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Versions: (draft-wang-avt-rfc3984bis) 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 6184

Obsoletes RFC 3984
Audio/Video Transport WG                                    Y.-K. Wang
Internet Draft                                     Huawei Technologies
Intended status: Standards track                               R. Even
Expires: April 2011                                      Self-employed
                                                         T. Kristensen
                                                              Tandberg
                                                               R. Jesup
                                               WorldGate Communications
                                                       October 9, 2010




                    RTP Payload Format for H.264 Video
                   draft-ietf-avt-rtp-rfc3984bis-12.txt


Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with
   the provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on April 9, 2009.



Copyright and License Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with
   respect to this document.  Code Components extracted from this
   document must include Simplified BSD License text as described in
   Section 4.e of the Trust Legal Provisions and are provided without
   warranty as described in the BSD License.



Abstract

   This memo describes an RTP Payload format for the ITU-T
   Recommendation H.264 video codec and the technically identical
   ISO/IEC International Standard 14496-10 video codec, excluding the
   Scalable Video Coding (SVC) extension and the Multivew Video Coding
   extension, for which the RTP payload formats are defined elsewhere.
   The RTP payload format allows for packetization of one or more
   Network Abstraction Layer Units (NALUs), produced by an H.264 video
   encoder, in each RTP payload.  The payload format has wide
   applicability, as it supports applications from simple low bit-rate
   conversational usage, to Internet video streaming with interleaved
   transmission, to high bit-rate video-on-demand.

   This memo obsoletes RFC 3984.  Changes from RFC 3984 are summarized
   in section 15.  Issues on backward compatibility to RFC 3984 are
   discussed in section 14.



Table of Contents


   Table of Contents................................................2
   1. Introduction..................................................4
      1.1. The H.264 Codec..........................................4
      1.2. Parameter Set Concept....................................6
      1.3. Network Abstraction Layer Unit Types.....................6
   2. Conventions...................................................7
   3. Scope.........................................................8
   4. Definitions and Abbreviations.................................8
      4.1. Definitions..............................................8
      4.2. Abbreviations...........................................10
   5. RTP Payload Format...........................................11



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      5.1. RTP Header Usage........................................11
      5.2. Payload Structures......................................13
      5.3. NAL Unit Header Usage...................................14
      5.4. Packetization Modes.....................................16
      5.5. Decoding Order Number (DON).............................18
      5.6. Single NAL Unit Packet..................................20
      5.7. Aggregation Packets.....................................21
   Table 4.  Type field for STAPs and MTAPs........................22
         5.7.1. Single-Time Aggregation Packet.....................23
         5.7.2. Multi-Time Aggregation Packets (MTAPs).............26
         5.7.3. Fragmentation Units (FUs)..........................29
   6. Packetization Rules..........................................33
      6.1. Common Packetization Rules..............................33
      6.2. Single NAL Unit Mode....................................34
      6.3. Non-Interleaved Mode....................................34
      6.4. Interleaved Mode........................................35
   7. De-Packetization Process.....................................35
      7.1. Single NAL Unit and Non-Interleaved Mode................35
      7.2. Interleaved Mode........................................36
         7.2.1. Size of the De-interleaving Buffer.................36
         7.2.2. De-interleaving Process............................37
      7.3. Additional De-Packetization Guidelines..................38
   8. Payload Format Parameters....................................39
      8.1. Media Type Registration.................................39
      8.2. SDP Parameters..........................................58
         8.2.1. Mapping of Payload Type Parameters to SDP..........58
         8.2.2. Usage with the SDP Offer/Answer Model..............59
         8.2.3. Usage in Declarative Session Descriptions..........69
      8.3. Examples................................................70
   Offer SDP:......................................................76
   Answer SDP:.....................................................76
      8.4. Parameter Set Considerations............................77
      8.5. Decoder Refresh Point Procedure using In-Band Transport of
      Parameter Sets (Informative).................................80
         8.5.1. IDR Procedure to Respond to a Request for a Decoder
         Refresh Point.............................................80
         8.5.2. Gradual Recovery Procedure to Respond to a Request for
         a Decoder Refresh Point...................................81
   9. Security Considerations......................................82
   10. Congestion Control..........................................82
   11. IANA Consideration..........................................83
   12. Informative Appendix: Application Examples..................83
      12.1. Video Telephony according to ITU-T Recommendation H.241
      Annex A......................................................84
      12.2. Video Telephony, No Slice Data Partitioning, No NAL Unit
      Aggregation..................................................84



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      12.3. Video Telephony, Interleaved Packetization Using NAL Unit
      Aggregation..................................................84
      12.4. Video Telephony with Data Partitioning.................85
      12.5. Video Telephony or Streaming with FUs and Forward Error
      Correction...................................................86
      12.6. Low Bit-Rate Streaming.................................88
      12.7. Robust Packet Scheduling in Video Streaming............89
   13. Informative Appendix: Rationale for Decoding Order Number...90
      13.1. Introduction...........................................90
      13.2. Example of Multi-Picture Slice Interleaving............90
      13.3. Example of Robust Packet Scheduling....................92
      13.4. Robust Transmission Scheduling of Redundant Coded Slices96
      13.5. Remarks on Other Design Possibilities..................96
   14. Backward Compatibility to RFC 3984..........................97
   15. Changes from RFC 3984.......................................99
   16. Acknowledgements...........................................101
   17. References.................................................101
      17.1. Normative References..................................101
      17.2. Informative References................................102
   18. Authors' Addresses.........................................104



1. Introduction

   This memo specifies an RTP payload specification for the video
   coding standard known as ITU-T Recommendation H.264 [1] and ISO/IEC
   International Standard 14496 Part 10 [2] (both also known as
   Advanced Video Coding, or AVC).  In this memo the name H.264 is
   used for the codec and the standard, but the memo is equally
   applicable to the ISO/IEC counterpart of the coding standard.

   This memo obsoletes RFC 3984.  Changes from RFC 3984 are summarized
   in section 15.  Issues on backward compatibility to RFC 3984 are
   discussed in section 14.

1.1. The H.264 Codec

   The H.264 video codec has a very broad application range that
   covers all forms of digital compressed video, from low bit-rate
   Internet streaming applications to HDTV broadcast and Digital
   Cinema applications with nearly lossless coding.  Compared to the
   current state of technology, the overall performance of H.264 is
   such that bit rate savings of 50% or more are reported.  Digital
   Satellite TV quality, for example, was reported to be achievable at




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   1.5 Mbit/s, compared to the current operation point of MPEG 2 video
   at around 3.5 Mbit/s [10].

   The codec specification [1] itself distinguishes conceptually
   between a video coding layer (VCL) and a network abstraction layer
   (NAL).  The VCL contains the signal processing functionality of the
   codec; mechanisms such as transform, quantization, and motion
   compensated prediction; and a loop filter.  It follows the general
   concept of most of today's video codecs, a macroblock-based coder
   that uses inter picture prediction with motion compensation and
   transform coding of the residual signal.  The VCL encoder outputs
   slices: a bit string that contains the macroblock data of an
   integer number of macroblocks, and the information of the slice
   header (containing the spatial address of the first macroblock in
   the slice, the initial quantization parameter, and similar
   information).  Macroblocks in slices are arranged in scan order
   unless a different macroblock allocation is specified, by using the
   so-called Flexible Macroblock Ordering syntax.  In-picture
   prediction is used only within a slice.  More information is
   provided in [10].

   The Network Abstraction Layer (NAL) encoder encapsulates the slice
   output of the VCL encoder into Network Abstraction Layer Units (NAL
   units), which are suitable for transmission over packet networks or
   use in packet oriented multiplex environments.  Annex B of H.264
   defines an encapsulation process to transmit such NAL units over
   byte-stream oriented networks.  In the scope of this memo, Annex B
   is not relevant.

   Internally, the NAL uses NAL units.  A NAL unit consists of a one-
   byte header and the payload byte string.  The header indicates the
   type of the NAL unit, the (potential) presence of bit errors or
   syntax violations in the NAL unit payload, and information
   regarding the relative importance of the NAL unit for the decoding
   process.  This RTP payload specification is designed to be unaware
   of the bit string in the NAL unit payload.

   One of the main properties of H.264 is the complete decoupling of
   the transmission time, the decoding time, and the sampling or
   presentation time of slices and pictures.  The decoding process
   specified in H.264 is unaware of time, and the H.264 syntax does
   not carry information such as the number of skipped frames (as is
   common in the form of the Temporal Reference in earlier video
   compression standards).  Also, there are NAL units that affect many
   pictures and that are, therefore, inherently timeless.  For this
   reason, the handling of the RTP timestamp requires some special



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   considerations for NAL units for which the sampling or presentation
   time is not defined or, at transmission time, unknown.

1.2. Parameter Set Concept

   One very fundamental design concept of H.264 is to generate self-
   contained packets, to make mechanisms such as the header
   duplication of RFC 4629 [11] or MPEG-4 Visual's Header Extension
   Code (HEC) [12] unnecessary.  This was achieved by decoupling
   information relevant to more than one slice from the media stream.
   This higher layer meta information should be sent reliably,
   asynchronously, and in advance from the RTP packet stream that
   contains the slice packets. (Provisions for sending this
   information in-band are also available for applications that do not
   have an out-of-band transport channel appropriate for the purpose.)
   The combination of the higher-level parameters is called a
   parameter set.  The H.264 specification includes two types of
   parameter sets: sequence parameter set and picture parameter set.
   An active sequence parameter set remains unchanged throughout a
   coded video sequence, and an active picture parameter set remains
   unchanged within a coded picture.  The sequence and picture
   parameter set structures contain information such as picture size,
   optional coding modes employed, and macroblock to slice group map.

   To be able to change picture parameters (such as the picture size)
   without having to transmit parameter set updates synchronously to
   the slice packet stream, the encoder and decoder can maintain a
   list of more than one sequence and picture parameter set.  Each
   slice header contains a codeword that indicates the sequence and
   picture parameter set to be used.

   This mechanism allows the decoupling of the transmission of
   parameter sets from the packet stream, and the transmission of them
   by external means (e.g., as a side effect of the capability
   exchange), or through a (reliable or unreliable) control protocol.
   It may even be possible that they are never transmitted but are
   fixed by an application design specification.

1.3. Network Abstraction Layer Unit Types

   Tutorial information on the NAL design can be found in [13], [14],
   and [15].

   All NAL units consist of a single NAL unit type octet, which also
   co-serves as the payload header of this RTP payload format.  The
   payload of a NAL unit follows immediately.



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   The syntax and semantics of the NAL unit type octet are specified
   in [1], but the essential properties of the NAL unit type octet are
   summarized below.  The NAL unit type octet has the following format:

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

   The semantics of the components of the NAL unit type octet, as
   specified in the H.264 specification, are described briefly below.

   F: 1 bit
       forbidden_zero_bit.  The H.264 specification declares a value of
       1 as a syntax violation.

   NRI: 2 bits
       nal_ref_idc.  A value of 00 indicates that the content of the
       NAL unit is not used to reconstruct reference pictures for inter
       picture prediction.  Such NAL units can be discarded without
       risking the integrity of the reference pictures.  Values greater
       than 00 indicate that the decoding of the NAL unit is required
       to maintain the integrity of the reference pictures.

   Type: 5 bits
       nal_unit_type.  This component specifies the NAL unit payload
       type as defined in Table 7-1 of [1], 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 [1].

   This memo introduces new NAL unit types, which are presented in
   section 5.2.  The NAL unit types defined in this memo are marked as
   unspecified in [1].  Moreover, this specification extends the
   semantics of F and NRI as described in section 5.3.

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 RFC 2119 [4].

   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).



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3. Scope

   This payload specification can only be used to carry the "naked"
   H.264 NAL unit stream over RTP, and not the bitstream format
   discussed in Annex B of H.264.  Likely, the first applications of
   this specification will be in the conversational multimedia field,
   video telephony or video conferencing, but the payload format also
   covers other applications, such as Internet streaming and TV over
   IP.

4. Definitions and Abbreviations

4.1. Definitions

   This document uses the definitions of [1].  The following terms,
   defined in [1], are summed up for convenience:

      access unit: A set of NAL units always containing a primary
      coded picture.  In addition to the primary coded picture, an
      access unit may also contain one or more redundant coded
      pictures 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.

      coded video sequence: A sequence of access units that consists,
      in decoding order, of an instantaneous decoding refresh (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.

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

      IDR picture: A coded picture containing only slices with I or SI
      slice types that causes a "reset" in the decoding process.
      After the decoding of an IDR picture, all following coded
      pictures in decoding order can be decoded without inter
      prediction from any picture decoded prior to the IDR picture.

      primary coded picture: The coded representation of a picture to
      be used by the decoding process for a bitstream conforming to
      H.264.  The primary coded picture contains all macroblocks of
      the picture.

      redundant coded picture: A coded representation of a picture or
      a part of a picture.  The content of a redundant coded picture



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      shall not be used by the decoding process for a bitstream
      conforming to H.264.  The content of a redundant coded picture
      may be used by the decoding process for a bitstream that
      contains errors or losses.

      VCL NAL unit: A collective term used to refer to coded slice and
      coded data partition NAL units.

   In addition, the following definitions apply:

      decoding order number (DON): A field in the payload structure or
      a derived variable indicating NAL unit decoding order.  Values
      of DON are in the range of 0 to 65535, inclusive.  After
      reaching the maximum value, the value of DON wraps around to 0.

      NAL unit decoding order: A NAL unit order that conforms to the
      constraints on NAL unit order given in section 7.4.1.2 in [1].

      NALU-time: The value that the RTP timestamp would have if the
      NAL unit would be transported in its own RTP packet.

      transmission order: The order of packets in ascending RTP
      sequence number order (in modulo arithmetic).  Within an
      aggregation packet, the NAL unit transmission order is the same
      as the order of appearance of NAL units in the packet.

      media aware network element (MANE): A network element, such as a
      middlebox or application layer gateway that is capable of
      parsing certain aspects of the RTP payload headers or the RTP
      payload and reacting to the contents.

         Informative note: The concept of a MANE goes beyond normal
         routers or gateways in that a MANE has to be aware of the
         signaling (e.g., to learn about the payload type mappings of
         the media streams), and in that it has to be trusted when
         working with SRTP.  The advantage of using MANEs is that they
         allow packets to be dropped according to the needs of the
         media coding.  For example, if a MANE has to drop packets due
         to congestion on a certain link, it can identify and remove
         those packets whose elimination produces the least adverse
         effect on the user experience.

      static macroblock: A certain amount of macroblocks in the video
      stream can be defined as static, as defined in section 8.3.2.8
      in  [3].  Static macroblocks free up additional processing
      cycles for the handling of non-static macroblocks.  Based on a



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      given amount of video processing resources and a given
      resolution, a higher number of static macroblocks enables a
      correspondingly higher frame rate.

      default sub-profile: The subset of coding tools, which may be
      all coding tools of one profile or the common subset of coding
      tools of more than one profile, indicated by the profile-level-
      id parameter.

      default level: The level indicated by the profile-level-id
      parameter, which consists of three octets, profile_idc, profile-
      iop, and level_idc.  The default level is indicated by level_idc
      in most cases, and, in some cases, additionally by profile-iop.

4.2. Abbreviations

      DON:        Decoding Order Number
      DONB:       Decoding Order Number Base
      DOND:       Decoding Order Number Difference
      FEC:        Forward Error Correction
      FU:         Fragmentation Unit
      IDR:        Instantaneous Decoding Refresh
      IEC:        International Electrotechnical Commission
      ISO:        International Organization for Standardization
      ITU-T:      International Telecommunication Union,
                  Telecommunication Standardization Sector
      MANE:       Media Aware Network Element
      MTAP:       Multi-Time Aggregation Packet
      MTAP16:     MTAP with 16-bit timestamp offset
      MTAP24:     MTAP with 24-bit timestamp offset
      NAL:        Network Abstraction Layer
      NALU:       NAL Unit
      SAR:        Sample Aspect Ratio
      SEI:        Supplemental Enhancement Information
      STAP:       Single-Time Aggregation Packet
      STAP-A:     STAP type A
      STAP-B:     STAP type B
      TS:         Timestamp
      VCL:        Video Coding Layer
      VUI:        Video Usability Information









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5. RTP Payload Format

5.1. RTP Header Usage

   The format of the RTP header is specified in RFC 3550 [5] and
   reprinted in Figure 1 for convenience.  This payload format uses
   the fields of the header in a manner consistent with that
   specification.

   When one NAL unit is encapsulated per RTP packet, the RECOMMENDED
   RTP payload format is specified in section 5.6.  The RTP payload
   (and the settings for some RTP header bits) for aggregation packets
   and fragmentation units are specified in sections 5.7 and 5.8,
   respectively.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |V=2|P|X|  CC   |M|     PT      |       sequence number         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           synchronization source (SSRC) identifier            |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |            contributing source (CSRC) identifiers             |
   |                             ....                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 1 RTP header according to RFC 3550

   The RTP header information to be set according to this RTP payload
   format is set as follows:

   Marker bit (M): 1 bit
       Set for the very last packet of the access unit indicated by the
       RTP timestamp, in line with the normal use of the M bit in video
       formats, to allow an efficient playout buffer handling.  For
       aggregation packets (STAP and MTAP), the marker bit in the RTP
       header MUST be set to the value that the marker bit of the last
       NAL unit of the aggregation packet would have been if it were
       transported in its own RTP packet.  Decoders MAY use this bit as
       an early indication of the last packet of an access unit, but
       MUST NOT rely on this property.

         Informative note: Only one M bit is associated with an
         aggregation packet carrying multiple NAL units.  Thus, if a



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         gateway has re-packetized an aggregation packet into several
         packets, it cannot reliably set the M bit of those packets.

   Payload type (PT): 7 bits
       The assignment of an RTP payload type for this new packet format
       is outside the scope of this document and will not be specified
       here.  The assignment of a payload type has to be performed
       either through the profile used or in a dynamic way.

   Sequence number (SN): 16 bits
       Set and used in accordance with RFC 3550.  For the single NALU
       and non-interleaved packetization mode, the sequence number is
       used to determine decoding order for the NALU.

   Timestamp: 32 bits
       The RTP timestamp is set to the sampling timestamp of the
       content.  A 90 kHz clock rate MUST be used.

       If the NAL unit has no timing properties of its own (e.g.,
       parameter set and SEI NAL units), the RTP timestamp is set to
       the RTP timestamp of the primary coded picture of the access
       unit in which the NAL unit is included, according to section
       7.4.1.2 of [1].

       The setting of the RTP Timestamp for MTAPs is defined in section
       5.7.2.

       Receivers SHOULD ignore any picture timing SEI messages included
       in access units that have only one display timestamp.  Instead,
       receivers SHOULD use the RTP timestamp for synchronizing the
       display process.

       If one access unit has more than one display timestamp carried
       in a picture timing SEI message, then the information in the SEI
       message SHOULD be treated as relative to the RTP timestamp, with
       the earliest event occurring at the time given by the RTP
       timestamp, and subsequent events later, as given by the
       difference in SEI message picture timing values.  Let tSEI1,
       tSEI2, ..., tSEIn be the display timestamps carried in the SEI
       message of an access unit, where tSEI1 is the earliest of all
       such timestamps.  Let tmadjst() be a function that adjusts the
       SEI messages time scale to a 90-kHz time scale.  Let TS be the
       RTP timestamp.  Then, the display time for the event associated
       with tSEI1 is TS.  The display time for the event with tSEIx,
       where x is [2..n] is TS + tmadjst (tSEIx - tSEI1).




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         Informative note: Displaying coded frames as fields is needed
         commonly in an operation known as 3:2 pulldown, in which film
         content that consists of coded frames is displayed on a
         display using interlaced scanning.  The picture timing SEI
         message enables carriage of multiple timestamps for the same
         coded picture, and therefore the 3:2 pulldown process is
         perfectly controlled.  The picture timing SEI message
         mechanism is necessary because only one timestamp per coded
         frame can be conveyed in the RTP timestamp.

5.2. Payload Structures

   The payload format defines three different basic payload structures.
   A receiver can identify the payload structure by the first byte of
   the RTP packet payload, which co-serves as the RTP payload header
   and, in some cases, as the first byte of the payload.  This byte is
   always structured as a NAL unit header.  The NAL unit type field
   indicates which structure is present.  The possible structures are
   as follows:

   Single NAL Unit Packet: Contains only a single NAL unit in the
   payload.  The NAL header type field will be equal to the original
   NAL unit type; i.e., in the range of 1 to 23, inclusive.  Specified
   in section 5.6.

