RFC 3640：MPEG-4 elementary stream RTP payload format标准

Payload

MPEG-4

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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]

RFC 3640

Transport of MPEG-4 Elementary Streams November 2003

2.11. Global Structure of Payload Format

The RTP payload following the RTP header, contains three octet-

aligned data sections, of which the first two MAY be empty, see

Figure 1.

+---------+-----------+-----------+---------------+

+---------+-----------+-----------+---------------+

<----------RTP Packet Payload----------->

Figure 1: Data sections within an RTP packet

The first data section is the AU (Access Unit) Header Section, that

contains one or more AU-headers; however, each AU-header MAY be

empty, in which case the entire AU Header Section is empty. The

second section is the Auxiliary Section, containing auxiliary data;

this section MAY also be configured empty. The third section is the

Access Unit Data Section, containing either a single fragment of one

Access Unit or one or more complete Access Units. The Access Unit

Data Section MUST NOT be empty.

2.12. Modes to Transport MPEG-4 Streams

While it is possible to build fully configurable receivers capable of

receiving any MPEG-4 stream, this specification also allows for the

design of simplified, but dedicated receivers, that are for example,

capable of receiving only one type of MPEG-4 stream. This is

achieved by requiring that specific modes be defined in order to use

this specification. Each mode may define constraints for transport

of one or more types of MPEG-4 streams, for instance on the payload

configuration.

The applied mode MUST be signaled. Signaling the mode is

particularly important for receivers that are only capable of

decoding one or more specific modes. Such receivers need to

determine whether the applied mode is supported, so as to avoid

problems with processing of payloads that are beyond the capabilities

of the receiver.

In this document several modes are defined for the transportation of

MPEG-4 CELP and AAC streams, as well as a generic mode that can be

used for any MPEG-4 stream. In the future, new RFCs may specify

other modes of using this specification. However, each mode MUST be

in full compliance with this specification (see

section 3.3.7).

van der Meer, et al. Standards Track [Page 9]

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