
   session and that have DID equal to DIDl. Let QIDh and TIDh be the
   greatest QID value and the great TID value, respectively, among all
   the NAL units that are conveyed in the RTP session and that have DID
   equal to DIDh.  The first set indicates the DID, QID and TID values
   of the lowest operation point, for which the DID, QID and TID values
   are equal to DIDl, QIDl, and TIDl, respectively.  The second set
   indicates the DID, QID and TID values of the highest operation
   point, for which the DID, QID and TID values are equal to DIDh,
   QIDh, and TIDh, respectively.

   scalable-layer-id:
   This parameter MAY be used to signal a receiver's choice of the
   offers or declared operation points or layers using sprop-
   scalability-info.  The value of scalable-layer-id is a base16
   representation of the layer_id[ i ] syntax element in the
   scalability information SEI message as specified in [SVC].
   [Edt. Note (TS): That is, a SDP capable receiver/middle-box must
   decode the sprop-scalabiltiy-info syntax, which is not specified in
   this memo, to select a scalable-layer-id. This is currently not
   addressed in the offer answer section!]


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   sprop-spatial-resolution: [Edt. Note: I know that framerate and
   bitrate SDP parameters are already available, but failed to find a
   spatial resolution SDP parameter. It would be good if this is
   already defined. Otherwise, it would be better to be defined
   somewhere else because it is a generic parameter.]

   This parameter MAY be used to indicate the property of a stream or
   the capability of a receiver or sender implementation.  The value is
   a base16 of the width and height of the spatial resolution, in
   pixels, separated by a comma.
   [Edt. Note (TS): Shouldn't this be a generic SDP parameter?]

      Encoding considerations:
                           This type is only defined for transfer
                           via RTP (RFC 3550).

      Security considerations:
                           See section 10 of RFC XXXX.

      Public specification:
                           Please refer to RFC XXXX and its section 14.

      Additional information:
                           None

      File extensions:     none
      Macintosh file type code: none
      Object identifier or OID: none
      Person & email address to contact for further information:
      Intended usage:      COMMON
      Author:
      Change controller:
                           IETF Audio/Video Transport working group
                           delegated from the IESG.

9.2. SDP Parameters

   [Edt. Note: For agreeing on a Layer or OP in unicast, an SDP can
   contain multiple m lines with bitrate, framerate and spatial
   resolution parameters available, in addition to sprop-scalability-
   info. The receive can select one of the m lines, or, for operation
   points that are not included in the m lines, one of the "scalable
   layers" specified by sprop-scalabiltiy-info using scalable-layer-id.

   For layered multicast, then the grouping signaling in
   I-D.ietf-mmusic-decoding-dependency is needed.


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   The above would conveniently support also the normal ROI use cases
   (with a few ROIs each indicated as a "scalable layer") but not the
   interactive ROI use cases.  The quality layer using priority_id use
   cases are not supported either. That would need one more optional
   media type parameter, to identify a quality layer.  The lightweight
   transcoding use cases should be supported well by using (multiple)
   normal AVC SDP offering messages.

   ]


9.2.1.    Mapping of Payload Type Parameters to SDP

   The media type video/H264-SVC string is mapped to fields in the
   Session Description Protocol (SDP) as follows:

   *  The media name in the "m=" line of SDP MUST be video.

   *  The encoding name in the "a=rtpmap" line of SDP MUST be H264-SVC
      (the media subtype).

   *  The clock rate in the "a=rtpmap" line MUST be 90000.

   *  The OPTIONAL parameters "profile-level-id", "max-mbps", "max-fs",
      "max-cpb", "max-dpb", "max-br", "redundant-pic-cap", "sprop-
      parameter-sets", "parameter-add", "packetization-mode", "sprop-
      interleaving-depth", "deint-buf-cap", "sprop-deint-buf-req",
      "sprop-init-buf-time", "sprop-max-don-diff", "max-rcmd-nalu-
      size", "sprop-prebuf-size", "sprop-prebuf-time",
      "sprop-layer-range", "sprop-scalability-info", "scalable-layer-
      id", "sprop-spatial-resolution" and "sprop-cl-don", when present,
      MUST be included in the "a=fmtp" line of SDP.  These parameters
      are expressed as a media type string, in the form of a semicolon
      separated list of parameter=value pairs.

9.2.2.    Usage with the SDP Offer/Answer Model

   When H.264 or SVC is offered over RTP using SDP in an Offer/Answer
   model [RFC3264] for negotiation for unicast usage, the following
   limitations and rules apply:

   o  The parameters identifying a media format configuration for H.264
   or SVC are "profile-level-id", "packetization-mode", and, if
   required by "packetization-mode", "sprop-deint-buf-req".  These
   three parameters MUST be used symmetrically; i.e., the answerer MUST
   either maintain all configuration parameters or remove the media


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   format (payload type) completely, if one or more of the parameter
   values are not supported.

      Informative note: The requirement for symmetric use applies only
      for the above three parameters and not for the other stream
      properties and capability parameters.


   To simplify handling and matching of these configurations, the same
   RTP payload type number used in the offer SHOULD also be used in the
   answer, as specified in [RFC3264].  An answer MUST NOT contain a
   payload type number used in the offer unless the configuration
   ("profile-level-id", "packetization-mode", and, if present, "sprop-
   deint-buf-req") is the same as in the offer.

       Informative note: An offerer, when receiving the answer, has to
       compare payload types not declared in the offer based on media
       type (i.e., video/H264-SVC) and the above three parameters with
       any payload types it has already declared, in order to determine
       whether the configuration in question is new or equivalent to a
       configuration already offered.

   An answerer MAY select from the layers offered in the "sprop-
   scalability-information" parameter by including "scalable-layer-id"
   or "sprop-layer-range" in the answer.[Edt. Note: do we need to
   additionally define behavior with snd/rcvonly parameter?]

   o  The parameters "sprop-parameter-sets", "sprop-deint-buf-req",
   "sprop-interleaving-depth", "sprop-max-don-diff", "sprop-init-buf-
   time", "sprop-prebuf-size", "sprop-prebuf-time",
   "sprop-scalability-information", "sprop-layer-range" describe the
   properties of the NAL unit stream that the offerer or answerer is
   sending for this media format configuration.  This differs from the
   normal usage of the Offer/Answer parameters: normally such
   parameters declare the properties of the stream that the offerer or
   the answerer is able to receive.  When dealing with H.264 or SVC,
   the offerer assumes that the answerer will be able to receive media
   encoded using the configuration being offered.

       Informative note: The above parameters apply for any stream sent
       by the declaring entity with the same configuration; i.e., they
       are dependent on their source.  Rather then being bound to the
       payload type, the values may have to be applied to another
       payload type when being sent, as they apply for the
       configuration.




