Demultiplexing and decoding an MPEG2 Transport Stream (MPEG2-TS) requires the knowledge of specific information about the transport stream, which we refer to as the MPEG2-TS Preamble. While this information is spread over different locations throughout the transport stream and can be eventually assembled after some time a receiver started receiving the MPEG2-TS, the time it takes to retrieve all this information (especially in multicast environments) may be long. Instead, having this information readily available as a Preamble and sending the Preamble to a receiver that will shortly start receiving the transport stream will virtually eliminate the waiting time and let the receiver start processing/decoding the MPEG2-TS sooner. In this document, we give an overview of the MPEG2-TS and the delay components in video systems, and motivate the need for constructing and using the MPEG2-TS Preamble for rapidly acquiring the source stream in RTP multicast sessions. We also define and register the RTP payload format for the MPEG2-TS Preamble.