SIP feature

The SIP communications protocol determines five attributes when establishing and terminating multimedia sessions:

User location.
User availability.
User capabilities.
Session setup.
Session management.
Different types of multimedia SIP sessions include internet telephony calls, video conferencing and other forms of unified communications. The protocol can be used to invite participants to unicast or multicast sessions that do not necessarily involve the initiator.

SIP itself does not provide communication services. Instead, the protocol's specification defines interoperable implementations of SIP features, called primitives, that can be used to facilitate different services. Primitives allow additional information to be embedded in a SIP message, such as linking a user's photo to directory information to enable media-rich caller ID.

SIP also supports name mapping and redirection services, which are two key ways the protocol enables mobility. Users and endpoints are detected with a single identifier, known as a uniform resource identifier (URI), which is independent of their network location. URIs are alphanumeric, using a syntax that looks more like an email address than a phone number or IP address.

In addition to real-time services, SIP can be used for asynchronous event notifications, such as automatic callbacks, message-waiting indicators and buddy lists based on presence.