Subject: Re: Question regarding protocol specific mtu for FCoE

From: Roland Dreier <[email-masked]>
Date: Wed, 03 Jun 2009 16:27:25 -0700

> Aren't all stations in the same broadcast domain "supposed" to have
 > the same MTU, at least down at L2? So, a station in the broadcast
 > domain just doing IP and a station in the broadcast domain doing
 > IP+FCoE "should" have the same MTU at the HW level right?
 > 
 > I could see where there would be lots of PMTU going-on if the
 > communications were to off-campus sites also had an FCoE upping their
 > MTU.  Otherwise, the MSS exchange at connection establishment is going
 > to preclude it right?  PMTU only "hits" when one has a so called
 > "dumb-bell" network which is "wider" at the ends than in the middle.

Yes, I think such dumb-bell networks would be pretty common (servers in
two different data centers with FCoE enabled, talking through an
old-school WAN with 1500 MTU).  And even if they're not as common I
think, we probably do want to have some way to handle this.

I'm probably not as up on old and/or obscure networking protocols, but
to me FCoE is the first time I've been forced to think about coexistence
of IP and non-IP protocols through the same netdev.

 - R.