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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 RFC 3984

 Network Working Group                                       S. Wenger
 Internet Draft                                        M.M. Hannuksela
 Document: draft-ietf-avt-rtp-h264-11.txt               T. Stockhammer
 Expires: February 2005                                  M. Westerlund
                                                             D. Singer
                                                           August 2004
 
 
 
 
 
                     RTP payload Format for H.264 Video
 
 
 
 Status of this Memo
 
    By submitting this Internet-Draft, I (we) certify that any
    applicable patent or other IPR claims of which I am (we are) aware
    have been disclosed, and any of which I (we) become aware will be
    disclosed, in accordance with RFC 3668 (BCP 79).
 
    By submitting this Internet-Draft, I (we) accept the provisions of
    Section 3 of RFC 3667 (BCP 78).
 
    Internet-Drafts are working documents of the Internet Engineering
    Task Force (IETF), its areas, and its working groups.  Note that
    other groups may also distribute working documents as Internet-
    Drafts.
 
    Internet-Drafts are draft documents valid for a maximum of six
    months and may be updated, replaced, or obsoleted by other documents
    at any time.  It is inappropriate to use Internet-Drafts as
    reference material or to cite them other than as "work in progress."
 
    The list of current Internet-Drafts can be accessed at
    http://www.ietf.org/1id-abstracts.txt
 
    The list of Internet-Draft Shadow Directories can be accessed at
    http://www.ietf.org/shadow.html
 
    This document is a submission of the IETF AVT WG.  Comments should
    be directed to the AVT WG mailing list, avt@ietf.org.
 
 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
 
 
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    supporting from simple low-bit rate conversational usage to Internet
    video streaming with interleaved transmission, all the way to high
    bit-rate video-on-demand applications.
 
 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
   5.4. Packetization Modes...........................................14
   5.5. Decoding Order Number (DON)...................................15
   5.6. Single NAL Unit Packet........................................17
   5.7. Aggregation Packets...........................................18
   5.8. Fragmentation Units (FUs).....................................26
 6. Packetization Rules...............................................29
   6.1. Common Packetization Rules....................................30
   6.2. Single NAL Unit Mode..........................................30
   6.3. Non-Interleaved Mode..........................................31
   6.4. Interleaved Mode..............................................31
 7. De-Packetization Process (Informative)............................31
   7.1. Single NAL Unit and Non-Interleaved Mode......................31
   7.2. Interleaved Mode..............................................32
   7.3. Additional De-Packetization Guidelines........................34
 8. Payload Format Parameters.........................................35
   8.1. MIME Registration.............................................35
   8.2. SDP Parameters................................................48
   8.3. Examples......................................................54
   8.4. Parameter Set Considerations..................................56
 9. Security Considerations...........................................58
 10. Congestion Control...............................................59
 11. IANA Consideration...............................................59
 12. Informative Appendix: Application Examples.......................59
   12.1. Video Telephony according to ITU-T Recommendation H.241  Annex
   A..................................................................60
   12.2. Video Telephony, No Slice Data Partitioning, No NAL Unit
   Aggregation........................................................60
   12.3. Video Telephony, Interleaved Packetization Using NAL Unit
   Aggregation........................................................60
   12.4. Video Telephony, with Data Partitioning......................61
   12.5. Video Telephony or Streaming, with FUs and Forward Error
   Correction.........................................................62
   12.6. Low-Bit-Rate Streaming.......................................64
 
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   12.7. Robust Packet Scheduling in Video Streaming..................64
 13. Informative Appendix: Rationale for Decoding Order Number........65
   13.1. Introduction.................................................65
   13.2. Example of Multi-Picture Slice Interleaving..................65
   13.3. Example of Robust Packet Scheduling..........................67
   13.4. Robust Transmission Scheduling of Redundant Coded Slices.....70
   13.5. Remarks on Other Design Possibilities........................71
 14. Acknowledgements.................................................72
 15. Full Copyright Statement.........................................72
 16. Intellectual Property Notice.....................................72
 17. References.......................................................73
   17.1. Normative References.........................................73
   17.2. Informative References.......................................73
 18. RFC Editor Considerations........................................75
 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 (both also known as Advanced
    Video Coding, AVC) [2].  Recommendation H.264 was approved by ITU-T
    on May 2003, and the approved draft specification is available for
    public review [9].  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 near loss-less coding.  The overall performance of
    H.264 is as such that bit rate savings of 50% or more, compared to
    the current state of technology, are reported.  Digital Satellite TV
    quality, for example, was reported to be achievable at 1.5 Mbit/s,
    compared to the current operation point of MPEG 2 video at around
    3.5 Mbit/s [10].
 
    The codec specification [1] itself distinguishes conceptually
    between a video coding layer (VCL), and a network abstraction layer
    (NAL).  The VCL contains the signal processing functionality of the
    codec, mechanisms such as transform, quantization, motion
    compensated prediction, and a loop filter.  It follows the general
    concept of most of today's video codecs, a macroblock-based coder
    that utilizes 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).
    Macroblocks in slices are ordered in scan order unless a different
    macroblock allocation is specified, using the so-called Flexible
 
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    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 the transmission over packet networks
    or the use in packet oriented multiplex environments.  Annex B of
    H.264 defines an encapsulation process to transmit such NAL units
    over byte-stream oriented networks.  In the scope of this memo Annex
    B is not relevant.
 
    Internally, the NAL uses NAL units.  A NAL unit consists of a one-
    byte header and the payload byte string.  The header indicates the
    type of the NAL unit, the (potential) presence of bit errors or
    syntax violations in the NAL unit payload, and information regarding
    the relative importance of the NAL unit for the decoding process.
    This RTP payload specification is designed to be unaware of the bit
    string in the NAL unit payload.
 
    One of the main properties of H.264 is the complete decoupling of
    the transmission time, the decoding time, and the sampling or
    presentation time of slices and pictures.  The decoding process
    specified in H.264 is unaware of time, and the H.264 syntax does not
    carry information such as the number of skipped frames (as common in
    the form of the Temporal Reference in earlier video compression
    standards).  Also, there are NAL units that affect many pictures and
    are, hence, inherently time-less.  For this reason, the handling of
    the RTP timestamp requires some special considerations for those 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 [12] or MPEG-4's Header Extension Code (HEC) [13]
    unnecessary.  The way that this was achieved is to decouple
    information that is 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 such 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
 
 
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    parameter set structures contain information such as picture size,
    optional coding modes employed, and macroblock to slice group map.
 
    In order to be able to change picture parameters (such as the
    picture size), without having the need to transmit parameter set
    updates synchronously to the slice packet stream, the encoder and
    decoder can maintain a list of more than one sequence and picture
    parameter set.  Each slice header contains a codeword that indicates
    the sequence and picture parameter set to be used.
 
    This mechanism allows the decoupling of the transmission of
    parameter sets from the packet stream, and the transmission of them
    by external means, e.g. as a side effect of the capability exchange,
    or through a (reliable or unreliable) control protocol.  It may even
    be possible that they get 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 [14],
    [15] and [16].
 
    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.
 
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    Type: 5 bits
       nal_unit_type.  This component specifies the NAL unit payload
       type as defined in table 7-1 of [1], and later within this memo.
       For a reference of all currently defined NAL unit types and
       their semantics please refer to section 7.4.1 in [1].
 
    This memo introduces new NAL unit types, which are presented in
    section 5.2.  The NAL unit types defined in this memo are marked as
    unspecified in [1].  Moreover, this specification extends the
    semantics of F and NRI as described in section 5.3.
 
 
 2. Conventions
 
    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
    document are to be interpreted as described in RFC 2119 [3].
 
    This specification uses the notion of setting and clearing a bit
    when handling bit fields.  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 below 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)
 
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       access unit followed zero or more non-IDR access units including
       all subsequent access units up to but not including any
       subsequent IDR access unit.
 
       IDR access unit: An access unit in which the primary coded
       picture is an IDR picture.
 
       IDR picture: A coded picture containing only slices with I or SI
       slice types that causes a "reset" in the decoding process.
       After the decoding of an IDR picture all following coded
       pictures in decoding order can be decoded without inter
       prediction from any picture decoded prior to the IDR picture.
 
       primary coded picture: The coded representation of a picture to
       be used by the decoding process for a bitstream conforming to
       H.264.  The primary coded picture contains all macroblocks of
       the picture.
 
       redundant coded picture: A coded representation of a picture or
       a part of a picture.  The content of a redundant coded picture
       shall not be used by the decoding process for a bitstream
       conforming to H.264.  The content of a redundant coded picture
       may be used by the decoding process for a bitstream that
       contains errors or losses.
 
