





Network Working Group                                          S. Wenger
Request for Comments: 3984                               M.M. Hannuksela
Category: Standards Track                                 T. Stockhammer
                                                           M. Westerlund
                                                               D. Singer
                                                           February 2005


                   RTP Payload Format for H.264 Video

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

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.  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.

Table of Contents

   1.  Introduction..................................................  3
       1.1.  The H.264 Codec.........................................  3
       1.2.  Parameter Set Concept...................................  4
       1.3.  Network Abstraction Layer Unit Types....................  5
   2.  Conventions...................................................  6
   3.  Scope.........................................................  6
   4.  Definitions and Abbreviations.................................  6
       4.1.  Definitions.............................................  6
   5.  RTP Payload Format............................................  8
       5.1.  RTP Header Usage........................................  8
       5.2.  Common Structure of the RTP Payload Format.............. 11
       5.3.  NAL Unit Octet Usage.................................... 12



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       5.4.  Packetization Modes..................................... 14
       5.5.  Decoding Order Number (DON)............................. 15
       5.6.  Single NAL Unit Packet.................................. 18
       5.7.  Aggregation Packets..................................... 18
       5.8.  Fragmentation Units (FUs)............................... 27
   6.  Packetization Rules........................................... 31
       6.1.  Common Packetization Rules.............................. 31
       6.2.  Single NAL Unit Mode.................................... 32
       6.3.  Non-Interleaved Mode.................................... 32
       6.4.  Interleaved Mode........................................ 33
   7.  De-Packetization Process (Informative)........................ 33
       7.1.  Single NAL Unit and Non-Interleaved Mode................ 33
       7.2.  Interleaved Mode........................................ 34
       7.3.  Additional De-Packetization Guidelines.................. 36
   8.  Payload Format Parameters..................................... 37
       8.1.  MIME Registration....................................... 37
       8.2.  SDP Parameters.......................................... 52
       8.3.  Examples................................................ 58
       8.4.  Parameter Set Considerations............................ 60
   9.  Security Considerations....................................... 62
   10. Congestion Control............................................ 63
   11. IANA Considerations........................................... 64
   12. Informative Appendix: Application Examples.................... 65
       12.1. Video Telephony according to ITU-T Recommendation H.241
             Annex A................................................. 65
       12.2. Video Telephony, No Slice Data Partitioning, No NAL
             Unit Aggregation........................................ 65
       12.3. Video Telephony, Interleaved Packetization Using NAL
             Unit Aggregation........................................ 66
       12.4. Video Telephony with Data Partitioning.................. 66
       12.5. Video Telephony or Streaming with FUs and Forward
             Error Correction........................................ 67
       12.6. Low Bit-Rate Streaming.................................. 69
       12.7. Robust Packet Scheduling in Video Streaming............. 70
   13. Informative Appendix: Rationale for Decoding Order Number..... 71
       13.1. Introduction............................................ 71
       13.2. Example of Multi-Picture Slice Interleaving............. 71
       13.3. Example of Robust Packet Scheduling..................... 73
       13.4. Robust Transmission Scheduling of Redundant Coded
             Slices.................................................. 77
       13.5. Remarks on Other Design Possibilities................... 77
   14. Acknowledgements.............................................. 78
   15. References.................................................... 78
       15.1. Normative References.................................... 78
       15.2. Informative References.................................. 79
   Authors' Addresses................................................ 81
   Full Copyright Statement.......................................... 83




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1.  Introduction

1.1.  The H.264 Codec

   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).  Recommendation H.264 was approved by ITU-T on
   May 2003, and the approved draft specification is available for
   public review [8].  In this memo the H.264 acronym is used for the
   codec and the standard, but the memo is equally applicable to the
   ISO/IEC counterpart of the coding standard.

   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 1.5 Mbit/s,
   compared to the current operation point of MPEG 2 video at around 3.5
   Mbit/s [9].

   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 [9].

   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.




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   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 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 2429 [10] or MPEG-4's Header Extension Code (HEC) [11]
   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.




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   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 [12], [13],
   and [14].

   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.

   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].





