Subject: Re: 802.1Q & ETHER_MAX_LEN
To: None <email@example.com>
From: Manuel Bouyer <firstname.lastname@example.org>
Date: 10/02/2000 11:48:03
On Mon, Oct 02, 2000 at 06:18:10PM +0900, email@example.com wrote:
> >I don't understant this. All interface have an MTU, rigth ?
> >I can't see what a IFF_ flag would give us. To enable/disable the "long frame"
> >feature ?
> >Anyway we have 2 things: the MTU we want to advertize to protocol layers,
> >and the MTU the interface can really do. We need a way to advertize both and
> >this, independantly from a IFF_ flag.
> ifp->if_mtu: MTU for L3 protocols
> ifp->if_hdrlen: header size for L2
> ifp->if_mtu + ifp->if_hdrlen: L2 frame size
> if we have single layer, it would be like this:
> mtu hdrlen total
> --- --- ---
> 1500 14 1514 normal ethernet frame
> 1496 18 1514 VLAN with normal ethernet frame
> 1500 18 1518 VLAN with larger frame
> the problems are:
> - we have two ways to do VLAN: decreasing MTU or using bigger L2 frame
If we want to talk to switches we have to use bigger L2 frames.
> - we would like to split vlan* and real interface handling clearly,
> and it tend to add some confusion
> - if_hdrlen needs to be more flexible?
Maybe we should just say if_hdrlen is the largest one possible, and let
if_ethersubr check that the frame size is OK for the protocol used ?
Manuel Bouyer, LIP6, Universite Paris VI. Manuel.Bouyer@lip6.fr