       VCL NAL unit: A collective term used to refer to coded slice and
       coded data partition NAL units.
 
    In addition, the following definitions apply:
 
       decoding order number (DON): A field in the payload structure or
       a derived variable indicating NAL unit decoding order.  Values
       of DON are in the range of 0 to 65535, inclusive.  After
       reaching the maximum value, the value of DON wraps around to 0.
 
       NAL unit decoding order: A NAL unit order that conforms to the
       constraints on NAL unit order given in section 7.4.1.2 in [1].
 
       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 on 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
           signalling (e.g. to learn about the payload type mappings of
 
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           the media streams) and that it has to be trusted when working
           with SRTP. The advantage of using MANEs is that they allow to
           drop packets according to the needs of the media coding. For
           example, if a MANE needs to drop packets due to congestion on
           a certain link, it can identify those packets whose dropping
           has the smallest negative impact on the user experience, and
           remove those 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 encapsulating one NAL unit per RTP packet, the RECOMMENDED RTP
    payload format is specified in section 5.6.  The RTP payload (and
    the settings for some RTP header bits) for aggregation packets and
    fragmentation units are specified in sections 5.7 and 5.8,
    respectively.
 
 
 
 
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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |V=2|P|X|  CC   |M|     PT      |       sequence number         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           timestamp                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           synchronization source (SSRC) identifier            |
    +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
    |            contributing source (CSRC) identifiers             |
    |                             ....                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
    Figure 1: RTP header according 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 and 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 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, and 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 needs to be performed
       either through the profile used or in a dynamic way.
 
    Sequence number (SN): 16 bits
       Set and used in accordance with RFC 3550.  For the single NALU
       and non-interleaved packetization mode, the sequence number is
       used to determine decoding order for the NALU.
 
    Timestamp: 32 bits
       The RTP timestamp is set to the sampling timestamp of the
       content.  A 90 kHz clock rate MUST be used.
 
 
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       If the NAL unit has no timing properties of its own (e.g.
       parameter set and SEI NAL units), the RTP timestamp is set to
       the RTP timestamp of the primary coded picture of the access
       unit in which the NAL unit is included according to section
       7.4.1.2 of [1].
 
       The setting of the RTP Timestamp for MTAPs is defined in section
       5.7.2.
 
       Receivers SHOULD ignore any picture timing SEI messages included
       in access units that have only one display timestamp.  Instead,
       receivers SHOULD use the RTP timestamp for synchronizing the
       display process.
 
       RTP senders SHOULD NOT transmit picture timing SEI messages for
       pictures that are not supposed to be displayed as multiple
       fields.
 
       In case that 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 where film
           content that consists of coded frames is displayed on an
           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: Due to the fact that 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 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.
 
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 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:
 
    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                                    -
 
 
       Informative note: This specification does not limit the size of
       NAL units encapsulated in single NAL unit packets and
       fragmentation units.  The maximum size of a NAL unit
       encapsulated in any aggregation packet is 65535 bytes.
 
 
 
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 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 violation may be present in
       the payload or in the NAL unit type octet.  The simplest decoder
       reaction to respond 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 compared to 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 can use this information to protect more important NAL
       units better than less important NAL units.  11 is the highest
       transport priority, followed by 10, then by 01 and, finally, 00
       is the lowest.
 
           Informative note: Any non-zero value of NRI is handled
           identically in H.264 decoders.  Therefore, receivers need not
 
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           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.
 
       An H.264 encoder SHOULD set the value of NRI for NAL units
       having nal_unit_type equal to 7 or 8 (indicating a sequence
       parameter set or a picture parameter set respectively) to 11 (in
       binary format). An H.264 encoder SHOULD set the value of NRI 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) to 11 (in binary format).
 
       The following example for a mapping of the remaining
       nal_unit_types to NRI values MAY be used and has been shown as
       efficient in a certain environment [15]. 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 bit stream conforms to a profile in which data
           partitioning is allowed, and not in streams that conform to
           the Main or Baseline profiles.
 
       Table 2: Example of NRI values for coded slices and coded slice
       data partitions of primary coded reference pictures
 
 
       NAL Unit Type     Content of NAL unit                 NRI
       (binary)
       ----------------------------------------------------------------
        1              non-IDR coded slice                         10
        2              Coded slice data partition A                10
        3              Coded slice data partition B                01
        4              Coded slice data partition C                01
 
           Informative note: As mentioned before, the NRI value of non-
           reference pictures is 00 as mandated by H.264/AVC.
 
       An H.264 encoder SHOULD set the value of NRI for coded slice and
       coded slice data partition NAL units of redundant coded
       reference pictures equal to 01 (in binary format).
 
       Definitions of the values for NRI for NAL unit types 24 to 29,
       inclusive, are given in sections 5.7 and 5.8 of this memo.
 
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       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, because 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 [17] (see section 12.1).  The
    non-interleaved mode is targeted for conversational systems that may
    not comply with ITU-T Recommendation H.241.  In the non-interleaved
    mode NAL units are transmitted in NAL unit decoding order.  The
    interleaved mode is targeted for systems that do not require very
    low end-to-end latency.  The interleaved mode allows transmission of
    NAL units out of NAL unit decoding order.
 
    The packetization mode in use MAY be signaled by the value of the
    OPTIONAL packetization-mode MIME parameter or by external means.
    The used packetization mode governs which NAL unit types are allowed
    in RTP payloads.  Table 3 summarizes the allowed NAL unit types for
    each packetization mode.  Some NAL unit type values (indicated as
    undefined in Table 3) are reserved for future extensions.  NAL units
    of those types SHOULD NOT be sent by a sender, and MUST be ignored
    by a receiver.  For example, the Types 1-23, with the associated
    packet type "NAL unit", are allowed in "Single NAL Unit Mode" and in
    "Non-Interleaved Mode", but disallowed in "Interleaved Mode".
    Packetization modes are explained in more detail in section 6.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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    Table 3. Summary of allowed NAL unit types for each packetization
    mode (yes = allowed, no = disallowed, ig = ignore).
 
    Type   Packet    Single NAL    Non-Interleaved    Interleaved
                     Unit Mode           Mode             Mode
    -------------------------------------------------------------
 
    0      undefined     ig               ig               ig
    1-23   NAL unit     yes              yes               no
    24     STAP-A        no              yes               no
    25     STAP-B        no               no              yes
    26     MTAP16        no               no              yes
    27     MTAP24        no               no              yes
    28     FU-A          no              yes              yes
    29     FU-B          no               no              yes
    30-31  undefined     ig               ig               ig
 
 5.5. Decoding Order Number (DON)
 
    In the interleaved packetization mode, the transmission order of NAL
    units is allowed to differ from the decoding order of the NAL units.
    Decoding order number (DON) is a field in the payload structure or a
    derived variable that indicates the NAL unit decoding order.
    Rationale and example 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.
 
       Informative note: When an FU-A occurs in interleaved mode, it
       always follows an FU-B which sets its DON.
 
 
 
 
 
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       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 and STAP-As, and FU-As MUST be the same as their
    NAL unit decoding order.  The NAL units within an STAP MUST appear
    in the NAL unit decoding order.  Thus the decoding order is first
    provided through the implicit order within a STAP, and second
    provided through the RTP sequence number for the order between
    STAPs, FUs, and single NAL unit packets.
 
    Signaling of the value of DON for NAL units carried in STAP-B, MTAP,
    and a series of fragmentation units starting with an FU-B is
    specified in sections 5.7.1, 5.7.2, and 5.8 respectively.  The DON
    value of the first NAL unit in transmission order MAY be set to any
    value.  Values of DON are in the range of 0 to 65535, inclusive.
    After reaching the maximum value, the value of DON wraps around to
    0.
 
    The decoding order of two NAL units contained in any STAP-B, MTAP,
    or a series of fragmentation units starting with an FU-B is
    determined as follows.  Let DON(i) be the decoding order number of
    the NAL unit having index i in the transmission order.  Function
    don_diff(m,n) is specified as follows:
 
    If DON(m) == DON(n), don_diff(m,n) = 0
 
    If (DON(m) < DON(n) and DON(n) - DON(m) < 32768),
    don_diff(m,n) = DON(n) - DON(m)
 
    If (DON(m) > DON(n) and DON(m) - DON(n) >= 32768),
    don_diff(m,n) = 65536 - DON(m) + DON(n)
 
    If (DON(m) < DON(n) and DON(n) - DON(m) >= 32768),
    don_diff(m,n) = - (DON(m) + 65536 - DON(n))
 
    If (DON(m) > DON(n) and DON(m) - DON(n) < 32768),
    don_diff(m,n) = - (DON(m) - DON(n))
 
    A positive value of don_diff(m,n) indicates that the NAL unit having
    transmission order index n follows, in decoding order, the NAL unit
    having transmission order index m.  When don_diff(m,n) is equal to
    0, then the NAL unit decoding order of the two NAL units can be in
    either order.  A negative value of don_diff(m,n) indicates that the
    NAL unit having transmission order index n precedes, in decoding
    order, the NAL unit having transmission order index m.
 
