RTCP and Type Of RTCP packets

RTCP 

RTP Control protocol is based on periodic transmission of control packets to all participants in the session using the same distribution mechanism as data packets. 

RTCP was first specified in RFC 1889 which is obsoleted by the RFC 3550. 

Typically, RTCP uses UDP protocol. RTCP doesn't not have a well known UDP port, instead port is allocated dynamically and then signalled using different protocols such as SDP and H245. 

Below are the major functions of RTP

- It provides feedback on the quality of data transmission. Different type of packets are used. 

- This carries persistent transport-level identifier for an RTP source called Cannonical Name or CNAME. SSRC may change from time to time, but CNAME remains the same. It used to identify a participant during a session. RTCP may also contain extra information about a participant like email. 

- By Having each participant send its control packets to all the others, each can independently observe the number of participants. This number is used to calculate the rate at which the packets are sent. More users in the session means an individual source may send the packets less frequently. 

Below are the types of RTCP packets

1. SR: Sender Report, for transmission and reception statistics from participants that are active senders. 

2. RR : Receiver Report : for reception statistics from participants that are NOT active senders. 

3. SDES: Source Description Items, including CNAME. 

4. BYE indicates the end of participation. 

5. APP : Application specific functions. 

References:
http://www.siptutorial.net/RTP/rtcp.html