   Aggregation Packet: Packet type used to aggregate multiple NAL
   units into a single RTP payload.  This packet exists in four
   versions, the Single-Time Aggregation Packet type A (STAP-A), the
   Single-Time Aggregation Packet type B (STAP-B), Multi-Time
   Aggregation Packet (MTAP) with 16-bit offset (MTAP16), and Multi-
   Time Aggregation Packet (MTAP) with 24-bit offset (MTAP24).  The
   NAL unit type numbers assigned for STAP-A, STAP-B, MTAP16, and
   MTAP24 are 24, 25, 26, and 27, respectively.  Specified in section
   5.7.

   Fragmentation Unit: Used to fragment a single NAL unit over
   multiple RTP packets.  Exists with two versions, FU-A and FU-B,
   identified with the NAL unit type numbers 28 and 29, respectively.
   Specified in section 5.8.

      Informative note: This specification does not limit the size of
      NAL units encapsulated in single NAL unit packets and
      fragmentation units.  The maximum size of a NAL unit
      encapsulated in any aggregation packet is 65535 bytes.





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   Table 1 summarizes NAL unit types and the corresponding RTP packet
   types when each of these NAL units is directly used as a packet
   payload, and where the types are described in this memo.

     Table 1.  Summary of NAL unit types and the corresponding packet
                                   types

      NAL Unit  Packet    Packet Type Name               Section
      Type      Type
      ---------------------------------------------------------
      0        reserved                                     -
      1-23     NAL unit  Single NAL unit packet             5.6
      24       STAP-A    Single-time aggregation packet     5.7.1
      25       STAP-B    Single-time aggregation packet     5.7.1
      26       MTAP16    Multi-time aggregation packet      5.7.2
      27       MTAP24    Multi-time aggregation packet      5.7.2
      28       FU-A      Fragmentation unit                 5.8
      29       FU-B      Fragmentation unit                 5.8
      30-31    reserved                                     -

5.3. NAL Unit Header Usage

   The structure and semantics of the NAL unit header were introduced
   in section 1.3.  For convenience, the format of the NAL unit header
   is reprinted below:

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

   This section specifies the semantics of F and NRI according to this
   specification.

   F: 1 bit
       forbidden_zero_bit.  A value of 0 indicates that the NAL unit
       type octet and payload should not contain bit errors or other
       syntax violations.  A value of 1 indicates that the NAL unit
       type octet and payload may contain bit errors or other syntax
       violations.

       MANEs SHOULD set the F bit to indicate detected bit errors in
       the NAL unit.  The H.264 specification requires that the F bit
       is equal to 0.  When the F bit is set, the decoder is advised
       that bit errors or any other syntax violations may be present in



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       the payload or in the NAL unit type octet.  The simplest decoder
       reaction to a NAL unit in which the F bit is equal to 1 is to
       discard such a NAL unit and to conceal the lost data in the
       discarded NAL unit.

   NRI: 2 bits
       nal_ref_idc.  The semantics of value 00 and a non-zero value
       remain unchanged from the H.264 specification.  In other words,
       a value of 00 indicates that the content of the NAL unit is not
       used to reconstruct reference pictures for inter picture
       prediction. Such NAL units can be discarded without risking the
       integrity of the reference pictures.  Values greater than 00
       indicate that the decoding of the NAL unit is required to
       maintain the integrity of the reference pictures.

       In addition to the specification above, according to this RTP
       payload specification, values of NRI indicate the relative
       transport priority, as determined by the encoder.  MANEs can use
       this information to protect more important NAL units better than
       they do less important NAL units.  The highest transport
       priority is 11, followed by 10, and then by 01; finally, 00 is
       the lowest.

         Informative note: Any non-zero value of NRI is handled
         identically in H.264 decoders.  Therefore, receivers need not
         manipulate the value of NRI when passing NAL units to the
         decoder.

       An H.264 encoder MUST set the value of NRI according to the
       H.264 specification (subclause 7.4.1) when the value of
       nal_unit_type is in the range of 1 to 12, inclusive.  In
       particular, the H.264 specification requires that the value of
       NRI SHALL be equal to 0 for all NAL units having nal_unit_type
       equal to 6, 9, 10, 11, or 12.

       For NAL units having nal_unit_type equal to 7 or 8 (indicating a
       sequence parameter set or a picture parameter set, respectively),
       an H.264 encoder SHOULD set the value of NRI to 11 (in binary
       format).  For coded slice NAL units of a primary coded picture
       having nal_unit_type equal to 5 (indicating a coded slice
       belonging to an IDR picture), an H.264 encoder SHOULD set the
       value of NRI to 11 (in binary format).

       For a mapping of the remaining nal_unit_types to NRI values, the
       following example MAY be used and has been shown to be efficient
       in a certain environment [14].  Other mappings MAY also be



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       desirable, depending on the application and the H.264/AVC Annex
       A profile in use.

         Informative note: Data Partitioning is not available in
         certain profiles; e.g., in the Main or Baseline profiles.
         Consequently, the NAL unit types 2, 3, and 4 can occur only
         if the video bitstream conforms to a profile in which data
         partitioning is allowed and not in streams that conform to
         the Main or Baseline profiles.

     Table 2.  Example of NRI values for coded slices and coded slice
            data partitions of primary coded reference pictures

      NAL Unit Type     Content of NAL unit              NRI (binary)
      ----------------------------------------------------------------
       1              non-IDR coded slice                         10
       2              Coded slice data partition A                10
       3              Coded slice data partition B                01
       4              Coded slice data partition C                01

         Informative note: As mentioned before, the NRI value of non-
         reference pictures is 00 as mandated by H.264/AVC.

       An H.264 encoder SHOULD set the value of NRI for coded slice and
       coded slice data partition NAL units of redundant coded
       reference pictures equal to 01 (in binary format).

       Definitions of the values for NRI for NAL unit types 24 to 29,
       inclusive, are given in sections 5.7 and 5.8 of this memo.

       No recommendation for the value of NRI is given for NAL units
       having nal_unit_type in the range of 13 to 23, inclusive,
       because these values are reserved for ITU-T and ISO/IEC.  No
       recommendation for the value of NRI is given for NAL units
       having nal_unit_type equal to 0 or in the range of 30 to 31,
       inclusive, as the semantics of these values are not specified in
       this memo.

5.4. Packetization Modes

   This memo specifies three cases of packetization modes:

   o  Single NAL unit mode

   o  Non-interleaved mode




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   o  Interleaved mode

   The single NAL unit mode is targeted for conversational systems
   that comply with ITU-T Recommendation H.241 [3]  (see section 12.1).
   The non-interleaved mode is targeted for conversational systems
   that may not comply with ITU-T Recommendation H.241.  In the non-
   interleaved mode, NAL units are transmitted in NAL unit decoding
   order.  The interleaved mode is targeted for systems that do not
   require very low end-to-end latency.  The interleaved mode allows
   transmission of NAL units out of NAL unit decoding order.

   The packetization mode in use MAY be signaled by the value of the
   OPTIONAL packetization-mode media type parameter.  The used
   packetization mode governs which NAL unit types are allowed in RTP
   payloads.  Table 3 summarizes the allowed packet payload types for
   each packetization mode.  Packetization modes are explained in more
   detail in section 6.

    Table 3.  Summary of allowed NAL unit types for each packetization
            mode (yes = allowed, no = disallowed, ig = ignore)

      Payload Packet    Single NAL    Non-Interleaved    Interleaved
      Type    Type      Unit Mode           Mode             Mode
      -------------------------------------------------------------
      0      reserved      ig               ig               ig
      1-23   NAL unit     yes              yes               no
      24     STAP-A        no              yes               no
      25     STAP-B        no               no              yes
      26     MTAP16        no               no              yes
      27     MTAP24        no               no              yes
      28     FU-A          no              yes              yes
      29     FU-B          no               no              yes
      30-31  reserved      ig               ig               ig


   Some NAL unit or payload type values (indicated as reserved in
   Table 3) are reserved for future extensions.  NAL units of those
   types SHOULD NOT be sent by a sender (direct as packet payloads, or
   as aggregation units in aggregation packets, or as fragmented units
   in FU packets) and MUST be ignored by a receiver.  For example, the
   payload types 1-23, with the associated packet type "NAL unit", are
   allowed in "Single NAL Unit Mode" and in "Non-Interleaved Mode",
   but disallowed in "Interleaved Mode".  However, NAL units of NAL
   unit types 1-23 can be used in "Interleaved Mode" as aggregation
   units in STAP-B, MTAP16 and MTAP24 packets as well as fragmented
   units in FU-A and FU-B packets.  Similarly, NAL units of NAL unit



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   types 1-23 can also be used in the "Non-Interleaved Mode" as
   aggregation units in STAP-A packets or fragmented units in FU-A
   packets, in addition to being directly used as packet payloads.

5.5. Decoding Order Number (DON)

   In the interleaved packetization mode, the transmission order of
   NAL units is allowed to differ from the decoding order of the NAL
   units.  Decoding order number (DON) is a field in the payload
   structure or a derived variable that indicates the NAL unit
   decoding order.  Rationale and examples of use cases for
   transmission out of decoding order and for the use of DON are given
   in section 13.

   The coupling of transmission and decoding order is controlled by
   the OPTIONAL sprop-interleaving-depth media type parameter as
   follows.  When the value of the OPTIONAL sprop-interleaving-depth
   media type parameter is equal to 0 (explicitly or per default), the
   transmission order of NAL units MUST conform to the NAL unit
   decoding order.  When the value of the OPTIONAL sprop-interleaving-
   depth media type parameter is greater than 0,

   o  the order of NAL units in an MTAP16 and an MTAP24 is not
      required to be the NAL unit decoding order, and

   o  the order of NAL units generated by de-packetizing STAP-Bs,
      MTAPs, and FUs in two consecutive packets is not required to be
      the NAL unit decoding order.

   The RTP payload structures for a single NAL unit packet, an STAP-A,
   and an FU-A do not include DON.  STAP-B and FU-B structures include
   DON, and the structure of MTAPs enables derivation of DON as
   specified in section 5.7.2.

      Informative note: When an FU-A occurs in interleaved mode, it
      always follows an FU-B, which sets its DON.

      Informative note: If a transmitter wants to encapsulate a single
      NAL unit per packet and transmit packets out of their decoding
      order, STAP-B packet type can be used.

   In the single NAL unit packetization mode, the transmission order
   of NAL units, determined by the RTP sequence number, MUST be the
   same as their NAL unit decoding order.  In the non-interleaved
   packetization mode, the transmission order of NAL units in single
   NAL unit packets, STAP-As, and FU-As MUST be the same as their NAL



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   unit decoding order.  The NAL units within an STAP MUST appear in
   the NAL unit decoding order.  Thus, the decoding order is first
   provided through the implicit order within a STAP, and second
   provided through the RTP sequence number for the order between
   STAPs, FUs, and single NAL unit packets.

   Signaling of the value of DON for NAL units carried in STAP-B, MTAP,
   and a series of fragmentation units starting with an FU-B is
   specified in sections 5.7.1, 5.7.2, and 5.8, respectively.  The DON
   value of the first NAL unit in transmission order MAY be set to any
   value.  Values of DON are in the range of 0 to 65535, inclusive.
   After reaching the maximum value, the value of DON wraps around to
   0.

   The decoding order of two NAL units contained in any STAP-B, MTAP,
   or a series of fragmentation units starting with an FU-B is
   determined as follows.  Let DON(i) be the decoding order number of
   the NAL unit having index i in the transmission order.  Function
   don_diff(m,n) is specified as follows:

         If DON(m) == DON(n), don_diff(m,n) = 0

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

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

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

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

   A positive value of don_diff(m,n) indicates that the NAL unit
   having transmission order index n follows, in decoding order, the
   NAL unit having transmission order index m.  When don_diff(m,n) is
   equal to 0, then the NAL unit decoding order of the two NAL units
   can be in either order.  A negative value of don_diff(m,n)
   indicates that the NAL unit having transmission order index n
   precedes, in decoding order, the NAL unit having transmission order
   index m.

   Values of DON related fields (DON, DONB, and DOND; see section 5.7)
   MUST be such that the decoding order determined by the values of
   DON, as specified above, conforms to the NAL unit decoding order.



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   If the order of two NAL units in NAL unit decoding order is
   switched and the new order does not conform to the NAL unit
   decoding order, the NAL units MUST NOT have the same value of DON.
   If the order of two consecutive NAL units in the NAL unit stream is
   switched and the new order still conforms to the NAL unit decoding
   order, the NAL units MAY have the same value of DON.  For example,
   when arbitrary slice order is allowed by the video coding profile
   in use, all the coded slice NAL units of a coded picture are
   allowed to have the same value of DON.  Consequently, NAL units
   having the same value of DON can be decoded in any order, and two
   NAL units having a different value of DON should be passed to the
   decoder in the order specified above.  When two consecutive NAL
   units in the NAL unit decoding order have a different value of DON,
   the value of DON for the second NAL unit in decoding order SHOULD
   be the value of DON for the first, incremented by one.

   An example of the de-packetization process to recover the NAL unit
   decoding order is given in section 7.

      Informative note: Receivers should not expect that the absolute
      difference of values of DON for two consecutive NAL units in the
      NAL unit decoding order will be equal to one, even in error-free
      transmission.  An increment by one is not required, as at the
      time of associating values of DON to NAL units, it may not be
      known whether all NAL units are delivered to the receiver.  For
      example, a gateway may not forward coded slice NAL units of non-
      reference pictures or SEI NAL units when there is a shortage of
      bit rate in the network to which the packets are forwarded.  In
      another example, a live broadcast is interrupted by pre-encoded
      content, such as commercials, from time to time.  The first
      intra picture of a pre-encoded clip is transmitted in advance to
      ensure that it is readily available in the receiver.  When
      transmitting the first intra picture, the originator does not
      exactly know how many NAL units will be encoded before the first
      intra picture of the pre-encoded clip follows in decoding order.
      Thus, the values of DON for the NAL units of the first intra
      picture of the pre-encoded clip have to be estimated when they
      are transmitted, and gaps in values of DON may occur.

5.6. Single NAL Unit Packet

   The single NAL unit packet defined here MUST contain only one NAL
   unit, of the types defined in [1].  This means that neither an
   aggregation packet nor a fragmentation unit can be used within a
   single NAL unit packet.  A NAL unit stream composed by de-
   packetizing single NAL unit packets in RTP sequence number order



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   MUST conform to the NAL unit decoding order.  The structure of the
   single NAL unit packet is shown in Figure 2.

      Informative note: The first byte of a NAL unit co-serves as the
      RTP payload header.

    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   |                                               |
   +-+-+-+-+-+-+-+-+                                               |
   |                                                               |
   |               Bytes 2..n of a Single NAL unit                 |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :...OPTIONAL RTP padding        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 2 RTP payload format for single NAL unit packet

5.7. Aggregation Packets

   Aggregation packets are the NAL unit aggregation scheme of this
   payload specification.  The scheme is introduced to reflect the
   dramatically different MTU sizes of two key target networks:
   wireline IP networks (with an MTU size that is often limited by the
   Ethernet MTU size; roughly 1500 bytes), and IP or non-IP (e.g.,
   ITU-T H.324/M) based wireless communication systems with preferred
   transmission unit sizes of 254 bytes or less.  To prevent media
   transcoding between the two worlds, and to avoid undesirable
   packetization overhead, a NAL unit aggregation scheme is introduced.

   Two types of aggregation packets are defined by this specification:

   o  Single-time aggregation packet (STAP): aggregates NAL units with
      identical NALU-time.  Two types of STAPs are defined, one
      without DON (STAP-A) and another including DON (STAP-B).

   o  Multi-time aggregation packet (MTAP): aggregates NAL units with
      potentially differing NALU-time.  Two different MTAPs are
      defined, differing in the length of the NAL unit timestamp
      offset.

   Each NAL unit to be carried in an aggregation packet is
   encapsulated in an aggregation unit.  Please see below for the four
   different aggregation units and their characteristics.



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   The structure of the RTP payload format for aggregation packets is
   presented in Figure 3.

    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   |                                               |
   +-+-+-+-+-+-+-+-+                                               |
   |                                                               |
   |             one or more aggregation units                     |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :...OPTIONAL RTP padding        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 3 RTP payload format for aggregation packets

   MTAPs and STAPs share the following packetization rules:  The RTP
   timestamp MUST be set to the earliest of the NALU-times of all the
   NAL units to be aggregated.  The type field of the NAL unit type
   octet MUST be set to the appropriate value, as indicated in Table 4.
   The F bit MUST be cleared if all F bits of the aggregated NAL units
   are zero; otherwise, it MUST be set.  The value of NRI MUST be the
   maximum of all the NAL units carried in the aggregation packet.

                 Table 4.  Type field for STAPs and MTAPs

      Type   Packet    Timestamp offset   DON related fields
                       field length       (DON, DONB, DOND)
                       (in bits)          present
      --------------------------------------------------------
      24     STAP-A       0                 no
      25     STAP-B       0                 yes
      26     MTAP16      16                 yes
      27     MTAP24      24                 yes

   The marker bit in the RTP header is set to the value that the
   marker bit of the last NAL unit of the aggregated packet would have
   if it were transported in its own RTP packet.

   The payload of an aggregation packet consists of one or more
   aggregation units.  See sections 5.7.1 and 5.7.2 for the four
   different types of aggregation units.  An aggregation packet can
   carry as many aggregation units as necessary; however, the total
   amount of data in an aggregation packet obviously MUST fit into an
   IP packet, and the size SHOULD be chosen so that the resulting IP



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   packet is smaller than the MTU size.  An aggregation packet MUST
   NOT contain fragmentation units specified in section 5.8.
   Aggregation packets MUST NOT be nested; i.e., an aggregation packet
   MUST NOT contain another aggregation packet.

5.7.1. Single-Time Aggregation Packet

   Single-time aggregation packet (STAP) SHOULD be used whenever NAL
   units are aggregated that all share the same NALU-time.  The
   payload of an STAP-A does not include DON and consists of at least
   one single-time aggregation unit, as presented in Figure 4.  The
   payload of an STAP-B consists of a 16-bit unsigned decoding order
   number (DON) (in network byte order) followed by at least one
   single-time aggregation unit, as presented in Figure 5.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   :                                               |
   +-+-+-+-+-+-+-+-+                                               |
   |                                                               |
   |                single-time aggregation units                  |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 4 Payload format for STAP-A

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   :  decoding order number (DON)  |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
   |                                                               |
   |                single-time aggregation units                  |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 5 Payload format for STAP-B

   The DON field specifies the value of DON for the first NAL unit in
   an STAP-B in transmission order.  For each successive NAL unit in
   appearance order in an STAP-B, the value of DON is equal to (the



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   value of DON of the previous NAL unit in the STAP-B + 1) % 65536,
   in which '%' stands for the modulo operation.

   A single-time aggregation unit consists of 16-bit unsigned size
   information (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 NAL unit), followed by the
   NAL unit itself, including its NAL unit type byte.  A single-time
   aggregation unit is byte aligned within the RTP payload, but it may
   not be aligned on a 32-bit word boundary.  Figure 6 presents the
   structure of the single-time aggregation unit.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   :        NAL unit size          |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
   |                                                               |
   |                           NAL unit                            |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 6 Structure for single-time aggregation unit

   Figure 7 presents an example of an RTP packet that contains an
   STAP-A.  The STAP contains two single-time aggregation units,
   labeled as 1 and 2 in the figure.




















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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          RTP Header                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |STAP-A NAL HDR |         NALU 1 Size           | NALU 1 HDR    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NALU 1 Data                           |
   :                                                               :
   +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               | NALU 2 Size                   | NALU 2 HDR    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NALU 2 Data                           |
   :                                                               :
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :...OPTIONAL RTP padding        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 7 An example of an RTP packet including an STAP-A containing
                     two single-time aggregation units

   Figure 8 presents an example of an RTP packet that contains an
   STAP-B.  The STAP contains two single-time aggregation units,
   labeled as 1 and 2 in the figure.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          RTP Header                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |STAP-B NAL HDR | DON                           | NALU 1 Size   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | NALU 1 Size   | NALU 1 HDR    | NALU 1 Data                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   :                                                               :
   +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               | NALU 2 Size                   | NALU 2 HDR    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       NALU 2 Data                             |
   :                                                               :
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :...OPTIONAL RTP padding        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 8 An example of an RTP packet including an STAP-B containing
                     two single-time aggregation units



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5.7.2. Multi-Time Aggregation Packets (MTAPs)

   The NAL unit payload of MTAPs consists of a 16-bit unsigned
   decoding order number base (DONB) (in network byte order) and one
   or more multi-time aggregation units, as presented in Figure 9.
   DONB MUST contain the value of DON for the first NAL unit in the
   NAL unit decoding order among the NAL units of the MTAP.

