YouTube goesfor two hours. Is the culprit a complete system failure or a sophisticated denial of service attack?
No. It's a single ISP in Pakistan trying to block access to YouTube in that country. The Pakistan government ordered access to YouTube shut down in that country after cartoons appeared on the site that some Muslims found offensive. Presumably by accident, the ISP took out YouTube everywhere.
On Sunday afternoon, YouTube was inaccessible for two hours. The company said that a network in Pakistan was to blame and that it was investigating.
There's an interesting blog by the people at Arbor Networks on how it might have happened.
Chief Research Officer at Arbor Networks , Danny McPherson guesses that there were two ways that ISP in question goofed and mistakenly started "announcing to the world that you provide destination reachability for the YouTube" IP address.
Either way, said McPherson in his blog post, an upstream provider almost certainly wasn't validating the ISP's prefix announcements. So the world is getting a message that the Pakistan-based ISP is providing access to YouTube.
"All the BGP speaking routers on the Internet believe Pakistan Telecom provides the best connectivity to YouTube," McPherson said. "The result is that you've not only taken YouTube offline within your little piece of the Internet, you've single-handedly taken YouTube completely off the Internet."