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   o  The capability parameters ("max-mbps", "max-fs", "max-cpb",
   "max-dpb", "max-br", ,"redundant-pic-cap", "max-rcmd-nalu-size") MAY
   be used to declare further capabilities.  Their interpretation
   depends on the direction attribute.  When the direction attribute is
   sendonly, then the parameters describe the limits of the RTP packets
   and the NAL unit stream that the sender is capable of producing.
   When the direction attribute is sendrecv or recvonly, then the
   parameters describe the limitations of what the receiver accepts.

   o  As specified above, an offerer has to include the size of the
   deinterleaving buffer in the offer for an interleaved H.264 or SVC
   stream.  To enable the offerer and answerer to inform each other
   about their capabilities for deinterleaving buffering, both parties
   are RECOMMENDED to include "deint-buf-cap".  This information MAY be
   used when the value for "sprop-deint-buf-req" is selected in a
   second round of offer and answer.  For interleaved streams, it is
   also RECOMMENDED to consider offering multiple payload types with
   different buffering requirements when the capabilities of the
   receiver are unknown.

   o  The "sprop-parameter-sets" parameter is used as described above.
   In addition, an answerer MUST maintain all parameter sets received
   in the offer in its answer.  Depending on the value of the
   "parameter-add" parameter, different rules apply: If "parameter-add"
   is false (0), the answer MUST NOT add any additional parameter sets.
   If "parameter-add" is true (1), the answerer, in its answer, MAY add
   additional parameter sets to the "sprop-parameter-sets" parameter.
   The answerer MUST also, independent of the value of "parameter-add",
   accept to receive a video stream using the sprop-parameter-sets it
   declared in the answer.

       Informative note: care must be taken when parameter sets are
       added not to cause overwriting of already transmitted parameter
       sets by using conflicting parameter set identifiers.

   For streams being delivered over multicast, the following rules
   apply in addition:

   o  The stream properties parameters ("sprop-parameter-sets", "sprop-
   deint-buf-req", "sprop-interleaving-depth",
   "sprop-max-don-diff", "sprop-init-buf-time", "sprop-prebuf-size",
   "sprop-prebuf-time", "sprop-scalability-information", and
   "sprop-layer-range") MUST NOT be changed by the answerer.  Thus, a
   payload type can either be accepted unaltered or removed.

   o  The receiver capability parameters "max-mbps", "max-fs", "max-
   cpb", "max-dpb", "max-br", and "max-rcmd-nalu-size" MUST be


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   supported by the answerer for all streams declared as sendrecv or
   recvonly; otherwise, one of the following actions MUST be performed:
   the media format is removed, or the session rejected.

   o  The receiver capability parameter redundant-pic-cap SHOULD be
   supported by the answerer for all streams declared as sendrecv or
   recvonly as follows:  The answerer SHOULD NOT include redundant
   coded pictures in the transmitted stream if the offerer indicated
   redundant-pic-cap equal to 0.  Otherwise (when redundant_pic_cap is
   equal to 1), it is beyond the scope of this memo to recommend how
   the answerer should use redundant coded pictures.

   Below are the complete lists of how the different parameters shall
   be interpreted in the different combinations of offer or answer and
   direction attribute.

   o  In offers and answers for which "a=sendrecv" or no direction
   attribute is used, or in offers and answers for which "a=recvonly"
   is used, the following interpretation of the parameters MUST be
   used.

   Declaring actual configuration or properties for receiving:

     - profile-level-id
     - packetization-mode

   Declaring actual properties of the stream to be sent (applicable
   only when "a=sendrecv" or no direction attribute is used):

     - sprop-deint-buf-req
     - sprop-interleaving-depth
     - sprop-parameter-sets
     - sprop-max-don-diff
     - sprop-init-buf-time
     - sprop-prebuf-size
     - sprop-prebuf-time
     - sprop-scalability-information
     - sprop-layer-range
     - scalable-layer-id
     - sprop-cl-don

   Declaring receiver implementation capabilities:

     - max-mbps
     - max-fs
     - max-cpb
     - max-dpb


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     - max-br
     - redundant-pic-cap
     - deint-buf-cap
     - max-rcmd-nalu-size

   Declaring how Offer/Answer negotiation shall be performed:

     - parameter-add

   o  In an offer or answer for which the direction attribute
   "a=sendonly" is included for the media stream, the following
   interpretation of the parameters MUST be used:

   Declaring actual configuration and properties of stream proposed to
   be sent:

     - profile-level-id
     - packetization-mode
     - sprop-deint-buf-req
     - sprop-max-don-diff
     - sprop-init-buf-time
     - sprop-parameter-sets
     - sprop-interleaving-depth
     - sprop-prebuf-size
     - sprop-prebuf-time
     - sprop-scalability-information
     - sprop-layer-range
     - sprop-spatial-resoltuion
     - sprop-cl-don

   Declaring how Offer/Answer negotiation shall be performed:

     - parameter-add

   Furthermore, the following considerations are necessary:

   o  Parameters used for declaring receiver capabilities are in
   general downgradable; i.e., they express the upper limit for a
   sender's possible behavior.  Thus a sender MAY select to set its
   encoder using only lower/lesser or equal values of these parameters.
   "sprop-parameter-sets" MUST NOT be used in a sender's declaration of
   its capabilities, as the limits of the values that are carried
   inside the parameter sets are implicit with the profile and level
   used.

   o  Parameters declaring a configuration point are not downgradable,
   with the exception of the level part of the "profile-level-id"


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   parameter.  This expresses values a receiver expects to be used and
   must be used verbatim on the sender side.

   o  When a sender's capabilities are declared, and non-downgradable
   parameters are used in this declaration, then these parameters
   express a configuration that is acceptable.  In order to achieve
   high interoperability levels, it is often advisable to offer
   multiple alternative configurations; e.g., for the packetization
   mode.  It is impossible to offer multiple configurations in a single
   payload type.  Thus, when multiple configuration offers are made,
   each offer requires its own RTP payload type associated with the
   offer.

   o  A receiver SHOULD understand all MIME parameters, even if it only
   supports a subset of the payload format's functionality.  This
   ensures that a receiver is capable of understanding when an offer to
   receive media can be downgraded to what is supported by receiver of
   the offer.

   o  An answerer MAY extend the offer with additional media format
   configurations.  However, to enable their usage, in most cases a
   second offer is required from the offerer to provide the stream
   properties parameters that the media sender will use.  This also has
   the effect that the offerer has to be able to receive this media
   format configuration, not only to send it.

   o  If an offerer wishes to have non-symmetric capabilities between
   sending and receiving, the offerer has to offer different RTP
   sessions; i.e., different media lines declared as "recvonly" and
   "sendonly", respectively.  This may have further implications on the
   system.

9.2.3.    Usage with Session Multiplexing

   If Session multiplexing is used, the rules on signaling media
   decoding dependency in SDP as defined in [I-D.ietf-mmusic-decoding-
   dependency] apply.
   [Edt. Note (TS): We may want to connect mid-value with e.g. lowest
   TDQ value.]