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   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 BCP 14, RFC 2119 [3].

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

3.  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.




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      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 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].

      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 those packets



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         whose dropping has the smallest negative impact on the user
         experience and remove them in order to remove the congestion
         and/or keep the delay low.

   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
      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

5.  RTP Payload Format

5.1.  RTP Header Usage

   The format of the RTP header is specified in RFC 3550 [4] 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.









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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |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
         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.



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      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.

      RTP senders SHOULD NOT transmit picture timing SEI messages for
      pictures that are not supposed to be displayed as multiple fields.

      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).

         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.

         Informative note: Because H.264 allows the decoding order to be
         different from the display order, values of RTP timestamps may
         not be monotonically non-decreasing as a function of RTP
         sequence numbers.  Furthermore, the value for interarrival
         jitter reported in the RTCP reports may not be a trustworthy
         indication of the network performance, as the calculation rules




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         for interarrival jitter (section 6.4.1 of RFC 3550) assume that
         the RTP timestamp of a packet is directly proportional to its
         transmission time.

5.2.  Common Structure of the RTP Payload Format

   The payload format defines three different basic payload structures.
   A receiver can identify the payload structure by the first byte of
   the RTP 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.

   Table 1.  Summary of NAL unit types and their payload structures

      Type   Packet    Type name                        Section
      ---------------------------------------------------------
      0      undefined                                    -
      1-23   NAL unit  Single NAL unit packet per H.264   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  undefined                                    -






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      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.

5.3.  NAL Unit Octet Usage

   The structure and semantics of the NAL unit octet were introduced in
   section 1.3.  For convenience, the format of the NAL unit type octet
   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 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 greater than 00 indicate the
      relative transport priority, as determined by the encoder.  MANEs



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      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 [13].  Other mappings MAY also be
      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




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         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
      o Interleaved mode

   The single NAL unit mode is targeted for conversational systems that
   comply with ITU-T Recommendation H.241 [15] (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 MIME parameter or by external means.  The
   used packetization mode governs which NAL unit types are allowed in
   RTP payloads.  Table 3 summarizes the allowed NAL unit types for each
   packetization mode.  Some NAL unit type values (indicated as
   undefined in Table 3) are reserved for future extensions.  NAL units
   of those types SHOULD NOT be sent by a sender and MUST be ignored by
   a receiver.  For example, the 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".
   Packetization modes are explained in more detail in section 6.






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

      Type   Packet    Single NAL    Non-Interleaved    Interleaved
                       Unit Mode           Mode             Mode
      -------------------------------------------------------------

      0      undefined     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  undefined     ig               ig               ig

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 MIME parameter as follows.  When
   the value of the OPTIONAL sprop-interleaving-depth MIME parameter is
   equal to 0 (explicitly or per default) or transmission of NAL units
   out of their decoding order is disallowed by external means, the
   transmission order of NAL units MUST conform to the NAL unit decoding
   order.  When the value of the OPTIONAL sprop-interleaving-depth MIME
   parameter is greater than 0 or transmission of NAL units out of their
   decoding order is allowed by external means,

   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 decapsulating 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.




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      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 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,



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   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.  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 decapsulation 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.





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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 decapsulating
   single NAL unit packets in RTP sequence number order 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.



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

   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.

   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.




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   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 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



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   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
   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























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   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.

       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 and two
                 single-time aggregation units



























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   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 and two
                 single-time aggregation units

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.














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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      :  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
   following NAL 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.

   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).











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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      :        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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      :        NALU 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 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 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.








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   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 and two multi-time aggregation
                  units






















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   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.

       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 and two multi-time aggregation
                  units

5.8.  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 picture
      and applying generic forward error correction as described in
      section 12.5.




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















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   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.

       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   |
      +---------------+




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   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 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.  A 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, where sometimes



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      several macroblocks occupy zero bits, this is undesirable and can
      add delay.  However, the (potential) use of zero-length NALUs
      should be carefully weighed against the increased risk of the loss
      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 permitted by the
      applicable profile defined in [1]; however, for delay-critical
      systems, they SHOULD be sent in their original coding order to
      minimize the delay.  Note that the coding order is not necessarily
      the scan order, but the order the NAL packets become available to
      the RTP stack.

