Re: [PATCH] 802.1p priority (fixed)
From: Brooks Davis (brooks_at_one-eyed-alien.net)
Date: 01/23/05
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Date: Sun, 23 Jan 2005 11:24:44 -0800 To: Jeremie Le Hen <jeremie@le-hen.org>
On Sun, Jan 23, 2005 at 12:22:19PM +0100, Jeremie Le Hen wrote:
> > > Having the possibility to test and set the 802.1p or TOS values
> > > separately would avoid making a "trust"/"override" subtlety and will
> > > obviously make it more flexible.
> >
> > I agree on this point. The one thing to be careful of is that 802.1p
> > priorities and TOS values work rather differently in that TOS values fit
> > in to an existing field of the packet and 802.1p values require
> > modifications to the header and adding data between the header and the
> > real body, possiably with a resuling reduction in MTU (though what
> > you're doing trying to use 802.1p priority with crappy nic I don't know
> > :-).
>
> I do not understand your point here. TOS is indeed an existing field
> of the IPv4 header but AFAIK, this is the same for the 802.1p header [1].
> There are already 3 bits reserved for priority (802.1p) near the 802.1q
> field which are both inside what they call "Tag Control Information".
At the point you are examining layer 3 state, you either have already
stripped off the ethernet header or have not created it yet so you can't
just modify it. At least according to what I've read, you may or may
not want to tag all traffic so if you strip the tags, you not want to
use a vlan tag on the packet. You do have the actual storage the TOS
values will use since you have the IP header. I'm basicly saying that
they aren't necessicairly as similar as you might think. It might make
sense to modify the TOS bits directly in the firewall, but it is simply
not possiable to modify the 802.1p bits at that point because there's no
where to put them.
-- Brooks
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