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Update bgp.rst #203

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2 changes: 1 addition & 1 deletion book-2nd/protocols/bgp.rst
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Expand Up @@ -441,7 +441,7 @@ The domains on the Internet can be divided in about four categories according to

- the core of the Internet is composed of a dozen-twenty `Tier-1` ISPs. A `Tier-1` is a domain that has no `provider`. Such an ISP has `shared-cost` peering relationships with all other `Tier-1` ISPs and `provider->customer` relationships with smaller ISPs. Examples of `Tier-1` ISPs include sprint_, level3_ or opentransit_
- the `Tier-2` ISPs are national or continental ISPs that are customers of `Tier-1` ISPs. These `Tier-2` ISPs have smaller customers and `shared-cost` peering relationships with other `Tier-2` ISPs. Example of `Tier-2` ISPs include France Telecom, Belgacom, British Telecom, ...
- the `Tier-3` networks are either stub domains such as enterprise or campus networks networks and smaller ISPs. They are customers of Tier-1 and Tier-2 ISPs and have sometimes `shared-cost` peering relationships
- the `Tier-3` networks are either stub domains such as enterprise or campus networks and smaller ISPs. They are customers of Tier-1 and Tier-2 ISPs and have sometimes `shared-cost` peering relationships
- the large content providers that are managing large datacenters. These content providers are producing a growing fraction of the packets exchanged on the global Internet [ATLAS2009]_. Some of these content providers are customers of Tier-1 or Tier-2 ISPs, but they often try to establish `shared-cost` peering relationships, e.g. at IXPs, with many Tier-1 and Tier-2 ISPs.

Due to this organisation 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.
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