Autonomous system

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An autonomous system ( short  AS ) is, according to the classical definition, a set of routers (the plurality of networks connect) with a common interior gateway protocol  (IGP) and common metrics to determine how packets within an AS provides be technical under a single Administration .

However, it is no longer uncommon to manage multiple  IGPs and multiple sets of metrics in one AS . An autonomous system is then a system that presents itself to other autonomous systems as if it only had a single internal routing plan in order to provide a constant picture of which goals (e.g. other networks) can be achieved by this system .

Autonomous systems are interconnected and thus form the Internet .

In addition, autonomous hardware agents ( autonomous driving , military robots , humanoid robots ) and software agents also belong to the autonomous systems.

administration

Each autonomous system is assigned a unique AS number ( Autonomous System Number , ASN). These are defined as 32-bit - integer ( RFC 6996 ; historically: 16 bits, RFC 1930 ). Private ASNs that are not intended for use on the Internet but only within an organization are in the range from 64512 to 65534 (FC00 hex … FFFE hex ) for 16-bit numbers or for 32-bit AS numbers 4200000000 to 4294967294 (FA56EA00 hex … FFFFFFFE hex ). At the end of August 2016, 54,969 AS numbers were assigned. The extension to 32-bit ASN has been completed and has also been supported by the RIPE NCC since 2007 . Since January 1, 2009, RIPE NCC has assigned 32-bit AS numbers as the standard, but it is still possible to apply for ASNs with a length of 16 bits (i.e. from the range 1-64511).

The Internet Assigned Numbers Authority (IANA) is responsible for managing the ASN . This delegates the allocation 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, Caribbean) and AfriNIC (Africa). In order to receive an AS number, an ISP must use a dynamic routing protocol with at least two other autonomous systems (usually BGP ; but also others such as the predecessor protocol of BGP ( EGP ) or theoretically also as Intradomain routing protocol usable EIGRP are conceivable). If routes are exchanged with only one AS, this can be done using private AS numbers, static routing or other solutions.

By dividing the Internet into autonomous systems, better scalability is achieved by reducing the storage space and the need to transmit the information necessary for routing ( hierarchical routing ): Since the network topology is no longer transmitted on the basis of individual routers, but rather on the basis of networks the amount of information required is clear.

Routing

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

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

Customers, peers, providers

In inter-AS routing, a distinction is typically made (on a meta level) between customers , peers and providers :

  • Another autonomous system is my customer (“customer”; “downstream”) if he pays me money to exchange data with me (and via me with the rest of the Internet) via a direct line (“link”) .
  • Conversely, another autonomous system is my provider (“upstream”) if I pay him money so that I can exchange data with him and the rest of the Internet via a direct line (“link”).
  • If two autonomous systems are of similar size, important, influential, and well connected, they can agree that they will share the costs of direct lines between themselves. In this case there is neither a customer nor a provider, but rather peers with equal rights . (These peers should not be confused with the peers of a peer-to-peer network.)
  • The very large Internet providers, which only have customers and peers, but do not assume the role of a customer anywhere, are also referred to as tier 1 providers . Autonomous systems that are exclusively customers of tier 1 providers are also called tier 2 providers. In general, belonging to tier- n could be defined as the customers of tier- (n-1) ; Usually, however, such distinctions are not made.
  • In addition, there are also so-called Sibling -relationships between autonomous systems (English for siblings ). For example, it occurs when one company is taken over by another, but the networks of the two companies each keep their own ASN.

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

Stub-AS, transit-AS, multihoming

Depending on whether an AS forms an end or an intermediate node in the higher-level network, a distinction is made between the following AS types:

  • Stub AS are connected to exactly one provider via exactly one link (end node). There should actually be no stub AS, since according to the allocation criteria for AS there must be at least two providers.
  • Dualhomed Stub-AS are connected to exactly one provider via more than one link.
  • Multihomed AS are connected to at least two different providers (end nodes) via several links for reasons of failure safety .
  • Transit ASs are connected to other Transit ASs and represent the service providers for the three types mentioned above in the form of Internet backbone networks (intermediate nodes ). A transit AS is therefore always a provider for at least one other AS.

Policy-based interdomain routing

The main features of common policies for forwarding routing information can be summarized as follows:

  • If an autonomous system is my customer, I tell them all of my routes that I know: I want to enable my customer to handle as much of their traffic through me because I earn money with it - normally the traffic between autonomous systems is slowed down Volume billed.
  • If an autonomous system is my provider, I tell them the routes to my customers so that my customers can be reached and I can earn money from them. I share my provider but not the routes to my peers or even to my other providers with: Otherwise, I would have for the transferred about me data more money to pay my provider, but would not earn more (with peers), or worse, would even pay twice (namely also to my second provider).
  • The same applies to peers: I only give a peer routes to my customers so that my customers can also be reached via the peer and I do not have to pay any money to my provider for these traffic routes. However, I do not tell my peer routes to my provider, as otherwise they can exchange data at my expense without me making any money. Most of the time, I do not give my peer any routes to my other peers, otherwise it will unnecessarily load my network without me making any profit.

Such a purely economically controlled policy-based routing usually results in routes that are technically not optimal. For example, two routers at different providers could theoretically exchange data via a router at a common customer and in this case would only be two hops apart - but such a scenario is out of the question: the customer will not allow such a route to be selected because he would suffer financial losses would.

Examples

Typically, ISPs, but also large international companies and some universities, have their own AS numbers. Examples:

BelWü AS553
DFN AS680
Deutsche Telekom AS3320
Vodafone AS3209
freenet.de AS5430
Wikimedia Foundation AS14907 (USA) or AS43821 (Europe)
University of Frankfurt AS20633

Web links

Individual evidence

  1. ↑ a b RFC 4271 : A Border Gateway Protocol 4 (BGP-4) (English, January 2006)
  2. ↑ RFC 1930 : Guidelines for creation, selection, and registration of an Autonomous System (AS) (English, March 1996)
  3. ↑ Tony Bates, Philip Smith, Geoff Huston. " CIDR report ( Memento from May 28, 2013 in the Internet Archive )". Retrieved August 26, 2016, archived at.
  4. ^ RIPE NCC : English announcement of RIPE NCC
  5. ↑ Nick Feamster: Multihoming and Multi-Path Routing. Retrieved November 23, 2016 .