As the federal government struggled on Wednesday to explain the technical problems experienced by would-be users of the health exchange Web site, www.healthcare.gov, computer security specialists say they had ruled out a cyberattack known as a denial of service, or DDoS, attack. Those occur when attackers fire huge amounts of traffic at a Web site until it collapses under the load. Such attacks typically entail hundreds of millions of data requests to a site per second. The federal health care site experienced 4.7 million unique visitors in the first 24 hours.
