rfc3640.txt

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   A packet SHALL carry either one or more complete Access Units, or a
   single fragment of an Access Unit.  Fragments of the same Access Unit
   have the same time stamp but different RTP sequence numbers.  The
   marker bit in the RTP header is 1 on the last fragment of an Access
   Unit, and 0 on all other fragments.

3.2.3.2.  Interleaving

   Unless prohibited by the signaled mode, a sender MAY interleave
   Access Units.  Receivers that are capable of receiving modes that
   support interleaving MUST be able to decode interleaved Access Units.

   When a sender interleaves Access Units, it needs to provide
   sufficient information to enable a receiver to unambiguously
   reconstruct the original order, even in the case of out-of-order
   packets, packet loss or duplication.  The information that senders
   need to provide depends on whether or not the Access Units have a
   constant time duration.  Access Units have a constant time duration,
   if:

   TS(i+1) - TS(i) = constant

       for any i, where:
          i indicates the index of the AU in the original order, and
          TS(i) denotes the time stamp of AU(i)

   The MIME parameter "constantDuration" SHOULD be used to signal that
   Access Units have a constant time duration; see section 4.1.

   If the "constantDuration" parameter is present, the receiver can
   reconstruct the original Access Unit timing based solely on the RTP
   timestamp and AU-Index-delta.  Accordingly, when transmitting Access
   Units of constant duration, the AU-Index, if present, MUST be set to



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   the value 0.  Receivers of constant duration Access Units MUST use
   the RTP timestamp to determine the index of the first AU in the RTP
   packet.  The AU-Index-delta header and the signaled
   "constantDuration" are used to reconstruct AU timing.

   If the "constantDuration" parameter is not present, then senders MAY
   signal AUs of constant duration by coding the AU-Index with zero in
   each RTP packet.  In the absence of the constantDuration parameter
   receivers MUST conclude that the AUs have constant duration if the
   AU-index is zero in two consecutive RTP packets.

   When transmitting Access Units of variable duration, then the
   "constantDuration" parameter MUST NOT be present, and the transmitter
   MUST use the AU-Index to encode the index information required for
   re-ordering, and the receiver MUST use that value to determine the
   index of each AU in the RTP packet.  The number of bits of the AU-
   Index field MUST be chosen so that valid index information is
   provided at the applied interleaving scheme, without causing problems
   due to roll-over of the AU-Index field.  In addition, the CTS-delta
   MUST be coded in the AU header for each non-first AU in the RTP
   packet, so that receivers can place the AUs correctly in time.

   When interleaving is applied, a de-interleave buffer is needed in
   receivers to put the Access Units in their correct logical
   consecutive decoding order.  This requires the computation of the
   time stamp for each Access Unit.  In case of a constant time duration
   per Access Unit, the time stamp of the i-th access unit in an RTP
   packet with RTP time stamp T is calculated as follows:

   Timestamp[0] = T
   Timestamp[i, i > 0] = T +(Sum(for k=1 to i of (AU-Index-delta[k]
                         + 1))) * access-unit-duration

   When AU-Index-delta is always 0, this reduces to T + i * (access-
   unit-duration).  This is the non-interleaved case, where the frames
   are consecutive in decoding order.  Note that the AU-Index field
   (present for the first Access Unit) is indeed not needed in this
   calculation.

3.2.3.3.  Constraints for Interleaving

   The size of the packets should be suitably chosen to be appropriate
   to both the path MTU and the capacity of the receiver's de-interleave
   buffer.  The maximum packet size for a session SHOULD be chosen to
   not exceed the path MTU.






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   To allow receivers to allocate sufficient resources for de-
   interleaving, senders MUST provide the information to receivers as
   specified in this section.

   AUs enter the decoder in decoding order.  The de-interleave buffer is
   used to re-order a stream of interleaved AUs back into decoding
   order.  When interleaving is applied, the decoding of "early" AUs has
   to be postponed until all AUs that precede it in decoding order are
   present.  Therefore, these "early" AUs are stored in the de-
   interleave buffer.  As an example in Figure 6, the interleaving
   pattern from section 2.5 is considered.

                             +--+--+--+--+--+--+--+--+--+--+--+-
   Interleaved AUs           | 0| 3| 6| 1| 4| 7| 2| 5| 8| 9|12|..
                             +--+--+--+--+--+--+--+--+--+--+--+-
   Storage of "early" AUs         3  3  3  3  3  3
                                     6  6  6  6  6  6
                                           4  4  4
                                              7  7  7
                                                            12 12

   Figure 6: Storage of "early" AUs in the de-interleave buffer per
             interleaved AU.

   AU(3) is to be delivered to the decoder after AU(0), AU(1) and AU(2);
   of these AUs, AU(2) arrives from the network last and hence AU(3)
   needs to be stored until AU(2) is present in the pattern.  Similarly,
   AU(6) is to be stored until AU(5) is present, while AU(4) and AU(7)
   are to be stored until AU(2) and AU(5) are present, respectively.
   Note that the fullness of the de-interleave buffer varies in time.
   In Figure 6, the de-interleave buffer contains at most 4, but often
   less AUs.

   So as to give a rough indication of the resources needed in the
   receiver for de-interleaving, the maximum displacement in time of an
   AU is defined.  For any AU(j) in the pattern, each AU(i) with i<j
   that is not yet present can be determined.  The maximum displacement
   in time of an AU is the maximum difference between the time stamp of
   an AU in the pattern and the time stamp of the earliest AU that is
   not yet present.  In other words, when considering a sequence of
   interleaved AUs, then:

   Maximum displacement = max{TS(i) - TS(j)}

       for any i and any j>i, where:
          i and j indicate the index of the AU in the interleaving
                pattern, and
          TS denotes the time stamp of the AU.



