Audio/Video Transport WG                                     Y.-K. Wang
Internet Draft                                      Huawei Technologies
Intended status: Standards track                                R. Even
Expires: August September 2009                                   Self-employed
                                                          T. Kristensen
                                                               Tandberg
                                                      February 23,
                                                          March 6, 2009

                    RTP Payload Format for H.264 Video
                   draft-ietf-avt-rtp-rfc3984bis-03.txt
                   draft-ietf-avt-rtp-rfc3984bis-04.txt

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Abstract

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

   This memo obsoletes RFC 3984.  Changes from RFC 3984 are summarized
   in section 18.  Issues on backward compatibility to RFC 3984 are
   discussed in section 17.

Table of Contents

   1. Introduction...................................................4
      1.1. The H.264 Codec...........................................4
      1.2. Parameter Set Concept.....................................5
      1.3. Network Abstraction Layer Unit Types......................6
   2. Conventions....................................................7
   3. Scope..........................................................7
   4. Definitions and Abbreviations..................................7
      4.1. Definitions...............................................7
      4.2. Abbreviations.............................................9
   5. RTP Payload Format............................................10
      5.1. RTP Header Usage.........................................10
      5.2. Payload Structures.......................................12
      5.3. NAL Unit Header Usage....................................14
      5.4. Packetization Modes......................................16
      5.5. Decoding Order Number (DON)..............................17
      5.6. Single NAL Unit Packet...................................20
      5.7. Aggregation Packets......................................21
         5.7.1. Single-Time Aggregation Packet......................23
         5.7.2. Multi-Time Aggregation Packets (MTAPs)..............25
         5.7.3. Fragmentation Units (FUs)...........................29
   6. Packetization Rules...........................................33
      6.1. Common Packetization Rules...............................33
      6.2. Single NAL Unit Mode.....................................34
      6.3. Non-Interleaved Mode.....................................34
      6.4. Interleaved Mode.........................................34
   7. De-Packetization Process......................................35
      7.1. Single NAL Unit and Non-Interleaved Mode.................35
      7.2. Interleaved Mode.........................................35
         7.2.1. Size of the De-interleaving Buffer..................36
         7.2.2. De-interleaving Process.............................36
      7.3. Additional De-Packetization Guidelines...................38
   8. Payload Format Parameters.....................................39
      8.1. Media Type Registration..................................39
      8.2. SDP Parameters...........................................56
         8.2.1. Mapping of Payload Type Parameters to SDP...........56
         8.2.2. Usage with the SDP Offer/Answer Model...............57
         8.2.3. Usage in Declarative Session Descriptions...........64
      8.3. Examples.................................................65
      8.4. Parameter Set Considerations.............................72
      8.5. Decoder Refresh Point Procedure using In-Band Transport of
      Parameter Sets (Informative)..................................74
         8.5.1. IDR Procedure to Respond to a Request for a Decoder
         Refresh Point..............................................75
         8.5.2. Gradual Recovery Procedure to Respond to a Request for a
         Decoder Refresh Point......................................75
   9. Security Considerations.......................................76
   10. Congestion Control...........................................77
   11. IANA Consideration...........................................77
   12. Informative Appendix: Application Examples...................78
      12.1. Video Telephony according to ITU-T Recommendation H.241
      Annex A.......................................................78
      12.2. Video Telephony, No Slice Data Partitioning, No NAL Unit
      Aggregation...................................................78
      12.3. Video Telephony, Interleaved Packetization Using NAL Unit
      Aggregation...................................................79
      12.4. Video Telephony with Data Partitioning..................79
      12.5. Video Telephony or Streaming with FUs and Forward Error
      Correction....................................................80
      12.6. Low Bit-Rate Streaming..................................82
      12.7. Robust Packet Scheduling in Video Streaming.............83
   13. Informative Appendix: Rationale for Decoding Order Number....84
      13.1. Introduction............................................84
      13.2. Example of Multi-Picture Slice Interleaving.............84
      13.3. Example of Robust Packet Scheduling.....................86
      13.4. Robust Transmission Scheduling of Redundant Coded Slices89
      13.5. Remarks on Other Design Possibilities...................90
   14. Acknowledgements.............................................91
   15. References...................................................91
      15.1. Normative References....................................91
      15.2. Informative References..................................92
   16. Authors' Addresses...........................................94
   17. Backward Compatibility to RFC 3984...........................94
   18. Changes from RFC 3984........................................96

1. Introduction

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

   This memo obsoletes RFC 3984.  Changes from RFC 3984 are summarized
   in section 18.   Issues on backward compatibility to RFC 3984 are
   discussed in section 17.

1.1. The H.264 Codec

   The H.264 video codec has a very broad application range that covers
   all forms of digital compressed video, from low bit-rate Internet
   streaming applications to HDTV broadcast and Digital Cinema
   applications with nearly lossless coding.  Compared to the current
   state of technology, the overall performance of H.264 is such that
   bit rate savings of 50% or more are reported.  Digital Satellite TV
   quality, for example, was reported to be achievable at 1.5 Mbit/s,
   compared to the current operation point of MPEG 2 video at around 3.5
   Mbit/s [10].

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

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

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

   One of the main properties of H.264 is the complete decoupling of the
   transmission time, the decoding time, and the sampling or
   presentation time of slices and pictures.  The decoding process
   specified in H.264 is unaware of time, and the H.264 syntax does not
   carry information such as the number of skipped frames (as is common
   in the form of the Temporal Reference in earlier video compression
   standards).  Also, there are NAL units that affect many pictures and
   that are, therefore, inherently timeless.  For this reason, the
   handling of the RTP timestamp requires some special 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 [11] or MPEG-4's Header Extension Code (HEC) [12]
   unnecessary.  This was achieved by decoupling information relevant to
   more than one slice from the media stream.  This higher layer meta
   information should be sent reliably, asynchronously, and in advance
   from the RTP packet stream that contains the slice packets.
   (Provisions for sending this information in-band are also available
   for applications that do not have an out-of-band transport channel
   appropriate for the purpose.)  The combination of the higher-level
   parameters is called a parameter set.  The H.264 specification
   includes two types of parameter sets: sequence parameter set and
   picture parameter set.  An active sequence parameter set remains
   unchanged throughout a coded video sequence, and an active picture
   parameter set remains unchanged within a coded picture.  The sequence
   and picture parameter set structures contain information such as
   picture size, optional coding modes employed, and macroblock to slice
   group map.

