[SR-Users] Negative ACK issue

Michael Broughton mbroughton at advanis.net
Wed Jan 8 20:41:36 CET 2020


Long time Kam/Ser user, first time poster.

I'm running into a problem with one of our telco providers when we make a
call that ends up being not in service or some other error. In this case
our ACK's are not working and the phone line stays open for a period
of time until something times out on their end.

They claim the issue is that our negative ACK message is dropping one of
the Via headers. This is the only case I can find in our setup where
Kamailio does this. But it does drop the first Via, which is the first hop
in our internal network.

I don't understand why this is a problem for them, and I'm still trying to
get a reasonable explanation out of them. Technically, I don't see why it
would be a problem. This behaviour is not an issue with our other telco
providers. Strangely enough, it is also not an issue for this provider when
we make the calls over their MPLS network (we are switching to the

My question is, can this behaviour be changed in Kamailio somehow? Is there
a way for it to keep all the Via headers for negative ACK's?

Or, do I just need to poke them harder to fix their issues?

My setup:

Source -> Kamailio -> Firewall (NAT, SIP ALG) -> Telco Provider

I hope I have provided enough information.

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