[SR-Users] Attended transfer in load-balanced multi-server setup

[Iñaki Baz Castillo]

2011/4/13 Pan B. Christensen <pan at ibidium.no>:
> Scenarios:
> A call comes in from Nortel/PSTN (user A) through Nortel1 to a SIP device
> (user B). User B pushes the transfer button (puts current call on hold and
> makes a new call) and dials a Nortel / PSTN number to user C. This call goes
> out through the nortel2 server. Users B and C talk a short while before user
> B pushes the Transfer button again to connect users A and C. In this
> scenario, the REFER is forwarded by kamailio to nortel1, which replies
> "SIP/2.0 481 Call leg/transaction does not exist." The call to join is of
> course on the other asterisk (nortel2).

The problem here is just in Nortel 1. It receives an in-dialog REFER
through the dialog between user A and B, and such dialog contains a
"replaces" parameter so user B can generate an INVITE with "Replaces"
header to user C and the attended transfer completes.

But for this to work you should use a single Nortel instance rather
than two or more (I assume that the Nortel behaves as a PBX). The
REFER Nortel (User 1) receives indicates it to call to another PSTN
number, so it would generate a new call to the PSTN (rather than
routing such an INVITE to the other Nortel box).

It's not possible your scenario. It's certainly hard to balance
traffic when you need a PBX role.



> A similar scenario is where user B transfers to another SIP user. This call
> will only exist in one of the kamailio servers, and Asterisk will give the
> same response to the REFER.
>
> A third scenario is where both calls are handled by the same Asterisk
> server. This scenario works.
>
> I'm assuming I'll have to build REFER-handling logic into kamailio, but am
> unsure of how to proceed. Any suggestions?

No, Kamailio can do nothing here. REFER is just an in-dialog request
Kamailio correctly routes in the loose_route section. The problem is
not the proxy, the problem is the explained above.


Regards.



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