rfc3640.txt

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   example one stream from local storage, another from an RTP streaming
   server), synchronization may become impossible if the receiver only
   knows the original time stamp relationships.  In such cases the time
   stamp relationship required for obtaining synchronization may be
   provided by out of band means.  The format of such information, as
   well as methods to convey such information, are beyond the scope of
   this specification.

   SSRC: set as described in RFC 3550 [2].

   CC and CSRC fields are used as described in RFC 3550 [2].

   RTCP SHOULD be used as defined in RFC 3550 [2].  Note that time
   stamps in RTCP Sender Reports may be used to synchronize multiple
   MPEG-4 elementary streams and also to synchronize MPEG-4 streams with
   non-MPEG-4 streams, in case the delivery of these streams uses RTP.

3.2.  RTP Payload Structure

3.2.1.  The AU Header Section

   When present, the AU Header Section consists of the AU-headers-length
   field, followed by a number of AU-headers, see Figure 2.

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- .. -+-+-+-+-+-+-+-+-+-+
      |AU-headers-length|AU-header|AU-header|      |AU-header|padding|
      |                 |   (1)   |   (2)   |      |   (n)   | bits  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- .. -+-+-+-+-+-+-+-+-+-+

                   Figure 2: The AU Header Section

   The AU-headers are configured using MIME format parameters and MAY be
   empty.  If the AU-header is configured empty, the AU-headers-length
   field SHALL NOT be present and consequently the AU Header Section is
   empty.  If the AU-header is not configured empty, then the AU-
   headers-length is a two octet field that specifies the length in bits
   of the immediately following AU-headers, excluding the padding bits.

   Each AU-header is associated with a single Access Unit (fragment)
   contained in the Access Unit Data Section in the same RTP packet.



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   For each contained Access Unit (fragment), there is exactly one AU-
   header.  Within the AU Header Section, the AU-headers are bit-wise
   concatenated in the order in which the Access Units are contained in
   the Access Unit Data Section.  Hence, the n-th AU-header refers to
   the n-th AU (fragment).  If the concatenated AU-headers consume a
   non-integer number of octets, up to 7 zero-padding bits MUST be
   inserted at the end in order to achieve octet-alignment of the AU
   Header Section.

3.2.1.1.  The AU-header

   Each AU-header may contain the fields given in Figure 3.  The length
   in bits of the fields, with the exception of the CTS-flag, the
   DTS-flag and the RAP-flag fields, is defined by MIME format
   parameters; see section 4.1.  If a MIME format parameter has the
   default value of zero, then the associated field is not present.  The
   number of bits for fields that are present and that represent the
   value of a parameter MUST be chosen large enough to correctly encode
   the largest value of that parameter during the session.

   If present, the fields MUST occur in the mutual order given in Figure
   3.  In the general case, a receiver can only discover the size of an
   AU-header by parsing it since the presence of the CTS-delta and DTS-
   delta fields is signaled by the value of the CTS-flag and DTS-flag,
   respectively.

      +---------------------------------------+
      |     AU-size                           |
      +---------------------------------------+
      |     AU-Index / AU-Index-delta         |
      +---------------------------------------+
      |     CTS-flag                          |
      +---------------------------------------+
      |     CTS-delta                         |
      +---------------------------------------+
      |     DTS-flag                          |
      +---------------------------------------+
      |     DTS-delta                         |
      +---------------------------------------+
      |     RAP-flag                          |
      +---------------------------------------+
      |     Stream-state                      |
      +---------------------------------------+

   Figure 3: The fields in the AU-header.  If used, the AU-Index field
             only occurs in the first AU-header within an AU Header
             Section; in any other AU-header, the AU-Index-delta field
             occurs instead.



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   AU-size: Indicates the size in octets of the associated Access Unit
      in the Access Unit Data Section in the same RTP packet.  When the
      AU-size is associated with an AU fragment, the AU size indicates
      the size of the entire AU and not the size of the fragment.  In
      this case, the size of the fragment is known from the size of the
      AU data section.  This can be exploited to determine whether a
      packet contains an entire AU or a fragment, which is particularly
      useful after losing a packet carrying the last fragment of an AU.

   AU-Index: Indicates the serial number of the associated Access Unit
      (fragment).  For each (in decoding order) consecutive AU or AU
      fragment, the serial number is incremented by 1.  When present,
      the AU-Index field occurs in the first AU-header in the AU Header
      Section, but MUST NOT occur in any subsequent (non-first) AU-
      header in that Section.  To encode the serial number in any such
      non-first AU-header, the AU-Index-delta field is used.

   AU-Index-delta: The AU-Index-delta field is an unsigned integer that
      specifies the serial number of the associated AU as the difference
      with respect to the serial number of the previous Access Unit.
      Hence, for the n-th (n>1) AU, the serial number is found from:

      AU-Index(n) = AU-Index(n-1) + AU-Index-delta(n) + 1

      If the AU-Index field is present in the first AU-header in the AU
      Header Section, then the AU-Index-delta field MUST be present in
      any subsequent (non-first) AU-header.  When the AU-Index-delta is
      coded with the value 0, it indicates that the Access Units are
      consecutive in decoding order.  An AU-Index-delta value larger
      than 0 signals that interleaving is applied.

