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Only in 802.11ac

A quick look at the 3.4.x and main-branch code seems to indicate that if a frame manages to be identified as 11ac, the duration is always calculated. (Look for have_duration being set to TRUE.)

If somebody has an 11ac capture where the 802.11 radio information shows the PHY as 11ac:

PHY type: 802.11ac (VHT)

and there's no duration information, please file an issue on the Wireshark issue list and attach a capture file containing at least one frame where that's the case, so we can determine why that's happening.

Note that a routine used when calculating the duration for 11ac has a comment that says:

"NOTE: this is a crude quick hack, need proper calculation of bits/symbols/FEC/etc"

so the duration might not be as accurate as you might like.

For 11n, the code will not try to calculate the duration if:

• the MCS index is not present in the radio metadata;
• the bandwidth is not presnet in the radio metadata;
• the "short GI" indication is not present in the radio metadata;
• the data is invalid (meaning that values used as indices in various tables are out of the range for the table in question.

and maybe 802.11ax

There is currently no code to calculate the duration for 11ax, because nobody's written it.