Autonomous system (Internet)

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In the Internet, an autonomous system (AS) is a collection of IP networks and routers under the control of one entity (or sometimes more) that presents a common routing policy to the Internet. See RFC 1930 for additional detail on this updated definition.

Originally, the definition required control by a single entity, typically an Internet service provider or a very large organization with independent connections to multiple networks, that adhere to a single and clearly defined routing policy. See RFC 1771, the original definition (now obsolete) of the Border Gateway Protocol (BGP). The newer definition of RFC 1930 came into use because multiple organizations can run BGP using private AS numbers to an ISP that connects all those organizations to the Internet. Even though there are multiple autonomous systems supported by the ISP, the Internet only sees the routing policy of the ISP. That ISP must have a public, registered ASN.

A unique AS number (or ASN) is allocated to each AS for use in BGP routing. With BGP, AS numbers are important because the ASN uniquely identifies each network on the Internet. The IANA has reserved ASN numbers 64512 through 65534 to be used for private purposes. The ASNs 0 and 65535 are reserved by the IANA and should not be used in a live environment.

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[edit] Assignment

AS numbers are assigned by the IANA, which also allocate IP addresses, to regional internet registries (RIRs) in blocks. The local RIR then assigns an AS number to an entity from the block assigned by the IANA. Entities wishing to receive an ASN must complete the application process of their local RIR and be approved before being assigned an ASN. Current IANA ASN assignments can be found on their website[1].

AS numbers were, until 2007, 16-bit integers, which allow for a maximum of 65536 assignments. AS numbers are divided into two ranges. The first are public AS numbers, which may be used on the Internet and range from 1 to 64511. The second range, from 64512 to 65534, are known as private numbers, and can only be used internally within an organization.

The RIRs started to issue 32-bit AS numbers in early 2007. These numbers will be written using a number format of <base-ten representation of the upper 16 bits>.<base-ten representation of the lower 16 bits>. RFC 4893 specificies how to use 32-bit AS numbers in BGP.

[edit] Types

Autonomous Systems can be grouped into three categories, depending on their connections and operation.

A multihomed AS is an AS that maintains connections to more than one other AS. This allows the AS to remain connected to the internet in the event of a complete failure of one of their connections. However, this type of AS would not allow traffic from one AS to pass through on its way to another AS.

A stub AS refers to an AS that is only connected to only one other AS. This may be a waste of an AS number if the network's routing policy is the same as its upstream AS's. There is often more to Internet routing: the apparently-stub AS may in fact have peering with other autonomous systems that is not reflected in public route-view servers. Specific examples include private interconnections in the financial and transportation sectors.

A transit AS is an AS that provides connections through itself to separate networks. That is, network A can use network B, the transit AS, to connect to network C. ISPs are always transit ASs, because they provide connections from one network to another. The ISP is considered to be 'selling transit service' to the customer network, thus the term transit AS.

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[edit] References

  1. ^ Autonomous System Numbers