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What is an IXP?

Euro-IX has accepted the industry definition of an IXP as:

A physical network infrastructure operated by a single entity with the purpose to facilitate the exchange of Internet traffic between Autonomous Systems. The number of Autonomous Systems connected should at least be three and there must be a clear and open policy for others to join.

So what does this definition mean, and what purpose do IXPs have?

In order to provide access to the 'global Internet', an Internet Service Provider (ISP) must, of course, have connectivity to the global Internet itself. With the exception of a small number of very large 'Tier 1' ISPs, (see 'Tier Hierarchy' in the Glossary) ISPs generally buy Internet access (often called 'upstream transit') from one or more of the Tier 1 ISPs. Due to the size of these Tier 1 networks, and their comprehensive interconnection with other networks, they can send and deliver Internet traffic to any network connected to the Internet; i.e. they have connectivity to the global Internet. It is this global connectivity that other ISPs buy (these ISPs are usually referred to as 'Tier 2' ISPs), and in turn sell access to the global Internet to their customers.

Now, consider the situation where there is traffic destined to travel between a customer of one Tier 2 ISP to a customer of another Tier 2 ISP. Both ISPs have to pay their upstream transit providers to deliver and receive this traffic. This is still the case even where both ISPs use the same upstream provider. Clearly, it would reduce costs, and reduce the amount of different networks (the number of 'hops') the traffic has to traverse, if the two ISPs networks were directly connected to each other. This solution is viable if there is considerable traffic between the two networks. Whilst upstream costs are reduced there is a cost involved in providing physical connection between the two ISP's networks, and this must be considered when calculating the savings by having a direct interconnection. There are, however many thousands of ISP networks in Europe alone. It would not be cost effective, scaleable or manageable to interconnect with all of them individually.

Internet Exchange Points (IXPs) provide a solution to this. An IXP is a single physical network infrastructure, (often an Ethernet local area network) to which many ISPs can connect. Any ISP that is connected to the IXP can exchange traffic with any of the other ISPs connected to the IXP, using a single physical connection to the IXP, thus overcoming the scaleability problem of individual interconnections. Also, by enabling traffic to take a more direct route between many ISP networks, an IXP can improve the efficiency of the Internet, resulting in a better service for the end user. Furthermore, since many networks have more than one connection to the Internet, it is not unusual to find several routes to the same network available at an IXP, thus providing a certain amount of redundancy.

Exchange of Internet traffic is known as 'peering' (hence IXPs are sometimes called 'peering points' or 'public peering points', and individual interconnections as described above are known as 'private peerings'). IXPs are not, generally, involved in the peering agreements between connected ISPs; whom an ISP peers with, and the conditions of that peering, are a matter for the two ISPs involved. IXPs do however have requirements that an ISP must meet to connect to the IXP, more about the joining requirements of Euro-IX members can be found here. Also, since the physical network infrastructure is shared by all the connected ISPs, and activities of one ISP can potentially affect the other connected ISPs, all IXPs have rules for correct use of the IXP.

Most European IXPs are non-commercial co-operatives funded by membership fees paid by the connected ISPs, and are operated for the benefit of the member ISPs and the Internet community at large. The membership fees are usually very modest in comparison with the amount of potential saving of upstream transit costs. The actual saving will depend on the cost of membership (the range of membership fees is quite wide) and amount of traffic that can be exchanged in relation to the ISPs total traffic, but savings of 10 to 20 times the membership fees are not unusual.

Whilst the role of IXPs is often invisible to the end user, they form a very important part of the overall infrastructure of the Internet. It is hoped that these pages will widen the understanding of IXPs function, and encourage ISPs to take advantage of the benefits of connecting to IXPs.