9.4.1 Routing Overlays
The simplest kind of overlay is one that exists purely to support an
alternative routing strategy; no additional application-level processing is
performed at the overlay nodes. You can view a virtual private network
(see Section 4.1.8) as an example of a routing overlay, but one that doesn’t
so much define an alternative strategy or algorithm as it does alternative
routing table entries to be processed by the standard IP forwarding algorithm.
In this particular case, the overlay is said to use “IP tunnels,” and
the ability to utilize these VPNs is supported in many commercial routers.
Suppose, however, you wanted to use a routing algorithm that commercial
router vendors were not willing to include in their products. How
would you go about doing it? You could simply run your algorithm on a
collection of end hosts, and tunnel through the Internet routers. These
hosts would behave like routers in the overlay network: As hosts they are
probably connected to the Internet by only one physical link, but as a
node in the overlay they would be connected to multiple neighbors via
tunnels.
9.4.1 Routing OverlaysThe simplest kind of overlay is one that exists purely to support analternative routing strategy; no additional application-level processing isperformed at the overlay nodes. You can view a virtual private network(see Section 4.1.8) as an example of a routing overlay, but one that doesn’tso much define an alternative strategy or algorithm as it does alternativerouting table entries to be processed by the standard IP forwarding algorithm.In this particular case, the overlay is said to use “IP tunnels,” andthe ability to utilize these VPNs is supported in many commercial routers.Suppose, however, you wanted to use a routing algorithm that commercialrouter vendors were not willing to include in their products. Howwould you go about doing it? You could simply run your algorithm on acollection of end hosts, and tunnel through the Internet routers. Thesehosts would behave like routers in the overlay network: As hosts they areprobably connected to the Internet by only one physical link, but as anode in the overlay they would be connected to multiple neighbors viatunnels.
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