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

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

1.3. Network Abstraction Layer Unit Types

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

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

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

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

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

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

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

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

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

2. Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC-2119 [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.

      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].

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

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

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

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

      static macroblock: A certain amount of macroblocks in the video
      stream can be defined as static, as defined in section 8.3.2.8 in
      [3].  Static macroblocks free up additional processing cycles for
      the handling of non-static macroblocks.  Based on a given amount
      of video processing resources and a given resolution, a higher
      number of static macroblocks enables a correspondingly higher
      frame rate.

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

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

4.2. Abbreviations

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

5. RTP Payload Format

5.1. RTP Header Usage

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

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |V=2|P|X|  CC   |M|     PT      |       sequence number         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           synchronization source (SSRC) identifier            |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |            contributing source (CSRC) identifiers             |
   |                             ....                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 1 RTP header according to RFC 3550
   The RTP header information to be set according to this RTP payload
   format is set as follows:

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

         Informative note: Only one M bit is associated with an
         aggregation packet carrying multiple NAL units.  Thus, if a
         gateway has re-packetized an aggregation packet into several
         packets, it cannot reliably set the M bit of those packets.

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

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

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

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

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

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

      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 inter-arrival
         jitter reported in the RTCP reports may not be a trustworthy
         indication of the network performance, as the calculation
         rules for inter-arrival 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. Payload Structures

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

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

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

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

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

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

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

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

5.3. NAL Unit Header Usage

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

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

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

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

      MANEs SHOULD set the F bit to indicate detected bit errors in the
      NAL unit.  The H.264 specification requires that the F bit is
      equal to 0.  When the F bit is set, the decoder is advised that
      bit errors or any other syntax violations may be present in the
      payload or in the NAL unit type octet.  The simplest decoder
      reaction to a NAL unit in which the F bit is equal to 1 is to
      discard such a NAL unit and to conceal the lost data in the
      discarded NAL unit.

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

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

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

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

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

      For a mapping of the remaining nal_unit_types to NRI values, the
      following example MAY be used and has been shown to be efficient
      in a certain environment [14].  Other mappings MAY also be
      desirable, depending on the application and the H.264/AVC Annex A
      profile in use.

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

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

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

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

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

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

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

5.4. Packetization Modes

   This memo specifies three cases of packetization modes:

   o  Single NAL unit mode

   o  Non-interleaved mode

   o  Interleaved mode

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

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

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

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

   Some NAL unit or payload type values (indicated as reserved in
   Table 3) are reserved for future extensions.  NAL units of those
   types SHOULD NOT be sent by a sender (direct as packet payloads, or
   as aggregation units in aggregation packets, or as fragmented units
   in FU packets) and MUST be ignored by a receiver.  For example, the
   payload types 1-23, with the associated packet type "NAL unit", are
   allowed in "Single NAL Unit Mode" and in "Non-Interleaved Mode", but
   disallowed in "Interleaved Mode".  However, NAL units of NAL unit
   types 1-23 can be used in "Interleaved Mode" as aggregation units in
   STAP-B, MTAP16 and MTAP14 packets as well as fragmented units in FU-A
   and FU-B packets.  Similarly, NAL units of NAL unit types 1-23 can
   also be used in the "Non-Interleaved Mode" as aggregation units in
   STAP-A packets or fragmented units in FU-A packets, in addition to
   being directly used as packet payloads.

5.5. Decoding Order Number (DON)

   In the interleaved packetization mode, the transmission order of NAL
   units is allowed to differ from the decoding order of the NAL units.
   Decoding order number (DON) is a field in the payload structure or a
   derived variable that indicates the NAL unit decoding order.

   Rationale and examples of use cases for transmission out of decoding
   order and for the use of DON are given in section 13.

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

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

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

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

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

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

   In the single NAL unit packetization mode, the transmission order of
   NAL units, determined by the RTP sequence number, MUST be the same as
   their NAL unit decoding order.  In the non-interleaved packetization
   mode, the transmission order of NAL units in single NAL unit packets,
   STAP-As, and FU-As MUST be the same as their NAL unit decoding order.
   The NAL units within an STAP MUST appear in the NAL unit decoding
   order.  Thus, the decoding order is first provided through the
   implicit order within a STAP, and second provided through the RTP
   sequence number for the order between STAPs, FUs, and single NAL unit
   packets.

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

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

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

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

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

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

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

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

   Values of DON related fields (DON, DONB, and DOND; see section 5.7)
   MUST be such that the decoding order determined by the values of DON,
   as specified above, conforms to the NAL unit decoding order.  If the
   order of two NAL units in NAL unit decoding order is switched and the
   new order does not conform to the NAL unit decoding order, the NAL
   units MUST NOT have the same value of DON.  If the order of two
   consecutive NAL units in the NAL unit stream is switched and the new
   order still conforms to the NAL unit decoding order, the NAL units
   MAY have the same value of DON.  For example, when arbitrary slice
   order is allowed by the video coding profile in use, all the coded
   slice NAL units of a coded picture are allowed to have the same value
   of DON.  Consequently, NAL units having the same value of DON can be
   decoded in any order, and two NAL units having a different value of
   DON should be passed to the decoder in the order specified above.
   When two consecutive NAL units in the NAL unit decoding order have a
   different value of DON, the value of DON for the second NAL unit in
   decoding order SHOULD be the value of DON for the first, incremented
   by one.

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

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

5.6. Single NAL Unit Packet

   The single NAL unit packet defined here MUST contain only one NAL
   unit, of the types defined in [1].  This means that neither an
   aggregation packet nor a fragmentation unit can be used within a
   single NAL unit packet.  A NAL unit stream composed by de-packetizing
   single NAL unit packets in RTP sequence number order MUST conform to
   the NAL unit decoding order.  The structure of the single NAL unit
   packet is shown in Figure 2.

