Network Element of SIP

Any communication would have a sender and a receiver. In SIP, these are generally called as User Agents (UA). The sender will initiate the request who is called precisely as User Agent Client (UAC) whereas the receiver who will receive the request is called by User Agent Server (UAS). The role of UAC and UAS are interchangeable and dynamic even within a single dialog.

Other than the UAC and UAS, there are other Network elements that can be involved in Call Routing, Call Processing, Interfacing. They are also called as SIP servers. They are,

Proxy server, which receives SIP request and forwards them on behalf of the requestor. 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.

Redirect server, which receives a SIP request and generates redirect response, directing the client to contact alternative set of URIs.

Registrar, a server which receives a SIP register request and places the information it receives in those requests into the location service for the domain it handles.

Location Service, can be implemented per the discretion of the designer. A location service is used by a SIP redirect or proxy server to obtain information about a callee’s possible location(s). It contains a list of bindings of address-of record keys to zero or more contact addresses. The bindings can be created and removed in many ways. One way is to use Register request.

Session Border Controllers(SBC), they serve as middle boxes between UA and SIP server for various types of functions, including network topology hiding, and assistance in NAT traversal.

It is an important concept that the segregation of SIP Servers is logical and not physical, which means implementations can integrate them all into one code or into multiple code serving from multiple machines.