radiotap.mcs.format erroneously = greenfield (1)

asked 2019-12-19 17:29:10 +0000

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Idiosyncrasy capturing WLAN frame Tx's between all of my AP's and all of my non-AP STA's with WireShark 3.0.6, 3.0.7 & now 3.2.0 with oldish MacBook Airport Extreme Broadcom 802.11b/a/g/n WLAN interface. None of the AP's & non-AP STA's (except possibly the MacBook itself) support Rx HT_GF preamble PPDU's.

All frames within all blocks of 2 or more unicast HT, QoS Data, A-MPDU frames, except the last frame of each block, have radiotap.mcs.format = greenfield (1) which is erroneous. The last frame of each block has it right: mixed (0).

Thus wlan_radio.11n.greenfield = True (erroneous) in all frames, including the last, of each block, & wlan_radio.preamble (which only appears in the 1st frame of each block) is 8 µs less than it would be for HT_MF preamble.

This does not happen with blocks of 1 frame (if that makes sense) i.e. wherein a single MPDU is nevertheless processed as an A-MPDU.

Just curious if anybody knows if this misconstruance could be coming from the MacBook's Airport Extreme/Broadcom 802.11a/b/g/n WLAN interface?

answered 2019-12-19 18:17:13 +0000

Guy Harris
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Just curious if anybody knows if this misconstruance could be coming from the MacBook's Airport Extreme/Broadcom 802.11a/b/g/n WLAN interface?

It's not coming from Wireshark - Wireshark doesn't construct the radiotap header, and neither does libpcap; it comes either from Apple's driver or from Apple's 802.11 kernel framework.

I'd suggest reporting that on Apple's Feedback Assistant - you'll need an Apple ID at minimum, and might have to sign up as an Apple Developer to use it.

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Ok. Thanks a million for that!

HT Greenfield ( 2019-12-19 19:59:07 +0000 )edit

Recap sent to Apple: Idiosyncrasy to report about capturing WLAN frames with WireShark 3.0.6, 3.0.7 & now 3.2.0 on this MacBook. This only applies to blocks of 2 or more unicast, HT, QoS Data, A-MPDU frames of HT_MF (non-HT_GF) PPDU’s: “radiotap.mcs.format” falsely = “greenfield (1)” in all frames except the last frame of each block. Only the last frame of each block has it right: “mixed (0).” Thus “wlan_radio.11n.greenfield” falsely = “True” in all frames, INCLUDING the last, of each block, & “wlan_radio.preamble” (which only appears in the 1st frame of each block) falsely = the HT_GF preamble length that would otherwise apply instead of the actual longer HT_MF. “radiotap.dbm_antsignal” is missing from all except the last frame of each block. Thus “wlan_radio.signal_dbm” & “wlan_radio.snr” are also missing from all but the last frame of each block. I ...(more)

HT Greenfield ( 2019-12-20 07:12:47 +0000 )edit

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