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

[Kevin Harwell (JIRA)]

    [ https://issues.asterisk.org/jira/browse/ASTERISK-28865?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=250611#comment-250611 ] 

Kevin Harwell commented on ASTERISK-28865:
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The 491's look correct here as Asterisk initiated a re-invite to the endpoint while at the same time the endpoint initiated one to Asterisk. So both have transactions in progress.

What's odd to me though, and I'm not sure what the correct or expected behavior is here is that the endpoint sends a subsequent re-invite attempting to renegotiate audio. Asterisk replies (200 OK) with a declined audio stream (m=audio 0 ....). Asterisk then attempts a similar re-invite to that endpoint renegotiating audio (using the declined stream again) which the endpoint essentially rejects.

Again the issuing of re-invites is acceptable as the a UA should fire off a timer to issue another re-invite after receiving a 491. So the above is probably happening due to that. However, I'm not sure why both try to negotiated audio again? Also why the declined stream?

Lastly it also appears during this no "trying" or anything is sent to the other endpoint so it re-transmits.

So all that to say is given the 491's seem correct in this scenario, what is the expected behavior in a fax scenario when this occurs? On the subsequent re-invite try negotiating fax again? Audio as it's doing?

> 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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