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rfc:rfc3984

Network Working Group S. Wenger Request for Comments: 3984 M.M. Hannuksela Category: Standards Track T. Stockhammer

                                                         M. Westerlund
                                                             D. Singer
                                                         February 2005
                 RTP Payload Format for H.264 Video

Status of This Memo

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

Copyright Notice

 Copyright (C) The Internet Society (2005).

Abstract

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

Table of Contents

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

Wenger, et al. Standards Track [Page 1] RFC 3984 RTP Payload Format for H.264 Video February 2005

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

Wenger, et al. Standards Track [Page 2] RFC 3984 RTP Payload Format for H.264 Video February 2005

1. Introduction

1.1. The H.264 Codec

 This memo specifies an RTP payload specification for the video coding
 standard known as ITU-T Recommendation H.264 [1] and ISO/IEC
 International Standard 14496 Part 10 [2] (both also known as Advanced
 Video Coding, or AVC).  Recommendation H.264 was approved by ITU-T on
 May 2003, and the approved draft specification is available for
 public review [8].  In this memo the H.264 acronym is used for the
 codec and the standard, but the memo is equally applicable to the
 ISO/IEC counterpart of the coding standard.
 The H.264 video codec has a very broad application range that covers
 all forms of digital compressed video from, low bit-rate Internet
 streaming applications to HDTV broadcast and Digital Cinema
 applications with nearly lossless coding.  Compared to the current
 state of technology, the overall performance of H.264 is such that
 bit rate savings of 50% or more are reported.  Digital Satellite TV
 quality, for example, was reported to be achievable at 1.5 Mbit/s,
 compared to the current operation point of MPEG 2 video at around 3.5
 Mbit/s [9].
 The codec specification [1] itself distinguishes conceptually between
 a video coding layer (VCL) and a network abstraction layer (NAL).
 The VCL contains the signal processing functionality of the codec;
 mechanisms such as transform, quantization, and motion compensated
 prediction; and a loop filter.  It follows the general concept of
 most of today's video codecs, a macroblock-based coder that uses
 inter picture prediction with motion compensation and transform
 coding of the residual signal.  The VCL encoder outputs slices: a bit
 string that contains the macroblock data of an integer number of
 macroblocks, and the information of the slice header (containing the
 spatial address of the first macroblock in the slice, the initial
 quantization parameter, and similar information).  Macroblocks in
 slices are arranged in scan order unless a different macroblock
 allocation is specified, by using the so-called Flexible Macroblock
 Ordering syntax.  In-picture prediction is used only within a slice.
 More information is provided in [9].
 The Network Abstraction Layer (NAL) encoder encapsulates the slice
 output of the VCL encoder into Network Abstraction Layer Units (NAL
 units), which are suitable for transmission over packet networks or
 use in packet oriented multiplex environments.  Annex B of H.264
 defines an encapsulation process to transmit such NAL units over
 byte-stream oriented networks.  In the scope of this memo, Annex B is
 not relevant.

Wenger, et al. Standards Track [Page 3] RFC 3984 RTP Payload Format for H.264 Video February 2005

 Internally, the NAL uses NAL units.  A NAL unit consists of a one-
 byte header and the payload byte string.  The header indicates the
 type of the NAL unit, the (potential) presence of bit errors or
 syntax violations in the NAL unit payload, and information regarding
 the relative importance of the NAL unit for the decoding process.
 This RTP payload specification is designed to be unaware of the bit
 string in the NAL unit payload.
 One of the main properties of H.264 is the complete decoupling of the
 transmission time, the decoding time, and the sampling or
 presentation time of slices and pictures.  The decoding process
 specified in H.264 is unaware of time, and the H.264 syntax does not
 carry information such as the number of skipped frames (as is common
 in the form of the Temporal Reference in earlier video compression
 standards).  Also, there are NAL units that affect many pictures and
 that are, therefore, inherently timeless.  For this reason, the
 handling of the RTP timestamp requires some special considerations
 for NAL units for which the sampling or presentation time is not
 defined or, at transmission time, unknown.

1.2. Parameter Set Concept

 One very fundamental design concept of H.264 is to generate self-
 contained packets, to make mechanisms such as the header duplication
 of RFC 2429 [10] or MPEG-4's Header Extension Code (HEC) [11]
 unnecessary.  This was achieved by decoupling information relevant to
 more than one slice from the media stream.  This higher layer meta
 information should be sent reliably, asynchronously, and in advance
 from the RTP packet stream that contains the slice packets.
 (Provisions for sending this information in-band are also available
 for applications that do not have an out-of-band transport channel
 appropriate for the purpose.)  The combination of the higher-level
 parameters is called a parameter set.  The H.264 specification
 includes two types of parameter sets: sequence parameter set and
 picture parameter set.  An active sequence parameter set remains
 unchanged throughout a coded video sequence, and an active picture
 parameter set remains unchanged within a coded picture.  The sequence
 and picture parameter set structures contain information such as
 picture size, optional coding modes employed, and macroblock to slice
 group map.
 To be able to change picture parameters (such as the picture size)
 without having to transmit parameter set updates synchronously to the
 slice packet stream, the encoder and decoder can maintain a list of
 more than one sequence and picture parameter set.  Each slice header
 contains a codeword that indicates the sequence and picture parameter
 set to be used.

Wenger, et al. Standards Track [Page 4] RFC 3984 RTP Payload Format for H.264 Video February 2005

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

1.3. Network Abstraction Layer Unit Types

 Tutorial information on the NAL design can be found in [12], [13],
 and [14].
 All NAL units consist of a single NAL unit type octet, which also
 co-serves as the payload header of this RTP payload format.  The
 payload of a NAL unit follows immediately.
 The syntax and semantics of the NAL unit type octet are specified in
 [1], but the essential properties of the NAL unit type octet are
 summarized below.  The NAL unit type octet has the following format:
    +---------------+
    |0|1|2|3|4|5|6|7|
    +-+-+-+-+-+-+-+-+
    |F|NRI|  Type   |
    +---------------+
 The semantics of the components of the NAL unit type octet, as
 specified in the H.264 specification, are described briefly below.
 F: 1 bit
    forbidden_zero_bit.  The H.264 specification declares a value of
    1 as a syntax violation.
 NRI: 2 bits
    nal_ref_idc.  A value of 00 indicates that the content of the NAL
    unit is not used to reconstruct reference pictures for inter
    picture prediction.  Such NAL units can be discarded without
    risking the integrity of the reference pictures.  Values greater
    than 00 indicate that the decoding of the NAL unit is required to
    maintain the integrity of the reference pictures.
 Type: 5 bits
    nal_unit_type.  This component specifies the NAL unit payload type
    as defined in table 7-1 of [1], and later within this memo.  For a
    reference of all currently defined NAL unit types and their
    semantics, please refer to section 7.4.1 in [1].

Wenger, et al. Standards Track [Page 5] RFC 3984 RTP Payload Format for H.264 Video February 2005

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

2. Conventions

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in BCP 14, RFC 2119 [3].
 This specification uses the notion of setting and clearing a bit when
 bit fields are handled.  Setting a bit is the same as assigning that
 bit the value of 1 (On).  Clearing a bit is the same as assigning
 that bit the value of 0 (Off).

3. Scope

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

4. Definitions and Abbreviations

4.1. Definitions

 This document uses the definitions of [1].  The following terms,
 defined in [1], are summed up for convenience:
    access unit: A set of NAL units always containing a primary coded
    picture.  In addition to the primary coded picture, an access unit
    may also contain one or more redundant coded pictures or other NAL
    units not containing slices or slice data partitions of a coded
    picture.  The decoding of an access unit always results in a
    decoded picture.
    coded video sequence: A sequence of access units that consists, in
    decoding order, of an instantaneous decoding refresh (IDR) access
    unit followed by zero or more non-IDR access units including all
    subsequent access units up to but not including any subsequent IDR
    access unit.
    IDR access unit: An access unit in which the primary coded picture
    is an IDR picture.

Wenger, et al. Standards Track [Page 6] RFC 3984 RTP Payload Format for H.264 Video February 2005

    IDR picture: A coded picture containing only slices with I or SI
    slice types that causes a "reset" in the decoding process.  After
    the decoding of an IDR picture, all following coded pictures in
    decoding order can be decoded without inter prediction from any
    picture decoded prior to the IDR picture.
    primary coded picture: The coded representation of a picture to be
    used by the decoding process for a bitstream conforming to H.264.
    The primary coded picture contains all macroblocks of the picture.
    redundant coded picture: A coded representation of a picture or a
    part of a picture.  The content of a redundant coded picture shall
    not be used by the decoding process for a bitstream conforming to
    H.264.  The content of a redundant coded picture may be used by
    the decoding process for a bitstream that contains errors or
    losses.
    VCL NAL unit: A collective term used to refer to coded slice and
    coded data partition NAL units.
 In addition, the following definitions apply:
    decoding order number (DON): A field in the payload structure, or
    a derived variable indicating NAL unit decoding order.  Values of
    DON are in the range of 0 to 65535, inclusive.  After reaching the
    maximum value, the value of DON wraps around to 0.
    NAL unit decoding order: A NAL unit order that conforms to the
    constraints on NAL unit order given in section 7.4.1.2 in [1].
    transmission order: The order of packets in ascending RTP sequence
    number order (in modulo arithmetic).  Within an aggregation
    packet, the NAL unit transmission order is the same as the order
    of appearance of NAL units in the packet.
    media aware network element (MANE): A network element, such as a
    middlebox or application layer gateway that is capable of parsing
    certain aspects of the RTP payload headers or the RTP payload and
    reacting to the contents.
       Informative note: The concept of a MANE goes beyond normal
       routers or gateways in that a MANE has to be aware of the
       signaling (e.g., to learn about the payload type mappings of
       the media streams), and in that it has to be trusted when
       working with SRTP.  The advantage of using MANEs is that they
       allow packets to be dropped according to the needs of the media
       coding.  For example, if a MANE has to drop packets due to
       congestion on a certain link, it can identify those packets

Wenger, et al. Standards Track [Page 7] RFC 3984 RTP Payload Format for H.264 Video February 2005

       whose dropping has the smallest negative impact on the user
       experience and remove them in order to remove the congestion
       and/or keep the delay low.
 Abbreviations
    DON:        Decoding Order Number
    DONB:       Decoding Order Number Base
    DOND:       Decoding Order Number Difference
    FEC:        Forward Error Correction
    FU:         Fragmentation Unit
    IDR:        Instantaneous Decoding Refresh
    IEC:        International Electrotechnical Commission
    ISO:        International Organization for Standardization
    ITU-T:      International Telecommunication Union,
                Telecommunication Standardization Sector
    MANE:       Media Aware Network Element
    MTAP:       Multi-Time Aggregation Packet
    MTAP16:     MTAP with 16-bit timestamp offset
    MTAP24:     MTAP with 24-bit timestamp offset
    NAL:        Network Abstraction Layer
    NALU:       NAL Unit
    SEI:        Supplemental Enhancement Information
    STAP:       Single-Time Aggregation Packet
    STAP-A:     STAP type A
    STAP-B:     STAP type B
    TS:         Timestamp
    VCL:        Video Coding Layer

5. RTP Payload Format

5.1. RTP Header Usage

 The format of the RTP header is specified in RFC 3550 [4] and
 reprinted in Figure 1 for convenience.  This payload format uses the
 fields of the header in a manner consistent with that specification.
 When one NAL unit is encapsulated per RTP packet, the RECOMMENDED RTP
 payload format is specified in section 5.6.  The RTP payload (and the
 settings for some RTP header bits) for aggregation packets and
 fragmentation units are specified in sections 5.7 and 5.8,
 respectively.

Wenger, et al. Standards Track [Page 8] RFC 3984 RTP Payload Format for H.264 Video February 2005

     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.

Wenger, et al. Standards Track [Page 9] RFC 3984 RTP Payload Format for H.264 Video February 2005

    If the NAL unit has no timing properties of its own (e.g.,
    parameter set and SEI NAL units), the RTP timestamp is set to the
    RTP timestamp of the primary coded picture of the access unit in
    which the NAL unit is included, according to section 7.4.1.2 of
    [1].
    The setting of the RTP Timestamp for MTAPs is defined in section
    5.7.2.
    Receivers SHOULD ignore any picture timing SEI messages included
    in access units that have only one display timestamp.  Instead,
    receivers SHOULD use the RTP timestamp for synchronizing the
    display process.
    RTP senders SHOULD NOT transmit picture timing SEI messages for
    pictures that are not supposed to be displayed as multiple fields.
    If one access unit has more than one display timestamp carried in
    a picture timing SEI message, then the information in the SEI
    message SHOULD be treated as relative to the RTP timestamp, with
    the earliest event occurring at the time given by the RTP
    timestamp, and subsequent events later, as given by the difference
    in SEI message picture timing values.  Let tSEI1, tSEI2, ...,
    tSEIn be the display timestamps carried in the SEI message of an
    access unit, where tSEI1 is the earliest of all such timestamps.
    Let tmadjst() be a function that adjusts the SEI messages time
    scale to a 90-kHz time scale.  Let TS be the RTP timestamp.  Then,
    the display time for the event associated with tSEI1 is TS.  The
    display time for the event with tSEIx, where x is [2..n] is TS +
    tmadjst (tSEIx - tSEI1).
       Informative note: Displaying coded frames as fields is needed
       commonly in an operation known as 3:2 pulldown, in which film
       content that consists of coded frames is displayed on a display
       using interlaced scanning.  The picture timing SEI message
       enables carriage of multiple timestamps for the same coded
       picture, and therefore the 3:2 pulldown process is perfectly
       controlled.  The picture timing SEI message mechanism is
       necessary because only one timestamp per coded frame can be
       conveyed in the RTP timestamp.
       Informative note: Because H.264 allows the decoding order to be
       different from the display order, values of RTP timestamps may
       not be monotonically non-decreasing as a function of RTP
       sequence numbers.  Furthermore, the value for interarrival
       jitter reported in the RTCP reports may not be a trustworthy
       indication of the network performance, as the calculation rules

Wenger, et al. Standards Track [Page 10] RFC 3984 RTP Payload Format for H.264 Video February 2005

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

5.2. Common Structure of the RTP Payload Format

 The payload format defines three different basic payload structures.
 A receiver can identify the payload structure by the first byte of
 the RTP payload, which co-serves as the RTP payload header and, in
 some cases, as the first byte of the payload.  This byte is always
 structured as a NAL unit header.  The NAL unit type field indicates
 which structure is present.  The possible structures are as follows:
 Single NAL Unit Packet: Contains only a single NAL unit in the
 payload.  The NAL header type field will be equal to the original NAL
 unit type; i.e., in the range of 1 to 23, inclusive.  Specified in
 section 5.6.
 Aggregation packet: Packet type used to aggregate multiple NAL units
 into a single RTP payload.  This packet exists in four versions, the
 Single-Time Aggregation Packet type A (STAP-A), the Single-Time
 Aggregation Packet type B (STAP-B), Multi-Time Aggregation Packet
 (MTAP) with 16-bit offset (MTAP16), and Multi-Time Aggregation Packet
 (MTAP) with 24-bit offset (MTAP24).  The NAL unit type numbers
 assigned for STAP-A, STAP-B, MTAP16, and MTAP24 are 24, 25, 26, and
 27, respectively.  Specified in section 5.7.
 Fragmentation unit: Used to fragment a single NAL unit over multiple
 RTP packets.  Exists with two versions, FU-A and FU-B, identified
 with the NAL unit type numbers 28 and 29, respectively.  Specified in
 section 5.8.
 Table 1.  Summary of NAL unit types and their payload structures
    Type   Packet    Type name                        Section
    ---------------------------------------------------------
    0      undefined                                    -
    1-23   NAL unit  Single NAL unit packet per H.264   5.6
    24     STAP-A    Single-time aggregation packet     5.7.1
    25     STAP-B    Single-time aggregation packet     5.7.1
    26     MTAP16    Multi-time aggregation packet      5.7.2
    27     MTAP24    Multi-time aggregation packet      5.7.2
    28     FU-A      Fragmentation unit                 5.8
    29     FU-B      Fragmentation unit                 5.8
    30-31  undefined                                    -