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   As an example in Figure 7, the interleaving pattern from section 2.5
   is considered.  For each AU in the pattern, the index is given of the
   earliest of any earlier AUs not yet present.  Hence for each AU(n) in
   the interleaving pattern the smallest index k (with k<n) of not yet
   delivered AUs is indicated.  A "-" indicates that all previous AUs
   are present.  If the AU period is constant, the maximum displacement
   equals 5 AU periods, as found for AU(6) and AU(7).

                                 +--+--+--+--+--+--+--+--+--+--+--+-
   Interleaved AUs               | 0| 3| 6| 1| 4| 7| 2| 5| 8| 9|12|..
                                 +--+--+--+--+--+--+--+--+--+--+--+-

   Earliest not yet present AU     -  1  1  -  2  2  -  -  -  - 10

   Figure 7: For each AU in the interleaving pattern, the earliest of
             any earlier AUs not yet present

   When interleaving, senders MUST signal the maximum displacement in
   time during the session via the MIME format parameter
   "maxDisplacement"; see section 4.1.

   An estimate of the size of the de-interleave buffer is found by
   multiplying the maximum displacement by the maximum bit rate:

   size(de-interleave buffer) = {(maxDisplacement) * Rate(max)} / (RTP
                                clock frequency),

       where:
          Rate(max) is the maximum bit-rate of the transported stream.

   Note that receivers can derive Rate(max) from the MIME format
   parameters streamType, profile-level-id, and config.

   However, this calculation estimates the size of the de-interleave
   buffer and the required size may differ from the calculated value.
   If this calculation under-estimates the size of the
   de-interleave buffer, then senders, when interleaving, MUST signal a
   size of the de-interleave buffer via the MIME format parameter
   "de-interleaveBufferSize"; see section 4.1.  If the calculation
   over-estimates the size of the de-interleave buffer, then senders,
   when interleaving, MAY signal a size of the de-interleave buffer via
   the MIME format parameter "de-interleaveBufferSize".









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   The signaled size of the de-interleave buffer MUST be large enough to
   contain all "early" AUs at any point in time during the session.
   That is:

   minimum de-interleave buffer size = max [sum {if TS(i) > TS(j) then
                                       AU-size(i) else 0}]

       for any j and any i<j, where:
          i and j indicate the index of an AU in the interleaving
                pattern,
          TS(i) denotes the time stamp of AU(i), and
          AU-size(i) denotes the size of AU(i) in number of octets.

   If the "de-interleaveBufferSize" parameter is present, then the
   applied buffer for de-interleaving in a receiver MUST have a size
   that is at least equal to the signaled size of the de-interleave
   buffer, else a size that is at least equal to the calculated size of
   the de-interleave buffer.

   No matter what interleaving scheme is used, the scheme must be
   analyzed to calculate the applicable maxDisplacement value, as well
   as the required size of the de-interleave buffer.  Senders SHOULD
   signal values that are not larger than the strictly required values;
   if larger values are signaled, the receiver will buffer excessively.

   Note that for low bit-rate material, the applied interleaving may
   make packets shorter than the MTU size.

3.2.3.4.  Crucial and Non-Crucial AUs with MPEG-4 System Data

   Some Access Units with MPEG-4 system data, called "crucial" AUs,
   carry information whose loss cannot be tolerated, either in the
   presentation or in the decoder.  At each crucial AU in an MPEG-4
   system stream, the stream state changes.  The stream-state MAY remain
   constant at non-crucial AUs.  In ISO/IEC 14496-1, MPEG-4 system
   streams use the AU_SequenceNumber to signal stream states.

   Example: Given three AUs, AU1 = "Insertion of node X", AU2 = "Set
   position of node X", AU3 = "Set position of node X".  AU1 is crucial,
   since if it is lost, AU2 cannot be executed.  However, AU2 is not
   crucial, since AU3 can be executed even if AU2 is lost.

   When a crucial AU is (possibly) lost, the stream is corrupted.  For
   example, when an AU is lost and the stream state has changed at the
   next received AU, then it is possible that the lost AU was crucial.
   Once corrupted, the stream remains corrupted until the next random
   access point.  Note that loss of non-crucial AUs does not corrupt the
   stream.  When a decoder starts receiving a stream, the decoder MUST



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   consider the stream corrupted until an AU is received that provides a
   random access point.

   An AU that provides a random access point, as signaled by the RAP-
   flag, may or may not be crucial.  Non-crucial RAP AUs provide a
   "repeated" random access point for use by decoders that recently
   joined the stream or that need to re-start decoding after a stream
   corruption.  Non-crucial RAP AUs MUST include all updates since the
   last crucial RAP AU.

   Upon receiving AUs, decoders are to react as follows:

   a) if the RAP-flag is set to 1 and the stream-state changes, then the
      AU is a crucial RAP AU, and the AU MUST be decoded.

   b) if the RAP-flag is set to 1 and the stream state does not change,
      then the AU is a non-crucial RAP AU, and the receiver SHOULD
      decode it if the stream is corrupted.  Otherwise, the decoder MUST
      ignore the AU.

   c) if the RAP-flag is set to 0, then the AU MUST be decoded, unless
      the stream is corrupted, in which case the AU MUST be ignored.