Date: Fri, 19 Jul 2002 14:02:38 -0500 (CDT) From: Mike Silbersack <silby@silby.com> To: Garrett Wollman <wollman@lcs.mit.edu> Cc: Jonathan Lemon <jlemon@flugsvamp.com>, <net@FreeBSD.org> Subject: Re: cvs commit: src/sys/netinet tcp_timer.h Message-ID: <20020719134639.S95326-100000@patrocles.silby.com> In-Reply-To: <200207182133.g6ILXHNl007758@khavrinen.lcs.mit.edu>
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On Thu, 18 Jul 2002, Garrett Wollman wrote: > - 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 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 Now that I've had a bit more time to think it over, I believe that Matt's 200ms slop + no floor on rtomin provides us with a very good system. In effect, Matt has seperated the delay necessary to avoid retrans because of delayed acks (200ms or less on modern systems) from rtt (quite variable.) This is an approach that seems quite obvious in hindsight, and should allow the tcp stack to adapt to varying link types quite dynamically. The main improvement upon this I could see is dynamically detecting the delayed ack period of the receiver, as suggesed by DG. Unfortunately, I suspect that such detection would be nearly impossible to get correct. In place of this, I'd suggest that the slop be bumped from 200ms up to 220ms; both linux and windows use a 200ms delayed ACK period, and we don't want to be exactly synchronized to that time period. Mike "Silby" Silbersack To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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