[Serusers] ACK processing (was: [Serdev] loose_route behaviour, detecting single?Route with myself)

Nils Ohlmeier nils at iptel.org
Thu Jul 12 13:25:06 CEST 2007

On Thursday 12 July 2007 12:54:58 Martin Hoffmann wrote:
> Nils Ohlmeier wrote:
> > On Thursday 12 July 2007 09:26:37 Martin Hoffmann wrote:
> > > Is there any argument against putting a
> > >
> > >    if (method == "ACK") {
> > >       t_relay();
> > >       drop();
> > >    }
> > >
> > > somewhere way up in your config?
> >
> > And how do you distinguish between in-dialog ACKs and out-of-dialog ACKs?
> Techically, there is no such thing. There is only ACKs to 2xx and
> non-2xx responses. The former are always in-dialog requests, since the
> 2xx creates the dialog should it not yet exists. The latter is a matter
> of the transaction layer which doesn't know of dialogs.

You call them non-2xx ACKs, I called them out-of-dialog ACKs (because the 
typical scenario for them is the acknowledgment of the 40[17] for the INVITE, 
and there is no dialog yet). But we are speaking about the same here.

> In practice, an ACK to a non-2xx response always needs to be eaten by
> tm. This t_relay() does. Any ACK to a 2xx response needs to be routed
> like any other in-dialog request to the endpoint (save for some
> elaborate processing such as authentication). Which means, my code
> snipped does indeed lack a loose_route() before t_relay().

Ok, what is the loose_route suppose to do in this case?
A 2xx ACK would be "routed" according to the Route headers by loose_route(), 
and relayed by t_relay() then. And what about the pre-loaded non-2xx ACKs?
I would treat them exactly the same way like the 2xx ACK. loose_route() will 
consum the Route header and t_relay() then forwards it to the RURI (which 
should be your proxy). When you receive it again, their is no Route in it any 
more, so it should hit your normal non-dialog code where it hopefully sooner 
or later will hit a t_relay.

> > Note: they both have a To-tag and even worse if the original INVITE
> > contained a pre-loaded Route header (for outbound proxy) the ACK has to
> > contain this Route as well!
> Depends. A non-2xx ACK indeed has to. But a 2xx ACK musn't. It is to be
> created like any other in-dialog request. The Route set thus MUST be the
> reversed set of the Record-Route headers of the 2xx response, possibly
> be empty.

You are not telling me anything new here. But as I explained in my other reply 
to Klaus it is quite tricky to distinguish a non-2xx ACK with pre-loaded 
Route, from a 2xx ACK with just one Route header. Then the only difference is 
the RURI.
Because of this I prefer the approach of "routing" pre-loaded requests via 
loose_route() to the RURI. That is IMHO the cleanest solution.

> Book keeping: Limited to seruser and changed subject.

I'm not sure if that was the right limitation, but anyway ;-)


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