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Autonomous system (Internet)

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Anautonomous system(AS) is a collection of connectedInternet Protocol(IP)routingprefixes under the control of one or more network operators on behalf of a single administrative entity or domain, that presents a common and clearly defined routing policy to the Internet.[1]Each AS is assigned anautonomous system number(ASN), for use inBorder Gateway Protocol(BGP) routing. Autonomous System Numbers are assigned toLocal Internet Registries(LIRs) and end-user organizations by their respectiveRegional Internet Registries(RIRs), which in turn receive blocks of ASNs for reassignment from theInternet Assigned Numbers Authority(IANA). The IANA also maintains a registry of ASNs which are reserved for private use (and should therefore not be announced to the global Internet).

Originally, the definition required control by a single entity, typically anInternet service provider(ISP) or a very large organization with independent connections to multiple networks, that adhered to a single and clearly defined routing policy.[2]In March 1996, the newer definition came into use because multiple organizations can runBGPusing private AS numbers to anISPthat connects all those organizations to the Internet. Even though there may be multiple autonomous systems supported by the ISP, the Internet only sees the routing policy of the ISP. That ISP must have an officially registered ASN.

Until 2007, AS numbers were defined as 16-bit integers, which allowed for a maximum of 65,536 assignments. Since then,[3]theIANAhas begun to also assign 32-bit AS numbers toregional Internet registries(RIRs). These numbers are written preferably as simple integers, in a notation referred to as "asplain",[4]ranging from 0 to 4,294,967,295 (hexadecimal0xFFFF FFFF). Or, alternatively, in the form called "asdot+" which looks likex.y,wherexandyare 16-bit numbers. Numbers of the form0.yare exactly the old 16-bit AS numbers. The special 16-bit ASN 23456 ( "AS_TRANS" )[5]was assigned by IANA as a placeholder for 32-bit ASN values for the case when 32-bit-ASN capable routers ( "new BGP speakers" ) send BGP messages to routers with older BGP software ( "old BGP speakers" ) which do not understand the new 32-bit ASNs.[6]

The first and last ASNs of the original 16-bit integers (0 and 65,535) and the last ASN of the 32-bit numbers (4,294,967,295) are reserved[7][8][9]and should not be used by operators; AS0 is used by all five RIRs to invalidate unallocated space.[10]ASNs 64,496 to 64,511 of the original 16-bit range and 65,536 to 65,551 of the 32-bit range are reserved for use in documentation.[11]ASNs 64,512 to 65,534 of the original 16-bit AS range, and 4,200,000,000 to 4,294,967,294 of the 32-bit range are reserved for Private Use.[12]

The number of unique autonomous networks in the routing system of the Internet exceeded 5,000 in 1999, 30,000 in late 2008, 35,000 in mid-2010, 42,000 in late 2012, 54,000 in mid-2016 and 60,000 in early 2018.[13] The number of allocated ASNs exceeded 100,000 as of March 2021.[14]

Assignment[edit]

AS numbers are assigned in blocks byInternet Assigned Numbers Authority(IANA) toregional Internet registries(RIRs). The appropriate RIR then assigns ASNs to entities within its designated area from the block assigned by IANA. Entities wishing to receive an ASN must complete the application process of their RIR, LIR or upstream service provider[15][16]and be approved before being assigned an ASN. Current IANA ASN assignments to RIRs can be found on the IANA website.[17]RIRs, as part ofNRO,can revoke AS numbers as part of theirInternet governanceabilities.[18]

There are other sources for more specific data:

ASN table[edit]

A complete table of available 16-bit and 32-bit ASN:[17]

Number Bits Description Reference
0 16 Reserved forRPKIunallocated space invalidation[19] RFC6483,RFC7607
1–23455 16 Public ASNs
23456 16 Reserved for AS Pool Transition RFC6793
23457–64495 16 Public ASNs
64496–64511 16 Reserved for use in documentation and sample code RFC5398
64512–65534 16 Reserved for private use RFC1930,RFC6996
65535 16 Reserved RFC7300
65536–65551 32 Reserved for use in documentation and sample code RFC5398,RFC6793
65552–131071 32 Reserved
131072–4199999999 32 Public 32-bit ASNs
4200000000–4294967294 32 Reserved for private use RFC6996
4294967295 32 Reserved RFC7300

Types[edit]

Autonomous systems (AS) can be grouped into four categories, depending on their connectivity and operating policy.

  1. multihomed: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, unlike a transit AS, this type of AS would not allow traffic from one AS to pass through on its way to another AS.
  2. stub: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 havepeeringwith other autonomous systems that is not reflected in publicroute-view servers.Specific examples include private interconnections in the financial and transportation sectors.
  3. transit:An AS that acts as a router between two ASes is called a transit. Since not all ASes are directly connected with every other AS, a transit AS carries data traffic between one AS to another AS to which it has links.[20]
  4. Internet Exchange Point(IX or IXP): A physical infrastructure through which ISPs orcontent delivery networks(CDNs) exchange Internet traffic between their networks (autonomous systems). These are often groups of local ISPs that band together to exchange data by splitting the costs of a local networking hub, avoiding the higher costs (and bandwidth charges) of a Transit AS. IXP ASNs are usually transparent. By having presence in an IXP, ASes shorten the transit path to other participating ASes, thereby reducingnetwork latencyand improvinground-trip delay.[20][21]

AS-SET[edit]

