L2TP is a combination of PPTP and L2F. It is defined in RFCs 2661 and 3438. L2TP took the best of both PPTP and L2F and integrated them into a single protocol. Like PPTP, L2TP uses PPP to encapsulate user data, allowing the multiple protocols to be sent across a tunnel. L2TP, like PPTP, extends the PPP protocol. As an additional security enhancement, L2TP can be placed in the payload of an IPsec packet, combining the security advantages of IPsec and the benefits of user authentication, tunnel address assignment and configuration, and multiple protocol support with PPP. This combination is commonly referred to as L2TP over IPsec or L2TP/IPsec. The remainder of this chapter is devoted to an overview of L2TP, how it is implemented, and the ...

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