iPad Fails at Networking: WiFi DHCP Issues

This image described by iPad, ipad support, Photo-4

Anyone who has had WiFi problems with their ‚Äòmagic‚Äô device, or read the tech blogs lately, knows the iPad is beset with a number of woes in regard its connectivity abilities. 

Word came out last week that Princeton was threatening to block iPads from the school‚Äôs WiFi network. "These devices are continuing to use an IP address they have been leased well beyond the time they should," according to a note from Princeton's IT department. "This behavior causes a disruption on the campus network." 

Some iPads were using IP addresses after the DHCP lease had run out. 

So what's going on?

Like most internet-capable devices, iPads obtain an IP address using the Dynamic Host Configuration Protocol (DHCP) when they connect to a WiFi network. That address isn't the iPad's to keep, it comes with an ‚Äòexpiration date‚Äô. When the lease expires, typically after several hours, the device must stop using it. To avoid inconvenient network interruptions, systems will contact the DHCP server and renew the lease. 

This doesn't happen on the iPad, at least not when the screen is off. The problem is hat the DHCP server has usually given the same address to another system. 

Two systems using the same address will break the connection for one of them, or probably both.

As long as iPad is being used, there‚Äôs no problem: the iPad conforms to the DHCP protocol and renews its IP address as required. If the iPad is turned off, or WiFi is disabled, the device disassociates itself from the WiFi network and stops using the IP address. 

However, the iPad will keep the WiFi connection alive if the screen is only turned off. It does this whether the screen turns off automatically or if the user pushes the wake/sleep button. The lower-level IP stack also remains active, as the iPad responds to ping packets.

The problem is, with the screen only asleep, the iPad does not pay attention to the DHCP lease time. It keeps using the same address even after the expiration point. 

Waking up the iPad doesn't solve the problem, either. It just keeps using the address. 

At Princeton 41 iPads have been seen, and 22 have exhibited the problem, 8 to the point of being blocked from the network.

If you experience this problem, just turn off WiFi on the iPad for a few moments then turn it back on. The iPad will ask the DHCP server for a new address. 

There are several ways to avoid further problems:

On the server side, some DHCP servers have the option of pinging addresses to make sure they're not used before they're given out. Setting the DHCP lease time on the server to something very high will reduce the impact of the problem.

Another solution is to give the iPad a fixed address. This is done on the iPad in the network settings for a specific network, also on a DHCP server or home router. 

In the case of your home router, you associate the iPad's MAC address ("Airport ID" or "Wi-Fi Address" in AppleSpeak, found in Settings > General > About) with a fixed address. In both cases, make sure the fixed address is outside the range of addresses given out using DHCP. For instance, DHCP may give out addresses in the range x.x.x.2 – x.x.x.200. Then give the iPad address x.x.x.201. Be careful, this is not a cure-all and YMMV. You could munge your network settings. Be sure to change the network settings from a system other than the iPad itself.

What all this underlines is that there are obviously real WiFi issues with the device, and Apple needs to address them. Though the tumult over the problem has subsided somewhat, the issues remain.

via Ars Technica

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