By this definition, a Tier 1 Network is a Transit-Free 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.
It is trivial to objectively prove (or disprove) a network is transit free. The fourteen (14) networks listed below, and only those fourteen, are transit free (as of July 2008). The most widely quoted source is Renesys Corporation http://renesys.com/, but the base information to prove the claim is publicly accessible from many locations, such as the RIPE RIS database http://www.ripe.net/ris/, the Oregon Route Views servers, the Packet Clearing House, and others.
It is impossible for an outside authority to confirm that a network is not paying settlements of any type because such business agreements are frequently not public information, or even covered under a Non-Disclosure Agreement. The information presented here is the best collective knowledge of the Internet peering community. There is little disagreement amongst the community itself, even though there is no quotable source for the information. (For clarity, here we will define the "peering community" as the set of peering coordinators for networks which are present at Internet Exchanges on at least two continents.)
It is commonly believed that observing this definition strictly would result in every network being disqualified. For instance, many large telephone companies who are also Tier 1 Networks buy, sell, or swap fiber amongst themselves. Even if it were possible to list every transaction, it is not possible to know if some of those transactions were required for or in payment of a peering connection.
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:
By definition, a Tier 1 network does not purchase IP transit from any other network or pay settlements to any other network to reach any other portion of the Internet. Therefore, in order to be a Tier 1, a network must peer with every other Tier 1 network. A new network cannot become a Tier 1 without the implicit approval of every other Tier 1 network, since any one network's refusal to peer with it will prevent the new network from being considered a Tier 1.
Tier 1 networks typically seek to protect their relatively rare status by preventing new networks from becoming Tier 1s and thus potentially competing. The networks often accomplish this by setting "peering requirements" which are intended to be too high for new networks to meet. Some experts in the field of Internet interconnections have compared the collective behaviors and motivations of Tier 1 networks to those of a cartel, in that they attempt to reduce competition in Internet bandwidth pricing through tacit collusion, and attempt to restrict the admission of new members. When one Tier 1 is perceived to be "cheating" the cartel by selling transit for too low a price, or by "dumping" too much outbound heavy bandwidth (which is significantly easier to deliver for the sending network than the receiving network), other members may move to de-peer that network.
It is important to remark here that Tier-2 (and lower) ISPs and their customers are normally unaffected by these partitions because they can have traffic with more than one tier-1 provider.
Because the "tier" ranking system is used in marketing and sales, a long-held though generally misguided view among customers is that they should "only purchase from a Tier 1". Because of this, many networks claim to be Tier 1 even though they are not, while honest networks may lose business to those who only wish to purchase from a Tier 1. The frequent misuse of the term has led to a corruption of the meaning, whereby almost every network claims to be a Tier 1 even though it is not. The issue is further complicated by the almost universal use of non-disclosure agreements among Tier 1 networks, which prevent the disclosure of details regarding their settlement-free interconnections.
Some of the incorrect measurements which are commonly cited include numbers of routers, route miles of fiber optic cable, or number of customers using a particular network. These are all valid ways to measure the size, scope, capacity, and importance of a network, but they have no direct relationship to Tier 1 status.
Another common area of debate is whether it is possible to become a Tier 1 through the purchase of "paid peering", or settlement-based interconnections, whereby a network "buys" the status of Tier 1 rather than achieving it through settlement-free means. While this may simulate the routing behaviors of a Tier 1 network, it does not simulate the financial or political peering motivations, and is thus considered by most Peering Coordinators to not be a true Tier 1 for most discussions.
A typical scenario for this behavior 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"s.
While the classification of "regional Tier 1" does hold 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.
The original Internet backbone was the ARPANET. It was replaced in 1989 by the NSFNet backbone. This was similar to a Tier 1 backbone. The Internet could be defined as anything able to send datagrams to this backbone.
When the Internet went private, a new network architecture based on decentralized routing (EGP/BGP) was developed. The Tier 1 ISPs and the peer connections made the NSFNet redundant and later obsolete. On April 30, 1995, the NSFNet backbone was shut down.
Currently, Tier 1 ISPs form the closest thing to a backbone.
The following 7 networks are believed to be Tier 1 Networks (i.e. they do not have an overt settlement on any peering link with any other network) by the overwhelming majority of the peering community:
|Name||AS Number||September, 2007 degree||Peering policy|
|AT&T||7018||1382||AT&T Peering policy|
|Global Crossing (GBLX)||3549||499||Global Crossing Peering policy (2003)|
|Level 3 Communications (L3)||3356|
|NTT Communications (Verio) (as2914 was originally TLGnet ; merged in 1999 after assets were bought)||2914||254||NTT Communications Routing Policy and Procedures|
|Qwest||209||828||North America; International|
|Verizon Business (formerly UUNET)||701||1452||Verizon UUNET Peering policy 701, 702, 703|
Most Tier 1 networks are headquartered in the United States, except for Global Crossing, which is headquartered in Hamilton, Bermuda, and NTT, which purchased the US network Verio to become a Tier 1 Network and is headquartered in Tokyo, Japan. (NTT is partially owned by the Japanese government.)
The following networks were Tier 1 Networks and may still be, but there is some question in the community as to whether they are now paying settlements to one or more of their peers:
|Name||AS Number||September, 2007 degree||Peering policy|
|AOL Transit Data Network (ATDN)||1668||ATDN Peering Policy|
The following networks are Transit-Free Networks, even though they have settlement based or paid peering with one or more other networks:
|Name||AS Number||September, 2007 degree||Settlement Peer|
|Cogent Communications||174||Sprint/as1239 and possibly Level 3 Communications (L3)/as3356|
|Teleglobe (part of VSNL)||6453||Sprint/as1239|
|XO Communications||2828||Sprint/as1239 and Level 3 Communications (L3)/as3356|
Due to the marketing considerations mentioned above, many people mistakenly believe that other networks are Tier 1 when they are not. Because of this, many online resources and forums incorrectly list several non-qualifying networks as Tier 1. Below is a list of some of these Tier 2 networks which are often listed as Tier 1, along with their upstream providers: