SIP can be used to control Internet multimedia conferences, Internet telephone calls and multimedia distribution, and is likely to be used in both the core and the periphery of the future communications network. SIP includes the following features. SIP invitations are used to create sessions and carry session descriptions that allow participants to agree on a set of compatible media types. In this way SIP is not restricted to any particular media type, and can therefore handle the expanding range of media technologies. SIP enables user mobility through a mechanism that allows requests to be proxied or redirected to the user's current location.
Users can register their current location with their home server.
Members in a SIP session can communicate using multicast or unicast relations, or a combination of these. SIP can be extended with additional capabilities and is actively being exploited for many media applications. A User Agent is the end-point of a SIP call. It initiates SIP requests as instructed by the user and, on receipt of a SIP request, contacts the user and responds to the request on their behalf. A Proxy is used to route requests and enforce policy or firewalls.