However, knowing the call state might be useful for certain applications (e.g., when billing is involved, when firewall permissions need to be set etc.). Session timer extension has been defined to solve this. The draft can be found at http://www.cs.columbia.edu/~hgs/sip/drafts/draft-ietf-sip-session-timer-01.txt and it basically allows the servers indicate a desired refresh interval. The call is considered terminated if a re-INVITE is not received within that interval.