Wenger, et al. Standards Track [Page 11] RFC 3984 RTP Payload Format for H.264 Video February 2005

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

5.3. NAL Unit Octet Usage

 The structure and semantics of the NAL unit octet were introduced in
 section 1.3.  For convenience, the format of the NAL unit type octet
 is reprinted below:
    +---------------+
    |0|1|2|3|4|5|6|7|
    +-+-+-+-+-+-+-+-+
    |F|NRI|  Type   |
    +---------------+
 This section specifies the semantics of F and NRI according to this
 specification.
 F: 1 bit
    forbidden_zero_bit.  A value of 0 indicates that the NAL unit type
    octet and payload should not contain bit errors or other syntax
    violations.  A value of 1 indicates that the NAL unit type octet
    and payload may contain bit errors or other syntax violations.
    MANEs SHOULD set the F bit to indicate detected bit errors in the
    NAL unit.  The H.264 specification requires that the F bit is
    equal to 0.  When the F bit is set, the decoder is advised that
    bit errors or any other syntax violations may be present in the
    payload or in the NAL unit type octet.  The simplest decoder
    reaction to a NAL unit in which the F bit is equal to 1 is to
    discard such a NAL unit and to conceal the lost data in the
    discarded NAL unit.
 NRI: 2 bits
    nal_ref_idc.  The semantics of value 00 and a non-zero value
    remain unchanged from the H.264 specification.  In other words, a
    value of 00 indicates that the content of the NAL unit is not used
    to reconstruct reference pictures for inter picture prediction.
    Such NAL units can be discarded without risking the integrity of
    the reference pictures.  Values greater than 00 indicate that the
    decoding of the NAL unit is required to maintain the integrity of
    the reference pictures.
    In addition to the specification above, according to this RTP
    payload specification, values of NRI greater than 00 indicate the
    relative transport priority, as determined by the encoder.  MANEs

Wenger, et al. Standards Track [Page 12] RFC 3984 RTP Payload Format for H.264 Video February 2005

    can use this information to protect more important NAL units
    better than they do less important NAL units.  The highest
    transport priority is 11, followed by 10, and then by 01; finally,
    00 is the lowest.
       Informative note: Any non-zero value of NRI is handled
       identically in H.264 decoders.  Therefore, receivers need not
       manipulate the value of NRI when passing NAL units to the
       decoder.
    An H.264 encoder MUST set the value of NRI according to the H.264
    specification (subclause 7.4.1) when the value of nal_unit_type is
    in the range of 1 to 12, inclusive.  In particular, the H.264
    specification requires that the value of NRI SHALL be equal to 0
    for all NAL units having nal_unit_type equal to 6, 9, 10, 11, or
    12.
    For NAL units having nal_unit_type equal to 7 or 8 (indicating a
    sequence parameter set or a picture parameter set, respectively),
    an H.264 encoder SHOULD set the value of NRI to 11 (in binary
    format).  For coded slice NAL units of a primary coded picture
    having nal_unit_type equal to 5 (indicating a coded slice
    belonging to an IDR picture), an H.264 encoder SHOULD set the
    value of NRI to 11 (in binary format).
    For a mapping of the remaining nal_unit_types to NRI values, the
    following example MAY be used and has been shown to be efficient
    in a certain environment [13].  Other mappings MAY also be
    desirable, depending on the application and the H.264/AVC Annex A
    profile in use.
       Informative note: Data Partitioning is not available in certain
       profiles; e.g., in the Main or Baseline profiles.
       Consequently, the nal unit types 2, 3, and 4 can occur only if
       the video bitstream conforms to a profile in which data
       partitioning is allowed and not in streams that conform to the
       Main or Baseline profiles.
    Table 2.  Example of NRI values for coded slices and coded slice
    data partitions of primary coded reference pictures
    NAL Unit Type     Content of NAL unit              NRI (binary)
    ----------------------------------------------------------------
     1              non-IDR coded slice                         10
     2              Coded slice data partition A                10
     3              Coded slice data partition B                01
     4              Coded slice data partition C                01

Wenger, et al. Standards Track [Page 13] RFC 3984 RTP Payload Format for H.264 Video February 2005

       Informative note: As mentioned before, the NRI value of non-
       reference pictures is 00 as mandated by H.264/AVC.
    An H.264 encoder SHOULD set the value of NRI for coded slice and
    coded slice data partition NAL units of redundant coded reference
    pictures equal to 01 (in binary format).
    Definitions of the values for NRI for NAL unit types 24 to 29,
    inclusive, are given in sections 5.7 and 5.8 of this memo.
    No recommendation for the value of NRI is given for NAL units
    having nal_unit_type in the range of 13 to 23, inclusive, because
    these values are reserved for ITU-T and ISO/IEC.  No
    recommendation for the value of NRI is given for NAL units having
    nal_unit_type equal to 0 or in the range of 30 to 31, inclusive,
    as the semantics of these values are not specified in this memo.

5.4. Packetization Modes

 This memo specifies three cases of packetization modes:
    o Single NAL unit mode
    o Non-interleaved mode
    o Interleaved mode
 The single NAL unit mode is targeted for conversational systems that
 comply with ITU-T Recommendation H.241 [15] (see section 12.1).  The
 non-interleaved mode is targeted for conversational systems that may
 not comply with ITU-T Recommendation H.241.  In the non-interleaved
 mode, NAL units are transmitted in NAL unit decoding order.  The
 interleaved mode is targeted for systems that do not require very low
 end-to-end latency.  The interleaved mode allows transmission of NAL
 units out of NAL unit decoding order.
 The packetization mode in use MAY be signaled by the value of the
 OPTIONAL packetization-mode MIME parameter or by external means.  The
 used packetization mode governs which NAL unit types are allowed in
 RTP payloads.  Table 3 summarizes the allowed NAL unit types for each
 packetization mode.  Some NAL unit type values (indicated as
 undefined in Table 3) are reserved for future extensions.  NAL units
 of those types SHOULD NOT be sent by a sender and MUST be ignored by
 a receiver.  For example, the Types 1-23, with the associated packet
 type "NAL unit", are allowed in "Single NAL Unit Mode" and in "Non-
 Interleaved Mode", but disallowed in "Interleaved Mode".
 Packetization modes are explained in more detail in section 6.

Wenger, et al. Standards Track [Page 14] RFC 3984 RTP Payload Format for H.264 Video February 2005

 Table 3.  Summary of allowed NAL unit types for each packetization
 mode (yes = allowed, no = disallowed, ig = ignore)
    Type   Packet    Single NAL    Non-Interleaved    Interleaved
                     Unit Mode           Mode             Mode
    -------------------------------------------------------------
    0      undefined     ig               ig               ig
    1-23   NAL unit     yes              yes               no
    24     STAP-A        no              yes               no
    25     STAP-B        no               no              yes
    26     MTAP16        no               no              yes
    27     MTAP24        no               no              yes
    28     FU-A          no              yes              yes
    29     FU-B          no               no              yes
    30-31  undefined     ig               ig               ig

5.5. Decoding Order Number (DON)

 In the interleaved packetization mode, the transmission order of NAL
 units is allowed to differ from the decoding order of the NAL units.
 Decoding order number (DON) is a field in the payload structure or a
 derived variable that indicates the NAL unit decoding order.
 Rationale and examples of use cases for transmission out of decoding
 order and for the use of DON are given in section 13.
 The coupling of transmission and decoding order is controlled by the
 OPTIONAL sprop-interleaving-depth MIME parameter as follows.  When
 the value of the OPTIONAL sprop-interleaving-depth MIME parameter is
 equal to 0 (explicitly or per default) or transmission of NAL units
 out of their decoding order is disallowed by external means, the
 transmission order of NAL units MUST conform to the NAL unit decoding
 order.  When the value of the OPTIONAL sprop-interleaving-depth MIME
 parameter is greater than 0 or transmission of NAL units out of their
 decoding order is allowed by external means,
 o  the order of NAL units in an MTAP16 and an MTAP24 is NOT REQUIRED
    to be the NAL unit decoding order, and
 o  the order of NAL units generated by decapsulating STAP-Bs, MTAPs,
    and FUs in two consecutive packets is NOT REQUIRED to be the NAL
    unit decoding order.
 The RTP payload structures for a single NAL unit packet, an STAP-A,
 and an FU-A do not include DON.  STAP-B and FU-B structures include
 DON, and the structure of MTAPs enables derivation of DON as
 specified in section 5.7.2.

Wenger, et al. Standards Track [Page 15] RFC 3984 RTP Payload Format for H.264 Video February 2005

    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,

Wenger, et al. Standards Track [Page 16] RFC 3984 RTP Payload Format for H.264 Video February 2005

 then the NAL unit decoding order of the two NAL units can be in
 either order.  A negative value of don_diff(m,n) indicates that the
 NAL unit having transmission order index n precedes, in decoding
 order, the NAL unit having transmission order index m.
 Values of DON related fields (DON, DONB, and DOND; see section 5.7)
 MUST be such that the decoding order determined by the values of DON,
 as specified above, conforms to the NAL unit decoding order.  If the
 order of two NAL units in NAL unit decoding order is switched and the
 new order does not conform to the NAL unit decoding order, the NAL
 units MUST NOT have the same value of DON.  If the order of two
 consecutive NAL units in the NAL unit stream is switched and the new
 order still conforms to the NAL unit decoding order, the NAL units
 MAY have the same value of DON.  For example, when arbitrary slice
 order is allowed by the video coding profile in use, all the coded
 slice NAL units of a coded picture are allowed to have the same value
 of DON.  Consequently, NAL units having the same value of DON can be
 decoded in any order, and two NAL units having a different value of
 DON should be passed to the decoder in the order specified above.
 When two consecutive NAL units in the NAL unit decoding order have a
 different value of DON, the value of DON for the second NAL unit in
 decoding order SHOULD be the value of DON for the first, incremented
 by one.
 An example of the decapsulation process to recover the NAL unit
 decoding order is given in section 7.
    Informative note: Receivers should not expect that the absolute
    difference of values of DON for two consecutive NAL units in the
    NAL unit decoding order will be equal to one, even in error-free
    transmission.  An increment by one is not required, as at the time
    of associating values of DON to NAL units, it may not be known
    whether all NAL units are delivered to the receiver.  For example,
    a gateway may not forward coded slice NAL units of non-reference
    pictures or SEI NAL units when there is a shortage of bit rate in
    the network to which the packets are forwarded.  In another
    example, a live broadcast is interrupted by pre-encoded content,
    such as commercials, from time to time.  The first intra picture
    of a pre-encoded clip is transmitted in advance to ensure that it
    is readily available in the receiver.  When transmitting the first
    intra picture, the originator does not exactly know how many NAL
    units will be encoded before the first intra picture of the pre-
    encoded clip follows in decoding order.  Thus, the values of DON
    for the NAL units of the first intra picture of the pre-encoded
    clip have to be estimated when they are transmitted, and gaps in
    values of DON may occur.

Wenger, et al. Standards Track [Page 17] RFC 3984 RTP Payload Format for H.264 Video February 2005

5.6. Single NAL Unit Packet

 The single NAL unit packet defined here MUST contain only one NAL
 unit, of the types defined in [1].  This means that neither an
 aggregation packet nor a fragmentation unit can be used within a
 single NAL unit packet.  A NAL unit stream composed by decapsulating
 single NAL unit packets in RTP sequence number order MUST conform to
 the NAL unit decoding order.  The structure of the single NAL unit
 packet is shown in Figure 2.
    Informative note: The first byte of a NAL unit co-serves as the
    RTP payload header.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |F|NRI|  type   |                                               |
    +-+-+-+-+-+-+-+-+                                               |
    |                                                               |
    |               Bytes 2..n of a Single NAL unit                 |
    |                                                               |
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :...OPTIONAL RTP padding        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Figure 2.  RTP payload format for single NAL unit packet

5.7. Aggregation Packets

 Aggregation packets are the NAL unit aggregation scheme of this
 payload specification.  The scheme is introduced to reflect the
 dramatically different MTU sizes of two key target networks:
 wireline IP networks (with an MTU size that is often limited by the
 Ethernet MTU size; roughly 1500 bytes), and IP or non-IP (e.g., ITU-T
 H.324/M) based wireless communication systems with preferred
 transmission unit sizes of 254 bytes or less.  To prevent media
 transcoding between the two worlds, and to avoid undesirable
 packetization overhead, a NAL unit aggregation scheme is introduced.
 Two types of aggregation packets are defined by this specification:
 o  Single-time aggregation packet (STAP): aggregates NAL units with
    identical NALU-time.  Two types of STAPs are defined, one without
    DON (STAP-A) and another including DON (STAP-B).
 o  Multi-time aggregation packet (MTAP): aggregates NAL units with
    potentially differing NALU-time.  Two different MTAPs are defined,
    differing in the length of the NAL unit timestamp offset.

Wenger, et al. Standards Track [Page 18] RFC 3984 RTP Payload Format for H.264 Video February 2005

 The term NALU-time is defined as the value that the RTP timestamp
 would have if that NAL unit would be transported in its own RTP
 packet.
 Each NAL unit to be carried in an aggregation packet is encapsulated
 in an aggregation unit.  Please see below for the four different
 aggregation units and their characteristics.
 The structure of the RTP payload format for aggregation packets is
 presented in Figure 3.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |F|NRI|  type   |                                               |
    +-+-+-+-+-+-+-+-+                                               |
    |                                                               |
    |             one or more aggregation units                     |
    |                                                               |
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :...OPTIONAL RTP padding        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Figure 3.  RTP payload format for aggregation packets
 MTAPs and STAPs share the following packetization rules:  The RTP
 timestamp MUST be set to the earliest of the NALU times of all the
 NAL units to be aggregated.  The type field of the NAL unit type
 octet MUST be set to the appropriate value, as indicated in Table 4.
 The F bit MUST be cleared if all F bits of the aggregated NAL units
 are zero; otherwise, it MUST be set.  The value of NRI MUST be the
 maximum of all the NAL units carried in the aggregation packet.
    Table 4.  Type field for STAPs and MTAPs
    Type   Packet    Timestamp offset   DON related fields
                     field length       (DON, DONB, DOND)
                     (in bits)          present
    --------------------------------------------------------
    24     STAP-A       0                 no
    25     STAP-B       0                 yes
    26     MTAP16      16                 yes
    27     MTAP24      24                 yes
 The marker bit in the RTP header is set to the value that the marker
 bit of the last NAL unit of the aggregated packet would have if it
 were transported in its own RTP packet.

Wenger, et al. Standards Track [Page 19] RFC 3984 RTP Payload Format for H.264 Video February 2005

 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

Wenger, et al. Standards Track [Page 20] RFC 3984 RTP Payload Format for H.264 Video February 2005

 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

Wenger, et al. Standards Track [Page 21] RFC 3984 RTP Payload Format for H.264 Video February 2005

 Figure 7 presents an example of an RTP packet that contains an STAP-
 A.  The STAP contains two single-time aggregation units, labeled as 1
 and 2 in the figure.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          RTP Header                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |STAP-A NAL HDR |         NALU 1 Size           | NALU 1 HDR    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         NALU 1 Data                           |
    :                                                               :
    +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               | NALU 2 Size                   | NALU 2 HDR    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         NALU 2 Data                           |
    :                                                               :
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :...OPTIONAL RTP padding        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Figure 7.  An example of an RTP packet including an STAP-A and two
               single-time aggregation units

Wenger, et al. Standards Track [Page 22] RFC 3984 RTP Payload Format for H.264 Video February 2005

 Figure 8 presents an example of an RTP packet that contains an STAP-
 B.  The STAP contains two single-time aggregation units, labeled as 1
 and 2 in the figure.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          RTP Header                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |STAP-B NAL HDR | DON                           | NALU 1 Size   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | NALU 1 Size   | NALU 1 HDR    | NALU 1 Data                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
    :                                                               :
    +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               | NALU 2 Size                   | NALU 2 HDR    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       NALU 2 Data                             |
    :                                                               :
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :...OPTIONAL RTP padding        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Figure 8.  An example of an RTP packet including an STAP-B and two
               single-time aggregation units

5.7.2. Multi-Time Aggregation Packets (MTAPs)

 The NAL unit payload of MTAPs consists of a 16-bit unsigned decoding
 order number base (DONB) (in network byte order) and one or more
 multi-time aggregation units, as presented in Figure 9.  DONB MUST
 contain the value of DON for the first NAL unit in the NAL unit
 decoding order among the NAL units of the MTAP.
    Informative note: The first NAL unit in the NAL unit decoding
    order is not necessarily the first NAL unit in the order in which
    the NAL units are encapsulated in an MTAP.

