[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)?

[Alex Balashov]

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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-- 
Alex Balashov | Principal | Evariste Systems LLC

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