- User Agents: End system acting on behalf of user e.g. client & server. Initiates SIP request & server receives requests and returns responses on behalf of user.
- Network Servers: Three types available:
- Registration server – receives client location updates.
- Proxy server – forwards receiving request to next-hop server.
- Redirect server – determines next-hop server, returns address of next-hop server to client instead of forwarding request.