Wenger, et al. Standards Track [Page 23] RFC 3984 RTP Payload Format for H.264 Video February 2005

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    :  decoding order number base   |               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
    |                                                               |
    |                 multi-time aggregation units                  |
    |                                                               |
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Figure 9.  NAL unit payload format for MTAPs
 Two different multi-time aggregation units are defined in this
 specification.  Both of them consist of 16 bits unsigned size
 information of the following NAL unit (in network byte order), an 8-
 bit unsigned decoding order number difference (DOND), and n bits (in
 network byte order) of timestamp offset (TS offset) for this NAL
 unit, whereby n can be 16 or 24.  The choice between the different
 MTAP types (MTAP16 and MTAP24) is application dependent: the larger
 the timestamp offset is, the higher the flexibility of the MTAP, but
 the overhead is also higher.
 The structure of the multi-time aggregation units for MTAP16 and
 MTAP24 are presented in Figures 10 and 11, respectively.  The
 starting or ending position of an aggregation unit within a packet is
 NOT REQUIRED to be on a 32-bit word boundary.  The DON of the
 following NAL unit is equal to (DONB + DOND) % 65536, in which %
 denotes the modulo operation.  This memo does not specify how the NAL
 units within an MTAP are ordered, but, in most cases, NAL unit
 decoding order SHOULD be used.
 The timestamp offset field MUST be set to a value equal to the value
 of the following formula: If the NALU-time is larger than or equal to
 the RTP timestamp of the packet, then the timestamp offset equals
 (the NALU-time of the NAL unit - the RTP timestamp of the packet).
 If the NALU-time is smaller than the RTP timestamp of the packet,
 then the timestamp offset is equal to the NALU-time + (2^32 - the RTP
 timestamp of the packet).

Wenger, et al. Standards Track [Page 24] RFC 3984 RTP Payload Format for H.264 Video February 2005

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :        NAL unit size          |      DOND     |  TS offset    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  TS offset    |                                               |
    +-+-+-+-+-+-+-+-+              NAL unit                         |
    |                                                               |
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Figure 10.  Multi-time aggregation unit for MTAP16
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :        NALU unit size         |      DOND     |  TS offset    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         TS offset             |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
    |                              NAL unit                         |
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Figure 11.  Multi-time aggregation unit for MTAP24
 For the "earliest" multi-time aggregation unit in an MTAP the
 timestamp offset MUST be zero.  Hence, the RTP timestamp of the MTAP
 itself is identical to the earliest NALU-time.
    Informative note: The "earliest" multi-time aggregation unit is
    the one that would have the smallest extended RTP timestamp among
    all the aggregation units of an MTAP if the aggregation units were
    encapsulated in single NAL unit packets.  An extended timestamp is
    a timestamp that has more than 32 bits and is capable of counting
    the wraparound of the timestamp field, thus enabling one to
    determine the smallest value if the timestamp wraps.  Such an
    "earliest" aggregation unit may not be the first one in the order
    in which the aggregation units are encapsulated in an MTAP.  The
    "earliest" NAL unit need not be the same as the first NAL unit in
    the NAL unit decoding order either.

Wenger, et al. Standards Track [Page 25] RFC 3984 RTP Payload Format for H.264 Video February 2005

 Figure 12 presents an example of an RTP packet that contains a
 multi-time aggregation packet of type MTAP16 that contains two
 multi-time aggregation units, labeled as 1 and 2 in the figure.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          RTP Header                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |MTAP16 NAL HDR |  decoding order number base   | NALU 1 Size   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  NALU 1 Size  |  NALU 1 DOND  |       NALU 1 TS offset        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  NALU 1 HDR   |  NALU 1 DATA                                  |
    +-+-+-+-+-+-+-+-+                                               +
    :                                                               :
    +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               | NALU 2 SIZE                   |  NALU 2 DOND  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       NALU 2 TS offset        |  NALU 2 HDR   |  NALU 2 DATA  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
    :                                                               :
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :...OPTIONAL RTP padding        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Figure 12.  An RTP packet including a multi-time aggregation
                packet of type MTAP16 and two multi-time aggregation
                units

Wenger, et al. Standards Track [Page 26] RFC 3984 RTP Payload Format for H.264 Video February 2005

 Figure 13 presents an example of an RTP packet that contains a
 multi-time aggregation packet of type MTAP24 that contains two
 multi-time aggregation units, labeled as 1 and 2 in the figure.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          RTP Header                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |MTAP24 NAL HDR |  decoding order number base   | NALU 1 Size   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  NALU 1 Size  |  NALU 1 DOND  |       NALU 1 TS offs          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |NALU 1 TS offs |  NALU 1 HDR   |  NALU 1 DATA                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
    :                                                               :
    +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               | NALU 2 SIZE                   |  NALU 2 DOND  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       NALU 2 TS offset                        |  NALU 2 HDR   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  NALU 2 DATA                                                  |
    :                                                               :
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :...OPTIONAL RTP padding        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Figure 13.  An RTP packet including a multi-time aggregation
                packet of type MTAP24 and two multi-time aggregation
                units

5.8. Fragmentation Units (FUs)

 This payload type allows fragmenting a NAL unit into several RTP
 packets.  Doing so on the application layer instead of relying on
 lower layer fragmentation (e.g., by IP) has the following advantages:
 o  The payload format is capable of transporting NAL units bigger
    than 64 kbytes over an IPv4 network that may be present in pre-
    recorded video, particularly in High Definition formats (there is
    a limit of the number of slices per picture, which results in a
    limit of NAL units per picture, which may result in big NAL
    units).
 o  The fragmentation mechanism allows fragmenting a single picture
    and applying generic forward error correction as described in
    section 12.5.

Wenger, et al. Standards Track [Page 27] RFC 3984 RTP Payload Format for H.264 Video February 2005

 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

Wenger, et al. Standards Track [Page 28] RFC 3984 RTP Payload Format for H.264 Video February 2005

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

Wenger, et al. Standards Track [Page 29] RFC 3984 RTP Payload Format for H.264 Video February 2005

 S: 1 bit
    When set to one, the Start bit indicates the start of a fragmented
    NAL unit.  When the following FU payload is not the start of a
    fragmented NAL unit payload, the Start bit is set to zero.
 E: 1 bit
    When set to one, the End bit indicates the end of a fragmented NAL
    unit, i.e., the last byte of the payload is also the last byte of
    the fragmented NAL unit.  When the following FU payload is not the
    last fragment of a fragmented NAL unit, the End bit is set to
    zero.
 R: 1 bit
    The Reserved bit MUST be equal to 0 and MUST be ignored by the
    receiver.
 Type: 5 bits
    The NAL unit payload type as defined in table 7-1 of [1].
 The value of DON in FU-Bs is selected as described in section 5.5.
    Informative note: The DON field in FU-Bs allows gateways to
    fragment NAL units to FU-Bs without organizing the incoming NAL
    units to the NAL unit decoding order.
 A fragmented NAL unit MUST NOT be transmitted in one FU; i.e., the
 Start bit and End bit MUST NOT both be set to one in the same FU
 header.
 The FU payload consists of fragments of the payload of the fragmented
 NAL unit so that if the fragmentation unit payloads of consecutive
 FUs are sequentially concatenated, the payload of the fragmented NAL
 unit can be reconstructed.  The NAL unit type octet of the fragmented
 NAL unit is not included as such in the fragmentation unit payload,
 but rather the information of the NAL unit type octet of the
 fragmented NAL unit is conveyed in F and NRI fields of the FU
 indicator octet of the fragmentation unit and in the type field of
 the FU header.  A FU payload MAY have any number of octets and MAY be
 empty.
    Informative note: Empty FUs are allowed to reduce the latency of a
    certain class of senders in nearly lossless environments.  These
    senders can be characterized in that they packetize NALU fragments
    before the NALU is completely generated and, hence, before the
    NALU size is known.  If zero-length NALU fragments were not
    allowed, the sender would have to generate at least one bit of
    data of the following fragment before the current fragment could
    be sent.  Due to the characteristics of H.264, where sometimes

Wenger, et al. Standards Track [Page 30] RFC 3984 RTP Payload Format for H.264 Video February 2005

    several macroblocks occupy zero bits, this is undesirable and can
    add delay.  However, the (potential) use of zero-length NALUs
    should be carefully weighed against the increased risk of the loss
    of the NALU because of the additional packets employed for its
    transmission.
 If a fragmentation unit is lost, the receiver SHOULD discard all
 following fragmentation units in transmission order corresponding to
 the same fragmented NAL unit.
 A receiver in an endpoint or in a MANE MAY aggregate the first n-1
 fragments of a NAL unit to an (incomplete) NAL unit, even if fragment
 n of that NAL unit is not received.  In this case, the
 forbidden_zero_bit of the NAL unit MUST be set to one to indicate a
 syntax violation.

6. Packetization Rules

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

6.1. Common Packetization Rules

 All senders MUST enforce the following packetization rules regardless
 of the packetization mode in use:
 o  Coded slice NAL units or coded slice data partition NAL units
    belonging to the same coded picture (and thus sharing the same RTP
    timestamp value) MAY be sent in any order permitted by the
    applicable profile defined in [1]; however, for delay-critical
    systems, they SHOULD be sent in their original coding order to
    minimize the delay.  Note that the coding order is not necessarily
    the scan order, but the order the NAL packets become available to
    the RTP stack.
 o  Parameter sets are handled in accordance with the rules and
    recommendations given in section 8.4.
 o  MANEs MUST NOT duplicate any NAL unit except for sequence or
    picture parameter set NAL units, as neither this memo nor the
    H.264 specification provides means to identify duplicated NAL
    units.  Sequence and picture parameter set NAL units MAY be
    duplicated to make their correct reception more probable, but any
    such duplication MUST NOT affect the contents of any active
    sequence or picture parameter set.  Duplication SHOULD be

Wenger, et al. Standards Track [Page 31] RFC 3984 RTP Payload Format for H.264 Video February 2005

    performed on the application layer and not by duplicating RTP
    packets (with identical sequence numbers).
 Senders using the non-interleaved mode and the interleaved mode MUST
 enforce the following packetization rule:
 o  MANEs MAY convert single NAL unit packets into one aggregation
    packet, convert an aggregation packet into several single NAL unit
    packets, or mix both concepts, in an RTP translator.  The RTP
    translator SHOULD take into account at least the following
    parameters: path MTU size, unequal protection mechanisms (e.g.,
    through packet-based FEC according to RFC 2733 [18], especially
    for sequence and picture parameter set NAL units and coded slice
    data partition A NAL units), bearable latency of the system, and
    buffering capabilities of the receiver.
    Informative note: An RTP translator is required to handle RTCP as
    per RFC 3550.

6.2. Single NAL Unit Mode

 This mode is in use when the value of the OPTIONAL packetization-mode
 MIME parameter is equal to 0, the packetization-mode is not present,
 or no other packetization mode is signaled by external means.  All
 receivers MUST support this mode.  It is primarily intended for low-
 delay applications that are compatible with systems using ITU-T
 Recommendation H.241 [15] (see section 12.1).  Only single NAL unit
 packets MAY be used in this mode.  STAPs, MTAPs, and FUs MUST NOT be
 used.  The transmission order of single NAL unit packets MUST comply
 with the NAL unit decoding order.

6.3. Non-Interleaved Mode

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

Wenger, et al. Standards Track [Page 32] RFC 3984 RTP Payload Format for H.264 Video February 2005

6.4. Interleaved Mode

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

7. De-Packetization Process (Informative)

 The de-packetization process is implementation dependent.  Therefore,
 the following description should be seen as an example of a suitable
 implementation.  Other schemes may be used as well.  Optimizations
 relative to the described algorithms are likely possible.  Section
 7.1 presents the de-packetization process for the single NAL unit and
 non-interleaved packetization modes, whereas section 7.2 describes
 the process for the interleaved mode.  Section 7.3 includes
 additional decapsulation guidelines for intelligent receivers.
 All normal RTP mechanisms related to buffer management apply.  In
 particular, duplicated or outdated RTP packets (as indicated by the
 RTP sequences number and the RTP timestamp) are removed.  To
 determine the exact time for decoding, factors such as a possible
 intentional delay to allow for proper inter-stream synchronization
 must be factored in.

7.1. Single NAL Unit and Non-Interleaved Mode

 The receiver includes a receiver buffer to compensate for
 transmission delay jitter.  The receiver stores incoming packets in
 reception order into the receiver buffer.  Packets are decapsulated
 in RTP sequence number order.  If a decapsulated packet is a single
 NAL unit packet, the NAL unit contained in the packet is passed
 directly to the decoder.  If a decapsulated packet is an STAP-A, the
 NAL units contained in the packet are passed to the decoder in the
 order in which they are encapsulated in the packet.  If a
 decapsulated packet is an FU-A, all the fragments of the fragmented
 NAL unit are concatenated and passed to the decoder.
    Informative note: If the decoder supports Arbitrary Slice Order,
    coded slices of a picture can be passed to the decoder in any
    order regardless of their reception and transmission order.

Wenger, et al. Standards Track [Page 33] RFC 3984 RTP Payload Format for H.264 Video February 2005

7.2. Interleaved Mode

 The general concept behind these de-packetization rules is to reorder
 NAL units from transmission order to the NAL unit decoding order.
 The receiver includes a receiver buffer, which is used to compensate
 for transmission delay jitter and to reorder packets from
 transmission order to the NAL unit decoding order.  In this section,
 the receiver operation is described under the assumption that there
 is no transmission delay jitter.  To make a difference from a
 practical receiver buffer that is also used for compensation of
 transmission delay jitter, the receiver buffer is here after called
 the deinterleaving buffer in this section.  Receivers SHOULD also
 prepare for transmission delay jitter; i.e., either reserve separate
 buffers for transmission delay jitter buffering and deinterleaving
 buffering or use a receiver buffer for both transmission delay jitter
 and deinterleaving.  Moreover, receivers SHOULD take transmission
 delay jitter into account in the buffering operation; e.g., by
 additional initial buffering before starting of decoding and
 playback.
 This section is organized as follows: subsection 7.2.1 presents how
 to calculate the size of the deinterleaving buffer.  Subsection 7.2.2
 specifies the receiver process how to organize received NAL units to
 the NAL unit decoding order.

