Hi,

If you take a look at the documentation for t_check_trans(), you'll see
that it behaves in very different ways depending on the type of message
it is handling, and is supposed to return a negative/false value in some
scenarios or abort script execution -- context is very important:

https://kamailio.org/docs/modules/stable/modules/tm.html#tm.f.t_check_trans

There are two types of ACKs - hop-by-hop ACKs, which acknowledge
negative final replies, and end-to-end ACKs, which are for successful
final 2xx replies and meant to be relayed by Kamailio end-to-end, in the
manner of a normal in-dialog request.

An ACK of this nature is a separate request and a separate transaction,
though its CSeq is identical to that of the INVITE transaction it is
meant to acknowledge. However, a 2xx final reply will close an INVITE
transaction, subject to `max_inv_lifetime` and `wt_timer`[1] and all
that stuff:

The documentation speaks to this point:

    "For end-to-end ACKs (ACKs to 2xx responses for forwarded
    INVITE transactions) it will return true if the corresponding
    INVITE transaction is found and still active and false if not.

    Note that the e2e ACK matching is more of a hint then a
    certainty. A delayed e2e ACK might arrive after the
    transaction wait time elapses, when the INVITE transaction
    no longer exists and thus would not match anything. There
    are also cases when tm would not keep all the information
    needed for e2e ACK matching (since this is not needed for
    a stateful proxy and it requires additional memory,
    tm will not keep this information unless needed by
    some other module or callbacks)."

So, in other words, you should not expect t_check_trans() to return true
for an end-to-end ACK. It is reasonable to expect that in the vast
majority of cases, it will return false. The ACK should be relayed in
the same manner as any other loose-routed in-dialog request, e.g. a
reinvite or a BYE. If you follow the logic of the stock configuration
that Kamailio ships with, this will happen correctly by default.

-- Alex

[1] https://kamailio.org/docs/modules/stable/modules/tm.html#tm.p.wt_timer

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--
Alex Balashov | Principal | Evariste Systems LLC

Tel: +1-706-510-6800 / +1-800-250-5920 (toll-free)
Web: http://www.evaristesys.com/, http://www.csrpswitch.com/

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