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The Duration field is a derived value. My experience with monitor mode captures created on my macOS systems is that the meta data used to calculate the Duration is occasionally missing on some packets. When I filter for !wlan_radio.duration these packets have a PHY type of 802.11ac (VHT). A quick survey of a handful of monitor mode pcaps with a display filter of wlan_radio.phy == 8 (802.11ac) no Duration fields are generated.

The Duration field is a derived value. My experience with monitor mode captures created on my macOS systems is that the meta data used to calculate the Duration is occasionally missing on some packets. When I filter for !wlan_radio.duration these packets have a PHY type of 802.11ac (VHT). A quick survey of a handful of monitor mode pcaps with a display filter of wlan_radio.phy == 8 (802.11ac) shows that no Duration fields are generated.