7.2.1. Size of the Deinterleaving Buffer

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

Wenger, et al. Standards Track [Page 34] RFC 3984 RTP Payload Format for H.264 Video February 2005

7.2.2. Deinterleaving Process

 There are two buffering states in the receiver: initial buffering and
 buffering while playing.  Initial buffering occurs when the RTP
 session is initialized.  After initial buffering, decoding and
 playback is started, and the buffering-while-playing mode is used.
 Regardless of the buffering state, the receiver stores incoming NAL
 units, in reception order, in the deinterleaving buffer as follows.
 NAL units of aggregation packets are stored in the deinterleaving
 buffer individually.  The value of DON is calculated and stored for
 all NAL units.
 The receiver operation is described below with the help of the
 following functions and constants:
 o  Function AbsDON is specified in section 8.1.
 o  Function don_diff is specified in section 5.5.
 o  Constant N is the value of the OPTIONAL sprop-interleaving-depth
    MIME type parameter (see section 8.1) incremented by 1.
 Initial buffering lasts until one of the following conditions is
 fulfilled:
 o  There are N VCL NAL units in the deinterleaving buffer.
 o  If sprop-max-don-diff is present, don_diff(m,n) is greater than
    the value of sprop-max-don-diff, in which n corresponds to the NAL
    unit having the greatest value of AbsDON among the received NAL
    units and m corresponds to the NAL unit having the smallest value
    of AbsDON among the received NAL units.
 o  Initial buffering has lasted for the duration equal to or greater
    than the value of the OPTIONAL sprop-init-buf-time MIME parameter.
 The NAL units to be removed from the deinterleaving buffer are
 determined as follows:
 o  If the deinterleaving buffer contains at least N VCL NAL units,
    NAL units are removed from the deinterleaving buffer and passed to
    the decoder in the order specified below until the buffer contains
    N-1 VCL NAL units.

Wenger, et al. Standards Track [Page 35] RFC 3984 RTP Payload Format for H.264 Video February 2005

 o  If sprop-max-don-diff is present, all NAL units m for which
    don_diff(m,n) is greater than sprop-max-don-diff are removed from
    the deinterleaving buffer and passed to the decoder in the order
    specified below.  Herein, n corresponds to the NAL unit having the
    greatest value of AbsDON among the received NAL units.
 The order in which NAL units are passed to the decoder is specified
 as follows:
 o  Let PDON be a variable that is initialized to 0 at the beginning
    of the an RTP session.
 o  For each NAL unit associated with a value of DON, a DON distance
    is calculated as follows.  If the value of DON of the NAL unit is
    larger than the value of PDON, the DON distance is equal to DON -
    PDON.  Otherwise, the DON distance is equal to 65535 - PDON + DON
    + 1.
 o  NAL units are delivered to the decoder in ascending order of DON
    distance.  If several NAL units share the same value of DON
    distance, they can be passed to the decoder in any order.
 o  When a desired number of NAL units have been passed to the
    decoder, the value of PDON is set to the value of DON for the last
    NAL unit passed to the decoder.

7.3. Additional De-Packetization Guidelines

 The following additional de-packetization rules may be used to
 implement an operational H.264 de-packetizer:
 o  Intelligent RTP receivers (e.g., in gateways) may identify lost
    coded slice data partitions A (DPAs).  If a lost DPA is found, a
    gateway may decide not to send the corresponding coded slice data
    partitions B and C, as their information is meaningless for H.264
    decoders.  In this way a MANE can reduce network load by
    discarding useless packets without parsing a complex bitstream.
 o  Intelligent RTP receivers (e.g., in gateways) may identify lost
    FUs.  If a lost FU is found, a gateway may decide not to send the
    following FUs of the same fragmented NAL unit, as their
    information is meaningless for H.264 decoders.  In this way a MANE
    can reduce network load by discarding useless packets without
    parsing a complex bitstream.

Wenger, et al. Standards Track [Page 36] RFC 3984 RTP Payload Format for H.264 Video February 2005

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

8. Payload Format Parameters

 This section specifies the parameters that MAY be used to select
 optional features of the payload format and certain features of the
 bitstream.  The parameters are specified here as part of the MIME
 subtype registration for the ITU-T H.264 | ISO/IEC 14496-10 codec.  A
 mapping of the parameters into the Session Description Protocol (SDP)
 [5] is also provided for applications that use SDP.  Equivalent
 parameters could be defined elsewhere for use with control protocols
 that do not use MIME or SDP.
 Some parameters provide a receiver with the properties of the stream
 that will be sent.  The name of all these parameters starts with
 "sprop" for stream properties.  Some of these "sprop" parameters are
 limited by other payload or codec configuration parameters.  For
 example, the sprop-parameter-sets parameter is constrained by the
 profile-level-id parameter.  The media sender selects all "sprop"
 parameters rather than the receiver.  This uncommon characteristic of
 the "sprop" parameters may not be compatible with some signaling
 protocol concepts, in which case the use of these parameters SHOULD
 be avoided.

8.1. MIME Registration

 The MIME subtype for the ITU-T H.264 | ISO/IEC 14496-10 codec is
 allocated from the IETF tree.
 The receiver MUST ignore any unspecified parameter.
 Media Type name:     video
 Media subtype name:  H264
 Required parameters: none

Wenger, et al. Standards Track [Page 37] RFC 3984 RTP Payload Format for H.264 Video February 2005

 OPTIONAL parameters:
     profile-level-id:
                      A base16 [6] (hexadecimal) representation of
                      the following three bytes in the sequence
                      parameter set NAL unit specified in [1]: 1)
                      profile_idc, 2) a byte herein referred to as
                      profile-iop, composed of the values of
                      constraint_set0_flag, constraint_set1_flag,
                      constraint_set2_flag, and reserved_zero_5bits
                      in bit-significance order, starting from the
                      most significant bit, and 3) level_idc.  Note
                      that reserved_zero_5bits is required to be
                      equal to 0 in [1], but other values for it may
                      be specified in the future by ITU-T or ISO/IEC.
                      If the profile-level-id parameter is used to
                      indicate properties of a NAL unit stream, it
                      indicates the profile and level that a decoder
                      has to support in order to comply with [1] when
                      it decodes the stream.  The profile-iop byte
                      indicates whether the NAL unit stream also
                      obeys all constraints of the indicated profiles
                      as follows.  If bit 7 (the most significant
                      bit), bit 6, or bit 5 of profile-iop is equal
                      to 1, all constraints of the Baseline profile,
                      the Main profile, or the Extended profile,
                      respectively, are obeyed in the NAL unit
                      stream.
                      If the profile-level-id parameter is used for
                      capability exchange or session setup procedure,
                      it indicates the profile that the codec
                      supports and the highest level
                      supported for the signaled profile.  The
                      profile-iop byte indicates whether the codec
                      has additional limitations whereby only the
                      common subset of the algorithmic features and
                      limitations of the profiles signaled with the
                      profile-iop byte and of the profile indicated
                      by profile_idc is supported by the codec.  For
                      example, if a codec supports only the common
                      subset of the coding tools of the Baseline
                      profile and the Main profile at level 2.1 and
                      below, the profile-level-id becomes 42E015, in
                      which 42 stands for the Baseline profile, E0
                      indicates that only the common subset for all
                      profiles is supported, and 15 indicates level
                      2.1.

Wenger, et al. Standards Track [Page 38] RFC 3984 RTP Payload Format for H.264 Video February 2005

                          Informative note: Capability exchange and
                          session setup procedures should provide
                          means to list the capabilities for each
                          supported codec profile separately.  For
                          example, the one-of-N codec selection
                          procedure of the SDP Offer/Answer model can
                          be used (section 10.2 of [7]).
                      If no profile-level-id is present, the Baseline
                      Profile without additional constraints at Level
                      1 MUST be implied.
     max-mbps, max-fs, max-cpb, max-dpb, and max-br:
                      These parameters MAY be used to signal the
                      capabilities of a receiver implementation.
                      These parameters MUST NOT be used for any other
                      purpose.  The profile-level-id parameter MUST
                      be present in the same receiver capability
                      description that contains any of these
                      parameters.  The level conveyed in the value of
                      the profile-level-id parameter MUST be such
                      that the receiver is fully capable of
                      supporting.  max-mbps, max-fs, max-cpb, max-
                      dpb, and max-br MAY be used to indicate
                      capabilities of the receiver that extend the
                      required capabilities of the signaled level, as
                      specified below.
                      When more than one parameter from the set (max-
                      mbps, max-fs, max-cpb, max-dpb, max-br) is
                      present, the receiver MUST support all signaled
                      capabilities simultaneously.  For example, if
                      both max-mbps and max-br are present, the
                      signaled level with the extension of both the
                      frame rate and bit rate is supported.  That is,
                      the receiver is able to decode NAL unit
                      streams in which the macroblock processing rate
                      is up to max-mbps (inclusive), the bit rate is
                      up to max-br (inclusive), the coded picture
                      buffer size is derived as specified in the
                      semantics of the max-br parameter below, and
                      other properties comply with the level
                      specified in the value of the profile-level-id
                      parameter.
                      A receiver MUST NOT signal values of max-
                      mbps, max-fs, max-cpb, max-dpb, and max-br that
                      meet the requirements of a higher level,

Wenger, et al. Standards Track [Page 39] RFC 3984 RTP Payload Format for H.264 Video February 2005

                      referred to as level A herein, compared to the
                      level specified in the value of the profile-
                      level-id parameter, if the receiver can support
                      all the properties of level A.
                          Informative note: When the OPTIONAL MIME
                          type parameters are used to signal the
                          properties of a NAL unit stream, max-mbps,
                          max-fs, max-cpb, max-dpb, and max-br are
                          not present, and the value of profile-
                          level-id must always be such that the NAL
                          unit stream complies fully with the
                          specified profile and level.
     max-mbps:        The value of max-mbps is an integer indicating
                      the maximum macroblock processing rate in units
                      of macroblocks per second.  The max-mbps
                      parameter signals that the receiver is capable
                      of decoding video at a higher rate than is
                      required by the signaled level conveyed in the
                      value of the profile-level-id parameter.  When
                      max-mbps is signaled, the receiver MUST be able
                      to decode NAL unit streams that conform to the
                      signaled level, with the exception that the
                      MaxMBPS value in Table A-1 of [1] for the
                      signaled level is replaced with the value of
                      max-mbps.  The value of max-mbps MUST be
                      greater than or equal to the value of MaxMBPS
                      for the level given in Table A-1 of [1].
                      Senders MAY use this knowledge to send pictures
                      of a given size at a higher picture rate than
                      is indicated in the signaled level.
     max-fs:          The value of max-fs is an integer indicating
                      the maximum frame size in units of macroblocks.
                      The max-fs parameter signals that the receiver
                      is capable of decoding larger picture sizes
                      than are required by the signaled level conveyed
                      in the value of the profile-level-id parameter.
                      When max-fs is signaled, the receiver MUST be
                      able to decode NAL unit streams that conform to
                      the signaled level, with the exception that the
                      MaxFS value in Table A-1 of [1] for the
                      signaled level is replaced with the value of
                      max-fs.  The value of max-fs MUST be greater
                      than or equal to the value of MaxFS for the
                      level given in Table A-1 of [1].  Senders MAY
                      use this knowledge to send larger pictures at a

Wenger, et al. Standards Track [Page 40] RFC 3984 RTP Payload Format for H.264 Video February 2005

                      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.

Wenger, et al. Standards Track [Page 41] RFC 3984 RTP Payload Format for H.264 Video February 2005

     max-dpb:         The value of max-dpb is an integer indicating
                      the maximum decoded picture buffer size in
                      units of 1024 bytes.  The max-dpb parameter
                      signals that the receiver has more memory than
                      the minimum amount of decoded picture buffer
                      memory required by the signaled level conveyed
                      in the value of the profile-level-id parameter.
                      When max-dpb is signaled, the receiver MUST be
                      able to decode NAL unit streams that conform to
                      the signaled level, with the exception that the
                      MaxDPB value in Table A-1 of [1] for the
                      signaled level is replaced with the value of
                      max-dpb.  Consequently, a receiver that signals
                      max-dpb MUST be capable of storing the
                      following number of decoded frames,
                      complementary field pairs, and non-paired
                      fields in its decoded picture buffer:
                      Min(1024 * max-dpb / ( PicWidthInMbs *
                      FrameHeightInMbs * 256 * ChromaFormatFactor ),
                      16)
                      PicWidthInMbs, FrameHeightInMbs, and
                      ChromaFormatFactor are defined in [1].
                      The value of max-dpb MUST be greater than or
                      equal to the value of MaxDPB for the level
                      given in Table A-1 of [1].  Senders MAY use
                      this knowledge to construct coded video streams
                      with improved compression.
                          Informative note: This parameter was added
                          primarily to complement a similar codepoint
                          in the ITU-T Recommendation H.245, so as to
                          facilitate signaling gateway designs.  The
                          decoded picture buffer stores reconstructed
                          samples and is a property of the video
                          decoder only.  There is no relationship
                          between the size of the decoded picture
                          buffer and the buffers used in RTP,
                          especially de-interleaving and de-jitter
                          buffers.
     max-br:          The value of max-br is an integer indicating
                      the maximum video bit rate in units of 1000
                      bits per second for the VCL HRD parameters (see
                      A.3.1 item i of [1]) and in units of 1200 bits

Wenger, et al. Standards Track [Page 42] RFC 3984 RTP Payload Format for H.264 Video February 2005

                      per second for the NAL HRD parameters (see
                      A.3.1 item j of [1]).
                      The max-br parameter signals that the video
                      decoder of the receiver is capable of decoding
                      video at a higher bit rate than is required by
                      the signaled level conveyed in the value of the
                      profile-level-id parameter.  The value of max-
                      br MUST be greater than or equal to the value
                      of MaxBR for the level given in Table A-1 of
                      [1].
                      When max-br is signaled, the video codec of the
                      receiver MUST be able to decode NAL unit
                      streams that conform to the signaled level,
                      conveyed in the profile-level-id parameter,
                      with the following exceptions in the limits
                      specified by the level:
                      o The value of max-br replaces the MaxBR value
                        of the signaled level (in Table A-1 of [1]).
                      o When the max-cpb parameter is not present,
                        the result of the following formula replaces
                        the value of MaxCPB in Table A-1 of [1]:
                        (MaxCPB of the signaled level) * max-br /
                        (MaxBR of the signaled level).
                      For example, if a receiver signals capability
                      for Level 1.2 with max-br equal to 1550, this
                      indicates a maximum video bitrate of 1550
                      kbits/sec for VCL HRD parameters, a maximum
                      video bitrate of 1860 kbits/sec for NAL HRD
                      parameters, and a CPB size of 4036458 bits
                      (1550000 / 384000 * 1000 * 1000).
                      The value of max-br MUST be greater than or
                      equal to the value MaxBR for the signaled level
                      given in Table A-1 of [1].
                      Senders MAY use this knowledge to send higher
                      bitrate video as allowed in the level
                      definition of Annex A of H.264, to achieve
                      improved video quality.
                          Informative note: This parameter was added
                          primarily to complement a similar codepoint
                          in the ITU-T Recommendation H.245, so as to
                          facilitate signaling gateway designs.  No
                          assumption can be made from the value of

Wenger, et al. Standards Track [Page 43] RFC 3984 RTP Payload Format for H.264 Video February 2005

                          this parameter that the network is capable
                          of handling such bit rates at any given
                          time.  In particular, no conclusion can be
                          drawn that the signaled bit rate is
                          possible under congestion control
                          constraints.
    redundant-pic-cap:
                      This parameter signals the capabilities of a
                      receiver implementation.  When equal to 0, the
                      parameter indicates that the receiver makes no
                      attempt to use redundant coded pictures to
                      correct incorrectly decoded primary coded
                      pictures.  When equal to 0, the receiver is not
                      capable of using redundant slices; therefore, a
                      sender SHOULD avoid sending redundant slices to
                      save bandwidth.  When equal to 1, the receiver
                      is capable of decoding any such redundant slice
                      that covers a corrupted area in a primary
                      decoded picture (at least partly), and therefore
                      a sender MAY send redundant slices.  When the
                      parameter is not present, then a value of 0
                      MUST be used for redundant-pic-cap.  When
                      present, the value of redundant-pic-cap MUST be
                      either 0 or 1.
                      When the profile-level-id parameter is present
                      in the same capability signaling as the
                      redundant-pic-cap parameter, and the profile
                      indicated in profile-level-id is such that it
                      disallows the use of redundant coded pictures
                      (e.g., Main Profile), the value of redundant-
                      pic-cap MUST be equal to 0.  When a receiver
                      indicates redundant-pic-cap equal to 0, the
                      received stream SHOULD NOT contain redundant
                      coded pictures.
                          Informative note: Even if redundant-pic-cap
                          is equal to 0, the decoder is able to
                          ignore redundant codec pictures provided
                          that the decoder supports such a profile
                          (Baseline, Extended) in which redundant
                          coded pictures are allowed.
                          Informative note: Even if redundant-pic-cap
                          is equal to 1, the receiver may also choose
                          other error concealment strategies to

Wenger, et al. Standards Track [Page 44] RFC 3984 RTP Payload Format for H.264 Video February 2005

                          replace or complement decoding of redundant
                          slices.
     sprop-parameter-sets:
                      This parameter MAY be used to convey
                      any sequence and picture parameter set NAL
                      units (herein referred to as the initial
                      parameter set NAL units) that MUST precede any
                      other NAL units in decoding order.  The
                      parameter MUST NOT be used to indicate codec
                      capability in any capability exchange
                      procedure.  The value of the parameter is the
                      base64 [6] representation of the initial
                      parameter set NAL units as specified in
                      sections 7.3.2.1 and 7.3.2.2 of [1].  The
                      parameter sets are conveyed in decoding order,
                      and no framing of the parameter set NAL units
                      takes place.  A comma is used to separate any
                      pair of parameter sets in the list.  Note that
                      the number of bytes in a parameter set NAL unit
                      is typically less than 10, but a picture
                      parameter set NAL unit can contain several
                      hundreds of bytes.
                         Informative note: When several payload
                         types are offered in the SDP Offer/Answer
                         model, each with its own sprop-parameter-
                         sets parameter, then the receiver cannot
                         assume that those parameter sets do not use
                         conflicting storage locations (i.e.,
                         identical values of parameter set
                         identifiers).  Therefore, a receiver should
                         double-buffer all sprop-parameter-sets and
                         make them available to the decoder instance
                         that decodes a certain payload type.
     parameter-add:   This parameter MAY be used to signal whether
                      the receiver of this parameter is allowed to
                      add parameter sets in its signaling response
                      using the sprop-parameter-sets MIME parameter.
                      The value of this parameter is either 0 or 1.
                      0 is equal to false; i.e., it is not allowed to
                      add parameter sets.  1 is equal to true; i.e.,
                      it is allowed to add parameter sets.  If the
                      parameter is not present, its value MUST be 1.

