Re: What's the difference between NdisMediumBare80211 (DLT_IEEE802_11) and NdisMediumRadio80211 (DLT_IEEE802_11_RADIO)

[Guy Harris <guy () alum mit edu>]

On Apr 6, 2016, at 5:41 PM, Yang Luo <hsluoyb () gmail com> wrote:

> I wonder why this mail went to my spam.. I don't know anything about radiotap header so I'm afraid i'm not supplying 
> it.

It's a way to provide "radio metadata" for packets; see

        http://www.radiotap.org

for a description of it.

If you were to implement that in the future, you'd get the "Media-Specific OOB Data for Received 802.11 Packets":

        https://msdn.microsoft.com/en-us/library/windows/hardware/ff559169(v=vs.85).aspx

in a DOT11_EXTSTA_RECV_CONTEXT structure:

        https://msdn.microsoft.com/en-us/library/windows/hardware/ff548626(v=vs.85).aspx

when you receive a packet.  Then you'd provide a link-layer header type of DLT_IEEE802_11_RADIO, and synthesize a 
radiotap header.  When you open the device, you'd have to fetch the device's data rate mapping table with the 
OID_DOT11_DATA_RATE_MAPPING_TABLE OID:

        https://msdn.microsoft.com/en-us/library/windows/hardware/ff569139(v=vs.85).aspx

        https://msdn.microsoft.com/en-us/library/windows/hardware/ff547679(v=vs.85).aspx

and associate that with the private data for the pcap_t.

Then, for each received packet:

   if DOT11_RECV_FLAG_RAW_PACKET_TIMESTAMP is set in uReceiveFlags, provide a radiotap TSFT field with the value from 
the ullTimestamp field of the structure;

   provide a radiotap Flags field with 0x10 set if the frame includes the FCS (you'll probably have to experiment a bit 
to see whether you get the FCS or not - the answer might differ for data and management frames, based on Network 
Monitor's behavior) and with 0x40 set if DOT11_RECV_FLAG_RAW_PACKET_FCS_FAILURE is set in uReceiveFlags;

   provide a radiotap Rate field whose value is the result of looking up the ucDataRate field's value in the data rate 
mapping table and returning the usDataRateValue value from that table - if it's not found, don't provide the Rate field;

   provide a radiotap Channel field where the frequency value is the uChCenterFrequency field of the structure and the 
flags are derived from the uChCenterFrequency and uPhyId fields of the structure - assuming that the uPhyId value is 
one of the ones from

        https://msdn.microsoft.com/en-us/library/windows/hardware/ff548741(v=vs.85).aspx

   then the mapping would be:

        dot11_phy_type_fhss - set 0x0800 in the flags (11 legacy FHSS);

        dot11_phy_type_ofdm - set 0x0040 in the flags (11a);

        dot11_phy_type_hrdsss - set 0x0020 in the flags (11b);

        dot11_phy_type_erp - set 0x0040 in the flags (11g, unknown whether it's pure or not);

   and, unless it's dot11_phy_type_irbaseband, set 0x0100 if the frequency is in the 5 GHz range or set 0x0080 if it's 
in the 2.4 GHz range;

   provide a radiotap Antenna signal field whose value is the value of the lRSSI field in the structure;

   if the phy is dot11_phy_type_ht, provide a radiotap MCS field where the known field is 0 and the other fields are 
also zeroed out (i.e., it's 11n, but we don't know anything else about it);

   if the phy is dot11_phy_type_vht, provide a radiotap VHT field where the known field is 0 and the other fields are 
also zeroed out (i.e., it's 11ac, but we don't know anything else about it).

> And I have confirmed that my captured packets are parsed well using NdisMediumBare80211. In Wireshark it shows "IEEE 
> 802.11 Data".

That means that you're just supplying packets that begin with an 802.11 header, with no form of radio information 
preceding it, so...

> So I think I will just use this value.

...that is exactly what you should be doing.
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