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WorldCom's Internet Technical Support Centre hotline is advising customers that the company is experiencing interruptions and working on correcting the problem.
WorldCom has restored service to some customers and is working with its vendors to determine the outage's cause, said spokeswoman Jennifer Baker. An update to WorldCom's hotline still warns of network problems, and several Internet monitoring services continue to show significant back-ups between major ISPs and their connections to UUNet.
"I personally have never seen anything like this. This is a bad event that's occurring," said Tom Ohlsson, vice-president of marketing and business development for monitoring firm Matrix NetSystems.
The problem occurred suddenly, Ohlsson said: One minute, UUNet was fine; the next, massive delays hit the system.
"What we believe happened is that overnight WorldCom network engineers loaded new operating system software onto most or many of their border routers," he said.
Border routers connect one system to another, in this case, linking UUNet to other regional and national ISPs. UUNet is an Internet backbone, carrying traffic originating from a number of ISPs; Matrix NetSystems estimates that 65% of the world's Internet traffic flows over UUNet.
One indicator tracked by Matrix NetSystems is packet loss, a measure of how much data is being lost in transmission. UUNet's highest packet loss for the entire day was 0.04%. UUNet's packet loss rate suddenly shot up to 22% - a change consistent with Matrix NetSystems' hypothesis that new, buggy software was abruptly activated, Ohlsson said.
Baker declined to comment on whether new software has been installed, reiterating that WorldCom continues to work with its technicians and vendors to identify the outage's cause.