[SR-Users] Negative ACK issue

[Daniel-Constantin Mierla]

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
> <mailto: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 <mailto:loi.dangthanh at gmail.com>
>
>
>     On Thu, Jan 9, 2020 at 2:42 AM Michael Broughton
>     <mbroughton at advanis.net <mailto: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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-- 
Daniel-Constantin Mierla -- www.asipto.com
www.twitter.com/miconda -- www.linkedin.com/in/miconda
Kamailio World Conference - April 27-29, 2020, in Berlin -- www.kamailioworld.com

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