IPhones flooding wireless LAN at Duke University

The Wi-Fi connection on Apple’s recently released iPhone seems to be the source of a big headache for network administrators at Duke University.

The built-in 802.11b/g adapters on several iPhones periodically flood sections of the Durham, N.C., school’s pervasive wireless LAN with MAC address requests, temporarily knocking out anywhere from a dozen to 30 wireless access points at a time. The campus network staff is talking with Cisco, the main WLAN provider, and have opened a help desk ticket with Apple. But so far, the precise cause of the problem remains unknown. (UPDATE: Readers speculate on the cause)

“Because of the time of year for us, it’s not a severe problem,” says Kevin Miller, assistant director, communications infrastructure, with Duke’s Office of Information Technology. “But from late August through May, our wireless net is critical. My concern is how many students will be coming back in August with iPhones? It’s a pretty big annoyance, right now, with 20-30 access points signaling they’re down, and then coming back up a few minutes later. But in late August, this would be devastating.”

That’s because the misbehaving iPhones flood the access points with up to 18,000 address requests per second, nearly 10Mbps of bandwidth, and monopolizing the AP’s airtime.

The access points show up as “out of service.” For 10-15 minutes, there’s no way to communicate with them, Miller says. “When the problem occurs, we see dozens of access points in that condition,” Miller says. The network team began capturing wireless traffic for analysis and that’s when they discovered that the offending devices were iPhones. Right now, Miller says, there are about 150 of the Apple devices registered on the campus WLAN.

The requests are for what is, at least for Duke’s network, an invalid router address. Devices use the Address Resolution Protocol (ARP) to request the MAC address of the destination node, for which it already has the IP address. When it doesn’t get an answer, the iPhone just keeps asking.