Call hangup at 3600 seconds

I am making a call between two asterisk servers via sip uri. The call hangups after 3600 seconds. I have set the absolute timeout to 7200 seconds in my agi.

$max_duration = 7200;

The origination server is initiating the hangup request at exact 3600 secs. In wireshark trace, I found

X-Asterisk-HangupCause: Normal Clearing
X-Asterisk-HangupCauseCode: 16

Is there any other parameter which can hangup the call?

OK. I got it!
It was the Session-Expires parameter being added in the header by the destination server in 200 OK message on reply to my INVITE packet.