      Informative note: The first NAL unit in the NAL unit decoding
      order is not necessarily the first NAL unit in the order in
      which the NAL units are encapsulated in an MTAP.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   :  decoding order number base   |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
   |                                                               |
   |                 multi-time aggregation units                  |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 9 NAL unit payload format for MTAPs

   Two different multi-time aggregation units are defined in this
   specification.  Both of them consist of 16 bits unsigned size
   information of the following NAL unit (in network byte order), an
   8-bit unsigned decoding order number difference (DOND), and n bits
   (in network byte order) of timestamp offset (TS offset) for this
   NAL unit, whereby n can be 16 or 24.  The choice between the
   different MTAP types (MTAP16 and MTAP24) is application dependent:
   the larger the timestamp offset is, the higher the flexibility of
   the MTAP, but the overhead is also higher.

   The structure of the multi-time aggregation units for MTAP16 and
   MTAP24 are presented in Figures 10 and 11, respectively.  The
   starting or ending position of an aggregation unit within a packet
   is not required to be on a 32-bit word boundary.  The DON of the
   NAL unit contained in a multi-time aggregation unit is equal to
   (DONB + DOND) % 65536, in which % denotes the modulo operation.
   This memo does not specify how the NAL units within an MTAP are
   ordered, but, in most cases, NAL unit decoding order SHOULD be used.





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   The timestamp offset field MUST be set to a value equal to the
   value of the following formula: If the NALU-time is larger than or
   equal to the RTP timestamp of the packet, then the timestamp offset
   equals (the NALU-time of the NAL unit - the RTP timestamp of the
   packet).  If the NALU-time is smaller than the RTP timestamp of the
   packet, then the timestamp offset is equal to the NALU-time + (2^32
   - the RTP timestamp of the 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :        NAL unit size          |      DOND     |  TS offset    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  TS offset    |                                               |
   +-+-+-+-+-+-+-+-+              NAL unit                         |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 10  Multi-time aggregation unit for MTAP16

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :        NAL unit size         |      DOND     |  TS offset    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         TS offset             |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                              NAL unit                         |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 11  Multi-time aggregation unit for MTAP24

   For the "earliest" multi-time aggregation unit in an MTAP the
   timestamp offset MUST be zero.  Hence, the RTP timestamp of the
   MTAP itself is identical to the earliest NALU-time.

      Informative note: The "earliest" multi-time aggregation unit is
      the one that would have the smallest extended RTP timestamp
      among all the aggregation units of an MTAP if the NAL units
      contained in the aggregation units were encapsulated in single
      NAL unit packets.  An extended timestamp is a timestamp that has
      more than 32 bits and is capable of counting the wraparound of



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      the timestamp field, thus enabling one to determine the smallest
      value if the timestamp wraps.  Such an "earliest" aggregation
      unit may not be the first one in the order in which the
      aggregation units are encapsulated in an MTAP.  The "earliest"
      NAL unit need not be the same as the first NAL unit in the NAL
      unit decoding order either.

   Figure 12 presents an example of an RTP packet that contains a
   multi-time aggregation packet of type MTAP16 that contains two
   multi-time aggregation units, labeled as 1 and 2 in the figure.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          RTP Header                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |MTAP16 NAL HDR |  decoding order number base   | NALU 1 Size   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  NALU 1 Size  |  NALU 1 DOND  |       NALU 1 TS offset        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  NALU 1 HDR   |  NALU 1 DATA                                  |
   +-+-+-+-+-+-+-+-+                                               +
   :                                                               :
   +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               | NALU 2 SIZE                   |  NALU 2 DOND  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       NALU 2 TS offset        |  NALU 2 HDR   |  NALU 2 DATA  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
   :                                                               :
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :...OPTIONAL RTP padding        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 12  An RTP packet including a multi-time aggregation packet
        of type MTAP16 containing two multi-time aggregation units

   Figure 13 presents an example of an RTP packet that contains a
   multi-time aggregation packet of type MTAP24 that contains two
   multi-time aggregation units, labeled as 1 and 2 in the figure.










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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          RTP Header                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |MTAP24 NAL HDR |  decoding order number base   | NALU 1 Size   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  NALU 1 Size  |  NALU 1 DOND  |       NALU 1 TS offs          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |NALU 1 TS offs |  NALU 1 HDR   |  NALU 1 DATA                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   :                                                               :
   +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               | NALU 2 SIZE                   |  NALU 2 DOND  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       NALU 2 TS offset                        |  NALU 2 HDR   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  NALU 2 DATA                                                  |
   :                                                               :
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :...OPTIONAL RTP padding        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 13  An RTP packet including a multi-time aggregation packet
        of type MTAP24 containing two multi-time aggregation units

5.7.3. Fragmentation Units (FUs)

   This payload type allows fragmenting a NAL unit into several RTP
   packets.  Doing so on the application layer instead of relying on
   lower layer fragmentation (e.g., by IP) has the following
   advantages:

   o  The payload format is capable of transporting NAL units bigger
      than 64 kbytes over an IPv4 network that may be present in pre-
      recorded video, particularly in High Definition formats (there
      is a limit of the number of slices per picture, which results in
      a limit of NAL units per picture, which may result in big NAL
      units).

   o  The fragmentation mechanism allows fragmenting a single NAL unit
      and applying generic forward error correction as described in
      section 12.5.

   Fragmentation is defined only for a single NAL unit and not for any
   aggregation packets.  A fragment of a NAL unit consists of an



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   integer number of consecutive octets of that NAL unit.  Each octet
   of the NAL unit MUST be part of exactly one fragment of that NAL
   unit.  Fragments of the same NAL unit MUST be sent in consecutive
   order with ascending RTP sequence numbers (with no other RTP
   packets within the same RTP packet stream being sent between the
   first and last fragment).  Similarly, a NAL unit MUST be
   reassembled in RTP sequence number order.

   When a NAL unit is fragmented and conveyed within fragmentation
   units (FUs), it is referred to as a fragmented NAL unit.  STAPs and
   MTAPs MUST NOT be fragmented.  FUs MUST NOT be nested; i.e., an FU
   MUST NOT contain another FU.

   The RTP timestamp of an RTP packet carrying an FU is set to the
   NALU-time of the fragmented NAL unit.

   Figure 14 presents the RTP payload format for FU-As.  An FU-A
   consists of a fragmentation unit indicator of one octet, a
   fragmentation unit header of one octet, and a fragmentation unit
   payload.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | FU indicator  |   FU header   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   |                         FU payload                            |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :...OPTIONAL RTP padding        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 14  RTP payload format for FU-A

   Figure 15 presents the RTP payload format for FU-Bs.  An FU-B
   consists of a fragmentation unit indicator of one octet, a
   fragmentation unit header of one octet, a decoding order number
   (DON) (in network byte order), and a fragmentation unit payload.
   In other words, the structure of FU-B is the same as the structure
   of FU-A, except for the additional DON field.








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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | FU indicator  |   FU header   |               DON             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
   |                                                               |
   |                         FU payload                            |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :...OPTIONAL RTP padding        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 15  RTP payload format for FU-B

   NAL unit type FU-B MUST be used in the interleaved packetization
   mode for the first fragmentation unit of a fragmented NAL unit.
   NAL unit type FU-B MUST NOT be used in any other case.  In other
   words, in the interleaved packetization mode, each NALU that is
   fragmented has an FU-B as the first fragment, followed by one or
   more FU-A fragments.

   The FU indicator octet has the following format:

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

   Values equal to 28 and 29 in the Type field of the FU indicator
   octet identify an FU-A and an FU-B, respectively.  The use of the F
   bit is described in section 5.3.  The value of the NRI field MUST
   be set according to the value of the NRI field in the fragmented
   NAL unit.

   The FU header has the following format:

      +---------------+
      |0|1|2|3|4|5|6|7|
      +-+-+-+-+-+-+-+-+
      |S|E|R|  Type   |
      +---------------+

   S: 1 bit
       When set to one, the Start bit indicates the start of a
       fragmented NAL unit.  When the following FU payload is not the



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       start of a fragmented NAL unit payload, the Start bit is set to
       zero.

   E: 1 bit
       When set to one, the End bit indicates the end of a fragmented
       NAL unit, i.e., the last byte of the payload is also the last
       byte of the fragmented NAL unit.  When the following FU payload
       is not the last fragment of a fragmented NAL unit, the End bit
       is set to zero.

   R: 1 bit
       The Reserved bit MUST be equal to 0 and MUST be ignored by the
       receiver.

   Type: 5 bits
       The NAL unit payload type as defined in Table 7-1 of [1].

   The value of DON in FU-Bs is selected as described in section 5.5.

       Informative note: The DON field in FU-Bs allows gateways to
       fragment NAL units to FU-Bs without organizing the incoming NAL
       units to the NAL unit decoding order.

   A fragmented NAL unit MUST NOT be transmitted in one FU; i.e., the
   Start bit and End bit MUST NOT both be set to one in the same FU
   header.

   The FU payload consists of fragments of the payload of the
   fragmented NAL unit so that if the fragmentation unit payloads of
   consecutive FUs are sequentially concatenated, the payload of the
   fragmented NAL unit can be reconstructed.  The NAL unit type octet
   of the fragmented NAL unit is not included as such in the
   fragmentation unit payload, but rather the information of the NAL
   unit type octet of the fragmented NAL unit is conveyed in F and NRI
   fields of the FU indicator octet of the fragmentation unit and in
   the type field of the FU header.  An FU payload MAY have any number
   of octets and MAY be empty.

       Informative note: Empty FUs are allowed to reduce the latency of
       a certain class of senders in nearly lossless environments.
       These senders can be characterized in that they packetize NALU
       fragments before the NALU is completely generated and, hence,
       before the NALU size is known.  If zero-length NALU fragments
       were not allowed, the sender would have to generate at least one
       bit of data of the following fragment before the current
       fragment could be sent.  Due to the characteristics of H.264,



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       where sometimes several macroblocks occupy zero bits, this is
       undesirable and can add delay.  However, the (potential) use of
       zero-length NALU fragments should be carefully weighed against
       the increased risk of the loss of at least a part of the NALU
       because of the additional packets employed for its transmission.

   If a fragmentation unit is lost, the receiver SHOULD discard all
   following fragmentation units in transmission order corresponding
   to the same fragmented NAL unit.

   A receiver in an endpoint or in a MANE MAY aggregate the first n-1
   fragments of a NAL unit to an (incomplete) NAL unit, even if
   fragment n of that NAL unit is not received.  In this case, the
   forbidden_zero_bit of the NAL unit MUST be set to one to indicate a
   syntax violation.

6. Packetization Rules

   The packetization modes are introduced in section 5.2.  The
   packetization rules common to more than one of the packetization
   modes are specified in section 6.1.  The packetization rules for
   the single NAL unit mode, the non-interleaved mode, and the
   interleaved mode are specified in sections 6.2, 6.3, and 6.4,
   respectively.

6.1. Common Packetization Rules

   All senders MUST enforce the following packetization rules
   regardless of the packetization mode in use:

   o  Coded slice NAL units or coded slice data partition NAL units
      belonging to the same coded picture (and thus sharing the same
      RTP timestamp value) MAY be sent in any order; however, for
      delay-critical systems, they SHOULD be sent in their original
      decoding order to minimize the delay.  Note that the decoding
      order is the order of the NAL units in the bitstream.

   o  Parameter sets are handled in accordance with the rules and
      recommendations given in section 8.4.










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   o  MANEs MUST NOT duplicate any NAL unit except for sequence or
      picture parameter set NAL units, as neither this memo nor the
      H.264 specification provides means to identify duplicated NAL
      units.  Sequence and picture parameter set NAL units MAY be
      duplicated to make their correct reception more probable, but
      any such duplication MUST NOT affect the contents of any active
      sequence or picture parameter set.  Duplication SHOULD be
      performed on the application layer and not by duplicating RTP
      packets (with identical sequence numbers).

   Senders using the non-interleaved mode and the interleaved mode
   MUST enforce the following packetization rule:

   o  MANEs MAY convert single NAL unit packets into one aggregation
      packet, convert an aggregation packet into several single NAL
      unit packets, or mix both concepts, in an RTP translator.  The
      RTP translator SHOULD take into account at least the following
      parameters: path MTU size, unequal protection mechanisms (e.g.,
      through packet-based FEC according to RFC 2733 [18], especially
      for sequence and picture parameter set NAL units and coded slice
      data partition A NAL units), bearable latency of the system, and
      buffering capabilities of the receiver.

         Informative note: An RTP translator is required to handle
         RTCP as per RFC 3550.

6.2. Single NAL Unit Mode

   This mode is in use when the value of the OPTIONAL packetization-
   mode media type parameter is equal to 0 or the packetization-mode
   is not present.  All receivers MUST support this mode.  It is
   primarily intended for low-delay applications that are compatible
   with systems using ITU-T Recommendation H.241 [3] (see section
   12.1).  Only single NAL unit packets MAY be used in this mode.
   STAPs, MTAPs, and FUs MUST NOT be used.  The transmission order of
   single NAL unit packets MUST comply with the NAL unit decoding
   order.

6.3. Non-Interleaved Mode

   This mode is in use when the value of the OPTIONAL packetization-
   mode media type parameter is equal to 1.  This mode SHOULD be
   supported.  It is primarily intended for low-delay applications.
   Only single NAL unit packets, STAP-As, and FU-As MAY be used in
   this mode.  STAP-Bs, MTAPs, and FU-Bs MUST NOT be used.  The




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   transmission order of NAL units MUST comply with the NAL unit
   decoding order.

6.4. Interleaved Mode

   This mode is in use when the value of the OPTIONAL packetization-
   mode media type parameter is equal to 2.  Some receivers MAY
   support this mode.  STAP-Bs, MTAPs, FU-As, and FU-Bs MAY be used.
   STAP-As and single NAL unit packets MUST NOT be used.  The
   transmission order of packets and NAL units is constrained as
   specified in section 5.5.


7. De-Packetization Process

   The de-packetization process is implementation dependent.
   Therefore, the following description should be seen as an example
   of a suitable implementation.  Other schemes may be used as well as
   long as the output for the same input is the same as the process
   described below.  The same output means that the resulting NAL
   units, and their order, are identical.  Optimizations relative to
   the described algorithms are likely possible.  Section 7.1 presents
   the de-packetization process for the single NAL unit and non-
   interleaved packetization modes, whereas section 7.2 describes the
   process for the interleaved mode.  Section 7.3 includes additional
   de-packetization guidelines for intelligent receivers.

   All normal RTP mechanisms related to buffer management apply.  In
   particular, duplicated or outdated RTP packets (as indicated by the
   RTP sequence number and the RTP timestamp) are removed.  To
   determine the exact time for decoding, factors such as a possible
   intentional delay to allow for proper inter-stream synchronization
   must be factored in.

7.1. Single NAL Unit and Non-Interleaved Mode

   The receiver includes a receiver buffer to compensate for
   transmission delay jitter.  The receiver stores incoming packets in
   reception order into the receiver buffer.  Packets are de-
   packetized in RTP sequence number order.  If a de-packetized packet
   is a single NAL unit packet, the NAL unit contained in the packet
   is passed directly to the decoder.  If a de-packetized packet is an
   STAP-A, the NAL units contained in the packet are passed to the
   decoder in the order in which they are encapsulated in the packet.
   For all the FU-A packets containing fragments of a single NAL unit,
   the de-packetized fragments are concatenated in their sending order
   to recover the NAL unit, which is then passed to the decoder.


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      Informative note: If the decoder supports Arbitrary Slice Order,
      coded slices of a picture can be passed to the decoder in any
      order regardless of their reception and transmission order.

7.2. Interleaved Mode

   The general concept behind these de-packetization rules is to
   reorder NAL units from transmission order to the NAL unit decoding
   order.

   The receiver includes a receiver buffer, which is used to
   compensate for transmission delay jitter and to reorder NAL units
   from transmission order to the NAL unit decoding order.  In this
   section, the receiver operation is described under the assumption
   that there is no transmission delay jitter.  To make a difference
   from a practical receiver buffer that is also used for compensation
   of transmission delay jitter, the receiver buffer is here after
   called the de-interleaving buffer in this section.  Receivers
   SHOULD also prepare for transmission delay jitter; i.e., either
   reserve separate buffers for transmission delay jitter buffering
   and de-interleaving buffering or use a receiver buffer for both
   transmission delay jitter and de-interleaving.  Moreover, receivers
   SHOULD take transmission delay jitter into account in the buffering
   operation; e.g., by additional initial buffering before starting of
   decoding and playback.

   This section is organized as follows: subsection 7.2.1 presents how
   to calculate the size of the de-interleaving buffer.  Subsection
   7.2.2 specifies the receiver process on how to organize received
   NAL units to the NAL unit decoding order.

7.2.1. Size of the De-interleaving Buffer

   In either Offer/Answer or declarative SDP usage, the sprop-deint-
   buf-req media type parameter signals the requirement for the de-
   interleaving buffer size.  It is therefore RECOMMENDED to set the
   de-interleaving buffer size, in terms of number of bytes, equal to
   or greater than the value of sprop-deint-buf-req media type
   parameter.

   When the SDP Offer/Answer model or any other capability exchange
   procedure is used in session setup, the properties of the received
   stream SHOULD be such that the receiver capabilities are not
   exceeded.  In the SDP Offer/Answer model, the receiver can indicate
   its capabilities to allocate a de-interleaving buffer with the
   deint-buf-cap media type parameter.  See section 8.1 for further



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   information on deint-buf-cap and sprop-deint-buf-req media type
   parameters and section 8.2.2 for further information on their use
   in the SDP Offer/Answer model.

7.2.2. De-interleaving Process

   There are two buffering states in the receiver: initial buffering
   and buffering while playing.  Initial buffering occurs when the RTP
   session is 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, in reception order, in the de-interleaving buffer as follows.
   NAL units of aggregation packets are stored in the de-interleaving
   buffer individually.  The value of DON is calculated and stored for
   each NAL unit.

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

   o  Function AbsDON is specified in section 8.1.

   o  Function don_diff is specified in section 5.5.

   o  Constant N is the value of the OPTIONAL sprop-interleaving-depth
      media type parameter (see section 8.1) 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-interleaving buffer.

   o  If sprop-max-don-diff is present, don_diff(m,n) is greater than
      the value of sprop-max-don-diff, 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.

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





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   o  If the de-interleaving buffer contains at least N VCL NAL units,
      NAL units are removed from the de-interleaving 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 is present, all NAL units m for which
      don_diff(m,n) is greater than sprop-max-don-diff are removed
      from the de-interleaving 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-interleaving buffer.

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

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

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

   o  NAL units are delivered to the decoder in ascending order of DON
      distance.  If several NAL units share the same value of 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 DON for the
      last NAL unit passed to the decoder.

7.3. Additional De-Packetization Guidelines

   The following additional de-packetization rules may be used to
   implement an operational H.264 de-packetizer:

   o  Intelligent RTP receivers (e.g., in gateways) may identify lost
      coded slice data partitions A (DPAs).  If a lost DPA is detected,
      after taking into account possible retransmission and FEC, a
      gateway may decide not to send the corresponding coded slice
      data partitions B and C, as their information is meaningless for
      H.264 decoders.  In this way a MANE can reduce network load by
      discarding useless packets without parsing a complex bitstream.





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   o  Intelligent RTP receivers (e.g., in gateways) may identify lost
      FUs.  If a lost FU is found, a gateway may decide not to send
      the following FUs of the same fragmented NAL unit, as their
      information is meaningless for H.264 decoders.  In this way a
      MANE can reduce network load by discarding useless packets
      without parsing a complex bitstream.

   o  Intelligent receivers having to discard packets or NALUs should
      first discard all packets/NALUs in which the value of the NRI
      field of the NAL unit type octet is equal to 0.  This will
      minimize the impact on user experience and keep the reference
      pictures intact.  If more packets have to be discarded, then
      packets with a numerically lower NRI value should be discarded
      before packets with a numerically higher NRI value.  However,
      discarding any packets with an NRI bigger than 0 very likely
      leads to decoder drift and SHOULD be avoided.

8. 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
   subtype registration for the ITU-T H.264 | ISO/IEC 14496-10 codec.
   A mapping of the parameters into the Session Description Protocol
   (SDP) [6] 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.

8.1. Media Type Registration

   The media subtype for the ITU-T H.264 | ISO/IEC 14496-10 codec is
   allocated from the IETF tree.

   Media Type name:     video

   Media subtype name:  H264

   Required parameters: none




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   OPTIONAL parameters:

       profile-level-id:
          A base16 [7] (hexadecimal) representation of the following
          three bytes in the sequence parameter set NAL unit specified
          in [1]: 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 [1], but other values for it may be
          specified in the future by ITU-T or ISO/IEC.

