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POInterpreting Frame Control bytes in 802.11 Wireshark trace
 

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  1. POInterpreting Frame Control bytes in 802.11 Wireshark trace
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    <p>I have a Wi-Fi capture (<code>.pcap</code>) that I'm analysing and have run across what appear to me to be inconsistencies between the 802.11 spec and Wireshark's interpretation of the data. Specifically what I'm trying to pull apart is the 2-byte 802.11 <em>Frame Control</em> field.</p> <p>Taken from <a href="http://www4.ncsu.edu/~aliu3/802.bmp" rel="noreferrer">http://www4.ncsu.edu/~aliu3/802.bmp</a>, the format of the Frame Control field's subfields are as follows:</p> <p><img src="https://i.stack.imgur.com/4E5DQ.png" alt="Frame control subfields."></p> <p>And below is a Wireshark screen cap of the packet that has me confused: </p> <p><img src="https://i.stack.imgur.com/SEPyr.png" alt="Confusing Frame Control in Wireshark"></p> <p>So as per the Wireshark screenshot, the flags portion (last 8 bits) of the Frame Control field is 0x22, which is fine. How the Version/Type/Subtype being <code>0x08</code> matches up with Wireshark's description of the frame is what has me confused. </p> <p><code>0x08</code> = <code>0000 1000b</code>, which I thought would translate to Version = <code>00</code>, Type = <code>00</code> (which I thought meant <em>management</em> not <em>data</em> frame) and Subtype = <code>1000</code> (which I thought would be a beacon frame). So I would expect this frame to be a management frame and more specifically, a beacon frame. Wireshark however reports it as a Data frame. The second thing that is confusing me is where Wireshark is even pulling <code>0x20</code> from in the line <code>Type/Subtype: Data (0x20)</code>. </p> <p>Can anyone clarify my interpretation of the 802.11 spec/Wireshark capture for me and why the two aren't consistent?</p>
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    1. COMy best guess is that this byte is interpreted in reverse, just like Ethernet frames are transmitted LSB first. Thus you'd get: subtype = `0000b`, type = `10b` / `0x2`, version = `00b`, and a reasonable way to represent type and subtype together would be `type << 4 & subtype`, i.e. `0x20` in this case. I'd be happy to have this confirmed or denied by someone knowledgeable, though.
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