There are five different reports used by RTCP: I will refer you to the RFC for a detailed explanation of how the ratio is calculated, but it’s enough to know that they are sent infrequently compared to RTP. The ratio for RTCP to RTP is not static, but dependent on a number of factors including bandwidth and the number of participants in a conversation. Third, while you may see thousands of RTP messages for a fairly short call, you will only see a handful of RTCP packets. These packets will be small and are sent using UDP. Second, there are a ton of RTP messages generated by the sender and the receiver. In fact, you can usually get by with only five messages. First, there are very few SIP messages involved in setting up a call.
If you were to do a Wireshark trace of a basic SIP telephone call, you will notice a few things. It may put you to sleep, but it’s the ultimate authority on both RTP and RTCP. For those of you who require a deeper dive, I suggest you take a look at RFC 3550. This article is meant to be a high level explanation of RTCP. For example, if RTP is sent on port 5000, RTCP will be sent on port 5001. Instead, RTCP packets are used to report statistics about the ongoing call.Īlthough I don’t know if this is required, I have always seen RTP sent on an even numbered port and RTCP on the adjacent odd numbered port. However, unlike RTP, they don’t contain media. As data are being sent, RTCP packets are periodically generated by both the sender and the receiver. As you already know, RTP is used to transmit media between peers. I am going to press my luck a little further and write about RTP’s sister protocol, Real-Time Control Protocol ( RTCP). Clearly, I am not the only geek around these parts because all three articles received quite a few views and continue to trend fairly high.
In November, I took you on a tour of a SIP conference in Dissecting a SIP Conference Call and in December you got to see the nitty-gritty of transfer in Dissecting SIP Transfer and media transmission in A Wireshark View of Real-Time Protocol (RTP). I’ve been on a Wireshark binge these past few weeks.
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