9.2.4.    Usage in Declarative Session Descriptions

   When H.264 or SVC over RTP is offered with SDP in a declarative
   style, as in RTSP [RFC2326] or SAP [RFC2974], the following
   considerations are necessary.




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   o  All parameters capable of indicating the properties of both a NAL
   unit stream and a receiver are used to indicate the properties of a
   NAL unit stream.  For example, in this case, the parameter "profile-
   level-id" declares the values used by the stream, instead of the
   capabilities of the sender.  This results in that the following
   interpretation of the parameters MUST be used:

   Declaring actual configuration or properties:

   - profile-level-id
   - sprop-parameter-sets
   - packetization-mode
   - sprop-interleaving-depth
   - sprop-deint-buf-req
   - sprop-max-don-diff
   - sprop-init-buf-time
   - sprop-prebuf-size
   - sprop-prebuf-time
   - sprop-layer-range
   - sprop-spatial-resolution
   - sprop-scalability-info
   - sprop-cl-don

   Not usable:

   - max-mbps
   - max-fs
   - max-cpb
   - max-dpb
   - max-br
   - redundant-pic-cap
   - max-rcmd-nalu-size
   - parameter-add
   - deint-buf-cap
   - scalable-layer-id

   o  A receiver of the SDP is required to support all parameters and
   values of the parameters provided; otherwise, the receiver MUST
   reject (RTSP) or not participate in (SAP) the session.  It falls on
   the creator of the session to use values that are expected to be
   supported by the receiving application.

9.3. Examples

9.3.1.    Example for offering a single SVC session

  Offerer -> Answerer SDP message:


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   m = video 20000 RTP/AVP 96 97 98
   a = rtpmap:96 H264/90000
   a = fmtp:96 profile-level-id=4d400a; packetization-mode=1; \
        sprop-parameter-sets=Z01ACprLFicg,aP4Eag= =;
   a = rtpmap:97 H264-SVC/90000
   a = fmtp:97 profile-level-id=53000c; packetization-mode=1; \
        sprop-parameter-sets=Z01ACprLFicg,Z1MADEsA1NZYWCWQ,aP4Eag==, \
        aEvgRqA=,aGvgRiA=;
   a = rtpmap:98 H264-SVC/90000
   a = fmtp:98 profile-level-id=53000c; packetization-mode=2; \
        init-buf-time=156320; sprop-parameter-sets=Z01ACprLFicg, \
        Z1MADEsA1NZYWCWQ,aP4Eag= =,aEvgRqA=,aGvgRiA=;

9.3.2.    Example for offering session multiplexing

  Offerer -> Answerer SDP message:

  a = group:DDP 1 2 3

  m = video 20000 RTP/AVP 96 97 98
  a = rtpmap:96 H264/90000
  a = fmtp:96 profile-level-id=4d400a; packetization-mode=0; \
       sprop-parameter-sets=Z01ACprLFicg,aP4Eag==;
  a = rtpmap:97 H264/90000
  a = fmtp:97 profile-level-id=53000c; packetization-mode=1; \
       sprop-parameter-sets=Z01ACprLFicg, \
       Z1MADEsA1NZYWCWQ,aP4Eag= =,aEvgRqA=,aGvgRiA=;
  a = rtpmap:98 H264-SVC/90000
  a = fmtp:98 profile-level-id=53000c; packetization-mode=2; \
       init-buf-time=156320; sprop-parameter-sets=Z01ACprLFicg, \
       Z1MADEsA1NZYWCWQ,aP4Eag= =,aEvgRqA=,aGvgRiA=;
  a = mid:1

  m = video 20002 RTP/AVP 99
  a = rtpmap:99 H264-SVC/90000
  a = fmtp:99 profile-level-id=53000c; packetization-mode=1; \
       sprop-parameter-sets=Z01ACprLFicg, \
       Z1MADEsA1NZYWCWQ,aP4Eag= =,aEvgRqA=,aGvgRiA=;
  a = mid:2
   a = depend:99 lay 1:97,98;

  m = video 20004 RTP/AVP 100
  a = rtpmap:100 H264-SVC/90000
  a = fmtp:100 profile-level-id=53000c; packetization-mode=1; \
       sprop-parameter-sets=Z01ACprLFicg, \
       Z1MADEsA1NZYWCWQ,aP4Eag= =,aEvgRqA=,aGvgRiA=;
  a = mid:3
   a = depend:100 lay 1:97,98 2:99;


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9.4. Parameter Set Considerations

   Please see section 8.4 of [RFC3984].

10.  Security Considerations

   Section 9 of [RFC3984] applies.  Additionally, the following
   applies.

   Decoders MUST exercise caution with respect to the handling of
   reserved NAL unit types and reserved SEI messages, particularly if
   they contain active elements, and MUST restrict their domain of
   applicability to the presentation containing the stream.  The safest
   way is to simply discard these NAL units and SEI messages.

   When integrity protection is applied, care MUST be taken that the
   stream being transported may be scalable; hence a receiver may be
   able to access only part of the entire stream.

          Informative note: Other security aspects, including
          confidentiality, authentication, and denial-of-service
          threat, for SVC are similar as H.264/AVC, as discussed in
          section 9 of [RFC3984].

11.  Congestion Control

   Within any given RTP session carrying payload according to this
   specification, the provisions of section 12 of [RFC3984] apply.
   Reducing the session bandwidth is possible by one or more of the
   following means, listed in an order that, in most cases, will assure
   the least negative impact to the user experience:

   a) within the highest Layer identified by the DID field, utilize the
      TID and/or QID fields in the NAL unit header to drop NAL units
      with lower importance for the decoding process or human
      perception.
   b) drop all NAL units belonging to the highest enhancement Layer as
      identified by the highest DID value.
   c) dropping NAL units according to their importance for the decoding
      process, as indicated by the fields in the NAL unit header of the
      NAL units or in the prefix NAL units.
   d) dropping NAL units or entire packets not according to the
      aforementioned rules (media-unaware stream thinning).  This



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      results in the reception of a non-compliant bitstream and, most
      likely, in very annoying artifacts

          Informative note: The discussion above is centered on NAL
          units and not on packets, primarily because that is the level
          where senders can meaningfully manipulate the scalable
          bitstream.  The mapping of NAL units to RTP packets is fairly
          flexible when using aggregation packets.  Depending on the
          nature of the congestion control algorithm, the "dimension"
          of congestion measurement (packet count or bitrate) and
          reaction to it (reducing packet count or bitrate or both) can
          be adjusted accordingly.

   All aforementioned means are available to the RTP sender, regardless
   whether that sender is located in the sending endpoint or in a mixer
   based MANE.