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |F|NRI|  Type   |                                               |
   +-+-+-+-+-+-+-+-+                                               |
   |                                                               |
   |               Bytes 2..n of a Single NAL unit                 |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :...OPTIONAL RTP padding        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 2 RTP payload format for single NAL unit packet

5.7. Aggregation Packets

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

   Two types of aggregation packets are defined by this specification:

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

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

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

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |F|NRI|  Type   |                                               |
   +-+-+-+-+-+-+-+-+                                               |
   |                                                               |
   |             one or more aggregation units                     |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :...OPTIONAL RTP padding        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 3 RTP payload format for aggregation packets

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

                 Table 4.  Type field for STAPs and MTAPs

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

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

   The payload of an aggregation packet consists of one or more
   aggregation units.  See sections 5.7.1 and 5.7.2 for the four
   different types of aggregation units.  An aggregation packet can
   carry as many aggregation units as necessary; however, the total
   amount of data in an aggregation packet obviously MUST fit into an IP
   packet, and the size SHOULD be chosen so that the resulting IP packet
   is smaller than the MTU size.  An aggregation packet MUST NOT contain
   fragmentation units specified in section 5.8.  Aggregation packets
   MUST NOT be nested; i.e., an aggregation packet MUST NOT contain
   another aggregation packet.

5.7.1. Single-Time Aggregation Packet

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   :                                               |
   +-+-+-+-+-+-+-+-+                                               |
   |                                                               |
   |                single-time aggregation units                  |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 4 Payload format for STAP-A

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   :  decoding order number (DON)  |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
   |                                                               |
   |                single-time aggregation units                  |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 5 Payload format for STAP-B

   The DON field specifies the value of DON for the first NAL unit in an
   STAP-B in transmission order.  For each successive NAL unit in
   appearance order in an STAP-B, the value of DON is equal to (the
   value of DON of the previous NAL unit in the STAP-B + 1) % 65536, in
   which '%' stands for the modulo operation.

   A single-time aggregation unit consists of 16-bit unsigned size
   information (in network byte order) that indicates the size of the
   following NAL unit in bytes (excluding these two octets, but
   including the NAL unit type octet of the NAL unit), followed by the
   NAL unit itself, including its NAL unit type byte.  A single-time
   aggregation unit is byte aligned within the RTP payload, but it may
   not be aligned on a 32-bit word boundary.  Figure 6 presents the
   structure of the single-time aggregation unit.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   :        NAL unit size          |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
   |                                                               |
   |                           NAL unit                            |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 6 Structure for single-time aggregation unit

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          RTP Header                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |STAP-A NAL HDR |         NALU 1 Size           | NALU 1 HDR    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NALU 1 Data                           |
   :                                                               :
   +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               | NALU 2 Size                   | NALU 2 HDR    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         NALU 2 Data                           |
   :                                                               :
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :...OPTIONAL RTP padding        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 7 An example of an RTP packet including an STAP-A containing
                     two single-time aggregation units
   Figure 8 presents an example of an RTP packet that contains an STAP-B.
   The STAP contains two single-time aggregation units, labeled as 1 and
   2 in the figure.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          RTP Header                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |STAP-B NAL HDR | DON                           | NALU 1 Size   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | NALU 1 Size   | NALU 1 HDR    | NALU 1 Data                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   :                                                               :
   +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               | NALU 2 Size                   | NALU 2 HDR    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       NALU 2 Data                             |
   :                                                               :
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :...OPTIONAL RTP padding        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

5.7.2. Multi-Time Aggregation Packets (MTAPs)

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

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   :  decoding order number base   |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
   |                                                               |
   |                 multi-time aggregation units                  |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 9 NAL unit payload format for MTAPs

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

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

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :        NAL unit size          |      DOND     |  TS offset    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  TS offset    |                                               |
   +-+-+-+-+-+-+-+-+              NAL unit                         |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 10  Multi-time aggregation unit for MTAP16

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :        NAL unit size         |      DOND     |  TS offset    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         TS offset             |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                              NAL unit                         |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 11  Multi-time aggregation unit for MTAP24

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

      Informative note: The "earliest" multi-time aggregation unit is
      the one that would have the smallest extended RTP timestamp among
      all the aggregation units of an MTAP if the NAL units contained
      in the aggregation units were encapsulated in single NAL unit
      packets.  An extended timestamp is a timestamp that has more than
      32 bits and is capable of counting the wraparound of the
      timestamp field, thus enabling one to determine the smallest
      value if the timestamp wraps.  Such an "earliest" aggregation
      unit may not be the first one in the order in which the
      aggregation units are encapsulated in an MTAP.  The "earliest"
      NAL unit need not be the same as the first NAL unit in the NAL
      unit decoding order either.

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          RTP Header                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |MTAP16 NAL HDR |  decoding order number base   | NALU 1 Size   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  NALU 1 Size  |  NALU 1 DOND  |       NALU 1 TS offset        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  NALU 1 HDR   |  NALU 1 DATA                                  |
   +-+-+-+-+-+-+-+-+                                               +
   :                                                               :
   +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               | NALU 2 SIZE                   |  NALU 2 DOND  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       NALU 2 TS offset        |  NALU 2 HDR   |  NALU 2 DATA  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
   :                                                               :
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :...OPTIONAL RTP padding        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          RTP Header                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |MTAP24 NAL HDR |  decoding order number base   | NALU 1 Size   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  NALU 1 Size  |  NALU 1 DOND  |       NALU 1 TS offs          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |NALU 1 TS offs |  NALU 1 HDR   |  NALU 1 DATA                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   :                                                               :
   +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               | NALU 2 SIZE                   |  NALU 2 DOND  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       NALU 2 TS offset                        |  NALU 2 HDR   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  NALU 2 DATA                                                  |
   :                                                               :
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :...OPTIONAL RTP padding        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

5.7.3. Fragmentation Units (FUs)

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

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

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

   Fragmentation is defined only for a single NAL unit and not for any
   aggregation packets.  A fragment of a NAL unit consists of an integer
   number of consecutive octets of that NAL unit.  Each octet of the NAL
   unit MUST be part of exactly one fragment of that NAL unit.
   Fragments of the same NAL unit MUST be sent in consecutive order with
   ascending RTP sequence numbers (with no other RTP packets within the
   same RTP packet stream being sent between the first and last
   fragment).  Similarly, a NAL unit MUST be reassembled in RTP sequence
   number order.

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

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

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | FU indicator  |   FU header   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   |                         FU payload                            |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :...OPTIONAL RTP padding        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 14  RTP payload format for FU-A

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | FU indicator  |   FU header   |               DON             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
   |                                                               |
   |                         FU payload                            |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :...OPTIONAL RTP padding        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 15  RTP payload format for FU-B

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

   The FU indicator octet has the following format:

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

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

   The FU header has the following format:

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

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

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

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

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

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

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

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

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

      Informative note: Empty FUs are allowed to reduce the latency of
      a certain class of senders in nearly lossless environments.
      These senders can be characterized in that they packetize NALU
      fragments before the NALU is completely generated and, hence,
      before the NALU size is known.  If zero-length NALU fragments
      were not allowed, the sender would have to generate at least one
      bit of data of the following fragment before the current fragment
      could be sent.  Due to the characteristics of H.264, where
      sometimes several macroblocks occupy zero bits, this is
      undesirable and can add delay.  However, the (potential) use of
      zero-length NALU fragments should be carefully weighed against
      the increased risk of the loss of at least a part of the NALU
      because of the additional packets employed for its transmission.

