rfc3984.txt

RTP英文标准文档

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


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

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119 [3].

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

3.  Scope

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

4.  Definitions and Abbreviations

4.1.  Definitions

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

      access unit: A set of NAL units always containing a primary coded
      picture.  In addition to the primary coded picture, an access unit
      may also contain one or more redundant coded pictures or other NAL
      units not containing slices or slice data partitions of a coded
      picture.  The decoding of an access unit always results in a
      decoded picture.

      coded video sequence: A sequence of access units that consists, in
      decoding order, of an instantaneous decoding refresh (IDR) access
      unit followed by zero or more non-IDR access units including all
      subsequent access units up to but not including any subsequent IDR
      access unit.

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




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

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

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

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

   In addition, the following definitions apply:

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

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

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

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

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



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

   Abbreviations

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

5.  RTP Payload Format

5.1.  RTP Header Usage

   The format of the RTP header is specified in RFC 3550 [4] and
   reprinted in Figure 1 for convenience.  This payload format uses the
   fields of the header in a manner consistent with that specification.

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









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

      Figure 1.  RTP header according to RFC 3550

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

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

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

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

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

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



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

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

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

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

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

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

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




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