D-Link DFL-2500 User Guide - Page 247
Pptp/ L2tp
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228 Chapter 22. VPN Protocols & Tunnels 22.2 PPTP/ L2TP As introduced in the previous sections, IPsec provides methods for two endpoints to transport data packets as they are connecting by a "private channel ". Such technique is often called Tunneling. Like the functions of IPsec we have discussed, the tunneling protocols offer the standards for encapsulation, transmission, and decapsulation to the data transfer process. The endpoints of the tunnel must agree on the same tunneling protocol to be able to communicate. IPsec features the Tunnel mode ESP encapsulation with encryption and authentication and becomes widely used for very secure VPN implementations. However, there are some limitations of using IPsec tunneling, for example, it is not supported by all systems and it can be hard to configure. In contrast, PPTP and L2TP tunneling protocols are widely supported and easier to configure than IPSec. 22.2.1 PPTP Point-to-Point Tunneling Protocol(PPTP) is built on Point-to-Point protocol(PPP), Generic Routing Encapsulation (GRE), and TCP/IP. PPTP tunneling format PPTP relies on the PPP protocol to encapsulate datagrams (see 9.4.1 PPP). The PPP frame is then encapsulated into GRE packet with routing information included, which is in turn packed with an IP header to conform to the Internet addressing convention, shown in Table 22.1. The Layer 2 data frame is the basic transport unit. Data-link layer header and trailer are put onto the PPTP encapsulated packet to form the tunneling data. PPTP uses TCP port 1723 for it's control connection and GRE (IP protocol 47) for the PPP data. IP Header GRE Header PPP Payload PPP Frame Table 22.1: PPTP Encapsulation. D-Link Firewalls User's Guide
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