          The profile-level-id parameter indicates the default sub-
          profile, i.e. the subset of coding tools that may have been
          used to generate the stream or that the receiver supports,
          and the default level of the stream or the receiver supports.

          The default sub-profile is indicated collectively by the
          profile_idc byte and some fields in the profile-iop byte.
          Depending on the values of the fields in the profile-iop byte,
          the default sub-profile may be the set of coding tools
          supported by one profile, or a common subset of coding tools
          of multiple profiles, as specified in subsection 7.4.2.1.1 of
          [1].  The default level is indicated by the level_idc byte,
          and, when profile_idc is equal to 66, 77 or 88 (the Baseline,
          Main, or Extended profile) and level_idc is equal to 11,
          additionally by bit 4 (constraint_set3_flag) of the profile-
          iop byte.  When profile_idc is equal to 66, 77 or 88 (the
          Baseline, Main, or Extended profile) and level_idc is equal
          to 11, and bit 4 (constraint_set3_flag) of the profile-iop
          byte is equal to 1, the default level is level 1b.

          Table 5 lists all profiles defined in Annex A of [1] and, for
          each of the profiles, the possible combinations of
          profile_idc and profile-iop that represent the same sub-
          profile.











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             Table 5.  Combinations of profile_idc and profile-iop
             representing the same sub-profile corresponding to the
             full set of coding tools supported by one profile.  In
             the following, x may be either 0 or 1, while the profile
             names are indicated as follows. CB: Constrained Baseline
             profile, B: Baseline profile, M: Main profile, E:
             Extended profile, H: High profile, H10: High 10 profile,
             H42: High 4:2:2 profile, H44: High 4:4:4 Predictive
             profile, H10I: High 10 Intra profile, H42I: High 4:2:2
             Intra profile, H44I: High 4:4:4 Intra profile, and C44I:
             CAVLC 4:4:4 Intra profile.

               Profile     profile_idc             profile-iop
                           (hexadecimal)           (binary)

               CB          42 (B)                  x1xx0000
                  same as: 4D (M)                  1xxx0000
                  same as: 58 (E)                  11xx0000
                  same as: 64 (H), 6E (H10),       1xx00000
                           7A (H42), or F4 (H44)
               B           42 (B)                  x0xx0000
                  same as: 58 (E)                  10xx0000
               M           4D (M)                  0x0x0000
                  same as: 64 (H), 6E (H10),       01000000
                           7A (H42), or F4 (H44)
               E           58                      00xx0000
               H           64                      00000000
               H10         6E                      00000000
               H42         7A                      00000000
               H44         F4                      00000000
               H10I        64                      00010000
               H42I        7A                      00010000
               H44I        F4                      00010000
               C44I        2C                      00010000

          For example, in the table above, profile_idc equal to 58
          (Extended) with profile-iop equal to 11xx0000 indicates the
          same sub-profile corresponding to profile_idc equal to 42
          (Baseline) with profile-iop equal to x1xx0000.  Note that
          other combinations of profile_idc and profile-iop (not listed
          in Table 5) may represent a sub-profile equivalent to the
          common subset of coding tools for more than one profile.
          Note also that a decoder conforming to a certain profile may
          be able to decode bitstreams conforming to other profiles.
          For example, a decoder conforming to the High 4:4:4 profile
          at certain level must be able to decode bitstreams conforming



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          to the Constrained Baseline, Main, High, High 10 or High
          4:2:2 profile at the same or a lower level.

          If the profile-level-id parameter is used to indicate
          properties of a NAL unit stream, it indicates that, to decode
          the stream, the minimum subset of coding tools a decoder has
          to support is the default sub-profile, and the lowest level
          the decoder has to support is the default level.

          If the profile-level-id parameter is used for capability
          exchange or session setup procedure, it indicates the subset
          of coding tools, which is equal to the default sub-profile,
          that the codec supports for both receiving and sending. If
          max-recv-level is not present, the default level from
          profile-level-id indicates the highest level the codec wishes
          to support.  If max-recv-level is present it indicates the
          highest level the codec supports for receiving.  For either
          receiving or sending, all levels that are lower than the
          highest level supported MUST also be supported.

             Informative note: Capability exchange and session setup
             procedures should provide means to list the capabilities
             for each supported sub-profile separately.  For example,
             the one-of-N codec selection procedure of the SDP
             Offer/Answer model can be used (section 10.2 of [8]).
             The one-of-N codec selection procedure may also be used
             to provide different combinations of profile_idc and
             profile-iop that represent the same sub-profile.  When
             there are many different combinations of profile_idc and
             profile-iop that represent the same sub-profile, using
             the one-of-N codec selection procedure may result into a
             fairly large SDP message.  Therefore, a receiver should
             understand the different equivalent combinations of
             profile_idc and profile-iop that represent the same sub-
             profile, and be ready to accept an offer using any of the
             equivalent combinations.

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

       max-recv-level:
          This parameter MAY be used to indicate the highest level a
          receiver supports when the highest level is higher than the
          default level (the level indicated by profile-level-id).  The
          value of max-recv-level is a base16 (hexadecimal)
          representation of the two bytes after the syntax element



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          profile_idc in the sequence parameter set NAL unit specified
          in [1]: profile-iop (as defined above) and level_idc.  If
          (the level_idc byte of max-recv-level is equal to 11 and bit
          4 of the profile-iop byte of max-recv-level is equal to 1) or
          (the level_idc byte of max-recv-level is equal to 9 and bit 4
          of the profile-iop byte of max-recv-level is equal to 0), the
          highest level the receiver supports is level 1b.  Otherwise,
          the highest level the receiver supports is equal to the
          level_idc byte of max-recv-level divided by 10.

          max-recv-level MUST NOT be present if the highest level the
          receiver supports is not higher than the default level.

       max-mbps, max-smbps, max-fs, max-cpb, max-dpb, and max-br:
          These parameters MAY be used to signal the capabilities of a
          receiver implementation. These parameters MUST NOT be used
          for any other purpose.  The highest level conveyed in the
          value of the profile-level-id parameter or the max-recv-level
          parameter MUST be such that the receiver is fully capable of
          supporting.  max-mbps, max-smbps,  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 highest level, as specified below.

          When more than one parameter from the set (max-mbps, max-
          smbps , 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 highest 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
          highest level specified in the value of the profile-level-id
          parameter or the max-recv-level parameter.

          If a receiver can support all the properties of level A, the
          highest level specified in the value of the profile-level-id
          parameter or the max-recv-level parameter MUST be level A
          (i.e. MUST NOT be lower than level A).  In other words, a
          receiver MUST NOT signal values of max-mbps, max-fs, max-cpb,
          max-dpb, and max-br that taken together meet the requirements
          of a higher level compared to the highest level specified in




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          the value of the profile-level-id parameter or the max-recv-
          level parameter.

             Informative note: When the OPTIONAL media type parameters
             are used to signal the properties of a NAL unit stream,
             max-mbps, max-smbps, 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 highest level conveyed in the value
          of the profile-level-id parameter or the max-recv-level
          parameter.  When max-mbps is signaled, the receiver MUST be
          able to decode NAL unit streams that conform to the signaled
          highest level, with the exception that the MaxMBPS value in
          Table A-1 of [1] for the signaled highest 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 given in Table
          A-1 of [1] for the highest level.  Senders MAY use this
          knowledge to send pictures of a given size at a higher
          picture rate than is indicated in the signaled highest level.

       max-smbps: The value of max-smbps is an integer indicating the
          maximum static macroblock processing rate in units of static
          macroblocks per second, under the hypothetical assumption
          that all macroblocks are static macroblocks.  When max-smbps
          is signaled the MaxMBPS value in Table A-1 of [1] should be
          replaced with the result of the following computation:

          o If the parameter max-mbps is signaled, set a variable
            MaxMacroblocksPerSecond to the value of max-mbps.
            Otherwise, set MaxMacroblocksPerSecond equal to the value
            of MaxMBPS in Table A-1 [1] for the signaled highest level
            conveyed in the value of the profile-level-id parameter or
            the max-recv-level parameter.

          o Set a variable P_non-static to the proportion of non-
            static macroblocks in picture n.

          o Set a variable P_static to the proportion of static
            macroblocks in picture n.




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          o The value of MaxMBPS in Table A-1 of [1] should be
            considered by the encoder to be equal to:

             MaxMacroblocksPerSecond * max-smbps / (P_non-static *
             max-smbps + P_static * MaxMacroblocksPerSecond)

          The encoder should recompute this value for each picture. The
          value of max-smbps MUST be greater than or equal to the value
          of MaxMBPS given explicitly  as the value of the max-mbps
          parameter or implicitly in Table A-1 of [1] for the signaled
          highest level.  Senders MAY use this knowledge to send
          pictures of a given size at a higher picture rate than is
          indicated in the signaled highest 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 highest level
          conveyed in the value of the profile-level-id parameter or
          the max-recv-level parameter.  When max-fs is signaled, the
          receiver MUST be able to decode NAL unit streams that conform
          to the signaled highest level, with the exception that the
          MaxFS value in Table A-1 of [1] for the signaled highest
          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
          given in Table A-1 of [1] for the highest level.  Senders MAY
          use this knowledge to send larger pictures at a
          proportionally lower frame rate than is indicated in the
          signaled highest 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 of [1]) and in units
          of 1200 bits for the NAL HRD parameters (see A.3.1 item j of
          [1]).  The max-cpb parameter signals that the receiver has
          more memory than the minimum amount of coded picture buffer
          memory required by the signaled highest level conveyed in the
          value of the profile-level-id parameter or the max-recv-level
          parameter.  When max-cpb is signaled, the receiver MUST be
          able to decode NAL unit streams that conform to the signaled
          highest level, with the exception that the MaxCPB value in
          Table A-1 of [1] for the signaled highest level is replaced
          with the value of max-cpb.  The value of max-cpb MUST be
          greater than or equal to the value of MaxCPB given in Table
          A-1 of [1] for the highest level.  Senders MAY use this
          knowledge to construct coded video streams with greater



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          variation of bit rate than can be achieved with the MaxCPB
          value in Table A-1 of [1].

             Informative note: The coded picture buffer is used in the
             hypothetical reference decoder (Annex C) of H.264.  The
             use of the hypothetical reference decoder is recommended
             in H.264 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 H.264, 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 highest level conveyed in the
          value of the profile-level-id parameter or the max-recv-level
          parameter.  When max-dpb is signaled, the receiver MUST be
          able to decode NAL unit streams that conform to the signaled
          highest level, with the exception that the MaxDPB value in
          Table A-1 of [1] for the signaled highest 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 ), 16)

          PicWidthInMbs, FrameHeightInMbs, and ChromaFormatFactor are
          defined in [1].

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





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             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 of [1]) and in units of 1200
          bits per second for the NAL HRD parameters (see A.3.1 item j
          of [1]).

          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 highest level conveyed in
          the value of the profile-level-id parameter or the max-recv-
          level 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 highest level, with the following exceptions in the
          limits specified by the highest level:

          o The value of max-br replaces the MaxBR value in Table A-1
            of [1] for the highest level.

          o When the max-cpb parameter is not present, the result of
            the following formula replaces the value of MaxCPB in
            Table A-1 of [1]: (MaxCPB of the signaled level) * max-br
            / (MaxBR of the signaled highest 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 given in Table A-1 of [1] for the signaled
          highest level.






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          Senders MAY use this knowledge to send higher bitrate video
          as allowed in the level definition of Annex A of H.264, 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 in the same
          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.



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       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 can be placed in the NAL unit
          stream to precede any other NAL units in decoding order.  The
          parameter MUST NOT be used to indicate codec capability in
          any capability exchange procedure.  The value of the
          parameter is a comma (',') separated list of base64 [7]
          representations of parameter set NAL units as specified in
          sections 7.3.2.1 and 7.3.2.2 of [1].  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 conflicting storage
             locations (i.e., identical values of parameter set
             identifiers).  Therefore, a receiver should buffer all
             sprop-parameter-sets and make them available to the
             decoder instance that decodes a certain payload type.

          The "sprop-parameter-sets" parameter MUST only contain
          parameter sets that are conforming to the profile-level-id,
          i.e., the subset of coding tools indicated by any of the
          parameter sets MUST be equal to the default sub-profile, and
          the level indicated by any of the parameter sets MUST be
          equal to the default level.

       sprop-level-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 can be placed in the NAL unit
          stream to precede any other NAL units in decoding order and
          that are associated with one or more levels different than
          the default level.  The parameter MUST NOT be used to
          indicate codec capability in any capability exchange
          procedure.

          The sprop-level-parameter-sets parameter contains parameter
          sets for one or more levels which are different than the
          default level.  All parameter sets associated with one level
          are clustered and prefixed with a three-byte field which has
          the same syntax as profile-level-id.  This enables the
          receiver to install the parameter sets for one level and



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          discard the rest.  The three-byte field is named PLId, and
          all parameter sets associated with one level are named PSL,
          which has the same syntax as sprop-parameter-sets.  Parameter
          sets for each level are represented in the form of PLId:PSL,
          i.e., PLId followed by a colon (':') and the base64 [7]
          representation of the initial parameter set NAL units for the
          level.  Each pair of PLId:PSL is also separated by a colon.
          Note that a PSL can contain multiple parameter sets for that
          level, separated with commas (',').

          The subset of coding tools indicated by each PLId field MUST
          be equal to the default sub-profile, and the level indicated
          by each PLId field MUST be different than the default level.
          All sequence parameter sets contained in each PSL MUST have
          the three bytes from profile_idc to level_idc, inclusive,
          equal to the preceding PLId.

             Informative note: This parameter allows for efficient
             level downgrade or upgrade in SDP Offer/Answer and out-
             of-band transport of parameter sets, simultaneously.

       use-level-src-parameter-sets:
          This parameter MAY be used to indicate a receiver capability.
          The value MAY be equal to either 0 or 1.  When the parameter
          is not present, the value MUST be inferred to be equal to 0.
          The value 0 indicates that the receiver does not understand
          the sprop-level-parameter-sets parameter, and does not
          understand the "fmtp" source attribute as specified in
          section 6.3 of [9], and will ignore sprop-level-parameter-
          sets when present, and will ignore sprop-parameter-sets when
          conveyed using the "fmtp" source attribute.  The value 1
          indicates that the receiver understands the sprop-level-
          parameter-sets parameter, and understands the "fmtp" source
          attribute as specified in section 6.3 of [9], and is capable
          of using parameter sets contained in the sprop-level-
          parameter-sets or contained in the sprop-parameter-sets that
          is conveyed using the "fmtp" source attribute.

             Informative note: An RFC 3984 receiver does not
             understand sprop-level-parameter-sets, use-level-src-
             parameter-sets, or the "fmtp" source attribute as
             specified in section 6.3 of [9].  Therefore, during SDP
             Offer/Answer, an RFC 3984 receiver as the answerer will
             simply ignore sprop-level-parameter-sets, when present in
             an offer, and sprop-parameter-sets conveyed using the
             "fmtp" source attribute as specified in section 6.3 of



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             [9].  Assume that the offered payload type was accepted
             at a level lower than the default level.  If the offered
             payload type included sprop-level-parameter-sets or
             included sprop-parameter-sets conveyed using the "fmtp"
             source attribute, and the offerer sees that the answerer
             has not included use-level-src-parameter-sets equal to 1
             in the answer, the offerer knows that in-band transport
             of parameter sets is needed.

       in-band-parameter-sets:
          This parameter MAY be used to indicate a receiver capability.
          The value MAY be equal to either 0 or 1.  The value 1
          indicates that the receiver discards out-of-band parameter
          sets in sprop-parameter-sets and sprop-level-parameter-sets,
          therefore the sender MUST transmit all parameter sets in-band.
          The value 0 indicates that the receiver utilizes out-of-band
          parameter sets included in sprop-parameter-sets and/or sprop-
          level-parameter-sets.  However, in this case, the sender MAY
          still choose to send parameter sets in-band.  When in-band-
          parameter-sets is equal to 1, use-level-src-parameter-sets
          MUST NOT be present or MUST be equal to 0.  When the
          parameter is not present, this receiver capability is not
          specified, and therefore the sender MAY send out-of-band
          parameter sets only, or it MAY send in-band-parameter-sets
          only, or it MAY send both.

       level-asymmetry-allowed:
          This parameter MAY be used in SDP Offer/Answer to indicate
          whether level asymmetry, i.e., sending media encoded at a
          different level in the offerer-to-answerer direction than the
          level in the answerer-to-offerer direction, is allowed.  The
          value MAY be equal to either 0 or 1.  When the parameter is
          not present, the value MUST be inferred to be equal to 0.
          The value 1 in both the offer and the answer indicates that
          level asymmetry is allowed.  The value of 0 in either the
          offer or the answer indicates the level asymmetry is not
          allowed.

          If "level-asymmetry-allowed" is equal to 0 (or not present)
          in either the offer or the answer, level asymmetry is not
          allowed.  In this case, the level to use in the direction
          from the offerer to the answerer MUST be the same as the
          level to use in the opposite direction.

       packetization-mode:
          This parameter signals the properties of an RTP payload type



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          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 MUST
          be used.  This mode is in use in standards using ITU-T
          Recommendation H.241 [3] (see section 12.1).  When the value
          of packetization-mode is equal to 1, the non-interleaved mode
          MUST be used.  When the value of packetization-mode is equal
          to 2, the interleaved mode 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 packetization-mode is
          not present or the value of packetization-mode is equal to 0
          or 1.  This parameter MUST be present when the value of
          packetization-mode is equal to 2.

          This parameter signals the properties of an RTP packet stream.
          It specifies the maximum number of VCL NAL units that precede
          any VCL NAL unit in the RTP packet 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.

          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 packetization-mode is
          not present or the value of packetization-mode is equal to 0
          or 1.  It MUST be present when the value of packetization-
          mode is equal to 2.

          sprop-deint-buf-req signals the required size of the de-
          interleaving buffer for the RTP packet 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 de-
          interleaving buffer that is specified in section 7.2.  It is
          guaranteed that receivers can perform the de-interleaving of



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          interleaved NAL units into NAL unit decoding order, when the
          de-interleaving buffer size is at least the value of sprop-
          deint-buf-req in terms of bytes.

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

             Informative note: sprop-deint-buf-req indicates the
             required size of the de-interleaving buffer only.  When
             network jitter can occur, an appropriately sized jitter
             buffer has to be provisioned for as well.

       deint-buf-cap:
          This parameter signals the capabilities of a receiver
          implementation and indicates the amount of de-interleaving
          buffer space in units of bytes that the receiver has
          available for reconstructing the NAL unit decoding order.  A
          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 de-interleaving 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 an RTP
          packet stream.  The parameter MUST NOT be present, if the
          value of packetization-mode is equal to 0 or 1.

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





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          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.

       sprop-max-don-diff:
          This parameter MAY be used to signal the properties of an RTP
          packet stream.  It MUST NOT be used to signal transmitter or
          receiver or codec capabilities.  The parameter MUST NOT be
          present if the value of packetization-mode is equal to 0 or 1.
          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.  sprop-max-don-diff is
          calculated as follows:

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





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          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 order.  The decoding order
          number is specified in section 5.5.

             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




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          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.

       sar-understood:
          This parameter MAY be used to indicate a receiver capability
          and not anything else.  The parameter indicates the maximum
          value of aspect_ratio_idc (specified in [1]) smaller than 255
          that the receiver understands.  Table E-1 of [1] specifies
          aspect_ratio_idc equal to 0 as "unspecified", 1 to 16,
          inclusive, as specific Sample Aspect Ratios (SARs), 17 to 254,
          inclusive, as "reserved", and 255 as the Extended SAR, for
          which SAR width and SAR height are explicitly signaled.
          Therefore, a receiver with a decoder according to [1]
          understands aspect_ratio_idc in the range of 1 to 16,
          inclusive and aspect_ratio_idc equal to 255, in the sense
          that the receiver knows what exactly the SAR is.  For such a
          receiver, the value of sar-understood is 16.  If in the
          future Table E-1 of [1] is extended, e.g., such that the SAR
          for aspect_ratio_idc equal to 17 is specified, then for a
          receiver with a decoder that understands the extension, the
          value of sar-understood is 17.  For a receiver with a decoder
          according to the 2003 version of [1], the value of sar-
          understood is 13, as the minimum reserved aspect_ratio_idc
          therein is 14.