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

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

6. Packetization Rules

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

6.1. Common Packetization Rules

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

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

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

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

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

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

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

6.2. Single NAL Unit Mode

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

6.3. Non-Interleaved Mode

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

6.4. Interleaved Mode

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

7. De-Packetization Process

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

   All normal RTP mechanisms related to buffer management apply.  In
   particular, duplicated or outdated RTP packets (as indicated by the
   RTP 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 de-packetized
   in RTP sequence number order.  If a de-packetized packet is a single
   NAL unit packet, the NAL unit contained in the packet is passed
   directly to the decoder.  If a de-packetized packet is an STAP-A, the
   NAL units contained in the packet are passed to the decoder in the
   order in which they are encapsulated in the packet.  For all the FU-A
   packets containing fragments of a single NAL unit, the de-packetized
   fragments are concatenated in their sending order to recover the NAL
   unit, which is then passed to the decoder.

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

7.2. Interleaved Mode

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

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

   This section is organized as follows: subsection 7.2.1 presents how o
   calculate the size of the de-interleaving 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 De-interleaving Buffer

   When the SDP Offer/Answer model or any other capability exchange
   procedure is used in session setup, the properties of the received
   stream SHOULD be such that the receiver capabilities are not exceeded.
   In the SDP Offer/Answer model, the receiver can indicate its
   capabilities to allocate a de-interleaving buffer with the deint-buf-
   cap media type parameter.  The sender indicates the requirement for
   the de-interleaving buffer size with the sprop-deint-buf-req media
   type parameter.  It is therefore RECOMMENDED to set the de-
   interleaving buffer size, in terms of number of bytes, equal to or
   greater than the value of sprop-deint-buf-req media type parameter.
   See section 8.1 for further information on deint-buf-cap and sprop-
   deint-buf-req media type parameters and section 8.2.2 for further
   information on their use in the SDP Offer/Answer model.

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

7.2.2. De-interleaving Process

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

   Regardless of the buffering state, the receiver stores incoming NAL
   units, in reception order, in the de-interleaving buffer as follows.
   NAL units of aggregation packets are stored in the de-interleaving
   buffer individually.  The value of DON is calculated and stored for
   each NAL unit.

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

   o  Function AbsDON is specified in section 8.1.

   o  Function don_diff is specified in section 5.5.

   o  Constant N is the value of the OPTIONAL sprop-interleaving-depth
      media type parameter (see section 8.1) incremented by 1.

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

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

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

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

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

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

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

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

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

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

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

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

7.3. Additional De-Packetization Guidelines

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

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

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

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

8. Payload Format Parameters

   This section specifies the parameters that MAY be used to select
   optional features of the payload format and certain features of the
   bitstream.  The parameters are specified here as part of the media
   subtype registration for the ITU-T H.264 | ISO/IEC 14496-10 codec.  A
   mapping of the parameters into the Session Description Protocol (SDP)
   [6] is also provided for applications that use SDP.  Equivalent
   parameters could be defined elsewhere for use with control protocols
   that do not use SDP.

   Some parameters provide a receiver with the properties of the stream
   that will be sent.  The names of all these parameters start with
   "sprop" for stream properties.  Some of these "sprop" parameters are
   limited by other payload or codec configuration parameters.  For
   example, the sprop-parameter-sets parameter is constrained by the
   profile-level-id parameter.  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. Media Type Registration

   The media 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

   OPTIONAL parameters:

      profile-level-id:
         A base16 [7] (hexadecimal) representation of the following
         three bytes in the sequence parameter set NAL unit specified
         in [1]: 1) profile_idc, 2) a byte herein referred to as
         profile-iop, composed of the values of constraint_set0_flag,
         constraint_set1_flag,constraint_set2_flag,
         constraint_set3_flag, and reserved_zero_4bits in bit-
         significance order, starting from the most significant bit,
         and 3) level_idc.  Note that reserved_zero_4bits is required
         to be equal to 0 in [1], but other values for it may be
         specified in the future by ITU-T or ISO/IEC.

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

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

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

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

              Profile     profile_idc             profile-iop
                          (hexadecimal)           (binary)

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

         For example, in the table above, profile_idc equal to 58
         (Extended) with profile-iop equal to 11xx0000 indicates the
         same sub-profile corresponding to profile_idc equal to 42
         (Baseline) with profile-iop equal to x1xx0000.  Note that
         other combinations of profile_idc and profile-iop (note listed
         in Table 5) may represent a sub-profile equivalent to the
         common subset of coding tools for more than one profile.  Note
         also that a decoder conforming to a certain profile may be
         able to decode bitstreams conforming to other profiles.  For
         example, a decoder conforming to the High 4:4:4 profile at
         certain level must be able to decode bitstreams confirming to
         the Constrained Baseline, Main, High, High 10 or High 4:2:2
         profile at the same or a lower level.

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

         If the profile-level-id parameter is used for capability
         exchange or session setup procedure, it indicates the subset
         of coding tools, which is equal to the default sub-profile,
         and the highest level, which is equal to the default level,
         that the codec supports.  All levels lower than the default
         level are also supported by the codec.

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

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

      max-mbps, max-smbps, max-fs, max-cpb, max-dpb, and max-br:
         These parameters MAY be used to signal the capabilities of a
         receiver implementation. These parameters MUST NOT be used for
         any other purpose.  The 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-smbps,
         max-fs, max-cpb, max-dpb, and max-br MAY be used to indicate
         capabilities of the receiver that extend the required
         capabilities of the signaled level, as specified below.

         When more than one parameter from the set (max-mbps, max-
         smbps , max-fs, max-cpb, max-dpb, max-br) is present, the
         receiver MUST support all signaled capabilities simultaneously.
         For example, if both max-mbps and max-br are present, the
         signaled 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.

         If a receiver can support all the properties of level A, the
         level specified in the value of the profile-level-id MUST be
         level A (i.e. MUST NOT be lower than level A).  In other words,
         a sender or 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 compared to the level specified in the value of
         the profile-level-id parameter.

