Retransmission Timeout

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We have a Genband C3 Switch and a couple of customers that operate asterisk PBXes connected via SIP Trunk. All of them still use some 1.6.X asterisk and this works fine.

One customer uses a 1.8 version and has a very strange problem:

Asterisk 1.8.10.0-1digium1~lucid built by pbuilder @ nighthawk on a x86_64 running Linux on 2012-03-06 01:51:21 UTC

When a call reaches this asterisk the call get’s dropped after a couple of
seconds with this message:

[Mar 27 13:47:35] WARNING[29806]: chan_sip.c:3641 retrans_pkt: Retransmission timeout reached on transmission F47F6F67@7f33ff47 for seqno 907689273
(Critical Response) — See https://wiki.asterisk.org/wiki/display/AST/SIP+Retransmissions Packet timed out after 6400ms with no response
[Mar 27 13:47:35] WARNING[29806]: chan_sip.c:3670 retrans_pkt: Hanging up call

Well, the Wiki is not very helpfull, as the problems explained there do not seem to be the cause.

Well apparently the C3, from my point of view sends correct Acks to the Asterisk, but somehow the Asterisk ignores them.

I did try to get more information with debug and verbose level set to ’99’, but I don’t see more messages

Does anyone have a clue, why acks could be not accepted by asterisk 1.8.10.0 ?

The other way round (asterisk => c3) the calls work fine.

Regards