GGSN returns a Create PDP Context Response message to the SGSN with the assigned IP address, if applicable. Information describing the various users, applications, files, printers, and other resources accessible from a network is often collected into a spe- cial database that is sometimes called a directory. As the number of differ- ent networks and applications has grown, the number of specialized directories of information has also grown, resulting in islands of informa- tion that are difficult to share and manage. If all of this information could be maintained and accessed in a consistent and controlled manner, it would Figure 10-14 GTP Tunnel (APN) The GGSN role. Authentication: Radius What is your password? IP Backbone Authorization: What you can do Address Allocation: Radius Use this address Tunneling: Starting a VPN tunnel Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. SGSN-to-GGSN (Gn) and GGSN-to-PDN (Gi) Interface SGSN-to-GGSN (G) and GGSN-to-PDN (G) Interface provide a focal point for integrating a distributed environment into a con- sistent and seamless system. The Lightweight Directory Access Protocol (LDAP) is an open industry standard that has evolved to meet these needs. LDAP defines a standard method for accessing and updating information in a directory. LDAP gained wide acceptance as the directory access method of the Internet and is there- fore strategic within corporate intranets. It is supported by a growing num- ber of software vendors and is being incorporated into a growing number of applications. For example, the two most popular Web browsers, Netscape Navigator/Communicator and Microsoft Internet Explorer, support LDAP functionality as a base feature. What Is a Directory? A directory is a collection of information about objects arranged in some order that gives details about each object. Popular examples are a city