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

   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



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      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
   MIME parameter is equal to 0, the packetization-mode is not present,
   or no other packetization mode is signaled by external means.  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 [15] (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
   MIME parameter is equal to 1 or the mode is turned on by external
   means.  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 transmission order of NAL units MUST comply with the
   NAL unit decoding order.











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6.4.  Interleaved Mode

   This mode is in use when the value of the OPTIONAL packetization-mode
   MIME parameter is equal to 2 or the mode is turned on by external
   means.  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 (Informative)

   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.  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 decapsulation 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 sequences 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 decapsulated
   in RTP sequence number order.  If a decapsulated packet is a single
   NAL unit packet, the NAL unit contained in the packet is passed
   directly to the decoder.  If a decapsulated 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.  If a
   decapsulated packet is an FU-A, all the fragments of the fragmented
   NAL unit are concatenated and passed to the decoder.

      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.








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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 packets 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 deinterleaving buffer in this section.  Receivers SHOULD also
   prepare for transmission delay jitter; i.e., either reserve separate
   buffers for transmission delay jitter buffering and deinterleaving
   buffering or use a receiver buffer for both transmission delay jitter
   and deinterleaving.  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 deinterleaving buffer.  Subsection 7.2.2
   specifies the receiver process how to organize received NAL units to
   the NAL unit decoding order.

7.2.1.  Size of the Deinterleaving Buffer

   When 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 deinterleaving buffer with the deint-
   buf-cap MIME parameter.  The sender indicates the requirement for the
   deinterleaving buffer size with the sprop-deint-buf-req MIME
   parameter.  It is therefore RECOMMENDED to set the deinterleaving
   buffer size, in terms of number of bytes, equal to or greater than
   the value of sprop-deint-buf-req MIME parameter.  See section 8.1 for
   further information on deint-buf-cap and sprop-deint-buf-req MIME
   parameters and section 8.2.2 for further information on their use in
   SDP Offer/Answer model.

   When a declarative session description is used in session setup, the
   sprop-deint-buf-req MIME parameter signals the requirement for the
   deinterleaving buffer size.  It is therefore RECOMMENDED to set the
   deinterleaving buffer size, in terms of number of bytes, equal to or
   greater than the value of sprop-deint-buf-req MIME parameter.




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7.2.2.  Deinterleaving 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 is 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 deinterleaving buffer as follows.
   NAL units of aggregation packets are stored in the deinterleaving
   buffer individually.  The value of DON is calculated and stored for
   all NAL units.

   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
      MIME type parameter (see section 8.1) incremented by 1.

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

   o  There are N VCL NAL units in the deinterleaving 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 MIME parameter.

   The NAL units to be removed from the deinterleaving buffer are
   determined as follows:

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







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   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 deinterleaving 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 received NAL units.

   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 an 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 found, 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.

   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.







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   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 MIME
   subtype registration for the ITU-T H.264 | ISO/IEC 14496-10 codec.  A
   mapping of the parameters into the Session Description Protocol (SDP)
   [5] is also provided for applications that use SDP.  Equivalent
   parameters could be defined elsewhere for use with control protocols
   that do not use MIME or SDP.

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

8.1.  MIME Registration

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

   The receiver MUST ignore any unspecified parameter.

   Media Type name:     video

   Media subtype name:  H264

   Required parameters: none







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   OPTIONAL parameters:
       profile-level-id:
                        A base16 [6] (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, and reserved_zero_5bits
                        in bit-significance order, starting from the
                        most significant bit, and 3) level_idc.  Note
                        that reserved_zero_5bits 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.

                        If the profile-level-id parameter is used to
                        indicate properties of a NAL unit stream, it
                        indicates the profile and level that a decoder
                        has to support in order to comply with [1] when
                        it decodes the stream.  The profile-iop byte
                        indicates whether the NAL unit stream also
                        obeys all constraints of the indicated profiles
                        as follows.  If bit 7 (the most significant
                        bit), bit 6, or bit 5 of profile-iop is equal
                        to 1, all constraints of the Baseline profile,
                        the Main profile, or the Extended profile,
                        respectively, are obeyed in the NAL unit
                        stream.

