Implemention Of VPDN Technology Based On L2TP Protocol

With the tremendous growth of Internet, TCP/IP is becoming more and more popular around the world, thus, many enterprises choose to build their private network with this technology. The best way to build a VPN is to use VPDN (Virtual Private Dial Network), which is more convenient and economic to construct and more easily to operate. VPDN uses special network encryption and communication protocols so as to build a safe and confidential network within the public network for dial-up users only.By far, L2TP (Layer 2 Tunnel Protocol) has been the optimal protocol to implement a VPDN based on the tunnel technology. So, the quality of the L2TP, mainly considering its framework, data structure and module constructions becomes the key factory of a highly-reliable VPDN.In the project on vanlink VL3600/2600 routers, a VPDN based on L2TP protocol is designed and implemented based on tunnel technology. The main contributions of the study are as follows:1. The framework of an L2TP based VPN is designed, consisting 4 modules, namely, the PPP, UDP, MMI and inter_clock timer.2. Data structure of the VPDN is designed. According to the definition of L2TP, three kinds of data structure were constructed, such as the global data structure, tunnel data structure and session data structure.3. System structure of the L2TP based VPDN is decided which contains 6 modules complementing the 6 following procedures, separately: configurable message process, UDP message process, tunnel establishment and maintenance, and session establish and maintenance, data transport, inter_clock timer.Necessary programs are coded to let run the system smoothly. Also the function of such a VPDN system based on L2TP is sufficiently tested on the VL3600/2600 serial routers.Finally, some ideas and solutions to further improving the security of L2TP are proposed.