   When a translator-based MANE is employed, then the MANE MAY
   manipulate the session only on the MANE's outgoing path, so that the
   sensed end-to-end congestion falls within the permissible envelope.
   As all translators, in this case the MANE needs to rewrite RTCP RRs
   to reflect the manipulations it has performed on the session.

      Informative note: Applications MAY also implement, in addition or
      separately, other congestion control mechanisms, e.g. as
      described in [RFC3450] and [Yan].

12.  IANA Consideration

   [Edt. Note: A new media type should be registered from IANA.]

13.  Informative Appendix: Application Examples

13.1.     Introduction

   Scalable video coding is a concept that has been around at least
   since MPEG-2 [MPEG2], which goes back as early as 1993.
   Nevertheless, it has never gained wide acceptance; perhaps partly
   because applications didn't materialize in the form envisioned
   during standardization.

   ISO/IEC MPEG and ITU-T VCEG, respectively, performed a requirement
   analysis for the SVC project.  Dozens of scenarios have been
   studied.  While some of the scenarios appear not to follow the most
   basic design principles of the Internet, e.g. as discussed in
   section 13.5, -- and are therefore not appropriate for IETF
   standardization -- others are clearly in the scope of IETF work.  Of


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   these, this draft chooses the following subset for immediate
   consideration.  The MPEG and VCEG requirement documents are
   available in [JVT-N026] and [JVT-N027], respectively.

   With these remarks, we now introduce three main application
   scenarios that we consider as relevant, and that are implementable
   with this specification.

13.2.     Layered Multicast

   This well-understood form of the use of layered coding [McCanne]
   implies that all layers are individually conveyed in their own RTP
   packet streams, each carried in its own RTP session using the IP
   (multicast) address and port number as the single demultiplexing
   point.  Receivers "tune" into the layers by subscribing to the IP
   multicast, normally by using IGMP [IGMP].  Depending on the
   application scenario, it is also possible to convey a number of
   layers in one RTP session, when finer operation points within the
   subset of layers are not needed.

   Layered multicast has the great advantage of simplicity and easy
   implementation.  However, it has also the great disadvantage of
   utilizing many different transport addresses.  While we consider
   this not to be a major problem for a professionally maintained
   content server, receiving client endpoints need to open many ports
   to IP multicast addresses in their firewalls.  This is a practical
   problem from a firewall and network address translation (NAT)
   viewpoint.  Furthermore, even today IP multicast is not as widely
   deployed as many wish.

   We consider layered multicast an important application scenario for
   the following reasons.  First, it is well understood and the
   implementation constraints are well known.  Second, there may well
   be large scale IP networks outside the immediate Internet context
   that may wish to employ layered multicast in the future.  One
   possible example could be a combination of content creation and
   core-network distribution for the various mobile TV services, e.g.
   those being developed by 3GPP (MBMS) [MBMS] and DVB (DVB-H) [DVB-H].

13.3.     Streaming of an SVC scalable stream

   In this scenario, a streaming server has a repository of stored SVC
   coded layers for a given content.  At the time of streaming, and
   according to the capabilities, connectivity, and congestion
   situation of the client(s), the streaming server generates and
   serves a scalable stream.  Both unicast and multicast serving is
   possible.  At the same time, the streaming server may use the same


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   repository of stored layers to compose different streams (with a
   different set of layers) intended for other audiences.

   As every endpoint receives only a single SVC RTP session, the number
   of firewall pinholes can be optimized to one.

   The main difference between this scenario and straightforward
   simulcasting lies in the architecture and the requirements of the
   streaming server, and is therefore out of the scope of IETF
   standardization.  However, compelling arguments can be made why such
   a streaming server design makes sense.  One possible argument is
   related to storage space and channel bandwidth.  Another is
   bandwidth adaptability without transcoding -- a considerable
   advantage in a congestion controlled network.  When the streaming
   server learns about congestion, it can reduce sending bitrate by
   choosing fewer layers, when composing the layered stream; see
   section 11.  SVC is designed to gracefully support both bandwidth
   rampdown and bandwidth rampup with a considerable dynamic range.
   This payload format is designed to allow for bandwidth flexibility
   in the mentioned sense.  While, in theory, a transcoding step could
   achieve a similar dynamic range, the computational demands are
   impractically high and video quality is typically lowered --
   therefore, few (if any) streaming servers implement full
   transcoding.

13.4.     Multicast to MANE, SVC scalable stream to endpoint

   This scenario is a bit more complex, and designed to optimize the
   network traffic in a core network, while still requiring only a
   single pinhole in the endpoint's firewall.  One of its key
   applications is the mobile TV market.

   Consider a large private IP network, e.g. the core network of 3GPP.
   Streaming servers within this core network can be assumed to be
   professionally maintained.  We assume that these servers can have
   many ports open to the network and that layered multicast is a real
   option.  Therefore, we assume that the streaming server multicasts
   SVC scalable layers, instead of simulcasting different
   representations of the same content at different bit rates.

   Also consider many endpoints of different classes.  Some of these
   endpoints may not have the processing power or the display size to
   meaningfully decode all layers; others may have these capabilities.
   Users of some endpoints may not wish to pay for high quality and are
   happy with a base service, which may be cheaper or even free.  Other
   users are willing to pay for high quality.  Finally, some connected
   users may have a bandwidth problem in that they can't receive the


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   bandwidth they would want to receive -- be it through congestion,
   connectivity, change of service quality, or for whatever other
   reasons.  However, all these users have in common that they don't
   want to be exposed too much, and therefore the number of firewall
   pinholes need to be small.

   This situation can be handled best by introducing middleboxes close
   to the edge of the core network, which receive the layered multicast
   streams and compose the single SVC scalable bit stream according to
   the needs of the endpoint connected.  These middleboxes are called
   MANEs throughout this specification.  In practice, we envision the
   MANE to be part of (or at least physically and topologically close
   to) the base station of a mobile network, where all the signaling
   and media traffic necessarily are multiplexed on the same physical
   link.  This is why we do not worry too much about decomposition
   aspects of the MANE as such.

   MANEs necessarily need to be fairly complex devices.  They certainly
   need to understand the signaling, so, for example, to associate the
   PT octet in the RTP header with the SVC payload type.

   A MANE may aggregate multiple RTP streams, possibly from multiple
   RTP sessions, thus to reduce the number of firewall pinholes
   required at the endpoints. This type of MANEs is conceptually easy
   to implement and can offer powerful features, primarily because it
   necessarily can "see" the payload (including the RTP payload
   headers), utilize the wealth of layering information available
   therein, and manipulate it.

   While such an MANE operation in its most trivial form (combining
   multiple RTP packet streams into a single one) can be implemented
   comparatively simply -- reordering the incoming packets according to
   the DON and sending them in the appropriate order -- more complex
   forms can also be envisioned.  For example, a MANE can be optimizing
   the outgoing RTP stream to the MTU size of the outgoing path by
   utilizing the aggregation and fragmentation mechanisms of this memo.

