Tier 1 network
||This article may contain original research. (July 2008)|
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.
When the Internet was opened to the commercial markets, and for-profit Internet backbone and access providers emerged, the network routing architecture was decentralized with new exterior routing protocols, in particular the Border Gateway Protocol. New tier 1 ISPs and their peering agreements supplanted the government-sponsored NSFNet, a program that was officially terminated on April 30, 1995.
Internet traffic between any two tier 1 networks is critically dependent on the peering relationship of the partners, because a tier 1 network does not have any alternate transit paths. If two tier 1 networks discontinue peering with each other, single-homed customers of each network will not be able to reach the customers of other networks. This effectively partitions the Internet and traffic between certain parts of the Internet is interrupted. This has happened several times during the history of the Internet. Those portions of the Internet typically remain partitioned until one side purchases transit, or until the collective pain of the outage or threat of litigation motivates the two networks to resume voluntary peering.[original research?]
Lower tier ISPs and their customers may be unaffected by these partitions because they may have redundant interconnections with more than one tier-1 provider.
The term tier 1 is often misused as a marketing slogan, rather than being an accurate technical description of a network, because there is no formal definition or authoritative body which determines who is and is not a tier 1 network. Frequent misconceptions of the tier hierarchy include:
- Tier 1 networks are closer to the backbone of the Internet.
- In reality, tier 1 networks usually have only a small number of peers (typically only other tier 1 networks and very large tier 2 networks), while tier 2 networks are motivated to peer with many other tier 2 and end-user networks. Thus a tier 2 Network with good peering is frequently much closer to most end users than a tier 1.
- Tier 1 networks by definition offer better quality Internet connectivity.
- By definition, there are networks which tier 1 networks have only one path to, and if they lose that path, they have no backup transit which preserves their continuous connectivity.
- Some tier 2 networks are significantly larger than some tier 1 networks, and are often able to provide more or better connectivity.
- Tier 2 networks are resellers of services from tier 1 networks.
- Only tier 3 networks (who provide Internet access) are true resellers, while many large tier 2 networks peer with the majority or even vast majority of the Internet directly except for a small portion of the Internet which is reached via a transit provider.
Because the tier-based 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 agreements. 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.
Regional tier 1 networks 
A common point of contention among people[who?] discussing 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.[original research?]
List of tier 1 networks 
These networks are believed to be tier 1 networks, in that they do not have overt settlements with any other network.
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 to being geographically regional players in North America and do not have a global network footprint especially when it comes to IP Transit services e.g. AT&T and CenturyLink. 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.
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|
Other major networks 
|Name||Headquarters||AS Number||August 2012 degree||Settlement Peer|
|Cogent Communications||USA||174||3537||Sprint/AS1239 and Level 3 Communications (L3)/AS3356|
|Hurricane Electric||USA||6939||2180||Global Crossing (GBLX)/AS3549 and TeliaSonera/AS1299 for IPv4—transit-free for IPv6|
|OpenTransit (France Telecom)||France||5511||146||Possibly Verizon Business/AS701 & Sprint Nextel Corporation/AS1239|
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:
- Allstream/AS15290 (Verizon Business/AS701 transit, AT&T/AS7018 transit, Level 3 Communications (L3)/AS3356 transit)
- British Telecom/AS5400 (Global Crossing (GBLX)/AS3549 transit, Level 3 Communications (L3)/AS3356 transit)
- Cable and Wireless/AS1273 (Level 3 Communications (L3)/AS3356, TeliaSonera/AS1299 transit)
- PCCWGlobal/AS3491 (Global Crossing (GBLX)/AS3549 transit)
- tw telecom/AS4323 (Sprint Nextel Corporation/AS1239 transit, paid peering with Level 3 Communications (L3)/AS3356)
See also 
- "How the 'Net works: an introduction to peering and transit: Page 4". 2008-09-02. Retrieved 2008-11-04. "Tier 1 networks are those networks that don't pay any other network for transit yet still can reach all networks connected to the internet."
- CAIDA AS ranking
- Visualizing Internet Topology at a Macroscopic Scale April 2005
- Markoff, John (2008-08-30). "Internet Traffic Begins to Bypass the U.S". The New York Times.