Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 208
RTP Message Format

Vers indicates the version, which as of this writing is 2 (version 1 was the draft spec and 0 was used for the public domain Visual Audio Tool [VAT]). The P bit indicates that padding is used at the end of the datagram and the last byte indicates how many bytes of padding. The X bit is the extension bit, which indicates that the RTP fixed header is followed by an extension header. This has limited use, but it does allow for extensibility. An extension mechanism is provided to allow individual implementations to experiment with new payload-format-independent functions that require additional information to be carried in the RTP data packet header. The sequence number is 16 bits long and increments by 1 for each message sent. The start number is like TCP and can be started anywhere within a 16-bit range. The timestamp indicates a number reflective of the time of the transmission of the first byte in the RTP data packet and increments sequentially. Timestamps are used to exact the timing as it was sent from the source. Several messages may have the same timestamp, which could indicate they were sent at the same time and belong to the same video frame.

Synchronization Source field is a 32-bit number that indicates the originator of the message that inserted the sequence number and the timestamp for the data so as not to be dependent on the IP address. As shown in the previous slide on mixers, there are two sources of audio data. Each packet will contain their address for the packets that they send. The selection of the identifier is beyond the scope of this book, but there is a random number generated for this field by the source, thereby allows each source to be unique. If two sources do select the same identifier, RTP does have the mechanisms with which to detect and correct this. This field could indicate an alternate source if the received message was originated by a mixer. If the two packets entered a mixer, the mixer would insert its 32-bit number as the source and push the previous SSRC numbers into the Contributing Source Identifier.

RTP Message Format

The Contributing Source Identifier indicates a source or sources (the original IDs of the sources) of a stream of RTP packets that were involved in the combined stream produced by a mixer.