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

      max-mbps: The value of max-mbps is an integer indicating the
         maximum macroblock processing rate in units of macroblocks per
         second.  The max-mbps parameter signals that the receiver is
         capable of decoding video at a higher rate than is required by
         the signaled 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-smbps: The value of max-smbps is an integer indicating the
         maximum static macroblock processing rate in units of static
         macroblocks per second, under the hypothetical assumption that
         all macroblocks are static macroblocks.  When max-smbps is
         signalled the MaxMBPS value in Table A-1 of [1] should be
         replaced with the result of the following computation:

         o If the parameter max-mbps is signalled, set a variable
            MaxMacroblocksPerSecond to the value of max-mbps.
            Otherwise, set MaxMacroblocksPerSecond equal to the value
            of MaxMBPS for the level in Table A-1 [1].

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

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

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

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

         The encoder should recompute this value for each picture. The
         value of max-smbps MUST be greater than 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 signalled 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 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.

      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.  There
            is no relationship between the size of the decoded picture
            buffer and the buffers used in RTP, especially de-
            interleaving and de-jitter buffers.

      max-br: The value of max-br is an integer indicating the maximum
         video bit rate in units of 1000 bits per second for the VCL
         HRD parameters (see A.3.1 item i of [1]) and in units of 1200
         bits per second for the NAL HRD parameters (see A.3.1 item j
         of [1]).

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

         When max-br is signaled, the video codec of the receiver MUST
         be able to decode NAL unit streams that conform to the
         signaled level, conveyed in the profile-level-id parameter,
         with the following exceptions in the limits specified by the
         level:

         o The value of max-br replaces the MaxBR value of the signaled
            level (in Table A-1 of [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 this parameter
            that the network is capable of handling such bit rates at
            any given time.  In particular, no conclusion can be drawn
            that the signaled bit rate is possible under congestion
            control constraints.

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

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

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

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

      sprop-parameter-sets:
         This parameter MAY be used to convey any sequence and picture
         parameter set NAL units (herein referred to as the initial
         parameter set NAL units) that can be placed in the NAL unit
         stream to precede any other NAL units in decoding order.  The
         parameter MUST NOT be used to indicate codec capability in any
         capability exchange procedure.  The value of the parameter is
         a comma (',') separated list of base64 [7] representations of
         parameter set NAL units as specified in sections 7.3.2.1 and
         7.3.2.2 of [1].  Note that the number of bytes in a parameter
         set NAL unit is typically less than 10, but a picture
         parameter set NAL unit can contain several hundreds of bytes.

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

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

      sprop-level-parameter-sets:
         This parameter MAY be used to convey any sequence and picture
         parameter set NAL units (herein referred to as the initial
         parameter set NAL units) that can be placed in the NAL unit
         stream to precede any other NAL units in decoding order and
         that are associated with one or more levels lower than the
         default level.  The parameter MUST NOT be used to indicate
         codec capability in any capability exchange procedure.

         The sprop-level-parameter-sets parameter contains parameter
         sets for one or more levels which are lower than the default
         level.  All parameter sets associated with one level are
         clustered and prefixed with a three-byte field which has the
         same syntax as profile-level-id.  This enables the receiver to
         install the parameter sets for one level and discard the rest.
         The three-byte field is named PLId, and all parameter sets
         associated with one level are named PSL, which has the same
         syntax as sprop-parameter-sets.  Parameter sets for each level
         are represented in the form of PLId:PSL, i.e., PLId followed
         by a colon (':') and the base64 [7] representation of the
         initial parameter set NAL units for the level.  Each pair of
         PLId:PSL is also separated by a colon.  Note that a PSL can
         contain multiple parameter sets for that level, separated with
         commas (',').

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

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

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

            Informative note: An RFC 3984 receiver does not understand
            sprop-level-parameter-sets, use-level-src-parameter-sets,
            or the "fmtp" source attribute as specified in section 6.3
            of [9].  Therefore, during SDP Offer/Answer, an RFC 3984
            receiver as the answerer will simply ignore sprop-level-
            parameter-sets, when present in an offer, and sprop-
            parameter-sets, when conveyed using the "fmtp" source
            attribute as specified in section 6.3 of [9].  Assume that
            the offered payload type was accepted at a level lower than
            the default level.  If the offered payload type included
            sprop-level-parameter-sets or included sprop-parameter-sets
            conveyed using the "fmtp" source attribute, and the offerer
            sees that the answerer has not included use-level-src-
            parameter-sets equal to 1 in the answer, the offerer gets
            to know that in-band transport of parameter sets is needed.

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

      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 [3]
         (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 an RTP packet stream.
         It specifies the maximum number of VCL NAL units that precede
         any VCL NAL unit in the RTP packet stream in transmission
         order and follow the VCL NAL unit in decoding order.
         Consequently, it is guaranteed that receivers can reconstruct
         NAL unit decoding order when the buffer size for NAL unit
         decoding order recovery is at least the value of sprop-
         interleaving-depth + 1 in terms of VCL NAL units.

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

      sprop-deint-buf-req:
         This parameter MUST NOT be present when packetization-mode is
         not present or the value of packetization-mode is equal to 0
         or 1.  It MUST be present when the value of packetization-mode
         is equal to 2.

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

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

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

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

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

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

      sprop-init-buf-time:
         This parameter MAY be used to signal the properties of an RTP
         packet stream.  The parameter MUST NOT be present, if the
         value of packetization-mode is equal to 0 or 1.

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

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

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

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

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

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

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

         Thus, in terms of intervals of NAL unit transmission times,
         the value of sprop-init-buf-time in this example is 1.  The
         parameter is coded as a non-negative base10 integer
         representation in clock ticks of a 90-kHz clock.  If the
         parameter is not present, then no initial buffering time value
         is defined.  Otherwise the value of sprop-init-buf-time MUST
         be an integer in the range of 0 to 4294967295, inclusive.

