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Based on these studies and [ATLAS2009]_, the AS-level Internet topology can be summarized as shown in the figure below.
The layered structure of the global Internet
The domains on the Internet can be divided in about four categories according to their role and their position in the AS-level topology.
Due to this organization of the Internet and due to the BGP decision process, most AS-level paths on the Internet have a length of 3-5 AS hops.
Footnotes
An analysis of the evolution of the number of domains on the global Internet during the last ten years may be found in http://www.potaroo.net/tools/asn32/.
Several web sites collect and analyze data about the evolution of BGP in the global Internet. http://bgp.potaroo.net provides lots of statistics and analyzes that are updated daily.
See http://as-rank.caida.org/ for an analysis of the interconnections between domains based on measurements collected in the global Internet.
Two routers that are attached to the same IXP exchange packets only when the owners of their domains have an economical incentive to exchange packets on this IXP. Usually, a router on an IXP is only able to exchange packets with a small fraction of the routers that are present on the same IXP.
See ftp://ftp.ripe.net/ripe/dbase for the RIPE database that contains the import and export policies of many European ISPs.
In this text, we consider Autonomous System and domain as synonyms. In practice, a domain may be divided into several Autonomous Systems, but we ignore this detail.
The BGP sessions and the underlying TCP connection are typically established by the routers when they boot based on information found in their configuration. The BGP sessions are rarely released, except if the corresponding peering link fails or one of the endpoints crashes or needs to be rebooted.
90 seconds is the default delay recommended by :rfc:`4271`. However, two BGP peers can negotiate a different timer during the establishment of their BGP session. Using a too small interval to detect BGP session failures is not recommended. BFD [KW2009]_ can be used to replace BGP's KEEPALIVE mechanism if fast detection of interdomain link failures is required.
A link is said to be flapping if it switches several times between an operational state and a disabled state within a short period of time. A router attached to such a link would need to frequently send routing messages.
Researchers such as [MUF+2007]_ have shown that modeling the Internet topology at the AS-level requires more than the `shared-cost` and `customer->provider` peering relationships. However, there is no publicly available model that goes beyond these classical peering relationships.
BGP data is often collected by establishing BGP sessions between Unix hosts running a BGP daemon and BGP routers in different ASes. The Unix hosts stores all BGP messages received and regular dumps of its BGP routing table. See http://www.routeviews.org, http://www.ripe.net/ris, http://bgp.potaroo.net or http://irl.cs.ucla.edu/topology/.

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../../protocols/bgp.rst:492
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a year ago
Source string age
a year ago
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locale/fr/LC_MESSAGES/protocols/bgp.po, string 142