cing the threat of malware. The fact that we can deploy malware detection and removal technologies on a proxy server is very attractive. Authentication, Authorization, and Accounting An important aspect of security is the authentication of users and access control, which is commonly handled by an AAA server. An AAA server is a network server used for access control. Authentication identifies the user, authorization implements policies that determine which resources and services a valid user may access, and accounting keeps track of time and data resources used for billing and analysis. The AAA server, also called 3A software, typically interacts with a network access server (NAS) and gateway servers, as well as with databases and directories that contain user information. RADIUS, the current standard by which devices or applications communicate with an AAA server, is an AAA protocol for applications such as network access or IP mobility. This UDP-based protocol is intended to work in both local and roam- ing situations and is considered suitable for high-volume service control applica- tions, such as regulation of dialup or VPN services. Increased use of RADIUS is occurring due to the introduction of 802.1X port security (discussed in Chapter 15, "WMANs, WLANs, and WPANs") for wired and wireless LANs. In fact, Microsoft has included 802. 1X security in Windows TCP/IP, and every enterprise PC might require authentication before being granted access to the LAN. RADIUS servers are designed to block unauthorized access by remote users, and they rely on authentication schemes such as Challenge Handshake Authentica- tion Protocol (CHAP), which means there's a back-and-forth dialog to verify a user's identity. In fact, RADIUS makes use of CHAP, which uses a three-way hand- shake to periodically verify the identity of the peer throughout the connection. The server sends a random token to the remote workstation. The token is then encrypted, using the user's password, and sent back to the server. The server per- forms a lo