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Tier 1 network

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A Tier 1 network is an IP network (typically but not necessarily an Internet Service Provider) which connects to the entire Internet solely via Settlement Free Interconnection, commonly known as peering. Another name for a Tier 1 network is "transit-free", because it does not receive a full transit table from any other network.

Although there is no formal definition of the "Internet Tier hierarchy", the generally accepted definition among networking professionals is:

  • Tier 1 - A network that can reach every other network on the Internet without purchasing IP transit.
  • Tier 2 - A network that peers with some networks, but still purchases IP transit to reach at least some portion of the Internet.
  • Tier 3 - A network that solely purchases transit from other networks to reach the Internet.

Politics

There are many reasons why networking professionals use the "Tier Hierarchy" to describe networks, but the most important one is better understanding of a particular network's political and economic motivations in relationship to how and with whom it peers.

By definition, a Tier 1 network does not purchase IP transit from 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.

Routing issues

Because a Tier 1 does not have any alternate transit paths, Internet traffic between any two Tier 1 networks is critically dependent on the peering relationship. If two Tier 1 networks arrive at an impasse and discontinue peering with each other (usually in a unilateral decision by one side), single-homed customers of each network will not be able to reach the customers of the other network. This effectively "partitions" the Internet, so that one portion cannot talk to another portion, which has happened several times during the history of the Internet. Those portions of the Internet typically remain partitioned until one side purchases transit (thus losing its "Tier 1" status), or until the collective pain of the outage and/or threat of litigation motivates the two networks to resume voluntary peering.

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.

Marketing issues

Because there is no formal definition or authoritative body which determines who is and is not a Tier 1, the term is often misused as a marketing slogan rather than an accurate technical description of a network. Frequent misconceptions of the "tier hierarchy" include:

  • Tier 1 networks are closer to the "center" of the Internet.
    • In reality, Tier 1 networks usually have only a small number of peers (typically only other Tier 1s and very large Tier 2s), 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 or content 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 would preserve their full 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 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" 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.

Global issues

See also: Internet Exchange Point

A common point of contention among people discussing Tier 1 networks is the concept of a "regional Tier 1". 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 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.

History

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.

List of Tier 1 IPv4 ISPs

The following 9 networks are the only Tier 1 ISPs:

Name AS Number September, 2007 degree[1][2] Peering policy
AOL Transit Data Network (ATDN) 1668 74 ATDN Peering policy
AT&T 7018 1382 AT&T Peering policy
Global Crossing (GBLX) 3549 499
Level 3 3356 753
Verizon Business (formerly UUNET) 701 1452 Verizon UUNET Peering policy 701, 702, 703
NTT Communications 2914 254
Qwest 209 828 North America; International
SAVVIS (AS3561 was originally C&W Worldwide network ; merged in 2004 after assets were bought) 3561 295 SAVVIS Peering Policy
Sprint 1239 880
TeliaSonera International Carrier 1299 508 [1]

Most Tier 1 networks are headquartered in the United States, except for Global Crossing, which is headquartered in Hamilton, Bermuda, and NTT, which is headquartered in Tokyo, Japan and is partially owned by the Japanese government.

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:

See also

Notes

  1. ^ CAIDA AS ranking
  2. ^ Visualizing Internet Topology at a Macroscopic Scale April 2005
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