many regions of the network are being advertised by BGP, and how often is the information about various parts of the network changing?

  • Measured reachability. Rather than relying on BGP, one can measure Internet connectivity directly by attempting to communicate with a number of systems scattered throughout the network and reporting on one’s actual ability to exchange data.

In the following paragraphs, these metrics are used to examine some recent Internet events that most people consider exceptional and to compare them with those of September 11.

Operator Error

Network operators often joke that a single misplaced comma in an appropriate configuration file could take down the Internet. While that was certainly true in the late 1980s,1 operators today have well-defined procedures and methods for checking configurations before putting them into their networks. Furthermore, most operators employ systems to protect their network from configuration errors in other networks. However, operational errors do still occur from time to time, and some of these have major effects.

To illustrate how local errors can have global impact, let us consider an example from the Domain Name System (DNS)—a distributed database that keeps the name-to-address mappings for the Internet. If a Web browser needs to find the Internet address of the name <>, for example, the browser queries the DNS.

The DNS is a hierarchical database that makes heavy use of caching. To explain the process by simplifying somewhat, the way that a name such as <> is looked up in the DNS is as follows: the browser asks a local DNS server if it knows the name <>. If the local server knows the name, it returns the IP address for <>; if not, the server consults 1 of 13 root servers. The root servers act as query managers; though they rarely answer a query themselves, they tell the local server what DNS server it should consult to get the definitive answer about <>.

What makes the DNS work and keeps the root servers from being overwhelmed with queries is the system’s use of caching. Once a local


In the late 1980s, the Internet often suffered from so-called black-hole problems—routers misconfigured to erroneously report to other routers that they have the best possible route to every point on the Internet. A black hole effectively encourages all nearby routers to send all traffic to it and then discards all the incoming traffic as undeliverable.

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