New Page 3 X.25 ITU-T standard that defines how connections between DTE and DCE are maintained for remote terminal access and computer communications in PDNs. X.25 specifies LAPB, a data link layer protocol, and PLP, a network layer protocol. Frame Relay has to some degree superseded X.25. X.29 ITU-T recommendation that defines the form for control information in the terminal-to-PAD interface used in X.25 networks. X.500 ITU-T recommendation specifying a standard for distributed maintenance of files and directories. X.509 Constitutes a widely accepted basis for a PKI infrastructure, defining data formats and procedures related to the distribution of public keys using certificates digitally signed by CAs. X.509v3 certificates Certificate support that allows the IPSec-protected network to scale by providing the equivalent of a digital ID card to each device. When two devices wish to communicate, they exchange digital certificates to prove their identity (thus removing the need to manually exchange public keys with each peer or to manually specify a shared key at each peer). These certificates are obtained from a CA. X.509 is part of the X.500 standard. Xauth Deploys IPSec VPNs using Terminal Access Controller Access Control System Plus (TACACS+) or Remote Authentication Dial-In User Service (RADIUS) as your user authentication method. This feature, which is designed for VPN clients, provides a user authentication by prompting the user for username and password and verifies them with the information stored in your TACACS+ or RADIUS database. XML eXtensible Markup Language. The universal format for structured documents and data on the Web. (http://www.w3.org/XML/) Xwindow Distributed, network-transparent, device-independent, multitasking windowing and graphics system originally developed by MIT for communication between X terminals and UNIX workstations.