Wenger, et al. Standards Track [Page 45] RFC 3984 RTP Payload Format for H.264 Video February 2005

     packetization-mode:
                      This parameter signals the properties of an
                      RTP payload type or the capabilities of a
                      receiver implementation.  Only a single
                      configuration point can be indicated; thus,
                      when capabilities to support more than one
                      packetization-mode are declared, multiple
                      configuration points (RTP payload types) must
                      be used.
                      When the value of packetization-mode is equal
                      to 0 or packetization-mode is not present, the
                      single NAL mode, as defined in section 6.2 of
                      RFC 3984, MUST be used.  This mode is in use in
                      standards using ITU-T Recommendation H.241 [15]
                      (see section 12.1).  When the value of
                      packetization-mode is equal to 1, the non-
                      interleaved mode, as defined in section 6.3 of
                      RFC 3984, MUST be used.  When the value of
                      packetization-mode is equal to 2, the
                      interleaved mode, as defined in section 6.4 of
                      RFC 3984, MUST be used.  The value of
                      packetization mode MUST be an integer in the
                      range of 0 to 2, inclusive.
     sprop-interleaving-depth:
                      This parameter MUST NOT be present
                      when packetization-mode is not present or the
                      value of packetization-mode is equal to 0 or 1.
                      This parameter MUST be present when the value
                      of packetization-mode is equal to 2.
                      This parameter signals the properties of a NAL
                      unit stream.  It specifies the maximum number
                      of VCL NAL units that precede any VCL NAL unit
                      in the NAL unit stream in transmission order
                      and follow the VCL NAL unit in decoding order.
                      Consequently, it is guaranteed that receivers
                      can reconstruct NAL unit decoding order when
                      the buffer size for NAL unit decoding order
                      recovery is at least the value of sprop-
                      interleaving-depth + 1 in terms of VCL NAL
                      units.
                      The value of sprop-interleaving-depth MUST be
                      an integer in the range of 0 to 32767,
                      inclusive.

Wenger, et al. Standards Track [Page 46] RFC 3984 RTP Payload Format for H.264 Video February 2005

     sprop-deint-buf-req:
                      This parameter MUST NOT be present when
                      packetization-mode is not present or the value
                      of packetization-mode is equal to 0 or 1.  It
                      MUST be present when the value of
                      packetization-mode is equal to 2.
                      sprop-deint-buf-req signals the required size
                      of the deinterleaving buffer for the NAL unit
                      stream.  The value of the parameter MUST be
                      greater than or equal to the maximum buffer
                      occupancy (in units of bytes) required in such
                      a deinterleaving buffer that is specified in
                      section 7.2 of RFC 3984.  It is guaranteed that
                      receivers can perform the deinterleaving of
                      interleaved NAL units into NAL unit decoding
                      order, when the deinterleaving buffer size is
                      at least the value of sprop-deint-buf-req in
                      terms of bytes.
                      The value of sprop-deint-buf-req MUST be an
                      integer in the range of 0 to 4294967295,
                      inclusive.
                          Informative note: sprop-deint-buf-req
                          indicates the required size of the
                          deinterleaving buffer only.  When network
                          jitter can occur, an appropriately sized
                          jitter buffer has to be provisioned for
                          as well.
     deint-buf-cap:   This parameter signals the capabilities of a
                      receiver implementation and indicates the
                      amount of deinterleaving buffer space in units
                      of bytes that the receiver has available for
                      reconstructing the NAL unit decoding order.  A
                      receiver is able to handle any stream for which
                      the value of the sprop-deint-buf-req parameter
                      is smaller than or equal to this parameter.
                      If the parameter is not present, then a value
                      of 0 MUST be used for deint-buf-cap.  The value
                      of deint-buf-cap MUST be an integer in the
                      range of 0 to 4294967295, inclusive.
                          Informative note: deint-buf-cap indicates
                          the maximum possible size of the
                          deinterleaving buffer of the receiver only.

Wenger, et al. Standards Track [Page 47] RFC 3984 RTP Payload Format for H.264 Video February 2005

                          When network jitter can occur, an
                          appropriately sized jitter buffer has to
                          be provisioned for as well.
     sprop-init-buf-time:
                      This parameter MAY be used to signal the
                      properties of a NAL unit stream.  The parameter
                      MUST NOT be present, if the value of
                      packetization-mode is equal to 0 or 1.
                      The parameter signals the initial buffering
                      time that a receiver MUST buffer before
                      starting decoding to recover the NAL unit
                      decoding order from the transmission order.
                      The parameter is the maximum value of
                      (transmission time of a NAL unit - decoding
                      time of the NAL unit), assuming reliable and
                      instantaneous transmission, the same
                      timeline for transmission and decoding, and
                      that decoding starts when the first packet
                      arrives.
                      An example of specifying the value of sprop-
                      init-buf-time follows.  A NAL unit stream is
                      sent in the following interleaved order, in
                      which the value corresponds to the decoding
                      time and the transmission order is from left to
                      right:
                      0  2  1  3  5  4  6  8  7 ...
                      Assuming a steady transmission rate of NAL
                      units, the transmission times are:
                      0  1  2  3  4  5  6  7  8 ...
                      Subtracting the decoding time from the
                      transmission time column-wise results in the
                      following series:
                      0 -1  1  0 -1  1  0 -1  1 ...
                      Thus, in terms of intervals of NAL unit
                      transmission times, the value of
                      sprop-init-buf-time in this
                      example is 1.

Wenger, et al. Standards Track [Page 48] RFC 3984 RTP Payload Format for H.264 Video February 2005

                      The parameter is coded as a non-negative base10
                      integer representation in clock ticks of a 90-
                      kHz clock.  If the parameter is not present,
                      then no initial buffering time value is
                      defined.  Otherwise the value of sprop-init-
                      buf-time MUST be an integer in the range of 0
                      to 4294967295, inclusive.
                      In addition to the signaled sprop-init-buf-
                      time, receivers SHOULD take into account the
                      transmission delay jitter buffering, including
                      buffering for the delay jitter caused by
                      mixers, translators, gateways, proxies,
                      traffic-shapers, and other network elements.
     sprop-max-don-diff:
                      This parameter MAY be used to signal the
                      properties of a NAL unit stream.  It MUST NOT
                      be used to signal transmitter or receiver or
                      codec capabilities.  The parameter MUST NOT be
                      present if the value of packetization-mode is
                      equal to 0 or 1.  sprop-max-don-diff is an
                      integer in the range of 0 to 32767, inclusive.
                      If sprop-max-don-diff is not present, the value
                      of the parameter is unspecified.  sprop-max-
                      don-diff is calculated as follows:
                      sprop-max-don-diff = max{AbsDON(i) -
                      AbsDON(j)},
                      for any i and any j>i,
                      where i and j indicate the index of the NAL
                      unit in the transmission order and AbsDON
                      denotes a decoding order number of the NAL
                      unit that does not wrap around to 0 after
                      65535.  In other words, AbsDON is calculated as
                      follows: Let m and n be consecutive NAL units
                      in transmission order.  For the very first NAL
                      unit in transmission order (whose index is 0),
                      AbsDON(0) = DON(0).  For other NAL units,
                      AbsDON is calculated as follows:
                      If DON(m) == DON(n), AbsDON(n) = AbsDON(m)
                      If (DON(m) < DON(n) and DON(n) - DON(m) <
                      32768),
                      AbsDON(n) = AbsDON(m) + DON(n) - DON(m)

Wenger, et al. Standards Track [Page 49] RFC 3984 RTP Payload Format for H.264 Video February 2005

                      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

Wenger, et al. Standards Track [Page 50] RFC 3984 RTP Payload Format for H.264 Video February 2005

                      unit will be more efficient.  A gateway may
                      terminate IP; thus, MTU discovery will normally
                      not work beyond the gateway.
                          Informative note: Setting this parameter to
                          a lower than necessary value may have a
                          negative impact.
 Encoding considerations:
                      This type is only defined for transfer via RTP
                      (RFC 3550).
                      A file format of H.264/AVC video is defined in
                      [29].  This definition is utilized by other
                      file formats, such as the 3GPP multimedia file
                      format (MIME type video/3gpp) [30] or the MP4
                      file format (MIME type video/mp4).
 Security considerations:
                      See section 9 of RFC 3984.
 Public specification:
                      Please refer to RFC 3984 and its section 15.
 Additional information:
                      None
 File extensions:     none
 Macintosh file type code: none
 Object identifier or OID: none
 Person & email address to contact for further information:
                      stewe@stewe.org
 Intended usage:      COMMON
 Author:
                      stewe@stewe.org
 Change controller:
                      IETF Audio/Video Transport working group
                      delegated from the IESG.

Wenger, et al. Standards Track [Page 51] RFC 3984 RTP Payload Format for H.264 Video February 2005

8.2. SDP Parameters

8.2.1. Mapping of MIME Parameters to SDP

 The MIME media type video/H264 string is mapped to fields in the
 Session Description Protocol (SDP) [5] as follows:
 o  The media name in the "m=" line of SDP MUST be video.
 o  The encoding name in the "a=rtpmap" line of SDP MUST be H264 (the
    MIME subtype).
 o  The clock rate in the "a=rtpmap" line MUST be 90000.
 o  The OPTIONAL parameters "profile-level-id", "max-mbps", "max-fs",
    "max-cpb", "max-dpb", "max-br", "redundant-pic-cap", "sprop-
    parameter-sets", "parameter-add", "packetization-mode", "sprop-
    interleaving-depth", "deint-buf-cap", "sprop-deint-buf-req",
    "sprop-init-buf-time", "sprop-max-don-diff", and "max-rcmd-nalu-
    size", when present, MUST be included in the "a=fmtp" line of SDP.
    These parameters are expressed as a MIME media type string, in the
    form of a semicolon separated list of parameter=value pairs.
 An example of media representation in SDP is as follows (Baseline
 Profile, Level 3.0, some of the constraints of the Main profile may
 not be obeyed):
    m=video 49170 RTP/AVP 98
    a=rtpmap:98 H264/90000
    a=fmtp:98 profile-level-id=42A01E;
              sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==

8.2.2. Usage with the SDP Offer/Answer Model

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

Wenger, et al. Standards Track [Page 52] RFC 3984 RTP Payload Format for H.264 Video February 2005

       Informative note: The requirement for symmetric use applies
       only for the above three parameters and not for the other
       stream properties and capability parameters.
    To simplify handling and matching of these configurations, the
    same RTP payload type number used in the offer SHOULD also be used
    in the answer, as specified in [7].  An answer MUST NOT contain a
    payload type number used in the offer unless the configuration
    ("profile-level-id", "packetization-mode", and, if present,
    "sprop-deint-buf-req") is the same as in the offer.
       Informative note: An offerer, when receiving the answer, has to
       compare payload types not declared in the offer based on media
       type (i.e., video/h264) and the above three parameters with any
       payload types it has already declared, in order to determine
       whether the configuration in question is new or equivalent to a
       configuration already offered.
 o  The parameters "sprop-parameter-sets", "sprop-deint-buf-req",
    "sprop-interleaving-depth", "sprop-max-don-diff", and "sprop-
    init-buf-time" describe the properties of the NAL unit stream that
    the offerer or answerer is sending for this media format
    configuration.  This differs from the normal usage of the
    Offer/Answer parameters: normally such parameters declare the
    properties of the stream that the offerer or the answerer is able
    to receive.  When dealing with H.264, the offerer assumes that the
    answerer will be able to receive media encoded using the
    configuration being offered.
       Informative note: The above parameters apply for any stream
       sent by the declaring entity with the same configuration; i.e.,
       they are dependent on their source.  Rather then being bound to
       the payload type, the values may have to be applied to another
       payload type when being sent, as they apply for the
       configuration.
 o  The capability parameters ("max-mbps", "max-fs", "max-cpb", "max-
    dpb", "max-br", ,"redundant-pic-cap", "max-rcmd-nalu-size") MAY be
    used to declare further capabilities.  Their interpretation
    depends on the direction attribute.  When the direction attribute
    is sendonly, then the parameters describe the limits of the RTP
    packets and the NAL unit stream that the sender is capable of
    producing.  When the direction attribute is sendrecv or recvonly,
    then the parameters describe the limitations of what the receiver
    accepts.

Wenger, et al. Standards Track [Page 53] RFC 3984 RTP Payload Format for H.264 Video February 2005

 o  As specified above, an offerer has to include the size of the
    deinterleaving buffer in the offer for an interleaved H.264
    stream.  To enable the offerer and answerer to inform each other
    about their capabilities for deinterleaving buffering, both
    parties are RECOMMENDED to include "deint-buf-cap".  This
    information MAY be used when the value for "sprop-deint-buf-req"
    is selected in a second round of offer and answer.  For
    interleaved streams, it is also RECOMMENDED to consider offering
    multiple payload types with different buffering requirements when
    the capabilities of the receiver are unknown.
 o  The "sprop-parameter-sets" parameter is used as described above.
    In addition, an answerer MUST maintain all parameter sets received
    in the offer in its answer.  Depending on the value of the
    "parameter-add" parameter, different rules apply: If "parameter-
    add" is false (0), the answer MUST NOT add any additional
    parameter sets.  If "parameter-add" is true (1), the answerer, in
    its answer, MAY add additional parameter sets to the "sprop-
    parameter-sets" parameter.  The answerer MUST also, independent of
    the value of "parameter-add", accept to receive a video stream
    using the sprop-parameter-sets it declared in the answer.
       Informative note: care must be taken when parameter sets are
       added not to cause overwriting of already transmitted parameter
       sets by using conflicting parameter set identifiers.
 For streams being delivered over multicast, the following rules apply
 in addition:
 o  The stream properties parameters ("sprop-parameter-sets", "sprop-
    deint-buf-req", "sprop-interleaving-depth", "sprop-max-don-diff",
    and "sprop-init-buf-time") MUST NOT be changed by the answerer.
    Thus, a payload type can either be accepted unaltered or removed.
 o  The receiver capability parameters "max-mbps", "max-fs", "max-
    cpb", "max-dpb", "max-br", and "max-rcmd-nalu-size" MUST be
    supported by the answerer for all streams declared as sendrecv or
    recvonly; otherwise, one of the following actions MUST be
    performed: the media format is removed, or the session rejected.
 o  The receiver capability parameter redundant-pic-cap SHOULD be
    supported by the answerer for all streams declared as sendrecv or
    recvonly as follows:  The answerer SHOULD NOT include redundant
    coded pictures in the transmitted stream if the offerer indicated
    redundant-pic-cap equal to 0.  Otherwise (when redundant_pic_cap
    is equal to 1), it is beyond the scope of this memo to recommend
    how the answerer should use redundant coded pictures.