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



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                            Informative note: Capability exchange and
                            session setup procedures should provide
                            means to list the capabilities for each
                            supported codec profile separately.  For
                            example, the one-of-N codec selection
                            procedure of the SDP Offer/Answer model can
                            be used (section 10.2 of [7]).

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

       max-mbps, max-fs, max-cpb, max-dpb, and max-br:
                        These parameters MAY be used to signal the
                        capabilities of a receiver implementation.
                        These parameters MUST NOT be used for any other
                        purpose.  The profile-level-id parameter MUST
                        be present in the same receiver capability
                        description that contains any of these
                        parameters.  The level conveyed in the value of
                        the profile-level-id parameter MUST be such
                        that the receiver is fully capable of
                        supporting.  max-mbps, max-fs, max-cpb, max-
                        dpb, and max-br MAY be used to indicate
                        capabilities of the receiver that extend the
                        required capabilities of the signaled level, as
                        specified below.

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

                        A receiver MUST NOT signal values of max-
                        mbps, max-fs, max-cpb, max-dpb, and max-br that
                        meet the requirements of a higher level,



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                        referred to as level A herein, compared to the
                        level specified in the value of the profile-
                        level-id parameter, if the receiver can support
                        all the properties of level A.

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

       max-mbps:        The value of max-mbps is an integer indicating
                        the maximum macroblock processing rate in units
                        of macroblocks per second.  The max-mbps
                        parameter signals that the receiver is capable
                        of decoding video at a higher rate than is
                        required by the signaled level conveyed in the
                        value of the profile-level-id parameter.  When
                        max-mbps is signaled, the receiver MUST be able
                        to decode NAL unit streams that conform to the
                        signaled level, with the exception that the
                        MaxMBPS value in Table A-1 of [1] for the
                        signaled level is replaced with the value of
                        max-mbps.  The value of max-mbps MUST be
                        greater than or equal to the value of MaxMBPS
                        for the level given in Table A-1 of [1].
                        Senders MAY use this knowledge to send pictures
                        of a given size at a higher picture rate than
                        is indicated in the signaled level.

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



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                        proportionally lower frame rate than is
                        indicated in the signaled level.

       max-cpb          The value of max-cpb is an integer indicating
                        the maximum coded picture buffer size in units
                        of 1000 bits for the VCL HRD parameters (see
                        A.3.1 item i 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 level conveyed in the value of
                        the profile-level-id parameter.  When max-cpb
                        is signaled, the receiver MUST be able to
                        decode NAL unit streams that conform to the
                        signaled level, with the exception that the
                        MaxCPB value in Table A-1 of [1] for the
                        signaled 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 for the
                        level given in Table A-1 of [1].  Senders MAY
                        use this knowledge to construct coded video
                        streams with greater variation of bit rate
                        than can be achieved with the
                        MaxCPB value in Table A-1 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.




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

                        Min(1024 * max-dpb / ( PicWidthInMbs *
                        FrameHeightInMbs * 256 * ChromaFormatFactor ),
                        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 for the level
                        given in Table A-1 of [1].  Senders MAY use
                        this knowledge to construct coded video streams
                        with improved compression.

                            Informative note: This parameter was added
                            primarily to complement a similar codepoint
                            in the ITU-T Recommendation H.245, so as to
                            facilitate signaling gateway designs.  The
                            decoded picture buffer stores reconstructed
                            samples and is a property of the video
                            decoder only.  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




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                        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 level conveyed in the value of the
                        profile-level-id parameter.  The value of max-
                        br MUST be greater than or equal to the value
                        of MaxBR for the level given in Table A-1 of
                        [1].

                        When max-br is signaled, the video codec of the
                        receiver MUST be able to decode NAL unit
                        streams that conform to the signaled level,
                        conveyed in the profile-level-id parameter,
                        with the following exceptions in the limits
                        specified by the level:
                        o The value of max-br replaces the MaxBR value
                          of the signaled level (in Table A-1 of [1]).
                        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 level).