 
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    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
    NAL unit in decoding order incremented by one.
 
    An example 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 is equal to one even in case of error-
       free transmission.  An increment by one is not required, because
       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 bitrate 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.
       At the time of transmitting the first intra picture, the
       originator does not exactly know how many NAL units are going to
       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 at the time of transmitting them and gaps
       in values of DON may occur.
 
 
 5.6. Single NAL Unit Packet
 
    The single NAL unit packet defined here MUST contain one and 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
 
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    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.  In order 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 one including DON (STAP-B).
    o Multi-time aggregation packet (MTAP) aggregates NAL units with
      potentially differing NALU-time.  Two different MTAPs are defined
      that differ in the length of the NAL unit timestamp offset.
 
    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 three different
    aggregation units and their characteristics.
 
    The structure of the RTP payload format for aggregation packets is
    presented in Figure 3.
 
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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |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 the marker bit
    of the last NAL unit of the aggregated packet would have if it were
    transported in its own RTP packet.
 
    The payload of an aggregation packet consists of one or more
    aggregation units.  See section 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 such 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.
 
 
 
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 5.7.1. Single-Time Aggregation Packet
 
    Single-time aggregation packet (STAP) SHOULD be used whenever
    aggregating NAL units that all share the same NALU-time.  The
    payload of an STAP-A does not include DON and consists of at least
    one single-time aggregation unit as presented in Figure 4.  The
    payload of an STAP-B consists of a 16-bit unsigned decoding order
    number (DON) (in network byte order) followed by at least one
    single-time aggregation unit as presented in Figure 5.
 
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    :                                               |
    +-+-+-+-+-+-+-+-+                                               |
    |                                                               |
    |                single-time aggregation units                  |
    |                                                               |
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
    Figure 4. Payload format for STAP-A.
 
 
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    :  decoding order number (DON)  |               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
    |                                                               |
    |                single-time aggregation units                  |
    |                                                               |
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
    Figure 5. Payload format for STAP-B.
 
    The DON field specifies the value of DON for the first NAL unit in
    an STAP-B in transmission order.  The value of DON for each
    successive NAL unit in appearance order in an STAP-B 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
 
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    not be aligned on a 32-bit word boundary.  Figure 6 presents the
    structure of the single-time aggregation unit.
 
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    :        NAL unit size          |               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
    |                                                               |
    |                           NAL unit                            |
    |                                                               |
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
    Figure 6. Structure for single-time aggregation unit.
 
 
    Figure 7 presents an example of an RTP packet that contains an STAP-
    A.  The STAP contains two single-time aggregation units, labeled as
    1 and 2 in the figure.
 
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          RTP Header                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |STAP-A NAL HDR |         NALU 1 Size           | NALU 1 HDR    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         NALU 1 Data                           |
     :                                                               |
     +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               | NALU 2 Size                   | NALU 2 HDR    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         NALU 2 Data                           |
     |                                                               |
     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               :...OPTIONAL RTP padding        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
    Figure 7. An example of an RTP packet including a STAP-A and two
    single-time aggregation units.
 
 
    Figure 8 presents an example of an RTP packet that contains an STAP-
    B.  The STAP contains two single-time aggregation units, labeled as
    1 and 2 in the figure.
 
 
 
 
 
 
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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          RTP Header                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |STAP-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 the NAL
       units are encapsulated in an MTAP.
 
 
 
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    :  decoding order number base   |               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
    |                                                               |
    |                 multi-time aggregation units                  |
    |                                                               |
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
    Figure 9. NAL unit payload format for MTAPs.
 
 
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    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 is the flexibility of the
    MTAP, but the higher is also the overhead.
 
    The structure of the multi-time aggregation units for MTAP16 and
    MTAP24 are presented in Figure 10 and Figure 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.  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).
 
    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
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    :        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 has 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 wrap around of the timestamp field, thus enabling
       one to actually determine the smallest value if the timestamp
       wraps.  Such an "earliest" aggregation unit may not be the first
       one in the order the aggregation units are encapsulated in an
       MTAP.  The "earliest" NAL unit need not be the same as the first
       NAL unit in the NAL unit decoding order either.
 
    Figure 12 presents an example of an RTP packet that contains a
    multi-time aggregation packet of type MTAP16 that contains two
    multi-time aggregation units, labeled as 1 and 2 in the figure.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          RTP Header                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |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 example of an RTP packet including a multi-time
    aggregation packet of type MTAP16 and two multi-time aggregation
    units.
 
 
    Figure 13 presents an example of an RTP packet that contains a
    multi-time aggregation packet of type MTAP24 that contains two
    multi-time aggregation units, labeled as 1 and 2 in the figure.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          RTP Header                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |MTAP16 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 example of an RTP packet including a multi-time
    aggregation packet of type MTAP16 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.
 
    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
 
 
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    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 fragmented NAL unit.  STAPs and
    MTAPs MUST NOT be fragmented.  FUs MUST NOT be nested, i.e., an FU
    MUST NOT contain another FU.
 
    The RTP timestamp of an RTP packet carrying an FU is set to the NALU
    time of the fragmented NAL unit.
 
    Figure 14 presents the RTP payload format for FU-As.  An FU-A
    consists of a fragmentation unit indicator of one octet, a
    fragmentation unit header of one octet, and a fragmentation unit
    payload.
 
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | FU indicator  |   FU header   |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
    |                                                               |
    |                         FU payload                            |
    |                                                               |
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :...OPTIONAL RTP padding        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
    Figure 14. RTP payload format for FU-A.
 
    Figure 15 presents the RTP payload format for FU-Bs.  An FU-B
    consists of a fragmentation unit indicator of one octet, a
    fragmentation unit header of one octet, a decoding order number
    (DON) (in network byte order), and a fragmentation unit payload.  In
    other words, the structure of FU-B is the same as the structure of
    FU-A except for the additional DON field.
 
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | FU indicator  |   FU header   |               DON             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
    |                                                               |
    |                         FU payload                            |
    |                                                               |
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               :...OPTIONAL RTP padding        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
    Figure 15. RTP payload format for FU-B.
 
 
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    NAL unit type FU-B MUST be used in the interleaved packetization
    mode for the first fragmentation unit of a fragmented NAL unit.  NAL
    unit type FU-B MUST NOT be used in any other case.  In other words,
    in the interleaved packetization mode, each NALU that is fragmented
    has an FU-B as the first fragment, followed by one or more FU-A
    fragments.
 
    The FU indicator octet has the following format:
 
    +---------------+
    |0|1|2|3|4|5|6|7|
    +-+-+-+-+-+-+-+-+
    |F|NRI|  Type   |
    +---------------+
 
    Values equal to 28 and 29 in the Type field of the FU indicator
    octet identify an FU-A and an FU-B, respectively.  The use of the F
    bit is described in section 5.3.  The value of the NRI field MUST be
    set according to the value of the NRI field in the fragmented NAL
    unit.
 
    The FU header has the following format:
 
    +---------------+
    |0|1|2|3|4|5|6|7|
    +-+-+-+-+-+-+-+-+
    |S|E|R|  Type   |
    +---------------+
 
    S: 1 bit
       The Start bit, when one, indicates the start of a fragmented NAL
       unit.  Otherwise, 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
       The End bit, when one, 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.  Otherwise, 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.
 
 
 
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       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., 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 such that if the fragmentation unit payloads of
    consecutive FUs are sequentially concatenated, the payload of the
    fragmented NAL unit is 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 near loss-less environments.
       Those senders can be characterized in that they packetize NALU
       fragments before the NALU is completely generated and hence,
       before the NALU size if known.  If zero-length NALU fragments
       were not allowed, the sender would have to generate at least one
       bit of data of the following fragment before the current
       fragment could be sent.  Due to the characteristics of H.264,
       where sometimes several macroblocks occupy zero bits, this is
       undesirable and can add delay.  However, the (potential) use of
       zero-length NALUs should be carefully weighted against the
       increase of the risk of the loss of the NALU, because of the
       additional packets that are 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 that are 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.
 
