Date: Thu, 18 Jul 2002 17:33:17 -0400 (EDT) From: Garrett Wollman <wollman@lcs.mit.edu> To: Jonathan Lemon <jlemon@flugsvamp.com> Cc: net@FreeBSD.org Subject: Re: cvs commit: src/sys/netinet tcp_timer.h Message-ID: <200207182133.g6ILXHNl007758@khavrinen.lcs.mit.edu> In-Reply-To: <20020718154341.D78047@prism.flugsvamp.com> References: <local.mail.freebsd-committers/20020717211230.G83269-100000@patrocles.silby.com> <200207181608.g6IG8dN82437@prism.flugsvamp.com> <200207181732.g6IHW9rY018853@apollo.backplane.com> <20020718133111.B78047@prism.flugsvamp.com> <200207181857.g6IIv9ei019345@apollo.backplane.com> <20020718125410.K91443@nexus.root.com> <20020718154341.D78047@prism.flugsvamp.com>
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[Trying desparately to move this discussion to the correct list....] I spent a few minutes talking to Dave Clark about this question this afternoon. Here's my paraphrase of his opinion: - He disclaims completely up-to-date knowledge of the current research results. - He feels that 1000 ms is commonly used only because the historical BSD implementation couldn't do any better (it actually did 750 ms +/- 250 ms), and supports the idea that retransmits could occur more frequently. He has traces which show where a longer RTO would be both beneficial and harmful. - He questioned whether the traditional VJ `srtt + 4*rttvar' computation captures enough of the variance in the real Internet to avoid unnecessary slow retransmits. - He notes that Microsoft's TCP had a serious problem wherein it would slow-retransmit too aggressively, which resulted in almost any network transient triggering sufficient dupacks to cause fast retransmit to engage. (The result was that every data packet would be sent twice.) He suggests that, to avoid this, it may be necessary to lengthen the slow-retransmit timeout after a fast retransmit is triggered. - He also notes that there have not been screams of protest since Linux adopted the 200-ms minimum, which suggests that it's not a completely hare-brained value. -GAWollman To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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