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 4:13 PM, Richard Sharpe <realrichardsharpe-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:

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

My inclination is 1) not to add the presence bits for suggested fields to any code that processes radiotap headers unless there are capture files or devices that use them and 2) not to add preferences unless there are capture files or devices that one interpretation of the presence bit in question *and* capture files or devices that use the other interpretation.

If you're doing this because you're working on something that *generates* radiotap headers, I would suggest that you use only defined fields if possible and, if you need to add information that's *not* supported by any defined field, try to get a field defined - preferably with its own presence bit assigned.

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

As the top-level radiotap page says:

	Radiotap requires that all fields in the radiotap header are aligned to natural boundaries. For radiotap, that means all 8-, 16-, 32-, and 64-bit fields must begin on 8-, 16-, 32-, and 64-bit boundaries, respectively. In this way, generators and parsers can avoid unaligned accesses to radiotap capture fields. Radiotap-compliant generators must insert padding before a capture field to ensure its natural alignment. Radiotap-compliant packet parsers, such as tcpdump(8), expect and skip the padding.

RX flags:

	http://www.radiotap.org/fields/RX%20flags.html

is a 2-byte field, requiring 2-byte alignment.

Timestamp:

	http://www.radiotap.org/fields/timestamp.html

requires 8-byte alignment, as it includes an 8-byte numerical value.

So if you have an RX flags field that happens to be aligned on an 8-byte boundary, followed immediately by a timestamp field, there needs to be 6 bytes of padding after the RX flags field to align the timestamp field on an 8-byte boundary.