Internet Overview: Tiers and ASs
The Internet is built by many different networks globally connected to each other. But how is Internet organized and technically implemented?
The Internet consists of Autonomous Systems (AS). An AS is a network domain administrated by a single entity, for example an operator, an Internet Service Provider (ISP), etc. Each AS has its own routing policies implemented internally and it operates independently from the other ASs. An AS is identified by an AS number (ASN) that is globally unique.
There are different interconnection agreements between the different ISPs (where each ISP usually is an AS domain). For example “peering” agreement means the two providers agree to a partnership in order to allow traffic to be exchanged between them for the purpose of expanding their Internet reachability. There is also the “transit” service which means that an operator will purchase network interconnection from a larger provider. Based on this setup, the different networks on the Internet are classified in “Tiers”, where Tier 1 is usually a large carrier network which only has peering agreements, Tier 2 is a network provider and has some peering agreements but also purchases transit services from others, and Tier 3 is a provider that only uses transit services in order to reach other networks on the Internet.
Different types of routing protocols are used to establish routing within an AS (intra-domain or interior gateway protocols) such as RIP, OSPF, EIGRP, IS-IS and routing between different ASs (inter-domain or exterior gateway protocols) such as BGP. A network domain can be connected to other ISPs either as single-homed (a single upstream connection) or multi-homed (multiple upstream connections). In the case of connecting to a single upstream provider there is no need to use public ASN, and private ASNs are usually used. The picture below shows an example interconnection setup on the Internet:
It is common that two ISPs use Service-Level Agreements (SLA) in order to agree upon terms and policies of their interconnection contract, for example bandwidth limitations, Quality-of-Service (QoS), etc.
In later posts, I will describe some of these interconnect concepts and routing protocols more in detail.