          When sar-understood is not present, the value MUST be
          inferred to be equal to 13.

       sar-supported:
          This parameter MAY be used to indicate a receiver capability
          and not anything else.  The value of this parameter is an
          integer in the range of 1 to sar-understood, inclusive, equal
          to 255.  The value of sar-supported equal to N smaller than
          255 indicates that the receiver supports all the SARs
          corresponding to H.264 aspect_ratio_idc values (see Table E-1
          of [1]) in the range from 1 to N, inclusive, without
          geometric distortion.  The value of sar-supported equal to



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          255 indicates that the receiver supports all sample aspect
          ratios which are expressible using two 16-bit integer values
          as the numerator and denominator, i.e., those that are
          expressible using the H.264 aspect_ratio_idc value of 255
          (Extended_SAR, see Table E-1 of [1]), without geometric
          distortion.

          H.264 compliant encoders SHOULD NOT send an aspect_ratio_idc
          equal to 0, or an aspect_ratio_idc larger than sar-understood
          and smaller than 255.  H.264 compliant encoders SHOULD send
          an aspect_ratio_idc that the receiver is able to display
          without geometrical distortion.  However, H.264 compliant
          encoders MAY choose to send pictures using any SAR.

          Note that the actual sample aspect ratio or extended sample
          aspect ratio, when present, of the stream is conveyed in the
          Video Usability Information (VUI) part of the sequence
          parameter set.

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

       Security considerations:
          See section 9 of RFC xxxx.

       Public specification:
          Please refer to RFC xxxx and its section 15.

       Additional information:
          None

       File extensions:     none

       Macintosh file type code: none

       Object identifier or OID: none

       Person & email address to contact for further information:
          Ye-Kui Wang, yekuiwang@huawei.com

       Intended usage:      COMMON

       Author:
          Ye-Kui Wang, yekuiwang@huawei.com





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       Change controller:
          IETF Audio/Video Transport working group delegated from the
          IESG.

8.2. SDP Parameters

   The receiver MUST ignore any parameter unspecified in this memo.

8.2.1. Mapping of Payload Type Parameters to SDP

   The media type video/H264 string is mapped to fields in the Session
   Description Protocol (SDP) [6] as follows:

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

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

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

   o  The OPTIONAL parameters "profile-level-id", "max-recv-level",
      "max-mbps", "max-smbps", "max-fs", "max-cpb", "max-dpb", "max-
      br", "redundant-pic-cap", "use-level-src-parameter-sets", "in-
      band-parameter-sets", "level-asymmetry-allowed", "packetization-
      mode", "sprop-interleaving-depth", "sprop-deint-buf-req",
      "deint-buf-cap", "sprop-init-buf-time", "sprop-max-don-diff",
      "max-rcmd-nalu-size", "sar-understood", and "sar-supported",
      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.

   o  The OPTIONAL parameters "sprop-parameter-sets" and "sprop-level-
      parameter-sets", when present, MUST be included in the "a=fmtp"
      line of SDP or conveyed using the "fmtp" source attribute as
      specified in section 6.3 of [9].  For a particular media format
      (i.e., RTP payload type), a "sprop-parameter-sets" or "sprop-
      level-parameter-sets" MUST NOT be both included in the "a=fmtp"
      line of SDP and conveyed using the "fmtp" source attribute.
      When 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.  When conveyed using
      the "fmtp" source attribute, these parameters are only
      associated with the given source and payload type as parts of
      the "fmtp" source attribute.





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          Informative note: Conveyance of "sprop-parameter-sets" and
          "sprop-level-parameter-sets" using the "fmtp" source
          attribute allows for out-of-band transport of parameter sets
          in topologies like Topo-Video-switch-MCU [29].

   An example of media representation in SDP is as follows (Baseline
   Profile, Level 3.0, some of the constraints of the Main profile may
   not be obeyed):

      m=video 49170 RTP/AVP 98
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42A01E;
                packetization-mode=1;
                sprop-parameter-sets=<parameter sets data>

8.2.2. Usage with the SDP Offer/Answer Model

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

   o  The parameters identifying a media format configuration for
      H.264 are "profile-level-id" and "packetization-mode".  These
      media format configuration parameters (except for the level part
      of "profile-level-id") MUST be used symmetrically; i.e., the
      answerer MUST either maintain all configuration parameters or
      remove the media format (payload type) completely, if one or
      more of the parameter values are not supported.  Note that the
      level part of "profile-level-id" includes level_idc, and, for
      indication of level 1b when profile_idc is equal to 66, 77 or 88,
      bit 4 (constraint_set3_flag) of profile-iop.  The level part of
      "profile-level-id" is changeable.

          Informative note: The requirement for symmetric use does not
          apply for the level part of "profile-level-id", and does not
          apply for the other stream properties and capability
          parameters.

          Informative note: In H.264 [1], all the levels except for
          level 1b are equal to the value of level_idc divided by 10.
          Level 1b is a level higher than level 1.0 but lower than
          level 1.1, and is signaled in an ad-hoc manner, due to that
          the level was specified after level 1.0 and level 1.1.  For
          the Baseline, Main and Extended profiles (with profile_idc
          equal to 66, 77 and 88, respectively), level 1b is indicated
          by level_idc equal to 11 (i.e. same as level 1.1) and



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          constraint_set3_flag equal to 1.  For other profiles, level
          1b is indicated by level_idc equal to 9 (but note that level
          1b for these profiles are still higher than level 1, which
          has level_idc equal to 10, and lower than level 1.1).  In SDP
          Offer/Answer, an answer to an offer may indicate a level
          equal to or lower than the level indicated in the offer.  Due
          to the ad-hoc indication of level 1b, offerers and answerers
          must check the value of bit 4 (constraint_set3_flag) of the
          middle octet of the parameter "profile-level-id", when
          profile_idc is equal to 66, 77 or 88 and level_idc is equal
          to 11.

      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 [8].  An answer MUST NOT
      contain a payload type number used in the offer unless the
      configuration is exactly the same as in the offer.

          Informative note: When an offerer receives an answer, it has
          to compare payload types not declared in the offer based on
          the media type (i.e., video/H264) and the above media
          configuration parameters with any payload types it has
          already declared.  This will enable it to determine whether
          the configuration in question is new or if it is equivalent
          to configuration already offered, since a different payload
          type number may be used in the answer.

   o  The parameter "max-recv-level", when present, declares the
      highest level supported for receiving.  In case "max-recv-level"
      is not present, the highest level supported for receiving is
      equal to the default level indicated by the level part of
      "profile-level-id".  "max-recv-level", when present, MUST be
      higher than the default level.

   o  The parameter "level-asymmetry-allowed" indicates whether level
      asymmetry is allowed.

      If "level-asymmetry-allowed" is equal to 0 (or not present) in
      either the offer or the answer, level asymmetry is not allowed.
      In this case, the level to use in the direction from the offerer
      to the answerer MUST be the same as the level to use in the
      opposite direction, and the common level to use is equal to the
      lower value of the default level in the offer and the default
      level in the answer.





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      Otherwise ("level-asymmetry-allowed" equals to 1 in both the
      offer and the answer), level asymmetry is allowed.  In this case,
      the level to use in the offerer-to-answerer direction MUST be
      equal to the highest level the answerer supports for receiving,
      and the level to use in the answerer-to-offerer direction MUST
      be equal to the highest level the offerer supports for receiving.

      When level asymmetry is not allowed, level upgrade is not
      allowed, i.e. the default level in the answer MUST be equal to
      or lower than the default level in the offer.

   o  The parameters "sprop-deint-buf-req", "sprop-interleaving-depth",
      "sprop-max-don-diff", and "sprop-init-buf-time" describe the
      properties of the RTP packet stream that the offerer or answerer
      is sending for the 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, 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 than 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.

   o  The capability parameters "max-mbps", "max-smbps", "max-fs",
      "max-cpb", "max-dpb", "max-br", ,"redundant-pic-cap", "max-rcmd-
      nalu-size", "sar-understood", "sar-supported" MAY be used to
      declare further capabilities of the offerer or answerer for
      receiving.  These parameters MUST NOT be present when the
      direction attribute is sendonly, and the parameters describe the
      limitations of what the offerer or answerer accepts for
      receiving streams.

   o  An offerer has to include the size of the de-interleaving buffer,
      "sprop-deint-buf-req", in the offer for an interleaved H.264
      stream.  To enable the offerer and answerer to inform each other
      about their capabilities for de-interleaving buffering in
      receiving streams, both parties are RECOMMENDED to include
      "deint-buf-cap".  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.



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   o  The "sprop-parameter-sets" or "sprop-level-parameter-sets"
      parameter, when present (included in the "a=fmtp" line of SDP or
      conveyed using the "fmtp" source attribute as specified in
      section 6.3 of [9]), is used for out-of-band transport of
      parameter sets.  However, when out-of-band transport of
      parameter sets is used, parameter sets MAY still be additionally
      transported in-band.

      The answerer MAY use either out-of-band or in-band transport of
      parameter sets for the stream it is sending, regardless of
      whether out-of-band parameter sets transport has been used in
      the offerer-to-answerer direction.  Parameter sets included in
      an answer are independent of those parameter sets included in
      the offer, as they are used for decoding two different video
      streams, one from the answerer to the offerer, and the other in
      the opposite direction.

      The following rules apply to transport of parameter sets in the
      offerer-to-answerer direction.

         o An offer MAY include either or both of "sprop-parameter-
           sets" and "sprop-level-parameter-sets".  If neither "sprop-
           parameter-sets" nor "sprop-level-parameter-sets" is present
           in the offer, then only in-band transport of parameter sets
           is used.

         o If the answer includes "in-band-parameter-sets" equal to 1,
           then the offerer MUST transmit parameter sets in-band.
           Otherwise, the following applies.

              o If the level to use in the offerer-to-answerer
                 direction is equal to the default level in the offer,
                 the following applies.

                     When there is a "sprop-parameter-sets" included
                     in the "a=fmtp" line in the offer, the answerer
                     MUST be prepared to use the parameter sets
                     included in the "sprop-parameter-sets" for
                     decoding the incoming NAL unit stream.

                     When there is a "sprop-parameter-sets" conveyed
                     using the "fmtp" source attribute in the offer,
                     the following applies.  If the answer includes
                     "use-level-src-parameter-sets" equal to 1 or the
                     "fmtp" source attribute, the answerer MUST be
                     prepared to use the parameter sets included in



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                     the "sprop-parameter-sets" for decoding the
                     incoming NAL unit stream;  Otherwise, the offerer
                     MUST transmit parameter sets in-band.

                     When "sprop-parameter-sets" is not present in the
                     offer, the offerer MUST transmit parameter sets
                     in-band.

                     The answerer MUST ignore "sprop-level-parameter-
                     sets", when present (either included in the
                     "a=fmtp" line or conveyed using the "fmtp" source
                     attribute) in the offer.

              o Otherwise (the level to use in the offerer-to-answerer
                direction is not equal to the default level in the
                offer), the following applies.

                     The answerer MUST ignore "sprop-parameter-sets",
                     when present (either included in the "a=fmtp"
                     line or conveyed using the "fmtp" source
                     attribute) in the offer.

                     When neither "use-level-src-parameter-sets" equal
                     to 1 nor the "fmtp" source attribute is present
                     in the answer, the answerer MUST ignore "sprop-
                     level-parameter-sets", when present in the offer,
                     and the offerer MUST transmit parameter sets in-
                     band.

                     When either "use-level-src-parameter-sets" equal
                     to 1 or the "fmtp" source attribute is present in
                     the answer, the answerer MUST be prepared to use
                     the parameter sets that are included in "sprop-
                     level-parameter-sets" for the accepted level (i.e.
                     the default level in the answer), when present in
                     the offer, for decoding the incoming NAL unit
                     stream, and ignore all other parameter sets
                     included in "sprop-level-parameter-sets".

                     When no parameter sets for the level to use in
                     the offerer-to-answerer direction are present in
                     "sprop-level-parameter-sets" in the offer, the
                     offerer MUST transmit parameter sets in-band.

      The following rules apply to transport of parameter sets in the
      answerer-to-offerer direction.



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         o An answer MAY include either "sprop-parameter-sets" or
           "sprop-level-parameter-sets", but MUST NOT include both of
           the two.  If neither "sprop-parameter-sets" nor "sprop-
           level-parameter-sets" is present in the answer, then only
           in-band transport of parameter sets is used.

         o If the offer includes "in-band-parameter-sets" equal to 1,
           the answerer MUST NOT include "sprop-parameter-sets" or
           "sprop-level-parameter-sets" in the answer and MUST
           transmit parameter sets in-band.  Otherwise, the following
           applies.

              o If the level to use in the answerer-to-offerer
                 direction is equal to the default level in the answer,
                 the following applies.

                     When there is a "sprop-parameter-sets" included
                     in the "a=fmtp" line in the answer, the offerer
                     MUST be prepared to use the parameter sets
                     included in the "sprop-parameter-sets" for
                     decoding the incoming NAL unit stream.

                     When there is a "sprop-parameter-sets" conveyed
                     using the "fmtp" source attribute in the answer,
                     the following applies. If the offer includes
                     "use-level-src-parameter-sets" equal to 1 or the
                     "fmtp" source attribute, the offerer MUST be
                     prepared to use the parameter sets included in
                     the "sprop-parameter-sets" for decoding the
                     incoming NAL unit stream;  Otherwise, the
                     answerer MUST transmit parameter sets in-band.

                     When "sprop-parameter-sets" is not present in the
                     answer, the answerer MUST transmit parameter sets
                     in-band.

                     The offerer MUST ignore "sprop-level-parameter-
                     sets", when present (either included in the
                     "a=fmtp" line or conveyed using the "fmtp" source
                     attribute) in the answer.

              o Otherwise (the level to use in the answerer-to-offerer
                direction is not equal to the default level in the
                answer), the following applies.





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                     The offerer MUST ignore "sprop-parameter-sets",
                     when present (either included in the "a=fmtp"
                     line of SDP or conveyed using the "fmtp" source
                     attribute) in the answer.

                     When neither "use-level-src-parameter-sets" equal
                     to 1 nor the "fmtp" source attribute is present
                     in the offer, the offerer MUST ignore "sprop-
                     level-parameter-sets", when present, and the
                     answerer MUST transmit parameter sets in-band.

                     When either "use-level-src-parameter-sets" equal
                     to 1 or the "fmtp" source attribute is present in
                     the offer, the offerer MUST be prepared to use
                     the parameter sets that are included in "sprop-
                     level-parameter-sets" for the level to use in the
                     answerer-to-offerer direction, when present in
                     the answer, for decoding the incoming NAL unit
                     stream, and ignore all other parameter sets
                     included in "sprop-level-parameter-sets" in the
                     answer.

                     When no parameter sets for the level to use in
                     the answerer-to-offerer direction are present in
                     "sprop-level-parameter-sets" in the answer, the
                     answerer MUST transmit parameter sets in-band.

      When "sprop-parameter-sets" or "sprop-level-parameter-sets" is
      conveyed using the "fmtp" source attribute as specified in
      section 6.3 of [9], the receiver of the parameters MUST store
      the parameter sets included in the "sprop-parameter-sets" or
      "sprop-level-parameter-sets" for the accepted level and
      associate them to the source given as a part of the "fmtp"
      source attribute.  Parameter sets associated with one source
      MUST only be used to decode NAL units conveyed in RTP packets
      from the same source.  When this mechanism is in use, SSRC
      collision detection and resolution MUST be performed as
      specified in [9].

          Informative note: Conveyance of "sprop-parameter-sets" and
          "sprop-level-parameter-sets" using the "fmtp" source
          attribute may be used in topologies like Topo-Video-switch-
          MCU [29] to enable out-of-band transport of parameter sets.

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



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   o  The media format configuration is identified by "profile-level-
      id", including the level part, and "packetization-mode".  These
      media format configuration parameters (including the level part
      of "profile-level-id") MUST be used symmetrically; i.e., the
      answerer MUST either maintain all configuration parameters or
      remove the media format (payload type) completely.  Note that
      this implies that the level part of "profile-level-id" for
      Offer/Answer in multicast is not changeable.

      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 [8].  An answer MUST NOT
      contain a payload type number used in the offer unless the
      configuration is the same as in the offer.

   o  Parameter sets received MUST be associated with the originating
      source, and MUST be only used in decoding the incoming NAL unit
      stream from the same source.

   o  The rules for other parameters are the same as above for unicast
      as long as the above rules are obeyed.

   Table 6 lists the interpretation of all the media type parameters
   that MUST be used for the different direction attributes.

























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       Table 6. Interpretation of parameters for different direction
                                attributes.

                                              sendonly --+
                                           recvonly --+  |
                                        sendrecv --+  |  |
                                                   |  |  |
                profile-level-id                   C  C  P
                max-recv-level                     R  R  -
                packetization-mode                 C  C  P
                sprop-deint-buf-req                P  -  P
                sprop-interleaving-depth           P  -  P
                sprop-max-don-diff                 P  -  P
                sprop-init-buf-time                P  -  P
                max-mbps                           R  R  -
                max-smbps                          R  R  -
                max-fs                             R  R  -
                max-cpb                            R  R  -
                max-dpb                            R  R  -
                max-br                             R  R  -
                redundant-pic-cap                  R  R  -
                deint-buf-cap                      R  R  -
                max-rcmd-nalu-size                 R  R  -
                sar-understood                     R  R  -
                sar-supported                      R  R  -
                in-band-parameter-sets             R  R  -
                use-level-src-parameter-sets       R  R  -
                level-asymmetry-allowed            O  -  -
                sprop-parameter-sets               S  -  S
                sprop-level-parameter-sets         S  -  S

             Legend:

             C: configuration for sending and receiving streams
             O: offer/answer mode
             P: properties of the stream to be sent
             R: receiver capabilities




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             S: out-of-band parameter sets
             -: not usable, when present SHOULD be ignored

   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/less or equal values of these parameters.

   Parameters declaring a configuration point are not changeable, with
   the exception of the level part of the "profile-level-id" parameter
   for unicast usage.  These express values a receiver expects to be
   used and must be used verbatim on the sender side.

   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 for the sender to
   receive streams.  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.

   A receiver SHOULD understand all media type 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 the
   receiver of the offer.

   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
   property 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.

   If an offerer wishes to have non-symmetric capabilities between
   sending and receiving, the offerer can allow asymmetric levels via
   "level-asymmetry-allowed" equal to 1.  Alternatively, the offerer
   could offer different RTP sessions; i.e., different media lines
   declared as "recvonly" and "sendonly", respectively.  This may have
   further implications on the system, and may require additional
   external semantics to associate the two media lines.






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8.2.3. Usage in Declarative Session Descriptions

   When H.264 over RTP is offered with SDP in a declarative style, as
   in RTSP [27] or SAP [28], the following considerations are
   necessary.

   o  All parameters capable of indicating both stream properties and
      receiver capabilities are used to indicate only stream
      properties.  For example, in this case, the parameter "profile-
      level-id" declares only the values used by the stream, not the
      capabilities for receiving streams.  This results in that the
      following interpretation of the parameters MUST be used:

      Declaring actual configuration or stream properties:

          - profile-level-id
          - packetization-mode
          - sprop-interleaving-depth
          - sprop-deint-buf-req
          - sprop-max-don-diff
          - sprop-init-buf-time

      Out-of-band transporting of parameter sets:

          - sprop-parameter-sets
          - sprop-level-parameter-sets

      Not usable(when present, they SHOULD be ignored):

          - max-mbps
          - max-smbps
          - max-fs
          - max-cpb
          - max-dpb
          - max-br
          - max-recv-level
          - redundant-pic-cap
          - max-rcmd-nalu-size
          - deint-buf-cap
          - sar-understood
          - sar-supported
          - in-band-parameter-sets
          - level-asymmetry-allowed
          - use-level-src-parameter-sets





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   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.

8.3. Examples

   An SDP Offer/Answer exchange wherein both parties are expected to
   both send and receive could look like the following.  Only the
   media codec specific parts of the SDP are shown.  Some lines are
   wrapped due to text constraints.

      Offerer -> Answerer SDP message:

      m=video 49170 RTP/AVP 100 99 98
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42A01E; packetization-mode=0;
        sprop-parameter-sets=<parameter sets data#0>
      a=rtpmap:99 H264/90000
      a=fmtp:99 profile-level-id=42A01E; packetization-mode=1;
        sprop-parameter-sets=<parameter sets data#1>
      a=rtpmap:100 H264/90000
      a=fmtp:100 profile-level-id=42A01E; packetization-mode=2;
        sprop-parameter-sets=<parameter sets data#2>;
        sprop-interleaving-depth=45; sprop-deint-buf-req=64000;
        sprop-init-buf-time=102478; deint-buf-cap=128000

   The above offer presents the same codec configuration in three
   different packetization formats.  PT 98 represents single NALU mode,
   PT 99 represents non-interleaved mode, and PT 100 indicates the
   interleaved mode.  In the interleaved mode case, the interleaving
   parameters that the offerer would use if the answer indicates
   support for PT 100 are also included.  In all three cases the
   parameter "sprop-parameter-sets" conveys the initial parameter sets
   that are required by the answerer when receiving a stream from the
   offerer when this configuration is accepted.  Note that the value
   for "sprop-parameter-sets" could be different for each payload type.