         In addition to the signaled sprop-init-buf-time, receivers
         SHOULD take into account the transmission delay jitter
         buffering, including buffering for the delay jitter caused by
         mixers, translators, gateways, proxies, traffic-shapers, and
         other network elements.

      sprop-max-don-diff:
         This parameter MAY be used to signal the properties of an RTP
         packet stream.  It MUST NOT be used to signal transmitter or
         receiver or codec capabilities.  The parameter MUST NOT be
         present if the value of packetization-mode is equal to 0 or 1.
         sprop-max-don-diff is an integer in the range of 0 to 32767,
         inclusive.  If sprop-max-don-diff is not present, the value of
         the parameter is unspecified.  sprop-max-don-diff is
         calculated as follows:

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

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

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

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

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

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

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

         where DON(i) is the decoding order number of the NAL unit
         having index i in the transmission order.  The decoding order
         number is specified in section 5.5 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 unit will be more efficient.  A gateway may
         terminate IP; thus, MTU discovery will normally not work
         beyond the gateway.

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

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

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

      sar-supported:
         This parameter MAY be used to indicate a receiver capability
         and not anything else.  The value of this parameter is an
         integer in the range of 1 to sar-understood, inclusive, equal
         to 255.  The value of sar-supported equal to N smaller than
         255 indicates that the reciever supports all the SARs
         corresponding to H.264 aspect_ratio_idc values (see Table E-1
         of [1]) in the range from 1 to N, inclusive, without geometric
         distortion.  The value of sar-supported equal to 255 indicates
         that the receiver supports all sample aspect ratios which are
         expressible using two 16-bit integer values as the numerator
         and denominator, i.e., those that are expressible using the
         H.264 aspect_ratio_idc value of 255 (Extended_SAR, see Table
         E-1 of [1]), without geometric distortion.

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

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

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

      Security considerations:
         See section 9 of RFC xxxx.

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

      Additional information:
         None

      File extensions:     none

      Macintosh file type code: none

      Object identifier or OID: none
      Person & email address to contact for further information:
         Ye-Kui Wang, yekuiwang@huawei.com

      Intended usage:      COMMON

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

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

8.2. SDP Parameters

8.2.1. Mapping of Payload Type Parameters to SDP

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

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

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

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

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

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

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

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

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

8.2.2. Usage with the SDP Offer/Answer Model

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

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

         Informative note: The requirement for symmetric use applies
         only for the above media format configuration parameters
         excluding the level part of "profile-level-id", and not for
         the other stream properties and capability parameters.

         Informative note: In H.264 [1], all the levels except for
         level 1b are equal to the value of level_idc divided by 10.
         Level 1b is a level higher than level 1.0 but lower than level
         1.1, and is signaled in an ad-hoc manner, due to that the
         level was specified after level 1.0 and level 1.1.  For the
         Baseline, Main and Extended profiles (with profile_idc equal
         to 66, 77 and 88, respectively), level 1b is indicated by
         level_idc equal to 11 (i.e. same as level 1.1) and
         constraint_set3_flag equal to 1.  For other profiles, level 1b
         is indicated by level_idc equal to 9 (but note that level 1b
         for these profiles are still higher than level 1, which has
         level_idc equal to 10, and lower than level 1.1).  In SDP
         Offer/Answer, an answer to an offer may indicate a level equal
         to or lower than the level indicated in the offer.  Due to the
         ad-hoc indication of level 1b, offerers and answerers must
         check the value of bit 4 (constraint_set3_flag) of the middle
         octet of the parameter "profile-level-id", when profile_idc is
         equal to 66, 77 or 88 and level_idc is equal to 11.

      To simplify handling and matching of these configurations, the
      same RTP payload type number used in the offer SHOULD also be
      used in the answer, as specified in [8].  An answer MUST NOT
      contain a payload type number used in the offer unless the
      configuration is exactly the same as in the offer or the
      configuration in the answer only differs from that in the offer
      with a level lower than the default level offered.

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

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

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

   o  The capability parameters ("max-mbps", "max-smbps", "max-fs",
      "max-cpb", "max-dpb", "max-br", ,"redundant-pic-cap", "max-rcmd-
      nalu-size", "sar-understood", "sar-supported") MAY be used to
      declare further capabilities of the offerer or answerer for
      receiving.  These parameters can only be present when the
      direction attribute is sendrecv or recvonly, and the parameters
      describe the limitations of what the offerer or answerer accepts
      for receiving streams.

   o  An offerer has to include the size of the de-interleaving buffer,
      "sprop-deint-buf-req", in the offer for an interleaved H.264
      stream.  To enable the offerer and answerer to inform each other
      about their capabilities for de-interleaving buffering in
      receiving streams, both parties are RECOMMENDED to include "deint-
      buf-cap".  For interleaved streams, it is also RECOMMENDED to
      consider offering multiple payload types with different buffering
      requirements when the capabilities of the receiver are unknown.

   o  The "sprop-parameter-sets" or "sprop-level-parameter-sets"
      parameter, when present (included in the "a=fmtp" line of SDP or
      conveyed using the "fmtp" source attribute as specified in section
      6.3 of [9]), is used for out-of-band transport of parameter sets.
      However, when out-of-band transport of parameter sets is used,
      parameter sets MAY still be additionally transported in-band.  If
      neither "sprop-parameter-sets" nor "sprop-level-parameter-sets" is
      present, then only in-band transport of parameter sets is used.

      An offer MAY include either or both of "sprop-parameter-sets" and
      "sprop-level-parameter-sets".  An answer MAY include "sprop-
      parameter-sets", and MUST NOT include "sprop-level-parameter-
      sets".

      If the answer includes "in-band-parameter-sets" equal to 1, then
      the sender MUST transmit parameter sets in-band.

      Otherwise, the following applies.

        o When an offered payload type is accepted without level
           downgrade, i.e. the default level is accepted, the following
           applies.

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

             o When there is a "sprop-parameter-sets" conveyed using
                the "fmtp" source attribute as specified in section 6.3
                of [9], and the answerer understands the "fmtp" source
                attribute, it MUST be prepared to use the parameter
                sets included in "sprop-parameter-sets" for decoding
                the incoming NAL unit stream, and it MUST include
                either "use-level-src-parameter-sets" equal to 1 or the
                "fmtp" source attribute in the answer.

             o When there is a "sprop-parameter-sets" conveyed using
                the "fmtp" source attribute as specified in section 6.3
                of [9], and the answerer does not understand the "fmtp"
                source attribute, the sender MUST transmit parameter
                sets in-band, and the answerer MUST NOT include "use-level-
               src-parameter-sets" "use-
                level-src-parameter-sets" equal to 1 or the "fmtp"
                source attribute in the answer.

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

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

        o When level downgrade is in use, i.e., a level lower than the
           default level offered is accepted, the following applies.

