the provider-customer relationship

Routing on the Internet is shaped less by geography than by money. Two of the three main business relationships between networks are the provider-customer pair: a customer pays a provider for transit — the service of carrying the customer's traffic to and from the rest of the Internet. It is exactly like paying an ISP for your home connection, but at the scale of whole networks: a regional ISP pays a bigger backbone to reach everywhere it can't reach by itself.

These relationships drive what a network is willing to advertise, captured by a famous rule of thumb sometimes called the Gao-Rexford rules. A network advertises ALL the routes it knows to its customers (the customer is paying to reach everywhere, so give them everything). But to its providers and its settlement-free peers, a network advertises only its OWN routes and its CUSTOMERS' routes — never the routes it learned from another provider or peer. The reason is economic: re-advertising a provider's routes to a peer would mean carrying that traffic for free, doing unpaid work. This single asymmetry is what makes Internet money flow downhill and routes flow according to who pays whom.

A consequence is the valley-free property of well-behaved Internet paths: traffic goes UP from customer to provider, possibly ACROSS one peer link, then DOWN from provider to customer, but never up-then-down-then-up again (which would force someone to provide free transit). When this discipline breaks — usually by accident — you get a route leak, where a network mistakenly announces routes it shouldn't, and traffic floods through a network never meant to carry it.