[SR-Users] Is it possible to change a positive (2xx) response to a negative one (4xx) during kamailio processing of a reply (g711 fallback drama)?

[George Diamantopoulos]

I know, but I'm that desperate.

On 21 May 2018 at 22:17, Alex Balashov <abalashov at evaristesys.com> wrote:

> I think this is the wrong job for Kamailio.
>
> On Mon, May 21, 2018 at 10:16:20PM +0300, George Diamantopoulos wrote:
>
> > Hello all,
> >
> > I've been hitting a wall in getting T.38 fallback to G.711 to work in a
> > specific case, and I was wondering if I could use kamailio to the rescue,
> > since all my other endeavours have failed miserably.
> >
> > In short, this is this the scenario where I need to have this
> manipulation
> > happen:
> > - During an established dialog between a SIP endpoint and asterisk (with
> > kamailio in the middle acting as proxy), the latter (asterisk) sends a
> > re-INVITE with a single offer for m=image with T.38 in the SDP.
> > - The other (eccentric) endpoint (yes, it's a Cisco) replies with 200 OK,
> > but will reject the T.38 offer in the SDP body instead by including a
> port
> > number of '0'
> > - Instead of falling back to G.711 as configured, asterisk in that case
> > will do nothing, and the dialog will eventually end with a BYE (don't
> > remember which of the two endpoints will send that, but it doesn't matter
> > much)
> >
> > The thing here is that asterisk *WILL* fallback to G711 with a second
> > re-INVITE, if the other endpoint replies to the first T.38 re-INVITE
> with a
> > negative response (488) instead of a positive one (200) declining the
> media
> > in the SDP body. I've looked at using freeswitch for a fax gateway
> instead,
> > but apparently FS doesn't support fallback at all even in the 488 reply
> > case, much less in the inactive media stream in 200 OK corner case I'm
> > investigating.
> >
> > So, I thought I'd do that in kamailio and convert those 200 OKs to 488s
> > with change_reply_status() from the textopsx module. Of course, the
> > universe hates me, and this will fail spectacularly:
> >
> > > ERROR: textopsx [textopsx.c:301]: ki_change_reply_status(): the class
> of
> > provisional or positive final replies cannot be changed
> >
> > So you can't change a reply's "class" with change_reply_status(), only
> the
> > response code within that class can be manipulated.
> >
> > I guess my question is now whether there is any way to do this sort of
> > transformation with kamailio, perhaps with some other function or method?
> >
> > If anyone else has had to deal with this problem and come up with a
> > different solution I'd be glad to hear it, as neither my head nor the
> > surrounding walls can take any more mutual banging at this point. Thanks!
> >
> > BR,
> > George.
> >
> > P.S. I hate fax
>
> > _______________________________________________
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> > sr-users at lists.kamailio.org
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>
>
> --
> 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/
>
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