Wenger, et al. Standards Track [Page 54] RFC 3984 RTP Payload Format for H.264 Video February 2005

 Below are the complete lists of how the different parameters shall be
 interpreted in the different combinations of offer or answer and
 direction attribute.
 o  In offers and answers for which "a=sendrecv" or no direction
    attribute is used, or in offers and answers for which "a=recvonly"
    is used, the following interpretation of the parameters MUST be
    used.
    Declaring actual configuration or properties for receiving:
  1. profile-level-id
  2. packetization-mode
    Declaring actual properties of the stream to be sent (applicable
    only when "a=sendrecv" or no direction attribute is used):
  1. sprop-deint-buf-req
  2. sprop-interleaving-depth
  3. sprop-parameter-sets
  4. sprop-max-don-diff
  5. sprop-init-buf-time
    Declaring receiver implementation capabilities:
  1. max-mbps
  2. max-fs
  3. max-cpb
  4. max-dpb
  5. max-br
  6. redundant-pic-cap
  7. deint-buf-cap
  8. max-rcmd-nalu-size
    Declaring how Offer/Answer negotiation shall be performed:
  1. parameter-add
 o  In an offer or answer for which the direction attribute
    "a=sendonly" is included for the media stream, the following
    interpretation of the parameters MUST be used:
    Declaring actual configuration and properties of stream proposed
    to be sent:
  1. profile-level-id
  2. packetization-mode
  3. sprop-deint-buf-req

Wenger, et al. Standards Track [Page 55] RFC 3984 RTP Payload Format for H.264 Video February 2005

  1. sprop-max-don-diff
  2. sprop-init-buf-time
  3. sprop-parameter-sets
  4. sprop-interleaving-depth
    Declaring the capabilities of the sender when it receives a
    stream:
  1. max-mbps
  2. max-fs
  3. max-cpb
  4. max-dpb
  5. max-br
  6. redundant-pic-cap
  7. deint-buf-cap
  8. max-rcmd-nalu-size
    Declaring how Offer/Answer negotiation shall be performed:
  1. parameter-add
 Furthermore, the following considerations are necessary:
 o  Parameters used for declaring receiver capabilities are in general
    downgradable; i.e., they express the upper limit for a sender's
    possible behavior.  Thus a sender MAY select to set its encoder
    using only lower/lesser or equal values of these parameters.
    "sprop-parameter-sets" MUST NOT be used in a sender's declaration
    of its capabilities, as the limits of the values that are carried
    inside the parameter sets are implicit with the profile and level
    used.
 o  Parameters declaring a configuration point are not downgradable,
    with the exception of the level part of the "profile-level-id"
    parameter.  This expresses values a receiver expects to be used
    and must be used verbatim on the sender side.
 o  When a sender's capabilities are declared, and non-downgradable
    parameters are used in this declaration, then these parameters
    express a configuration that is acceptable.  In order to achieve
    high interoperability levels, it is often advisable to offer
    multiple alternative configurations; e.g., for the packetization
    mode.  It is impossible to offer multiple configurations in a
    single payload type.  Thus, when multiple configuration offers are
    made, each offer requires its own RTP payload type associated with
    the offer.

Wenger, et al. Standards Track [Page 56] RFC 3984 RTP Payload Format for H.264 Video February 2005

 o  A receiver SHOULD understand all MIME parameters, even if it only
    supports a subset of the payload format's functionality.  This
    ensures that a receiver is capable of understanding when an offer
    to receive media can be downgraded to what is supported by the
    receiver of the offer.
 o  An answerer MAY extend the offer with additional media format
    configurations.  However, to enable their usage, in most cases a
    second offer is required from the offerer to provide the stream
    properties parameters that the media sender will use.  This also
    has the effect that the offerer has to be able to receive this
    media format configuration, not only to send it.
 o  If an offerer wishes to have non-symmetric capabilities between
    sending and receiving, the offerer has to offer different RTP
    sessions; i.e., different media lines declared as "recvonly" and
    "sendonly", respectively.  This may have further implications on
    the system.

8.2.3. Usage in Declarative Session Descriptions

 When H.264 over RTP is offered with SDP in a declarative style, as in
 RTSP [27] or SAP [28], the following considerations are necessary.
 o  All parameters capable of indicating the properties of both a NAL
    unit stream and a receiver are used to indicate the properties of
    a NAL unit stream.  For example, in this case, the parameter
    "profile-level-id" declares the values used by the stream, instead
    of the capabilities of the sender.  This results in that the
    following interpretation of the parameters MUST be used:
    Declaring actual configuration or properties:
  1. profile-level-id
  2. sprop-parameter-sets
  3. packetization-mode
  4. sprop-interleaving-depth
  5. sprop-deint-buf-req
  6. sprop-max-don-diff
  7. sprop-init-buf-time

Wenger, et al. Standards Track [Page 57] RFC 3984 RTP Payload Format for H.264 Video February 2005

    Not usable:
  1. max-mbps
  2. max-fs
  3. max-cpb
  4. max-dpb
  5. max-br
  6. redundant-pic-cap
  7. max-rcmd-nalu-size
  8. parameter-add
  9. deint-buf-cap
 o  A receiver of the SDP is required to support all parameters and
    values of the parameters provided; otherwise, the receiver MUST
    reject (RTSP) or not participate in (SAP) the session.  It falls
    on the creator of the session to use values that are expected to
    be supported by the receiving application.

8.3. Examples

 A SIP Offer/Answer exchange wherein both parties are expected to both
 send and receive could look like the following.  Only the media codec
 specific parts of the SDP are shown.  Some lines are wrapped due to
 text constraints.
    Offerer -> Answer SDP message:
    m=video 49170 RTP/AVP 100 99 98
    a=rtpmap:98 H264/90000
    a=fmtp:98 profile-level-id=42A01E; packetization-mode=0;
              sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==
    a=rtpmap:99 H264/90000
    a=fmtp:99 profile-level-id=42A01E; packetization-mode=1;
              sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==
    a=rtpmap:100 H264/90000
    a=fmtp:100 profile-level-id=42A01E; packetization-mode=2;
               sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==;
               sprop-interleaving-depth=45; sprop-deint-buf-req=64000;
               sprop-init-buf-time=102478; deint-buf-cap=128000
 The above offer presents the same codec configuration in three
 different packetization formats.  PT 98 represents single NALU mode,
 PT 99 non-interleaved mode; PT 100 indicates the interleaved mode.
 In the interleaved mode case, the interleaving parameters that the
 offerer would use if the answer indicates support for PT 100 are also
 included.  In all three cases the parameter "sprop-parameter-sets"
 conveys the initial parameter sets that are required for the answerer
 when receiving a stream from the offerer when this configuration

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 (profile-level-id and packetization mode) is accepted.  Note that the
 value for "sprop-parameter-sets", although identical in the example
 above, could be different for each payload type.
   Answerer -> Offerer SDP message:
   m=video 49170 RTP/AVP 100 99 97
   a=rtpmap:97 H264/90000
   a=fmtp:97 profile-level-id=42A01E; packetization-mode=0;
             sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==,As0DEWlsIOp==,
             KyzFGleR
   a=rtpmap:99 H264/90000
   a=fmtp:99 profile-level-id=42A01E; packetization-mode=1;
             sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==,As0DEWlsIOp==,
             KyzFGleR; max-rcmd-nalu-size=3980
   a=rtpmap:100 H264/90000
   a=fmtp:100 profile-level-id=42A01E; packetization-mode=2;
             sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==,As0DEWlsIOp==,
             KyzFGleR; sprop-interleaving-depth=60;
             sprop-deint-buf-req=86000; sprop-init-buf-time=156320;
             deint-buf-cap=128000; max-rcmd-nalu-size=3980
 As the Offer/Answer negotiation covers both sending and receiving
 streams, an offer indicates the exact parameters for what the offerer
 is willing to receive, whereas the answer indicates the same for what
 the answerer accepts to receive.  In this case the offerer declared
 that it is willing to receive payload type 98.  The answerer accepts
 this by declaring a equivalent payload type 97; i.e., it has
 identical values for the three parameters "profile-level-id",
 packetization-mode, and "sprop-deint-buf-req".  This has the
 following implications for both the offerer and the answerer
 concerning the parameters that declare properties.  The offerer
 initially declared a certain value of the "sprop-parameter-sets" in
 the payload definition for PT=98.  However, as the answerer accepted
 this as PT=97, the values of "sprop-parameter-sets" in PT=98 must now
 be used instead when the offerer sends PT=97.  Similarly, when the
 answerer sends PT=98 to the offerer, it has to use the properties
 parameters it declared in PT=97.
 The answerer also accepts the reception of the two configurations
 that payload types 99 and 100 represent.  It provides the initial
 parameter sets for the answerer-to-offerer direction, and for
 buffering related parameters that it will use to send the payload
 types.  It also provides the offerer with its memory limit for
 deinterleaving operations by providing a "deint-buf-cap" parameter.
 This is only useful if the offerer decides on making a second offer,
 where it can take the new value into account.  The "max-rcmd-nalu-
 size" indicates that the answerer can efficiently process NALUs up to

Wenger, et al. Standards Track [Page 59] RFC 3984 RTP Payload Format for H.264 Video February 2005

 the size of 3980 bytes.  However, there is no guarantee that the
 network supports this size.
 Please note that the parameter sets in the above example do not
 represent a legal operation point of an H.264 codec.  The base64
 strings are only used for illustration.

8.4. Parameter Set Considerations

 The H.264 parameter sets are a fundamental part of the video codec
 and vital to its operation; see section 1.2.  Due to their
 characteristics and their importance for the decoding process, lost
 or erroneously transmitted parameter sets can hardly be concealed
 locally at the receiver.  A reference to a corrupt parameter set has
 normally fatal results to the decoding process.  Corruption could
 occur, for example, due to the erroneous transmission or loss of a
 parameter set data structure, but also due to the untimely
 transmission of a parameter set update.  Therefore, the following
 recommendations are provided as a guideline for the implementer of
 the RTP sender.
 Parameter set NALUs can be transported using three different
 principles:
 A. Using a session control protocol (out-of-band) prior to the actual
    RTP session.
 B. Using a session control protocol (out-of-band) during an ongoing
    RTP session.
 C. Within the RTP stream in the payload (in-band) during an ongoing
    RTP session.
 It is necessary to implement principles A and B within a session
 control protocol.  SIP and SDP can be used as described in the SDP
 Offer/Answer model and in the previous sections of this memo.  This
 section contains guidelines on how principles A and B must be
 implemented within session control protocols.  It is independent of
 the particular protocol used.  Principle C is supported by the RTP
 payload format defined in this specification.
 The picture and sequence parameter set NALUs SHOULD NOT be
 transmitted in the RTP payload unless reliable transport is provided
 for RTP, as a loss of a parameter set of either type will likely
 prevent decoding of a considerable portion of the corresponding RTP

Wenger, et al. Standards Track [Page 60] RFC 3984 RTP Payload Format for H.264 Video February 2005

 stream.  Thus, the transmission of parameter sets using a reliable
 session control protocol (i.e., usage of principle A or B above) is
 RECOMMENDED.
 In the rest of the section it is assumed that out-of-band signaling
 provides reliable transport of parameter set NALUs and that in-band
 transport does not.  If in-band signaling of parameter sets is used,
 the sender SHOULD take the error characteristics into account and use
 mechanisms to provide a high probability for delivering the parameter
 sets correctly.  Mechanisms that increase the probability for a
 correct reception include packet repetition, FEC, and retransmission.
 The use of an unreliable, out-of-band control protocol has similar
 disadvantages as the in-band signaling (possible loss) and, in
 addition, may also lead to difficulties in the synchronization (see
 below).  Therefore, it is NOT RECOMMENDED.
 Parameter sets MAY be added or updated during the lifetime of a
 session using principles B and C.  It is required that parameter sets
 are present at the decoder prior to the NAL units that refer to them.
 Updating or adding of parameter sets can result in further problems,
 and therefore the following recommendations should be considered.
  1. When parameter sets are added or updated, principle C is

vulnerable to transmission errors as described above, and

    therefore principle B is RECOMMENDED.
  1. When parameter sets are added or updated, care SHOULD be taken to

ensure that any parameter set is delivered prior to its usage. It

    is common that no synchronization is present between out-of-band
    signaling and in-band traffic.  If out-of-band signaling is used,
    it is RECOMMENDED that a sender does not start sending NALUs
    requiring the updated parameter sets prior to acknowledgement of
    delivery from the signaling protocol.
  1. 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).
  1. When new parameter sets are added, previously unused parameter set

identifiers are used. This avoids the problem identified in the

Wenger, et al. Standards Track [Page 61] RFC 3984 RTP Payload Format for H.264 Video February 2005

    previous paragraph.  However, in a multiparty session, unless a
    synchronized control protocol is used, there is a risk that
    multiple entities try to add different parameter sets for the same
    identifier, which has to be avoided.
  1. Adding or modifying parameter sets by using both principles B and

C in the same RTP session may lead to inconsistencies of the

    parameter sets because of the lack of synchronization between the
    control and the RTP channel.  Therefore, principles B and C MUST
    NOT both be used in the same session unless sufficient
    synchronization can be provided.
 In some scenarios (e.g., when only the subset of this payload format
 specification corresponding to H.241 is used), it is not possible to
 employ out-of-band parameter set transmission.  In this case,
 parameter sets have to be transmitted in-band.  Here, the
 synchronization with the non-parameter-set-data in the bitstream is
 implicit, but the possibility of a loss has to be taken into account.
 The loss probability should be reduced using the mechanisms discussed
 above.
  1. When parameter sets are initially provided using principle A and

then later added or updated in-band (principle C), there is a risk

    associated with updating the parameter sets delivered out-of-band.
    If receivers miss some in-band updates (for example, because of a
    loss or a late tune-in), those receivers attempt to decode the
    bitstream using out-dated parameters.  It is RECOMMENDED that
    parameter set IDs be partitioned between the out-of-band and in-
    band parameter sets.
 To allow for maximum flexibility and best performance from the H.264
 coder, it is recommended, if possible, to allow any sender to add its
 own parameter sets to be used in a session.  Setting the "parameter-
 add" parameter to false should only be done in cases where the
 session topology prevents a participant to add its own parameter
 sets.

9. Security Considerations

 RTP packets using the payload format defined in this specification
 are subject to the security considerations discussed in the RTP
 specification [4], and in any appropriate RTP profile (for example,
 [16]).  This implies that confidentiality of the media streams is
 achieved by encryption; for example, through the application of SRTP
 [26].  Because the data compression used with this payload format is
 applied end-to-end, any encryption needs to be performed after
 compression.

