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.
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Definition
There is no authority that defines tiers of networks participating in the Internet. However, the most common definition of a tier 1 network is a network 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 published by 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 when the business agreements are not public information, or are covered under a non-disclosure agreement. The Internet peering community is roughly the set of peering coordinators present at the Internet exchange points 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 networks 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 is 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 network: 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 network: A network that solely purchases transit from other networks to participate in the Internet.
History
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.
When the Internet was opened to the commercial markets, multiple for-profit Internet backbone and access providers emerged. The network routing architecture then became decentralized and attained a need for exterior routing protocols, in particular the Border Gateway Protocol emerged. New tier 1 ISPs and their peering agreements supplanted the government-sponsored NSFNet, a program that was officially terminated on April 30, 1995.
Routing
A bilateral private peering agreement typically involves a direct physical link between two partners. Traffic from one network to the other is then primarily routed through that direct link. A tier 1 network may have various such links to other tier 1 networks. Peering is founded on the principle of equality of traffic between the partners and as such disagreements may arise between partners in which usually one of the partners unilaterally disconnects the link in order to force the other into a payment scheme. Such disruptive de-peering has happened several times during the first decade of the 21st century. When this involves large-scale networks involving many millions of customers this may effectively partition a part of the Internet involving those carriers, especially if they decide to play dirty and disallow routing through alternate routes. Essentially then, this is not largely a technical issue but a matter of company politics in which a financial dispute is fought out using the other party's customers as hostages to obtain a better negotiating position. In the worst case, single-homed customers of each network will not be able to reach the other network at all. The de-peering party then hopes that the other network's customers will be hurt more by the decision than its own customers which may eventually conclude the negotiations in its favor.
Lower tier ISPs and other parties not involved in the dispute may be unaffected by such a partition as there exist typically multiple routes onto the same network. The disputes referenced have also typically involved transit-free peering in which one player only exchanged data with the other that involved each other's networks--there was no data transiting through the other's network destined for other parts of the Internet. By the strict definition of peering and the strict definition of a tier 1 network, a tier 1 network only peers with other tier 1 networks and has no transit routes going anywhere. More practically speaking, tier 1 networks serve as transit networks for lower tier networks and only peer with other tier 1 networks that offer the same services on an adequate scale--effectively being "peers" in the truest sense of the word.
More appropriately then, peering means the exchange of an equitable and fair amount of data-miles between two networks, agreements of which do not preclude any pay-for-transit contracts to exist between the very same parties. On the subject of routing, settlement-free peering involves conditions disallowing the abuse of the other's network by sending it traffic not destined for that network (i.e. intended for transit). Transit agreements however would typically cater for just such outbound packets. Tier 1 providers are more central to the Internet backbone and would only purchase transit from other tier 1 providers, while selling transit to providers of all tiers. Given their huge networks, tier 1 providers do not participate in public Internet Exchanges but rather sell transit services to such participants. In the most logical definition, a tier 1 provider will never pay for transit because the set of all tier 1 providers sells transit to all of the lower tier providers everywhere, and because (a) all tier 1 providers peer with every other tier 1 provider globally and (b) the peering agreement allows access to all of the transit customers, this means that (c) the tier 1 network contains all hosts everywhere that are connected to the global Internet. As such, by the peering agreement, all the customers of any tier 1 provider already have access to all the customers of all the other tier 1 providers without the tier 1 provider itself having to pay transit costs to the other networks. Effectively, the actual transit costs incurred by provider A on behalf of provider B are logically identical to the transit costs incurred by provider B on behalf of provider A -- hence there not being any payment required.
Regional tier 1 networks
A common point of contention regarding tier 1 networks is the concept of a regional tier 1 network. A regional tier 1 network is a network which is not transit free globally, but which maintains many of the classic behaviors and motivations of a tier 1 network within a specific region.
A typical scenario for this characteristic involves a network that was the incumbent telecommunications company in a specific country or region, usually tied to some level of government-supported monopoly. Within their specific countries or regions of origin, these networks maintain peering policies which mimic those of tier 1 networks (such as lack of openness to new peering relationships and having existing peering with every other major network in that region). However, this network may then extend to another country, region, or continent outside of its core region of operations, where it may purchase transit or peer openly like a tier 2 network.
A commonly cited example of these behaviors involves the incumbent carriers within Australia, who will not peer with new networks in Australia under any circumstances, but who will extend their networks to the United States and peer openly with many networks. Less extreme examples of much less restrictive peering requirements being set for regions in which a network peers, but does not sell services or have a significant market share, are relatively common among many networks, not just regional tier 1 networks.
While the classification regional tier 1 holds some merit for understanding the peering motivations of such a network within different regions, these networks do not meet the requirements of a true global tier 1 because they are not transit free globally.
List of tier 1 networks
These networks are recognised by the Internet community as tier 1 networks, even if some of them appear to have transit providers in CAIDA ranking.
While most of these Tier-1 providers offer global coverage (based on the published network map on their respective public websites), there are some which are restricted geographically. However these do offer global coverage for mobiles and IP-VPN type services which are unrelated to being a Tier-1 provider.
A 2008 report shows Internet traffic relying less on U.S. networks than previously.
Other major networks
Below is a list of some of these tier 2 networks which are often listed as tier 1, along with their upstream providers:
Source of the article : Wikipedia
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