Date: Tue, 13 Jul 2010 17:33:43 +0200 From: Andre Oppermann <andre@freebsd.org> To: Maxim Dounin <mdounin@mdounin.ru> Cc: freebsd-net@freebsd.org, Igor Sysoev <is@rambler-co.ru> Subject: Re: net.inet.tcp.slowstart_flightsize in 8-STABLE Message-ID: <4C3C8757.2080705@freebsd.org> In-Reply-To: <20100713140051.GV99657@mdounin.ru> References: <20100512124702.GJ2679@rambler-co.ru> <20100713140051.GV99657@mdounin.ru>
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On 13.07.2010 16:01, Maxim Dounin wrote: > Hello! > > On Wed, May 12, 2010 at 04:47:02PM +0400, Igor Sysoev wrote: > >> It seems that net.inet.tcp.slowstart_flightsize does not work in 8-STABLE. >> For a long time I used slowstart_flightsize=2 on FreeBSD 4, 6, and 7 hosts. >> However, FreeBSD-8 always starts with the single packet. >> I saw this on different versions of 8-STABLE since 8 Oct 2009 till >> 04 Apr 2010. > > Finally I had some time to look into it (sorry for long delay). > > 1. Slow start isn't used on recent FreeBSD versions for initial snd_cwnd > calculations as long as you have rfc3390 support switched on (default since > Jan 06 23:29:46 2004, at least in 7.*). It effectively sets initial > snd_cwnd to 3*MSS on common networks and shouldn't cause any problems. > Slowstart_flightsize only affects connection restarts. > > 2. Due to bug in syncache code (patch below) all accepted connections has > their snd_cwnd reset to 1*MSS (since r171639, 7.0+ AFAIR). > > 3. Support for rfc3465 introduced in r187289 uncovered (2) as > ACK to SYN/ACK no longer causes snd_cwnd increase by MSS (actually, this > increase shouldn't happen as it's explicitly forbidden by rfc 3390, but > it's another issue). Snd_cwnd remains really small (1*MSS + 1) and this > causes really bad interaction with delayed acks on other side. > > As a workaround to delayed acks interaction problems you may disable > rfc3465 by setting net.inet.tcp.rfc3465 to 0. Correct fix would be to apply > the patch below. > > To Andre Oppermann: could you please take a look at the patch and > commit it if found appropriate? Thanks for your debugging. I will look into it and commit if there are no further questions. -- Andre > # HG changeset patch > # User Maxim Dounin<mdounin@mdounin.ru> > # Date 1279028684 -14400 > # Node ID 93699203f408fa8e22c971769bde9c26bd14d410 > # Parent e2cf8c51a6294a0d09d5628d96c6ab3ab626957e > Fix cwnd resetting problem introduced in r171639. > > Sc_rxmits counts timer engagements, not actual retransmits, and value 1 > corresponds to no retransmits. Revert check to correct one as present > before r171639. > > diff --git a/netinet/tcp_syncache.c b/netinet/tcp_syncache.c > --- a/netinet/tcp_syncache.c > +++ b/netinet/tcp_syncache.c > @@ -804,8 +804,10 @@ syncache_socket(struct syncache *sc, str > > /* > * If the SYN,ACK was retransmitted, reset cwnd to 1 segment. > + * Note that sc_rxmits counts timer engagements, not actual > + * retransmits. > */ > - if (sc->sc_rxmits) > + if (sc->sc_rxmits> 1) > tp->snd_cwnd = tp->t_maxseg; > tcp_timer_activate(tp, TT_KEEP, tcp_keepinit); > > > Maxim Dounin >
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