On Mon, Oct 02, 2000 at 06:18:10PM +0900, itojun@iijlab.net 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
--