What Is SIP Infrastructure And Terminology?

User Agent Client (UAC)
      A logical entity that creates a new request and sends it
     The role lasts only for the duration of a transaction
User Agent Server (UAS)
     A logical entity that generates a response to a SIP request
     The role lasts only for the duration of a transaction
User Agent (UA)
      A logical entity that can act as both a user agent client and user agent server

Transaction
   A SIP transaction comprises of all messages from the first request sent up to a final        response

Final Response
A response that terminates a SIP transaction, as opposed to a provisional response that does not
Server
A network element that receives requests in order to service them and sends back responses to those          requests.  
Proxies, user agent servers, redirect servers, and registrars

Proxy Server
An intermediary entity acting as both a server and a client making requests on behalf of other clients
A proxy server primarily plays the role of routing, ensuring that a request is sent to another entity "closer" to the targeted user
Used for enforcing policy (for example, making sure a user is allowed to make a call)
A proxy interprets, and, if necessary, rewrites specific parts of a request message before forwarding it
Can be classified as
Stateful Proxy
Stateless Proxy