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vpn - CHAPTER 18 Chapter Goals Understand the basics of how...

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C H A P T E R 18-1 Internetworking Technologies Handbook 1-58705-001-3 18 Chapter Goals Understand the basics of how L2TP can be used to build a VPN. Learn how L2TP’s Layer 2 protocols enable secure passage through unsecured networks. Explain the relationship between L2TP and IPSec. Virtual Private Networks Background Virtual private networks (VPNs) are a fairly quixotic subject; there is no single defining product, nor even much of a consensus among VPN vendors as to what comprises a VPN. Consequently, everyone knows what a VPN is, but establishing a single definition can be remarkably difficult. Some definitions are sufficiently broad as to enable one to claim that Frame Relay qualifies as a VPN when, in fact, it is an overlay network. Although an overlay network secures transmissions through a public network, it does so passively via logical separation of the data streams. VPNs provide a more active form of security by either encrypting or encapsulating data for transmission through an unsecured network. These two types of security—encryption and encapsulation—form the foundation of virtual private networking. However, both encryption and encapsulation are generic terms that describe a function that can be performed by a myriad of specific technologies. To add to the confusion, these two sets of technologies can be combined in different implementation topologies. Thus, VPNs can vary widely from vendor to vendor. This chapter provides an overview of building VPNs using the Layer 2 Tunneling Protocol (L2TP), and it explores the possible implementation topologies. Layer 2 Tunneling Protocol The Internet Engineering Task Force (IETF) was faced with competing proposals from Microsoft and Cisco Systems for a protocol specification that would secure the transmission of IP datagrams through uncontrolled and untrusted network domains. Microsoft’s proposal was an attempt to standardize the Point-to-Point Tunneling Protocol (PPTP), which it had championed. Cisco, too, had a protocol designed to perform a similar function. The IETF combined the best elements of each proposal and specified the open standard L2TP.
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18-2 Book Title Appears Here 1-58705-001-3 Chapter 18 Virtual Private Networks
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