asterisk users - Apr 2023 - Asterisk translates 200 OK + SDP into 488 not acceptable here after both side agreed on codec.

Hi List

Asterisk 16.28.0 in use.

PJSIP in use
Two endpoints
Both using IPv6

One Endpoint on UDP, the other via TLS.

Both with:

t38_udptl=yes
;fax_detect=yes
;fax_detect_timeout=30
rtp_ipv6=yes

Both sides are T.38 capable and detect fax tone so no need for fax
detection on asterisk.

Voice calls between the two work fine.

But on a Fax call, I see this situation:

A <=> Asterisk <=> B

A: INVITE + Audio SDP => Asterisk => (same SDP) => B

B: 200 OK + Audio SDP => Asterisk => (same SDP) => A

* B Detects Fax-Tone!

B: Re-Invite + UDPTL => Asterisk => (same SDP) => A

A: 200 OK + UDPTL => Asterisk => 488 => B

I tweakted the udptl setting in various ways, but I am unable to figure
out, why Asterisk is sending a 488 to B, after it first happily
forwarded the SDP to A and got confirmation from A it was happy to
accept that DSP.

Any hint?

-- 
Mit freundlichen Grüssen

-Benoît Panizzon- @ HomeOffice und normal erreichbar
-- 
I m p r o W a r e   A G    -    Leiter Commerce Kunden
______________________________________________________

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On Fri, Apr 28, 2023 at 10:43 AM Benoît Panizzon <benoit.panizzon at
imp.ch>
wrote:
> Hi List
>
> Asterisk 16.28.0 in use.
>
> PJSIP in use
> Two endpoints
> Both using IPv6
>
> One Endpoint on UDP, the other via TLS.
>
> Both with:
>
> t38_udptl=yes
> ;fax_detect=yes
> ;fax_detect_timeout=30
> rtp_ipv6=yes
>
> Both sides are T.38 capable and detect fax tone so no need for fax
> detection on asterisk.
>
> Voice calls between the two work fine.
>
> But on a Fax call, I see this situation:
>
> A <=> Asterisk <=> B
>
> A: INVITE + Audio SDP => Asterisk => (same SDP) => B
>
> B: 200 OK + Audio SDP => Asterisk => (same SDP) => A
>
> * B Detects Fax-Tone!
>
> B: Re-Invite + UDPTL => Asterisk => (same SDP) => A
>
> A: 200 OK + UDPTL => Asterisk => 488 => B
>
> I tweakted the udptl setting in various ways, but I am unable to figure
> out, why Asterisk is sending a 488 to B, after it first happily
> forwarded the SDP to A and got confirmation from A it was happy to
> accept that DSP.
>
>
You could enable core debug and see if there's any insight, otherwise
you'd
have to actually provide the full traces. Asterisk also doesn't forward
SDPs between sides so they're not the same SDP.

-- 
Joshua C. Colp
Asterisk Project Lead
Sangoma Technologies
Check us out at www.sangoma.com and www.asterisk.org
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