asterisk users - Jun 2017 - asterisk 13.16 / pjsip / t.38: res_pjsip_t38.c:207 t38_automatic_reject: Automatically rejecting T.38 request on channel 'PJSIP/91-00000007'

On Sun, Jun 11, 2017, at 08:16 AM, Michael Maier wrote:

<snip>
 
> I did some further investigations and fixed a local problem. Now,
> asterisk is able to successfully activate T.38 - unfortunately just for
> very shot time because of a phantom package it receives!
What was the local problem?
 
> Let's go into details:
> I'm starting at the point where asterisk / fax client receives the T.38
> reininvite from the server from the provider 195.185.37.60:5060 for the
> fax client (extension 91):
<snip>
> 
> But now, something strange happens: Asterisk "receives" a T.38
reinvite
> package from provider!
> Why is it strange? Because *this package doesn't exist at all* ! It
> can't be seen in tcpdump. I did 4 tests - always the same! Where does
> this package come from? It's exactly the same package which can be seen
> at the beginning of the trace excerpted here! Isn't it been deleted
> after it has been processed the first time?
PJSIP uses a dispatch model. The request is queued up, acted on, and
then that's it. The act of acting on it removes it from the queue. The
only reason I'd expect to see it again is if there was a retransmission
or something somehow requeued it up - but I don't think we do that
anywhere. Not quite sure why it would be happening...

-- 
Joshua Colp
Digium, Inc. | Senior Software Developer
445 Jan Davis Drive NW - Huntsville, AL 35806 - US
Check us out at: www.digium.com & www.asterisk.org

On 06/11/2017 at 01:29 PM Joshua Colp wrote:
> On Sun, Jun 11, 2017, at 08:16 AM, Michael Maier wrote:
> 
> <snip>
>  
>> I did some further investigations and fixed a local problem. Now,
>> asterisk is able to successfully activate T.38 - unfortunately just for
>> very shot time because of a phantom package it receives!
> 
> What was the local problem?
Update of t38modem from 3.13 to 3.15
>> Let's go into details:
>> I'm starting at the point where asterisk / fax client receives the
T.38
>> reininvite from the server from the provider 195.185.37.60:5060 for the
>> fax client (extension 91):
> 
> <snip>
> 
>>
>> But now, something strange happens: Asterisk "receives" a
T.38 reinvite
>> package from provider!
>> Why is it strange? Because *this package doesn't exist at all* ! It
>> can't be seen in tcpdump. I did 4 tests - always the same! Where
does
>> this package come from? It's exactly the same package which can be
seen
>> at the beginning of the trace excerpted here! Isn't it been deleted
>> after it has been processed the first time?
> 
> PJSIP uses a dispatch model. The request is queued up, acted on, and
> then that's it. The act of acting on it removes it from the queue.
That's the *expected* behavior ... . I rechecked again and again. All
existing tcpdumps. The "resent" package isn't part of any tcpdump
(wireshark doesn't show it) - and during tcpdump no package was dropped.
> The
> only reason I'd expect to see it again is if there was a retransmission
> or something somehow requeued it up - but I don't think we do that
> anywhere. Not quite sure why it would be happening...
But even if this package would have really been sent (as retransmission)
- shouldn't there be another response? T.38 has been successfully
enabled before and the faxclient has already sent a valid 200 ok
including complete SDP information to asterisk.

All in all it looks really odd to me.


Thanks,
regards,
Michael

On Sun, Jun 11, 2017, at 11:34 AM, Michael Maier wrote:

<snip>
> > 
> > PJSIP uses a dispatch model. The request is queued up, acted on, and
> > then that's it. The act of acting on it removes it from the queue.
> 
> That's the *expected* behavior ... . I rechecked again and again. All
> existing tcpdumps. The "resent" package isn't part of any
tcpdump
> (wireshark doesn't show it) - and during tcpdump no package was
dropped.
> 
> > The
> > only reason I'd expect to see it again is if there was a
retransmission
> > or something somehow requeued it up - but I don't think we do that
> > anywhere. Not quite sure why it would be happening...
> 
> But even if this package would have really been sent (as retransmission)
> - shouldn't there be another response? T.38 has been successfully
> enabled before and the faxclient has already sent a valid 200 ok
> including complete SDP information to asterisk.
> 
> All in all it looks really odd to me.
Depends on how we handle that scenario. I don't think we have any tests
to cover it, so it's entirely possible that we wouldn't respond like
that. Why it's happening in the first place I still don't know though,
haven't seen anything like it.

-- 
Joshua Colp
Digium, Inc. | Senior Software Developer
445 Jan Davis Drive NW - Huntsville, AL 35806 - US
Check us out at: www.digium.com & www.asterisk.org

On 06/11/2017 at 04:34 PM Michael Maier wrote:
> On 06/11/2017 at 01:29 PM Joshua Colp wrote:
>> On Sun, Jun 11, 2017, at 08:16 AM, Michael Maier wrote:
>>
>> <snip>
>>  
>>> I did some further investigations and fixed a local problem. Now,
>>> asterisk is able to successfully activate T.38 - unfortunately just
for
>>> very shot time because of a phantom package it receives!
>>
>> What was the local problem?
> 
> Update of t38modem from 3.13 to 3.15
I forgot to mention the main thing: I initially used *two* trunks (and
one number), one "normal" trunk and another trunk_fax additionally
enabling udptl. Outbound not a problem - but inbound used the normal
trunk - this can't work. Merging both trunks to one trunk has been the
solution, which isn't a problem with udptl, because it's initially
handled by the extension (t38modem). That's why I came across line
option for the other provider I'm using and which wasn't that easy to
use with FreePBX, because it is not yet directly supported and
pjsip.registration_custom_post.conf unfortunately doesn't work (I'm not
the only one experiencing this problem). Therefore I'm using now a trick
which works especially for the registration problem within FreePBX as
long as it isn't supported natively.



Thanks,
Michael