North Korea’s already tenuous links to the Internet went completely dark on Monday after days of instability, in what Internet monitors described as one of the worst North Korean network failures in years. The loss of service came just days after President Obama pledged that the United States would launch a “proportional response” to the recent attacks on Sony Pictures, which government officials have linked to North Korea. While an attack on North Korea’s networks was suspected, there was no definitive evidence of it. Doug Madory, the director of Internet analysis at Dyn Research, an Internet performance management company, said...