   A MANE can also perform stream thinning, so to adhere to congestion
   control principles as discussed in section 11.  While the
   implementation of the forward (media) channel of such a MANE appears
   to be comparatively simple, the need to rewrite RTCP RRs makes even
   such a MANE a complex device.

   While the implementation complexity of either case of a MANE, as
   discussed above, is fairly high, the computational demands are
   comparatively low.  In particular, SVC and/or this specification
   contain means to easily generate the correct inter-layer decoding


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   order of NAL units.  No serious bit-oriented processing is required
   and no significant state information (beyond that of the signaling
   and perhaps the SVC sequence parameter sets) need to be kept.

13.5.     Scenarios currently not considered for being unaligned with
          IP philosophy

   Remarks have been made that the current draft does not take into
   consideration at least one application scenario which some JVT folks
   considered important.  In particular, their idea was to make the RTP
   payload format (or the media stream itself) self-contained enough
   that a stateless, non-signaling-aware device can "thin" an RTP
   session to meet the bandwidth demands of the endpoint.  They called
   this device a "Router" or "Gateway", and sometimes a MANE.
   Obviously, it's not a Router or Gateway in the IETF sense.  To
   distinguish it from a MANE as defined in RFC 3984 and in this
   specification, let's call it an MDfH (Magic Device from Heaven).

   To simplify discussions, let's assume point-to-point traffic only.
   The endpoint has a signaling relationship with the streaming server,
   but it is known that the MDfH is somewhere in the media path (e.g.
   because the physical network topology ensures this).  It has been
   requested, at least implicitly through MPEG's and JVT's requirements
   document, that the MDfH should be capable to intercept the SVC
   scalable bit stream, modify it by dropping packets or parts thereof,
   and forwarding the resulting packet stream to the receiving
   endpoint.  It has been requested that this payload specification
   contains protocol elements facilitating such an operation, and the
   argument has been made that the NRI field of RFC 3984 serves exactly
   the same purpose.

   The authors of this I-D do not consider the scenario above to be
   aligned with the most basic design philosophies the IETF follows,
   and therefore have not addressed the comments made (except through
   this section).  In particular, we see the following problems with
   the MDfH approach):

   - As the very minimum, the MDfH would need to know which RTP
     streams are carrying SVC.  We don't see how this could be
     accomplished but by using a static payload type.  None of the
     IETF defined RTP profiles envision static payload types for SVC,
     and even the de-facto profiles developed by some application
     standard organizations (3GPP for example) do not use this
     outdated concept.  Therefore, the MDfH necessarily needs to be at
     least "listening" to the signaling.
   - If the RTP packet payload were encrypted, it would be impossible
     to interpret the payload header and/or the first bytes of the


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     media stream.  We understand that there are crypto schemes under
     discussion that encrypt only the last n bytes of an RTP payload,
     but we are more than unsure that this is fully in line with the
     IETF's security vision.

   Even if the above two problems would have been overcome through
   standardization outside of the IETF, we still foresee serious design
   flaws:

   - An MDfH can't simply dump RTP packets it doesn't want to forward.
     It either needs to act as a full RTP Translator (implying that it
     rewrites RTCP RRs and such), or it needs to patch the RTP
     sequence numbers to fulfill the RTP specification.  Not doing
     either would, for the receiver, look like the gaps in the
     sequence numbers occurred due to unintentional erasures, which
     has interesting effects on congestion control (if implemented),
     will break pretty much every meta-payload ever developed, and so
     on.  (Many more points could be made here).

   In summary, based on our current knowledge we are not willing to
   specify protocol mechanisms that support an operation point that has
   so little in common with classic RTP use.

13.6.     SSRC Multiplexing

   The authors have played with the idea of introducing SSRC
   multiplexing, i.e. allowing sending multiple RTP packet streams
   containing layers in the same RTP session, differentiated by SSRC
   values.  Our intention was to minimize the number of firewall
   pinholes in an endpoint to one, by using MANEs to aggregate multiple
   outgoing sessions stemming from a server into a single session (with
   SSRC multiplexed packet streams).  We were hoping that would be
   feasible even with encrypted packets in an SRTP context.

   While an implementation along these lines indeed appears to be
   feasible for the forward media path, the RTCP RR rewrite cannot be
   implemented in the way necessary for this scheme to work.  This
   relates to the need to authenticate the RTCP RRs as per SRTP
   [RFC3711].  While the RTCP RR itself does not need to be rewritten
   by the scheme we envisioned, its transport addresses needs to be
   manipulated.  This, in turn, is incompatible with the mandatory
   authentication of RTCP RRs.  As a result, there would be a
   requirement that a MANE needs to be in the RTCP security context of
   the sessions, which was not envisioned in our use case.





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   As the envisioned use case cannot be implemented, we refrained to
   add the considerable document complexity to support SSRC
   multiplexing herein.

14.  References

14.1.     Normative References

[H.264]     ITU-T Recommendation H.264, "Advanced video coding for
            generic audiovisual services", Version 4, July 2005.
[I-D.ietf-mmusic-decoding-dependency]
            Schierl, T., and Wenger, S., "Signaling media decoding
            dependency in Session Description Protocol (SDP)",
            draft-ietf-mmusic-decoding-dependency-01 (work in
            progress), February 2008.
[MPEG4-10]  ISO/IEC International Standard 14496-10:2005.
[RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
            With Session Description Protocol (SDP)", RFC 3264, June
            2002.
[RFC3548]   Josefsson, S., "The Base16, Base32, and Base64 Data
            Encodings", RFC 3548, July 2003.
[RFC3550]   Schulzrinne, H., Casner, S., Frederick, R., and
            Jacobson, V., "RTP: A Transport Protocol for Real-Time
            Applications", STD 64, RFC 3550, July 2003.
[RFC3984]   Wenger, S., Hannuksela, M., Stockhammer, T., Westerlund,M.,
            and Singer, D., "RTP Payload Format for H.264 Video", RFC
            3984, February 2005.
[RFC4566]   Handley, M., Jacobson, V., and Perkins, C., "SDP: Session
            Description Protocol", RFC 4566, July 2006.
[SVC]       Joint Video Team, "Joint Draft 11 of SVC Amendment",
            available from http://ftp3.itu.ch/av-arch/jvt-site
            /2007_06_Geneva/JVT-X201.zip, Geneva, Switzerland, June
            2007.

14.2.     Informative References

[DVB-H]     DVB - Digital Video Broadcasting (DVB); DVB-H
            Implementation Guidelines, ETSI TR 102 377, 2005.
            ITU-T Rec. H.241, "Extended video procedures and control
            signals for H.300-series terminals", May 2006.
[IGMP]      Cain, B., Deering S., Kovenlas, I., Fenner, B., and
            Thyagarajan, A., "Internet Group Management Protocol,
            Version 3", RFC 3376, October 2002.
[JVT-N026]  Ohm J.-R., Koenen, R., and Chiariglione, L. (ed.), "SVC
            requirements specified by MPEG (ISO/IEC JTC1 SC29 WG11)",
            JVT-N026, available from http://ftp3.itu.ch/av-arch/jvt-
            site/2005_01_HongKongGeneva/JVT-N026.doc, Hong Kong, China,
            January 2005.