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

             o When "use-level-src-parameter-sets" equal to 1 and the
                "fmtp" source attribute are not present in the answer
                for the accepted payload type, the answerer MUST ignore
                "sprop-level-parameter-sets", when present, and the
                sender MUST transmit parameter sets in-band.

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

             o When no parameter sets for the accepted level are
                present in the "sprop-level-parameter-sets", the sender
                MUST transmit parameter sets in-band.

      The answerer MAY or MAY not include "sprop-parameter-sets", i.e.,
      the answerer MAY use either out-of-band or in-band transport of
      parameter sets for the stream it is sending, regardless of
      whether out-of-band parameter sets transport has been used in the
      offerer-to-answerer direction.  When the offer includes "in-band-
      parameter-sets" equal to 1, the answerer MUST not include "sprop-
      parameter-sets" and MUST transmit parameter sets in-band.  All
      parameter sets included in the "sprop-parameter-sets", when
      present, for the accepted payload type in an answer MUST be
      associated with the accepted level, as indicated by the profile-
      level-id in the answer for the accepted payload type.

      Parameter sets included in "sprop-parameter-sets" in an answer
      are independent of those parameter sets included in the offer, as
      they are used for decoding two different video streams, one from
      the answerer to the offerer, and the other in the opposite
      direction.  The offerer MUST be prepared to use the parameter
      sets included in the answer's "sprop-parameter-sets", when
      present, for decoding the incoming NAL unit stream.

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

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

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

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

      To simplify handling and matching of these configurations, the
      same RTP payload type number used in the offer SHOULD also be
      used in the answer, as specified in [8].  An answer MUST NOT
      contain a payload type number used in the offer unless the
      configuration is the same as in the offer.

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

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

   Table 6 lists the interpretation of all the 20 media type parameters
   that MUST be used for the different direction attributes.

       Table 6. Interpretation of parameters for different direction
                                attributes.

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

             Legend:

             C: configuration for sending and receiving streams
             P: properties of the stream to be sent
             R: receiver capabilities
             S: out-of-band parameter sets
             -: not usable, when present SHOULD be ignored

   Parameters used for declaring receiver capabilities are in general
   downgradable; i.e., they express the upper limit for a sender's
   possible behavior.  Thus a sender MAY select to set its encoder using
   only lower/less or equal values of these parameters.

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

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

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

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

   If an offerer wishes to have non-symmetric capabilities between
   sending and receiving, the offerer should 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 both stream properties and
      receiver capabilities are used to indicate only stream properties.
      For example, in this case, the parameter "profile-level-id"
      declares only the values used by the stream, not the capabilities
      for receiving streams.  This results in that the following
      interpretation of the parameters MUST be used:

      Declaring actual configuration or stream properties:

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

      Out-of-band transporting of parameter sets:

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

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

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

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

8.3. Examples

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

      Offerer -> Answerer SDP message:

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

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

      Answerer -> Offerer SDP message:

      m=video 49170 RTP/AVP 100 99 97
      a=rtpmap:97 H264/90000
      a=fmtp:97 profile-level-id=42A01E; packetization-mode=0;
        sprop-parameter-sets=<parameter sets data#3>
      a=rtpmap:99 H264/90000
      a=fmtp:99 profile-level-id=42A01E; packetization-mode=1;
        sprop-parameter-sets=<parameter sets data#4>;
        max-rcmd-nalu-size=3980
      a=rtpmap:100 H264/90000
      a=fmtp:100 profile-level-id=42A01E; packetization-mode=2;
        sprop-parameter-sets=<parameter sets data#5>;
        sprop-interleaving-depth=60;
        sprop-deint-buf-req=86000; sprop-init-buf-time=156320;
        deint-buf-cap=128000; max-rcmd-nalu-size=3980

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

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

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

      Offer SDP:

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

      Answer SDP:

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

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

      Offer SDP:

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

      Answer SDP:

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

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

      Offer SDP:

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

      Answer SDP:

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

   In the following example, the offer is accepted without level
   downgrading, and "sprop-parameter-sets" is present in the offer.
   Parameter sets in sprop-parameter-sets=<parameter sets data#0> must
   be stored and used used by the encoder of the offerer and the decoder
   of the answerer, and parameter sets in sprop-parameter-
   sets=<parameter sets data#1>must be used by the encoder of the
   answerer and the decoder of the offerer.  Note that sprop-parameter-
   sets=<parameter sets data#0> is basically independent of sprop-
   parameter-sets=<parameter sets data#1>.

      Offer SDP:

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

      Answer SDP:

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

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

      Offer SDP:

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

      Answer SDP:

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

   In the following example, the offer is accepted with level
   downgrading and "sprop-parameter-sets" is present in the offer.  As
   sprop-parameter-sets=<parameter sets data#0> contains level_idc
   indicating Level 3.0, therefore cannot be used as the answerer wants
   Level 2.0 and must be ignored by the answerer, and in-band parameter
   sets must be used.

      Offer SDP:

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

      Answer SDP:

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

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

      Offer SDP:

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

      Answer SDP:

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

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

      Offer SDP:

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

      Answer SDP:

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

8.4. Parameter Set Considerations

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

   Parameter set NALUs can be transported using three different
   principles:

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

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

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

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

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

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

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

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

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

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

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

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

   In some scenarios (e.g., when only the subset of this payload format
   specification corresponding to H.241 is used) or topologies, it is
   not possible to employ out-of-band parameter set transmission.  In
   this case, parameter sets have to be transmitted in-band.  Here, the
   synchronization with the non-parameter-set-data in the bitstream is
   implicit, but the possibility of a loss has to be taken into account.
   The loss probability should be reduced using the mechanisms discussed
   above.  In case a loss of a parameter set is detected, recovery may
   be achieved by using a Decoder Refresh Point procedure, for example,
   using RTCP feedback Full Intra Request (FIR) [30].  Two example
   Decoder Refresh Point procedures are provided in the informative
   Section 8.5.

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

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

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

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

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

   The encoder shall, in the order presented here:

   1) Immediately prepare to send an IDR picture.

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

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

   4) Send the IDR picture.

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

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

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

   The encoder shall, in the order presented here:

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

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

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

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

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

9. Security Considerations

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

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

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

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

10. Congestion Control

   Congestion control for RTP SHALL be used in accordance with RFC 3550
   [5], and with any applicable RTP profile; e.g., RFC 3551 [16].  An
   additional requirement if best-effort service is being used is: users
   of this payload format MUST monitor packet loss to ensure that the
   packet loss rate is within acceptable parameters.  Packet loss is
   considered acceptable if a TCP flow across the same network path, and
   experiencing the same network conditions, would achieve an average
   throughput, measured on a reasonable timescale that is not less than
   the RTP flow is achieving.  This condition can be satisfied by
   implementing congestion control mechanisms to adapt the transmission
   rate (or the number of layers subscribed for a layered multicast
   session), or by arranging for a receiver to leave the session if the
   loss rate is unacceptably high.

