[SR-Users] Negative ACK issue

Tue Jan 14 16:46:01 CET 2020

FortiGate 200E, OS 6.0.6.

I opened a support ticket, but they didn't seem to worry about it once I
found a workaround.

On Tue, Jan 14, 2020 at 2:21 AM Daniel-Constantin Mierla <miconda at gmail.com>
wrote:

> Curious about the ALG vendor/model/version, if it is some
> carrier/enterprise grade firewall, just to be aware when meeting it ... of
> course, if you can and want to share such details...
>
> Cheers,
> Daniel
> On 14.01.20 08:42, Lợi Đặng wrote:
>
> 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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>> sr-users at lists.kamailio.org
>> https://lists.kamailio.org/cgi-bin/mailman/listinfo/sr-users
>>
>
> _______________________________________________
> Kamailio (SER) - Users Mailing Listsr-users at lists.kamailio.orghttps://lists.kamailio.org/cgi-bin/mailman/listinfo/sr-users
>
> --
> 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
>
> _______________________________________________
> Kamailio (SER) - Users Mailing List
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> https://lists.kamailio.org/cgi-bin/mailman/listinfo/sr-users
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