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I want Windows Wi-Fi client to detect wireless connection as metered connection. I found that 802.11 protocol supports vendor-specific information elements (IEs). Those IEs are injected into beacon and probe response wireless frames.

I found that Microsoft defined It's own vendor-specific IE to inform clients about metered connection over Wi-Fi (Network Cost Element) and my access point's hostapd supports vendor-specific features.

According to documentation for (Network Cost Element), I added vendor-specific field to hostapd on my OpenWRT router:

0xDD (Vendor-specific record)
0x08 (Record length : 8 bytes)
0x00 0x50 0xF2 (Vendor: Microsoft)
0x11 (OUI Type: Network Cost)
0x00 0x00 0x00 0x02 (Portable Hotspot Default: Metered network; limit unknown or not yet reached; matches Windows default for mobile broadband connections.)

hostapd on openwrt commands:

ubus call hostapd.wlan24 set_vendor_elements '{"vendor_elements": "dd080050f21100000002"}'
ubus call hostapd.wlan24 update_beacon 

Using CommView, I capture wireless traffic and I see that in Probe Response and Beacon frames there is data I supplied to hostapd (last 10 bytes).

0x0000   80 00 00 00 FF FF FF FF-FF FF XX XX XX XX XX XX   €...ÿÿÿÿÿÿXXXXXX
0x0010   EC 41 18 50 A7 35 00 8E-99 81 50 23 98 00 00 00   ìA.P§5.Ž™.P#˜...
0x0020   64 00 31 04 00 1B YY YY-YY YY YY YY YY YY YY YY   d.1...YYYYYYYYYY
0x0030   YY YY YY YY YY YY YY YY-YY YY YY YY YY YY YY YY   YYYYYYYYYYYYYYYY
0x0040   XX 01 08 82 84 8B 96 0C-12 18 24 03 01 0B 05 04   Y..‚„‹–...$.....
0x0050   00 02 00 00 2A 01 00 32-04 30 48 60 6C 30 14 01   ....*..2.0H`l0..
0x0060   00 00 0F AC 04 01 00 00-0F AC 04 01 00 00 0F AC   ...¬.....¬.....¬
0x0070   02 0C 00 0B 05 04 00 08-00 00 3B 02 51 00 2D 1A   ..........;.Q.-.
0x0080   EC 01 17 FF FF 00 00 00-00 00 00 00 00 00 00 01   ì..ÿÿ...........
0x0090   00 00 00 00 00 00 00 00-00 00 3D 16 0B 00 04 00   ..........=.....
0x00A0   00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00   ................
0x00B0   00 00 7F 08 04 00 00 00-00 00 01 40 DD 18 00 50   ...........@Ý..P
0x00C0   F2 02 01 01 01 00 03 A4-00 00 27 A4 00 00 42 43   ò......¤..'¤..BC
0x00D0   5E 00 62 32 2F 00 DD 08-00 50 F2 11 00 00 00 02   ^.b2/.Ý..Pò.....

I connect to this AP and check if connection is metered using MS Powershell on Windows PC:

[void][Windows.Networking.Connectivity.NetworkInformation, Windows, ContentType = WindowsRuntime]
[Windows.Networking.Connectivity.NetworkInformation]::GetInternetConnectionProfile().GetConnectionCost()

And get response

ApproachingDataLimit          : False
NetworkCostType               : Unrestricted
OverDataLimit                 : False
Roaming                       : False
BackgroundDataUsageRestricted : False

Where NetworkCostType is either Unrestricted or Unknown instead of expected Fixed.

When I manually set on Windows PC this connection as metered, powershell says NetworkCostType : Fixed


Some time later I found another AP (not mine, hidden) that is broadcasting beacons. In those beacons, I found that same vendor-specific extension 0x11 by Microsoft is 16 bytes length and is

DD 10 00 50 F2 11 | 01 10 02 28 2C 30 00 00 00 01 00 00
or
DD 10 00 50 F2 11 | 01 10 02 28 2C 30 00 00 00 00 00 00 (deffers 3rd byte from the end)

0x0000   80 00 00 00 FF FF FF FF-FF FF 62 45 BD 07 36 9A   €...ÿÿÿÿÿÿbE½.6š
0x0010   62 45 BD 07 36 9A 80 71-87 80 70 00 00 00 00 00   bE½.6š€q‡€p.....
0x0020   64 00 31 C6 00 00 DD 10-00 50 F2 11 01 10 02 28   d.1Æ..Ý..Pò....(
0x0030   2C 30 00 00 00 01 00 00-                          ,0......
---
0x0000   80 00 00 00 FF FF FF FF-FF FF 62 45 BD 07 36 9A   €...ÿÿÿÿÿÿbE½.6š
0x0010   62 45 BD 07 36 9A D0 B4-16 FE 38 03 00 00 00 00   bE½.6šÐ´.þ8.....
0x0020   64 00 31 C6 00 00 DD 10-00 50 F2 11 01 10 02 28   d.1Æ..Ý..Pò....(
0x0030   2C 30 00 00 00 00 00 00-                          ,0......

I found that OUI 0x11 and Vendor 0x00 0x50 0xF2 are same - maybe MS Docs are wrong? I've tried same on my access point with same result - data is in wireless frames, but Windows 10 does not sets connection as metered.

Any help? Maybe I made mistake somewhere? Maybe It is Wireless driver-dependent? Maybe MS have docs but did not implemented it in desktop systems?


Setting Metered connection manually is not the way I want to solve this problem.

1 Answer 1

3
  1. There is was a mistake in MS Docs: Correct packet is DD080050F211??00**00, where ?? is Cost Level and ** is Cost Flags

Cost Levels:

  • 00 Unknown - The connection cost is unknown.
  • 01 Unrestricted - The connection is unlimited and has unrestricted usage constraints.
  • 02 Fixed - Usage counts toward a fixed allotment of data which the user has already paid for (or agreed to pay for).
  • 04 Variable - The connection cost is on a per-byte basis.

Cost Flags:

  • 00 Unknown - The usage is unknown or unrestricted.
  • 01 Over Data Limit - Usage has exceeded the data limit of the metered network; different network costs or conditions might apply.
  • 02 Congested - The network operator is experiencing or expecting heavy load.
  • 04 Roaming - The tethering connection is roaming outside the provider's home network or affiliates.
  • 08 Approaching Data Limit - Usage is near the data limit of the metered network; different network costs or conditions might apply once the limit is reached.

Correct MS Docs MS-NCT


  1. Seems that when you manually trigger the Metered connection switch, Windows ignores 802.11 IE until you forget network (or maybe not).

For OpenWRT access point my epic issue resulting in 4 commands

uci add_list wireless.radio0.hostapd_options='vendor_elements=DD080050F21102000200'
uci add_list wireless.radio1.hostapd_options='vendor_elements=DD080050F21102000200'
uci commit
reboot

I've added Fixed cost level with Congested flag. Don't know if Congested affects anything.

hostapd_options is undocumented parameter in /etc/config/wireless (or uci radio?.hostapd_options), but it exists since 2017

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