[Serusers] Troubles using b2bua as a accounting and call duration limiting module for SER router

[Maxim Sobolev]

Dear Sirs,

I am having some strange problems when trying to use b2bua for
accounting and call duration limiting with SER proxy server. The idea
is simple: since the SER can route SIP messages depending on their
source address, we can force incoming SIP messages to be passed to
B2BUA for accounting purposes first, and after the same request
re-enters the proxy from B2BUA pass it to the final destination. Call
flow looks like the following:

          -----
          |UA2|
          -----
            ^
            |
            |4
            |
          -----     3      -------
-----  1  |   |<-----------|     |
|UA1|---->|SER|     2      |B2BUA|
-----     |   |----------->|     |
          -----            -------

For some reason, it doesn't work in such configuration. The problem is
that B2BUA's UAC keeps resending `200 OK' replies ingnoring ACKs it
receives from UA2 until timeout hits, after which it considers the
call dead, despite the fact that both UA1 and UA2 think that the call 
is established. Maybe it has something to do with the fact that it
sends to and receives messages from the same host (SER), but I don't
think that this should be a problem, since those two call legs have
different call id's, so that b2bua should be able to distinguish
between them easily. Attached please find tcpdump logs of one such
session, here 192.168.1.1 is UA1 (originating), 192.168.0.9 is UA2,
192.168.1.100 is the host running both SER and B2BUA (the former uses
port 5060, while the latter - 5065). There are two files:
ser-b2bua.log is the log of udp exchange between SER and B2BUA and
ser-ua1ua2.log - log of exchange between SER and UA1/UA2.

Any ideas are appreciated.

Thanks!

-Maxim
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