SIP (Session Initiation Protocol) is a signalling protocol and is primarily used in IP (Internet Protocol) telecommunications. Its primary uses in IP telephony include a means for a telephone to register and identify itself, to setup and tear down calls, and provide a means for the source and destination endpoints to negotiate call options.
Because SIP is built to use TCP/IP and is an open standard, it can be continually developed and new features implemented. Looking at traditional telephony, its signalling cannot undergo any major changes due to its design and its requirement to remain compatible with standard analog telephones. For this reason alone SIP can provide a rich subset of features and will always remain scalable.