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

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

                        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



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

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

                            Informative note: Even if redundant-pic-cap
                            is equal to 1, the receiver may also choose
                            other error concealment strategies to




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                            replace or complement decoding of redundant
                            slices.

       sprop-parameter-sets:
                        This parameter MAY be used to convey
                        any sequence and picture parameter set NAL
                        units (herein referred to as the initial
                        parameter set NAL units) that MUST 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 the
                        base64 [6] representation of the initial
                        parameter set NAL units as specified in
                        sections 7.3.2.1 and 7.3.2.2 of [1].  The
                        parameter sets are conveyed in decoding order,
                        and no framing of the parameter set NAL units
                        takes place.  A comma is used to separate any
                        pair of parameter sets in the list.  Note that
                        the number of bytes in a parameter set NAL unit
                        is typically less than 10, but a picture
                        parameter set NAL unit can contain several
                        hundreds of bytes.

                           Informative note: When several payload
                           types are offered in the SDP Offer/Answer
                           model, each with its own sprop-parameter-
                           sets parameter, then the receiver cannot
                           assume that those parameter sets do not use
                           conflicting storage locations (i.e.,
                           identical values of parameter set
                           identifiers).  Therefore, a receiver should
                           double-buffer all sprop-parameter-sets and
                           make them available to the decoder instance
                           that decodes a certain payload type.

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






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

                        When the value of packetization-mode is equal
                        to 0 or packetization-mode is not present, the
                        single NAL mode, as defined in section 6.2 of
                        RFC 3984, MUST be used.  This mode is in use in
                        standards using ITU-T Recommendation H.241 [15]
                        (see section 12.1).  When the value of
                        packetization-mode is equal to 1, the non-
                        interleaved mode, as defined in section 6.3 of
                        RFC 3984, MUST be used.  When the value of
                        packetization-mode is equal to 2, the
                        interleaved mode, as defined in section 6.4 of
                        RFC 3984, MUST be used.  The value of
                        packetization mode MUST be an integer in the
                        range of 0 to 2, inclusive.

       sprop-interleaving-depth:
                        This parameter MUST NOT be present
                        when 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 a NAL
                        unit stream.  It specifies the maximum number
                        of VCL NAL units that precede any VCL NAL unit
                        in the NAL unit stream in transmission order
                        and follow the VCL NAL unit in decoding order.
                        Consequently, it is guaranteed that receivers
                        can reconstruct NAL unit decoding order when
                        the buffer size for NAL unit decoding order
                        recovery is at least the value of sprop-
                        interleaving-depth + 1 in terms of VCL NAL
                        units.

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




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       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 deinterleaving buffer for the NAL unit
                        stream.  The value of the parameter MUST be
                        greater than or equal to the maximum buffer
                        occupancy (in units of bytes) required in such
                        a deinterleaving buffer that is specified in
                        section 7.2 of RFC 3984.  It is guaranteed that
                        receivers can perform the deinterleaving of
                        interleaved NAL units into NAL unit decoding
                        order, when the deinterleaving buffer size is
                        at least the value of sprop-deint-buf-req in
                        terms of bytes.

                        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
                            deinterleaving 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 deinterleaving 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
                            deinterleaving buffer of the receiver only.



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                            When network jitter can occur, an
                            appropriately sized jitter buffer has to
                            be provisioned for as well.

       sprop-init-buf-time:
                        This parameter MAY be used to signal the
                        properties of a NAL unit stream.  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 buffer before
                        starting decoding to recover the NAL unit
                        decoding order from the transmission order.
                        The parameter is the maximum value of
                        (transmission time of a NAL unit - decoding
                        time of the NAL unit), assuming reliable and
                        instantaneous transmission, the same
                        timeline for transmission and decoding, and
                        that decoding starts when the first packet
                        arrives.

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

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

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

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

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

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

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





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                        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 a NAL unit 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,

                        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)




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                        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 of RFC 3984.

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

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

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

                        This parameter is motivated by, for example, an
                        IP to H.223 video telephony gateway, where
                        NALUs smaller than the H.223 transport data



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                        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.

