2.1.1.3.6 RTP Encapsulation of MPEG-2 Elementary Streams

This section defines how MPEG-2 ES content is encapsulated in the RTP Payload Format for Windows Media Audio and Video [WMRTP] and how the encryption parameters are communicated using SDP.

General RTP encapsulation rules for MPEG-2 ES Content

Media Access Unit (MAU): A Media Access Unit (MAU) for encapsulation of MPEG-2 ES content with the payload format for Windows Media Audio and Video [WMRTP] is defined as a single frame of video or an audio frame.

When MPEG video headers are present, they MUST precede the subsequent frame. This requirement is similar to the fragmentation rules outlined in Section 3.1 of [RFC2250]. Specifically:

Unlike [RFC2250], if a MAU containing video is fragmented, there is no requirement to perform the fragmentation at a slice boundary.

MAU Fragmentation: MAUs can be fragmented across multiple Real-Time Transport Protocol (RTP) packets for different reasons. The most common reasons are:

  • RTP packet size restrictions.

  • Differences in encryption parameters for specific portions of the MAU.

RTP Header Fields: The RTP Header fields MUST be interpreted according to section 3.5 of the Payload Format for Windows Media Audio and Video [WMRTP]. The following clarifications apply:

  • The Timestamp field in the RTP header MUST be set to the PTS of the sample with an accuracy of 90 kHz.

  • The Payload Type (PT) field MUST be set according to out-of-band negotiation mechanisms (for example, using SDP).

Packet Specific Info section: The Packet Specific Info section MUST be interpreted according to section 3.6.1 of the Payload Format for Windows Media Audio and Video [WMRTP].

The following clarifications apply:

  • The presence of the Send Time field is optional.

  • The presence of the Correspondence field is optional.

  • The Bit Field 2 Present bit (B2P) MUST be set in case the payload contains a portion of a MAU which is encrypted, or a fragment of a MAU which is encrypted.

MAU Properties section: The MAU Properties section MUST be interpreted according to section 3.6.2 of [WMRTP]. The following clarifications apply:

  • The Sync Point bit (S) MUST be set when the MAU contains a video I-frame or an audio frame.

  • The Discontinuity bit (D1) MUST be set when one or more MAUs are missing. For example, when video frames were dropped by a frame dropping transmitter.

  • The use of the Droppable bit (D2) is optional. Defining the cases in which it SHOULD be used is outside of the scope of this specification.

  • The Encryption bit (E) MUST be set in case the payload contains a portion of a MAU that is encrypted or a fragment of a MAU that is encrypted.

MAU Timing section: The MAU Timing section MUST be interpreted according to section 3.6.3 of the RTP Payload Format for Windows Media Audio and Video specification [WMRTP]. The following clarifications apply:

  • The Decode Time field is optional. If used, it MUST contain the DTS of the MAU.

  • The Presentation Time field is optional.

  • The NPT field is optional.

  • The Extension Present bit (X) MUST be set in case the payload contains a portion of a MAU which is encrypted, or a fragment of a MAU which is encrypted.