Siemens optiPoint 410 entry Administrator's Manual page 36

Technical Overview
SIP Components
SIP is a peer-to-peer protocol. The peers in a session are called user agents
(UAs).
SIP Clients
SIP clients include the following:
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SIP Servers
SIP servers include the following:
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Additional Components
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Telephones — Act as UAS and UAC. The advance can initiate SIP re-
quests and respond to requests.
Gateways — Provide call control. Gateways provide many services, the
most common being translation between SIP conferencing endpoints
of transmission format, communications procedures, and codecs.
Other functions include call setup and clearing on both the LAN side
and the switched-circuit network side.
Proxy servers — Receive SIP requests from a client and forward them
to the next SIP server in the network. Proxy servers can provide func-
tions such as authentication, authorization, network access control,
routing, reliable request retransmission, and security.
Registrar servers — Process requests from UACs for registration of
their current location. Registrar servers are often colocated with redi-
rect or proxy servers.
DHCP server
Distributes IP data and further information in a network automatically
(list of distributed information ! page 175).
SNTP server
Provides time, date, daylight saving and timezone information.
Messaging server
For recording and reading messages.
SNMP server
Logging and maintenance of network components.
FTP server
For up- and download of files from and to the phone. These include
configuration files and music files.
PC with internet browser
Enables the administration of the phone by using a Web Client such as
Internet Explorer.