\begin{abstract}
The Session Initiation Protocol (SIP) is an application-layer control
(signaling) protocol for creating, modifying and terminating sessions
with one or more participants.  These sessions include Internet
multimedia conferences, Internet telephone calls and multimedia
distribution.  Members in a session can communicate via multicast or via
a mesh of unicast relations, or a combination of these. 

SIP invitations used to create sessions carry session descriptions which
allow participants to agree on a set of compatible media types.  SIP
supports user mobility by proxying and redirecting requests to the
user's current location.  Users can register their current location. 
SIP is not tied to any particular conference control protocol.  SIP is
designed to be independent of the lower-layer transport protocol and can
be extended with additional capabilities.
\end{abstract}