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 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 hence sharing the same
      RTP timestamp value) MAY be sent in any order permitted by the
      applicable profile defined in [1], although, 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, because neither this memo nor the
      H.264 specification provides means to identify duplicated NAL
      units.  Sequence and picture parameter set NAL units MAY be
      duplicated to make their correct reception more probable, but any
      such duplication MUST NOT affect the contents of any active
      sequence or picture parameter set.  Duplication SHOULD be
      performed on the application layer, and not by duplicating RTP
      packets (with identical sequence numbers).
 
    Senders according to 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 [21], 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 or packetization-mode is not
    present or no other packetization mode is signaled by external
 
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    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 [17] (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.
 
 
 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.  Hence,
    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
 
 
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    The receiver includes a receiver buffer to compensate 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
    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.
 
 
 
 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 assuming that there is no
    transmission delay jitter.  To make a difference between a practical
    receiver buffer that is also used for compensation of transmission
    delay jitter, the receiver buffer is hereinafter 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
 
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    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.
 
 
 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 into the deinterleaving buffer as follows.
    NAL units of aggregation packets are stored into 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:
 
 
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    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.
    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.
    o
 
    The order that 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.
 
    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
 
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      pictures intact.  If more packets need 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
    bit stream.  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 those 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 is going to 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
 
    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
 
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                         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 for
                         indicating 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
                         decoding 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 that is
                         supported for the signaled profile.  The
                         profile-iop byte indicates whether the codec
                         has such additional limitations that only the
                         common subset of the algorithmic features and
                         limitations of the profiles signaled with the
                         profile-iop byte and 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.
 
                             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 [8]).
 
                         If no profile-level-id is present, the Baseline
                         Profile without additional constraints at Level
                         1 MUST be implied.
 
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        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 such
                         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 such 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 such values of max-
                         mbps, max-fs, max-cpb, max-dpb, and max-br that
                         meet the requirements of a higher level,
                         referred to as level A herein, compared to the
                         level specified in the value of the profile-
                         level-id parameter, if the receiver can support
                         all the properties of level A.
 
                             Informative note: When the OPTIONAL 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.
 
 
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        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
                         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
                         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 required by the signaled level conveyed in
                         the value of the profile-level-id parameter.
                         When max-fs is signaled, the receiver MUST be
                         able to decode NAL unit streams that conform to
                         the signaled level with the exception that the
                         MaxFS value in Table A-1 of [1] for the
                         signaled level is replaced with the value of
                         max-fs.  The value of max-fs MUST be greater
                         than or equal to the value of MaxFS for the
                         level given in Table A-1 of [1].  Senders MAY
                         use this knowledge to send larger pictures at a
                         proportionally lower frame rate than 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
 
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                         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 bitrate
                         compared to which 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
                             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 from any other
                             potential buffers in the receiver,
                             including de-interleaving and de-jitter
                             buffers.  The coded picture buffer need not
                             be implemented in decoders as specified in
                             Annex C of H.264, but rather standard-
                             compliant decoders can have any buffering
                             arrangements provided that they can decode
                             standard-compliant bitstreams.  Thus, in
                             practice, the input buffer for video
                             decoder can be integrated with de-
                             interleaving and de-jitter buffers of the
                             receiver.
 
        max-dpb:         The value of max-dpb is an integer indicating
                         the maximum decoded picture buffer size in
                         units of 1024 bytes.  The max-dpb parameter
                         signals that the receiver has more memory than
                         the minimum amount of decoded picture buffer
                         memory required by the signaled level conveyed
                         in the value of the profile-level-id parameter.
                         When max-dpb is signaled, the receiver MUST be
                         able to decode NAL unit streams that conform to
                         the signaled level with the exception that the
                         MaxDPB value in Table A-1 of [1] for the
                         signaled level is replaced with the value of
                         max-dpb.  Consequently, a receiver that signals
                         max-dpb MUST be capable of storing the
                         following number of decoded frames,
                         complementary field pairs, and non-paired
                         fields in its decoded picture buffer:
 
 
 
 
 
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                         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
                         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 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
 
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                           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 4,036,458 bits
                         (1550000 / 384000 * 1000 * 1000).
 
                         The value of max-br MUST be grater than or
                         equal to the value MaxBR for the signaled level
                         given in Table A-1 of [1].
 
                         Senders MAY use this knowledge to send higher
                         bitrate video as allowed in the level
                         definition of Annex A of H.264, to achieve
                         improved video quality.
 
                             Informative note: This parameter was added
                             primarily to complement a similar codepoint
                             in the ITU-T Recommendation H.245, so as to
                             facilitate signaling gateway designs.  No
                             assumption can be made from the value of
                             this parameter that the network is capable
                             of handling such bit rates at any given
                             time.  In particular, no conclusion can be
                             drawn that the signaled bit rate is
                             possible under congestion control
                             constraints.
 
       redundant-pic-cap: This parameter signals the capabilities of a
                         receiver implementation.  When equal to 0, the
                         parameter indicates 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, hence 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 hence 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.
 
 
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                         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 profile
                             (Baseline, Extended) in which redundant
                             coded pictures are allowed.
 
                             Informative note: Even if redundant-pic-cap
                             is equal to 1, the receiver may also choose
                             other error concealment strategies to
                             replace or complement decoding of redundant
                             slices.
 
        sprop-parameter-sets:   This parameter MAY be used to convey
                         such 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 bytes, 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
 
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                             identifiers).  Hence, 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.
 
        packetization-mode: This parameter signals the properties of a
                         RTP payload type or the capabilities of a
                         receiver implementation.  Only a single
                         configuration point can be indicated, thus for
                         when declaring capabilities to support more
                         than one packetization-mode, 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 XXXX MUST be used.  This mode is in use in
                         standards using ITU-T Recommendation H.241 [17]
                         (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 XXXX MUST be used.  When the value of
                         packetization-mode is equal to 2, the
                         interleaved mode as defined in section 6.4 of
                         RFC XXXX 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
 
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                         can reconstruct NAL unit decoding order, when
                         the buffer size for NAL unit decoding order
                         recovery is at least the value of sprop-
                         interleaving-depth + 1 in terms of VCL NAL
                         units.
 
                         The value of sprop-interleaving-depth MUST be
                         an integer in the range of 0 to 32767,
                         inclusive.
 
        sprop-deint-buf-req:   This parameter MUST NOT be present when
                         packetization-mode is not present or the value
                         of packetization-mode is equal to 0 or 1.  It
                         MUST be present when the value of
                         packetization-mode is equal to 2.
 
                         sprop-deint-buf-req signals the required size
                         of the 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 XXXX.  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 4 294 967 295,
                         inclusive.
 
                             Informative note: sprop-deint-buf-req
                             indicates the required size of the
                             deinterleaving buffer only.  When network
                             jitter can occur, additionally an
                             appropriately sized jitter buffer has to be
                             provisioned for.
 
        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
 
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                         of deint-buf-cap MUST be an integer in the
                         range of 0 to 4 294 967 295, inclusive.
 
                             Informative note: deint-buf-cap indicates
                             the maximum possible size of the
                             deinterleaving buffer of the receiver only.
                             When network jitter can occur, additionally
                             an appropriately sized jitter buffer has to
                             be provisioned for.
 
 
        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 starting of
                         decoding 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 into the
                         following series:
                         0 -1  1  0 -1  1  0 -1 1 ...
 
                         Thus, the value of sprop-init-buf-time in this
                         example is 1 in terms of intervals of NAL unit
                         transmission times.
 
                         The parameter is coded as a non-negative base10
                         integer representation in clock ticks of a 90-
 
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                         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 4 294 967 295, 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 such decoding order number of the NAL
                         unit that does not wrap around to 0 after
                         65535.  In other words, AbsDON is calculated as
                         follows: Let m and n be consecutive NAL units
                         in transmission order.  For the very first NAL
                         unit in transmission order (whose index is 0),
                         AbsDON(0) = DON(0).  For other NAL units,
                         AbsDON is calculated as follows:
 
                         If DON(m) == DON(n), AbsDON(n) = AbsDON(m)
 
                         If (DON(m) < DON(n) and DON(n) - DON(m) <
                         32768),
                         AbsDON(n) = AbsDON(m) + DON(n) - DON(m)
 
                         If (DON(m) > DON(n) and DON(m) - DON(n) >=
                         32768),
                         AbsDON(n) = AbsDON(m) + 65536 - DON(m) + DON(n)
 
                         If (DON(m) < DON(n) and DON(n) - DON(m) >=
                         32768),
 
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                         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 XXXX.
 
                             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
                         higher cost than NALUs following the
                         limitation.
 
