A system and method for uniquely identifying an SIP device extends the SIP communications protocol with an end point identifier, carried for example in the header of an SIP transmission. The end point identifier is useful for routing, registration, subscription, and authentication. The end point (device) of a given user epid can be uniquely identified by creating a key from an epid and a user's address-of-record (URI). This in turn enables improved connection management and security association management when the connections/IP addresses are transient, such as when HTTPS tunneling is used.