      Answerer -> Offerer SDP message:

      m=video 49170 RTP/AVP 100 99 97
      a=rtpmap:97 H264/90000
      a=fmtp:97 profile-level-id=42A01E; packetization-mode=0;
        sprop-parameter-sets=<parameter sets data#3>
      a=rtpmap:99 H264/90000



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      a=fmtp:99 profile-level-id=42A01E; packetization-mode=1;
        sprop-parameter-sets=<parameter sets data#4>;
        max-rcmd-nalu-size=3980
      a=rtpmap:100 H264/90000
      a=fmtp:100 profile-level-id=42A01E; packetization-mode=2;
        sprop-parameter-sets=<parameter sets data#5>;
        sprop-interleaving-depth=60;
        sprop-deint-buf-req=86000; sprop-init-buf-time=156320;
        deint-buf-cap=128000; max-rcmd-nalu-size=3980

   As the Offer/Answer negotiation covers both sending and receiving
   streams, an offer indicates the exact parameters for what the
   offerer is willing to receive, whereas the answer indicates the
   same for what the answerer accepts to receive.  In this case the
   offerer declared that it is willing to receive payload type 98.
   The answerer accepts this by declaring an equivalent payload type
   97; i.e., it has identical values for the two parameters "profile-
   level-id" and "packetization-mode" (since "packetization-mode" is
   equal to 0, "sprop-deint-buf-req" is not present).  As the offered
   payload type 98 is accepted, the answerer needs to store parameter
   sets included in sprop-parameter-sets=<parameter sets data#0> in
   case the offer finally decides to use this configuration. In the
   answer, the answerer includes the parameter sets in sprop-
   parameter-sets=<parameter sets data#3> that the answerer would use
   in the stream sent from the answerer if this configuration is
   finally used.

   The answerer also accepts the reception of the two configurations
   that payload types 99 and 100 represent.  Again, the answerer needs
   to store parameter sets included in sprop-parameter-sets=<parameter
   sets data#1> and sprop-parameter-sets=<parameter sets data#2> in
   case the offer finally decides to use either of these two
   configurations.  The answerer provides the initial parameter sets
   for the answerer-to-offerer direction, i.e. the parameter sets in
   sprop-parameter-sets=<parameter sets data#4> and sprop-parameter-
   sets=<parameter sets data#5>, for payload types 99 and 100,
   respectively, that it will use to send the payload types.  The
   answerer also provides the offerer with its memory limit for de-
   interleaving operations by providing a "deint-buf-cap" parameter.
   This is only useful if the offerer decides on making a second offer,
   where it can take the new value into account.  The "max-rcmd-nalu-
   size" indicates that the answerer can efficiently process NALUs up
   to the size of 3980 bytes.  However, there is no guarantee that the
   network supports this size.





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   In the following example, the offer is accepted without level
   downgrading (i.e. the default level, 3.0, is accepted), and both
   "sprop-parameter-sets" and "sprop-level-parameter-sets" are present
   in the offer.  The answerer must ignore sprop-level-parameter-
   sets=<parameter sets data#1> and store parameter sets in sprop-
   parameter-sets=<parameter sets data#0> for decoding the incoming
   NAL unit stream.  The offerer must store the parameter sets in
   sprop-parameter-sets=<parameter sets data#2> in the answer for
   decoding the incoming NAL unit stream.  Note that in this example,
   parameter sets in sprop-parameter-sets=<parameter sets data#2> must
   be associated with level 3.0.

      Offer SDP:

      m=video 49170 RTP/AVP 98
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
        packetization-mode=1;
        sprop-parameter-sets=<parameter sets data#0>;
        sprop-level-parameter-sets=<parameter sets data#1>

      Answer SDP:

      m=video 49170 RTP/AVP 98
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
        packetization-mode=1;
        sprop-parameter-sets=<parameter sets data#2>

   In the following example, the offer (Baseline profile, level 1.1)
   is accepted with level downgrading (the accepted level is 1b), and
   both "sprop-parameter-sets" and "sprop-level-parameter-sets" are
   present in the offer.  The answerer must ignore sprop-parameter-
   sets=<parameter sets data#0> and all parameter sets not for the
   accepted level (level 1b) in sprop-level-parameter-sets=<parameter
   sets data#1>, and must store parameter sets for the accepted level
   (level 1b) in sprop-level-parameter-sets=<parameter sets data#1>
   for decoding the incoming NAL unit stream.  The offerer must store
   the parameter sets in sprop-parameter-sets=<parameter sets data#2>
   in the answer for decoding the incoming NAL unit stream.  Note that
   in this example, parameter sets in sprop-parameter-sets=<parameter
   sets data#2> must be associated with level 1b.

      Offer SDP:





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      m=video 49170 RTP/AVP 98
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42A00B; //Baseline profile, Level 1.1
        packetization-mode=1;
        sprop-parameter-sets=<parameter sets data#0>;
        sprop-level-parameter-sets=<parameter sets data#1>

      Answer SDP:

      m=video 49170 RTP/AVP 98
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42B00B; //Baseline profile, Level 1b
        packetization-mode=1;
        sprop-parameter-sets=<parameter sets data#2>;
        use-level-src-parameter-sets=1

   In the following example, the offer (Baseline profile, level 1.1)
   is accepted with level downgrading (the accepted level is 1b), and
   both "sprop-parameter-sets" and "sprop-level-parameter-sets" are
   present in the offer.  However, the answerer is a legacy RFC 3984
   implementation and does not understand "sprop-level-parameter-sets",
   hence it does not include "use-level-src-parameter-sets" (which the
   answerer does not understand, either) in the answer.  Therefore,
   the answerer must ignore both sprop-parameter-sets=<parameter sets
   data#0> and sprop-level-parameter-sets=<parameter sets data#1>, and
   the offerer must transport parameter sets in-band.

      Offer SDP:

      m=video 49170 RTP/AVP 98
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42A00B; //Baseline profile, Level 1.1
        packetization-mode=1;
        sprop-parameter-sets=<parameter sets data#0>;
        sprop-level-parameter-sets=<parameter sets data#1>

      Answer SDP:

      m=video 49170 RTP/AVP 98
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42B00B; //Baseline profile, Level 1b
        packetization-mode=1


   In the following example, the offer is accepted without level
   downgrading, and "sprop-parameter-sets" is present in the offer.



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   Parameter sets in sprop-parameter-sets=<parameter sets data#0> must
   be stored and used used by the encoder of the offerer and the
   decoder of the answerer, and parameter sets in sprop-parameter-
   sets=<parameter sets data#1>must be used by the encoder of the
   answerer and the decoder of the offerer.  Note that sprop-
   parameter-sets=<parameter sets data#0> is basically independent of
   sprop-parameter-sets=<parameter sets data#1>.

      Offer SDP:

      m=video 49170 RTP/AVP 98
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
        packetization-mode=1;
        sprop-parameter-sets=<parameter sets data#0>

      Answer SDP:

      m=video 49170 RTP/AVP 98
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
        packetization-mode=1;
        sprop-parameter-sets=<parameter sets data#1>

   In the following example, the offer is accepted without level
   downgrading, and neither "sprop-parameter-sets" nor "sprop-level-
   parameter-sets" is present in the offer, meaning that there is no
   out-of-band transmission of parameter sets, which then have to be
   transported in-band.

      Offer SDP:

      m=video 49170 RTP/AVP 98
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
        packetization-mode=1

      Answer SDP:

      m=video 49170 RTP/AVP 98
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
        packetization-mode=1

   In the following example, the offer is accepted with level
   downgrading and "sprop-parameter-sets" is present in the offer.  As



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   sprop-parameter-sets=<parameter sets data#0> contains level_idc
   indicating Level 3.0, therefore cannot be used as the answerer
   wants Level 2.0 and must be ignored by the answerer, and in-band
   parameter sets must be used.

      Offer SDP:

      m=video 49170 RTP/AVP 98
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
        packetization-mode=1;
        sprop-parameter-sets=<parameter sets data#0>

      Answer SDP:

      m=video 49170 RTP/AVP 98
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42A014; //Baseline profile, Level 2.0
        packetization-mode=1

   In the following example, the offer is also accepted with level
   downgrading, and neither "sprop-parameter-sets" nor "sprop-level-
   parameter-sets" is present in the offer, meaning that there is no
   out-of-band transmission of parameter sets, which then have to be
   transported in-band.

      Offer SDP:

      m=video 49170 RTP/AVP 98
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
        packetization-mode=1

      Answer SDP:

      m=video 49170 RTP/AVP 98
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42A014; //Baseline profile, Level 2.0
        packetization-mode=1

   In the following example, the offer is accepted with level
   upgrading, and neither "sprop-parameter-sets" nor "sprop-level-
   parameter-sets" is present in the offer or the answer, meaning that
   there is no out-of-band transmission of parameter sets, which then
   have to be transported in-band.  The level to use in the offerer-
   to-answerer direction is Level 3.0, and the level to use in the



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   answerer-to-offerer direction is Level 2.0.  The answerer is
   allowed to send at any level up to and including level 2.0, and the
   offerer is allowed to send at any level up to and including level
   3.0.

      Offer SDP:

      m=video 49170 RTP/AVP 98
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42A014; //Baseline profile, Level 2.0
        packetization-mode=1; level-asymmetry-allowed=1


      Answer SDP:

      m=video 49170 RTP/AVP 98
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
        packetization-mode=1; level-asymmetry-allowed=1

   In the following example, the offerer is a Multipoint Control Unit
   (MCU) in a Topo-Video-switch-MCU like topology [29], offering
   parameter sets received (using out-of-band transport) from three
   other participants B, C, and D, and receiving parameter sets from
   the participant A, which is the answerer.  The participants are
   identified by their values of CNAME, which are mapped to different
   SSRC values.  The same codec configuration is used by all the four
   participants.  The participant A stores and associates the
   parameter sets included in <parameter sets data#B>, <parameter sets
   data#C>, and <parameter sets data#D> to participants B, C, and D,
   respectively, and uses <parameter sets data#B> for decoding NAL
   units carried in RTP packets originated from participant B only,
   uses <parameter sets data#C> for decoding NAL units carried in RTP
   packets originated from participant C only, and uses <parameter
   sets data#D> for decoding NAL units carried in RTP packets
   originated from participant D only.

      Offer SDP:

      m=video 49170 RTP/AVP 98
      a=ssrc:SSRC-B cname:CNAME-B
      a=ssrc:SSRC-C cname:CNAME-C
      a=ssrc:SSRC-D cname:CNAME-D
      a=ssrc:SSRC-B fmtp:98
        sprop-parameter-sets=<parameter sets data#B>
      a=ssrc:SSRC-C fmtp:98



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        sprop-parameter-sets=<parameter sets data#C>
      a=ssrc:SSRC-D fmtp:98
        sprop-parameter-sets=<parameter sets data#D>
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
        packetization-mode=1

      Answer SDP:

      m=video 49170 RTP/AVP 98
      a=ssrc:SSRC-A cname:CNAME-A
      a=ssrc:SSRC-A fmtp:98
        sprop-parameter-sets=<parameter sets data#A>
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42A01E; //Baseline profile, Level 3.0
        packetization-mode=1

8.4. Parameter Set Considerations

   The H.264 parameter sets are a fundamental part of the video codec
   and vital to its operation; see section 1.2.  Due to their
   characteristics and their importance for the decoding process, lost
   or erroneously transmitted parameter sets can hardly be concealed
   locally at the receiver.  A reference to a corrupt parameter set
   has normally fatal results to the decoding process.  Corruption
   could occur, for example, due to the erroneous transmission or loss
   of a parameter set NAL unit, but also due to the untimely
   transmission of a parameter set update.  A parameter set update
   refers to a change of at least one parameter in a picture parameter
   set or sequence parameter set for which the picture parameter set
   or sequence parameter set identifier remains unchanged.  Therefore,
   the following recommendations are provided as a guideline for the
   implementer of the RTP sender.

   Parameter set NALUs can be transported using three different
   principles:

   A. Using a session control protocol (out-of-band) prior to the
      actual RTP session.

   B. Using a session control protocol (out-of-band) during an ongoing
      RTP session.

   C. Within the RTP packet stream in the payload (in-band) during an
      ongoing RTP session.




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   It is recommended to implement principles A and B within a session
   control protocol.  SIP and SDP can be used as described in the SDP
   Offer/Answer model and in the previous sections of this memo.
   Section 8.2.2 includes a detailed discussion on transport of
   parameter sets in-band or out-of-band in SDP Offer/Answer using
   media type parameters "sprop-parameter-sets", "sprop-level-
   parameter-sets", "use-level-src-parameter-sets" and "in-band-
   parameter-sets".  This section contains guidelines on how
   principles A and B should be implemented within session control
   protocols.  It is independent of the particular protocol used.
   Principle C is supported by the RTP payload format defined in this
   specification.  There are topologies like Topo-Video-switch-MCU [29]
   for which the use of principle C may be desirable.

   If in-band signaling of parameter sets is used, the picture and
   sequence parameter set NALUs SHOULD be transmitted in the RTP
   payload using a reliable method of delivering of RTP (see below),
   as a loss of a parameter set of either type will likely prevent
   decoding of a considerable portion of the corresponding RTP packet
   stream.

   If in-band signaling of parameter sets is used, the sender SHOULD
   take the error characteristics into account and use mechanisms to
   provide a high probability for delivering the parameter sets
   correctly.  Mechanisms that increase the probability for a correct
   reception include packet repetition, FEC, and retransmission.  The
   use of an unreliable, out-of-band control protocol has similar
   disadvantages as the in-band signaling (possible loss) and, in
   addition, may also lead to difficulties in the synchronization (see
   below).  Therefore, it is NOT RECOMMENDED.

   Parameter sets MAY be added or updated during the lifetime of a
   session using principles B and C.  It is required that parameter
   sets are present at the decoder prior to the NAL units that refer
   to them.  Updating or adding of parameter sets can result in
   further problems, and therefore the following recommendations
   should be considered.












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   - When parameter sets are added or updated, care SHOULD be taken
      to ensure that any parameter set is delivered prior to its usage.
      When new parameter sets are added, previously unused parameter
      set identifiers are used.  It is common that no synchronization
      is present between out-of-band signaling and in-band traffic.
      If out-of-band signaling is used, it is RECOMMENDED that a
      sender does not start sending NALUs requiring the added or
      updated parameter sets prior to acknowledgement of delivery from
      the signaling protocol.

   - When parameter sets are updated, the following synchronization
      issue should be taken into account.  When overwriting a
      parameter set at the receiver, the sender has to ensure that the
      parameter set in question is not needed by any NALU present in
      the network or receiver buffers.  Otherwise, decoding with a
      wrong parameter set may occur.  To lessen this problem, it is
      RECOMMENDED either to overwrite only those parameter sets that
      have not been used for a sufficiently long time (to ensure that
      all related NALUs have been consumed), or to add a new parameter
      set instead (which may have negative consequences for the
      efficiency of the video coding).

         Informative note: In some topologies like Topo-Video-switch-
         MCU [29] the origin of the whole set of parameter sets may
         come from multiple sources that may use non-unique parameter
         sets identifiers.  In this case an offer may overwrite an
         existing parameter set if no other mechanism that enables
         uniqueness of the parameter sets in the out-of-band channel
         exists.

   - In a multiparty session, one participant MUST associate
      parameter sets coming from different sources with the source
      identification whenever possible, e.g. by conveying out-of-band
      transported parameter sets, as different sources typically use
      independent parameter set identifier value spaces.

   - Adding or modifying parameter sets by using both principles B
      and C in the same RTP session may lead to inconsistencies of the
      parameter sets because of the lack of synchronization between
      the control and the RTP channel.  Therefore, principles B and C
      MUST NOT both be used in the same session unless sufficient
      synchronization can be provided.

   In some scenarios (e.g., when only the subset of this payload
   format specification corresponding to H.241 is used) or topologies,
   it is not possible to employ out-of-band parameter set transmission.



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   In this case, parameter sets have to be transmitted in-band.  Here,
   the synchronization with the non-parameter-set-data in the
   bitstream is implicit, but the possibility of a loss has to be
   taken into account.  The loss probability should be reduced using
   the mechanisms discussed above.  In case a loss of a parameter set
   is detected, recovery may be achieved by using a Decoder Refresh
   Point procedure, for example, using RTCP feedback Full Intra
   Request (FIR) [30].  Two example Decoder Refresh Point procedures
   are provided in the informative Section 8.5.

   - When parameter sets are initially provided using principle A and
      then later added or updated in-band (principle C), there is a
      risk associated with updating the parameter sets delivered out-
      of-band.  If receivers miss some in-band updates (for example,
      because of a loss or a late tune-in), those receivers attempt to
      decode the bitstream using out-dated parameters.  It is
      therefore RECOMMENDED that parameter set IDs be partitioned
      between the out-of-band and in-band parameter sets.

8.5. Decoder Refresh Point Procedure using In-Band Transport of
   Parameter Sets (Informative)

   When a sender with a video encoder according to [1] receives a
   request for a decoder refresh point, the encoder shall enter the
   fast update mode by using one of the procedures specified
   in Section 8.5.1 or 8.5.2 below.  The procedure in 8.5.1 is the
   preferred response in a lossless transmission environment.  Both
   procedures satisfy the requirement to enter the fast update mode
   for H.264 video encoding.

8.5.1. IDR Procedure to Respond to a Request for a Decoder Refresh
   Point

   This section gives one possible way to respond to a request for a
   decoder refresh point.

   The encoder shall, in the order presented here:

   1) Immediately prepare to send an IDR picture.

   2) Send a sequence parameter set to be used by the IDR picture to
      be sent. The encoder may optionally also send other sequence
      parameter sets.






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   3) Send a picture parameter set to be used by the IDR picture to be
      sent. The encoder may optionally also send other picture
      parameter sets.

   4) Send the IDR picture.

   5) From this point forward in time, send any other sequence or
      picture parameter sets that have not yet been sent in this
      procedure, prior to their reference by any NAL unit, regardless
      of whether such parameter sets were previously sent prior to
      receiving the request for a decoder refresh point.  As needed,
      such parameter sets may be sent in a batch, one at a time, or in
      any combination of these two methods.  Parameter sets may be re-
      sent at any time for redundancy.  Caution should be taken when
      parameter set updates are present, as described above in Section
      8.4.

8.5.2. Gradual Recovery Procedure to Respond to a Request for a
   Decoder Refresh Point

   This section gives another possible way to respond to a request for
   a decoder refresh point.

   The encoder shall, in the order presented here:

   1) Send a recovery point SEI message (see Sections D.1.7 and D.2.7
      of [1]).

   2) Repeat any sequence and picture parameter sets that were sent
      before the recovery point SEI message, prior to their reference
      by a NAL unit.

   The encoder shall ensure that the decoder has access to all
   reference pictures for inter prediction of pictures at or after the
   recovery point, which is indicated by the recovery point SEI
   message, in output order, assuming that the transmission from now
   on is error-free.

   The value of the recovery_frame_cnt syntax element in the recovery
   point SEI message should be small enough to ensure a fast recovery.

   As needed, such parameter sets may be re-sent in a batch, one at a
   time, or in any combination of these two methods.  Parameter sets
   may be re-sent at any time for redundancy.  Caution should be taken
   when parameter set updates are present, as described above in
   Section 8.4.



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9. Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [5], and in any appropriate RTP profile (for example,
   [16]).  This implies that confidentiality of the media streams is
   achieved by encryption; for example, through the application of
   SRTP [26].  Because the data compression used with this payload
   format is applied end-to-end, any encryption needs to be performed
   after compression.  A potential denial-of-service threat exists for
   data encodings using compression techniques that have non-uniform
   receiver-end computational load.  The attacker can inject
   pathological datagrams into the stream that are complex to decode
   and that cause the receiver to be overloaded.  H.264 is
   particularly vulnerable to such attacks, as it is extremely simple
   to generate datagrams containing NAL units that affect the decoding
   process of many future NAL units.  Therefore, the usage of data
   origin authentication and data integrity protection of at least the
   RTP packet is RECOMMENDED; for example, with SRTP [26].

   Note that the appropriate mechanism to ensure confidentiality and
   integrity of RTP packets and their payloads is very dependent on
   the application and on the transport and signaling protocols
   employed.  Thus, although SRTP is given as an example above, other
   possible choices exist.

