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There is the list of key features of RADIUS:
NAS will work as a client for RADIUS server.
RADIUS server is responsible for getting user connection requests, authenticating the user and then returning all configuration information necessary for the client to deliver service to the user.
A RADIUD server can act as a proxy client to other RADIUS servers.
Transactions between client and server are authenticated through the use of a shared key and this key is never sent over the network.
Password is encrypted before sending it over network.
RADIUS supports following protocols for authentication purpose:
Point-to-Point Protocal - PPP
Password authentication protocol - PAP
Challenge-handshake authentication protocol - CHAP
Simple UNIX Login
RADIUS is extensible; most vendors of RADIUS hardware and software implement their own dialects.