2.2.1 RTP Packets

The syntax of the RTP header is as specified in [RFC3550]. The fields of the fixed RTP header have their usual meaning (specified in [RFC3550] and [RFC3551]) with the following additional notes.

Marker bit (M): In audio streams, if silence suppression is enabled, the Marker bit (M) SHOULD be one for the first packet of a talk spurt and zero for all other packets; failure to do so can result in reduced audio quality at the receiving end. If silence suppression is disabled, the Marker bit MAY be one for the first packet in the stream but SHOULD be zero for all other packets. In video streams, the Marker bit MUST be one for the last packet of each video frame and zero for all other packets.

Payload type (PT): The Payload type field identifies the format of the RTP payload and determines its interpretation by the application. Codecs that are not assigned to static payload types MUST be assigned to a payload type within the dynamic range (that is, between 0x60 and 0x7F). Additionally, DTMF payloads MUST use the same payload type for the send and receive directions. Codecs with payload type numbers on the static range MUST be used as specified in the following table; codecs with payload types on the dynamic range MAY use a different payload type number but MUST be used with the clock rate, packetization time (P-time), and number of channels as specified in the following table.

Audio:

Payload type	Codec	Clock rate	P-times	Channels
0x00	G.711 µ-Law	8000	20	1
0x03	GSM 6.10	8000	40	1
0x04	G.723.1	8000	30, 60, 90	1
0x05	DVI4	8000	20	1
0x06	DVI4	16000	20	1
0x08	G.711 A-Law	8000	20	1
0x 6F	Siren	16000	20, 40	1
0x 70	G.722.1	16000	20	1

Video:

Payload type	Codec	Clock rate
0x1F	H.261	90000
0x22	H.263	90000

Synchronization Source (SSRC): The SSRC field identifies the synchronization source. This identifier SHOULD be chosen randomly but MUST not be zero. The loop detection and collision resolution algorithms from [RFC3550] section 8.2 MAY be used. Regardless of loops or collisions, the SSRC SHOULD not be changed within 2 seconds of the start of the stream or a previous SSRC change, in order to prevent packets from being ignored by the throttling algorithm described in section 3.1.

CSRC list: The CSRC list identifies the contributing sources (CSRC) for the payload contained in this packet, as defined by [RFC3550] section 5.1.