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The ‘net’ in ‘internet’ stands for network. So technically it’s an internetwork—a network of computer networks. Are you confused yet? When we discuss routing, we refer to these independent computer networks as autonomous systems. A single autonomous system routes packets internally, while packets that travel across the internet typically traverse many autonomous systems. Think about it: internet routing happens across autonomous systems, not single computers. Thanks to the Internet Assigned Numbers Authority (IANA), each AS gets their own unique 16 digit identification number or the ASN. Smaller networks like your house have much simpler interactions between the network and the internet. When you buy an internet service plan, your ISP (internet service provider) gives you a DSL or one of those old school cable modems, which allows you to access “the entire internet.” The only thing concerning the router is that on one side you have your local computers+devices, and on the other hand of the network link you have the entire internet. So Why Create Autonomous Systems?For mere mortals, that’s enough to explain how the internet works. But when you want to avoid being beholden to a single internet service provider or your internet connection is not as good as you need it; when you want to ‘expand your parameters’ of possibility, as they say, you create your own AS. Having your own AS can be beneficial for your network in several ways, including:
How to Create an Autonomous SystemsCreating an autonomous system is not that difficult and only takes a few steps. If you wish to create an autonomous system, here’s how you do it: Step 1: Found a company – A legal entity is required to create an AS, so start brainstorming on a business name. Step 2: Get a public address space for yourself – This might be the toughest step. You need to acquire a block of public IP address which will be large enough for advertising over BGP. Three are no more IPv4 addresses left, so you will have to buy an IPv6 address which can be pretty costly. Step 3: Find peers – The tricky part about the internet game is that you need to be interconnected with one part of it to be able to reach anything. If you peered with just a single other AS, there would be no need to run BGP. But if you did, you can just use a private autonomous system number which your upstream provider can easily replace with their own. Then they’ll pass your routes to the rest of the internet. Step 4: Get a router with the capability of handling the entire Internet routing table – This is one powerful router you can’t buy at your local depot. A solution would be to build a router by yourself out of a server that runs a router operating system. How Hackers Use ASAs a company grows and invests into its own AS, security concerns over your network and traffic start coming into play. You probably have loads of private corporate information which you want to keep private. Hackers hunt that information, and with enough skill, they can access your network, intercept your packets, and get remote access to all your computers to install some malicious code onto your server. It’s not hard to find own the owner of the IP range. Plenty of services give broad information about organizations such as WHOIS, CIDR, and so on. Knowing this info can help you identify connections between companies, figure out the attack surface, and run a nasty targeted DDoS attack. That’s where the cybersecurity market comes in. There exist tools which showcase vulnerabilities and help remove malicious software from your network. However, few of these tools focus on actually preventing attacks. Word on the market is that an up and coming cybersecurity company called Spyse is developing a solution based on the collection of mass data from the internet. Spyse uses this data to create a vast map of vulnerabilities of a network. This tool aims to help security specialists predict vulnerabilities, staying one step ahead of the hackers, and preventing potential threats to the system. Recently, Spyse has released several tools for security engineers, pentesters, sysadmins, and business analysts which are running in beta test mode. ASlookup is one of their most recent creations, which allows you to monitor your organization’s, network’s, or company’s infrastructure. The Spyse team knows that the best way to address threats is to prevent them beforehand; therefore, their services are aimed at helping you determine the attack surface and identify vulnerabilities before they’re exposed. Plus, they are giving away 3 free credits to all new users.
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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. Inter-AS RoutingA routing policy defines how routing information is exchanged between the ASes. For example, suppose two ASes, ASx and ASy, are connected to each other via a link between two gateway routers. Suppose that ASx knows how to reach some network, NET–1, that is defined by an IP prefix. This network may be within ASx or may be external to it. Suppose that ASy knows how to reach some other network, NET–2. For systems on NET–1 to be able to send messages to systems on NET–2, and vice versa, traffic will need to flow between ASx and ASy. This means that ASx needs to announce to ASy that it has a route to NET–1 and ASy needs to announce to ASx that it has a route to NET–2. The exterior gateway protocol is used to do this. ASx and ASy can then decide whether to accept this information or discard it (if, for example, they have better routes to those networks). AS assignmentAn AS has a globally unique 32-bit number associated with it[1], called an ASN (Autonomous System Number). If an Autonomous System exchanges routing information with other Autonomous Systems on the public Internet, it needs to have a unique ASN. This number is used in exchanging exterior routing information, particularly in identifying paths through multiple ASes via BGP, the Border Gateway Protocol. AS numbers are assigned to organizations by the Regional Internet Registry (RIR) for their country (e.g., ARIN for the U.S. and Canada, for example, for a $500 fee). Each RIR gets blocks of available AS numbers from the IANA (Internet Assigned Numbers Authority). The process is similar to that of assigning IP addresses. There are currently over 42,000 autonomous systems. The top-level list of IANA assignments of AS numbers can be fond at http://www.iana.org/assignments/as-numbers/as-numbers.xml An ASN is not needed for a network that has a single connection to an ISP and a single block of IP addresses (single prefix). The prefix of this network should be managed by the AS of the provider since the external routing policy of this network is the same as that of its provider. Autonomous systems may be connected with multiple links and one autonomous system will often be connected to several others for fault tolerance, to access different parts of the network, and to route traffic between these different parts. Each autonomous system can decide who they will exchange traffic with (that is, whether they will allow another AS to route traffic through it). Internet Tiers, Peering, and TransitSince most ASes are not connected with each other, they need to route their traffic through other ASes. Peering is when a pair of ASes establish a reciprocal agreement to connect with each other to exchange traffic with each other, without charge. The assumption that each has an interest in connecting to the other’s customers, similar to how postal systems throughout the world do not charge when mail is routed from one country to another. Tier 1 ISPs are those that do not have to pay any other network for transit. They peer with all other tier 1 networks (there are only about fourteen of these worldwide). Given any IP address, a Tier 1 ISP will be able to connect directly to a top-level ISP that can route to that address. The United States has eight interconnection regions[2] that create a “default free zone” where Tier 1 ISPs connect their networks together in peering relationships. Peering agreements are not necessarily transitive. If AS1 peers with AS2 and AS2 peers with AS3, AS2 is not necessarily obligated to carry traffic to AS3. Whether this is permitted or not is a business, rather than technical, decision. A Transit relationship is when an ISP (an AS) sells access to the Internet. It is when an AS agrees to act as a router, carrying traffic from one AS and out to some other AS to which it has a link. The complete data path may, of course involve multiple transit hops through different ASes. An AS will typically meter the traffic on each link and charge a transit fee. Depending on policy, an organization in one AS may be charged for traffic even to the connected AS. A Tier 2 ISP is one that needs to purchase Transit to connect to at least some part of the Internet. Because of transit fees, many Tier 2 ISPs will try to establish peering relations directly with as many Tier 1 and other Tier 2 ISPs as they can so they can exchange traffic with those ISPs for no fee (although, even in those cases, there may still be a peering fee — it’s a business decision). For example, it is common for cable and phone companies to peer with content providers such as Google, Amazon, and Microsoft. Internet BackboneThe Internet Backbone is the collection of major connections (routers and links) that connects large autonomous systems, typically Tier 1 networks, together. Like the Internet, it s a distributed infrastructure that is managed by numerous companies, universities, and other organizations. Categories of Autonomous SystemsAs with IP addresses, there are also ranges of AS numbers that are reserved for private use for organizations to use if they want to organize their internal systems into internal ASes and use BGP to compute routes between them. References
This document is an updated version of one that was originally written on April 5, 2013. |
What is the purpose of an autonomous system
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