SIP Trapezoid

SIP Trapezoid^†

^† From the lecture notes “SIP Tutorial: Introduction to SIP” by Henry Sinnreich and Alan Johnston, formerly at http://smuhandouts.com/8393/SIPTutorial.pdf

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Transcript

[slide139] So, in the end, this forms what's called the SIP trapezoid. So, we have SIP requests go to the outbound proxy, which reach an inbound proxy, which look things up in the location server, sending the sent message to the other user agent, the callee's user agent. Messages come back, and then what happens? We set up a media session, but it goes directly between the two parties. There's no reason that it has to go across that other path. Why is this very significant for us? [student answers:] Because we have to find the address of the address of the address of the UA. Right. We'll find that via SIP our outbound proxy will take care of the work for B. The difference is, our signaling and our media follow two completely different paths. So, we can't rely upon the knowledge of the signaling nodes to tell us anything about what the quality of the call is going to be. Right? It takes a completely different path. It also means that these messages which are about setting up the call, how time-sensitive are they? Not very time-sensitive. But the media stream, I'd like to have low delay and I'd like to have low jitter for that. That's perfect because that means that I could put my SIP server, in my case, in Sweden, and I could be in Germany, and I send the signaling message via my outbound proxy, which might be in Sweden, to someone else's inbound proxy, and I don't know or care where they are. But now the media session gets set up directly between the two of us. Right? This is wonderful. As we saw earlier, this is why the SIP provider has very low cost. They're only dealing with the signaling messages. But it means they have no idea where the media is going. And they have no real control over it.