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From: Richard Sharpe <>
Subject: Re: Would it be useful to have preferences for radiotap to allow handling of captures with conflicting presence flags?
Date: Thu, 14 Dec 2017 16:13:54 -0800
Message-ID: <> (raw)
In-Reply-To: <481F8A87-D5A1-440F-832F-7E433179A7F9-FrUbXkNCsVf2fBVCVOL8/>

On Thu, Dec 14, 2017 at 3:24 PM, Guy Harris <guy-FrUbXkNCsVf2fBVCVOL8/> wrote:
> On Dec 14, 2017, at 1:06 PM, Richard Sharpe <> wrote:
>> I notice on this page
>> it mentions that there are some conflicting usages of flag bits for radiotap.
>> Currently, Wireshark knows about bit 14 in the presence flags, but
>> hard-codes it for rx-Flags.
> No, it has a preference for that, defaulting to rx-flags.  Tcpdump has no preference for it, hardwiring it to rx-flags.

Ahhh, I see you are correct. I looked for the radiotap protocol at
first and didn't see it. After you responded I looked in the code and
then discovered it was called 802.11 Radiotap. That's possibly a bit
onerous for users but that is a separate issue.

>> It also doesn't know about tx-Flags (or
>> hardware-queue).
>> Would it be useful to allow the user to specify they want to be able
>> to switch between usages or are we trying to stamp out such
>> conflicting uses?
> The conflicts are historical; I think the intent for radiotap is that further conflicts be at least *very strongly* discouraged, if not simply forbidden.  I vote for "forbidden".  As the page you cite says:
>         Note that some fields currently have numbers assigned already, this is due to different OSes defining different bits unilaterally. *Such mistakes should not be repeated in the future.*
> (emphasis mine).
> The radiotap spec doesn't control what code that processes radiotap headers does; it neither requires nor forbids preferences.  I don't think it should require or forbid them as a way of dealing with existing conflicts, but it should (and does) say that no requirement for a preference should be introduced in the future.

Great. I will get to work adding some of the missing bits and maybe a
preference around the tx_flags and hardward_queue.

BTW, why is there what looks like 6-bytes of un-interpreted data
between the rx-flags and timestamp info?

I can send a capture if you would like to look.

Richard Sharpe

  parent reply index

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-12-14 21:06 Richard Sharpe
     [not found] ` <>
2017-12-14 23:24   ` Guy Harris
     [not found]     ` <481F8A87-D5A1-440F-832F-7E433179A7F9-FrUbXkNCsVf2fBVCVOL8/>
2017-12-15  0:13       ` Richard Sharpe [this message]
     [not found]         ` <>
2017-12-15  1:52           ` Guy Harris
     [not found]             ` <7A209774-9919-455B-9E04-BDAF26BE99DA-FrUbXkNCsVf2fBVCVOL8/>
2017-12-15  8:24               ` Johannes Berg

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