Verification of the SIP Transaction Using Coloured Petri Nets

The Session Initiation Protocol (SIP) is one of the leading protocols for multimedia control over the Internet, including initiating, maintaining and terminating multimedia sessions. The protocol uses transactions to complete the control tasks. In this paper we focus on the INVITE transaction of SIP, which is used to initiate a session. SIP is designed to operate over a transport protocol that can be reliable or unreliable. Our previous work has verified the functional properties of the INVITE transaction over a reliable transport medium, using Coloured Petri Nets (CPNs). In this paper, we use CPNs to model and analyse SIP INVITE transaction when the medium is unreliable. The verification reveals similar problem as that in the case of a reliable medium, i.e. the transaction may terminate in an undesirable state while one communication party is still waiting for a response from its peer. Additionally with an unreliable medium, the transaction has undesirable terminal states in which retransmitted requests may lead to erroneous operation. This result provides theoretical evidence and timely support for the Internet Draft that has been recently submitted to the Internet Engineering Task Force to propose updates to SIP INVITE transaction.