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 sendrecv
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,
Paul
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