Autonomous system (Internet)

In the Internet, an Autonomous System (AS) is a collection of connected IP routing prefixes under the control of one or more network operators that presents a common, clearly defined routing policy to the Internet, cf. RFC 1930, Section 3.

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 an officially registered Autonomous System Number (ASN).

A unique 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.

AS numbers were, until 2007, 16-bit integers, which allowed for a maximum of 65536 assignments. The Internet Assigned Numbers Authority (IANA) has designated ASN numbers 64512 through 65534 to be used for private purposes. The ASNs 0, 54272-64511, and 65535 are reserved by the IANA and should not be used in any routing environment. ASN 0 may be used to label non-routed networks. All other ASNs (1 - 54271) are subject to assignment by IANA, and, as of 2008-09-09, only 49152-54271 remained unassigned. RFC 4893 introduced 32-bit AS numbers, which IANA has begun to allocate. 32-bit AS numbers are written in the form x.y, where x and y are 16-bit numbers. 0.y AS numbers are exactly the old 16-bit AS numbers, 1.y numbers and 65535.65535 are reserved and the remainder of the space is available for allocation.

Assignment

AS numbers are assigned in blocks by the Internet Assigned Numbers Authority (IANA), which also allocates IP addresses, to Regional Internet Registries (RIRs). The appropriate 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 [http://www.iana.org/assignments/as-numbers Autonomous System Numbers] ] .

Types

Autonomous Systems can be grouped into three categories, depending on their connectivity and operating policy.

A "multihomed" Autonomous System 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" Autonomous System refers to an AS that is connected to only one other AS. This may be an apparent waste of an AS number if the network's routing policy is the same as its upstream AS's. However, the 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" Autonomous System is an AS that provides connections through itself to other 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".

ee also

* Internet Assigned Numbers Authority
* Regional Internet Registry
* Routing
* Routing Assets Database (RADB)
* Administrative distance

External links

* [http://www.apnic.net/services/asn_guide.html ASN Resource Guide]
* [http://logbud.com/visual_trace Traceroute with ASN detection]
* [http://www.asnumber.networx.ch AS Number plugin for Mozilla Firefox]
* [http://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_9-1/autonomous_system_numbers.html Exploring Autonomous System Numbers]
* [http://ietfreport.isoc.org/idref/draft-michaelson-4byte-as-representation/ Draft reports on 4-byte AS number representation]
* [http://www.menog.net/meetings/menog2/presentations/philip-smith-32bit-asn.pdf 32-bit ASN presentation at Doha, Qatar in 11-2007]

References