   Decoders MUST exercise caution with respect to the handling of user
   data SEI messages, particularly if they contain active elements,
   and MUST restrict their domain of applicability to the presentation
   containing the stream.

   End-to-End security with either authentication, integrity or
   confidentiality protection will prevent a MANE from performing
   media-aware operations other than discarding complete packets.  And
   in the case of confidentiality protection it will even be prevented
   from performing discarding of packets in a media aware way.  To
   allow any MANE to perform its operations, it will be required to be
   a trusted entity which is included in the security context
   establishment.

10. Congestion Control

   Congestion control for RTP SHALL be used in accordance with RFC
   3550 [5], and with any applicable RTP profile; e.g., RFC 3551 [16].
   An additional requirement if best-effort service is being used is:
   users of this payload format MUST monitor packet loss to ensure



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   that the packet loss rate is within acceptable parameters.  Packet
   loss is considered acceptable if a TCP flow across the same network
   path, and experiencing the same network conditions, would achieve
   an average throughput, measured on a reasonable timescale, that is
   not less than the RTP flow is achieving.  This condition can be
   satisfied by implementing congestion control mechanisms to adapt
   the transmission rate (or the number of layers subscribed for a
   layered multicast session), or by arranging for a receiver to leave
   the session if the loss rate is unacceptably high.

   The bit rate adaptation necessary for obeying the congestion
   control principle is easily achievable when real-time encoding is
   used.  However, when pre-encoded content is being transmitted,
   bandwidth adaptation requires the availability of more than one
   coded representation of the same content, at different bit rates,
   or the existence of non-reference pictures or sub-sequences [22] in
   the bitstream.  The switching between the different representations
   can normally be performed in the same RTP session; e.g., by
   employing a concept known as SI/SP slices of the Extended Profile,
   or by switching streams at IDR picture boundaries.  Only when non-
   downgradable parameters (such as the profile part of the
   profile/level ID) are required to be changed does it become
   necessary to terminate and re-start the media stream.  This may be
   accomplished by using a different RTP payload type.

   MANEs MAY follow the suggestions outlined in section 7.3 and remove
   certain unusable packets from the packet stream when that stream
   was damaged due to previous packet losses.  This can help reduce
   the network load in certain special cases.

11. IANA Consideration

   The H264 media subtype name specified by RFC 3984 should be updated
   as defined in section 8.1 of this memo.

12. Informative Appendix: Application Examples

   This payload specification is very flexible in its use, in order to
   cover the extremely wide application space anticipated for H.264.
   However, this great flexibility also makes it difficult for an
   implementer to decide on a reasonable packetization scheme.  Some
   information on how to apply this specification to real-world
   scenarios is likely to appear in the form of academic publications
   and a test model software and description in the near future.
   However, some preliminary usage scenarios are described here as
   well.



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12.1. Video Telephony according to ITU-T Recommendation H.241 Annex A

   H.323-based video telephony systems that use H.264 as an optional
   video compression scheme are required to support H.241 Annex A [3]
   as a packetization scheme.  The packetization mechanism defined in
   this Annex is technically identical with a small subset of this
   specification.

   When a system operates according to H.241 Annex A, parameter set
   NAL units are sent in-band.  Only Single NAL unit packets are used.
   Many such systems are not sending IDR pictures regularly, but only
   when required by user interaction or by control protocol means;
   e.g., when switching between video channels in a Multipoint Control
   Unit or for error recovery requested by feedback.

12.2. Video Telephony, No Slice Data Partitioning, No NAL Unit
   Aggregation

   The RTP part of this scheme is implemented and tested (though not
   the control-protocol part; see below).

   In most real-world video telephony applications, picture parameters
   such as picture size or optional modes never change during the
   lifetime of a connection.  Therefore, all necessary parameter sets
   (usually only one) are sent as a side effect of the capability
   exchange/announcement process, e.g., according to the SDP syntax
   specified in section 8.2 of this document.  As all necessary
   parameter set information is established before the RTP session
   starts, there is no need for sending any parameter set NAL units.
   Slice data partitioning is not used, either.  Thus, the RTP packet
   stream basically consists of NAL units that carry single coded
   slices.

   The encoder chooses the size of coded slice NAL units so that they
   offer the best performance.  Often, this is done by adapting the
   coded slice size to the MTU size of the IP network.  For small
   picture sizes, this may result in a one-picture-per-one-packet
   strategy.  Intra refresh algorithms clean up the loss of packets
   and the resulting drift-related artifacts.

12.3. Video Telephony, Interleaved Packetization Using NAL Unit
   Aggregation

   This scheme allows better error concealment and is used in H.263
   based designs using RFC 4629 packetization [11].  It has been
   implemented, and good results were reported [13].



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   The VCL encoder codes the source picture so that all macroblocks
   (MBs) of one MB line are assigned to one slice.  All slices with
   even MB row addresses are combined into one STAP, and all slices
   with odd MB row addresses into another.  Those STAPs are
   transmitted as RTP packets.  The establishment of the parameter
   sets is performed as discussed above.

   Note that the use of STAPs is essential here, as the high number of
   individual slices (18 for a CIF picture) would lead to unacceptably
   high IP/UDP/RTP header overhead (unless the source coding tool FMO
   is used, which is not assumed in this scenario).  Furthermore, some
   wireless video transmission systems, such as H.324M and the IP-
   based video telephony specified in 3GPP, are likely to use
   relatively small transport packet size.  For example, a typical MTU
   size of H.223 AL3 SDU is around 100 bytes [17].  Coding individual
   slices according to this packetization scheme provides further
   advantage in communication between wired and wireless networks, as
   individual slices are likely to be smaller than the preferred
   maximum packet size of wireless systems.  Consequently, a gateway
   can convert the STAPs used in a wired network into several RTP
   packets with only one NAL unit, which are preferred in a wireless
   network, and vice versa.

12.4. Video Telephony with Data Partitioning

   This scheme has been implemented and has been shown to offer good
   performance, especially at higher packet loss rates [13].

   Data Partitioning is known to be useful only when some form of
   unequal error protection is available.  Normally, in single-session
   RTP environments, even error characteristics are assumed; i.e., the
   packet loss probability of all packets of the session is the same
   statistically.  However, there are means to reduce the packet loss
   probability of individual packets in an RTP session.  A FEC packet
   according to RFC 2733 [18], for example, specifies which media
   packets are associated with the FEC packet.

   In all cases, the incurred overhead is substantial but is in the
   same order of magnitude as the number of bits that have otherwise
   been spent for intra information.  However, this mechanism does not
   add any delay to the system.

   Again, the complete parameter set establishment is performed
   through control protocol means.





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12.5. Video Telephony or Streaming with FUs and Forward Error
   Correction

   This scheme has been implemented and has been shown to provide good
   performance, especially at higher packet loss rates [19].

   The most efficient means to combat packet losses for scenarios
   where retransmissions are not applicable is forward error
   correction (FEC).  Although application layer, end-to-end use of
   FEC is often less efficient than an FEC-based protection of
   individual links (especially when links of different
   characteristics are in the transmission path), application layer,
   end-to-end FEC is unavoidable in some scenarios.  RFC 5109 [18]
   provides means to use generic, application layer, end-to-end FEC in
   packet-loss environments.  A binary forward error correcting code
   is generated by applying the XOR operation to the bits at the same
   bit position in different packets.  The binary code can be
   specified by the parameters (n,k) in which k is the number of
   information packets used in the connection and n is the total
   number of packets generated for k information packets; i.e., n-k
   parity packets are generated for k information packets.

   When a code is used with parameters (n,k) within the RFC 5109
   framework, the following properties are well known:

   a) If applied over one RTP packet, RFC 5109 provides only packet
      repetition.

   b) RFC 5109 is most bit rate efficient if XOR-connected packets
      have equal length.

   c) At the same packet loss probability p and for a fixed k, the
      greater the value of n is, the smaller the residual error
      probability becomes.  For example, for a packet loss probability
      of 10%, k=1, and n=2, the residual error probability is about 1%,
      whereas for n=3, the residual error probability is about 0.1%.

   d) At the same packet loss probability p and for a fixed code rate
      k/n, the greater the value of n is, the smaller the residual
      error probability becomes.  For example, at a packet loss
      probability of p=10%, k=1 and n=2, the residual error rate is
      about 1%, whereas for an extended Golay code with k=12 and n=24,
      the residual error rate is about 0.01%.

   For applying RFC 5109 in combination with H.264 baseline coded
   video without using FUs, several options might be considered:



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   1) The video encoder produces NAL units for which each video frame
      is coded in a single slice.  Applying FEC, one could use a
      simple code; e.g., (n=2, k=1).  That is, each NAL unit would
      basically just be repeated.  The disadvantage is obviously the
      bad code performance according to d), above, and the low
      flexibility, as only (n, k=1) codes can be used.

   2) The video encoder produces NAL units for which each video frame
      is encoded in one or more consecutive slices.  Applying FEC, one
      could use a better code, e.g., (n=24, k=12), over a sequence of
      NAL units.  Depending on the number of RTP packets per frame, a
      loss may introduce a significant delay, which is reduced when
      more RTP packets are used per frame.  Packets of completely
      different length might also be connected, which decreases bit
      rate efficiency according to b), above.  However, with some care
      and for slices of 1kb or larger, similar length (100-200 bytes
      difference) may be produced, which will not lower the bit
      efficiency catastrophically.

   3) The video encoder produces NAL units, for which a certain frame
      contains k slices of possibly almost equal length.  Then,
      applying FEC, a better code, e.g., (n=24, k=12), can be used
      over the sequence of NAL units for each frame.  The delay
      compared to that of 2), above, may be reduced, but several
      disadvantages are obvious.  First, the coding efficiency of the
      encoded video is lowered significantly, as slice-structured
      coding reduces intra-frame prediction and additional slice
      overhead is necessary.  Second, pre-encoded content or, when
      operating over a gateway, the video is usually not appropriately
      coded with k slices such that FEC can be applied.  Finally, the
      encoding of video producing k slices of equal length is not
      straightforward and might require more than one encoding pass.

   Many of the mentioned disadvantages can be avoided by applying FUs
   in combination with FEC.  Each NAL unit can be split into any
   number of FUs of basically equal length; therefore, FEC with a
   reasonable k and n can be applied, even if the encoder made no
   effort to produce slices of equal length.  For example, a coded
   slice NAL unit containing an entire frame can be split to k FUs,
   and a parity check code (n=k+1, k) can be applied.  However, this
   has the disadvantage that unless all created fragments can be
   recovered, the whole slice will be lost.  Thus a larger section is
   lost than would be if the frame had been split into several slices.

   The presented technique makes it possible to achieve good
   transmission error tolerance, even if no additional source coding



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   layer redundancy (such as periodic intra frames) is present.
   Consequently, the same coded video sequence can be used to achieve
   the maximum compression efficiency and quality over error-free
   transmission and for transmission over error-prone networks.
   Furthermore, the technique allows the application of FEC to pre-
   encoded sequences without adding delay.  In this case, pre-encoded
   sequences that are not encoded for error-prone networks can still
   be transmitted almost reliably without adding extensive delays.  In
   addition, FUs of equal length result in a bit rate efficient use of
   RFC 5109.

   If the error probability depends on the length of the transmitted
   packet (e.g., in case of mobile transmission [15]), the benefits of
   applying FUs with FEC are even more obvious.  Basically, the
   flexibility of the size of FUs allows appropriate FEC to be applied
   for each NAL unit and unequal error protection of NAL units.

   When FUs and FEC are used, the incurred overhead is substantial but
   is in the same order of magnitude as the number of bits that have
   to be spent for intra-coded macroblocks if no FEC is applied.  In
   [19], it was shown that the overall performance of the FEC-based
   approach enhanced quality when using the same error rate and same
   overall bit rate, including the overhead.

12.6. Low Bit-Rate Streaming

   This scheme has been implemented with H.263 and non-standard RTP
   packetization and has given good results [20].  There is no
   technical reason why similarly good results could not be achievable
   with H.264.

   In today's Internet streaming, some of the offered bit rates are
   relatively low in order to allow terminals with dial-up modems to
   access the content.  In wired IP networks, relatively large packets,
   say 500 - 1500 bytes, are preferred to smaller and more frequently
   occurring packets in order to reduce network congestion.  Moreover,
   use of large packets decreases the amount of RTP/UDP/IP header
   overhead.  For low bit-rate video, the use of large packets means
   that sometimes up to few pictures should be encapsulated in one
   packet.

   However, loss of a packet including many coded pictures would have
   drastic consequences for visual quality, as there is practically no
   other way to conceal a loss of an entire picture than to repeat the
   previous one.  One way to construct relatively large packets and
   maintain possibilities for successful loss concealment is to



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   construct MTAPs that contain interleaved slices from several
   pictures.  An MTAP should not contain spatially adjacent slices
   from the same picture or spatially overlapping slices from any
   picture.  If a packet is lost, it is likely that a lost slice is
   surrounded by spatially adjacent slices of the same picture and
   spatially corresponding slices of the temporally previous and
   succeeding pictures.  Consequently, concealment of the lost slice
   is likely to be relatively successful.

12.7. Robust Packet Scheduling in Video Streaming

   Robust packet scheduling has been implemented with MPEG-4 Part 2
   and simulated in a wireless streaming environment [21].  There is
   no technical reason why similar or better results could not be
   achievable with H.264.

   Streaming clients typically have a receiver buffer that is capable
   of storing a relatively large amount of data.  Initially, when a
   streaming session is established, a client does not start playing
   the stream back immediately.  Rather, it typically buffers the
   incoming data for a few seconds.  This buffering helps maintain
   continuous playback, as, in case of occasional increased
   transmission delays or network throughput drops, the client can
   decode and play buffered data.  Otherwise, without initial
   buffering, the client has to freeze the display, stop decoding, and
   wait for incoming data.  The buffering is also necessary for either
   automatic or selective retransmission in any protocol level.  If
   any part of a picture is lost, a retransmission mechanism may be
   used to resend the lost data.  If the retransmitted data is
   received before its scheduled decoding or playback time, the loss
   is recovered perfectly.  Coded pictures can be ranked according to
   their importance in the subjective quality of the decoded sequence.
   For example, non-reference pictures, such as conventional B
   pictures, are subjectively least important, as their absence does
   not affect decoding of any other pictures.  In addition to non-
   reference pictures, the ITU-T H.264 | ISO/IEC 14496-10 standard
   includes a temporal scalability method called sub-sequences [22].
   Subjective ranking can also be made on coded slice data partition
   or slice group basis.  Coded slices and coded slice data partitions
   that are subjectively the most important can be sent earlier than
   their decoding order indicates, whereas coded slices and coded
   slice data partitions that are subjectively the least important can
   be sent later than their natural coding order indicates.
   Consequently, any retransmitted parts of the most important slices
   and coded slice data partitions are more likely to be received




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   before their scheduled decoding or playback time compared to the
   least important slices and slice data partitions.

13. Informative Appendix: Rationale for Decoding Order Number

13.1. Introduction

   The Decoding Order Number (DON) concept was introduced mainly to
   enable efficient multi-picture slice interleaving (see section 12.6)
   and robust packet scheduling (see section 12.7).  In both of these
   applications, NAL units are transmitted out of decoding order.  DON
   indicates the decoding order of NAL units and should be used in the
   receiver to recover the decoding order.  Example use cases for
   efficient multi-picture slice interleaving and for robust packet
   scheduling are given in sections 13.2 and 13.3, respectively.
   Section 13.4 describes the benefits of the DON concept in error
   resiliency achieved by redundant coded pictures.  Section 13.5
   summarizes considered alternatives to DON and justifies why DON was
   chosen to this RTP payload specification.

13.2. Example of Multi-Picture Slice Interleaving

   An example of multi-picture slice interleaving follows.  A subset
   of a coded video sequence is depicted below in output order.  R
   denotes a reference picture, N denotes a non-reference picture, and
   the number indicates a relative output time.

      ... R1 N2 R3 N4 R5 ...

   The decoding order of these pictures from left to right is as
   follows:

      ... R1 R3 N2 R5 N4 ...

   The NAL units of pictures R1, R3, N2, R5, and N4 are marked with a
   DON equal to 1, 2, 3, 4, and 5, respectively.

   Each reference picture consists of three slice groups that are
   scattered as follows (a number denotes the slice group number for
   each macroblock in a QCIF frame):









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      0 1 2 0 1 2 0 1 2 0 1
      2 0 1 2 0 1 2 0 1 2 0
      1 2 0 1 2 0 1 2 0 1 2
      0 1 2 0 1 2 0 1 2 0 1
      2 0 1 2 0 1 2 0 1 2 0
      1 2 0 1 2 0 1 2 0 1 2
      0 1 2 0 1 2 0 1 2 0 1
      2 0 1 2 0 1 2 0 1 2 0
      1 2 0 1 2 0 1 2 0 1 2

   For the sake of simplicity, we assume that all the macroblocks of a
   slice group are included in one slice.  Three MTAPs are constructed
   from three consecutive reference pictures so that each MTAP
   contains three aggregation units, each of which contains all the
   macroblocks from one slice group.  The first MTAP contains slice
   group 0 of picture R1, slice group 1 of picture R3, and slice group
   2 of picture R5.  The second MTAP contains slice group 1 of picture
   R1, slice group 2 of picture R3, and slice group 0 of picture R5.
   The third MTAP contains slice group 2 of picture R1, slice group 0
   of picture R3, and slice group 1 of picture R5.  Each non-reference
   picture is encapsulated into an STAP-B.

   Consequently, the transmission order of NAL units is the following:

      R1, slice group 0, DON 1, carried in MTAP,RTP SN: N
      R3, slice group 1, DON 2, carried in MTAP,RTP SN: N
      R5, slice group 2, DON 4, carried in MTAP,RTP SN: N
      R1, slice group 1, DON 1, carried in MTAP,RTP SN: N+1
      R3, slice group 2, DON 2, carried in MTAP,RTP SN: N+1
      R5, slice group 0, DON 4, carried in MTAP,RTP SN: N+1
      R1, slice group 2, DON 1, carried in MTAP,RTP SN: N+2
      R3, slice group 1, DON 2, carried in MTAP,RTP SN: N+2
      R5, slice group 0, DON 4, carried in MTAP,RTP SN: N+2
      N2, DON 3, carried in STAP-B, RTP SN: N+3
      N4, DON 5, carried in STAP-B, RTP SN: N+4

   The receiver is able to organize the NAL units back in decoding
   order based on the value of DON associated with each NAL unit.

   If one of the MTAPs is lost, the spatially adjacent and temporally
   co-located macroblocks are received and can be used to conceal the
   loss efficiently.  If one of the STAPs is lost, the effect of the
   loss does not propagate temporally.




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13.3. Example of Robust Packet Scheduling

   An example of robust packet scheduling follows.  The communication
   system used in the example consists of the following components in
   the order that the video is processed from source to sink:

      o camera and capturing
      o pre-encoding buffer
      o encoder
      o encoded picture buffer
      o transmitter
      o transmission channel
      o receiver
      o receiver buffer
      o decoder
      o decoded picture buffer
      o display

   The video communication system used in the example operates as
   follows.  Note that processing of the video stream happens
   gradually and at the same time in all components of the system.
   The source video sequence is shot and captured to a pre-encoding
   buffer.  The pre-encoding buffer can be used to order pictures from
   sampling order to encoding order or to analyze multiple
   uncompressed frames for bit rate control purposes, for example.  In
   some cases, the pre-encoding buffer may not exist; instead, the
   sampled pictures are encoded right away.  The encoder encodes
   pictures from the pre-encoding buffer and stores the output; i.e.,
   coded pictures, to the encoded picture buffer.  The transmitter
   encapsulates the coded pictures from the encoded picture buffer to
   transmission packets and sends them to a receiver through a
   transmission channel.  The receiver stores the received packets to
   the receiver buffer.  The receiver buffering process typically
   includes buffering for transmission delay jitter.  The receiver
   buffer can also be used to recover correct decoding order of coded
   data.  The decoder reads coded data from the receiver buffer and
   produces decoded pictures as output into the decoded picture buffer.
   The decoded picture buffer is used to recover the output (or
   display) order of pictures.  Finally, pictures are displayed.

   In the following example figures, I denotes an IDR picture, R
   denotes a reference picture, N denotes a non-reference picture, and
   the number after I, R, or N indicates the sampling time relative to
   the previous IDR picture in decoding order.  Values below the
   sequence of pictures indicate scaled system clock timestamps.  The
   system clock is initialized arbitrarily in this example, and time



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   runs from left to right.  Each I, R, and N picture is mapped into
   the same timeline compared to the previous processing step, if any,
   assuming that encoding, transmission, and decoding take no time.
   Thus, events happening at the same time are located in the same
   column throughout all example figures.

