netdev - Re: Question regarding protocol specific mtu for FCoE

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Date:	Wed, 03 Jun 2009 15:36:49 -0700
From:	Rick Jones <rick.jones2@...com>
To:	Roland Dreier <rdreier@...co.com>
CC:	"Zou\, Yi" <yi.zou@...el.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"Leech\, Christopher" <christopher.leech@...el.com>,
	"Dev\, Vasu" <vasu.dev@...el.com>,
	"Love\, Robert W" <robert.w.love@...el.com>,
	"Ma\, Steve" <steve.ma@...el.com>,
	"Waskiewicz Jr\, Peter P" <peter.p.waskiewicz.jr@...el.com>,
	"Kirsher\, Jeffrey T" <jeffrey.t.kirsher@...el.com>
Subject: Re: Question regarding protocol specific mtu for FCoE

Roland Dreier wrote:
>  > So FCoE cannot say "fcoe_mtu = min(OPTIMAL_FCOEMTU,netdev->mtu)" and
>  > send-down frames based on that?
> 
> I think the point is that FCoE wants to use OPTIMAL_FCOEMTU (2KB + a bit
> for headers) even when netdev->mtu is 1500.  (eg datacenter network
> supports baby jumbo frames so FCoE traffic that stays within the network
> should use OPTIMAL_FCOEMTU, while lots of IP traffic is going out onto a
> 1500-byte MTU campus and having TCP doing lots of PMTU discovery is a
> pain)

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.
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