Wenger, et al. Standards Track [Page 62] RFC 3984 RTP Payload Format for H.264 Video February 2005

 A potential denial-of-service threat exists for data encodings using
 compression techniques that have non-uniform receiver-end
 computational load.  The attacker can inject pathological datagrams
 into the stream that are complex to decode and that cause the
 receiver to be overloaded.  H.264 is particularly vulnerable to such
 attacks, as it is extremely simple to generate datagrams containing
 NAL units that affect the decoding process of many future NAL units.
 Therefore, the usage of data origin authentication and data integrity
 protection of at least the RTP packet is RECOMMENDED; for example,
 with SRTP [26].
 Note that the appropriate mechanism to ensure confidentiality and
 integrity of RTP packets and their payloads is very dependent on the
 application and on the transport and signaling protocols employed.
 Thus, although SRTP is given as an example above, other possible
 choices exist.
 Decoders MUST exercise caution with respect to the handling of user
 data SEI messages, particularly if they contain active elements, and
 MUST restrict their domain of applicability to the presentation
 containing the stream.
 End-to-End security with either authentication, integrity or
 confidentiality protection will prevent a MANE from performing
 media-aware operations other than discarding complete packets.  And
 in the case of confidentiality protection it will even be prevented
 from performing discarding of packets in a media aware way.  To allow
 any MANE to perform its operations, it will be required to be a
 trusted entity which is included in the security context
 establishment.

10. Congestion Control

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

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 The bit rate adaptation necessary for obeying the congestion control
 principle is easily achievable when real-time encoding is used.
 However, when pre-encoded content is being transmitted, bandwidth
 adaptation requires the availability of more than one coded
 representation of the same content, at different bit rates, or the
 existence of non-reference pictures or sub-sequences [22] in the
 bitstream.  The switching between the different representations can
 normally be performed in the same RTP session; e.g., by employing a
 concept known as SI/SP slices of the Extended Profile, or by
 switching streams at IDR picture boundaries.  Only when non-
 downgradable parameters (such as the profile part of the
 profile/level ID) are required to be changed does it become necessary
 to terminate and re-start the media stream.  This may be accomplished
 by using a different RTP payload type.
 MANEs MAY follow the suggestions outlined in section 7.3 and remove
 certain unusable packets from the packet stream when that stream was
 damaged due to previous packet losses.  This can help reduce the
 network load in certain special cases.

11. IANA Consideration

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

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12. Informative Appendix: Application Examples

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

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

     Annex A
 H.323-based video telephony systems that use H.264 as an optional
 video compression scheme are required to support H.241 Annex A [15]
 as a packetization scheme.  The packetization mechanism defined in
 this Annex is technically identical with a small subset of this
 specification.
 When a system operates according to H.241 Annex A, parameter set NAL
 units are sent in-band.  Only Single NAL unit packets are used.  Many
 such systems are not sending IDR pictures regularly, but only when
 required by user interaction or by control protocol means; e.g., when
 switching between video channels in a Multipoint Control Unit or for
 error recovery requested by feedback.

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

     Aggregation
 The RTP part of this scheme is implemented and tested (though not the
 control-protocol part; see below).
 In most real-world video telephony applications, picture parameters
 such as picture size or optional modes never change during the
 lifetime of a connection.  Therefore, all necessary parameter sets
 (usually only one) are sent as a side effect of the capability
 exchange/announcement process, e.g., according to the SDP syntax
 specified in section 8.2 of this document.  As all necessary
 parameter set information is established before the RTP session
 starts, there is no need for sending any parameter set NAL units.
 Slice data partitioning is not used, either.  Thus, the RTP packet
 stream basically consists of NAL units that carry single coded
 slices.
 The encoder chooses the size of coded slice NAL units so that they
 offer the best performance.  Often, this is done by adapting the
 coded slice size to the MTU size of the IP network.  For small

Wenger, et al. Standards Track [Page 65] RFC 3984 RTP Payload Format for H.264 Video February 2005

 picture sizes, this may result in a one-picture-per-one-packet
 strategy.  Intra refresh algorithms clean up the loss of packets and
 the resulting drift-related artifacts.

12.3. Video Telephony, Interleaved Packetization Using NAL Unit

     Aggregation
 This scheme allows better error concealment and is used in H.263
 based designs using RFC 2429 packetization [10].  It has been
 implemented, and good results were reported [12].
 The VCL encoder codes the source picture so that all macroblocks
 (MBs) of one MB line are assigned to one slice.  All slices with even
 MB row addresses are combined into one STAP, and all slices with odd
 MB row addresses into another.  Those STAPs are transmitted as RTP
 packets.  The establishment of the parameter sets is performed as
 discussed above.
 Note that the use of STAPs is essential here, as the high number of
 individual slices (18 for a CIF picture) would lead to unacceptably
 high IP/UDP/RTP header overhead (unless the source coding tool FMO is
 used, which is not assumed in this scenario).  Furthermore, some
 wireless video transmission systems, such as H.324M and the IP-based
 video telephony specified in 3GPP, are likely to use relatively small
 transport packet size.  For example, a typical MTU size of H.223 AL3
 SDU is around 100 bytes [17].  Coding individual slices according to
 this packetization scheme provides further advantage in communication
 between wired and wireless networks, as individual slices are likely
 to be smaller than the preferred maximum packet size of wireless
 systems.  Consequently, a gateway can convert the STAPs used in a
 wired network into several RTP packets with only one NAL unit, which
 are preferred in a wireless network, and vice versa.

12.4. Video Telephony with Data Partitioning

 This scheme has been implemented and has been shown to offer good
 performance, especially at higher packet loss rates [12].
 Data Partitioning is known to be useful only when some form of
 unequal error protection is available.  Normally, in single-session
 RTP environments, even error characteristics are assumed; i.e., the
 packet loss probability of all packets of the session is the same
 statistically.  However, there are means to reduce the packet loss
 probability of individual packets in an RTP session.  A FEC packet
 according to RFC 2733 [18], for example, specifies which media
 packets are associated with the FEC packet.

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 In all cases, the incurred overhead is substantial but is in the same
 order of magnitude as the number of bits that have otherwise been
 spent for intra information.  However, this mechanism does not add
 any delay to the system.
 Again, the complete parameter set establishment is performed through
 control protocol means.

12.5. Video Telephony or Streaming with FUs and Forward Error

     Correction
 This scheme has been implemented and has been shown to provide good
 performance, especially at higher packet loss rates [19].
 The most efficient means to combat packet losses for scenarios where
 retransmissions are not applicable is forward error correction (FEC).
 Although application layer, end-to-end use of FEC is often less
 efficient than an FEC-based protection of individual links
 (especially when links of different characteristics are in the
 transmission path), application layer, end-to-end FEC is unavoidable
 in some scenarios.  RFC 2733 [18] provides means to use generic,
 application layer, end-to-end FEC in packet-loss environments.  A
 binary forward error correcting code is generated by applying the XOR
 operation to the bits at the same bit position in different packets.
 The binary code can be specified by the parameters (n,k) in which k
 is the number of information packets used in the connection and n is
 the total number of packets generated for k information packets;
 i.e., n-k parity packets are generated for k information packets.
 When a code is used with parameters (n,k) within the RFC 2733
 framework, the following properties are well known:
 a) If applied over one RTP packet, RFC 2733 provides only packet
    repetition.
 b) RFC 2733 is most bit rate efficient if XOR-connected packets have
    equal length.
 c) At the same packet loss probability p and for a fixed k, the
    greater the value of n is, the smaller the residual error
    probability becomes.  For example, for a packet loss probability
    of 10%, k=1, and n=2, the residual error probability is about 1%,
    whereas for n=3, the residual error probability is about 0.1%.
 d) At the same packet loss probability p and for a fixed code rate
    k/n, the greater the value of n is, the smaller the residual error
    probability becomes.  For example, at a packet loss probability of
    p=10%, k=1 and n=2, the residual error rate is about 1%, whereas

Wenger, et al. Standards Track [Page 67] RFC 3984 RTP Payload Format for H.264 Video February 2005

    for an extended Golay code with k=12 and n=24, the residual error
    rate is about 0.01%.
 For applying RFC 2733 in combination with H.264 baseline coded video
 without using FUs, several options might be considered:
 1) The video encoder produces NAL units for which each video frame is
    coded in a single slice.  Applying FEC, one could use a simple
    code; e.g., (n=2, k=1).  That is, each NAL unit would basically
    just be repeated.  The disadvantage is obviously the bad code
    performance according to d), above, and the low flexibility, as
    only (n, k=1) codes can be used.
 2) The video encoder produces NAL units for which each video frame is
    encoded in one or more consecutive slices.  Applying FEC, one
    could use a better code, e.g., (n=24, k=12), over a sequence of
    NAL units.  Depending on the number of RTP packets per frame, a
    loss may introduce a significant delay, which is reduced when more
    RTP packets are used per frame.  Packets of completely different
    length might also be connected, which decreases bit rate
    efficiency according to b), above.  However, with some care and
    for slices of 1kb or larger, similar length (100-200 bytes
    difference) may be produced, which will not lower the bit
    efficiency catastrophically.
 3) The video encoder produces NAL units, for which a certain frame
    contains k slices of possibly almost equal length.  Then, applying
    FEC, a better code, e.g., (n=24, k=12), can be used over the
    sequence of NAL units for each frame.  The delay compared to that
    of 2), above,  may be reduced, but several disadvantages are
    obvious.  First, the coding efficiency of the encoded video is
    lowered significantly, as slice-structured coding reduces intra-
    frame prediction and additional slice overhead is necessary.
    Second, pre-encoded content or, when operating over a gateway, the
    video is usually not appropriately coded with k slices such that
    FEC can be applied.  Finally, the encoding of video producing k
    slices of equal length is not straightforward and might require
    more than one encoding pass.
 Many of the mentioned disadvantages can be avoided by applying FUs in
 combination with FEC.  Each NAL unit can be split into any number of
 FUs of basically equal length; therefore, FEC with a reasonable k and
 n can be applied, even if the encoder made no effort to produce
 slices of equal length.  For example, a coded slice NAL unit
 containing an entire frame can be split to k FUs, and a parity check
 code (n=k+1, k) can be applied.  However, this has the disadvantage

Wenger, et al. Standards Track [Page 68] RFC 3984 RTP Payload Format for H.264 Video February 2005

 that unless all created fragments can be recovered, the whole slice
 will be lost.  Thus a larger section is lost than would be if the
 frame had been split into several slices.
 The presented technique makes it possible to achieve good
 transmission error tolerance, even if no additional source coding
 layer redundancy (such as periodic intra frames) is present.
 Consequently, the same coded video sequence can be used to achieve
 the maximum compression efficiency and quality over error-free
 transmission and for transmission over error-prone networks.
 Furthermore, the technique allows the application of FEC to pre-
 encoded sequences without adding delay.  In this case, pre-encoded
 sequences that are not encoded for error-prone networks can still be
 transmitted almost reliably without adding extensive delays.  In
 addition, FUs of equal length result in a bit rate efficient use of
 RFC 2733.
 If the error probability depends on the length of the transmitted
 packet (e.g., in case of mobile transmission [14]), the benefits of
 applying FUs with FEC are even more obvious.  Basically, the
 flexibility of the size of FUs allows appropriate FEC to be applied
 for each NAL unit and unequal error protection of NAL units.
 When FUs and FEC are used, the incurred overhead is substantial but
 is in the same order of magnitude as the number of bits that have to
 be spent for intra-coded macroblocks if no FEC is applied.  In [19],
 it was shown that the overall performance of the FEC-based approach
 enhanced quality when using the same error rate and same overall bit
 rate, including the overhead.

12.6. Low Bit-Rate Streaming

 This scheme has been implemented with H.263 and non-standard RTP
 packetization and has given good results [20].  There is no technical
 reason why similarly good results could not be achievable with H.264.
 In today's Internet streaming, some of the offered bit rates are
 relatively low in order to allow terminals with dial-up modems to
 access the content.  In wired IP networks, relatively large packets,
 say 500 - 1500 bytes, are preferred to smaller and more frequently
 occurring packets in order to reduce network congestion.  Moreover,
 use of large packets decreases the amount of RTP/UDP/IP header
 overhead.  For low bit-rate video, the use of large packets means
 that sometimes up to few pictures should be encapsulated in one
 packet.

Wenger, et al. Standards Track [Page 69] RFC 3984 RTP Payload Format for H.264 Video February 2005

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

12.7. Robust Packet Scheduling in Video Streaming

 Robust packet scheduling has been implemented with MPEG-4 Part 2 and
 simulated in a wireless streaming environment [21].  There is no
 technical reason why similar or better results could not be
 achievable with H.264.
 Streaming clients typically have a receiver buffer that is capable of
 storing a relatively large amount of data.  Initially, when a
 streaming session is established, a client does not start playing the
 stream back immediately.  Rather, it typically buffers the incoming
 data for a few seconds.  This buffering helps maintain continuous
 playback, as, in case of occasional increased transmission delays or
 network throughput drops, the client can decode and play buffered
 data.  Otherwise, without initial buffering, the client has to freeze
 the display, stop decoding, and wait for incoming data.  The
 buffering is also necessary for either automatic or selective
 retransmission in any protocol level.  If any part of a picture is
 lost, a retransmission mechanism may be used to resend the lost data.
 If the retransmitted data is received before its scheduled decoding
 or playback time, the loss is recovered perfectly.  Coded pictures
 can be ranked according to their importance in the subjective quality
 of the decoded sequence.  For example, non-reference pictures, such
 as conventional B pictures, are subjectively least important, as
 their absence does not affect decoding of any other pictures.  In
 addition to non-reference pictures, the ITU-T H.264 | ISO/IEC
 14496-10 standard includes a temporal scalability method called sub-
 sequences [22].  Subjective ranking can also be made on coded slice
 data partition or slice group basis.  Coded slices and coded slice
 data partitions that are subjectively the most important can be sent
 earlier than their decoding order indicates, whereas coded slices and
 coded slice data partitions that are subjectively the least important
 can be sent later than their natural coding order indicates.
 Consequently, any retransmitted parts of the most important slices

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 and coded slice data partitions are more likely to be received before
 their scheduled decoding or playback time compared to the least
 important slices and slice data partitions.

13. Informative Appendix: Rationale for Decoding Order Number

13.1. Introduction

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

13.2. Example of Multi-Picture Slice Interleaving

 An example of multi-picture slice interleaving follows.  A subset of
 a coded video sequence is depicted below in output order.  R denotes
 a reference picture, N denotes a non-reference picture, and the
 number indicates a relative output time.
    ... R1 N2 R3 N4 R5 ...
 The decoding order of these pictures from left to right is as
 follows:
    ... R1 R3 N2 R5 N4 ...
 The NAL units of pictures R1, R3, N2, R5, and N4 are marked with a
 DON equal to 1, 2, 3, 4, and 5, respectively.

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 Each reference picture consists of three slice groups that are
 scattered as follows (a number denotes the slice group number for
 each macroblock in a QCIF frame):
    0 1 2 0 1 2 0 1 2 0 1
    2 0 1 2 0 1 2 0 1 2 0
    1 2 0 1 2 0 1 2 0 1 2
    0 1 2 0 1 2 0 1 2 0 1
    2 0 1 2 0 1 2 0 1 2 0
    1 2 0 1 2 0 1 2 0 1 2
    0 1 2 0 1 2 0 1 2 0 1
    2 0 1 2 0 1 2 0 1 2 0
    1 2 0 1 2 0 1 2 0 1 2
 For the sake of simplicity, we assume that all the macroblocks of a
 slice group are included in one slice.  Three MTAPs are constructed
 from three consecutive reference pictures so that each MTAP contains
 three aggregation units, each of which contains all the macroblocks
 from one slice group.  The first MTAP contains slice group 0 of
 picture R1, slice group 1 of picture R3, and slice group 2 of
 picture R5.  The second MTAP contains slice group 1 of picture R1,
 slice group 2 of picture R3, and slice group 0 of picture R5.  The
 third MTAP contains slice group 2 of picture R1, slice group 0 of
 picture R3, and slice group 1 of picture R5.  Each non-reference
 picture is encapsulated into an STAP-B.
 Consequently, the transmission order of NAL units is the following:
    R1, slice group 0, DON 1, carried in MTAP,   RTP SN: N
    R3, slice group 1, DON 2, carried in MTAP,   RTP SN: N
    R5, slice group 2, DON 4, carried in MTAP,   RTP SN: N
    R1, slice group 1, DON 1, carried in MTAP,   RTP SN: N+1
    R3, slice group 2, DON 2, carried in MTAP,   RTP SN: N+1
    R5, slice group 0, DON 4, carried in MTAP,   RTP SN: N+1
    R1, slice group 2, DON 1, carried in MTAP,   RTP SN: N+2
    R3, slice group 1, DON 2, carried in MTAP,   RTP SN: N+2
    R5, slice group 0, DON 4, carried in MTAP,   RTP SN: N+2
    N2,                DON 3, carried in STAP-B, RTP SN: N+3
    N4,                DON 5, carried in STAP-B, RTP SN: N+4
 The receiver is able to organize the NAL units back in decoding order
 based on the value of DON associated with each NAL unit.
 If one of the MTAPs is lost, the spatially adjacent and temporally
 co-located macroblocks are received and can be used to conceal the
 loss efficiently.  If one of the STAPs is lost, the effect of the
 loss does not propagate temporally.

