Real-Time Transport Control Protocol - what & why Explained

 
I had came across this questions few times in since last week, actually one of colleague is working on a traffic priotitization requirement for SIP based telephony system and as we all know this could not be completed without RTCP.

So, What is RTCP ?

Well, RTCP is Real-Time Transport Control Protocol, which works hand in hand with RTP (Real-Time Transport Protocol) and is used for real time traffic like voice and video. So, RTCP gives us a control mechanism that provides feedback to the sender about the quality of the data it is sending – how much packets the remote side received, what got lost along the way.

 RTCP is accompanied with two of its extended features - 

Lets discuss these features in detail - 

Extended Reports -  RTCP is a kind of reporting tool. It monitors the traffic flow and sends a report every now and then but by default it provides a report with basic information only. So, it basically provides us little guidance is controlling the issues like congestion, Call Status detection etc. Extended Reports compliments the existing features of the RTCP protocol. Its added functinality includes - 

Extended Reports can provide more useful information like how many packets were lost, What was the sequence number of the lost packets, If the retransmit of the lost packets would do some help in improving the quality or can automatically judge the MOS and take the appropriate decisions rapidly. So, Isnt this impressive ?? 

Now lets talk about another feature - 

Feedback - Today RTP is used by all major real time transmission protocols like SIP, XMPP, H.323 etc. negotiation of different bitrates, request for frames and the likes are usually done over the signaling protocols: H.245 when it comes to H.323, INFO and SDP when it comes to SIP. While this means that mapping protocols and gatewaying between them is a hassle, there’s an even bigger implication: We want media to go point-to-point as much as possible to reduce latency and increase quality, which means we would like all control mechanisms of the media to traverse the same route – and signaling is often split from the media and takes a different route through a different set of entities over the network. 

To solve these issues, RTCP-FB comes in the picture, which is also known as RTP/AVPF. This defines a way for applications to send commands over RTCP from the receiver of the media to the sender. So, basically it sits on top of the RTP to monitor and control the to and fro of control messages, prioritization and other management feaures. Kind of tough job....

So you see, RTCP is a lot more than just an aside to RTP – it is a fully fledged protocol that is bound to be used and adopted as voice/video calling becomes commonplace.

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