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[JVT-N027]  Sullivan, G., and Wiegand, T. (ed.), "SVC requirements
            specified by VCEG (ITU-T SG16 Q.6)", JVT-N027, available
            from http://ftp3.itu.ch/av-arch/jvt-
            site/2005_01_HongKongGeneva/JVT-N027.doc, Hong Kong, China,
            January 2005.
[McCanne]   McCanne, S., Jacobson, V., and Vetterli, M., "Receiver-
            driven layered multicast", in Proc. of ACM SIGCOMM'96,
            pages 117--130, Stanford, CA, August 1996.
[MBMS]      3GPP - Technical Specification Group Services and System
            Aspects; Multimedia Broadcast/Multicast Service (MBMS);
            Protocols and codecs (Release 6), December 2005.
[MPEG2]     ISO/IEC International Standard 13818-2:1993.
[RFC2326]   Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
            Streaming Protocol (RTSP)", RFC 2326, April 1998.
[RFC2974]   Handley, M., Perkins, C., and E. Whelan, "Session
            Announcement Protocol", RFC 2974, October 2000.
[RFC3450]   Luby, M., Gemmell, J., Vicisano, L., Rizzo, L., and
            Crowcroft, J., "Asynchronous layered coding (ALC) protocol
            instantiation", RFC 3450, December 2002.
[RFC3711]   Baugher, M., McGrew, D, Naslund, M., Carrara, E., and
            Norrman, K., "The secure real-time transport protocol
            (SRTP)", RFC 3711, March 2004.
[Yan]       Yan, J., Katrinis, K., May, M., and Plattner, R., "Media-
            And TCP-friendly congestion control for scalable video
            streams", in IEEE Trans. Multimedia, pages 196--206, April
            2006.

15.  Author's Addresses

  Stephan Wenger
  Nokia
  955 Page Mill Road
  Palo Alto, CA 94304
  USA
  Phone: +1-650-862-7368
  Email: stewe@stewe.org

  Ye-Kui Wang
  Nokia Research Center
  P.O. Box 100
  FIN-33721 Tampere
  Finland
  Phone: +358-50-486-7004
  Email: ye-kui.wang@nokia.com

  Thomas Schierl
  Fraunhofer HHI
  Einsteinufer 37
  D-10587 Berlin
  Germany


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

16.  Copyright Statement

Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

17.  Disclaimer of Validity

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


18.  Intellectual Property Statement

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.


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19.  Acknowledgement

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).


20.  RFC Editor Considerations

   none

21.  Open Issues

  1) Cross layer decoding order dependency - two suggested solutions
      on the table. Need to agree if use one or both. In the case of
      both how to resolve interoperability. Initial step is to update
      text explaining the usage.
  2) Backward compatibility to H.264, enabling H.264 (RFC 3984 single
      NAL unit mode) to interoperate with SVC using base layer. Need
      more definition.
  3) Clarify the PACSI packet since there were changes between the
      draft revision
  4) Review the SDP parameters.
  5) Changed semantics between RFC 3984 and svc like sprop-deint-buf-
      req - probably will need new parameters.
  6) What to do with bugs in RFC 3984.
  7) Clarify the usage of the new parameters like sprop-scalability-
      info, relation to SEI and usage in offer/answer.
  8) The text should be clear enough to allow an implementer to use it
      for creating the payload without having to read the H.264 SVC
      document.
  9) Non-VCL NAL units, e.g. SEI messages and parameter sets, may be
      needed by an enhancement layer but not the base layer. However,
      according to SVC, within an access unit, these non-VCL NAL units
      must precede VCL NAL units in decoding order. In session
      multiplexing, should non-VCL NAL units be transported in the same
      session as the layer that requires the non-VCL NAL unit, or
      should they be always transported in the base session? It may be
      impossible to find out without parsing details which session
      respectively SPS/subset SPS a picture parameter set belongs to.
      It may make sense for simplicity to allow a MANE to include all
      of the non-VCL NAL units within all the sessions.
  10) sprop-spatial-resolution: in this draft or a more generic draft?
  11) Shall we allow NAL unit types 14 and 15 to be present in the RTP
      stream carrying the T0 base Layer, when RFC3984 encapsulated?
  12) Further to the comments 9 and 11 above, if different sessions
      carry different temporal enhancement layers, then who should get,
      e.g., the subset SPS. It is actually possible that none does, if
      transmitted out-of-band. We should enumerate the possibilities
      and leave no doubt about how it is supposed to work. This can be
      done in the definition of the RTP sessions in 5.1.2, but it's


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      even better if specific text is added (after
      discussion/approval).
  13) Do we need to describe the filler NAL unit insertion?
  14) Current draft allows for interleaving capabilities within non-
      interleaved packetization mode (packetization-mode==1) via PACSIs
      with CL-DON.  There is no need for mixing the capabilities of
      decoding order recovery for session multiplexing and
      interleaving.  Shouldn't these two features (decoding order
      recovery and interleaving) be clearly separated, e.g. via the
      explicit indication of packetization-mode==2 as specified by
      RFC3984?



22.  Changes Log

Version 00

- 29.08.2005, YkW: Initial version
- 29.09.2005, Miska: Reviewed and commented throughout the document
- 05.10.2006, StW: Editorial changes through the document, and
formatted the document in RFC payload format style

From -00 to -01

- 04.02.2006, StW: Added details to scope
- 04.02.2006, StW: Added short subsection 6.1 "Design Principles"
- 04.02.2006, StW: Added section 15, "Application Examples"
- 06.02 - 03.03.2006, YkW: Various modifications throughout the
document
- 13.02.2006 - 03.03.2006 , ThS: Added definitions and additional
information to section 3.3, 5.1, 7 and 8, parameters in section 9.1 and
added section 14 for NAL unit re-ordering for layered multicast.
Further modifications throughout the document

From -01 to -02

- 06.03.2006, StW: Editorial improvements
- 26.05.2006, YkW: Updated NAL unit header syntax and semantics
according to the latest draft SVC spec
- 20.06.2006, Miska/YkW: Added section 6.10 "Payload Content
Scalability Information (PACSI) NAL Unit"
- 20.06.2006, YkW: Updated the NAL unit reordering process for layered
multicast (removed the old section 14 "Informative Appendix: NAL Unit
Re-ordering for Layered Multicast" and added the new section 13 "NAL
Unit Reordering for Layered Multicast")

