Each ISP is responsible for running its own interior routing protocols and for exchanging routing information via the BGP exterior routing protocol at NAPs and MAEs and any other connection points between individual ISPs. In Figure 10.1, each of the ISPs are autonomous systems, as defined in next topic. The dispute was resolved, and they began accepting traffic from each other and the rest of the Internet again. The two ISPs had a peering agreement but one complained that the other was sending it more traffic than it should so it demanded payment and stopped accepting traffic, leaving large portions of the network isolated from the rest of the Internet. In October, 2005, an argument between two national ISPs shut down 45 million Web sites for a week. And of course, a local ISP will charge individuals like us for access! This is true to some extent, but higher-level ISPs normally charge lower-level ISPs to transmit their data (e.g., a national will charge a regional and a regional will charge a local). It is peering that makes the Internet work and has led to the belief that the Internet is free. Figure 10.1 shows several examples of peering. That is, a national tier 1 ISP does not charge another national tier 1 ISP to transmit its messages. In general, ISPs at the same level do not charge one another for transferring messages they exchange across a NAP or MAE. In this way, they are not dependent on just one higher-level ISP.

Regional and local ISPs often will have several connections into other national, regional, and local ISPs to provide backup connections in case one Internet connection fails. Indiana University, for example, which might be considered a local ISP because it provides Internet access for about 40,000 individuals, has a direct connection into the Chicago NAP, as do several other universities and large corporations. Today there are about 50 MAEs in the United States.īecause most NAPs, MAEs, and ISPs now are run by commercial firms, many of the early restrictions on who could connect to whom have been lifted. MAEs are smaller versions of NAPs and typically link a set of regional ISPs whose networks come together in major cities (Figure 10.1). As the number of ISPs grew, a new form of NAP called a metropolitan area exchange (MAE) emerged.

Regional ISPs, in turn, provide services to their customers and to local ISPs (sometimes called tier 3 ISPs) who sell Internet access to individuals. These regional ISPs rely on the national ISPs to transmit their messages to national ISPs in other countries. These national ISPs in turn provide services for their customers and also to regional ISPs (sometimes called tier 2 ISPs) such as Cogent Communications or France Telcom. NAPs were originally designed to connect only national ISPs. ISP = Internet service provider MAE = metropolitan area exchange NAP = network access point

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