CONTINUED:
YouTube back on in Pakistan

PCCW denied that it had been responsible for the outage, telling CNET.co.uk's sister site ZDNet.co.uk on Wednesday that the actions of a downstream customer had caused the outage.

"PCCW Global operates a large resilient global IP backbone network with many national service providers worldwide as customers," said an email statement sent to ZDNet.co.uk by a PCCW spokesperson. "One of [PCCW's] downstream customers erred in routing configuration in a manner that apparently affected YouTube traffic. PCCW Global acted swiftly to correct the problem by disconnecting the source network which allowed immediate traffic normalisation."

Researchers at security company Arbor Networks said YouTube could have been knocked out by ISPs in Pakistan announcing to a Border Gateway Protocol (BGP) community that it was blocking YouTube.

A Pakistani ISP could have statically routed YouTube IPs to a null or discard interface on its routers, according to a blog post by Arbor Networks chief research officer Danny McPherson. However, this could have resulted in redistribution of all configured static routes into sets of globally advertised BGP routes, redirecting all YouTube traffic to the null interface.

"The net-net is that you're announcing reachability to your upstream for 208.85.153.0/24 [YouTube IP addresses], and your upstream provider, who is obviously not validating your prefix announcements based on Regional Internet Registry (RIR) allocations or even Internet Routing Registry (IRR) objects, is conveying to the rest of the world, via the Border Gateway Protocol (BGP), that you, AS 17557 (PKTELECOM-AS-AP Pakistan Telecom), provide reachability for the Internet address space (prefix) that actually belongs to YouTube, AS 36561," wrote McPherson. "YouTube [becomes] unavailable because all the BGP speaking routers on the internet believe Pakistan Telecom provides the best connectivity to YouTube."

The PTA on Wednesday denied that it had caused the YouTube outage, insisting it had been caused by a "malfunction" elsewhere, the BBC reported.

"We are not hackers. Why would we do that?" Shahzada Alam Malik, head of the PTA, told the AP news service.