draft-ietf-avt-mpeg4-simple-06.txt

网络MPEG4IP流媒体开发源代码

   SSRC: set as described in RFC 1889 [2]. 

   CC and CSRC fields are used as described in RFC 1889 [2].

   RTCP SHOULD be used as defined in RFC 1889 [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.  


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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. 
   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 above 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.



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   +---------------------------------------+
   |     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.

   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 with 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 




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         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 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. 

   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 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: Indicates when set to 1 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 stream-state MUST be 
         incremented by one. 

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



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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.    

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- .. -+-+-+-+-+-+-+-+-+
   | 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.











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

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


   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, but 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 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

   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

   Access Units MAY be interleaved. Senders MAY perform interleaving.
   Receivers MUST support interleaving, except if the receiver only 
   supports modes in which no interleaving is allowed. When Access  
   Units are interleaved, it SHALL be implemented using the AU-Index 
   and the AU-Index-delta fields in the AU-header.

   When a sender interleaves Access Units, then the transmitter needs
   to provide sufficient information to enable a receiver to 
   unambiguously reconstruct the original order, even in case of 
   out-of-order packets, packet loss or duplication. The information 


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   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)