                         The value of max-rcmd-nalu-size MUST be an
                         integer in the range of 0 to 4 294 967 295,
                         inclusive.  If this parameter is not specified,
                         no known limitation to the NALU size exists.
                         Senders still need to consider the MTU size
                         available between the sender and the receiver
                         and SHOULD run MTU discovery for this purpose.
 
                         This parameter is motivated by, for example, an
                         IP to H.223 video telephony gateway, where
                         NALUs smaller than the H.223 transport data
                         unit will be more efficient.  A gateway may
                         terminate IP, thus MTU discovery will normally
                         not work beyond the gateway.
 
                             Informative note: Setting this parameter to
                             a lower than necessary value may have a
                             negative impact.
 
    Encoding considerations:
                         This type is only defined for transfer via RTP
                         (RFC 3550).
 
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                         A file format of H.264/AVC video is defined in
                         [32].  This definition is utilized by other
                         file formats such as the 3GPP multimedia file
                         format (MIME type video/3gpp) [33] or the MP4
                         file format (MIME type video/mp4).
 
    Security considerations:
                         See section 9 of RFC XXXX.
 
    Public specification:
                         Please refer to RFC XXXX and its section 17.
 
    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/Change controller:
                         stewe@stewe.org
                         IETF Audio/Video transport working group
 
 
 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.
 
 
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      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 offering H.264 over RTP using SDP in an Offer/Answer model [8]
    for negotiation for unicast usage, the following limitations and
    rules apply:
 
    o The parameters identifying a media format configuration for H.264
      are "profile-level-id", "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.
 
          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 [8].  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, needs
          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
 
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      properties of the stream 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.  As they apply for the
          configuration, rather then being bound to the payload type,
          the values may need to be applied to another payload type when
          sending.
 
    o The capability parameters ("max-mbps", "max-fs", "max-cpb", "max-
      dpb", "max-br", ,"redundant-pic-cap", "max-rcmd-nalu-size") MAY be
      used to declare further capabilities.  Their interpretation
      depends on the direction attribute.  When the direction attribute
      is sendonly, then the parameters describe the limits of the RTP
      packets and the NAL unit stream that the sender is capable of
      producing.  When the direction attribute is sendrecv or recvonly,
      then the parameters describe the limitations of what the receiver
      accepts.
 
    o As specified above, an offerer needs 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 utilized when selecting the value for "sprop-
      deint-buf-req" 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 adding parameter
          sets 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.
 
 
 
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    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.
      Hence, 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.
 
    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 when "a=sendrecv", or no direction attribute
      is used, or in offers and answers where "a=recvonly" is used, the
      following interpretation of the parameters MUST be used.
 
      Declaring actual configuration or properties for receiving:
         - profile-level-id
         - packetization-mode
 
      Declaring actual properties of the stream to be sent (applicable
      only when "a=sendrecv" or no direction attribute is used):
         - sprop-deint-buf-req
         - sprop-interleaving-depth
         - sprop-parameter-sets
         - sprop-max-don-diff
         - sprop-init-buf-time
 
      Declaring receiver implementation capabilities:
         - max-mbps
         - max-fs
         - max-cpb
         - max-dpb
         - max-br
         - redundant-pic-cap
         - deint-buf-cap
         - max-rcmd-nalu-size
 
 
 
 
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      Declaring how Offer/Answer negotiation shall be performed:
         - parameter-add
 
    o In an Offer or Answer where the direction attribute "a=sendonly"
      is included for the media stream, the following interpretation of
      the parameters MUST be used:
 
      Declaring actual configuration and properties of stream proposed
      to be sent:
         - profile-level-id
         - packetization-mode
         - sprop-deint-buf-req
         - sprop-max-don-diff
         - sprop-init-buf-time
         - sprop-parameter-sets
         - sprop-interleaving-depth
 
      Declaring the capabilities of the sender when it receives a
      stream:
         - max-mbps
         - max-fs
         - max-cpb
         - max-dpb
         - max-br
         - redundant-pic-cap
         - deint-buf-cap
         - max-rcmd-nalu-size
 
      Declaring how Offer/Answer negotiation shall be performed:
         - parameter-add
 
    Further 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 senders 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.  They express values a receiver expects to be used, and
      must be used verbatim on the sender side.
 
    o When declaring sender's capabilities, 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
 
 
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      multiple alternative configurations, e.g. for the packetization
      mode.  It is impossible to offer multiple configurations in a
      single payload type.  Hence, when multiple configuration offers
      are made, each offer requires its own RTP payload type associated
      with the offer.
 
    o A receiver SHOULD understand all MIME parameters even if it only
      supports a subset of the payload formats 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 the usage of these, a second
      offer from the offerer is required in most cases to provide the
      stream properties parameters that the media sender will use.  This
      also has the effect that the offerer needs to be able to receive
      this media format configuration, not only 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 offering H.264 over RTP using SDP in a declarative style as
    used in RTSP [30] or SAP [31], the following considerations are
    necessary.
 
      o All parameters that are capable of indicating both the
         properties of a NAL unit stream and the capabilities of 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
         capabilities of the sender.  This results in that the following
         interpretation of the parameters MUST be used:
         Declaring actual configuration or properties:
           - profile-level-id
           - sprop-parameter-sets
           - packetization-mode
           - sprop-interleaving-depth
           - sprop-deint-buf-req
           - sprop-max-don-diff
           - sprop-init-buf-time
 
 
 
 
 
 
 
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         Not usable:
           - max-mbps
           - max-fs
           - max-cpb
           - max-dpb
           - max-br
           - redundant-pic-cap
           - max-rcmd-nalu-size
           - parameter-add
           - deint-buf-cap
 
    o A receiver of the SDP is required to support all parameters and
      all values of the parameters provided, or 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 where 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 offers the same codec configuration in three
    different packetization formats.  PT 98 represents single NALU mode,
    99 non-interleaved mode, and 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 (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.
 
 
 
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    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, while 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 represents.  It provides the initial
    parameter sets for the answerer-to-offerer direction, and 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 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 are not
    representing a legal operation point of an H.264 codec -- the base64
    strings are only used for illustration.
 
 
 
 
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 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.  Hence, 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 how principles A and B must be
    implemented within session control protocols, and is independent of
    the particular protocol used.  Principle C is supported by the RTP
    payload format defined in this specification.
 
    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 likely prevents decoding of
    a considerable portion of the corresponding RTP 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, while 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) and is NOT RECOMMENDED.
 
 
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    Parameter sets MAY be added or updated during the lifetime of a
    session using principles B and C.  It is required that parameter
    sets are present at the decoder prior to the NAL units that refer to
    them.  Updating or adding of parameter sets can result in further
    problems, and therefore the following recommendations should be
    considered.
 
    - When adding or updating parameter sets, principle C is vulnerable
      to transmission errors as described above, and therefore principle
      B is RECOMMENDED.
 
    - When adding or updating parameter sets, 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 RECOMMEDED that a sender does not start sending NALUs
      requiring the updated parameter sets prior to acknowledgement of
      delivery from the signaling protocol.
 
    - When updating parameter sets, the following synchronization issue
      should be taken into account.  When overwriting a parameter set at
      the receiver, the sender needs ensure that the parameter set in
      question is not needed by any NALU present in the network or
      receiver buffers.  Otherwise decoding using a wrong parameter set
      may occur.  To lessen this problem, it is RECOMMENDED to either
      overwrite only those parameter sets that have not been used for a
      sufficiently long time (to ensure that all related NALUs have been
      consumed), or to add a new parameter set instead (which may have
      negative consequences for the efficiency of the video coding).
 
    - When adding new parameter sets, previously unused parameter set
      identifiers are used.  This avoids the problem identified in the
      previous paragraph.  However, in a multiparty session and 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 needs to be avoided.
 
    - Adding or modifying parameter sets by using both principles B and
      C in the same RTP session may lead to inconsistencies of the
      parameter sets because of the lack of synchronization between the
      control and the RTP channel.  Therefore principle 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 need 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 needs to be taken into
 
 
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    account and the loss probability should be reduced using the
    mechanisms discussed above.
 
    - When parameter sets are both provided initially using principle A
      and then later added or updated in-band (principle C), then there
      is a risk associated with updating the parameter sets delivered
      out-of-band.  If receivers miss some in-band updates, because of a
      loss or a late tune-in, for example, those receivers attempt to
      decode the bitstream using out-dated parameters.  It is
      RECOMMENDED that parameter set IDs are 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 any appropriate RTP profile (for example
    [18]).  This implies that confidentiality of the media streams is
    achieved by encryption, for example through the application of SRTP
    [29].  Because the data compression used with this payload format is
    applied end-to-end, encryption may be performed after compression so
    there is no conflict between the two operations.
 
