Session Initiation Protocol
Session Initiation Protocol (SIP) was designed from the bottom up to connect people and devices whenever and wherever they are in order to engage in a (possibly lengthy) exchange of information. Existing protocols, such as HTTP and SMTP, were not purpose-built for this essential human activity, and so SIP was born to fill the gap. However, SIP borrows from these two other protocols heavily. From using HTTP's message exchange pattern, message format and encoding, to SMTP's URI scheme. In 2002 a revised version of the SIP standard was formalised into the Internet Engineering Task Force's (IEFT) standardisation process as RFC3261.