The acronym SIP stands for Session Initiation Protocol and refers to a TCP/IP-based network protocol which can be used to establish and control communication connections of several subscribers. SIP is often used in Voice-over-IP telephony to establish the connection for telephone calls. The key features of Session Initiation Protocol are specified in RFC3261. Since Session Initiation Protocol is an open standard, the protocol is widely used worldwide. Almost all IP-based telephone systems have SIP.