Why have some AOL users been having problems with MSN services?

AOL Time Warner on Wednesday said full service had been temporarily restored for Road Runner and America Online subscribers locked out of Microsoft websites, as more details emerged about a glitch that inconvenienced millions of surfers for nearly two days.

"In the interest of our mutual consumers, AOL has stepped forward and proactively restored network connectivity strictly on a short-term interim basis," AOL spokesman Nicholas Graham said. "We will continue to work with Microsoft and other providers toward a long-term solution."

Connectivity issues between MSN and AOL surfaced on Tuesday when some Road Runner users complained they could not access a number of Microsoft sites, such as Hotmail and MSN. A representative for US-based Road Runner said at that time the outage occurred when its technicians were updating routing tables and as Microsoft was doing routine maintenance. By Wednesday AOL confirmed that some of its members could not access MSN sites and that the problems were related.

Some readers of silicon.com affected by the problem were quick to predict foul play between MSN and AOL, who are major rivals. However, an AOL UK spokesman promised there have been "no dark plots... just a technical issue".

Regardless of who is to blame, the problems between AOL and Microsoft were not entirely coincidental, according to sources familiar with the problem.

A source close to Microsoft who asked to remain anonymous said the lockout stemmed from changes in a bandwidth ‘peering’ agreement between America Online and Microsoft.

Since the beginning of the internet, service providers have piggybacked off each others' networks to deliver data more efficiently between end points. Companies, including AOL and Microsoft, have long established such peering relationships to efficiently route traffic.

In this scenario, the most bandwidth-intensive link between AOL and Microsoft, which used a Level 3 connection as an intermediary, was disconnected between AOL and Level 3, according to the source. This re-routed high amounts of data to a smaller-bandwidth connection, essentially flooding the pipe and causing inaccessibility for many AOL and Microsoft users.

"I believe that the peering link that AOL had with Level 3 was solely used for MSN communication," the source familiar with the details wrote in an email. "When this link went down, so did two-thirds of their Hotmail accessibility." Hotmail is Microsoft's web-based email service.

A Microsoft memo recited to silicon.com sister site CNET News.com from a source close to the company said the software giant was in "negotiations with AOL over the matter", but did not elaborate on the nature of the talks.

AOL's Graham declined to comment on the source's statement. A Level 3 spokesman did not return requests for comment.

A Microsoft spokeswoman would not comment on the peering relationship but expressed satisfaction that service had been restored.

Microsoft on Wednesday also reiterated a statement saying it did not make any changes to its services or its network infrastructure in advance of the blackout, passing responsibility to cable-access provider Road Runner and its parent company, AOL Time Warner.

"We have no further information at this time regarding the cause of any access issues for specific ISPs but suggest that if customers are experiencing issues accessing MSN sites that they contact their ISP's customer service directly for further information," a Microsoft spokeswoman wrote in an email.

Jim Hu writes for CNET News.com.