[SR-Users] Negative ACK issue

Lợi Đặng loi.dangthanh at gmail.com
Thu Jan 9 11:59:57 CET 2020

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.

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>

> 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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> Kamailio (SER) - Users Mailing List
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