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

                        A file format of H.264/AVC video is defined in
                        [29].  This definition is utilized by other
                        file formats, such as the 3GPP multimedia file
                        format (MIME type video/3gpp) [30] or the MP4
                        file format (MIME type video/mp4).

   Security considerations:
                        See section 9 of RFC 3984.

   Public specification:
                        Please refer to RFC 3984 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:
                        stewe@stewe.org

   Intended usage:      COMMON

   Author:
                        stewe@stewe.org
   Change controller:
                        IETF Audio/Video Transport working group
                        delegated from the IESG.










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8.2.  SDP Parameters

8.2.1.  Mapping of MIME Parameters to SDP

   The MIME media type video/H264 string is mapped to fields in the
   Session Description Protocol (SDP) [5] 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
      MIME subtype).

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

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

   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;
                sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==

8.2.2.  Usage with the SDP Offer/Answer Model

   When H.264 is offered over RTP using SDP in an Offer/Answer model [7]
   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", "packetization-mode", and, if required by
      "packetization-mode", "sprop-deint-buf-req".  These three
      parameters MUST be used symmetrically; i.e., the answerer MUST
      either maintain all configuration parameters or remove the media
      format (payload type) completely, if one or more of the parameter
      values are not supported.






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

      To simplify handling and matching of these configurations, the
      same RTP payload type number used in the offer SHOULD also be used
      in the answer, as specified in [7].  An answer MUST NOT contain a
      payload type number used in the offer unless the configuration
      ("profile-level-id", "packetization-mode", and, if present,
      "sprop-deint-buf-req") is the same as in the offer.

         Informative note: An offerer, when receiving the answer, has to
         compare payload types not declared in the offer based on media
         type (i.e., video/h264) and the above three parameters with any
         payload types it has already declared, in order to determine
         whether the configuration in question is new or equivalent to a
         configuration already offered.

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

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

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






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

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

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

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

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

   o  The receiver capability parameters "max-mbps", "max-fs", "max-
      cpb", "max-dpb", "max-br", and "max-rcmd-nalu-size" MUST be
      supported by the answerer for all streams declared as sendrecv or
      recvonly; otherwise, one of the following actions MUST be
      performed: the media format is removed, or the session rejected.

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




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

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

      Declaring actual configuration or properties for receiving:

         - profile-level-id
         - packetization-mode

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

         - sprop-deint-buf-req
         - sprop-interleaving-depth
         - sprop-parameter-sets
         - sprop-max-don-diff
         - sprop-init-buf-time

      Declaring receiver implementation capabilities:

         - max-mbps
         - max-fs
         - max-cpb
         - max-dpb
         - max-br
         - redundant-pic-cap
         - deint-buf-cap
         - max-rcmd-nalu-size

      Declaring how Offer/Answer negotiation shall be performed:

         - parameter-add

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

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

         - profile-level-id
         - packetization-mode
         - sprop-deint-buf-req



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         - sprop-max-don-diff
         - sprop-init-buf-time
         - sprop-parameter-sets
         - sprop-interleaving-depth

      Declaring the capabilities of the sender when it receives a
      stream:

         - max-mbps
         - max-fs
         - max-cpb
         - max-dpb
         - max-br
         - redundant-pic-cap
         - deint-buf-cap
         - max-rcmd-nalu-size

      Declaring how Offer/Answer negotiation shall be performed:

         - parameter-add

   Furthermore, the following considerations are necessary:

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

   o  Parameters declaring a configuration point are not downgradable,
      with the exception of the level part of the "profile-level-id"
      parameter.  This expresses values a receiver expects to be used
      and must be used verbatim on the sender side.

