T.38 negotiation failure

sacechovic · March 3, 2021, 5:47pm

I am trying to get t38 fax negotiation working between Asterisk 13.28.1 and a Streem T38 fax server (using dialogic). The path is PRI <=> Asterisk Server gw62(PJSIP) <=> Asterisk Server gw63(PJSIP)<=> Streem. When a fax is received over the PRI, the call is sent to Streem. The Streem endpoint has t38_udptl set to yes as well as the PJSIP endpoints between the servers. When Streem receives the fax it sends the T38 re-invite several times to Asterisk and Asterisk responds with SIP status 488 not acceptable here. Debug output shows t38 not enabled. Dialplan logic is as follows:

[FAXincoming] ; from PRI to Streem
exten = _X.,1,NoOp()
same = n,Macro(dialSTREEM,${EXTEN})
same = n,Hangup()

[macro-dialSTREEM]
exten = s,1,Dial(PJSIP/${ARG1}@streem)
exten = s,n,Hangup()

Debug output:
full.txt (60.1 KB)

david551 · March 3, 2021, 6:02pm

The only 488 in that log is from the Dialogic, not Asterisk.

That would have been in the channel driver configuration, which you haven’t provided.

github.com

asterisk/asterisk/blob/master/configs/samples/pjsip.conf.sample#L729


      
                                  ; to what is recommended in RFC3551. Note, 'g726aal2' also
                                  ; needs to be specified in the codec allow list
                                  ; (default: "no")
          ;inband_progress=no     ; Determines whether chan_pjsip will indicate ringing
                                  ; using inband progress (default: "no")
          ;call_group=    ; The numeric pickup groups for a channel (default: "")
          ;pickup_group=  ; The numeric pickup groups that a channel can pickup (default:
                          ; "")
          ;named_call_group=      ; The named pickup groups for a channel (default: "")
          ;named_pickup_group=    ; The named pickup groups that a channel can pickup
                                  ; (default: "")
          ;device_state_busy_at=0 ; The number of in use channels which will cause busy
                                  ; to be returned as device state (default: "0")
          ;t38_udptl=no   ; Whether T 38 UDPTL support is enabled or not (default: "no")
          ;t38_udptl_ec=none      ; T 38 UDPTL error correction method (default: "none")
          ;t38_udptl_maxdatagram=0        ; T 38 UDPTL maximum datagram size (default:
                                          ; "0")
          ;fax_detect=no  ; Whether CNG tone detection is enabled (default: "no")
          ;fax_detect_timeout=30  ; How many seconds into a call before fax_detect is
                                  ; disabled for the call.
                                  ; Zero disables the timeout.

sacechovic · March 3, 2021, 6:45pm

Thanks for the quick reply. I didn’t go far enough down in the debug so you don’t have the t38 rejection from asterisk back to dialogic. The initial failure is trying to negotiate standard fax protocol and the dialogic does not have a match for the codecs we offer. However, that doesn’t matter as the dialogic sends the re-invites for t38 (which I didn’t show). And then Asterisk responds to dialogic with a 488. I have included a packet capture and the channel configs for all endpoints involved.

gw62.pjsip.txt (5.9 KB)
gw63.pjsip.txt (5.9 KB) streem.pjsip.txt (5.9 KB)

sacechovic · March 4, 2021, 6:17pm

Just checking to see if you’ve had a chance to look at trace and channel info? Thx

system · April 3, 2021, 6:17pm

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