[asterisk-bugs] [Asterisk 0012437]: Asterisk negotiates only T.38 when answering even if the other end offers audio

[Asterisk Bug Tracker]

A NOTE has been added to this issue. 
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http://bugs.digium.com/view.php?id=12437 
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Reported By:                marsosa
Assigned To:                
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Project:                    Asterisk
Issue ID:                   12437
Category:                   Channels/chan_sip/T.38
Reproducibility:            always
Severity:                   major
Priority:                   normal
Status:                     confirmed
Asterisk Version:           1.4.18 
SVN Branch (only for SVN checkouts, not tarball releases): N/A 
SVN Revision (number only!):  
Disclaimer on File?:        N/A 
Request Review:              
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Date Submitted:             2008-04-14 08:31 CDT
Last Modified:              2008-09-09 12:40 CDT
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Summary:                    Asterisk negotiates only T.38 when answering even if
the other end offers audio
Description: 
One of our gateways (audiocodes mp-118) offers ulaw,g729 and t.38 when an
incoming call is sent to asterisk, and asterisk answer() with t.38 only,
instead of using ulaw. T.38 is enabled on the gateway because this is
needed for reinvites, if i disable it, the call works ok but fails later
when the ata wants to do reinvite for receiving faxes with t.38 '488 not
acceptable'.
The main problem here is that, after answering with t.38, asterisk sends
invites with t.38 only to the ip phones, and they rejected with not
acceptable.
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 (0092267) Hubguru (reporter) - 2008-09-09 12:40
 http://bugs.digium.com/view.php?id=12437#c92267 
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Hi All,
I have the same issue with Dialogic DMG 2000 firmware 6.0.103.
The DMG invite sends to asterisk:
m=audio 49016 RTP/AVP 0 101          [notice the m=audio]

And Asterisk responds with the 200 OK:
m=image 29475 udptl t38                   [notice the m=image]

Quote from Raj Jain:
That's how T.38 gateways typically work. This particular gateway seems
to have some advanced knowledge that the call may be converted into
T.38 later. They are offering m=image with port number set to 0 in the
INVITE. By doing this, they seem to be offering a sort "heads up" that
the call may be converted to T.38 later but no T.38 now because the
port number is zero. This hint is of no use to Asterisk. If you can
get the gateway to not send the m=image line in the first INVITE, they
you may be in luck.

This seems like a bug in Asterisk. Asterisk is encoding the SDP in 200
OK incorrectly on two fronts. It's dropping the m=audio line
completely and it's activating T.38 stream when the remote end hasn't
asked it to do so. 

Issue History 
Date Modified    Username       Field                    Change               
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2008-09-09 12:40 Hubguru        Note Added: 0092267                          
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