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





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

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

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

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

      Declaring actual configuration or properties:

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











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      Not usable:

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

   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

   A SIP 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 -> Answer 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=Z0IACpZTBYmI,aMljiA==
      a=rtpmap:99 H264/90000
      a=fmtp:99 profile-level-id=42A01E; packetization-mode=1;
                sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==
      a=rtpmap:100 H264/90000
      a=fmtp:100 profile-level-id=42A01E; packetization-mode=2;
                 sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==;
                 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 non-interleaved mode; 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 for the answerer
   when receiving a stream from the offerer when this configuration



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   (profile-level-id and packetization mode) is accepted.  Note that the
   value for "sprop-parameter-sets", although identical in the example
   above, 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=Z0IACpZTBYmI,aMljiA==,As0DEWlsIOp==,
               KyzFGleR
     a=rtpmap:99 H264/90000
     a=fmtp:99 profile-level-id=42A01E; packetization-mode=1;
               sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==,As0DEWlsIOp==,
               KyzFGleR; max-rcmd-nalu-size=3980
     a=rtpmap:100 H264/90000
     a=fmtp:100 profile-level-id=42A01E; packetization-mode=2;
               sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==,As0DEWlsIOp==,
               KyzFGleR; 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 a equivalent payload type 97; i.e., it has
   identical values for the three parameters "profile-level-id",
   packetization-mode, and "sprop-deint-buf-req".  This has the
   following implications for both the offerer and the answerer
   concerning the parameters that declare properties.  The offerer
   initially declared a certain value of the "sprop-parameter-sets" in
   the payload definition for PT=98.  However, as the answerer accepted
   this as PT=97, the values of "sprop-parameter-sets" in PT=98 must now
   be used instead when the offerer sends PT=97.  Similarly, when the
   answerer sends PT=98 to the offerer, it has to use the properties
   parameters it declared in PT=97.

   The answerer also accepts the reception of the two configurations
   that payload types 99 and 100 represent.  It provides the initial
   parameter sets for the answerer-to-offerer direction, and for
   buffering related parameters that it will use to send the payload
   types.  It also provides the offerer with its memory limit for
   deinterleaving 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



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   the size of 3980 bytes.  However, there is no guarantee that the
   network supports this size.

   Please note that the parameter sets in the above example do not
   represent a legal operation point of an H.264 codec.  The base64
   strings are only used for illustration.

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 data structure, but also due to the untimely
   transmission of a parameter set update.  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 stream in the payload (in-band) during an ongoing
      RTP session.

   It is necessary 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.  This
   section contains guidelines on how principles A and B must 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.

   The picture and sequence parameter set NALUs SHOULD NOT be
   transmitted in the RTP payload unless reliable transport is provided
   for RTP, as a loss of a parameter set of either type will likely
   prevent decoding of a considerable portion of the corresponding RTP






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   stream.  Thus, the transmission of parameter sets using a reliable
   session control protocol (i.e., usage of principle A or B above) is
   RECOMMENDED.

   In the rest of the section it is assumed that out-of-band signaling
   provides reliable transport of parameter set NALUs and that in-band
   transport does not.  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.

   -  When parameter sets are added or updated, principle C is
      vulnerable to transmission errors as described above, and
      therefore principle B is RECOMMENDED.

   -  When parameter sets are added or updated, care SHOULD be taken to
      ensure that any parameter set is delivered prior to its usage.  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 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).

   -  When new parameter sets are added, previously unused parameter set
      identifiers are used.  This avoids the problem identified in the



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      previous paragraph.  However, in a multiparty session, unless a
      synchronized control protocol is used, there is a risk that
      multiple entities try to add different parameter sets for the same
      identifier, which has to be avoided.

   -  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), it is not possible to
   employ out-of-band parameter set transmission.  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.

   -  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 RECOMMENDED that
      parameter set IDs be partitioned between the out-of-band and in-
      band parameter sets.

   To allow for maximum flexibility and best performance from the H.264
   coder, it is recommended, if possible, to allow any sender to add its
   own parameter sets to be used in a session.  Setting the "parameter-
   add" parameter to false should only be done in cases where the
   session topology prevents a participant to add its own parameter
   sets.

9.  Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [4], 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.




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   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
   [4], 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 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.





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   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

   IANA has registered one new MIME type; see section 8.1.




























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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.

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 [15]
   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



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   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 2429 packetization [10].  It has been
   implemented, and good results were reported [12].

   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 [12].

   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.





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   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.

