L2TP protocol is ongoing. L2TP, through the "virtual dial" business where the initial dial-up servers and dial-up connection agreements separate from the end. When the "virtual dial" customers began to access the remote user and their ISPs NAS established between a PPP link. ISP went on to end parts of the system or user identification to determine whether the user a "virtual dial-up" business, once identified the need, then the user name and VPN network will be linked to a central server. Then, NAS will check there is no central server to the L2TP VPN connection exists, and if not, then the L2TP protocol will end in the NAS to initiate a central server L2TP VPN tunnel negotiations. If negotiation is successful, the central server in the NAS and VPN established between a L2TP tunnel, and established between the centers of end users and VPN connections. L2TP protocol also defines a number of tunnel management and maintenance operations, such as periodically send Hello packets to determine the connectivity of the tunnel, using protocols provided by the sending and receiving serial number domain domain tunnel flow control and congestion control. Layer Two Tunneling Protocol with the advantages of simple, but they are not good scalability. More importantly, they do not provide inherent security mechanisms, they can not support the enterprise and external customers and suppliers the confidentiality of conversations between the demand, so they do not support the use to connect to corporate intranets and enterprise and external customers Supplier Extranet Extranet concept. Extranet encryption tunnel is required and the need for appropriate key management mechanism.
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