This document describes Session Initiation Protocol (SIP), an
application-layer control (signaling) protocol for creating,
modifying, and terminating sessions with one or more participants.
These sessions include Internet telephone calls, multimedia
distribution, and multimedia conferences.
SIP invitations 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. SIP runs on top of several different transport protocols.