What Exactly is the Internet? A Tour of Internet Routing and Peering - page 2
What Exactly is the Internet?
Most public exchanges have 100Mb or 1Gb connections participants can connect to. If you're a very large ISP, you need (many) 10Gb links to handle all your traffic. The problem is that public exchanges don't normally have the funds to provide this type of service reliably. Private peering provides an SLA (service level agreement), and most of the time that comes with better hardware to handle the traffic. There's always ongoing public vs. private peering debates that are quite entertaining, but never really lead anywhere. If you're going to need custom solutions, like trunking two links together, then private peering is where you'll likely end up.
Peering is the opposite of transit. Transit providers, like tier 1 ISPs, give you all the Internet routes you pay for (normally all of them), and take care of everything else. A company's ISP is a transit provider, and so is the ISP that services a smaller ISP. ISPs that have peering along with their transit will generally send everything they can through the peer, and everything else onto the transit provider.
The concept of a default-free router means that you have a router that gets a full Internet routing table. If your transit provider doesn't send you a route for some network, and you don't have any other knowledge of it through your other peers, then it's simply unreachable. There is no "default router" that you simply send everything to in hopes that it will do the right thing.
When an ISP becomes large enough that tier 1 ISPs are sending a big portion of their traffic to said ISP, then peering agreements will normally happen. Peering is really motivated by two factors: money, and traffic efficiency. Often times two ISPs will notice that they talk between each other quite often, but their traffic has to travel half way around the world before it reaches the other. If the growing ISP is geographically disperse enough, they will be able to peer in many different exchanges, and pretty soon most of their traffic will no longer have to use a transit provider anymore. Anyone with enough muscle to get peering arranged with the big players, and their own infrastructure in all of these places, is probably a major service provider already.
That's a very quick rundown of how the Internet works. There are a lot more layers involved than we've implied, but this is the gist of how the Internet works. Come back next week to discover how the routing works between all of these separate entities. It's quite fascinating.
Article courtesy of Enterprise Networking Planet, originally published June 29, 2006
When he's not writing for Enterprise Networking Planet or riding his motorcycle, Charlie Schluting is the Associate Director of Computing Infrastructure at Portland State University. Charlie also operates OmniTraining.net, and recently finished Network Ninja, a must-read for every network engineer.