RTCP packet
An RTCP packet is encapsulated in a UDP packet. Usually a UDP packet carries at least two RTP packets, and such a UDP packet is called a "compound RTCP packet."
Figure 32: Compound RTCP packet format
As shown in Figure 32, the random 32-bit prefix in the header exists only when the RTCP packet is encrypted. An encrypted RTCP packet no longer has the features of an RTCP packet and requires no special processing. Each packet in the figure represents an RTCP packet, and no space exists between two RTCP packets. The type of the first RTCP packet in a compound RTCP packet must be SR or RR.
Figure 33: Header format of an SR-type RTCP packet
The fields are described as follows:
V—2 bits, version number.
P—1 bits, padding flag.
RC—5 bits, the number of receiving report blocks in the RTCP packet.
PT—8 bits, RTCP packet type flag. This field is 200 for SR-type RTCP packets.
Length—16 bits, length of the RTCP packet.
SSRC of Sender—32 bits, SSRC of the sender.
The structure of the RR-type RTCP packet header is similar to that of the SR-type RTCP packet header, except the PT field of the RR type is 201.