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The first question of access is, What is accessing the VPN The VPN may see the smallest unit of access as an individual user machine, a specific LAN, or a site with routers and multiple media An individual host or physical site can be a member of more than one VPN, and may also be able to access a real corporate intranet and the Internet Access policies need to be determined very early in the process and to interact tightly with your security policy
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A basic definition for a customer domain is the set of network resources whose operation the customer contracts to you A given customer organization, of course, may have multiple domains with different requirements and characteristics, billed under one master contract A good example of differences in requirements involves Internet access Figure 131 shows a multisite corporate network where the real servers use private address space but the virtual servers are on the public Internet This is a very real-world situation, although the terminology might seem a little strange If you think of the public Internet as real and the intranet as virtual, then, in this example, the real servers are on the virtual network and the virtual servers are on the real network
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Figure 131 What is real
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In this example, the customer domain has two sites of its own, plus two external gateways/load directors offered by the provider The VPN includes two server clusters at two two-host sites As a value-added service, the provider operates an intelligent DNS that directs traffic to the least loaded server A separate internal emulated LAN allows synchronization among the servers (see Emulated LAN Service later in this chapter) In any event, the customer domain will contain one or more sites, which contain one or more hosts Some sites may be virtual, and some hosts may actually be operated by the provider
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As shown in Figure 132, a site is a collection of hosts that use a common connection to the provider The connection runs from a customer equipment (CE) device associated with the site to a provider equipment (PE) device associated with a provider POP In the VPN context, a site may involve more than one physical location, as long as the VPN provider is not responsible for intersite connectivity (see Virtual Sites ) That connection may support different VPN and Internet connectivity requirements, as well as connections to voice and other integrated services (see Figure 133)
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For site-oriented VPNs that use encryption, if a new site or router can authenticate itself with a certificate, it can be assumed to be part of the membership It is not much of a mental stretch to think of a virtual site as a set of hosts that pass the authentication criteria of an access wholesaler, with that access provider linking all these hosts to the VPN provider through a single connection Any host that can reach the CE is a member of the virtual site in Figure 134
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Figure 132 Site concept for VPNs
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Figure 133 Site with VPN and Internet access
Customer-Operated Hosts
A customer host may be a single physical machine with a single IP (or other protocol) address, one of several addresses on a single physical machine, or the outside address of a server cluster The VPN sees hosts as IP addresses A given customer host is associated with a single customer site The contents of this host, however, may be distributed into content caches and use content delivery mechanisms
Content Caching
Large content sources (for example, CNN) often desire to spread copies of their content to a set of caches closer to the user While the virtual source (for exam-
Host
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IPSec ISP Access Server CE
L2TP Tunnel
Figure 134 Host accessing a virtual site
VPNs and Related Services
ple, wwwcnncom) appears to be on the public Internet, there may very well be a VPN interconnecting the content source to local caches it preloads with content There is communication back to the source to keep it informed of hit rates and other usage statistics Content providers, however, are not the only organizations that may want to implement content caches ISPs can often both minimize upstream bandwidth requirements and improve customer access time by installing web caches at their POPs, or at least inside their infrastructure These reduce bandwidth by reducing the total number of content requests that must go to the end server, and improve access time by reducing the latency of the customer s access to data