RADIUS is a protocol that enables a single server to become responsible for all remote access authentication, authorization, and auditing (or accounting) services. The RADIUS protocol can be implemented as a vendor-specific product such as Microsoft's Internet Authentication Server (IAS).
RADIUS functions as a client/server system. The remote user dials in to the remote access server, which acts as a RADIUS client, or network access server (NAS), and connects to a RADIUS server. The RADIUS server performs authentication, authorization, and auditing (or accounting) functions and returns the information to the RADIUS client (which is a remote access server running RADIUS client software); the connection is either established or rejected based on the information received.
Kerberos
Seasoned administrators can tell you about the risks of sending clear-text, unencrypted passwords across any network. The Kerberos network authentication protocol is designed to ensure that the data sent across networks is safe from attack. Its purpose is to provide authentication for client/server applications.
Kerberos authentication works by assigning a unique key (called a ticket), to each client that successfully authenticates to a server. The ticket is encrypted and contains the password of the user, which is used to verify the user's identity when a particular network service is requested.
Kerberos was created at Massachusetts Institute of Technology to provide a solution to network security issues. With Kerberos, the client must prove its identity to the server, and the server must also prove its identity to the client. Kerberos provides a method to verify the identity of a computer system over an insecure network connection.
Kerberos is distributed freely, as is its source code, allowing anyone interested to view the source code directly. Kerberos is also available from many different vendors that provide additional support for its use.