From -02 to -03


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- 05.09.2006, YkW: Updated the NAL unit header syntax, definitions,
etc., according to the foreseen July JVT output.  Updated possible MANE
adaptation operations according to SPID, TL, DID and QL.  Clarified the
removal of single NAL unit packetiztaion mode.  Added the support of
SSRC multiplexing in layered multicast.
- 08.09.2006, StW: Editorial changes throughout the document
- 08.09.2006, YkW: Added the packetization rule for suffix NAL unit.
- 19.09.2006, YkW: Moved/updated SSRC multiplexing support to section
6.2 ``RTP header usage''. Moved/updated the cross layer DON constraint
to Section 6.6 ``Decoding order number''. Moved/updated the
packetization rule when a SVC bistream is transported over more than
one RTP session to Section 7 ``Packetization rules''. Removed Section
13 "Support of layered multicast".
- 16.10, TS: Added detailed four-byte NAL unit header description.
Change "AVC" to "H.264" conforming to 3984. Modifications throughout
the document. Extended description of 3rd byte of PACSI NAL unit.
Corrected terms RTP session and RTP packet stream in case of SSRC
multiplexing. Added terms in definition section on RTP multiplexing.
Constraints on optional media type parameters of 3984 for cross-layer
DON (DON section and media type parameters). Copied parts of SI paper
regarding mixer, translator and SSRC mux with SRTP to section
application examples. Added section on SDP usage with Session and SSRC
multiplexing. Added points in Design principles on translator/mixer and
RTP multiplexing. Added additional founding information in Ack-
section. Corrected reference for SVC and added reference for generic
signaling.
17.10, StW: Fixed many editorials, clarified MANE, mixer, translator
and RTP packet stream throughout doc (hopefully consistently)
18.10., removed comments, clarified B-Bit, changed definition of base-
layer (do not need to be of the lowest temporal resolution),

From -03 to draft-ietf-avt-rtp-svc-00

   - 23.11.06, StW: Editorials throughout the memo
   - 23.11.06, StW: removed all occurrences of the security
     discussions, as they are incorrect.  When using SRTP, the RTCP is
     authenticated, implying that a translator cannot rewrite RTCP
     RRs, implying that RRs would be incorrect as soon as the session
     is modified (i.e. packets are being removed), implying that SSRC-
     mux does not work in multicast.
   - 23.11.06, StW: rewrote congestion control
   - 23.11.06, StW: removed application scenario related to SRTP, as
     this does not work (see above
   - 23.11.06, StW: added informative reference to H.241
   - 27/29.11.06, YkW: editorial changes throughout the document
   - 27/29.11.06, YkW: alignment with the SVC specification


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   - 19.12.06, TS:
     TS: [SVC] is now the complete Joint Draft of H.264
     TS: Removed SSRC Multiplexing
     TS: Changed use cases for MANE as a translator
     TS: Editorials throughout the document, alignment with SVC spec.
   - 20-28.12.06, StW/TS/YkW: editorial changes throughout the
     document

From draft-ietf-avt-rtp-svc-00 to draft-ietf-avt-rtp-svc-01
   - 23.02.07, YkW/Miska Hannuksela: Added enhancements to PACSI NAL
     unit
   - 01.03.07, Jonathan Lennox/YkW: Added recommendatory packetization
     rules for SEI messages and non-VCL NAL units
   - 05.03.07, Thomas Wiegand/YkW: Added the fields of picture start,
     picture end, and Tl0PicIdx to PACSI NAL unit
   - 05.03.07, TS: Draft conforms to new I-D style

From draft-ietf-avt-rtp-svc-01 to draft-ietf-avt-rtp-svc-02
     25-June-2007: TS
     Clarified definitions Layer, Operation Points,
     Removed FGS
     Aligned with JVT-W201 spec
     Use of DON in de-packetization
     Congestion control
     25-June-2007: YkW
     Edit throughout the spec, aligned with JVT-X201 SVC spec
     09-July-2007: TS
     Further modifications and alignments with JVT-X201.
     05-Dec-2007: TS
     Formatting corrected, ref to signaling draft corrected
From draft-ietf-avt-rtp-svc-02 to draft-ietf-avt-rtp-svc-03
   - 21-Aug-2007 to 24-Sep-2007: YkW
     1) Resolved most of the comments sent to the AVT reflector and to
     the editors
     2) Updated the intro text for parameter sets
     3) Reordered the definitions according to alphabetical order and
     added some definitions
     4) Added the NAL unit order recovery process for layered multicast
     using CL-DON in the PACSI NAL unit, thus to allow for layered
     multicast without requiring the non-interleaved packetization
     mode. The detailed NAL unit order recovery process added to
     section 8.
     5) Added some packetization rules. Some of these were to resolve
     the "single NAL unit mode deprecation" issue.
     6) Added semantics of the media type parameters inherited from RFC
     3984, and added a couple of new parameters for negotiation of
     operation point.


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     7) Other edits throughout the document.
   - 16 to 18 November 2007: TS
     1) Added the NAL unit order recovery process for layered multicast
     without using CL-DON, thus to allow for layered multicast without
     requiring the non-interleaved packetization mode.
     2) Added the usages of the media type parameters, including SDP
     usage with offer/answer model, declarative usage, and examples.
   - 08 to 19 November 2007: YkW
     1) Aligned the spec with the final version of the SVC spec.
     2) Updated the congestion control part according to Colin Perkins'
     comment.
     3) Checked the parameter set considerations and confirmed that the
     text in RFC 3984 is OK.
     4) Updated the security considerations part.
     5) Added justifications for some fields in the PACSI NAL units.
From draft-ietf-avt-rtp-svc-03 to draft-ietf-avt-rtp-svc-04
   - 18 December 2007: TS
     1) Updated formatting in the Media Type Registration section
     2) Updated the semantics of sprop-layer-range
     3) Updated Open issues according to Roni's email
     4) Corrected usage of "depend" in SDP example
From draft-ietf-avt-rtp-svc-04 to draft-ietf-avt-rtp-svc-05
    08 January 2008: TS
     1) Back to original word source document for draft-ietf-avt-rtp-
       svc-03.
     2) Changed/corrected formatting of document:
          a. Word source document margins
          b. Footer/Header adjustment
          c. Formatting of RFC default sections
          d. Formatting of Author's section
          e. Formatting of Reference sections
          f. Corrected formatting of Media Type section
       After TXT document generation, document should be readable by
       Internet Draft submission tool.
     3) Fixed SDP example (fmtp:97 to 96) [Mike's comment]
     4) Changed "sprop-layer-range" definition according to Ye-Kui's
       internal proposal.
     5) Corrected usage of "depend" in SDP example
From draft-ietf-avt-rtp-svc-05 to draft-ietf-avt-rtp-svc-06
    16-17 January 2008: YkW
     1) Updated Sections 13.1 and 13.2 with some corrections, and added
       two informative references on SVC requirements.
     2) Added text (in Section 4) to explain
       a. When layered multicast (i.e. session multiplexing) should be
          used.
       b. Whether one or more layers should be carried in one RTP
          stream when layered multicast is used or not.


