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SIP FEATURES

The SIP protocol includes the following features:

SIP invitations are used to create sessions 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.
Members in a SIP session can communicate using multicast or unicast relations, or a combination of both. In addition, SIP is independent of the lower-layer transport protocol, which allows it to take advantage of new transport protocols.
SIP enables user mobility that allows requests to be redirected to the user's current location. Users can register their current location with their home server.
Software implementing the basic SIP protocol can be extended with additional capabilities and is actively being exploited for many media applications.

A SIP entity may operate in one of the following modes, all of which are implemented by Data Connection's SIP software, DC-SIP.

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. It accepts requests on behalf of a user and passes them on, modified as necessary, to the user.
A Redirect server may be used to provide user mobility. A Redirector accepts SIP requests and returns zero or more new addresses that should be contacted to fulfil the request. A redirector does not initiate SIP requests or accept SIP calls.
A Registrar accepts registration requests. These enable users to update their location and policy information as may be used to provide user mobility.