[asterisk-bugs] [JIRA] (ASTERISK-29167) T.38 fax relay won't pass through local channels in recent versions

[Gregory Massel (JIRA)]

Gregory Massel created ASTERISK-29167:
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             Summary: T.38 fax relay won't pass through local channels in recent versions
                 Key: ASTERISK-29167
                 URL: https://issues.asterisk.org/jira/browse/ASTERISK-29167
             Project: Asterisk
          Issue Type: Bug
      Security Level: None
          Components: Channels/chan_local, Resources/res_pjsip
    Affects Versions: 16.14.1
         Environment: Asterisk versions 16.11.1 through 16.14.1 on Ubuntu 18.04 LTS.
            Reporter: Gregory Massel


In versions up to and including 16.11.1, Asterisk is able to relay T.38 fax through local channels, however, in versions 16.12.0 and later, this functionality no longer works.

e.g. Fax call initiated from Grandstream/Cisco ATA to Asterisk box 1, received via PJSIP. In dialplan, logic such as 
[incoming]
exten => s,1,Dial(Local/100 at outgoing/n)
[outgoing]
exten => 100,1,Dial(PJSIP/asterisk_box_2)

Note that, in this case, Asterisk is NOT invoking T.38 gatewaying functionality; it is merely acting as a T.38 relay (i.e. forwarding T.38 packets between the incoming and outgoing PJSIP channels). Both the ATA and 2nd Asterisk box are able to talk T.38, hence there should be mere passthrough.

For versions prior to and including 16.11.1, there is a two-way flow of UDPTL packets.
For versions 16.12.0 and onwards (tested on 16.12.0, 16.13.0, 16.14.0 and 16.14.1), the first Asterisk box simply doesn't send out any UDPTL packets at all. Traces show that the 2nd asterisk box is sending UDPTL but the 1st one is not responding with UDPTL. Eventually the fax transmission dies and the call renegotiates to A-law and hangs up.

Note that the second Asterisk box in this case is running version 16.14.1 (and tested with older versions as well) but does NOT use Local channels at all. It suffers no such issue, whether using 16.11.1 or 16.14.1. The problem only appears on the 1st Asterisk box, which passes the fax calls through Local channels.

N.B. This bug was NOT caused by ASTERISK-28900. I have tested by applying that patch to version 16.11.1, and T.38 relay works correctly. So it was something else that changed between 16.11.1 and 16.12.0.



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