Lecture 15: Internet Topology and Structure

Modern Internet Structure

The Internet in the USA has always used a backbone topology, with the National Science Foundation Network (NSFNET) providing the original backbone network.

In 1995, this structure was privatised, with a variety of commercial organisations (National Service Providers (NSPs)) taking over the backbone functionality. Interconnections between these organisations occur at a number of Network Access Points (NAPs). A NAP can be independently owned. NSPs are said to peer with other NSPs at the NAPs, with (apparently) complex financial and/or cooperative arrangments in place to "carry each other's packets". The worldwide Internet is now structured similarly. We have:

NSPs (or tier-1 providers) provide ISP service to Regional Internet Service Providers (or tier-2 providers) -- ie, R-ISPs are customers of NSPs. R-ISPs can also negotiate private peering agreements, as indicated in the diagram. Retail (also called access, local, or tier-3) ISPs generally are clients of a Regional ISP and offer dial-in (and possibly permanent) services to the public. At each level, we say that a provider has a Point of Presence (POP) at a physical location if customers can connect to it at that location.

Note that the distinction between each of these classes of provider can become rather blurred, especially in Australia!

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