2010/3/18 Timo Reimann timo.reimann@1und1.de:
It could occur a corner case (very famous in SIPit events) in which two UAS (UAS1 and UAS2) reply 200 for the INVITE at the same time so the proxy must relay *both* 200 responses to UAC which must ACK both and later send a BYE for one of them. Until such BYE occurs there are two established dialogs in the proxy (or there must be).
Since it's up to the UAC what to do with these "concurrent dialogs" (e.g., immediately BYE one dialog, or keep both for a long period of time) the creation of another, regular dialog should take care of things. That is, when a response is about to be forwarded for which a dialog with the same Call-ID and From-tag already exists, create a completely new dialog. That way, a concurrent dialog created from such a race condition would be treated and managed just like another dialog created "regularly", including in-dialog routing as the dialog identifiers will differ.
The only issue one have to think about in these scenarios is how to deal with callbacks as one of the two concurrent dialogs hasn't gone through the regular "none/early/confirmed" transition but starts off right at being "confirmed". Maybe a flag or a brand new callback type could be used to enable users to hook up to dialogs established this very special way.
I still think that the approach with an unique dialog_in and multiple dialog_out is easier. I'm going to send right now a mail inviting the people to comment all this stuf. I've writen a new proposal for the Dialog moduel trying to achieve all the cases explained in this thread (even complex ones).
But AFAIK that would fail in case of serial forking as the new branch (created after the first one fails) doesn't inherit the callbacks.
Does serial forking imply deletion of the transaction and creation of a new one? If so, the dialog callbacks will in fact be destroyed. If not, each branch's provisional response will map to the same transaction and hence, to the same dialog. I believe the latter is the case but please correct me if I am wrong.
The new transaction (due to serial forking) should inherit the callbacks applied to the original transaction, but... shouldn't in fact such callbacks be inserted into the proxy *server* transaction? that's an unique transaciton even if the proxy forkes.
Agree. I'm trying to figure out the best design taking spirals into account. It will take some time to me as I don't find a proper solution yet (all the solutions have some corner cases or vulnerabilities when handling spirals).
Do you happen to have a list of these corner cases where spiraling or the dialog module in general fails?
I hope this case is covered in the wiki proposal, but please check it :)
I think it would prove really helpful if we make a checklist of items, including issues we have already discussed and and those which we haven't but which you may know. That way, we could verify whether any new approach discussed here is sufficiently good.
Additionally, I think that as soon as we have figured out the most important design choices for the improved dialog module, we should set up a wiki page to maintain the new dialog design and have a rough specification to implement from.
There we go! :)