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Lecture #1 - Introduction to Internetworking

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.

Small IP Network
  • Groups of computers (or "hosts") are connected to form sub-networks (or subnets for short). This may be achieved using any form of suitable transmission media including ancient coaxial cable, radio based technologies (such as 802.11 wireless networks), and UTP (twisted pair) cable with hubs (both dumb and "switched"):
     
    • Dumb hubs are gossips, they simply repeat everything they hear and forward all packets to every connected device.
    • "Switching hubs" (more commonly known as "switches") are more selective, they forward data packets only to their intended destination if known, else they broadcast them. A table that maps MAC addresses to ports is created by examining the source address of incoming Ethernet frames.
  • Subnets are connected together by routers to form networks. These networks are usually under the administration of one organisation eg. La Trobe University. This network is often referred to as an Autonomous System (AS). This organisational unit (the AS) is important when we examine routing between organisations (exterior routing). Within these networks, the routing decisions are implemented by "interior routing" protocols like RIP or OSPF.
  • Networks are connected together by routers to form "The Internet". At this level, the routing rules may be quite different to those appropriate to the smaller "networks", often having to reflect business policy decisions about whose traffic gets through (and to where). This is the job of the Border Gateway Protocol (BGP4).

How the Internet was Organised

Internet - Tiered Topology
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

Internet - Meshed Topology

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:

Opte Project 2005 Internet Map

An extended view of this data can be found in Wikimedia (1.27MB JPG).

A Brief Course in Network Topology

Mesh (Fully Connected) Networks

Mesh 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!).

[1] Subramanian, Agarwal, Rexford and Katz. Characterising the Internet Hierarchy from Multiple Vantage Points (PDF)

Tree (Minimally Connected) Networks

Tree Diagram
  • 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.

References

The Opte Project - http://www.opte.org/maps/

London INternet eXchange (LINX) - http://www.linx.net/

In particular, have a look at their traffic graphs!

Wikipedia's IXP Entry - http://en.wikipedia.org/wiki/Internet_exchange_point