links. This problem can further affect others that peer with these two providers and also cause those ASs to prefer the misconfigured link. In reality, this problem hardly ever occurs with large ISPs, as these ISPs tend to restrict what an end customer can advertise. However, any ISP not filtering customer advertisements can allow errant information to be advertised into the global routing table where it can affect even the large Tier-1 providers. The concept of BGP hijacking revolves around locating an ISP that is not filtering advertisements (intentionally or otherwise) or locating an ISP whose internal or ISP-to-ISP BGP session is susceptible to a man-in-the-middle attack. Once located, an attacker can potentially advertise any prefix they want, causing some or all traffic to be diverted from the real source towards the attacker. This can be done either to overload the ISP the attacker has infiltrated, or to perform a DoS or impersonation attack on the entity whose prefix is being advertised. It is not uncommon for an attacker to cause serious outages, up to and including a complete loss of connectivity. In early 2008, at least eight US Universities had their traffic diverted to Indonesia for about 90 minutes one morning in an attack kept mostly quiet by those involved. Also, in February 2008, a large portion of YouTube's address space was redirected to Pakistan when the PTA decided to block access to the site from inside the country, but accidentally blackholed the route in the global BGP table. While filtering and MD5/TTL protection is already available for most BGP implementations (thus preventing the source of most attacks), the problem stems from the concept that ISPs rarely ever filter advertisements from other ISPs, as there is no common or efficient way to determine the list of permissible prefixes each AS can originate. The penalty for allowing errant information to be advertised can range from simple filtering by other/larger ISPs to a complete shutdown of the BGP session by the neighboring ISP (causing the two ISPs to cease peering), and repeated problems often end in permanent termination of all peering agreements. It is also noteworthy that even causing a major provider to block or shutdown a smaller, problematic provider, the global BGP table will often reconfigure and reroute the traffic through other available routes until all peers take action, or until the errant ISP fixes the problem at the source.