Hi Richard,
I guess it's a very thin line between a loop and a spiral. They are very similar (even impossible to tell the difference from SIP point of view), but first is harmful and the second useful :-). I haven't went through the RFC, but IMHO is tricky for UAS detect properly a loop - I guess that's way everybody relies on MAX_FORWARD to end potential loops. So, depends a lot of what * consider a loop (how exactly does this) - because, as you said, all important hdrs are the same. Btw, is at least RURI changing between outgoing and returning call ( on *) ? The worst solution will be insert a very simple B2BUA in the middle to change at least some tags. But is ugly :-(
regards, Bogdan
Richard wrote:
Hi,
I am trying to use * as a PSTN gateway. When a call comes in from PSTN, it is forwarded to ser. If for some reason (e.g. call being forwarded back to PSTN), ser will just do a record route and send it back to *. When * gets this call, it thinks it is a loop and drop the call. I know that this question has been raised before. I'd like to see, just from sip respective, what's the theoretical way to solve it? Since all major fields are the same, e.g. fromuri, touri, cseq, callid, what is the right way to detect loop in SIP in this case? Btw, cisco router doesn't have this problem.
Thanks, Richard
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