Autonomous Systems (ASs) and IxPs

- Routing and Peering

[Rutgers University]: An Autonomous System (AS) is a collection of routers whose prefixes and routing policies are under common administrative control. This could be a network service provider, a large company, a university, a division of a company, or a group of companies. The AS represents a connected group of one or more blocks of IP addresses, called IP prefixes, that have been assigned to that organization and provides a single routing policy to systems outside the AS. An IP prefix is a group of IP addresses expressed in CIDR form (i.e., address/bits, such as 128.6.0.0/16). Autonomous Systems create a two-level hierarchy for routing in the Internet. Routing between Autonomous Systems (inter-AS routing) is external to the AS and allows one AS to send traffic to another AS. Note that most organizations do not interconect via autonomous systems but simply connect to a single ISP, which may be an autonomous system.

Routers within an AS use an Interior Gateway Protocol (IGP), which handles routing between nodes inside the AS. Common interior gateway protocols include RIP, OSPF, IS-IS, EIGRP, as well as some proprietary protocols such as IGRP. Routing within an Autonomous System (intra-AS routing) is internal to that AS and invisible to those outside it. The AS administrator decides what routing algorithm should run within it.

To get traffic from a host in one AS to a host in another AS, the autonomous systems need to be connected. Most ASes do not share a direct link with each other, in which case data traffic may be routed through the networks of other ASes that agree to carry the traffic. An Exterior Gateway Protocol (EGP) is a routing protocol that handles routing between Autonomous Systems (inter-AS routing). BGP version 4, the Border Gateway Protocol, is the standard EGP for inter-AS routing. At some point in the future, the Internet is expected to adopt IDRP, the OSI Inter-Domain Routing protocol.

The number of unique autonomous networks in the routing system of the Internet exceeded 5000 in 1999, 30,000 in late 2008, 35,000 in mid-2010, 42,000 in late 2012, and 54,000 in mid-2016 (List of Autonomous Numbers).

- IxPs and Autonomous Systems

 An Internet Exchange Point (IxP) is a network facility that enables the interconnection and exchange of Internet traffic between two independent Autonomous Systems. An IxP provides interconnection only for Autonomous Systems. An IxP does not require the Internet traffic passing between any pair of participating Autonomous Systems to pass through any third Autonomous System, nor does it alter or otherwise interfere with such traffic.

Generally speaking, there are three types of IXPs and IP connectivity: Private Network Interconnections (PI or PNI), Internet eXchange Point (IXP) and Regular IP Transit (IPT). “Quality” measured by: latency, available bandwith, and control - more recently we also consider mitigation against DDoS.

[More to come ...]