A SIP network uses the following components:

• SIP Proxy Server-The proxy server works as an intermediate device that receives SIP requests from a client and then forwards the requests on the behalf of the client. Proxy servers can provide functions such as authentication, authorization, network access control, routing, reliable request retransmission, and security.
• Redirect Server-The redirect server provides the client with information about the next hop or hops that a message should take, and the client then contacts the next hop server or user agent server directly.
• Registrar Server-The registrar server processes requests from user agent clients for registration of their current location. Redirect or proxy servers often contain registrar servers.
• User Agent (UA)-UA comprises a combination of user agent client (UAC) and user agent server (UAS) that initiates and receives calls. A UAC initiates a SIP request. A UAS, a server application, contacts the user when it receives a SIP request. The UAS then responds on behalf of the user. Cisco Unified Communications Manager can act as both a server and a client (a back-to-back user agent).

SIP uses a request/response method to establish communications between various components in the network and to ultimately establish a call or session between two or more endpoints. A single session may involve several clients and servers.

Identification of users in a SIP network works through:

• A unique phone or extension number.
• A unique SIP address that appears similar to an e-mail address and uses the format sip:<userID>@<domain>. The user ID can comprise either a user name or an E.164 address. Cisco Unified Communications Manager only supports E.164 addresses; it does not support e-mail addresses.
• An e-mail address format (employee@company.com) that is supported on Cisco Unified Communications Manager with SIP route patterns.