Server
A “server” in SIP is a logical entity which processes requests. Processing means generation of responses or forwarding.
The following servers are defined by RFC 3261 (SIP):
- user agent server
- proxy
- outbound proxy
- redirect server
- registrar
User agent server
A “user agent server” is an entity that generates a response to a request. In other words a SIP entity that handles inbound call
is called a “user agent server” in the context of processing of that call. A typical “user agent” (UA) acts as client (UAC) when
it starts outbound calls and also it acts as server (UAS) when receives inbound calls.
Proxy server
A network host which provides at least one of the following services:
- message routing
- authentication
- authorization
- call routing
- location services
Example of proxies are registrars and redirect
servers.
Outbound Proxy (provides message routing)
A network host that receives requests from user agents. The word “outbound” means that in a typical scheme any outbound request
generated by user agent msut be sent to outbound proxy which forwards the request further to destination hosts.
Registrar (provides location services)
A proxy server which The name “registrar” is defined for entities that can process REGISTER request. In other words a SIP proxy server which provides location services can also be called registrar.
Redirect server (provides call routing)
A proxy server which generates and sends back 3xx response to the request. This allows to redirect calls from user agent to
another host.
Stateless proxy
A type of proxy server which forwards every request and response.
Stateless proxies do not generate any messages.
Transaction stateful proxy
A type of proxy server which is also called “stateful proxy”. It maintains transaction state and may send
provisional responses (1xx) to the UAC for the incoming INVITE requests.
Call stateful proxy
A type of transaction stateful proxy which also maintains a call state.
These are logical entities and implementations usually have one or more logical functions.
RFC does explicitly define “SIP gateway” entity.
When “SIP gateway” name is used in applications it
usually has one of the following meanings.
- SIP Gateway is a registrar server
- SIP Gateway is a proxy server
- SIP Gateway is a Public Switched Telephone Network (PSTN) gateway
Also when SIP Gateway name is used it sometimes means