Autonomous systems can be included in one or more AS-SETs, for example AS-SET of RIPE NCC "AS-12655" has AS1, AS2 and AS3 as its members,[22]but AS1 is also included in other sets in ARIN (AS-INCAPSULA) and APNIC (AS-IMCL). Another AS-SET sources can be RADB, LEVEL3 (tier 1 networknow calledLumen Technologies) and also ARIN has ARIN-NONAUTH source of AS-SETs.[23]AS-SETs are created by network operators in anInternet Routing Registry(IRR), like other route objects, and can be included in other AS-SETs and even form cycles.[24][25]

AS-SET names usually start with "AS-", but can also have a hierarchical name. For example, the administrator of AS 64500 may create an AS-SET called "AS64500:AS-UPSTREAMS", to avoid conflict with other similarly named AS-SETs.[26]

AS-SETs are often used to simplify management of published routing policies. A routing policy is published in the IRR using "import" and "export" (or the newer "mp-import" and "mp-export" ) attributes, which each contain the source or destination AS number and the AS number imported or exported. Instead of single AS numbers, AS-SETs can be referenced in these attributes, which simplifies management of complex routing policies.

See also[edit]

References[edit]

  1. ^Hawkinson, John; Bates, Tony (March 1996).Guidelines for creation, selection, and registration of an Autonomous System (AS).IETF.sec. 3.doi:10.17487/RFC1930.RFC1930.
  2. ^Rekhter, Yakov; Li, Tony (March 1995).A Border Gateway Protocol 4 (BGP-4).IETF.doi:10.17487/RFC1771.RFC1771.(obsoleted byRFC4271)
  3. ^Vohra, Quaizar; Chen, Enke (May 2007).BGP Support for Four-octet AS Number Space.IETF.doi:10.17487/RFC4893.RFC4893.(obsoleted byRFC6793)
  4. ^Huston, Geoff; Michaelson, George (December 2008).Textual Representation of Autonomous System (AS) Numbers.IETF.doi:10.17487/RFC5396.RFC5396.
  5. ^Q. Vohra; E. Chen (December 2012).BGP Support for Four-Octet Autonomous System (AS) Number Space.IETF.doi:10.17487/RFC6793.RFC6793.
  6. ^"Using AS 23456: How BGP Uses Conversion or Truncation For Compatibility".2008-07-21. Archived fromthe originalon 2016-10-29.Retrieved2018-12-31.
  7. ^G. Huston; G. Michaelson (February 2012).Validation of Route Origination Using the Resource Certificate Public Key Infrastructure (PKI) and Route Origin Authorizations (ROAs).IETF.doi:10.17487/RFC6483.RFC6483.
  8. ^J. Haas; J. Mitchell (July 2014).Reservation of Last Autonomous System (AS) Numbers.IETF.doi:10.17487/RFC7300.ISSN2070-1721.BCP 6.RFC7300.
  9. ^W. Kumari; R. Bush; H. Schiller; K. Patel (August 2015).Codification of AS 0 Processing.IETF.doi:10.17487/RFC7607.RFC7607.
  10. ^"IRR explorer".irrexplorer.dashcare.nl.Retrieved2021-10-24.
  11. ^G. Huston (December 2008).Autonomous System (AS) Number Reservation for Documentation Use.Network Working Group.doi:10.17487/RFC5398.RFC5398.
  12. ^J. Mitchell (July 2013).Autonomous System (AS) Reservation for Private Use.IETF.doi:10.17487/RFC6996.ISSN2070-1721.BCP 6.RFC6996.
  13. ^Bates, Tony; Smith, Philip; Huston, Geoff."CIDR report".Retrieved2018-12-31.
  14. ^"World - Autonomous System Number statistics - Sorted by number".Regional Internet Registries Statistics.Retrieved2018-12-31.
  15. ^"How to setup a LIR".www.afrinic.net.Retrieved23 March2020.
  16. ^Mar 2017, Publication date: 14."Autonomous System (AS) Number Assignment Policies".RIPE Network Coordination Centre.Retrieved23 March2020.{{cite web}}:CS1 maint: numeric names: authors list (link)
  17. ^ab"Autonomous System (AS) Numbers".IANA.org.2018-12-07.Retrieved2018-12-31.
  18. ^"Revocation of Internet Resources allocation".www.lacnic.net.Retrieved2021-02-17.
  19. ^"Policy prop-132 (AS0 for unallocated space) deployed in service".APNIC Blog.2020-09-02.Retrieved2020-09-12.
  20. ^abKrzyzanowski, Paul (21 March 2016)."Understanding Autonomous Systems: Routing and Peering".Rutgers University,Department of Computer Science.Archivedfrom the original on 3 March 2022.Retrieved29 March2022.
  21. ^"Glossary: Internet exchange point (IXP)".CloudFlare.Archivedfrom the original on 30 March 2022.Retrieved29 March2022.
  22. ^"AS-12655 - bgp.he.net".bgp.he.net.Retrieved2021-10-24.
  23. ^"IRR explorer".irrexplorer.nlnog.net.Retrieved2022-01-30.
  24. ^"IRR explorer".irrexplorer.nlnog.net.Retrieved2022-01-30.
  25. ^"IDIDB - Объекты AS-SET".www.ididb.ru.Retrieved2021-10-24.
  26. ^"Descriptions of Primary Objects - RIPE Database Docs".Retrieved2023-04-05.

External links[edit]

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