The IMS: IP Multimedia Concepts And Services, Second Edition

15.1. RTP for real-time data delivery

15.1.1. RTP fixed header fields

The RTP fields that are present in an RTP packet (Figure 15.1) represent the following:

• Version (V) – the version of RTP. It is always set to "2" if the implementation is [RFC3550]-compliant.

• Padding (P) – if set, then the RTP packet contains padding octets. Padding may be needed by some encryption algorithms with fixed block sizes or for carrying multiple RTP packets in a lower layer protocol packet.

• Extension (X) – if set, then there is an RTP header extension.

• CSRC count (CC) – the number of contributing source (CSRC) IDs that follow the fixed header.

• Marker (M) – its interpretation is defined by a profile.

• Payload type (PT) – Identifies the payload format (codec).

• Sequence number – this increments by 1 for each RTP data packet. This sequence number is used by the receiver to re-order packets that arrive out of sequence. It also allows the receiver to determine whether packets have been lost.

• Timestamp – indicates the time when the first octet in the payload was sampled. This field is used along with the sequence number to remove jitter (see Section 15.1.2).

• Synchronization source (SSRC) – RTP is not dependent on the underlying Internet Protocol (IP) and, therefore, uses this field to identify the source of RTP packets.

• Contributing source (CSRC) – this field carries a list of SSRCs indicating the sources that have contributed to a mixed media stream, if a mixer has been involved. It allows between 0 and 15 SSRCs. ...