[SR-Users] Negative ACK issue

Michael Broughton mbroughton at advanis.net
Thu Jan 9 16:55:17 CET 2020

Thank you, this was a helpful sanity check.

We have been capturing SIP traces to try and debug this. I normally just
look at the traffic on our Kam box because it is convenient to do so, but I
have also taken traces on our firewall to check the ALG behaviour. The
provider techs are also tracing these calls on their network as well. The
ALG is new equipment in our setup, but as far as I can tell it is behaving

The one rather annoying discovery that I made is that when I call directly
out from the source (Freeswitch in this case) and bypass Kamailio, the
negative ACK's seem to work. I do not see any retransmissions of their
final response. And of course the only significant difference in the SIP
traces is the Via headers.

Anyway, thanks again for your input.

On Thu, Jan 9, 2020 at 4:04 AM Lợi Đặng <loi.dangthanh at gmail.com> wrote:

> Hi,
> You're not going to have the Via header from your `source` sent to your
> `telco provider` in the negative ACK when the call is not answered, because
> the ACK in the right hand side of the call is created by the kamailio
> itself, not a forwarding one by the `source`.
> Yes, you've guessed it, ACK for an answered call is a forwarding one which
> contains all the Via headers. It's the SIP spec, not kamailio, you may want
> to dive into rfc3261 for more details.
> In this case, your telco's expectation is not correct, my best guess is
> something went wrong with either your SIP ALG or Telco Provider. SIP
> capturing may help.
> rgds,
> Loi Dang Thanh
> Phone : +84. 774.735.448
> Email : loi.dangthanh at gmail.com
> On Thu, Jan 9, 2020 at 2:42 AM Michael Broughton <mbroughton at advanis.net>
> wrote:
>> Hello,
>> 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
>> internet).
>> 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.
>> Thanks!
>> Michael
>> _______________________________________________
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