The Structure of the Internet
The Internet is a global scale IP network which consists of many networks that have been interconnected to form the larger network. In turn each of these networks will generally consist of multiple sub-networks.
How the Internet was Organised
Legend NAP - Network Access Point (Telecommunications Company major switching point) NSP - Network Service Provider (Big Internet Service Provider) ISP - Internet Service Provider C - ISP Client networks
Everything is a hierarchy and the rules are dictated by the larger players. The NSPs connect "peer to peer" via the NAPs, the medium level ISPs are clients of the NSPs, the Local ISPs are clients of the medium level ISPs and the ISP Client networks are clients of the Local ISPs.
How the Internet is Organised Now
As telecommunication costs fell and bandwidths increased, many ISPs installed links (often through the use of Internet eXchange Points or IXPs) to their peers (equals) to offload traffic (and costs) from the bigger ISPs. Also, many ISPs "Multi Homed" their provider links for reliability and increased bandwidth. As a result, the distinction between levels became blurred.
The end result is that the inner part of the Internet has tended toward a fully connected "mesh". The outer "client" networks has tended to remain "tree like".
See "The Changing Structure of the Internet" by Geoff Huston (particularly slides 27 and 34 through 41).
An interesting view of the world has been provided by the Opte Project which mapped the Internet using traceroute. The following is one such example, from January 2005:
An extended view of this data can be found in Wikimedia (1.27MB JPG).
A Brief Course in Network Topology
Mesh (Fully Connected) Networks
Every node (router or even ISP if you like) is connected directly to every other:
- Mesh networks don't scale well; big networks have many links.
- Mesh networks are robust. There are many ways to get data to its destination (except in 2 node networks ;-). The loss of one or more links causes only minor problems.
- Great care is needed to avoid "routing loops".
- Routing rules are often rather complicated.
As an example of a mesh network[1], the "central core" of the Internet in the USA consists of about 20 large ISPs. In 2002, 15 of these form an almost complete mesh: only 3 links were missing (97% connected!).
Tree (Minimally Connected) Networks
- Tree networks have the minimum possible number of communication links.
- Tree networks are fragile - one broken link splits the network into two pieces that cannot communicate with one another.
- Routing loops are impossible.
- Routing rules are simple - there is only ever one route to a destination.
Copyright © 2009 Joel Sing