RTCP detailed tutorial

the RTCP packet contains below items 

8 bytes of UDP header 

12 bytes min of RTCP header

Variable, with 32 bit boundary of of RTCP data 

RTCP is delivered over RTP UDP port number + 1

(usually one port number per multi media session) 

Usually, RTCP packets are bundled and may contains several packets which is encapsulated in the same UDP data gram (to reduce the overhead due to headers) 

The RTCP header has the following components

version - 2 bits

Padding bit - 1 bits 

RR count - 5 bits

Packet Type - 8 bits 

Message Length - 16 bits 

The following are the possible packet types possible in the RTCP data 

192 - FIR => Full INTRA frame request 

193 - NACK => Negetive Acknowledgement 

200 - SR => Sender report for transmission and reception statistics from active senders (periodically transmitted) 

201 - RR => Receiver report for reception statistics from participants that are not active senders (periodically transmitted) 

202 - SDES => Source Description items (including CNAME - canonical name) 

203 - BYE => Good bye indicates end of participation 

204 - APP => Application specific functions 

207 - XR => RTCP extension 

RTCP Sender Report 

RTCP receivers provide reception quality feedback using a SR or a RR (if receiver also a sender) 

The Sender Report is having two main blocks apart from the header. 

1. Sender Information Block

2. Reception Report Block (for each source) 

Sender Information Block is having below items

- NTP timestamp (the absolute wall clock time when report was sent) 

- RTP timestamp (relative timestamp used in RTP packets) 

- Sender's packet count (RTP packets transmitted by this sender so far in this session) 

- Sender's octect count (same but the total number of RTP payloads)

Reception Report Block contains the below main items 

- Fraction lost (fraction of RTP packets from this source lost since last report) 

- Commutative number of lost packets

- Extended highest sequence number received 

- Inter arrival Jitter (estimate of jitter of RTP data from SSRC) 

- Last SR timestamp received from this source. 

- Delay since receiving the last SR report from this source. 

References:

http://medusa.sdsu.edu/network/CS596/Lectures/ch28_RT.pdf