From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rick Jones Subject: Re: Question regarding protocol specific mtu for FCoE Date: Wed, 03 Jun 2009 15:36:49 -0700 Message-ID: <4A26FB01.1020502@hp.com> References: <7C88852EF6F99F4EB538472FCFEBE2223A7E6F45@orsmsx509.amr.corp.intel.com> <4A26BAF7.3070301@hp.com> <7C88852EF6F99F4EB538472FCFEBE2223A7E6FBD@orsmsx509.amr.corp.intel.com> <4A26E4AE.4020607@hp.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: netdev-owner@vger.kernel.org To: Roland Dreier Cc: "Zou, Yi" , "netdev@vger.kernel.org" , "linux-scsi@vger.kernel.org" , "Leech, Christopher" , "Dev, Vasu" , "Love, Robert W" , "Ma, Steve" , "Waskiewicz Jr, Peter P" , "Kirsher, Jeffrey T" List-Id: linux-scsi@vger.kernel.org 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.