Autonomous System (Internet)

An autonomous system (AS) is a collection of IP networks that are managed as a unit and on a common internal routing protocol (IGP) (or more ) are connected. This network, in turn, may be composed of sub-networks. An AS is under a common management, typically from an Internet Service Provider (ISP ), an international company or a university. Autonomous systems are mutually connected to form the internet.

Management

Each autonomous system is assigned a unique AS number ( Autonomous System Number ASN ). It has a 16-bit integer; corresponding to 65536 possible AS. Public ASN, which may be used on the Internet, are in the range of 1 to 64511th private ASN, which may only be used within an organization and are intended for internal use, are in the range 64512-65535. At the moment over 37,000 numbers have been assigned. An extension to 32-bit ASN is planned and is supported by the RIPE NCC, among other things since the beginning of 2007. Since 1 January 2009 32 -bit AS numbers are assigned by default by RIPE NCC, but it is still possible to apply for ASN with a length of 16 bits ( ie from the area 1-64511 ).

The management of the ASN takes over the Internet Assigned Numbers Authority (IANA). This delegates the allocation on to the Regional Internet Registries (RIR ). These are ARIN ( North America ) RIPE NCC (Europe, Middle East and Central Asia), APNIC (Asia - Pacific), LACNIC (Latin America, the Caribbean) and AfriNIC (Africa). To obtain an AS number, an ISP has at least two other autonomous systems use a dynamic routing protocol (usually usefully BGP, but also others such as, for example, the precursor Protocol BGP (EGP ) or the theory as intra -domain routing protocol EIGRP usable are conceivable ). Be replaced with only one AS routes, this can be done via private AS numbers, via static routing or other solutions.

By dividing the Internet into autonomous systems better scalability by reducing the storage space and the demands for the transmission of necessary for routing information reaches ( Hierarchical routing ): There is no longer the network topology based on individual routers, but on the basis of networks is transmitted, reduced the amount of information dramatically.

Routing

For routing within an AS, the so-called intra -AS routing, the operator is responsible; for the inter-AS routing between autonomous systems, there are uniform standards. Inter -AS routing protocols are also called Exterior Gateway Protocols ( EGP). The only currently used worldwide EGP is the Border Gateway Protocol ( BGP). BGP is one of the so-called policy-based routing, which is further described below in a separate section.

Intra -AS routing protocols are also known as Interior Gateway Protocols ( IGP). Examples are the Routing Information Protocol (RIP), the Open Shortest Path First Protocol ( OSPF) or Intermediate System Intra- Domain to Intermediate System Routing Protocol (IS -IS).

Customers, peers, providers

In the inter-AS routing is typically distinguished ( at a meta - level) between customers, peers and providers:

• Another autonomous system is my customer ( "Customer "; " downstream" ), if he pays me money that he can have a direct line ( "Link" ) with me (and about me with the rest of the Internet ) to exchange data.
• Another autonomous system is inversely my provider ( "upstream" ), if I'm paying him money that I can exchange data via a direct line ( "Link" ) with him and the rest of the Internet.
• If two autonomous systems of similar size, important influential and well connected, so they can agree that they share the cost of direct lines to each other. In this case, there is neither customer nor provider, but we speak of equal peers. (This should not be confused with peers to peers of a peer-to- peer network. )
• The really big Internet providers, which have only customers and peers, but nowhere to assume the role of a customer, also referred to as Tier 1 provider. Autonomous systems that are exclusively clients of Tier 1 providers, also called Tier 2 providers. In general, the membership would define animal -n as the customers of animal ( n -1); usually such distinctions are not made ​​.
• In addition, there are also so-called sibling relationships between autonomous systems (English for siblings). It occurs, for example, when a company is taken over by another, but keep the networks of the two companies each have their own ASN.

The distinction between customers, providers and peers takes place only on a meta level - in the data transmitted by the routing protocol they reflected only indirectly, namely in particular in the definition of the routing policies.

Stub AS, transit AS, multihoming

Depending on whether an AS is a final or an intermediate node in the parent network, the following thin AS types:

• Stub AS are connected via a provider just a link to exactly ( end nodes ). Actually, there should not be stub AS, since, according to award criteria for AS at least two providers must be present.
• Multihomed stub AS connected for reasons of reliability through multiple links to exactly one provider ( end nodes ).
• Multihomed AS are connected for reasons of resiliency to multiple providers ( end nodes ).
• Transit AS are connected to other transit AS and represent the service provider for the above three types in the form of Internet backbone networks represent ( intermediate nodes ). A transit AS is thus always a provider for at least one other AS.

Policy -based inter-domain routing

The basic features of conventional policies for forwarding routing information can be summarized as follows:

• If an autonomous system my client, so I share it with all my routes, I know: I want to allow my customers to process as much as possible of its traffic over me because I earn money with it - usually the traffic between autonomous systems, namely by volume accounted for.
• If an autonomous system my provider, so I share it with the routes to my clients, my clients are reachable and I can make them. I do not share my provider but with the routes to my peers or even to my other providers: Otherwise, I would have to pay for the data transmitted over me more money to my provider, but would not earn more (with peers) or, even worse, would even pay twice ( also present at my second provider).
• The same applies to peers: I share a peer only routes to my customers so that my customers can be reached via the peer and I have to pay any money to my provider for these roads. However, I share my peer with no routes to my provider because it can exchange data at my expense, otherwise, without that I deserve it. Mostly I share my peer with no routes to my other peers, because otherwise it will be unnecessarily burdened my network without that I deserve it.

As you can clearly make easily, such a result, purely economically driven policy -based routing, mostly in ways that are technically not optimal. For example, could theoretically two routers with different providers to exchange data via a router with a common customer and would in this case only two hops away from each other - however, such a scenario for obvious reasons forbids; the customer will not allow such a routing, because he would thereby suffer massive financial losses.

Examples

Typically, ISPs, but also large international companies and some universities own AS numbers. Here are some examples of AS numbers: