Re: proposal: rename DLT_PRISM_HEADER

[David Young <dyoung () ojctech com>]

Sounds like hosttime is redundant.

Hmm. Seems like mactime should be some suitably high-resolution unit
for 802.11, such as 10ns or 1ns, or else a second header in the radio
header should tell the units. The point of DLT_IEEE802_11_RADIO is to
smooth over hardware differences.

(For some applications, it may be handy to know which frame feature
the mactime indicates. E.g., whether it is the start of the PLCP or MAC
header, or the end of frame.)

BTW, I keep thinking of reasons to adopt length-type-value tuples for
the radio header. There are three, so far.  First, records such as RSSI
are meaningless for transmitted frames, so they may as well be omitted.
Second, certain records are not supported by certain hardware.  Third,
the radio header is likely to change fast: one may desire to adopt a
new device driver without breaking compatibility with your tcpdump.
With LTVs, your tcpdump may skip records whose type it does not know.

Dave

On Mon, Dec 16, 2002 at 04:40:36PM -0500, Solomon Peachy wrote:
> On Mon, Dec 16, 2002 at 12:38:15PM -0800, Guy Harris wrote:
> > Unfortunately, you didn't CC Solomon on the mail, so he wouldn't see it
> > unless he's subscribed to tcpdump-workers, and he doesn't seem to be
> > subcribed to it, so he probably didn't see it.
>
> Okay, okay.  I should get off my ass and subscribe.  :) 
>
> [providing majordomo likes me this time, I'm now on tcpdump-workers..]
>
> > > What are the units for the mactime? The hosttime? It would be nice if
> > > the units were standardized (nanoseconds, picoseconds) or if the header
> > > carried some units indication. Also, is it desirable to indicate which
> > > packet feature the mactime marks (PLCP header, 802.11 header, packet end)?
>
> mactime is the high-resolution timestamp from the NIC.  There are no
> units, because it is highly hardware dependent.. 
>
> hosttime is the highest-resolution timestamp we can get from the host at
> the time the packet is received from the hardware, before it wades its
> way through the rest of the network stack.  On linux systems, this is
> the 'jiffies' counter.  
>
> Of course, I'm open to suggestions on how to improve this header.  The
> goal was to have it provide all the layer 2 information one could
> conceivably get from the NIC hardware that wouldn't necessarily be
> present in the 802.11 or os-specific headers. 
>
> > Also, why is the hosttime here in the first place?  How does that differ
> > from the regular frame time stamp that *all* frames passing through BPF
> > or the Linux networking stack get?
>
> Ah, Linux provides this, but do other systems?
>
> > > Is the dot11AntennaList given in the 802.11 spec?
>
> The dot11PhyAntenna table is documented on page 495 of the 802.11-1999
> spec, and the dot11AntennasList table is documented on page 503.
>
> > > Any idea how the version numbers be coordinated?
>
> Right now, through me. *chuckle* 
>
> Currently the "definitive" source is at:
>
> www.shaftnet.org/~pizza/software/capturefrm.txt
>
> I'm partial to making the "definitive" document part of the ethereal cvs
> tree, to be kept hand-in-hand with the dissector.
>
> On the other hand, 95% of a maintainer's job is saying "no", and I'd
> like to keep this from getting bloated with OS/NIC-specific things.
>
> The document as-is doesn't actually have any kind of revision
> information, that will obviously have to change.  Plus a few corrections
> that have cropped up..
>
> > > Any particular reason for the super-generous, 32-bit fields?
>
> So we don't have to worry about alignment and byte-packing issues.
> Simplicity is more important than saving a few bytes.
>
>  - Pizza
> -- 
> Solomon Peachy                        solomon () linux-wlan com
> AbsoluteValue Systems                 http://www.linux-wlan.com
> 715-D North Drive                     +1 (321) 259-0737  (office)
> Melbourne, FL 32934                   +1 (321) 259-0286  (fax)



-- 
David Young             OJC Technologies
dyoung () ojctech com      Engineering from the Right Brain
                        Urbana, IL * (217) 278-3933
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