  In recent years, Session Initiation Protocol (SIP) has become widely used in
current internet protocols. It is a text-based protocol much like Hyper Text
Transport Protocol (HTTP) and Simple Mail Transport Protocol (SMTP). SIP is a
strong enough signaling protocol on the internet for establishing, maintaining,
and terminating session. In this paper the areas of security and attacks in SIP
are discussed. We consider attacks from diverse related perspectives. The
authentication schemes are compared, the representative existing solutions are
highlighted, and several remaining research challenges are identified. Finally,
the taxonomy of SIP threat will be presented.
