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Figure 19 Arbitrary client to known server
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Remember that the client sees a server identifier, which is not necessarily the actual server that will process the request The potential ability to separate virtual from actual servers is the basis for many performance-enhancing or resource-conserving functions, including web caches and content distribution services Figure 110 shows a typical television scenario, in which the broadcasters transmissions are simplex The broadcasters (analogous to servers) receive no feedback from receivers (analogous to clients) A client selects a particular channel to receive There is a distinct similarity to a cnncom placing its servers on the Internet and making them accessible to arbitrary clients In the case of the Internet, minimal duplex communications are needed simply for reliable transmission of the web data Figure 111 introduces a more complex model, that of premium services in television In this model, it is not required that the content sources know which particular clients are listening to them It is required that the operator of the access control devices either enable subscription service or authorize and collect pay-per-view services It is not unreasonable to make an analogy between the access control providers for pay-per-view and the micro-cashtransfer services (for example, PayPal) evolving to provide paid passwords to specific Internet services Paying for specific passwords is subtly different than the next case of having pre-established relationships between clients and servers
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Figure 110 Television metaphor
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Figure 111 Premium television metaphor
Known Clients to Known Servers under Common Administration
Traditional enterprise-operated networks fall under this category, as do a variety of models in which the operation of some services is outsourced to providers Even the traditional model, however, involves a certain amount of operational outsourcing when the customer does not operate their own wide area links When multiple sites interconnect, the wide area transmission facilities among the sites are assumed to be under the control of a service provider In other words, there is an administrative boundary between the customer and provider Some technologies, such as asynchronous transfer mode (ATM), can be used both within and between sites, so it proves more useful to distinguish between site and wide area services with respect to administrative models rather than technologies In principle, the service provider is responsible for its own network, both its facilities and their administration Figure 112 shows that the service providers may have many customers, whose traffic flows over shared resources under provider control The provider has an operational support network for controlling these resources In Figure 112, there are sites that belong to the intranet of one enterprise only or to the extranet of either or both of enterprises 1 and 2 See the next section for a discussion of intranets and extranets
1
Enterprise 2 Intranet Extranet Extranet Enterprise B B
Public Internet versus private VPN overlay
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Public or
Enterprise 1 Intranet
Enterprise 1 Intranet
Shared Carrier Network
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Extranet Enterprise A
Public Internet access
Access to shared environment Single responsible operator
VPN operations
Enterprise 2 Intranet
Enterprise 2 Intranet
Figure 112 Provider structure
Introduction to VPNs
Managed service models still require significant customer involvement in defining requirements for connections between sites, the technical details of interconnection devices, and so on A model that reduces customer involvement is the intranet VPN (Figure 113) No single, universally accepted definition of a VPN yet exists When I taught Cisco University seminars for resellers that would offer VPNs to their customers, I observed that it had long been traditional for network sales to sell products that didn t exist Since VPNs didn t have actual existence if they did, they obviously wouldn t be virtual then sales should be able to sell infinite numbers of VPNs In this book, I define a VPN as a set of endpoints that are interconnected over shared facilities operated by a single contractually responsible service provider The customer has the illusion that it has a private network, and indeed different customers on the VPN are isolated from one another The shared infrastructure may be the public Internet If any appreciable performance/service guarantees are needed, however, it is quite unrealistic for the customer to expect any guarantees if the provider has no control of the underlying transmission facilities There are many vendor-specific interpretations of VPNs It is not meant as a criticism that vendors tend to emphasize their core competence in their definition of VPNs Those concerned with packet transport tend to emphasize the