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

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

11. IANA Consideration

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

12. Informative Appendix: Application Examples

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

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

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

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

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

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

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

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

12.3. Video Telephony, Interleaved Packetization Using NAL Unit
   Aggregation

   This scheme allows better error concealment and is used in H.263
   based designs using RFC 2429 packetization [11].  It has been
   implemented, and good results were reported [13].

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

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

12.4. Video Telephony with Data Partitioning

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   The presented technique makes it possible to achieve good
   transmission error tolerance, even if no additional source coding
   layer redundancy (such as periodic intra frames) is present.
   Consequently, the same coded video sequence can be used to achieve
   the maximum compression efficiency and quality over error-free
   transmission and for transmission over error-prone networks.
   Furthermore, the technique allows the application of FEC to pre-
   encoded sequences without adding delay.  In this case, pre-encoded
   sequences that are not encoded for error-prone networks can still be
   transmitted almost reliably without adding extensive delays.  In
   addition, FUs of equal length result in a bit rate efficient use of
   RFC 5109.

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

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

12.6. Low Bit-Rate Streaming

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

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

   However, loss of a packet including many coded pictures would have
   drastic consequences for visual quality, as there is practically no
   other way to conceal a loss of an entire picture than to repeat the
   previous one.  One way to construct relatively large packets and
   maintain possibilities for successful loss concealment is to
   construct MTAPs that contain interleaved slices from several pictures.
   An MTAP should not contain spatially adjacent slices from the same
   picture or spatially overlapping slices from any picture.  If a
   packet is lost, it is likely that a lost slice is surrounded by
   spatially adjacent slices of the same picture and spatially
   corresponding slices of the temporally previous and succeeding
   pictures.  Consequently, concealment of the lost slice is likely to
   be relatively successful.

12.7. Robust Packet Scheduling in Video Streaming

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

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

13. Informative Appendix: Rationale for Decoding Order Number

13.1. Introduction

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

13.2. Example of Multi-Picture Slice Interleaving

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

      ... R1 N2 R3 N4 R5 ...

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

      ... R1 R3 N2 R5 N4 ...

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

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

      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.

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
   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)).

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

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

              Figure 18  Received pictures in decoding order

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

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

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

                          Figure 19  Output order

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

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

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

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

       Figure 20  Interleaving: Early IDR pictures in sending order
   The OPTIONAL sprop-interleaving-depth media type parameter is set
   equal to 1 according to its definition.  (The value of sprop-
   interleaving-depth in this example can be derived as follows: Picture
   I00 is the only picture preceding picture N58 or N59 in transmission
   order and following it in decoding order.  Except for pictures I00,
   N58, and N59, the transmission order is the same as the decoding
   order of pictures.  As a coded picture is encapsulated into exactly
   one NAL unit, the value of sprop-interleaving-depth is equal to the
   maximum number of pictures preceding any picture in transmission
   order and following the picture in decoding order.)

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

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

                 Figure 21  Interleaving: Receiver buffer

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

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

         Figure 22  Interleaving: Receiver buffer after reordering

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

13.4. Robust Transmission Scheduling of Redundant Coded Slices

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

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

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

13.5. Remarks on Other Design Possibilities

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

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

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

   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

   Stephan Wenger, Miska Hannuksela, Thomas Stockhammer, Magnus
   Westerlund, and David Singer are thanked as the authors of RFC 3984.
   Dave Lindbergh, Philippe Gentric, Gonzalo Camarillo, Gary Sullivan,
   Joerg Ott, and Colin Perkins are thanked for careful review during
   the development of RFC 3984. Randell Jesup, Stephen Botzko, Magnus
   Westerlund, Alex Eleftheriadis, and Thomas Schierl are thanked for
   their valuable comments and inputs during the development of this
   memo.

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

15. References

15.1. Normative References

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

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

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

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

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

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

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

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

   [9]   Lennox, J., Ott, J., and Schierl, T., "Source-Specific Media
         Attributes in the Session Description Protocol", draft-ietf-
         mmusic-sdp-source-attributes-02 (work in progress), October
         2008.

15.2. Informative References

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

   [11]  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.

   [12]  ISO/IEC IS 14496-2.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

16. Authors' Addresses

   Ye-Kui Wang
   Huawei Technologies
   400 Somerset Corporate Blvd
   Bridgewater, NJ 08807
   USA

   Phone: +1-908-393-4758
   EMail: yekuiwang@huawei.com

   Roni Even
   14 David Hamelech
   Tel Aviv 64953
   Israel

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

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

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

17. Backward Compatibility to RFC 3984

   The current document is a revision of RFC 3984 and intends to
   obsolete it.  This section addresses the backward compatibility
   issues.

   The technical changes are listed in section 18.

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

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

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

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

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

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

   Item 7), allowance of conveying sprop-parameter-sets and sprop-level-
   parameter-sets using the "fmtp" source attribute as specified in
   section 6.3 of [9], is similar as item 4).  It does not incur any
   backward compatibility issues for SDP Offer/Answer based applications,
   as legacy RFC 3984 receivers ignore the "fmtp" source attribute, and
   it is fine for legacy RFC 3984 senders not to use the "fmtp" source
   attribute as it is optional.  However, there is a backward
   compatibility issue for SDP declarative usage based applications, e.g.
   those using RTSP and SAP, because the SDP receiver per RFC 3984
   cannot accept a session for which the SDP includes an unrecognized
   parameter (i.e., the "fmtp" source attribute).  Therefore, the RTSP
   or SAP server may have to prepare two sets of streams, one for legacy
   RFC 3984 receivers and one for receivers according to this memo.

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

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

18. Changes from RFC 3984

   Following is the list of technical changes (including bug fixes) from
   RFC 3984.  Besides this list of technical changes, numerous editorial
   changes have been made, but not documented in this memo.

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

   2) In subsection 7.2.2, changed the sentence

      There are N VCL NAL units in the deinterleaving buffer.

      to

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

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

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

      to

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

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

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

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

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

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

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

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

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

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

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

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

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