   CTS-flag: Indicates whether the CTS-delta field is present.  A value
      of 1 indicates that the field is present, a value of 0 indicates
      that it is not present.

      The CTS-flag field MUST be present in each AU-header if the length
      of the CTS-delta field is signaled to be larger than zero.  In
      that case, the CTS-flag field MUST have the value 0 in the first
      AU-header and MAY have the value 1 in all non-first AU-headers.
      The CTS-flag field SHOULD be 0 for any non-first fragment of an
      Access Unit.










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   CTS-delta: Encodes the CTS by specifying the value of CTS as a 2's
      complement offset (delta) from the time stamp in the RTP header of
      this RTP packet.  The CTS MUST use the same clock rate as the time
      stamp in the RTP header.

   DTS-flag: Indicates whether the DTS-delta field is present.  A value
      of 1 indicates that DTS-delta is present, a value of 0 indicates
      that it is not present.

      The DTS-flag field MUST be present in each AU-header if the length
      of the DTS-delta field is signaled to be larger than zero.  The
      DTS-flag field MUST have the same value for all fragments of an
      Access Unit.

   DTS-delta: Specifies the value of the DTS as a 2's complement offset
      (delta) from the CTS.  The DTS MUST use the same clock rate as the
      time stamp in the RTP header.  The DTS-delta field MUST have the
      same value for all fragments of an Access Unit.

   RAP-flag: When set to 1, indicates that the associated Access Unit
      provides a random access point to the content of the stream.  If
      an Access Unit is fragmented, the RAP flag, if present, MUST be
      set to 0 for each non-first fragment of the AU.

   Stream-state:  Specifies the state of the stream for an AU of an
      MPEG-4 system stream; each state is identified by a value of a
      modulo counter.  In ISO/IEC 14496-1, MPEG-4 system streams use the
      AU_SequenceNumber to signal stream states.  When the stream state
      changes, the value of the stream-state MUST be incremented by one.

      Note: no relation is required between stream-states of different
      streams.

3.2.2.  The Auxiliary Section

   The Auxiliary Section consists of the auxiliary-data-size field
   followed by the auxiliary-data field.  Receivers MAY (but are not
   required to) parse the auxiliary-data field; to facilitate skipping
   of the auxiliary-data field by receivers, the auxiliary-data-size
   field indicates the length in bits of the auxiliary-data.  If the
   concatenation of the auxiliary-data-size and the auxiliary-data
   fields consume a non-integer number of octets, up to 7 zero padding
   bits MUST be inserted immediately after the auxiliary data in order
   to achieve octet-alignment.  See Figure 4.







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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- .. -+-+-+-+-+-+-+-+-+
      | auxiliary-data-size   | auxiliary-data       |padding bits |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- .. -+-+-+-+-+-+-+-+-+

           Figure 4: The fields in the Auxiliary Section

   The length in bits of the auxiliary-data-size field is configurable
   by a MIME format parameter; see section 4.1.  The default length of
   zero indicates that the entire Auxiliary Section is absent.

   auxiliary-data-size: specifies the length in bits of the immediately
      following auxiliary-data field;

   auxiliary-data: the auxiliary-data field contains data of a format
      not defined by this specification.

3.2.3.  The Access Unit Data Section

   The Access Unit Data Section contains an integer number of complete
   Access Units or a single fragment of one AU.  The Access Unit Data
   Section is never empty.  If data of more than one Access Unit is
   present, then the AUs are concatenated into a contiguous string of
   octets.  See Figure 5.  The AUs inside the Access Unit Data Section
   MUST be in decoding order, though not necessarily contiguous in the
   case of interleaving.

   The size and number of Access Units SHOULD be adjusted such that the
   resulting RTP packet is not larger than the path MTU.  To handle
   larger packets, this payload format relies on lower layers for
   fragmentation, which may result in reduced performance.

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |AU(1)                                                          |
      +                                                               |
      |                                                               |
      |               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               |AU(2)                                          |
      +-+-+-+-+-+-+-+-+                                               |
      |                                                               |
      |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               | AU(n)                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |AU(n) continued|
      |-+-+-+-+-+-+-+-+

        Figure 5: Access Unit Data Section; each AU is octet-aligned.





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   When multiple Access Units are carried, the size of each AU MUST be
   made available to the receiver.  If the AU size is variable, then the
   size of each AU MUST be indicated in the AU-size field of the
   corresponding AU-header.  However, if the AU size is constant for a
   stream, this mechanism SHOULD NOT be used; instead, the fixed size
   SHOULD be signaled by the MIME format parameter "constantSize"; see
   section 4.1.

   The absence of both AU-size in the AU-header and the constantSize
   MIME format parameter indicates the carriage of a single AU
   (fragment), i.e., that a single Access Unit (fragment) is transported
   in each RTP packet for that stream.

3.2.3.1.  Fragmentation