Seems Pakistan Telecom blocked YouTube by hijacking its IP address and directing it to a so-called “black hole.” But when it sent that address out to the country’s Internet providers, it accidentally passed it on PCCW, one of Asia’s leading ISPs, as well. And PCCW propagated it to the rest of the world, and YouTube went down.

“It is exactly like the ‘game of telephone’ that kids play,” one network engineer explained: “For example, Pakistan Telecom says ‘I am responsible for 1.2.3.4 (some IP address)’ and then they tell PCCW. PCCW tells Verizon Business and NTT and others. NTT tells us, and so when my customers ask ‘Where is YouTube,’ we’re just answering based on what we’ve heard. … But all we know is that we heard it from NTT, who heard it from PCCW, who heard it from Pakistan Telecom. If Pakistan Telecom was lying (or made a mistake), we’d have no way to verify it.”

Apparently the transitive trust system on global routing is based can’t be trusted. “Whether accidental or not, the black-holing of YouTube by Pakistan Telecom demonstrates a serious weakness in the ‘longest prefix wins’ rule: There is no concept of trust contained in it,” Tomas Byrnes wrote in a message to the North American Network Operators Group mailing list. “Trust, whether implicit or explicit, is inherent in all human interactions, yet expressing it in cyberspace has continued to be troublesome. In routing decisions, once you are beyond a connected (either directly or multi-hop) peer, it becomes much more difficult.”