The RTP Header

The Real-Time Transport Protocol header contains a lot of necessary information for a VoIP (or any other real-time) communication. Major components include the RTP Timeclock, the RTP Timestamp, the Synchronization Source, the Contributing Source, the Payload Identification, the Sequence ID, and feedback on jitter.

The RTP Timeclock is an autonomous clock source that determines how many clock ticks have occurred. The RTP timestamp is a counter value that shows when a packet was created, and supports both silence suppression and jitter calculation. The Synchronization source (SSRC) is a unique, randomly-generated identifier that the RTP client uses to identify an RTP session, including which parties are talking and which are silent. The Contributing Source (CSRC) is an identifying list found after the RTP Packet Header, it preserves the identity of the original source. The Payload Identification is a traffic identifier, it identifies the CODEC being used. The Sequence ID is part of the header information, and contains information regarding the order that the packets were sent in. The Feedback on Jitter portion of the RTP header provides potential adjustments that will be required for the possible buffer adjustment.

Since RTP is a connectionless protocol it does not provide feedback to the sending computer. To address this issue the Real Time Control Protocol (RTCP) has been added. RTCP provides end-to-end delivery services for real-time traffic, including monitoring QoS and providing some feedback in QoS reports. It also prvides additional information about the sender, such as the name, telephone number, etc.