[SR-Users] Kamailio propagates 180 and 200 OK OUT OF ORDER

[Alex Balashov]

On Thu, Apr 09, 2020 at 08:14:02AM -0400, Luis Rojas G. wrote:

> Yes, I know that specifically in this case, from the point fo view of
> SIP, it's not "much" important. It's just a symptom than I can't rely
> on Kamailio to keep the ordering of messages when they are very very
> close in time. With this customer (a Brazilian mobile operator) I have
> seen scenarios where they send Re-Invite immediately after ACK, and
> sometimes it caused us problems. I can't think right now in other
> scenario,, but I'm afraid to find out in production. For what I see
> the Async module, as it is now, could help me to deal with requests.
> However, even though it's not a problem for SIP, the operator will
> complain, I know them. And also, they will not like to just drop the
> 180, because there will be scenarios with interworking, so it needs to
> propagate the ACM ISUP body, with parameters as backward call
> indicators.

Agreed that you need to conserve signalling messages end-to-end, as a
matter of principle; can't just drop things.

As I mentioned earlier, we had the same problem--ironically, with a
major mobile operator--sending reinvites and e2e ACKs almost
contemporaneously. We solved it with 'async' by delaying all reinvites
by 50 ms, and haven't had a single complaint since.

Aside from that specific scenario, we haven't seen ordering problems,
and have never had cause to call into question whether we can 'rely' on
Kamailio to conserve ordering. 

There are valid questions raised in this thread about whether any
user-space SIP element subject to the vicissitudes and realities of
packet-switched networking can be relied upon to preserve ordering in a
consistent and universal way.

At the end of the day, this is a network problem. PSTN interworking is
imperfect; it imposes synchronous assumptions upon asynchronous media,
and we see this play out in many areas, e.g. fax. Not much you can do
about it, but thankfully the corner cases are relatively few.

-- Alex

-- 
Alex Balashov | Principal | Evariste Systems LLC

Tel: +1-706-510-6800 / +1-800-250-5920 (toll-free)
Web: http://www.evaristesys.com/, http://www.csrpswitch.com/