netdev - Re: Question regarding protocol specific mtu for FCoE

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Message-ID: <adazlcp814o.fsf@cisco.com>
Date:	Wed, 03 Jun 2009 14:21:11 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	Rick Jones <rick.jones2@...com>
Cc:	"Zou\, Yi" <yi.zou@...el.com>,
	"netdev\@vger.kernel.org" <netdev@...r.kernel.org>,
	"linux-scsi\@vger.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

 > 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)

 - R.
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