[SR-Users] Negative ACK issue

[Lợi Đặng]

Nice, be ware of the ALG, sometimes it's an unpredictable foe.

rgds,
Loi Dang Thanh


On Tue, Jan 14, 2020 at 4:42 AM Michael Broughton <mbroughton at 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 at 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 at 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 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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>>>
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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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