The Session Initiation Protocol (SIP), defined in [RFC 3261; RFC 5411], is an open and lightweight protocol that does the following:
• It provides mechanisms for establishing calls between a caller and a callee over an IP network. It allows the caller to notify the callee that it wants to start a call. It allows the participants to agree on media encodings. It also allows participants to end calls.
• It provides mechanisms for the caller to determine the current IP address of the callee. Users do not have a single, fixed IPaddress because they may be assigned addresses dynamically (using DHCP) and because they may have multiple IP devices, each with a different IPaddress.
• It provides mechanisms for call management, such as adding new media streams during the call, changing the encoding during the call, inviting new participants during the call, call transfer, and call holding.