A trip through wake-on-wireless-LAN

For several months now I’ve been struggling with an issue that showed up after I managed to set up Wake on Wireless LAN (WoWLAN) on my desktop computer, and I thought the whole process it would make for a great blog post, so here we go!

Chapter 1: got it to work!

Getting WoWLAN to work wasn’t particularly hard, it basically boiled down to two things:

  • Make sure the BIOS would allow it.
  • Configure the wireless NIC settings in Windows.

The first step was about looking for the appropriate settings in my BIOS, and setting them to the correct values. Some people might not be able to complete this if their motherboard/NIC/BIOS doesn’t support WoWLAN, and in that case there’s not much to be done other than changing hardware (or making sure it’s not just a missing BIOS update, which it probably isn’t). In my case, the only relevant setting (and maybe not even that, since I only use WoWLAN with state S3 (sleep), not S4 (hibernate) nor S5 (soft-off)) was S4/S5 Wake on LAN.

BIOS options

For the second step I went to Device Manager, double-clicked my wireless card under “Network Adapters”, and made sure that Wake on Magic Packet and Wake on Pattern Match were set to Enabled in the Advanced Settings tab; and that “Allow this device to wake up the computer” and “Only allow a magic packet to wake up the computer” were checked in the Power Management tab.

NIC settings

NIC settings - power management

And voilĂ ! I was immediately able to put my computer to sleep, and wake it up with a Wake-on-LAN packet sent through the WiFi.

Chapter 2: an issue shows up

Things were great until I noticed that my computer was waking up on its own every night after I went to bed and put it to sleep.

I first went to Windows’ Event Viewer and found this sequence of events (the first one has the wrong time because Windows still thinks it’s the same moment as when the computer went to sleep, and the second event fixes that by syncing the OS clock with the hardware clock):

Wakeup Event 1

Wakeup Event 2

And a couple of entries later, this one:

Wakeup Event 3

It was clear that the NIC was responsible for waking up the computer, and sure enough, if I disabled its “Allow this device to wake up the computer” setting in Device Manager, the problem went away. But that setting is needed for WoWLAN to work, so I started looking for a solution.

Playing around with the other settings in Device Manager didn’t help. Intel provides some documentation on those that was pretty useful. For obvious reasons, of particular interest were NS offloading for WoWLAN, ARP offloading for WoWLAN, GTK rekeying for WoWLAN, and Sleep on WoWLAN disconnect. The first two let the OS “delegate” some work to the NIC when it is sleeping, so that some things can happen without it waking up. They are enabled by default, and it sounds like that’s the way it should be. The documentation for GTK rekeying for WoWLAN is not clear on what it does, but some additional research shows that it’s related to the PMWiFiRekeyOffload standard keyword for power management, which says “A value that describes whether the device should be enabled to offload group temporal key (GTK) rekeying for wake-on-wireless-LAN (WOL) when the computer enters a sleep state.” So just like the previous two, we want that enabled.

Finally, I just can’t wrap my head around what Sleep on WoWLAN disconnect is. The documentation says “Sleep on WoWLAN Disconnect is the ability to put the device to sleep/drop connection when WoWLAN is disconnected.” but I don’t understand what “WoWLAN is disconnected” means. I think of WoWLAN as an event, not a persistent connection. So I didn’t really mess around with this one. Maybe it’s supposed to say “disabled” instead of “disconnected”, and it lets the NIC go to sleep if WoWLAN is disabled…

I don’t remember what else I did to try and fix this, but if there was anything else, it didn’t work. After a while, I resigned myself and didn’t even try to put my computer to sleep before bed.

Chapter 3: a second attempt

Some time later I came back to the issue and this time my research first led me to the powercfg utility.

powercfg /lastwake didn’t give me any new information, it also said that it was the NIC waking up the computer:

powercfg lastwake

powercfg /waketimers (which needs to run in an elevated command prompt) said there were no active wake timers on my system, so nothing to do there:

powercfg waketimers

Just to be sure, I also went through all the tasks in Task Scheduler, trying to figure out if a scheduled action was the culprit. A couple of them seemed like potential candidates but few of them could wake up the computer, and they were disabled or had schedules that didn’t match the symptoms I was seeing.

Chapter 4: found the root cause!

Fast forward another month or so, and I found a new clue: the wake up from sleep didn’t happen only during the night, the time of day didn’t matter! My computer is usually on all day, so I hadn’t noticed that before. But putting it to sleep at any time during the day resulted in the same wake-up-on-its-own behavior after some time. And more importantly, the computer always woke up on the 41st minute of the hour.

Knowing that, I did some more research and found this question in the Intel forums, with a superbly documented reddit post by someone having the exact same problem.

The author of that post did A LOT of research and troubleshooting, and found out that his issue was related to the Group Key Update feature of WPA2, and concluded that the GTK rekeying for WoWLAN setting in the NIC probably had a bug, since it should have offloaded handling of the appropriate network packets to the NIC, without having to wake up the computer.

I wanted to really soak up all the information there and make sure I understood what was happening, so I followed the research on that post and applied it to my scenario.

My starting point was this document from Microsoft regarding WoWLAN on Windows and which specific things can wake up the computer. Besides receiving a WOL packet or WOL magic pattern, 4 things can do that:

  • AP Association Lost: i.e. the NIC loses its connection to the AP. My AP wasn’t restarting or anything similar, so that couldn’t be it.
  • GTK Handshake Error: (here I had to go and research what “GTK” was. It’s not super relevant to this post, but here I found a great explanation) I’m not sure what could cause an error of this sort, probably something like changing the WiFi pre-shared key on the AP? I wasn’t seeing any errors in my AP/Router’s log, and besides the wake-up issue, my WiFi worked fine, so I guessed it was probably not this.
  • 802.1x EAP-Request/Identity Packet Received: this only applies to WPA2-Enterprise, and since I’m using WPA2-Personal, it couldn’t be it.
  • Four-way Handshake Request Received: thanks to all the reading I had done up to this point I knew that 4-way handshake is the process by which the AP and a wireless client establish keys (PTK and GTK) to encrypt the packets sent between them, and that my AP was configured to update the GTK every hour. And my computer was restarting every hour. So… We probably have a winner!

I confirmed that this is probably the culprit by changing the GTK rekeying interval (referred to in my settings as “Group Key Update”) in my router. After that, the minute when my computer woke up changed to match the time of the AP restart, so I’m pretty confident that this is it.

Chapter 5: …but it still doesn’t work

Yet, just like for that other person having this issue, having GTK rekeying for WoWLAN enabled wasn’t helping, so I’m inclined to agree that there’s a bug somewhere in Windows or the NIC driver.

Speaking of which… I looked for updates to my NIC driver, and there was one but it didn’t help things.

A workaround for those that can do this, is to increase the GTK rekey interval in the router. I was going to set it to 12 hours (at 9am/pm) so it didn’t happen while I was asleep, but my router only allows up to 2 hours.

Conclusion

So I’m still leaving my computer on when I go to bed because I know it will wake up on its own not long after. I’ll keep my eye out for updates to the NIC driver and see if they help.

In any case, I got a lot out of this ordeal. I learned about low-level details of WiFi connections like the Beacon Frame, the Beacon Interval and DTIM, plus some other things mentioned above. So even if the problem hasn’t gone away, trying to solve it has been a very productive endeavor.

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