Date: Fri, 17 Oct 2003 21:19:08 +0200 From: Andre Oppermann <oppermann@pipeline.ch> To: Carl Mascott <cmascott@att.net> Cc: freebsd-net@freebsd.org Subject: Re: TCP window size issues Message-ID: <3F9040AC.7F117AFA@pipeline.ch> References: <200310171830.h9HIUnSr000572@callisto.local>
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Carl Mascott wrote: > > I have a few TCP window size issues. > > 1. In FreeBSD 4.8-R the kernel default recv window is 56 KB. This > is so large that it causes dropped packets due to queue overflow > with my V.90 link (BW 5 KB/s on compressed data). > > 2. The 4.4BSD TCP implementation has never had the correct precedence > rules for setting window sizes. App-set values should take > precedence over all others. Routing table values should take > precedence over kernel default values. The fix for PR 11966 > alters the behavior but still doesn't implement these precedence > rules. I'll fix this in my tcp_hostcache patch currently under review by Sam Leffler and Ruslan Ermilov (however that is for -CURRENT). > http://www.freebsd.org/cgi/query-pr.cgi?pr=11966 > > 3. RFC 793 (TCP) says that shrinking the receive window after > connection is established is "strongly discouraged". I'm > currently shrinking the receive window on my default route > (PPP link) from 32 KB to 4 KB with settings in > /etc/ppp/ppp.conf. (Had to back out patch in PR 11966.) > No problems noticed yet. To avoid shrinking the window I > believe I'd have to do the following: > > - set kernel default send & recv windows to smallest values > used on any route > > - modify all other routes on bootup (with script in > /usr/local/etc/rc.d ?) to set desired larger window sizes > > Keeping the script in sync with the routing table would be > a new maintenance headache. > > How likely am I to get into trouble by shrinking the TCP > receive window? Likely enough that I should make the > changes above? The patch in PR 11966 was kicking around > for about a year before the PR was filed. Was this patch > originally intended to solve a TCP compatibility problem > caused by shrinking the TCP receive window? -- Andre
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