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From: Rick Jones <rick.jones2@hp.com>
To: Roland Dreier <rdreier@cisco.com>
Cc: "Zou, Yi" <yi.zou@intel.com>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
	"Leech, Christopher" <christopher.leech@intel.com>,
	"Dev, Vasu" <vasu.dev@intel.com>,
	"Love, Robert W" <robert.w.love@intel.com>,
	"Ma, Steve" <steve.ma@intel.com>,
	"Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@intel.com>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@intel.com>
Subject: Re: Question regarding protocol specific mtu for FCoE
Date: Wed, 03 Jun 2009 15:36:49 -0700	[thread overview]
Message-ID: <4A26FB01.1020502@hp.com> (raw)
In-Reply-To: <adazlcp814o.fsf@cisco.com>

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.

WARNING: multiple messages have this Message-ID (diff)
From: Rick Jones <rick.jones2@hp.com>
To: Roland Dreier <rdreier@cisco.com>
Cc: "Zou\, Yi" <yi.zou@intel.com>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
	"Leech\, Christopher" <christopher.leech@intel.com>, "Dev\,
	Vasu" <vasu.dev@intel.com>, "Love\,
	Robert W" <robert.w.love@intel.com>, "Ma\,
	Steve" <steve.ma@intel.com>, "Waskiewicz Jr\,
	Peter P" <peter.p.waskiewicz.jr@intel.com>, "Kirsher\,
	Jeffrey T" <jeffrey.t.kirsher@intel.com>
Subject: Re: Question regarding protocol specific mtu for FCoE
Date: Wed, 03 Jun 2009 15:36:49 -0700	[thread overview]
Message-ID: <4A26FB01.1020502@hp.com> (raw)
In-Reply-To: <adazlcp814o.fsf@cisco.com>

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.

  parent reply	other threads:[~2009-06-03 22:36 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2009-06-03 17:56 Question regarding protocol specific mtu for FCoE Zou, Yi
2009-06-03 18:03 ` Rick Jones
2009-06-03 19:16   ` Zou, Yi
2009-06-03 19:27     ` Ben Hutchings
2009-06-03 20:43       ` Zou, Yi
2009-06-03 20:57         ` Ben Hutchings
2009-06-03 22:22           ` Zou, Yi
2009-06-03 21:01     ` Rick Jones
2009-06-03 21:17       ` Waskiewicz Jr, Peter P
2009-06-03 21:21       ` Roland Dreier
2009-06-03 21:21         ` Roland Dreier
2009-06-03 22:25         ` Zou, Yi
2009-06-03 22:36         ` Rick Jones [this message]
2009-06-03 22:36           ` Rick Jones
2009-06-03 23:00           ` Zou, Yi
2009-06-03 23:27           ` Roland Dreier
2009-06-03 23:27             ` Roland Dreier

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