RTCP, RTP Control Protocol

Protocol suite:	TCP/IP.
Protocol type:	Application layer protocol.
Related protocols:	RTP, Real-Time Transport Protocol.
Ports:	5005 (UDP).
MIME subtype:
SNMP MIBs:
Working groups:	avt, Audio/Video Transport.
Links:	IANA: RTP parameters. RTP page at Columbia University.

MAC header

IP header

UDP header

RTCP header

Data :::

RTCP header:

00	01	02	03	04	05	06	07	08	09	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31
Version		P	Count					Type								Length
Data :::

Version. 2 bits.
The version of RTP which is the same in RTCP packets as in RTP data packets.

P, Padding. 1 bit.
If set, this RTCP packet contains some additional padding bytes at the end which are not part of the control information. The last byte of the padding is a count of how many padding bytes should be ignored. Padding may be needed by some encryption algorithms with fixed block sizes. In a compound RTCP packet, padding should only be required on the last individual packet because the compound packet is encrypted as a whole.

Count. 5 bits. 0 to 31.
The number of reception report blocks contained in this packet.

Type. 8 bits.
RTCP packet type.

Type	Description	References
0 - 191
192	FIR, full INTRA-frame request.	RFC 2032
193	NACK, negative acknowledgement.	RFC 2032
194 - 199
200	SR, sender report.	RFC 3550
201	RR, receiver report.	RFC 3550
202	SDES, source description.	RFC 3550
203	BYE, goodbye.	RFC 3550
204	APP, application defined.	RFC 3550
205	RTPFB, Generic RTP Feedback.
206	PSFB, Payload-specific.
207	XR, RTCP extension.	RFC 3611
208 - 255

Length. 16 bits.
The length of this RTCP packet in 32 bit words minus one, including the header and any padding. The offset of one makes zero a valid length and avoids a possible infinite loop in scanning a compound RTCP packet, while counting 32 bit words avoids a validity check for a multiple of 4.

Glossary:

RFCs:

[RFC 2032] RTP Payload Format for H.261 Video Streams.