RTP Common Header

The common header begins with a fixed-length block (V to SSRC), followed by a variable-length CSRC list.
 

 0               1               2               3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X|  CC   |M|     PT      |      sequence number SN       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         timestamp TS                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           synchronisation source (SSRC) identifier            |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|            contributing source (CSRC) identifiers             |
|                             ....                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

V:

2 bits
This field identifies the version of RTP, currently 2.

P:

1 bit
The padding bit indicates whether the packet carries one or more padding octets at the end which are not part of the payload. The number of padding octets to be ignored is contained in the last padding octet.

X:

1 bit
The extension bit indicates whether the fixed header is followed by a (single) header extension.

CC:

4 bits
The CSRC count contains the number of CSRC identifiers that follow the SSRC identifier.

M:

(normally) 1 bit
The interpretation of the marker is profile-dependent. It can be used to mark significant events such as frame boundaries. A profile can trade additional marker bits for additional PT bits.

PT:

(normally) 7 bits
The payload type field identifies the format of the RTP payload, as specified in a profile. Byte codes for an initial set of payload types are defined in [RFC1890] and [RFC1700]. An RTP sender emits only a single RTP payload type at any given time; this field is not intended for multiplexing separate media streams.

SN:

16 bits
A sender increments the value of this sequence number field by one for each RTP data packet sent, starting with a random initial value. A receiver might use this field to detect packet loss, and to restore the original sequence of packets arrived out of order.

TS:

32 bits
The timestamp reflects the sampling instant of the first octet in the RTP data packet, derived from a clock that increments monotonically and linearly in time. The timestamp allows the receiver to do playout synchronisation and jitter calculations.

SSRC:

32 bits
The SSRC field identifies the synchronisation source. Although the SSRC identifiers are chosen randomly, it is not permissible that an RTP session participant chooses an SSRC identifier used by another session participant. Every RTP implementation must be programmed to detect and resolve any SSRC collision.

CSRC:

0 to 15 items, 32 bits each
CSRC identifiers are inserted by mixers, using the SSRC identifiers of sources that have contributed to the payload in the packet. The number of identifiers is given by the CC field.