    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 such pathological
    datagrams into the stream that are complex to decode and cause the
    receiver to be overloaded.  H.264 is particularly vulnerable to such
    attacks because it is extremely simple to generate datagrams
    containing NAL units that affect the decoding process of many future
    NAL units.  Therefore the usage of authentication of at least the
    RTP packet is RECOMMENDED, for example with SRTP [29].
 
    Note that the appropriate mechanism to ensure confidentiality and
    integrity of RTP packets and their payloads are very dependent on
    the application and the transport and signaling protocols employed.
    Hence, although SRTP is given as example above, other possible
    choices exist.
 
    As with any IP-based protocol, in some circumstances a receiver may
    be overloaded simply by the receipt of too many packets, either
    desired or undesired.  Network-layer authentication may be used to
    discard packets from undesired sources, but the processing cost of
 
 
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    the authentication itself may be too high.  In a multicast
    environment, pruning of specific sources may be implemented in
    future versions of IGMP [19] and in multicast routing protocols to
    allow a receiver to select which sources are allowed to reach it.
 
    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.
 
 
 10. Congestion Control
 
    Congestion control for RTP SHALL be used in accordance with RFC 3550
    [4], and any applicable RTP profile, e.g. RFC 3551 [18].  This means
    that congestion control is required for any transmission over
    unmanaged best-effort networks.
 
    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 [25] 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 if non-
    downgradable parameters, such as the profile part of the
    profile/level ID change, it becomes necessary to terminate and re-
    start the media stream, possibly using a different RTP payload type.
 
    MANEs MAY follow the suggestions outlined in section 7.3 and remove
    certain not usable packets from the packet stream when that stream
    was damaged due to previous packet losses.  This can help reducing
    the network load in certain special cases.
 
 11. IANA Consideration
 
    IANA is kindly requested to register one new MIME type, see section
    8.1.
 
 12. Informative Appendix: Application Examples
 
    This payload specification is very flexible in its use, to cover the
    extremely wide application space that is anticipated for H.264.
    However, such a 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.
 
 
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    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 [17]
    as a packetization scheme.  The packetization mechanism defined in
    this Annex is technically identical with a small subset of this
    specification.
 
    When operating according to H.241 Annex A, parameter sets 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, the picture
    parameters such as picture size or optional modes never change
    during the lifetime of a connection.  Hence, 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.  Since 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.  Hence, the RTP packet
    stream consists basically of NAL units that carry single coded
    slices.
 
    The encoder chooses the size of coded slice NAL units such that they
    offer the best performance.  Often, this is done by adapting the
    coded slice size to the MTU size of the IP network.  For small
    picture sizes this may result in a one-picture-per-one-packet
    strategy.  Intra refresh algorithms clean up the loss of packets and
    the resulting drift-related artifacts.
 
 
 12.3. Video Telephony, Interleaved Packetization Using NAL Unit
       Aggregation
 
 
 
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    This scheme allows better error concealment and is used in H.263
    based designed using RFC 2429 packetization [12].  It is also
    implemented and good results were reported [14].
 
    The VCL encoder codes the source picture such 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 STAP.  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, because 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 [20].
    Coding individual slices according to this packetization scheme
    provides a 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 to several RTP packets with only one NAL unit that are
    preferred in a wireless network and vice versa.
 
 
 12.4. Video Telephony, with Data Partitioning
 
    This scheme is implemented and was shown to offer good performance
    especially at higher packet loss rates [14].
 
    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 [21], for example, specifies which media
    packets are associated with the FEC packet.
 
    In all cases, the incurred overhead is substantial, but in the same
    order of magnitude as the number of bits that have otherwise be
    spent for intra information.  However, this mechanism is not adding
    any delay to the system.
 
    Again, the complete parameter set establishment is performed through
    control protocol means.
 
 
 
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 12.5. Video Telephony or Streaming, with FUs and Forward Error
       Correction
 
    This scheme is implemented and was shown to provide good performance
    especially at higher packet loss rates [22].
 
    The most efficient means to combat packet-losses for scenarios where
    retransmissions are not applicable is forward error correction
    (FEC).  Although the application layer, end-to-end use of FEC is
    often less efficient when compared to a 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 [21] 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 using a code 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 packet loss probability
       10%, k=1, and n=2, the residual error probability is about 1%,
       whereas for n=3, the residual error probability is about 0.1%.
    d) At the same packet loss probability p and for a fixed code rate
       k/n, the greater the value of n is, the smaller the residual
       error probability becomes.  For example, at a packet loss
       probability of p=10%, k=1 and n=2, the residual error rate is
       about 1%, whereas for an extended Golay code with k=12 and n=24,
       the residual error rate is about 0.01%.
 
    For applying RFC 2733 in combination with H.264 baseline coded video
    without using FUs several options might be considered:
    1) The video encoder produces NAL units where each video frame is
       coded in a single slice.  Applying FEC, one could use a simple
       code, e.g. (n=2, k=1), i.e., each NAL unit would basically just
       be repeated.  The disadvantage is obviously the bad code
       performance according to (d) and the low flexibility as only (n,
       k=1) codes can be used.
    2) The video encoder produces NAL units where 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
 
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       NAL units.  Depending on the number of RTP packets per frame, a
       loss may introduce a significant delay, which is reduced the more
       RTP packets per frame are used.  Packets of completely different
       length might also be connected, which decreases bit-rate
       efficiency according to (b).  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, where a certain frame
       contains k slices of possibly almost equal length.  Then,
       applying FEC, a better code, e.g. (n=24, k=12), over the sequence
       of NAL units for each frame can be used.  The delay compared to
       (2) may be reduced, but several disadvantages are obvious.
       Firstly, the coding efficiency of the encoded video is lowered
       significantly as slice-structured coding reduces intra-frame
       prediction and additional slice overhead is necessary.  Secondly,
       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, and therefore FEC with a
    reasonable k and n can be applied even if the encoder made no effort
    of producing slices of equal length.  For example, a coded slice NAL
    unit containing an entire frame can be split to k FUs and a parity
    check code (n=k+1, k) can be applied.  However this has the
    disadvantage that unless all created fragments can be recovered the
    whole slice will be lost.  Thus a larger section is lost, than would
    be the case 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 for
    achieving 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 addition, 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.
 
    In case that the error probability depends on the length of the
    transmitted packet, e.g. in case of mobile transmission [16], the
    benefits of applying FUs with FEC are even more obvious.  Basically,
    the flexibility of the size of FUs allows applying appropriate FEC
    for each NAL unit and even unequal error protection of NAL units.
 
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    The incurred overhead when using FUs and FEC is substantial, but 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 [22] it
    was shown that the overall performance at the same error rate and
    the same overall bit-rate including the overhead, the FEC-based
    approach can enhance the quality.
 
 
 12.6. Low-Bit-Rate Streaming
 
    This scheme has been implemented with H.263 and non-standard RTP
    packetization and gave good results [23].  There is no technical
    reason why similarly good results could not be achievable with
    H.264.
 
    In today's Internet streaming, some of the offered bit-rates are
    relatively low in order to allow terminals with dial-up modems to
    access the content.  In wired IP networks, relatively large packets,
    say 500 - 1500 bytes, are preferred to smaller and more frequently
    occurring packets in order to reduce network congestion.  Moreover,
    use of large packets decreases the amount of RTP/UDP/IP header
    overhead.  For low-bit-rate video, the use of large packets means
    that sometimes up to few pictures should be encapsulated in one
    packet.
 
    However, loss of a packet including many coded pictures would have
    drastic consequences in 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 slices from several pictures in an
    interleaved manner.  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 succeed relatively well.
 
 
 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 [24].  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
 
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    the stream back immediately, but rather it typically buffers the
    incoming data for a few seconds.  This buffering helps to maintain
    continuous playback, because, 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 perfectly
    recovered.  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, because 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 [25].  Subjective
    ranking can also be made on coded slice data partition or slice
    group basis.  Coded slices and coded slice data partitions that are
    subjectively the most important can be sent earlier than their
    decoding order indicates, whereas coded slices and coded slice data
    partitions that are subjectively the least important can be sent
    later than their natural coding order indicates.  Consequently, any
    retransmitted parts of the most important slices and coded slice
    data partitions are more likely to be received before their
    scheduled decoding or playback time compared to the least important
    slices and slice data partitions.
 
