The Session Initiation Protocol (SIP) is an IETF proposed standard for
multimedia conferencing over IP. SIP is
an application-layer control protocol that can be used to establish,
maintain, and terminate calls between two or more end points.
A SIP networks consits of clients and
a server. SIP clients (phones or softphones) initiate SIP requests and
respond to requests.
SIP servers include:
Proxy server: an intermediate device that receives SIP requests
from a client and then forwards them to the next SIP server in the
Redirect server: provides information about the next hop or hops
that a request should take.
The animation shows SIP operation in the different modes: a direct call
between two clients, a call through a proxy server and through a redirect