Hi Paul,My concern is not about fax machine, it is about my system when it
receives re-INVITE with no precondition, but it adds precondition to the 200
OK.Thanks,A.CSent from my Samsung Galaxy smartphone.
-------- Original message --------From: Paul Kyzivat <pkyzi...@alum.mit.edu>
Date: 4/18/19 23:21 (GMT+07:00) To: sip-implementors@lists.cs.columbia.edu
Subject: Re: [Sip-implementors] Precondition is added into 200OK re-INVITE On
4/17/19 10:50 PM, Anh Cao wrote:> Hi all,> > I meet a weird case with
precondition but can not find any document talk> about it:> > I make SIP call
from my system to the fax machine. In the initial INVITE,> my system includes
precondition in Supported header and status in SDP. But> fax machine does not
support precondition so it removes precondition from> Supported header and
status from SDP.> > After voice call is established, fax machine sends T.38
re-INVITE to my> system to start fax with Supported header has no precondition
and SDP has> no status.> And my system answers fax machine by 200 OK with
Supported has precondition> and T.38 SDP has status for precondition:> m=image
55314 udptl t38> a=T38FaxVersion:0> a=T38MaxBitRate:14400> a=curr:qos local sen
drecv> a=curr:qos remote sendrecv> a=des:qos none local sendrecv> a=des:qos
none remote sendrecv> > Is it the acceptable behavior? and there is any
document talk a bout this> case? with re-INVITE for voice all and for fax call,
the behaviors are the> same or different?I don't see anything wrong with this.
The "Supported" is declaring your *support* for preconditions. The fax machine
is acting as it should given that it doesn't support or understand
preconditions. It is apparently ignoring the a= lines carrying precondition
status, which is what an implementation is supposed to do with attributes it
doesn't understand.What else do you think should be done?This is basic behavior
for extensions. Thanks,
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