The first twelve octets are present in every RTP packet, while the list of CSRC identifiers is present only when inserted by a mixer.
This field identifies the version of RTP. The version defined by this specification is two (2).
If the padding bit is set, the packet contains one or more additional padding octets at the end which are not part of the payload.
If the extension bit is set, the fixed header is followed by exactly one header extension, with a format defined in Section 5.2.1.
The CSRC count contains the number of CSRC identifiers that follow the fixed header.
The interpretation of the marker is defined by a profile. It is intended to allow significant events such as frame boundaries to be marked in the packet stream.
This field identifies the format of the RTP payload and determines its interpretation by the application.
The sequence number increments by one for each RTP data packet sent, and may be used by the receiver to detect packet loss and to restore packet sequence.
The timestamp reflects the sampling instant of the first octet in the RTP data packet. The sampling instant must be derived from a clock that increments monotonically and linearly in time to allow synchronization and jitter calculations
The SSRC field identifies the synchronization source.
The CSRC list identifies the contributing sources for the payload contained in this packet. The number of identifiers is given by the CC field. If there are more than 15 contributing sources, only 15 may be identified. CSRC identifiers are inserted by mixers, using the SSRC identifiers of contributing sources.