   A subset of a sequence of coded pictures is depicted below in
   sampling order.

       ...  N58 N59 I00 N01 N02 R03 N04 N05 R06 ... N58 N59 I00 N01 ...
       ... --|---|---|---|---|---|---|---|---|- ... -|---|---|---|- ...
       ...  58  59  60  61  62  63  64  65  66  ... 128 129 130 131 ...

             Figure 16  Sequence of pictures in sampling order

   The sampled pictures are buffered in the pre-encoding buffer to
   arrange them in encoding order.  In this example, we assume that
   the non-reference pictures are predicted from both the previous and
   the next reference picture in output order, except for the non-
   reference pictures immediately preceding an IDR picture, which are
   predicted only from the previous reference picture in output order.
   Thus, the pre-encoding buffer has to contain at least two pictures,
   and the buffering causes a delay of two picture intervals.  The
   output of the pre-encoding buffering process and the encoding (and
   decoding) order of the pictures are as follows:

       ... N58 N59 I00 R03 N01 N02 R06 N04 N05 ...
       ... -|---|---|---|---|---|---|---|---|- ...
       ... 60  61  62  63  64  65  66  67  68  ...

         Figure 17  Re-ordered pictures in the pre-encoding buffer

   The encoder or the transmitter can set the value of DON for each
   picture to a value of DON for the previous picture in decoding
   order plus one.

   For the sake of simplicity, let us assume that:

   o  the frame rate of the sequence is constant,
   o  each picture consists of only one slice,
   o  each slice is encapsulated in a single NAL unit packet,
   o  there is no transmission delay, and
   o  pictures are transmitted at constant intervals (that is, 1 /
   (frame rate)).





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   When pictures are transmitted in decoding order, they are received
   as follows:

       ... N58 N59 I00 R03 N01 N02 R06 N04 N05 ...
       ... -|---|---|---|---|---|---|---|---|- ...
       ... 60  61  62  63  64  65  66  67  68  ...

              Figure 18  Received pictures in decoding order

   The OPTIONAL sprop-interleaving-depth media type parameter is set
   to 0, as the transmission (or reception) order is identical to the
   decoding order.

   The decoder has to buffer for one picture interval initially in its
   decoded picture buffer to organize pictures from decoding order to
   output order as depicted below:

        ... N58 N59 I00 N01 N02 R03 N04 N05 R06 ...
        ... -|---|---|---|---|---|---|---|---|- ...
        ... 61  62  63  64  65  66  67  68  69  ...

                          Figure 19  Output order

   The amount of required initial buffering in the decoded picture
   buffer can be signaled in the buffering period SEI message or with
   the num_reorder_frames syntax element of H.264 video usability
   information.  num_reorder_frames indicates the maximum number of
   frames, complementary field pairs, or non-paired fields that
   precede any frame, complementary field pair, or non-paired field in
   the sequence in decoding order and that follow it in output order.
   For the sake of simplicity, we assume that num_reorder_frames is
   used to indicate the initial buffer in the decoded picture buffer.
   In this example, num_reorder_frames is equal to 1.

   It can be observed that if the IDR picture I00 is lost during
   transmission and a retransmission request is issued when the value
   of the system clock is 62, there is one picture interval of time
   (until the system clock reaches timestamp 63) to receive the
   retransmitted IDR picture I00.

   Let us then assume that IDR pictures are transmitted two frame
   intervals earlier than their decoding position; i.e., the pictures
   are transmitted as follows:






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        ...  I00 N58 N59 R03 N01 N02 R06 N04 N05 ...
        ... --|---|---|---|---|---|---|---|---|- ...
        ...  62  63  64  65  66  67  68  69  70  ...

       Figure 20  Interleaving: Early IDR pictures in sending order

   The OPTIONAL sprop-interleaving-depth media type parameter is set
   equal to 1 according to its definition.  (The value of sprop-
   interleaving-depth in this example can be derived as follows:
   Picture I00 is the only picture preceding picture N58 or N59 in
   transmission order and following it in decoding order.  Except for
   pictures I00, N58, and N59, the transmission order is the same as
   the decoding order of pictures.  As a coded picture is encapsulated
   into exactly one NAL unit, the value of sprop-interleaving-depth is
   equal to the maximum number of pictures preceding any picture in
   transmission order and following the picture in decoding order.)

   The receiver buffering process contains two pictures at a time
   according to the value of the sprop-interleaving-depth parameter
   and orders pictures from the reception order to the correct
   decoding order based on the value of DON associated with each
   picture.  The output of the receiver buffering process is as
   follows:

       ... N58 N59 I00 R03 N01 N02 R06 N04 N05 ...
       ... -|---|---|---|---|---|---|---|---|- ...
       ... 63  64  65  66  67  68  69  70  71  ...

                 Figure 21  Interleaving: Receiver buffer

   Again, an initial buffering delay of one picture interval is needed
   to organize pictures from decoding order to output order, as
   depicted below:

        ... N58 N59 I00 N01 N02 R03 N04 N05 ...
        ... -|---|---|---|---|---|---|---|- ...
        ... 64  65  66  67  68  69  70  71  ...

         Figure 22  Interleaving: Receiver buffer after reordering

   Note that the maximum delay that IDR pictures can undergo during
   transmission, including possible application, transport, or link
   layer retransmission, is equal to three picture intervals.  Thus,
   the loss resiliency of IDR pictures is improved in systems
   supporting retransmission compared to the case in which pictures
   were transmitted in their decoding order.



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13.4. Robust Transmission Scheduling of Redundant Coded Slices

   A redundant coded picture is a coded representation of a picture or
   a part of a picture that is not used in the decoding process if the
   corresponding primary coded picture is correctly decoded.  There
   should be no noticeable difference between any area of the decoded
   primary picture and a corresponding area that would result from
   application of the H.264 decoding process for any redundant picture
   in the same access unit.  A redundant coded slice is a coded slice
   that is a part of a redundant coded picture.

   Redundant coded pictures can be used to provide unequal error
   protection in error-prone video transmission.  If a primary coded
   representation of a picture is decoded incorrectly, a corresponding
   redundant coded picture can be decoded.  Examples of applications
   and coding techniques using the redundant codec picture feature
   include the video redundancy coding [23] and the protection of "key
   pictures" in multicast streaming [24].

   One property of many error-prone video communications systems is
   that transmission errors are often bursty.  Therefore, they may
   affect more than one consecutive transmission packets in
   transmission order.  In low bit-rate video communication, it is
   relatively common that an entire coded picture can be encapsulated
   into one transmission packet.  Consequently, a primary coded
   picture and the corresponding redundant coded pictures may be
   transmitted in consecutive packets in transmission order.  To make
   the transmission scheme more tolerant of bursty transmission errors,
   it is beneficial to transmit the primary coded picture and
   redundant coded picture separated by more than a single packet.
   The DON concept enables this.

13.5. Remarks on Other Design Possibilities

   The slice header syntax structure of the H.264 coding standard
   contains the frame_num syntax element that can indicate the
   decoding order of coded frames.  However, the usage of the
   frame_num syntax element is not feasible or desirable to recover
   the decoding order, due to the following reasons:

   o  The receiver is required to parse at least one slice header per
      coded picture (before passing the coded data to the decoder).

   o  Coded slices from multiple coded video sequences cannot be
      interleaved, as the frame number syntax element is reset to 0 in
      each IDR picture.



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   o  The coded fields of a complementary field pair share the same
      value of the frame_num syntax element.  Thus, the decoding order
      of the coded fields of a complementary field pair cannot be
      recovered based on the frame_num syntax element or any other
      syntax element of the H.264 coding syntax.

   The RTP payload format for transport of MPEG-4 elementary streams
   [25] enables interleaving of access units and transmission of
   multiple access units in the same RTP packet.  An access unit is
   specified in the H.264 coding standard to comprise all NAL units
   associated with a primary coded picture according to subclause
   7.4.1.2 of [1].  Consequently, slices of different pictures cannot
   be interleaved, and the multi-picture slice interleaving technique
   (see section 12.6) for improved error resilience cannot be used.

14. Backward Compatibility to RFC 3984

   The current document is a revision of RFC 3984 and obsoletes it.
   The technical changes relative to RFC 3984 are listed in section 15.
   This section addresses the backward compatibility issues.

   It should be noted that for the majority of cases, there will be no
   compatibility issues for legacy implementations per RFC 3984 and
   new implementations per this document to interwork.  Compatibility
   issues may only occur when both of the following conditions are
   true: 1) legacy implementations and new implementations are
   interworking; and 2) parameter sets are transported out of band.
   Even when such compatibility issues occur, it is easy to debug and
   find out the reason according to the following analyses.

   Items 1), 2), 3), 7), 9), 10), 12) and 13) are bug-fix type of
   changes, and do not incur any backward compatibility issues.

   Item 4), addition of six new media type parameters, does not incur
   any backward compatibility issues for SDP Offer/Answer based
   applications, as legacy RFC 3984 receivers ignore these parameters,
   and it is fine for legacy RFC 3984 senders not to use these
   parameters as they are optional.  However, there is a backward
   compatibility issue for SDP declarative usage based applications
   (only for the parameter sprop-level-parameter-sets as the other
   five parameters are not usable in declarative usage), e.g. those
   using RTSP and SAP, because the SDP receiver per RFC 3984 cannot
   accept a session for which the SDP includes an unrecognized
   parameter.  Therefore, the RTSP or SAP server may have to prepare
   two sets of streams, one for legacy RFC 3984 receivers and one for
   receivers according to this memo.



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   Items 5), 6), and 11) are related to out-of-band transport of
   parameter sets.  There are following backward compatibility issues.

   1) When a legacy sender per RFC 3984 includes parameter sets for a
      level different than the default level indicated by profile-
      level-id to sprop-parameter-sets, the parameter value of sprop-
      parameter-sets is invalid to the receiver per this memo and
      therefore the session may be rejected.

   2) In SDP Offer/Answer between a legacy offerer per RFC 3984 and an
      answerer per this memo, when the answerer includes in the answer
      parameter sets that are not a superset of the parameter sets
      included in the offer, the parameter value of sprop-parameter-
      sets is invalid to the offerer and the session may not be
      initiated properly (related to change item 11).

   3) When one endpoint A per this memo includes in-band-parameter-
      sets equal to 1, the other side B per RFC 3984 does not
      understand that it must transmit parameter sets in-band and B
      may still exclude parameter sets in the in-band stream it is
      sending. Consequently endpoint A cannot decode the stream it
      receives.

   Item7), allowance of conveying sprop-parameter-sets and sprop-
   level-parameter-sets using the "fmtp" source attribute as specified
   in section 6.3 of [9], is similar as item 4).  It does not incur
   any backward compatibility issues for SDP Offer/Answer based
   applications, as legacy RFC 3984 receivers ignore the "fmtp" source
   attribute, and it is fine for legacy RFC 3984 senders not to use
   the "fmtp" source attribute as it is optional.  However, there is a
   backward compatibility issue for SDP declarative usage based
   applications, e.g. those using RTSP and SAP, because the SDP
   receiver per RFC 3984 cannot accept a session for which the SDP
   includes an unrecognized parameter (i.e., the "fmtp" source
   attribute).  Therefore, the RTSP or SAP server may have to prepare
   two sets of streams, one for legacy RFC 3984 receivers and one for
   receivers according to this memo.

   Item 14) removed that use of out-of-band transport of parameter
   sets is recommended.  As out-of-band transport of parameter sets is
   still allowed, this change does not incur any backward
   compatibility issues.

   Item 15) does not incur any backward compatibility issues as the
   added subsection 8.5 is informative.




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   Item 16) does not create any backward compatibility issues as the
   handling of default level is the same if either end is RFC 3984
   compliant, and furthermore, RFC 3984 compliant ends would simply
   ignore the new media type parameters, if present.

15. Changes from RFC 3984

   Following is the list of technical changes (including bug fixes)
   from RFC 3984.  Besides this list of technical changes, numerous
   editorial changes have been made, but not documented in this
   section.  Note that section 8.2.2 is where much of the important
   changes in this memo occurs and deserves particular attention.

   1) In subsections 5.4, 5.5, 6.2, 6,3 and 6.4, removed that the
      packetization mode in use may be signaled by external means.

   2) In subsection 7.2.2, changed the sentence

      There are N VCL NAL units in the deinterleaving buffer.

      to

      There are N or more VCL NAL units in the de-interleaving buffer.

   3) In subsection 8.1, the semantics of sprop-init-buf-time,
      paragraph 2, changed the sentence

      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.

      to

      The parameter is the maximum value of (decoding time of the NAL
      unit - transmission time of a NAL unit), assuming reliable and
      instantaneous transmission, the same timeline for transmission
      and decoding, and that decoding starts when the first packet
      arrives.

   4) Added media type parameters max-smbps, sprop-level-parameter-
      sets, use-level-src-parameter-sets, in-band-parameter-sets, sar-
      understood and sar-supported.





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   5) In subsection 8.1, removed the specification of parameter-add.
      Other descriptions of parameter-add (in subsections 8.2 and 8.4)
      are also removed.

   6) In subsection 8.1, added a constraint to sprop-parameter-sets
      such that it can only contain parameter sets for the same
      profile and level as indicated by profile-level-id.

   7) In subsection 8.2.1, added that sprop-parameter-sets and sprop-
      level-parameter-sets may be either included in the "a=fmtp" line
      of SDP or conveyed using the "fmtp" source attribute as
      specified in section 6.3 of [9].

   8) In subsection 8.2.2, removed sprop-deint-buf-req from being part
      of the media format configuration in usage with the SDP
      Offer/Answer model.

   9) In subsection 8.2.2, made it clear that level is downgradable in
      the SDP Offer/Answer model, i.e. the use of the level part of
      "profile-level-id" does not need to be symmetric (the level
      included in the answer can be lower than or equal to the level
      included in the offer).

   10)In subsection 8.2.2, removed that the capability parameters may
      be used to declare encoding capabilities.

   11)In subsection 8.2.2, added rules on how to use sprop-parameter-
      sets and sprop-level-parameter-sets for out-of-band transport of
      parameter sets, with or without level downgrading.

   12)In subsection 8.2.2, clarified the rules of using the media type
      parameters with SDP Offer/Answer for multicast.

   13)In subsection 8.2.2, completed and corrected the list of how
      different media type parameters shall be interpreted in the
      different combinations of offer or answer and direction
      attribute.

   14)In subsection 8.4, changed the text such that both out-of-band
      and in-band transport of parameter sets are allowed and neither
      is recommended or required.

   15)Added subsection 8.5 (informative) providing example methods for
      decoder refresh to handle parameter set losses.





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   16)Added media type parameters max-recv-level, and level-asymmetry-
      allowed, and adjusted associated text and examples for level
      upgrade and asymmetry.

16. Acknowledgements

   Stephan Wenger, Miska Hannuksela, Thomas Stockhammer, Magnus
   Westerlund, and David Singer are thanked as the authors of RFC 3984.
   Dave Lindbergh, Philippe Gentric, Gonzalo Camarillo, Gary Sullivan,
   Joerg Ott, and Colin Perkins are thanked for careful review during
   the development of RFC 3984. Stephen Botzko, Magnus Westerlund,
   Alex Eleftheriadis, Thomas Schierl, Tom Taylor, Ali Begen, Aaron
   Wells, Stuart Taylor, Robert Sparks, Dan Romascanu, and Niclas
   Comstedt are thanked for their valuable comments and inputs during
   the development of this memo.

   This document was prepared using 2-Word-v2.0.template.dot.

17. References

17.1. Normative References

   [1]   ITU-T Recommendation H.264, "Advanced video coding for
         generic audiovisual services", November 2007.

   [2]   ISO/IEC International Standard 14496-10:2008.

   [3]   ITU-T Recommendation H.241, "Extended video procedures and
         control signals for H.300 series terminals", May 2006.

   [4]   Bradner, S., "Key words for use in RFCs to Indicate
         Requirement Levels", BCP 14, RFC 2119, March 1997.

   [5]   Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
         "RTP: A Transport Protocol for Real-Time Applications", STD
         64, RFC 3550, July 2003.

   [6]   Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
         Description Protocol", RFC 4566, July 2006.

   [7]   Josefsson, S., "The Base16, Base32, and Base64 Data
         Encodings", RFC 4648, October 2006.

   [8]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
         Session Description Protocol (SDP)", RFC 3264, June 2002.




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   [9]   Lennox, J., Ott, J., and Schierl, T., "Source-Specific Media
         Attributes in the Session Description Protocol (SDP)", RFC
         5576, June 2009.

17.2. Informative References

   [10]  Luthra, A., Sullivan, G.J., and T. Wiegand (eds.), Special
         Issue on H.264/AVC. IEEE Transactions on Circuits and Systems
         on Video Technology, July 2003.

   [11]  Ott, J., Bormann, C., Sullivan, G., Wenger, S., and R. Even
         (Ed.), "RTP Payload Format for ITU-T Rec. H.263 Video", RFC
         4629, January 2007.

   [12]  ISO/IEC IS 14496-2.

   [13]  Wenger, S., "H.26L over IP", IEEE Transaction on Circuits and
         Systems for Video technology, Vol. 13, No. 7, July 2003.

   [14]  Wenger, S., "H.26L over IP: The IP Network Adaptation Layer",
         Proceedings Packet Video Workshop 02, April 2002.

   [15]  Stockhammer, T., Hannuksela, M.M., and S. Wenger, "H.26L/JVT
         Coding Network Abstraction Layer and IP-based Transport" in
         Proc. ICIP 2002, Rochester, NY, September 2002.

   [16]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
         Video Conferences with Minimal Control", STD 65, RFC 3551,
         July 2003.

   [17]  ITU-T Recommendation H.223, "Multiplexing protocol for low
         bit rate multimedia communication", July 2001.

   [18]  Li, A., "RTP Payload Format for Generic Forward Error
         Correction", RFC 5109, December 2007.

   [19]  Stockhammer, T., Wiegand, T., Oelbaum, T., and F. Obermeier,
         "Video Coding and Transport Layer Techniques for H.264/AVC-
         Based Transmission over Packet-Lossy Networks", IEEE
         International Conference on Image Processing (ICIP 2003),
         Barcelona, Spain, September 2003.

   [20]  Varsa, V. and M. Karczewicz, "Slice interleaving in
         compressed video packetization", Packet Video Workshop 2000.





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   [21]  Kang, S.H. and A. Zakhor, "Packet scheduling algorithm for
         wireless video streaming," International Packet Video
         Workshop 2002.

   [22]  Hannuksela, M.M., "Enhanced concept of GOP", JVT-B042,
         available http://ftp3.itu.int/av-arch/video-
         site/0201_Gen/JVT-B042.doc, anuary 2002.

   [23]  Wenger, S., "Video Redundancy Coding in H.263+", 1997
         International Workshop on Audio-Visual Services over Packet
         Networks, September 1997.

   [24]  Wang, Y.-K., Hannuksela, M.M., and M. Gabbouj, "Error
         Resilient Video Coding Using Unequally Protected Key
         Pictures", in Proc. International Workshop VLBV03, September
         2003.

   [25]  van der Meer, J., Mackie, D., Swaminathan, V., Singer, D.,
         and P. Gentric, "RTP Payload Format for Transport of MPEG-4
         Elementary Streams", RFC 3640, November 2003.

   [26]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
         Norrman, "The Secure Real-time Transport Protocol (SRTP)",
         RFC 3711, March 2004.

   [27]  Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
         Streaming Protocol (RTSP)", RFC 2326, April 1998.

   [28]  Handley, M., Perkins, C., and E. Whelan, "Session
         Announcement Protocol", RFC 2974, October 2000.

   [29]  Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
         January 2008.

   [30]  Wenger, S., Chandra, U., and M. Westerlund, "Codec Control
         Messages in the RTP Audio-Visual Profile with Feedback
         (AVPF)", RFC 5104, February 2008.












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18. Authors' Addresses

   Ye-Kui Wang
   Huawei Technologies
   400 Somerset Corp Blvd, Suite 602
   Bridgewater, NJ 08807
   USA

   Phone: +1-908-541-3518
   EMail: yekuiwang@huawei.com


   Roni Even
   14 David Hamelech
   Tel Aviv 64953
   Israel

   Phone: +972-545481099
   Email: ron.even.tlv@gmail.com


   Tom Kristensen
   TANDBERG
   Philip Pedersens vei 22
   N-1366 Lysaker
   Norway

   Phone: +47 67125125
   Email: tom.kristensen@tandberg.com, tomkri@ifi.uio.no


   Randell Jesup
   WorldGate Communications
   3190 Tremont Ave
   Trevose, PA 19053
   USA

   Phone: +1-215-354-5166
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