Proxy Servers It receives SIP requests and responses from SIP user agents. On behalf of the user agent, it forwards or responds to it. Typically, the SIP proxy has access to a database or a location service to aid in processing requests (determination of the next hop). Furthermore, proxy’s servers can support forking. This means that the proxy can handle number of (i.e. more than one) user registrations in parallel. An invitation message can be “forked” to different locations where the user is registered at the same time (e.g., ring several phones at different places at once until somebody takes the call at one of them). 

Redirect Servers In contrast to Proxy Servers, it does not forward requests. It sends a response back to the initiating SIP user agent that includes the actual SIP URL where the invited party is reachable or the SIP URL of the next hop SIP server. Like the Proxy Server, it accesses a database or location server to look up a user.  

Registration Server (Registrar) This type of server only accepts SIP user agent registration requests. These include the addressing information (URLs) for the user agent being known and alternative addresses or aliases for which it would like to accept requests. This contact information is made available to other SIP servers in the same administrative domain. For each invited SIP URL that had been registered before, these proxy and redirect servers will be able to forward the request correctly. The registration procedure is usually executed when a user becomes reachable (e.g., after start-up of a SIP client on a PC, or after the user changed his location). 

UAC (User Agent Client) The user agent client (UAC) is a program that initiates SIP requests such as initiating a call. A UAC is also known as the calling user agent. 

UAS (User Agent Server)A user agent server (UAS) is a program that receives SIP requests such as an incoming call and sends back responses to those requests. A UAS is also known as the called user agent. Each proxy server can either be call/transaction stateful or stateless:  

o        a call stateful proxy holds information about the call during the entire time the call is up;
o        a transaction stateful proxy maintains the state only during a SIP transaction;
o        a stateless proxy processes a message and then forgets everything until the next message arrives.

The most generic SIP operation involves a SIP user agent client (UAC) issuing a request, a SIP proxy server acting as end-user location discovery agent and a SIP user agent server (UAS) accepting the call.  A successful SIP invitation consists of two requests: INVITE answered by a Response (200 OK) that is again acknowledged by ACK. The INVITE message contains session description that informs the called party what type of media the inviting party can accept and where it wishes the media data to be sent to. SIP addresses are referred to as SIP Uniform Resource Locator (SIP-URL's), which are of the form sip:user@host.domain.  SIP message format is based on the Hyper Text Transport Protocol (HTTP) message format, which uses a human-readable, text-based encoding. In particular, SIP handles the following aspects of establishing and terminating multimedia communications: 

o        User location: determination of the end system to be used for communication;
o        User capabilities: determination of the media and media parameters to be used;
o        User availability: determination of the willingness of the called party to engage;
o        Call setup: “ringing”, the establishment of call parameters of both parties;
o        Call handling: including the transfer and termination of calls.

Since SIP is limited to the multi-media session control, there needs to be a description within a SIP request and response message about the multimedia session properties. IETF Session Description Protocol (SDP) is commonly used together with SIP to accomplish all the call signaling functions in IP telephony.