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13.3. Example of Robust Packet Scheduling

 An example of robust packet scheduling follows.  The communication
 system used in the example consists of the following components in
 the order that the video is processed from source to sink:
    o camera and capturing
    o pre-encoding buffer
    o encoder
    o encoded picture buffer
    o transmitter
    o transmission channel
    o receiver
    o receiver buffer
    o decoder
    o decoded picture buffer
    o display
 The video communication system used in the example operates as
 follows.  Note that processing of the video stream happens gradually
 and at the same time in all components of the system.  The source
 video sequence is shot and captured to a pre-encoding buffer.  The
 pre-encoding buffer can be used to order pictures from sampling order
 to encoding order or to analyze multiple uncompressed frames for bit
 rate control purposes, for example.  In some cases, the pre-encoding
 buffer may not exist; instead, the sampled pictures are encoded right
 away.  The encoder encodes pictures from the pre-encoding buffer and
 stores the output; i.e., coded pictures, to the encoded picture
 buffer.  The transmitter encapsulates the coded pictures from the
 encoded picture buffer to transmission packets and sends them to a
 receiver through a transmission channel.  The receiver stores the
 received packets to the receiver buffer.  The receiver buffering
 process typically includes buffering for transmission delay jitter.
 The receiver buffer can also be used to recover correct decoding
 order of coded data.  The decoder reads coded data from the receiver
 buffer and produces decoded pictures as output into the decoded
 picture buffer.  The decoded picture buffer is used to recover the
 output (or display) order of pictures.  Finally, pictures are
 displayed.
 In the following example figures, I denotes an IDR picture, R denotes
 a reference picture, N denotes a non-reference picture, and the
 number after I, R, or N indicates the sampling time relative to the
 previous IDR picture in decoding order.  Values below the sequence of
 pictures indicate scaled system clock timestamps.  The system clock
 is initialized arbitrarily in this example, and time runs from left
 to right.  Each I, R, and N picture is mapped into the same timeline
 compared to the previous processing step, if any, assuming that

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 encoding, transmission, and decoding take no time.  Thus, events
 happening at the same time are located in the same column throughout
 all example figures.
 A subset of a sequence of coded pictures is depicted below in
 sampling order.
     ...  N58 N59 I00 N01 N02 R03 N04 N05 R06 ... N58 N59 I00 N01 ...
     ... --|---|---|---|---|---|---|---|---|- ... -|---|---|---|- ...
     ...  58  59  60  61  62  63  64  65  66  ... 128 129 130 131 ...
    Figure 16.  Sequence of pictures in sampling order
 The sampled pictures are buffered in the pre-encoding buffer to
 arrange them in encoding order.  In this example, we assume that the
 non-reference pictures are predicted from both the previous and the
 next reference picture in output order, except for the non-reference
 pictures immediately preceding an IDR picture, which are predicted
 only from the previous reference picture in output order.  Thus, the
 pre-encoding buffer has to contain at least two pictures, and the
 buffering causes a delay of two picture intervals.  The output of the
 pre-encoding buffering process and the encoding (and decoding) order
 of the pictures are as follows:
              ... N58 N59 I00 R03 N01 N02 R06 N04 N05 ...
              ... -|---|---|---|---|---|---|---|---|- ...
              ... 60  61  62  63  64  65  66  67  68  ...
    Figure 17.  Re-ordered pictures in the pre-encoding buffer
 The encoder or the transmitter can set the value of DON for each
 picture to a value of DON for the previous picture in decoding order
 plus one.
 For the sake of simplicity, let us assume that:
 o  the frame rate of the sequence is constant,
 o  each picture consists of only one slice,
 o  each slice is encapsulated in a single NAL unit packet,
 o  there is no transmission delay, and
 o  pictures are transmitted at constant intervals (that is, 1 / frame
    rate).

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 When pictures are transmitted in decoding order, they are received as
 follows:
              ... N58 N59 I00 R03 N01 N02 R06 N04 N05 ...
              ... -|---|---|---|---|---|---|---|---|- ...
              ... 60  61  62  63  64  65  66  67  68  ...
    Figure 18.  Received pictures in decoding order
 The OPTIONAL sprop-interleaving-depth MIME type parameter is set to
 0, as the transmission (or reception) order is identical to the
 decoding order.
 The decoder has to buffer for one picture interval initially in its
 decoded picture buffer to organize pictures from decoding order to
 output order as depicted below:
                  ... N58 N59 I00 N01 N02 R03 N04 N05 R06 ...
                  ... -|---|---|---|---|---|---|---|---|- ...
                  ... 61  62  63  64  65  66  67  68  69  ...
    Figure 19.  Output order
 The amount of required initial buffering in the decoded picture
 buffer can be signaled in the buffering period SEI message or with
 the num_reorder_frames syntax element of H.264 video usability
 information.  num_reorder_frames indicates the maximum number of
 frames, complementary field pairs, or non-paired fields that precede
 any frame, complementary field pair, or non-paired field in the
 sequence in decoding order and that follow it in output order.  For
 the sake of simplicity, we assume that num_reorder_frames is used to
 indicate the initial buffer in the decoded picture buffer.  In this
 example, num_reorder_frames is equal to 1.
 It can be observed that if the IDR picture I00 is lost during
 transmission and a retransmission request is issued when the value of
 the system clock is 62, there is one picture interval of time (until
 the system clock reaches timestamp 63) to receive the retransmitted
 IDR picture I00.

Wenger, et al. Standards Track [Page 75] RFC 3984 RTP Payload Format for H.264 Video February 2005

 Let us then assume that IDR pictures are transmitted two frame
 intervals earlier than their decoding position; i.e., the pictures
 are transmitted as follows:
                     ...  I00 N58 N59 R03 N01 N02 R06 N04 N05 ...
                     ... --|---|---|---|---|---|---|---|---|- ...
                     ...  62  63  64  65  66  67  68  69  70  ...
    Figure 20.  Interleaving: Early IDR pictures in sending order
 The OPTIONAL sprop-interleaving-depth MIME type parameter is set
 equal to 1 according to its definition.  (The value of sprop-
 interleaving-depth in this example can be derived as follows:
 Picture I00 is the only picture preceding picture N58 or N59 in
 transmission order and following it in decoding order.  Except for
 pictures I00, N58, and N59, the transmission order is the same as the
 decoding order of pictures.  As a coded picture is encapsulated into
 exactly one NAL unit, the value of sprop-interleaving-depth is equal
 to the maximum number of pictures preceding any picture in
 transmission order and following the picture in decoding order.)
 The receiver buffering process contains two pictures at a time
 according to the value of the sprop-interleaving-depth parameter and
 orders pictures from the reception order to the correct decoding
 order based on the value of DON associated with each picture.  The
 output of the receiver buffering process is as follows:
                          ... N58 N59 I00 R03 N01 N02 R06 N04 N05 ...
                          ... -|---|---|---|---|---|---|---|---|- ...
                          ... 63  64  65  66  67  68  69  70  71  ...
    Figure 21.  Interleaving: Receiver buffer
 Again, an initial buffering delay of one picture interval is needed
 to organize pictures from decoding order to output order, as depicted
 below:
                              ... N58 N59 I00 N01 N02 R03 N04 N05 ...
                              ... -|---|---|---|---|---|---|---|- ...
                              ... 64  65  66  67  68  69  70  71  ...
    Figure 22.  Interleaving: Receiver buffer after reordering
 Note that the maximum delay that IDR pictures can undergo during
 transmission, including possible application, transport, or link
 layer retransmission, is equal to three picture intervals.  Thus, the

Wenger, et al. Standards Track [Page 76] RFC 3984 RTP Payload Format for H.264 Video February 2005

 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).

Wenger, et al. Standards Track [Page 77] RFC 3984 RTP Payload Format for H.264 Video February 2005

 o  Coded slices from multiple coded video sequences cannot be
    interleaved, as the frame number syntax element is reset to 0 in
    each IDR picture.
 o  The coded fields of a complementary field pair share the same
    value of the frame_num syntax element.  Thus, the decoding order
    of the coded fields of a complementary field pair cannot be
    recovered based on the frame_num syntax element or any other
    syntax element of the H.264 coding syntax.
 The RTP payload format for transport of MPEG-4 elementary streams
 [25] enables interleaving of access units and transmission of
 multiple access units in the same RTP packet.  An access unit is
 specified in the H.264 coding standard to comprise all NAL units
 associated with a primary coded picture according to subclause
 7.4.1.2 of [1].  Consequently, slices of different pictures cannot be
 interleaved, and the multi-picture slice interleaving technique (see
 section 12.6) for improved error resilience cannot be used.

14. Acknowledgements

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

15. References

15.1. Normative References

 [1]  ITU-T Recommendation H.264, "Advanced video coding for generic
      audiovisual services", May 2003.
 [2]  ISO/IEC International Standard 14496-10:2003.
 [3]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [4]  Schulzrinne, H.,  Casner, S., Frederick, R., and V. Jacobson,
      "RTP: A Transport Protocol for Real-Time Applications", STD 64,
      RFC 3550, July 2003.
 [5]  Handley, M. and V. Jacobson, "SDP: Session Description
      Protocol", RFC 2327, April 1998.
 [6]  Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
      RFC 3548, July 2003.

Wenger, et al. Standards Track [Page 78] RFC 3984 RTP Payload Format for H.264 Video February 2005

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

15.2. Informative References

 [8]  "Draft ITU-T Recommendation and Final Draft International
      Standard of Joint Video Specification (ITU-T Rec. H.264 |
      ISO/IEC 14496-10 AVC)", available from http://ftp3.itu.int/av-
      arch/jvt-site/2003_03_Pattaya/JVT-G050r1.zip, May 2003.
 [9]  Luthra, A., Sullivan, G.J., and T. Wiegand (eds.), Special Issue
      on H.264/AVC. IEEE Transactions on Circuits and Systems on Video
      Technology, July 2003.
 [10] Bormann, C., Cline, L., Deisher, G., Gardos, T., Maciocco, C.,
      Newell, D., Ott, J., Sullivan, G., Wenger, S., and C. Zhu, "RTP
      Payload Format for the 1998 Version of ITU-T Rec. H.263 Video
      (H.263+)", RFC 2429, October 1998.
 [11] ISO/IEC IS 14496-2.
 [12] Wenger, S., "H.26L over IP", IEEE Transaction on Circuits and
      Systems for Video technology, Vol. 13, No. 7, July 2003.
 [13] Wenger, S., "H.26L over IP: The IP Network Adaptation Layer",
      Proceedings Packet Video Workshop 02, April 2002.
 [14] Stockhammer, T., Hannuksela, M.M., and S. Wenger, "H.26L/JVT
      Coding Network Abstraction Layer and IP-based Transport" in
      Proc. ICIP 2002, Rochester, NY, September 2002.
 [15] ITU-T Recommendation H.241, "Extended video procedures and
      control signals for H.300 series terminals", 2004.
 [16] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
      Conferences with Minimal Control", STD 65, RFC 3551, July 2003.
 [17] ITU-T Recommendation H.223, "Multiplexing protocol for low bit
      rate multimedia communication", July 2001.
 [18] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format for
      Generic Forward Error Correction", RFC 2733, December 1999.
 [19] Stockhammer, T., Wiegand, T., Oelbaum, T., and F. Obermeier,
      "Video Coding and Transport Layer Techniques for H.264/AVC-Based
      Transmission over Packet-Lossy Networks", IEEE International
      Conference on Image Processing (ICIP 2003), Barcelona, Spain,
      September 2003.

Wenger, et al. Standards Track [Page 79] RFC 3984 RTP Payload Format for H.264 Video February 2005

 [20] Varsa, V. and M. Karczewicz, "Slice interleaving in compressed
      video packetization", Packet Video Workshop 2000.
 [21] Kang, S.H. and A. Zakhor, "Packet scheduling algorithm for
      wireless video streaming," International Packet Video Workshop
      2002.
 [22] Hannuksela, M.M., "Enhanced concept of GOP", JVT-B042, available
      http://ftp3.itu.int/av-arch/video-site/0201_Gen/JVT-B042.doc,
      January 2002.
 [23] Wenger, S., "Video Redundancy Coding in H.263+", 1997
      International Workshop on Audio-Visual Services over Packet
      Networks, September 1997.
 [24] Wang, Y.-K., Hannuksela, M.M., and M. Gabbouj, "Error Resilient
      Video Coding Using Unequally Protected Key Pictures", in Proc.
      International Workshop VLBV03, September 2003.
 [25] van der Meer, J., Mackie, D., Swaminathan, V., Singer, D., and
      P. Gentric, "RTP Payload Format for Transport of MPEG-4
      Elementary Streams", RFC 3640, November 2003.
 [26] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
      Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
      3711, March 2004.
 [27] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
      Protocol (RTSP)", RFC 2326, April 1998.
 [28] Handley, M., Perkins, C., and E. Whelan, "Session Announcement
      Protocol", RFC 2974, October 2000.
 [29] ISO/IEC 14496-15: "Information technology - Coding of audio-
      visual objects - Part 15: Advanced Video Coding (AVC) file
      format".
 [30] Castagno, R. and D. Singer, "MIME Type Registrations for 3rd
      Generation Partnership Project (3GPP) Multimedia files", RFC
      3839, July 2004.

Wenger, et al. Standards Track [Page 80] RFC 3984 RTP Payload Format for H.264 Video February 2005

Authors' Addresses

 Stephan Wenger
 TU Berlin / Teles AG
 Franklinstr. 28-29
 D-10587 Berlin
 Germany
 Phone: +49-172-300-0813
 EMail: stewe@stewe.org
 Miska M. Hannuksela
 Nokia Corporation
 P.O. Box 100
 33721 Tampere
 Finland
 Phone: +358-7180-73151
 EMail: miska.hannuksela@nokia.com
 Thomas Stockhammer
 Nomor Research
 D-83346 Bergen
 Germany
 Phone: +49-8662-419407
 EMail: stockhammer@nomor.de
 Magnus Westerlund
 Multimedia Technologies
 Ericsson Research EAB/TVA/A
 Ericsson AB
 Torshamsgatan 23
 SE-164 80 Stockholm
 Sweden
 Phone: +46-8-7190000
 EMail: magnus.westerlund@ericsson.com

Wenger, et al. Standards Track [Page 81] RFC 3984 RTP Payload Format for H.264 Video February 2005

 David Singer
 QuickTime Engineering
 Apple
 1 Infinite Loop MS 302-3MT
 Cupertino
 CA 95014
 USA
 Phone +1 408 974-3162
 EMail: singer@apple.com

Wenger, et al. Standards Track [Page 82] RFC 3984 RTP Payload Format for H.264 Video February 2005

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 assurances of licenses to be made available, or the result of an
 attempt made to obtain a general license or permission for the use of
 such proprietary rights by implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at ietf-
 ipr@ietf.org.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Wenger, et al. Standards Track [Page 83]

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