9.4 OVERLAY NETWORKS
From its inception, the Internet has adopted a clean model, in which the
routers inside the network are responsible for forwarding packets from
source to destination, and application programs run on the hosts connected
to the edges of the network. The client/server paradigm illustrated
by the applications discussed in the first two sections of this chapter
certainly adhere to this model.
In the last few years, however, the distinction between packet forwarding
and application processing has become less clear. New applications
are being distributed across the Internet, and in many cases these
applications make their own forwarding decisions. These new hybrid
applications can sometimes be implemented by extending traditional
routers and switches to support a modest amount of application-specific
processing. For example, so-called level-7 switches sit in front of server
clusters and forward HTTP requests to a specific server based on the
requested URL. However, overlay networks are quickly emerging as the
mechanism of choice for introducing new functionality into the Internet.
You can think of an overlay as a logical network implemented on top
of some underlying network. By this definition, the Internet started out as
an overlay network on top of the links provided by the old telephone network.
Figure 9.19 depicts an overlay implemented on top of an underlying
network. Each node in the overlay also exists in the underlying network;
9.4 OVERLAY NETWORKSFrom its inception, the Internet has adopted a clean model, in which therouters inside the network are responsible for forwarding packets fromsource to destination, and application programs run on the hosts connectedto the edges of the network. The client/server paradigm illustratedby the applications discussed in the first two sections of this chaptercertainly adhere to this model.In the last few years, however, the distinction between packet forwardingand application processing has become less clear. New applicationsare being distributed across the Internet, and in many cases theseapplications make their own forwarding decisions. These new hybridapplications can sometimes be implemented by extending traditionalrouters and switches to support a modest amount of application-specificprocessing. For example, so-called level-7 switches sit in front of serverclusters and forward HTTP requests to a specific server based on therequested URL. However, overlay networks are quickly emerging as themechanism of choice for introducing new functionality into the Internet.You can think of an overlay as a logical network implemented on topof some underlying network. By this definition, the Internet started out asan overlay network on top of the links provided by the old telephone network.Figure 9.19 depicts an overlay implemented on top of an underlyingnetwork. Each node in the overlay also exists in the underlying network;
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