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 2733 [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 2733
   framework, the following properties are well known:

   a) If applied over one RTP packet, RFC 2733 provides only packet
      repetition.

   b) RFC 2733 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



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      for an extended Golay code with k=12 and n=24, the residual error
      rate is about 0.01%.

   For applying RFC 2733 in combination with H.264 baseline coded video
   without using FUs, several options might be considered:

   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





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   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
   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 2733.

   If the error probability depends on the length of the transmitted
   packet (e.g., in case of mobile transmission [14]), 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.






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   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
   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



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   and coded slice data partitions are more likely to be received 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.














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   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):

      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 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



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   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 MIME 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.












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   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:

                       ...  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 MIME 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





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   loss resiliency of IDR pictures is improved in systems supporting
   retransmission compared to the case in which pictures were
   transmitted in their decoding order.

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).






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   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.

   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.  Acknowledgements

   The authors thank Roni Even, Dave Lindbergh, Philippe Gentric,
   Gonzalo Camarillo, Gary Sullivan, Joerg Ott, and Colin Perkins for
   careful review.

15.  References

15.1.  Normative References

   [1]  ITU-T Recommendation H.264, "Advanced video coding for generic
        audiovisual services", May 2003.

   [2]  ISO/IEC International Standard 14496-10:2003.

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

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

   [5]  Handley, M. and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, April 1998.

   [6]  Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
        RFC 3548, July 2003.





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RFC 3984           RTP Payload Format for H.264 Video      February 2005


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

15.2.  Informative References

   [8]  "Draft ITU-T Recommendation and Final Draft International
        Standard of Joint Video Specification (ITU-T Rec. H.264 |
        ISO/IEC 14496-10 AVC)", available from http://ftp3.itu.int/av-
        arch/jvt-site/2003_03_Pattaya/JVT-G050r1.zip, May 2003.

   [9]  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.

   [10] Bormann, C., Cline, L., Deisher, G., Gardos, T., Maciocco, C.,
        Newell, D., Ott, J., Sullivan, G., Wenger, S., and C. Zhu, "RTP
        Payload Format for the 1998 Version of ITU-T Rec. H.263 Video
        (H.263+)", RFC 2429, October 1998.

   [11] ISO/IEC IS 14496-2.

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

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

   [14] 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.

   [15] ITU-T Recommendation H.241, "Extended video procedures and
        control signals for H.300 series terminals", 2004.

   [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] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format for
        Generic Forward Error Correction", RFC 2733, December 1999.

   [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.



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RFC 3984           RTP Payload Format for H.264 Video      February 2005


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

   [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,
        January 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] ISO/IEC 14496-15: "Information technology - Coding of audio-
        visual objects - Part 15: Advanced Video Coding (AVC) file
        format".

   [30] Castagno, R. and D. Singer, "MIME Type Registrations for 3rd
        Generation Partnership Project (3GPP) Multimedia files", RFC
        3839, July 2004.











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Authors' Addresses

   Stephan Wenger
   TU Berlin / Teles AG
   Franklinstr. 28-29
   D-10587 Berlin
   Germany

   Phone: +49-172-300-0813
   EMail: stewe@stewe.org


   Miska M. Hannuksela
   Nokia Corporation
   P.O. Box 100
   33721 Tampere
   Finland

   Phone: +358-7180-73151
   EMail: miska.hannuksela@nokia.com


   Thomas Stockhammer
   Nomor Research
   D-83346 Bergen
   Germany

   Phone: +49-8662-419407
   EMail: stockhammer@nomor.de


   Magnus Westerlund
   Multimedia Technologies
   Ericsson Research EAB/TVA/A
   Ericsson AB
   Torshamsgatan 23
   SE-164 80 Stockholm
   Sweden

   Phone: +46-8-7190000
   EMail: magnus.westerlund@ericsson.com










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RFC 3984           RTP Payload Format for H.264 Video      February 2005


   David Singer
   QuickTime Engineering
   Apple
   1 Infinite Loop MS 302-3MT
   Cupertino
   CA 95014
   USA

   Phone +1 408 974-3162
   EMail: singer@apple.com









































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