Networks of independent networks.
WHAT IT IS
Tens of thousands of independently operated networks, interconnected, agreeing to carry each other's traffic.
WHAT AN AUTONOMOUS SYSTEM IS
One such network, under a single administrative authority, with its own routing policy.
WHAT IDENTIFIES IT
An autonomous system number, allocated by a regional registry.
WHAT THE TIERS DESCRIBE
Networks reaching the whole internet without paying anyone for transit Networks buying some transit and peering where they can Networks buying transit only
WHAT THAT HIERARCHY IS NOT
Formal. It is a description of commercial position, not a governing structure.
WHAT NOBODY DOES
Control it centrally.
WHAT IS COORDINATED
Address allocation, autonomous system numbers, and the naming system, through recognised bodies.
WHAT DETERMINES HOW TRAFFIC FLOWS
Commercial agreements, expressed as routing policy.
WHY THAT MATTERS PRACTICALLY
The shortest path is not chosen. The commercially preferred path is.
WHAT THAT EXPLAINS
Traffic between two nearby networks travelling a long way, because no direct arrangement exists.
WHAT TO UNDERSTAND
Routing is policy before it is geography.
