[asterisk-bugs] [JIRA] (ASTERISK-28865) FAX T.38 re-Invite failed on '491 Another INVITE transaction in progress'

[Alexei Gradinari (JIRA)]

    [ https://issues.asterisk.org/jira/browse/ASTERISK-28865?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=250612#comment-250612 ] 

Alexei Gradinari commented on ASTERISK-28865:
---------------------------------------------

There is nothing wrong that asterisk sends 491.

I see 2 issues with the next steps:

1. When FAX1 received 491 reply on T.38 re-Invite from the asterisk, the FAX1 sent a new re-Invite with audio SDP "m=audio 46082 RTP/AVP 0 8", but the asterisk rejected with reply
200 OK SDP "m=audio 0 RTP/AVP 0 8"
Why?
At the same time the asterisk didn't reply to FAX2 on T.38 re-INVITE, so FAX2 resent this re-Invite.

2. When the asterisk received resent T.38 re-INVITE from FAX2, the asterisk sent a new re-Invite to FAX1 with SDP "m=audio 0 RTP/AVP 0 8"
What is it? The asterisk should send a new T.38 re-INVITE to FAX1.
Of course FAX1 replied with "488 Not Acceptable Here" which the asterisk forwarded to FAX2.
So FAX2 decided that the asterisk not supported T.38 and negotiated to SDP voice "m=audio 19449 RTP/AVP 0"

I think this is related to ASTERISK-28811.
and res_pjsip_session: Reduce (and improve) SDP renegotiation.
https://gerrit.asterisk.org/c/asterisk/+/6576

The patch " fax: Fix crashes in PJSIP re-negotiation scenarios."
https://gerrit.asterisk.org/c/asterisk/+/14298
fixed off-nominal re-negotiation cases, but maybe not all cases.
This case with 491 led to crash and patch "stream: Enforce formats immutability and ensure formats exist."
https://gerrit.asterisk.org/c/asterisk/+/14276
allows to avoid crash as empty format list always exists.
May be because of empty format list the asterisk sends "m=audio 0".

The best result in this scenario - asterisk should allow to renegotiate T.38.


> FAX T.38 re-Invite failed on '491 Another INVITE transaction in progress'
> -------------------------------------------------------------------------
>
>                 Key: ASTERISK-28865
>                 URL: https://issues.asterisk.org/jira/browse/ASTERISK-28865
>             Project: Asterisk
>          Issue Type: Bug
>      Security Level: None
>          Components: Resources/res_pjsip_session, Resources/res_pjsip_t38
>    Affects Versions: 16.10.0
>            Reporter: Alexei Gradinari
>              Labels: fax, pjsip
>         Attachments: fax_491.txt
>
>
> On call between 2 PJSIP endpoints when fax tone is detected simultaneously by both endpoints there may be a case when both endpoints send T.38 re-Invite to the asterisk.
> One of this re-Invite is rejected by the asterisk with "491 Another INVITE transaction in progress".
> After that the asterisk no longer allows to negotiate T.38 on this session.
> There are strange reply/Invite from the asterisk with SDP line "m=audio 0 RTP/AVP 0 8"
> Attached SIP flow file 'fax_491.txt'



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