H3C Technologies H3C MSR 5600 - Manual (Page 89)

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Configuring SIP
Overview
The Session Initiation Protocol (SIP) is an application layer control protocol that can create, modify, and
terminate multimedia sessions such as voice and video calls over IP networks.
Terminology
User agent
A user agent (UA) is a SIP endpoint such as a phone, a gateway, or a router.
There are two types of UAs: user agent client (UAC) and user agent server (UAS). A UAC sends SIP
requests, and a User Agent Server (UAS) receives SIP requests and returns SIP responses. These roles of
UAC and UAS only last for the duration of a SIP transaction.
Proxy server
A proxy server primarily forwards session requests and responses. It can also provide call control,
accounting, and authorization functions.
Redirect server
A redirect server sends new addresses to UACs so the UAC sends session requests to the new addresses.
Location server
A location server provides UA information to proxy and redirect servers.
Registrar
A registrar receives registrations from UAs and generates UA information. The UA information is stored
on the location server.
SIP functions
SIP supports the following facets of establishing and terminating multimedia communications:
• User location—Determines the end system to be used for communication. SIP can use UA
information on the registrar, or use information provided by DNS or LDAP to locate end systems.
• User availability—Determines the willingness of the called party to engage in communications.
• User capabilities—Determines the media type and media parameters to be used. In a message
exchange process, each SIP endpoint advertises media information so that all other participants
can learn about its capabilities.
• Session setup—Establishes session parameters at both called and calling parties.
• Session management—Transfers and terminates sessions, modifies session parameters, and
invokes services.