From owner-freebsd-arch@FreeBSD.ORG Tue Dec 27 20:19:57 2005 Return-Path: X-Original-To: arch@freebsd.org Delivered-To: freebsd-arch@FreeBSD.ORG Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 85D3F16A41F; Tue, 27 Dec 2005 20:19:57 +0000 (GMT) (envelope-from peter@wemm.org) Received: from canning.wemm.org (canning.wemm.org [192.203.228.65]) by mx1.FreeBSD.org (Postfix) with ESMTP id E029B43D5E; Tue, 27 Dec 2005 20:19:56 +0000 (GMT) (envelope-from peter@wemm.org) Received: from fw.wemm.org (canning.wemm.org [192.203.228.65]) by canning.wemm.org (Postfix) with ESMTP id 942472A905; Tue, 27 Dec 2005 12:19:56 -0800 (PST) (envelope-from peter@wemm.org) Received: from peter-laptop.wemm.org (dhcp99.wemm.org [10.0.0.99]) by fw.wemm.org (Postfix) with ESMTP id 4559BE2B3; Tue, 27 Dec 2005 12:19:56 -0800 (PST) (envelope-from peter@wemm.org) Received: from peter-laptop.wemm.org (localhost [127.0.0.1]) by peter-laptop.wemm.org (8.13.4/8.13.4) with ESMTP id jBRKI6Gh034514; Tue, 27 Dec 2005 12:18:06 -0800 (PST) (envelope-from peter@wemm.org) Received: from localhost (localhost [[UNIX: localhost]]) by peter-laptop.wemm.org (8.13.4/8.13.3/Submit) id jBRKI5AC034513; Tue, 27 Dec 2005 12:18:05 -0800 (PST) (envelope-from peter@wemm.org) X-Authentication-Warning: peter-laptop.wemm.org: peter set sender to peter@wemm.org using -f From: Peter Wemm To: freebsd-arch@freebsd.org Date: Tue, 27 Dec 2005 12:18:04 -0800 User-Agent: KMail/1.8.3 References: <20051107140451.GU91530@cell.sick.ru> <20051111140926.GC733@empiric.icir.org> <20051111141519.GE1647@cell.sick.ru> In-Reply-To: <20051111141519.GE1647@cell.sick.ru> MIME-Version: 1.0 Content-Type: text/plain; charset="koi8-r" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200512271218.05201.peter@wemm.org> Cc: Bruce M Simpson , Gleb Smirnoff , arch@freebsd.org Subject: Re: ARP request retransmitting X-BeenThere: freebsd-arch@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Discussion related to FreeBSD architecture List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 27 Dec 2005 20:19:57 -0000 On Friday 11 November 2005 06:15 am, Gleb Smirnoff wrote: > On Fri, Nov 11, 2005 at 02:09:26PM +0000, Bruce M Simpson wrote: > B> On Mon, Nov 07, 2005 at 05:04:51PM +0300, Gleb Smirnoff wrote: > B> > I suggest to keep sending ARP requests while there is a demand for > B> > this (we are trying to transmit packets to this particular IP), > B> > ratelimiting these requests to one per second. This will help in a > B> > quite common case, when some host on net is rebooting, and we are > B> > waiting for him to come up, and notice this only after 1 - 20 seconds > B> > since the time it is reachable. > B> > Any objections? > B> > B> In response to the other replies to this thread citing broadcast > B> pollution on Ethernet-based networks: > B> Please add this functionality under a sysctl where it is turned off by > default. B> > B> It is desirable in situations where ARP entries cached further upstream > are B> stale, but it may cause flooding in an environment where the layer 2 > backbone B> hasn't been split or has not been segregated well. > B> > B> Other people cited examples where vendor switch implementations were > B> retransmitting across VLANs -- this week I've been offering moral > support B> to a friend who is dealing with similar VLAN brokenness at his > $DAYJOB B> (there was an extension to 802.1d to support multiple spanning > tree instances B> across VLANs which I think not everyone supports > correctly). > > I'd like to see a proven evidence that this functionality leads to a > measurable increase in broadcast traffic. Many modern operating systems > behave in such way and no-one complains. The increase of broadcast traffic > is very theoretical, it happens only when there are downed hosts. Personally, I think that the place that this can most benefit is small home/office/business networks of a small number of hosts. People with large networks already have to deal with this sort of problem anyway. FreeBSD systems retransmit once per second for 20 seconds, then take a short break, then resume the once-per-second retransmits again. The "short break" is useless IMHO and makes such a small difference in modern networks. The saddest thing I see these days is a constant stream of ARP traffic coming in my cable modem. About 20-30 per second. 09:36:27.040649 arp who-has 67.174.245.39 tell 67.174.244.1 09:36:27.104437 arp who-has 67.188.248.237 tell 67.188.240.1 09:36:27.128126 arp who-has 67.188.240.180 tell 67.188.240.1 09:36:27.162068 arp who-has 67.174.244.30 tell 67.174.244.1 09:36:27.162313 arp who-has 67.174.244.37 tell 67.174.244.1 09:36:27.166890 arp who-has 67.174.244.48 tell 67.174.244.1 09:36:27.167550 arp who-has 67.174.244.44 tell 67.174.244.1 09:36:27.168296 arp who-has 67.174.244.45 tell 67.174.244.1 09:36:27.168735 arp who-has 67.174.244.50 tell 67.174.244.1 09:36:27.168984 arp who-has 67.174.244.91 tell 67.174.244.1 09:36:27.170819 arp who-has 67.174.244.97 tell 67.174.244.1 09:36:27.171062 arp who-has 67.174.244.101 tell 67.174.244.1 09:36:27.171226 arp who-has 67.174.244.107 tell 67.174.244.1 09:36:27.171662 arp who-has 67.174.244.110 tell 67.174.244.1 09:36:27.171909 arp who-has 67.174.244.116 tell 67.174.244.1 09:36:27.174206 arp who-has 67.174.244.92 tell 67.174.244.1 09:36:27.174447 arp who-has 67.188.248.57 tell 67.188.240.1 09:36:27.174603 arp who-has 67.174.244.112 tell 67.174.244.1 09:36:27.176663 arp who-has 67.174.244.135 tell 67.174.244.1 09:36:27.177101 arp who-has 67.174.244.158 tell 67.174.244.1 09:36:27.177352 arp who-has 67.174.244.144 tell 67.174.244.1 09:36:27.178172 arp who-has 67.174.244.141 tell 67.174.244.1 09:36:27.178413 arp who-has 67.174.244.146 tell 67.174.244.1 09:36:27.180278 arp who-has 67.174.244.148 tell 67.174.244.1 09:36:27.180948 arp who-has 67.174.244.151 tell 67.174.244.1 09:36:27.181184 arp who-has 67.174.244.152 tell 67.174.244.1 09:36:27.716214 arp who-has 67.188.247.253 tell 67.188.240.1 09:36:27.765102 arp who-has 69.181.212.233 tell 69.181.212.1 09:36:27.799458 arp who-has 67.188.113.101 tell 67.188.112.1 09:36:27.848736 arp who-has 67.188.240.194 tell 67.188.240.1 09:36:27.854934 arp who-has 67.188.240.142 tell 67.188.240.1 09:36:27.897613 arp who-has 67.188.240.195 tell 67.188.240.1 09:36:27.997441 arp who-has 67.188.240.95 tell 67.188.240.1 I'm sure most of this is comcast's self-inflicted pain, but FreeBSD doesn't even make a dent in ARP traffic like this. Most of the ARP traffic I see at work on our corp network comes from routers trying to reach down hosts or re-arping up machines. But then again, we use vlans to limit the size of broadcast domains. I suspect most well managed "large" networks will have something similar. The difference between sending 20 arps per 40 seconds or 40 arps per 40 seconds for a down host isn't going to make a dent. What does seem to hurt is when some body does an nmap and you get thousands of arps from the router... -Peter