 
 13. Informative Appendix: Rationale for Decoding Order Number
 
 13.1. Introduction
 
    The Decoding Order Number (DON) concept was introduced mainly to
    enable efficient multi-picture slice interleaving (see section 12.6)
    and robust packet scheduling (see section 12.7).  In both of these
    applications NAL units are transmitted out of decoding order.  DON
    indicates the decoding order of NAL units and should be used in the
    receiver to recover the decoding order.  Example use cases for
    efficient multi-picture slice interleaving and for robust packet
    scheduling are given in sections 13.2 and 13.3 respectively.
    Section 13.4 describes the benefits of the DON concept in error
    resiliency achieved by redundant coded pictures.  Section 13.5
    summarizes considered alternatives to DON and justifies why DON was
    chosen to this RTP payload specification.
 
 
 13.2. Example of Multi-Picture Slice Interleaving
 
 
 
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    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 is from left to right as
    follows:
    ... R1 R3 N2 R5 N4 ...
 
    The NAL units of pictures R1, R3, N2, R5, and N4 are marked with a
    DON equal to 1, 2, 3, 4, and 5, respectively.
 
    Each reference picture consists of three slice groups that are
    scattered as follows (a number denotes the slice group number for
    each macroblock in a QCIF frame):
 
    0 1 2 0 1 2 0 1 2 0 1
    2 0 1 2 0 1 2 0 1 2 0
    1 2 0 1 2 0 1 2 0 1 2
    0 1 2 0 1 2 0 1 2 0 1
    2 0 1 2 0 1 2 0 1 2 0
    1 2 0 1 2 0 1 2 0 1 2
    0 1 2 0 1 2 0 1 2 0 1
    2 0 1 2 0 1 2 0 1 2 0
    1 2 0 1 2 0 1 2 0 1 2
 
    For the sake of simplicity, we assume that all the macroblocks of a
    slice group are included in one slice.  Three MTAPs are constructed
    from three consecutive reference pictures so that each MTAP contains
    three aggregation units, each of which contains all the macroblocks
    from one slice group.  The first MTAP contains slice group 0 of
    picture R1, slice group 1 of picture R3, and slice group 2 of
    picture R5.  The second MTAP contains slice group 1 of picture R1,
    slice group 2 of picture R3, and slice group 0 of picture R5.  The
    third MTAP contains slice group 2 of picture R1, slice group 0 of
    picture R3, and slice group 1 of picture R5.  Each non-reference
    picture is encapsulated into an STAP-B.
 
    Consequently, the transmission order of NAL units is the following:
      R1, slice group 0, DON 1, carried in MTAP,   RTP SN: N
      R3, slice group 1, DON 2, carried in MTAP,   RTP SN: N
      R5, slice group 2, DON 4, carried in MTAP,   RTP SN: N
      R1, slice group 1, DON 1, carried in MTAP,   RTP SN: N+1
      R3, slice group 2, DON 2, carried in MTAP,   RTP SN: N+1
      R5, slice group 0, DON 4, carried in MTAP,   RTP SN: N+1
      R1, slice group 2, DON 1, carried in MTAP,   RTP SN: N+2
      R3, slice group 1, DON 2, carried in MTAP,   RTP SN: N+2
      R5, slice group 0, DON 4, carried in MTAP,   RTP SN: N+2
      N2,                DON 3, carried in STAP-B, RTP SN: N+3
 
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      N4,                DON 5, carried in STAP-B, RTP SN: N+4
 
    The receiver is able to organize the NAL units back in decoding
    order based on the value of DON associated with each NAL unit.
 
    If one of the MTAPs is lost, the spatially adjacent and temporally
    co-located macroblocks are received and can be used to conceal the
    loss efficiently.  If one of the STAPs is lost, the effect of the
    loss does not propagate temporally.
 
 
 13.3. Example of Robust Packet Scheduling
 
    An example of robust packet scheduling follows.  The communication
    system used in the example consists of the following components in
    the order that the video is processed from source to sink:
    o camera and capturing
    o pre-encoding buffer
    o encoder
    o encoded picture buffer
    o transmitter
    o transmission channel
    o receiver
    o receiver buffer
    o decoder
    o decoded picture buffer
    o display
 
    The video communication system used in the example operates as
    follows.  Note that processing of the video stream happens gradually
    and at the same time in all components of the system.  The source
    video sequence is shot and captured to a pre-encoding buffer.  The
    pre-encoding buffer can be used to order pictures from sampling
    order to encoding order or to analyze multiple uncompressed frames
    for bitrate rate control purposes, for example.  In some cases the
    pre-encoding buffer may not exist, but rather 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.
 
 
 
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    In the following example figures, I denotes an IDR picture, R
    denotes a reference picture, N denotes a non-reference picture, and
    the number after I, R, or N indicates the sampling time relative to
    the previous IDR picture in decoding order.  Values below the
    sequence of pictures indicate scaled system clock timestamps.  The
    system clock is initialized arbitrarily in this example, and time
    runs from left to right.  Each I, R, and N picture is mapped into
    the same timeline compared to the previous processing step, if any,
    assuming that encoding, transmission, and decoding take no time.
    Thus, events happening at the same time are located in the same
    column throughout all example figures.
 
    A subset of a sequence of coded pictures is depicted below in
    sampling order.
 
    ...  N58 N59 I00 N01 N02 R03 N04 N05 R06 ... N58 N59 I00 N01 ...
    ... --|---|---|---|---|---|---|---|---|- ... -|---|---|---|- ...
    ...  58  59  60  61  62  63  64  65  66  ... 128 129 130 131 ...
 
    Figure 16. Sequence of pictures in sampling order
 
 
    The sampled pictures are buffered in the pre-encoding buffer to
    arrange them in encoding order.  In this example, we assume that the
    non-reference pictures are predicted from both the previous and the
    next reference picture in output order except for the non-reference
    pictures immediately preceding an IDR picture, which are predicted
    only from the previous reference picture in output order.  Thus, the
    pre-encoding buffer has to contain at least two pictures and the
    buffering causes a delay of two picture intervals.  The output of
    the pre-encoding buffering process and the encoding (and decoding)
    order of the pictures are as follows:
 
             ... N58 N59 I00 R03 N01 N02 R06 N04 N05 ...
             ... -|---|---|---|---|---|---|---|---|-  ...
             ... 60  61  62  63  64  65  66  67  68  ...
 
    Figure 17. Re-ordered pictures in the pre-encoding buffer
 
    The encoder or the transmitter can set the value of DON for each
    picture to a value of DON for the previous picture in decoding order
    plus one.
 
    For the sake of simplicity, let us assume that:
    o the frame rate of the sequence is constant,
    o each picture consists of only one slice,
    o each slice is encapsulated in a single NAL unit packet,
    o there is no transmission delay, and
    o pictures are transmitted at constant intervals (that is equal to 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, because 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 follow it in output order.  For the
    sake of simplicity, we assume that num_reorder_frames is used to
    indicate the initial buffer in the decoded picture buffer.  In this
    example, num_reorder_frames is equal to 1.
 
    It can be observed that if the IDR picture I00 is lost during
    transmission and a retransmission request is issued when the value
    of the system clock is 62, there is one picture interval of time
    (until the system clock reaches timestamp 63) to receive the
    retransmitted IDR picture I00.
 
    Let us then assume that IDR pictures are transmitted two frame
    intervals earlier than their decoding position, i.e., the pictures
    are transmitted as follows:
 
                    ...  I00 N58 N59 R03 N01 N02 R06 N04 N05 ...
                    ... --|---|---|---|---|---|---|---|---|- ...
                    ...  62  63  64  65  66  67  68  69  70  ...
 
    Figure 20. Interleaving: early IDR pictures in sending order
 
 
 
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    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.  Since 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 the following:
 
                         ... 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
 
    It can be observed that the maximum delay that IDR pictures can
    undergo during transmission, including possible application,
    transport, or link layer retransmission, is equal to three picture
    intervals.  Thus, the loss resiliency of IDR pictures is improved in
    systems supporting retransmission compared to the case in which
    pictures were transmitted in their decoding order.
 
 
 13.4. Robust Transmission Scheduling of Redundant Coded Slices
 
    A redundant coded picture is a coded representation of a picture or
    a part of a picture that is not used in the decoding process if the
    corresponding primary coded picture is correctly decoded.  There
    should be no noticeable difference between any area of the decoded
    primary picture and a corresponding area that would result from
    application of the H.264 decoding process for any redundant picture
 
 
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    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 utilizing the redundant codec picture feature
    include the video redundancy coding [26] and protection of "key
    pictures" in multicast streaming [27].
 
    One property of many error-prone video communications systems is
    that transmission errors are often bursty and therefore they may
    affect more than one consecutive transmission packets in
    transmission order.  In low bitrate 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.  In order to make the
    transmission scheme more tolerant of bursty transmission errors, it
    is beneficial to transmit a primary coded picture further apart from
    the corresponding redundant coded pictures.  The DON concept enables
    this.
 
