What is going wrong in this SIP call? Multiple NOTIFY messages in a row before RTP established

Question

the long string of NOTIFY messages happen after the called number answers. and after about 20-30 seconds the 503 happens and then the call connects fine with audio.

Answer 1

If that trace is for a single call it's an incredibly complex one. After spending a bit of time looking over it I don't think it is for a single call and instead there are a few different calls mixed up in it. It's complicated by the fact that 10.10.20.1 is a Back to Back User Agent (B2BUA) and is initiating its on calls in response to different events.

As to your question about the NOTIFY request it's originally generated by the UAC at 10.10.10.3 as part of what appears to be an attended transfer. The REFER request is the start of the transfer. An implicit subscription, which is what the NOTIFY request is part of, gets created for a REFER transaction (see http://tools.ietf.org/html/rfc3515 and also see http://tools.ietf.org/html/rfc4488 which deals with suppressing the implicit transaction).

For an attended transfer the NOTIFY request allows a call leg end point to indicate that the transfer has been processed successfully. In this case it looks like the user agent at 10.10.10.3 isn't happy to accept the transfer until it gets a response to its NOTIFY request. This is unusual behaviour as typically the NOTIFY requests are for just that, notifying agents of events not controlling call flow. Once 10.10.10.3 gets the 503 response to its NOTIFY request it finally starts sending the RTP to 10.10.20.4. It mustn't care what the response is as 503 is an error condition and would usually result in whatever was waiting for it to fail.