Re: Would it be useful to have preferences for radiotap to allow handling of captures with conflicting presence flags?

[Guy Harris <guy-FrUbXkNCsVf2fBVCVOL8/A@public.gmane.org>]

On Dec 14, 2017, at 1:06 PM, Richard Sharpe <realrichardsharpe-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:

> I notice on this page
> 
>  http://www.radiotap.org/fields/suggested
> 
> 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.

> 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.