[SR-Users] Negative ACK issue

Michael Broughton mbroughton at advanis.net
Thu Jan 9 17:27:42 CET 2020

I'm using 5.3.1+stretch from deb.kamailio.org for our new setup. Our old
setup was using 4.4.4+wheezy.

On Thu, Jan 9, 2020 at 9:16 AM Daniel-Constantin Mierla <miconda at gmail.com>

> Is it a recent version of kamailio, or an older one?
> Cheers,
> Daniel
> On 09.01.20 16:55, Michael Broughton wrote:
> 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
> correctly.
> 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
>>> _______________________________________________
>>> Kamailio (SER) - Users Mailing List
>>> sr-users at lists.kamailio.org
>>> https://lists.kamailio.org/cgi-bin/mailman/listinfo/sr-users
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> --
> Daniel-Constantin Mierla -- www.asipto.comwww.twitter.com/miconda -- www.linkedin.com/in/miconda
> Kamailio World Conference - April 27-29, 2020, in Berlin -- www.kamailioworld.com
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