RTCP (RTP Control protocol) is an application layer protocol for controlling the RTP. RTP based on UDP makes use of RTCP to make sure the packets are recontructed in order and to know the info about RTP packets.
Below is a block diagram for various field in an RTCP packet
Version: 2 bits:
The version of RTP. This value is same in RTP and RTCP packet
P, Padding bit
If set, contains some additional padding bytes at the end which are not part of control info. Last byte of padding is a count of how many padding bytes should be ignored. Padding is added for encryption algorithms with Fixed block sizes. In a compound RTCP packet, padding should be only required for the last individual packet because compound packet is encrypted as a whole.
Count:
This field is of 5 bits 0-31 . Contains the number of Reception reports contained in the RTCP packet.
Type: 8 bits : this indicates the RTCP packet type. Various types of RTCP packets are FIR (Full Intra frame Request), NACK (negative acknowledgement),
SMPTEC,SMPTE time code mapping., IJ , extended inter Jitter report , SR sender report, RR receiver report, SDES source description, BYE good bye, APP application defined, RTPFB general RTP feedback, PSFB payload specific, XR RTCP extension, AVB AVB RTCP packet, RSI receiver summary info etc.
Length (16 bits). This is the length of RTCP packet including the header and any padding bits.
references:
http://www.networksorcery.com/enp/protocol/rtcp.htm
Below is a block diagram for various field in an RTCP packet
Version: 2 bits:
The version of RTP. This value is same in RTP and RTCP packet
P, Padding bit
If set, contains some additional padding bytes at the end which are not part of control info. Last byte of padding is a count of how many padding bytes should be ignored. Padding is added for encryption algorithms with Fixed block sizes. In a compound RTCP packet, padding should be only required for the last individual packet because compound packet is encrypted as a whole.
Count:
This field is of 5 bits 0-31 . Contains the number of Reception reports contained in the RTCP packet.
Type: 8 bits : this indicates the RTCP packet type. Various types of RTCP packets are FIR (Full Intra frame Request), NACK (negative acknowledgement),
SMPTEC,SMPTE time code mapping., IJ , extended inter Jitter report , SR sender report, RR receiver report, SDES source description, BYE good bye, APP application defined, RTPFB general RTP feedback, PSFB payload specific, XR RTCP extension, AVB AVB RTCP packet, RSI receiver summary info etc.
Length (16 bits). This is the length of RTCP packet including the header and any padding bits.
references:
http://www.networksorcery.com/enp/protocol/rtcp.htm
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