Subject: Re: adding an 802.11 data link type
To: Bill Studenmund , Atsushi Onoe <>
From: Matt Thomas <>
List: tech-net
Date: 08/07/2002 10:48:32
At 10:34 AM 8/7/2002, Bill Studenmund wrote:
>On Tue, 6 Aug 2002, Atsushi Onoe wrote:
> > > > >  1) A driver supporting more than one DLT will call bpf with a new
> > > > >     procedure, bpfattach_multidlt, passing bpfattach_multidlt a mask
> > > > >     describing the DLTs the driver supports.
> > > >
> > > > Since the number of supported DLTs for a driver would be limited,
> > > > multiple calls to bpfattach_dlt() may be accepted.
> > > > This would help to implement streight forward implementation which 
> splits
> > > > bpf_if for each DLT.
> > >
> > > But do we want multiple bpf_if's? My original idea was there would be one
> > > bpf, just it could hand back different packet types.
> >
> > Hmm, I've found my previous implementation.  I admit it is something like
> > ad hoc, but it works well and it has complete compatibility.
>That code is missing one of the key points:
>+#if NBPFILTER > 0
>+               if (ic->ic_rawbpf)
>+                       bpf_mtap(ic->ic_rawbpf, m0);

I've been quiet on this for a while but no longer. :)

Since BPF gives the driver an opaque hook into the interface, if_bpf, there's
no need to change that part of the driver interface.  What you really want
are additional bpf calls,

         bpf_tap2(caddr_t if_bpf, caddr_t ptr, size_t len, int flags)
         bpf_mtap2(caddr_t if_bpf, struct mbuf *m0, int flags)

We will define flags so the bottom 16 bits are reserved for passing an
optional DLT type. Then if you need special processing, you can just do

         bpf_map2(ifp->if_bpf, m0, DLT_IEEE80211B | DLF_RX);

(Since we are extending BPF, we might as well add support for passing in
the direction of the packet).

Now, all the big changes are in BPF.  This can uses for other interfaces
which can change packet modes (for instance, between CHDLC and PPP).

Matt Thomas               Internet:
3am Software Foundry      WWW URL:
Cupertino, CA             Disclaimer: I avow all knowledge of this message