 
 13.5. Remarks on Other Design Possibilities
 
    The slice header syntax structure of the H.264 coding standard
    contains the frame_num syntax element that can indicate the decoding
    order of coded frames.  However, the usage of the frame_num syntax
    element is not feasible or desirable to recover the decoding order
    due to the following reasons:
    o The receiver is required to parse at least one slice header per
      coded picture (before passing the coded data to the decoder).
    o Coded slices from multiple coded video sequences cannot be
      interleaved, because 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
    [28] 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 consist of all NAL units
    that are 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
 
 
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    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, Joerg Ott, and Colin Perkins for careful review.
 
 
 15. Full Copyright Statement
 
    Copyright (C) The Internet Society (2004).  This document is subject
    to the rights, licenses and restrictions contained in BCP 78, and
    except as set forth therein, the authors retain all their rights.
 
    This document and the information contained herein are provided on
    an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
    REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
    INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
    IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
    THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
    WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
 
 
 16. Intellectual Property Notice
 
    The IETF takes no position regarding the validity or scope of any
    Intellectual Property Rights or other rights that might be claimed
    to pertain to the implementation or use of the technology described
    in this document or the extent to which any license under such
    rights might or might not be available; nor does it represent that
    it has made any independent effort to identify any such rights.
    Information on the procedures with respect to rights in RFC
    documents can be found in BCP 78 and BCP 79.
 
    Copies of IPR disclosures made to the IETF Secretariat and any
    assurances of licenses to be made available, or the result of an
    attempt made to obtain a general license or permission for the use
    of such proprietary rights by implementers or users of this
    specification can be obtained from the IETF on-line IPR repository
    at http://www.ietf.org/ipr.
 
    The IETF invites any interested party to bring to its attention any
    copyrights, patents or patent applications, or other proprietary
    rights that may cover technology that may be required to implement
    this standard.  Please address the information to the IETF at ietf-
    ipr@ietf.org.
 
 
 
 
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 17. References
 
 17.1. 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]  S. Bradner, "Key words for use in RFCs to Indicate Requirement
          Levels", BCP 14, RFC 2119, March 1997.
    [4]  H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson,
          "RTP: A Transport Protocol for Real-Time Applications", STD
          64, RFC 3550, July 2003.
    [5]  M. Handley and V. Jacobson, "SDP: Session Description
          Protocol", RFC 2327, April 1998.
    [6]  S. Josefsson, "The Base16, Base32, and Base64 Data Encodings",
          RFC 3548, July 2003.
    [7]  ITU-T Recommendation T.35, "Procedure for the allocation of
          ITU-T defined codes for non-standard facilities", February
          2000.
    [8]  J. Rosenberg, and H. Schulzrinne, "An Offer/Answer Model with
          the Session Description Protocol (SDP)", RFC 3264, June 2002.
 
 
 17.2. Informative References
 
    [9]  "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 ftp://ftp.imtc-
          files.org/jvt-experts/2003_03_Pattaya/JVT-G050r1.zip, May
          2003.
    [10] A. Luthra, G.J. Sullivan, and T. Wiegand (eds.), Special Issue
          on H.264/AVC. IEEE Transactions on Circuits and Systems on
          Video Technology, July 2003.
    [11] P. Borgwardt, "Handling Interlaced Video in H.26L", VCEG-
          N57r2, available from http://ftp3.itu.int/av-arch/video-
          site/0109_San/VCEG-N57r2.doc, September 2001.
    [12] C. Borman et. Al., "RTP Payload Format for the 1998 Version of
          ITU-T Rec. H.263 Video (H.263+)", RFC 2429, October 1998.
    [13] ISO/IEC IS 14496-2.
    [14] S. Wenger, "H.26L over IP", IEEE Transaction on Circuits and
          Systems for Video technology, July 2003.
    [15] S. Wenger, "H.26L over IP: The IP Network Adaptation Layer",
          Proceedings Packet Video Workshop 02, April 2002
    [16] T. Stockhammer, M.M. Hannuksela, and S. Wenger, "H.26L/JVT
          Coding Network Abstraction Layer and IP-based Transport" in
          Proc. ICIP 2002, Rochester, NY, September 2002.
    [17] ITU-T Recommendation H.241, "Extended video procedures and
          control signals for H.300 series terminals", 2004.
    [18] H. Schulzrinne and S. Casner, "RTP Profile for Audio and Video
          Conferences with Minimal Control", STD 65, RFC 3551,    July
          2003.
 
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 Internet Draft                                            August, 2004
 
    [19] B. Cain, S. Deering, I. Kouvelas, B. Fenner, and A.
          Thyagarajan, "Internet Group Management Protocol, Version 3",
          RFC 3376, October 2002.
    [20] ITU-T Recommendation H.223, "Multiplexing protocol for low bit
          rate multimedia communication", July 2001.
    [21] J. Rosenberg, H. Schulzrinne, "An RTP Payload Format for
          Generic Forward Error Correction", RFC 2733, December 1999.
    [22] T. Stockhammer, T. Wiegand, T. Oelbaum, 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.
    [23] V. Varsa, M. Karczewicz, "Slice interleaving in compressed
          video packetization", Packet Video Workshop 2000.
    [24] S.H. Kang and A. Zakhor, "Packet scheduling algorithm for
          wireless video streaming," International Packet Video Workshop
          2002, available http://www.pv2002.org.
    [25] M.M. Hannuksela, "Enhanced concept of GOP", JVT-B042,
          available http://ftp3.itu.int/av-arch/video-site/0201_Gen/JVT-
          B042.doc , January 2002.
    [26] S. Wenger, "Video Redundancy Coding in H.263+", 1997
          International Workshop on Audio-Visual Services over Packet
          Networks, September 1997.
    [27] Y.-K. Wang, M.M. Hannuksela, and M. Gabbouj, "Error Resilient
          Video Coding Using Unequally Protected Key Pictures", in Proc.
          International Workshop VLBV03, September 2003.
    [28] J. van der Meer, D. Mackie, V. Swaminathan, D. Singer, and P.
          Gentric, "RTP Payload Format for Transport of MPEG-4
          Elementary Streams", RFC 3640, November 2003.
    [29] Baugher, McGrew, Carrara, Naslund, and Norrman, "The Secure
          Real-time Transport Protocol," RFC 3711, Internet Engineering
          Task Force, March 2004.
    [30] H. Schulzrinne, A. Rao, R. Lanphier, "Real Time Streaming
          Protocol (RTSP)", RFC 2326, Internet Engineering Task Force,
          April 1998.
    [31] M. Handley, C. Perkins, E. Whelan, "Session Announcement
          Protocol", RFC 2974, Internet Engineering Task Force, June
          2001.
    [32] ISO/IEC 14496-15: "Information technology - Coding of audio-
          visual objects - Part 15: Advanced Video Coding (AVC) file
          format".
    [33] D. Singer, and R. Castagno, "MIME Type Registrations for 3GPP
          Multimedia files", Internet Draft,
          draft-singer-avt-3gpp-mime-01, Sep 2003.
 
 
    Author's Addresses
 
    Stephan Wenger                    Phone: +49-172-300-0813
    TU Berlin / Teles AG              Email: stewe@stewe.org
    Franklinstr. 28-29
 
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 Internet Draft                                            August, 2004
 
    D-10587 Berlin
    Germany
 
    Miska M. Hannuksela               Phone: +358-7180-73151
    Nokia Corporation                 Email: miska.hannuksela@nokia.com
    P.O. Box 100
    33721 Tampere
    Finland
 
    Thomas Stockhammer                Phone: +49-89-28923474
    Institute for Communications Eng. Email: stockhammer@ei.tum.de
    Munich University of Technology
    D-80290 Munich
    Germany
 
    Magnus Westerlund                 Phone: +46-8-7570000
    Multimedia Technologies           Email:
    Ericsson Research EAB/TVA/A       magnus.westerlund@ericsson.com
    Ericsson AB
    Torshamsgatan 23
    SE-164 80 Stockholm
    Sweden
 
    David Singer                      Phone +1 408 974-3162
    QuickTime Engineering             Email: singer@apple.com
    Apple
    1 Infinite Loop MS 302-3MT
    Cupertino
    CA 95014
    USA
 
 
 18. RFC Editor Considerations
 
    The RFC editor is requested to remove this section before
    publications as a RFC.  The RFC editor is also requested to replace
    all occurrences of XXXX with the RFC number this document receive.
 
    If available at the time of publication please do update reference
    33 with the assigned RFC number.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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