Date: Wed, 26 Sep 2012 20:55:01 +0200 From: Andre Oppermann <oppermann@networx.ch> To: Andrey Zonov <zont@FreeBSD.org> Cc: "freebsd-net@freebsd.org" <freebsd-net@freebsd.org>, "Eggert, Lars" <lars@netapp.com> Subject: Re: [patch] sysctls for TCP timers Message-ID: <50634F85.50901@networx.ch> In-Reply-To: <505F2C2D.5050904@FreeBSD.org> References: <505AC500.6060903@FreeBSD.org> <D4D47BCFFE5A004F95D707546AC0D7E906888619@SACEXCMBX01-PRD.hq.netapp.com> <505F2C2D.5050904@FreeBSD.org>
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On 23.09.2012 17:35, Andrey Zonov wrote: > On 9/20/12 11:35 AM, Eggert, Lars wrote: >> Hi, >> >> On Sep 20, 2012, at 9:25, Andrey Zonov <zont@freebsd.org> wrote: >>> Some of them may be read google's article about tuning TCP parameters >>> [1]. I convert most of TCP timers to sysctls [2] and we are using this >>> patch for few months. We tuned net.inet.tcp.rtobase and >>> net.inet.tcp.syncache.rexmttime and it gives good results (especially in >>> conjunction with cc_htcp(4)). >> >> can you share some measurements that quantify the results? >> > > When we set net.inet.tcp.syncache.rexmttime=200 and > net.inet.tcp.syncache.rexmtlimit=7 for our external web service, the > number of duplicated SYN was reduced in four times. This isn't surprising. You're simply trading retransmits by the client with retransmits by the server. Whether this is within the overall packet conservation principle is not clear. On the timeline it may be an advantage. I'm not comfortable with the rather low retransmit time you've chosen here. Considering higher RTT's (e.g. Hawaii or JP/CN) and the bufferbloat problem this may be too low. When it is to be tuned, then something in the range of 500-1000ms may be more realistic to avoid spurious retransmits. When a SYN or SYN/ACK retransmit happens, the initial CWND should be reduced per the applicable RFC's as this indicates packet loss on the downstream. -- Andre
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