A tier 1 network is an Internet Protocol (IP) network that participates in the Internet solely via settlement-free interconnection, also known as settlement-free peering.
Although there is no authority that defines tiers of networks participating in the Internet, the most common definition of a tier 1 network is one that can reach every other network on the Internet without purchasing IP transit or paying settlements.
By this definition, a tier 1 network is a transit-free network that peers with every other tier-1 network. But not all transit-free networks are tier 1 networks. It is possible to become transit-free by paying for peering or agreeing to settlements.
The most widely quoted source for identifying tier 1 networks is Renesys Corporation, but the base information to prove the claim is publicly accessible from many locations, such as the RIPE RIS database, the Oregon Route Views servers, the Packet Clearing House, and others.
It is difficult to determine whether a network is paying settlements if the business agreements are not public information, or covered under a non-disclosure agreement. The Internet "peering community" is roughly the set of peering coordinators present at Internet exchanges on more than one continent. The subset representing "tier 1" networks is collectively understood, but not published as such.
Strictly observing this definition of "tier 1" would exclude every network. For instance, many large telephone companies are tier 1 networks, but they buy, sell, or swap fiber amongst themselves. Payments between companies are not all known, nor whether they cover peering connections.
As a result, the term "tier 1 network" is used in the industry to mean a network with no overt settlements. An overt settlement would be a monetary charge for the amount, direction, or type of traffic sent between networks.
Common definitions of tier 2 and tier 3 networks:
- Tier 2: A network that peers with some networks, but still purchases IP transit or pays settlements to reach at least some portion of the Internet.
- Tier 3: A network that solely purchases transit from other networks to reach the Internet.
The original Internet backbone was the ARPANET when it provided the routing between most participating networks. It was replaced in 1989 with the NSFNet backbone. The Internet could be defined as the collection of all networks connected and able to interchange Internet Protocol datagrams with this backbone.
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