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     3) Added new Subsection 7.1 to contain packetization rules for
       layered multicast, and within the new subsection,
       a. Tried to improve the readability
       b. Added text to explain which packetization modes can be used
          in each decoder order recovery mode for layered multicast
     4) Aligned the definitions with the SVC specification, including
       the term base layer, and used the term T0 base layer for the
       minimum subset of the base layer.
     5) Systematically checked throughout the document the places that
       use the terms base layer or T0 base layer.
     6) Corrected the semantics of the reserved fields in the SVC NAL
       unit header extension.
     7) Renamed the CL-DON field with DONC, and systematically updated
       throughout the document texts containing instances of "CL-DON".
     8) Clarified numerous instances of "the remaining NAL units in the
       payload" in the PACSI NAL unit description by adding "of the
       aggregation packet".
     9) Moved the definition of "target NAL units" to be beginning of
       the semantics of PACSI NAL unit fields.
     10)  Removed the obsolete semantics of the T bit in the PACSI NAL
       unit.
     11)  Updated the semantics of the P bit in the PACSI NAL unit, to
       indicate that all the remaining NAL units in the payload of the
       aggregation packet are redundant slices.
     12)  Removed some obsolete text
       a. Two paragraphs in Subsection 3.1
       b. The editor note on max-mbps, max-fs, max-cpb, max-dpb, and
          max-br semantics.
     21 January 2008: TS
     1) New parameter sprop-cl-don indicating use of CL DON for decoding
       order recovery
     2) Non-CL DON mode is now referred to as "classical RTP decoding
       order recovery".
     3) Extended SVC definition section by target dependency and target
       layer representation
     4) Extended Skope section.
     5) Clarified rules for CL-DON mode and classical RTP mode.
     6) Extended usage of classical RTP mode for interleaved mode
     7) General depacketiztion decscription in section 8.1 for classical
       RTP and CL-DON mode.
     8) Removed rule for presence of SEI messages for enhancement
       layers.

     21 January 2008: YkW
     1) Updated sub-section 8.1.2.

From draft-ietf-avt-rtp-svc-06 to draft-ietf-avt-rtp-svc-07


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     28-29 January 2008: TS
     1) Removed/changed constraints on session multiplexing in section
       4. (scope)
     2) Removed constraint on non-VCL NAL units to be in the same
       session as related VCL NAL units in section 7.
     3) Removed some of the old constraints in section 7. on process
       I.(re-ordering without CL-DON).
     4) Removed decoding order constraints of section 8.1 and rewriting
       of 8.1.
     5) Rewriting of section 8.1.1.
     6) New local definitions: Operation Point representation, Base RTP
       session, Enhancement RTP session
     7) Changed meaning of sprop-prebuf-size and sprop-prebuf-time

     31 January 2008: YkW
     1) Clarified "subset of the base layer" per Roni's comment, in
       sections 3.1, 4, and 6.1.
     2) Updated Section 4 (Scope) per Roni's and Mike's comments.
       Session multiplexing is now considered a distinct thing as
       layered multicast, which uses session multiplexing.
     3) Commented text in section 8.1 and suggested an alternative text.
     4) Updated subsection 8.1.2 (CL-DON decoding order recovery mode).
     5) Corrected the semantics of sprop-cl-don.
     6) Updated open issues.

     1 February 2008: TS
     1) Updated text in section 8.1
     2) Updated open issues

From draft-ietf-avt-rtp-svc-07 to draft-ietf-avt-rtp-svc-08
     11 February 2008: AE (Alex Eleftheriadis)
     1) Updated text in sections 1, 3.1, 3.2, based on comments posted
        by the author.

     13 February 2008: YkW
     1) Made some corrections and improvements to the newly updated
        text in sections 1, 3.1, 3.2, removed the last paragraph in
        section 1 that was used to point out the cross-layer decoding
        order recovery issue, and added clarifications regarding
        picture parameter set usage in SVC.

     18 February 2008: AE
     1) Revised all definitions (5.1.2) based on extensive discussions
        with editors to ensure that the definitions are logically
        consistent, are inline with the SVC specification, and avoid
        confusion (to the extent possible given existing SVC
        terminology).


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     2) Major changes include: new 'layer' definition that is not
        temporal_id specific; new definitions of 'AVC base layer' and
        'SVC base layer' to distinguish with/without prefix NALs; T0
        used for all temporal subcomponents; new definitions of
        'enhancement layer'; new 'base RTP session' and 'enhancement
        RTP session'; introduction of 'SVC RTP session' to provide a
        reference to either base or enhancement; change of 'SVC NAL
        unit' to include type 15.
     3) Reviewed entire document to ensure consistent use of new
        terminology.
     4) Changed capitalization to be consistent throughout the
        document, and removed smart quotes where present.
     5) Changed definitions of 'S' and 'E' bits to indicate
        transmission order, rather than decoding order, per Mike's
        posting and this author's response.
     6) Identified as open issue the placement of non-VLC NAL units in
        the various RTP sessions. The draft is currently more or less
        silent about who should get what (e.g., if temporal_id 0 and 1
        sessions exist, who should get the SPS, both?). Current
        definitions of layers etc. are on purpose silent about this as
        various options exist - the definitions should not be locked to
        any particular choice unless we want them to be.

     18 February 2008: YkW
     1) Made a couple corrections in sections 3.1 and 3.2, added back
        the SVC usage of sequence parameter sets in section 3.2.
     2) Updated the definitions of base RTP session, enhancement RTP
        session, and cross-layer decoding order number.
     3) Added a few editing comments regarding the definitions of
        operation point and SVC RTP session and the use of "SVC
        context".
     4) Other various editorial changes throughout sections 1-6.

     19 February 2008: AE
     1) Corrected minor typos and some remaining instances of 'Layer'
        (wrong capitalization).
     2) Reworded 6.4, specifically the discussion about NRI use.
     3) Reworded 9.1, informative note about ignoring unspecified
        parameters.
     4) Ensured consistent change of the word 'greater' to 'higher' in
        the definitions section (5.1.2), when referring to values of D,
        T, or Q.

     20 February 2008: TS
     1) RTP base session definition modified
     2) Timestamps are derived from same clock instance for session
       multiplexing


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     3) Added FU-A to text in section 7.)
     4) Corrected Offer examples
     5) Added various [Edt. Note] - comments

     25 February 2008: TS
     1) Integrated/addressed Mike's comments sent by email on
       02/08/2008: 6), 7), 9), 10), 16), 17), 18), 19), 20), 21), 22),
       23), 24), 25), 26), 27). Other comments have been addressed by
       the authors earlier.
       Open comments: 8), 11)/12), 13), 15) ,
     2) Updated open issues.

     25 February 2008: YkW
     1) Various minor editorial changes.



































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