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 B6915138330 for ; Tue, 20 Sep 2016 17:38:50 +0000 (UTC) Received: from pigeon.gentoo.org (localhost [127.0.0.1]) by pigeon.gentoo.org (Postfix) with SMTP id 043DBE0B88; Tue, 20 Sep 2016 17:38:41 +0000 (UTC) Received: from km35626.keymachine.de (km35626.keymachine.de [87.118.86.27]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by pigeon.gentoo.org (Postfix) with ESMTPS id CD8E9E0B60 for ; Tue, 20 Sep 2016 17:38:39 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by km35626.keymachine.de (Postfix) with ESMTP id 57E9B1124786 for ; Tue, 20 Sep 2016 19:50:14 +0200 (CEST) X-Virus-Scanned: Debian amavisd-new at km35626.keymachine.de. Received: from km35626.keymachine.de ([127.0.0.1]) by localhost (km35626.keymachine.de [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id pzk2fOuCllbe for ; Tue, 20 Sep 2016 19:50:12 +0200 (CEST) Received: from grusum.endjinn.de (p4FC96D26.dip0.t-ipconnect.de [79.201.109.38]) by km35626.keymachine.de (Postfix) with ESMTPSA id 9C5691124785 for ; Tue, 20 Sep 2016 19:50:12 +0200 (CEST) Received: by grusum.endjinn.de (Postfix, from userid 500) id 2EE3A1702EF; Tue, 20 Sep 2016 19:37:36 +0200 (CEST) Date: Tue, 20 Sep 2016 19:38:02 +0200 From: David Haller To: gentoo-user@lists.gentoo.org Subject: Re: [gentoo-user] {OT} ISP requires MTU below 1500? Message-ID: <20160920173801.beu76zur3potnhrk@grusum.endjinn.de> Mail-Followup-To: gentoo-user@lists.gentoo.org References: <20160920024829.0d9b56ea@hal9000.localdomain> <20160920115258.GV7108@ns1.bonedaddy.net> <20160920144615.juwnp76jtqe6ec7y@grusum.endjinn.de> <20160920155650.2rlyahbqy4tduhtp@grusum.endjinn.de> 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 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: Organization: What? X-Clacks-Overhead: GNU Terry Pratchett User-Agent: Mutt/1.6.2 (2016-07-01) X-Archives-Salt: 306257cf-a7b8-40d4-b70c-03aa9f8a3628 X-Archives-Hash: 6c3d28d2ee9355b2b0da0ff512ef1762 Hello, On Tue, 20 Sep 2016, Grant wrote: >>>Strangely, I'm able to ping with that command even with a very high -s value: >>> >>>$ ping -c 4 -M dont -s 9999 www.dslreports.com >>>PING www.dslreports.com (64.91.255.98) 9999(10027) bytes of data. >>>10007 bytes from www.dslreports.com (64.91.255.98): icmp_seq=1 ttl=54 >>>time=331 ms >>>10007 bytes from www.dslreports.com (64.91.255.98): icmp_seq=2 ttl=54 >>>time=329 ms >>>10007 bytes from www.dslreports.com (64.91.255.98): icmp_seq=3 ttl=54 >>>time=329 ms >>>10007 bytes from www.dslreports.com (64.91.255.98): icmp_seq=4 ttl=54 >>>time=329 ms >>> >>>4 packets transmitted, 4 received, 0% packet loss, time 3003ms >>>rtt min/avg/max/mdev = 329.159/329.877/331.612/1.158 ms >> >> Look again! You're just looking at the _PING_ packets, not the ICMP/IP >> packets actually going over the interface! You'll need to run >> 'tcpdump icmp' in parallel! "My ping" also just reports 1 packet, but >> there's two IP packets actually going over the interface, due to the >> ping-packet being too large and being fragmented. >> >> Start the tcpdump in another (x)term before running the "ping" ... >> >> If you use '-M do', you should get the >> >> "Frag needed and DF set (mtu = NNNN)" > > >I switched to '-M do' and found that 1464 is the highest size I can >ping without the "Frag needed" error. This means I should add 28 to >that The overhead of 28 bytes is just specific to ping. It means that your upstream has a MTU of 1492 bytes. And it depends on your local needs if setting this MTU network-wide is the best course. I think I and others wrote enough for you to decide. >and set my MTU to 1492 across the network? Probably yes. I'd even say: unless you know otherwise for your local needs. It's a very small "pay" (-0.5% max throughput) locally for a potentially much bigger gain towards the 'net side, esp. when factoring in latency ... And BTW: changing the MTU is easy, why not start with one system? Even temporarily just using ifconfig/ip commandline (don't forget to set the default-route if you "down" the connection: 'route add default gw $GW_IP' ) and running some tests/benchmarks. HTH, -dnh -- Actually, NT is more like LSD with all the good effects filtered out. -- Andrew Maddox