Peering & Transit

Transport networks are rarely universal, to get there from here, you must often use a series of networks to make the trip.

We are familiar with this from real life experience, taking a bus to the airport to catch a plane to a location where you get a cab to the final destination.

Each step in the chain is paid seperately and there are locations where each network comes to meet, railway stations, bus terminals, airports and the like.

In general, you don't use any of these networks for "local travel," you might walk, cycle or drive.

You only need the services of the more specialised transport networks when you travel farther afield.

The process of getting yourself from one place to another (say by walking next door) without assistance from a commercial transport operator could be compared to "peering."

When you do need to use one of the commercial transport operators, you pay a fare (or may have it paid for you) that is akin to "transit."

This is all very common, and taken for granted in real world physical transport networks. No carrier expects everyone to be their exclusive customer and most carriers, even competitors will co-operate in establishing and using exchanges for the convenience and efficiency they provide.

This has not been the case in the relatively recent past of telecommunications. There a single carrier has done everything, but that job, "everything" has grown in the telecommunications market until it is beyond the capacity of any single organisation, and thus the Internet has grown at a fantastic rate, to fill the gaps left by the previously "universal" providers.

In an internetwork like the Internet, the situation will also arise where a customer or member of one self-contained (Autonomous System, an ISP or private) network will want to send, recieve or exchange information with a member of another independant network.

This is precisely the appeal of the Internet. The ability to easily exchange information with any other customer of the Internet. This is what you deserve to expect from an internetwork service provider, or ISP.

If you and and the other party are customers of networks who can efficiently and directly connect, then you peer with the other customer.

If you must pass through other networks to reach the other party's network, you are transiting those networks, and they need to be paid, by you and the other party.

There are many ways to explain peering and transit, we have a more formal, technical explanation, a physical transport interconnect example, and there are others.

Simon Blake, manager of CityLink's network operations group gave a presentation to staff on the subject after disruptions occured in June, 2004.