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Interdomain routing
As explained earlier, the Internet is composed of more than 45,000 different networks [#fasnum]_ called `domains`. Each domain is composed of a group of routers and hosts that are managed by the same organization. Example domains include belnet_, sprint_, level3_, geant_, abilene_, cisco_ or google_ ...
Each domain contains a set of routers. From a routing point of view, these domains can be divided into two classes : the `transit` and the `stub` domains. A `stub` domain sends and receives packets whose source or destination are one of its own hosts. A `transit` domain is a domain that provides a transit service for other domains, i.e. the routers in this domain forward packets whose source and destination do not belong to the transit domain. As of this writing, about 85% of the domains in the Internet are stub domains [#fpotaroo]_. A `stub` domain that is connected to a single transit domain is called a `single-homed stub` (e.g., `S1` in the figure below.). A `multihomed stub` is a `stub` domain connected to two or more transit providers (e.g., `S2`).
Transit and stub domains
The stub domains can be further classified by considering whether they mainly send or receive packets. An `access-rich` stub domain is a domain that contains hosts that mainly receive packets. Typical examples include small ADSL- or cable modem-based Internet Service Providers or enterprise networks. On the other hand, a `content-rich` stub domain is a domain that mainly produces packets. Examples of `content-rich` stub domains include google_, yahoo_, microsoft_, facebook_ or content distribution networks such as akamai_ or limelight_. For the last few years, we have seen a rapid growth of these `content-rich` stub domains. Recent measurements [ATLAS2009]_ indicate that a growing fraction of all the packets exchanged on the Internet are produced in the data centers managed by these content providers.
Domains need to be interconnected to allow a host inside a domain to exchange IP packets with hosts located in other domains. From a physical perspective, domains can be interconnected in two different ways. The first solution is to directly connect a router belonging to the first domain with a router inside the second domain. Such links between domains are called private interdomain links or `private peering links`. In practice, for redundancy or performance reasons, distinct physical links are usually established between different routers in the two domains that are interconnected.
Interconnection of two domains via a private peering link
Such `private peering links` are useful when, for example, an enterprise or university network needs to be connected to its Internet Service Provider. However, some domains are connected to hundreds of other domains [#fasrank]_ . For some of these domains, using only private peering links would be too costly. A better solution to allow many domains to cheaply interconnect are the `Internet eXchange Points` (:term:`IXP`). An :term:`IXP` is usually some space in a data center that hosts routers belonging to different domains. A domain willing to exchange packets with other domains present at the :term:`IXP` installs one of its routers on the :term:`IXP` and connects it to other routers inside its own network. The IXP contains a Local Area Network to which all the participating routers are connected. When two domains that are present at the IXP wish [#fwish]_ to exchange packets, they simply use the Local Area Network. IXPs are very popular in Europe and many Internet Service Providers and Content providers are present in these IXPs.
Interconnection of two domains at an Internet eXchange Point
In the early days of the Internet, domains would simply exchange all the routes they know to allow a host inside one domain to reach any host in the global Internet. However, in today's highly commercial Internet, this is no longer true as interdomain routing mainly needs to take into account the economical relationships between the domains. Furthermore, while intradomain routing usually prefers some routes over others based on their technical merits (e.g. prefer route with the minimum number of hops, prefer route with the minimum delay, prefer high bandwidth routes over low bandwidth ones, etc) interdomain routing mainly deals with economical issues. For interdomain routing, the cost of using a route is often more important than the quality of the route measured by its delay or bandwidth.
There are different types of economical relationships that can exist between domains. Interdomain routing converts these relationships into peering relationships between domains that are connected via peering links.
The first category of peering relationship is the `customer->provider` relationship. Such a relationship is used when a customer domain pays an Internet Service Provider to be able to exchange packets with the global Internet over an interdomain link. A similar relationship is used when a small Internet Service Provider pays a larger Internet Service Provider to exchange packets with the global Internet.
A simple Internet with peering relationships
To understand the `customer->provider` relationship, let us consider the simple internetwork shown in the figure above. In this internetwork, `AS7` is a stub domain that is connected to one provider : `AS4`. The contract between `AS4` and `AS7` allows a host inside `AS7` to exchange packets with any host in the internetwork. To enable this exchange of packets, `AS7` must know a route towards any domain and all the domains of the internetwork must know a route via `AS4` that allows them to reach hosts inside `AS7`. From a routing perspective, the commercial contract between `AS7` and `AS4` leads to the following routes being exchanged :
over a `customer->provider` relationship, the `customer` domain advertises to its `provider` its own prefixes and all the routes that it has learned from its own customers.
over a `provider->customer` relationship, the `provider` advertises all the routes that it knows to its `customer`.
The second rule ensures that the customer domain receives a route towards all destinations that are reachable via its provider. The first rule allows the prefixes of the customer domain to be distributed throughout the Internet.

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../../protocols/bgp.rst:7
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locale/pot/protocols/bgp.pot, string 2