From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from lists.gentoo.org (pigeon.gentoo.org [208.92.234.80]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by finch.gentoo.org (Postfix) with ESMTPS id 496EF138330 for ; Tue, 20 Sep 2016 03:34:51 +0000 (UTC) Received: from pigeon.gentoo.org (localhost [127.0.0.1]) by pigeon.gentoo.org (Postfix) with SMTP id 99AE0E0B5C; Tue, 20 Sep 2016 03:34:42 +0000 (UTC) Received: from icp-osb-irony-out4.external.iinet.net.au (icp-osb-irony-out4.external.iinet.net.au [203.59.1.220]) by pigeon.gentoo.org (Postfix) with ESMTP id BB56DE0B2A for ; Tue, 20 Sep 2016 03:34:39 +0000 (UTC) X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: A2D6BABIreBX/5ITRWpeGwEBAQMBAQEJAQEBgzsBAQEBAR6BAVKNM55+jEKCA4YeAoFZOBQBAgEBAQEBAQFeJ4RiAQEEax4LDQsuITYTBgIBAYgsAxa5AA2DJAEBCAIlhjeBfIJYgkeCSYUQBY8oihI1jGsBgnCBbod3FwyFYohViAgCHjYghFcxNIccAQEB X-IPAS-Result: A2D6BABIreBX/5ITRWpeGwEBAQMBAQEJAQEBgzsBAQEBAR6BAVKNM55+jEKCA4YeAoFZOBQBAgEBAQEBAQFeJ4RiAQEEax4LDQsuITYTBgIBAYgsAxa5AA2DJAEBCAIlhjeBfIJYgkeCSYUQBY8oihI1jGsBgnCBbod3FwyFYohViAgCHjYghFcxNIccAQEB X-IronPort-AV: E=Sophos;i="5.30,365,1470672000"; d="scan'208";a="120493426" Received: from unknown (HELO mail.vm.localdomain) ([106.69.19.146]) by icp-osb-irony-out4.iinet.net.au with ESMTP; 20 Sep 2016 11:34:37 +0800 Received: from localhost (localhost [127.0.0.1]) by mail.vm.localdomain (Postfix) with ESMTP id EC074577FF for ; Tue, 20 Sep 2016 11:34:38 +0800 (AWST) X-Virus-Scanned: amavisd-new at localdomain Received: from mail.vm.localdomain ([127.0.0.1]) by localhost (mail.vm.localdomain [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id 32QtXr6-0h4e for ; Tue, 20 Sep 2016 11:34:34 +0800 (AWST) Received: from [192.168.45.3] (unknown [192.168.45.3]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) (Authenticated sender: wdk) by mail.vm.localdomain (Postfix) with ESMTPSA id 65E5E577FB for ; Tue, 20 Sep 2016 11:34:34 +0800 (AWST) Subject: Re: [gentoo-user] {OT} ISP requires MTU below 1500? To: gentoo-user@lists.gentoo.org References: <20160920024829.0d9b56ea@hal9000.localdomain> <20160920043538.69d5bb65@hal9000.localdomain> From: Bill Kenworthy Message-ID: <30cd879b-f177-8ad7-ccbf-98e4f9e1a251@iinet.net.au> Date: Tue, 20 Sep 2016 11:34:32 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.3.0 Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-Id: Gentoo Linux mail X-BeenThere: gentoo-user@lists.gentoo.org Reply-to: gentoo-user@lists.gentoo.org MIME-Version: 1.0 In-Reply-To: <20160920043538.69d5bb65@hal9000.localdomain> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-Archives-Salt: 736d0acb-047c-4fdd-9a1a-443de33b097c X-Archives-Hash: c96d4657170d1a6e9e6d60dfbfdc38ca On 09/20/16 10:35, wabe wrote: > Grant wrote: > >>>> A while back I was having networking issues. I eventually tried >>>> drastically lowering the MTU of all the systems onsite and the >>>> issues disappeared. I always thought the issue was due to the MTU >>>> on our modem/router. Today I read that AT&T DSL requires a 1492 >>>> MTU so I increased the MTU of our systems up to 1492 and haven't >>>> had any issues. Do certain ISPs require you to change the MTU of >>>> your entire network, or is this likely due to our AT&T >>>> modem/router itself? >>> AFAIK the MTU is defined for every network interface separately. >>> For an ADSL connection it is common that a lower MTU is needed >>> because of the PPPoE header information that is encapsulated in the >>> ethernet frames. But in that case it is sufficient to lower the MTU >>> just for the WAN interface that is connected to the DSL modem. >>> If you don't use protocol encapsulation in your LAN then there >>> should be IMHO no reason for lowering the MTU of your internal >>> interfaces. >> >> So I should be OK with 1492 MTU on the modem/router and 1500 inside >> that LAN? That hasn't been my experience but I haven't tried in a >> while. Wouldn't that lead to fragmentation issues? Admittedly, my >> understanding of this is weak. > FWIR it is sufficient when all interfaces that are connected to a > layer 2 network are using the same MTU for the respective layer 3 > protocols. So it should be ok when the MTU of the (logical) ppp > interface is set to 1492 even when the MTU of the (physical) Ethernet > interface is set to 1500. This is the case for my router that is > connected to my DSL modem. I don't have any network problems and > always maximum internet speed. > > I'm not a network expert and don't understand all the details. Also > my English is not good enough to explain it in a better way. > But to be honest, I'm not sure that I could explain it better in my > native language. ;-) > > Probably there are other members on this ML that can give your more > useful information about this topic. > > -- > Regards > wabe > Rather than guess and take random values read on the net - measure it. Google calculate mtu - netgear and others show ways to test upstream to get the ideal size using ping You are looking for the largest MTU value before fragmentation starts to occur. BillK