netdev - RE: Question regarding protocol specific mtu for FCoE

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Date:	Wed, 3 Jun 2009 15:25:39 -0700
From:	"Zou, Yi" <yi.zou@...el.com>
To:	Roland Dreier <rdreier@...co.com>, Rick Jones <rick.jones2@...com>
CC:	"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

> > 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.

Yes, exactly, thanks for the clarification. Also, since FC max payload
of 2112 is not going to change, I guess the OPTIMAL_FCOEMTU is probably
going to stay unchanged forever.

Thanks.

yi
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