[sr-dev] ACKs to failure responses do not match transactions

[Hugh Waite]

We are seeing an issue with ACKs to failure responses.

Our config is set up to forward an INVITE to another destination. It 
replies with a 407 Auth Required. Kamailio sends an ACK to the 
destination and relays the 407 back to the source. The source sends an 
ACK, but this always fails the transaction match, so kamailio keeps 
retransmitting the 407 (when using UDP)

UA            KAM              UA

  -- INVITE -->   -- INVITE -->
<--   407  --
                  --   ACK  -->
<--   407  --
  --   ACK  -->
  ...
<--   407  -- (retrans)
  --   ACK  -->

I wondered if it was the same issue as the stateless reply (Flyspray 
#154), but this happens when we do a t_relay.

Basically, when t_check_trans() is called for the ACK, it does not 
terminate the script. I have worked my way into the t_check_trans() code 
and found that it does not find a matching transaction; matching_3261() 
in t_lookup.c returns 0 because there are no entries in that hash bucket.

Does anyone understand why this may be happening?

Regards,
Hugh

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Hugh Waite
Senior Design Engineer
Crocodile RCS Ltd.