>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
	- 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?

itojun