Some iPad Network Instabilities Narrowed Down to DHCP Bug
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We already knew that the iPad could be finicky when it comes to wireless connectivity. During the weekend, Princeton’s IT department added another entry to the connectivity laundry list by uncovering a problem with Apple’s implementation of the DHCP protocol in iPhone OS 3.2.
If you don’t know what DHCP is, it is a mechanism used to simplify the way devices connect to networks, and allows devices like the iPad to get automatically configured, even if they have no information about the network they are attempting to connect to. For instance, you likely use it at home every time your iPad connects to your wireless router, or when you connect your iPad to a public hot spot, like at Starbucks or at the airport.
Like with any other device, when your iPad connects, it is offered a temporary “right-of-way” called a “lease”, in order to use the network for a pre-defined amount of time. This lease is usually set to expire after 24 hours, and is automatically renewed as long as the device remains connected. That way, the network doesn’t get overloaded with “ghost” devices, and network resources can be re-assigned to other devices as they come. Problem is, Apple’s implementation of this protocol is flawed.
If by chance the lease happens to expire while the iPad is in sleep mode, the iPad forgets to renew it when it wakes up, like it should. Given that no renewal request is sent by the iPad, the network automatically assumes that the iPad is no longer connected, prunes it from the list of connected devices, and re-assigns the network resources to another device. From there, things start going downhill, the iPad still tries to use the network even if its right-of-way expired, and accidentally tries to steal the resources possibly assigned to another device.
Ultimately, for networks like Princeton’s, the iPads showing this behavior are simply denied further access. According to their IT department “When a DHCP client malfunctions this way repeatedly, Princeton blocks the device from using those campus network services which rely on the device’s DHCP client respecting lease times. These include our wireless services. We do this to protect other customers of those services from the disruptions caused by the malfunctioning devices.”
While a bit embarrassing given the fact that DHCP has been properly implemented with most Apple products for the last 15+ years, the issue is very easy to fix and will be corrected with the next iPhone OS update. Meanwhile, if you experience the problem, switching your iPad’s WiFi on and off a couple times should do the trick.
