Date: Mon, 7 Nov 2005 17:52:53 +0300 From: Gleb Smirnoff <glebius@FreeBSD.org> To: Robert Watson <rwatson@FreeBSD.org> Cc: arch@FreeBSD.org Subject: Re: ARP request retransmitting Message-ID: <20051107145253.GB91530@cell.sick.ru> In-Reply-To: <20051107143620.P9690@fledge.watson.org> References: <20051107140451.GU91530@cell.sick.ru> <20051107143620.P9690@fledge.watson.org>
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On Mon, Nov 07, 2005 at 02:40:43PM +0000, Robert Watson wrote: R> > I have a proposition on changing the behavior of ARP retransmitting. R> >Currently we after sending several ARP requests, sending ARP requests R> >for given IP is suppressed for some interval (by default 20 seconds). R> >Probably this feature was designed in early 90th, when sending one R> >additional broadcast packet was an expensive thing. R> > R> > I suggest to keep sending ARP requests while there is a demand for this R> >(we are trying to transmit packets to this particular IP), ratelimiting R> >these requests to one per second. This will help in a quite common case, R> >when some host on net is rebooting, and we are waiting for him to come R> >up, and notice this only after 1 - 20 seconds since the time it is R> >reachable. R> R> While networks have gotten a lot faster in the last few years, I've R> noticed that many of the ones I deal with have also gotten a lot larger in R> terms of number of hosts. This has been possible both because of the R> absolute increase in bandwidth, and also because of the widespread use of R> switches to suppress unnecessary unicast traffic to segments. I worry R> that significantly increasing the amount of broadcast traffic will be a R> problem for sites with large bridged network configurations. On the other R> hand, they already have to deal with things like windows network R> neighborhoods, various service discovery protocols, and so on. R> R> Do you have any information on what other operating systems may have done R> in terms of tweaking ARP for improved responsiveness? I imagine we can't R> be the first to discuss such changes, and if there is already a widely R> deployed system with ARP adaptations of this sort, it would be interesting R> to know if they've seen any problems or have recommendations. Frankly speaking, I have written my email after looking at Linux behavior. It sends one ARP request per second while there is demand for this ARP entry. After demand has disapparead, it sends a few retransmits driven by timer. If again there is a demand, it starts to retransmit again. Under "demand" here I mean a traffic to this particular IP, either locally generated or routed. -- Totus tuus, Glebius. GLEBIUS-RIPN GLEB-RIPE
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