Network Elements 1.1 User Agent A SIP User Agent (UA) is a logical network end-point used to create or receive SIP messages and thereby manage a SIP session. A SIP UA can perform the role of a User Agent Client (UAC), which sends SIP requests, and the User Agent Server (UAS), which receives the requests and returns a SIP response. These roles of UAC and UAS only last for the duration of a SIP transaction. 1.2 Proxy server An intermediary entity that acts as both a server (UAS) and a client (UAC) for the purpose of making requests on behalf of other clients. A proxy server primarily plays the role of routing, which means its job is to ensure that a request is sent to another entity "closer" to the targeted user. 1.3 Registrar A server that accepts REGISTER requests and places the information it receives in those requests into the location service for the domain it handles which registers one or more IP addresses to a certain SIP URI. SIP registrars are logical elements, and are commonly co-located with SIP proxies. But it is also possible and often good for network scalability to place this location service with a redirect server. 1.4 Redirect server A user agent server that generates 3xx (Redirection) responses to requests it receives, directing the client to contact an alternate set of URIs. The redirect server allows proxy servers to direct SIP session invitations to external domains. 1.5 Session border controller Session border controllers serve as middle boxes between UA and SIP server for various types of functions, including network topology hiding, and assistance in NAT traversal. 1.6 Gateway Gateways can be used to interface a SIP network to other networks, such as the public switched telephone network, which use different protocols or technologies. 1.7 Application Layer Gateway ALG is a SIP aware monitoring device commonly contained in Routers and or Firewalls. SIP ALGs can have the capabilities of changing SIP Messages and should be disabled if any issues are experienced with SIP Calls.