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RFC 3640
Transport of MPEG-4 Elementary Streams November 2003
2.7. State Indication of MPEG-4 System Streams
ISO/IEC 14496-1 defines states for MPEG-4 system streams. So as to
convey state information when transporting MPEG-4 system streams,
this payload format allows for the optional carriage in the RTP
payload of the stream state for each contained Access Unit. Stream
states are used to signal "crucial" AUs that carry information whose
loss cannot be tolerated and are also useful when repeating AUs
according to the carousel mechanism defined in ISO/IEC 14496-1.
2.8. Random Access Indication
Random access to the content of MPEG-4 elementary streams may be
possible at some but not all Access Units. To signal Access Units
where random access is possible, a random access point flag can
optionally be carried in the RTP payload for each contained Access
Unit. Carriage of random access points is particularly useful for
MPEG-4 system streams in combination with the stream state.
2.9. Carriage of Auxiliary Information
This payload format defines a specific field to carry auxiliary data.
The auxiliary data field is preceded by a field that specifies the
length of the auxiliary data, so as to facilitate the skipping of
data without parsing it. The coding of the auxiliary data is not
defined in this document; instead, the format, meaning and signaling
of auxiliary information is expected to be specified in one or more
future RFCs. Auxiliary information MUST NOT be transmitted until its
format, meaning and signaling have been specified and its use has
been signaled. Receivers that have knowledge of the auxiliary data
MAY decode the auxiliary data, but receivers without knowledge of
such data MUST skip the auxiliary data field.
2.10. MIME Format Parameters and Configuring Conditional Fields
To support the features described in the previous sections, several
fields are defined for carriage in the RTP payload. However, their
use strongly depends on the type of MPEG-4 elementary stream that is
carried. Sometimes a specific field is needed with a certain length,
while in other cases such a field is not needed. To be efficient in
either case, the fields to support these features are configurable by
means of MIME format parameters. In general, a MIME format parameter
defines the presence and length of the associated field. A length of
zero indicates absence of the field. As a consequence, parsing of
the payload requires knowledge of MIME format parameters. The MIME
format parameters are conveyed to the receiver via SDP [
5] messages,
as specified in
section 4.4.1, or through other means.
van der Meer, et al. Standards Track [Page 8]