[SR-Users] TM clarifications Re: CANCEL before INVITE + loose_route()

[Iñaki Baz Castillo]

2010/6/18 David <kamailio.org at spam.lublink.net>:
> With regards to the thread "Re: [SR-Users] CANCEL before INVITE". I added a
> t_newtran(); call after t_checktrans() and before t_relay(), this prevented
> t_relay() from sending a 100 Trying to the calling user. Why would the
> functionality of t_relay() change like this ? Is t_relay() assuming that I
> have already send the 100 Trying if the transaction exists?

Yes.


> 'Once the CANCEL is constructed, the client SHOULD check whether it has
> received any response (provisional or final) for the request being cancelled
> (herein referred to as the "original request"). ' - RFC3261 Section 9.
>
> So is this the check that tm 1.4.11 t_checktrans() is doing ? If not, how
> should I be doing this check.

This text is about client side. An UAC shouldn't send a CANCEL if it
has received no provisional response from the proxy/UAS. This is to
avoid race conditions:
In UDP the UAC could send the INVITE, followed by a CANCEL, and the
CANCEL could arrive to the proxy before the INVITE (race condition),
so the UAC must wait until having a provisiona response in order to
send a CANCEL.

But from the server/proxy point of view it doesn't matter. This is, if
the proxy has received an INVITE it doesn't matter if it has replied a
provisional response or not, it already *can* handle a CANCEL from the
client. The only requeriment is having a transaction created for such
INVITE (which occurs when calling t_relay or t_newtran).



> It looks like I should be be absorbing transactions pretty much at the
> beginning of my routing script after the sanity checks, is that right?
> Should the transactions be checked before I call the loose_route() method?

Yes, but usually it's done after loose_route section because such
section consumes very few resources (just pure routing). But in my
case I check for retransmissions at the beginning of the script.


> I read that loose_route() should be secured, would it be correct to request
> a proxy authentication before calling loose_route() ?

The dialog mechanism (call-id, from-tag and to-tag) should be secure
enough. Anyhow you can ask for auth in the proxy but take into account
that some UA's out there don't re-send an in-dialog request with
credentials.
Also, when asking for auth take into account that the From URI of the
sender could be different than the username of its credentials (i.e:
alice calls to 200 which is an alias to bob, bob later sends a BYE
with original From: "sip:200 at domain", so it wouldn't match the
credentials username "bob").


> Can a CANCEL request ever be routed using loose_route().

No, CANCEL is a hop-by-hop request, an UAC sends a CANCEL to the
inmediate proxy, and the proxy inmediately replies 200 to the UAC.
Later the proxy generates its ***own*** outgoing CANCEL's in order to
terminate pending branches (if they exist).
Also, CANCEL is not an in-dialog request (no To-tag, no route-set).
But you would do better by reading the CANCEL section in RFC 3261 in
which this subjet is very well explained.

Regards.


-- 
Iñaki Baz Castillo
<ibc at aliax.net>