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What Exactly is the Internet? A Tour of Internet Routing and Peering
What Exactly is the Internet?
November 7, 2008
Starting at the service provider level, let's use an example of a local ISP that isn't very big.
JoeBob ISP is going to need an ISP itself before it can ship a customer's traffic to the Internet. It should start by forming distinct relationships with two different tier 1 ISPs. These are the big players: Level 3, AT&T, Cogent, XO, etc. JoeBob ISP will "peer" with these ISPs, in a BGP sense. This means that they will form a BGP session, and exchange routes. The big ISP will send all of the Internet's routes to JoeBob, which amounts to just fewer than 200,000 routes at this time. We'll get into the details of how that works in the BGP article.
JoeBob ISP will pay a certain dollar amount per MB of traffic sent. It's connected to two ISPs, so it will likely have two different prices. BGP magic can be done to favor the cheaper link, in that case. The crux of the matter is that JoeBob ISP is getting routes from another ISP, and it can do what it wishes with them. Some traffic can be sent to either tier 1 ISPs, and some can be sent elsewhere.
Peering agreements aren't just for the ISP-to-ISP BGP sessions, they are also used to negotiate company to company traffic flows. Even the small ISP can sometimes hop onto a local exchange, and peer with other companies, schools or ISPs in the area. An exchange is a network connection point, and every city has at least one. All the major players in an area will connect to the exchange, and maintain their own routers within. If they decide to peer with a friend, they can simply have the exchange operators patch some fiber through to their friend's rack, and viola, free packets. There's normally a port charge to connect to exchanges, and I didn't mention anything about how you'd get traffic from your site to the exchange. That takes fiber (money) too.
On the topic of peering, there are two types of exchanges: public and private.
A public exchange will allow most anyone to become a member. Public peering doesn't imply that you all of a sudden get free routes to every ISP that's on the exchange; rather it simply means that you can connect to the exchange, sometimes for free. One very popular exchange is the SIX, or Seattle Internet Exchange. If you are already a tenant of the Westin building in Seattle, you can pay them to patch you into the SIX closet. Once you get an IP from the SIX operator, you're on. After that, however, you must form peering arrangements with the other BGP speakers in the exchange.
The SIX and the PAIX are the most popular west-coast exchanges. A few people operate the SIX with a few Cisco switches in a rack. Essentially every major carrier and service provider now connects to the SIX, including Google, XO and AT&T broadband. The PAIX started life in Palo Alto, and is operated by Switch and Data now. It too is a public exchange, and it also runs private peering alongside its public offerings.
Then there's private peering.