[SR-Users] Negative ACK issue

Daniel-Constantin Mierla miconda at gmail.com
Tue Jan 14 10:20:43 CET 2020


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 <mailto: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 <mailto:mbroughton at advanis.net>> wrote:
>
>         I'm using 5.3.1+stretch from deb.kamailio.org
>         <http://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 <mailto: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
>>             <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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>>
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>>
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>
>             -- 
>             Daniel-Constantin Mierla -- www.asipto.com <http://www.asipto.com>
>             www.twitter.com/miconda <http://www.twitter.com/miconda> -- www.linkedin.com/in/miconda <http://www.linkedin.com/in/miconda>
>             Kamailio World Conference - April 27-29, 2020, in Berlin -- www.kamailioworld.com <http://www.kamailioworld.com>
>
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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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