[Kamailio-Devel] Flags are not visible when processing CANCEL

[Iñaki Baz Castillo]

2009/5/22 Alex Balashov <abalashov at evaristesys.com>:

> Hmm, that's true.  End-to-end CANCELs aren't sent by the originating UA to
> the destination URI of the far end, are they?  Otherwise they would be
> routed like any other sequential in-dialog request, and they're not.

For generating an in-dialog request you need first having a dialog or
early-dialog (To tag). It could not occur (UAC sends INVITE, proxy
replies "100 Trying" and after a while the UAC sends a CANCEL).


> I'm curious, why are they designed that way?  Why not route end-to-end
> CANCELs the same way one routes end-to-end BYEs?

The CANCEL is sent to the proxy (or the SIP node in front of the UAC).
Imagine the UAC sends the INVITE, the proxy does parallel forking so
the UA Creceives various 1XX with different "To tags". If CANCEL would
be end-to-end and an in-dialog request, then the UAC should generate
so many CANCEL's as different "To tags" received in the 1XX responses.

So, when an UAC sends a CANCEL it is asking its proxy to CANCEL the
INVITE transaction at all, so the proxy replies "200 cancelled" and
later, the proxy handles all the pending branches by sending a CANCEL
to each one.

It's easier in this way :)



> --
> Alex Balashov
> Evariste Systems
> Web     : http://www.evaristesys.com/
> Tel     : (+1) (678) 954-0670
> Direct  : (+1) (678) 954-0671
>



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