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Message-ID: <4A26FB01.1020502@hp.com>
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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