SIP is a peer-to-peer protocol providing call processing functions and features similar to public switched telephone networks. The SIP protocol (or protocol family rather) is not a conventional Internet protocol, because it is not based on the traditional client-server model. Although there are prioritized servers for performing certain tasks, in most cases SIP phones function as both clients and servers on the network. Consequently, the protocol does not use the usual request/response based communication, and that has important consequences in perimeter defense.