[SR-Users] processing ACK

[Maxim Sobolev]

Well technically there could be SDP in ACK as well, in the body-less INVITE
scenario. I am not sure if it's the case here, though. Other than that,
it's correct, final positive ACK in RFC3261 is end-to-end, so that proxy is
just passive retranslator for that.
On Mar 9, 2016 5:07 PM, "Alex Balashov" <abalashov at evaristesys.com> wrote:

> Hello,
>
> I think you may be missing a couple of key fundamentals of SIP protocol
> mechanics:
>
> 1) By definition, one cannot respond to an ACK; an ACK garners no
> responses.
>
> 2) End-to-end ACKs for 2xx INVITE responses are generated by a SIP UA; a
> proxy simply cannot behave as though it were an endpoint since it has no
> hand in SDP negotiation.
>
> So, you really have no choice but to forward an end-to-end ACK to the
> destination for which it is intended. If you intercepted it at the proxy
> and absorbed it somehow, the UA that is waiting for it (the callee) would
> eventually end the dialog because it remains in established-nonconfirmed
> state.
>
> Now, as far as Kamailio goes: an ACK is a separate transaction from the
> INVITE to which it corresponds, and accordingly, does not have access to
> transaction-persistent variables associated with that INVITE. There are two
> or three ways to maintain dialog-persistent variables:
>
> 1) Stuff them into Record-Route parameters.
>
> 2) Put them in htable entries.
>
> 3) Use $dlg_var():
>
> http://www.kamailio.org/wiki/cookbooks/4.3.x/pseudovariables#dlg_var_key
>
> However, whatever you're trying to do here, it doesn't sound like it's
> going to work. I would encourage you to acquire a deeper understanding of
> SIP before proceeding further.
>
> -- Alex
>
> --
> Alex Balashov | Principal | Evariste Systems LLC
> 1447 Peachtree Street NE, Suite 700
> Atlanta, GA 30309
> United States
>
> Tel: +1-800-250-5920 (toll-free) / +1-678-954-0671 (direct)
> Web: http://www.evaristesys.com/, http://www.csrpswitch.com/
>
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