On Tue, Jan 14, 2020 at 4:42 AM Michael Broughton <mbroughton@advanis.net> wrote:

Just to provide some closure to this, the problem did end up being with the Via headers and our firewall ALG.

In the top Via header of the INVITE requests the ALG was transforming the internal proxy address to our external address and adding port 5060. In subsequent negative ACK and CANCEL requests, the ALG was transforming the internal proxy address to our external address with no port number. Thus the Via's did not exactly match, and this prevented our telco from matching the existing transaction.

I was able to fix the issue by modifying our Kam config with the advertise parameter:

listen = 10.x.y.z advertise 10.x.y.z:5060

With this setting in place the ALG is forced to behave itself.
On Thu, Jan 9, 2020 at 9:27 AM Michael Broughton <mbroughton@advanis.net> wrote:
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@gmail.com> wrote:
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@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@gmail.com

On Thu, Jan 9, 2020 at 2:42 AM Michael Broughton <mbroughton@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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