ISP Tiers

Internet Service Provider 3-Tier Model

ISP 3-Tier Model

The global Internet is a collection of separate, but interconnected networks, each of which is managed as a single administrative domain called an Autonomous Systems (AS). There are over sixty thousand AS numbers (ASNs) assigned to a wide variety of businesses, educational, non-profit and government entities. The AS networks that form the primary transport for the Internet are independently controlled by Internet Service Providers (ISPs), each with its own business policies, internal network topologies, services, and customer profiles. The binding glue of the Internet is that all AS share a standard Internet Protocol (IP) addressing scheme and global Border Gateway Protocol (BGP) routing framework that allows all these networks to interconnect with each other directly or indirectly.

ISPs provide transport of Internet traffic on behalf of other ISPs, businesses or other non-ISP organizations, and individuals. ISPs are classified into a 3-tier model that categorizes them depending on the type of Internet services they provide.

Tier 1 ISPs are the networks that provide the backbone of the Internet. They are sometimes referred to as backbone Internet providers. These ISPs build infrastructure such as the Atlantic Internet sea cables. They provide traffic to all other ISPs, not end users. Tier 1 ISPs own and manage their operating infrastructure, including the routers and other intermediate devices (e.g., switches) that make up the Internet backbone. Key Tier 1 ISPs include AT&T, Verizon, Sprint, NTT, Singtel, PCCW, Telstra, Deutsche Telekom and British Telecom.

A Tier 1 ISP only exchanges Internet traffic with other Tier 1 providers on a non-commercial basis via private settlement-free peering interconnections. They also interconnect at Internet Exchange Points (IXPs). Tier 1 ISPs can deliver the best network throughput over the Internet backbone through these private peering connections because they own their network infrastructure and have direct control over how traffic flows through these connections.

Tier 1 networks support very high traffic volumes, large customer bases, with a large number of routers and are typically comprised of many Autonomous Systems (ASs). Key attributes of a global Tier 1 ISPs include:

  • They don't pay to have their traffic delivered though similar-sized networks.
  • They can deliver traffic to the entire Internet routing table solely through their peering relationships.
  • They peer on more than one continent.
  • They own or lease transoceanic fiber optic transport.
  • They deliver packets to and from customers and to and from peers around the world.

A Tier 2 ISP is a service provider that utilizes a combination of paid transit via Tier 1 ISPs and peering with other Tier 2 ISPs to deliver Internet traffic to end customers through Tier 3 ISPs. Tier 2 ISPs are typically regional or national providers. Only a few Tier 2 ISPs can provide service to customers on more than two continents. Often, they have slower access speeds than Tier 1 ISPs and are at least one router hop away from the backbone of the Internet.

A Tier 3 ISP is a provider that strictly purchases Internet transit. A Tier 3 provider is by definition primarily engaged in delivering Internet access to end customers. Tier 3 ISPs focus on local business and consumer markets. They provide the "on-ramp" or local access to the Internet for end customers, through cable, DSL, fiber or wireless access networks. Their coverage is limited to specific countries or sub regions, such as a metro area. Tier 3 ISPs utilize and pay higher-tier ISPs for access to the rest of the Internet.

Public, Private Peering, and Transit Connections

Because the Internet is made up of this complex interconnection of separate networks, rules are applied to consistently manage the technical and business aspects of these connections. There are three forms of interconnection rules: public peering, private peering, and transit.

There are two types of peering. Private peering is the direct connection between two ISPs over a Layer 1 or Layer 2 link with similar network capacity and traffic levels and is a bilateral agreement with traffic exchange dedicated exclusively to the two ISPs connecting. A large percentage of the Internet's traffic is exchanged via private Tier 1 peering agreements.

Public peering is the typical model for exchanging traffic between ISPs. It was originally accomplished at network access points (NAPs) primarily in the United States. This role is now complemented by about 250 commercial and non-profit Internet Exchange (IX) facilities around the world. NAPs and IXs facilitate the exchange of traffic over Layer 2 fabrics and may collocate dozens or even hundreds of ISPs across all three tiers in a single physical location.

Transit is a simpler form of interconnection where Tier 1 ISPs and some large Tier 2 ISPs sell dedicated access to their networks via private leased-line telecommunications circuits. Transit costs include both the circuit required for the ISP and the variable cost associated with the traffic carried upstream to the Internet.

ThousandEyes addresses many of the challenges associated with understanding ISP interconnection. To assure optimal performance of apps and services, organizations that deliver or consume services and apps over the Internet need detailed and accurate network path visibility, along with routing and application layer data.