SIP, the session intitation protocol, is the IETF protocol for VOIP and other text and multimedia sessions, like instant messaging, video, online games and other services.

This is a very versatile protocol that can be adapted to the need of any client. This versatility enables hosting of the SIP service on various means such as local hosting on a SIP server, remote hosting and software based hosting. This can also be compiled with a PBX to enable use of both PSTN and VOIP based connectivity.

SIP phone invitations are used to create sessions carry session descriptions that allow participants to agree on a set of compatible media types. SIP makes use of elements called proxy servers to help route requests to the user's current location, authenticate and authorize users for services, implement provider call-routing policies, and provide features to users. SIP also provides a registration function that allows users to upload their current locations for use by proxy servers.