Date: Wed, 14 Aug 2002 00:44:28 -0500 (CDT) From: Mike Silbersack <silby@silby.com> To: Oleg Polyakov <opolyakov@yahoo.com> Cc: freebsd-net@freebsd.org Subject: Re: Initial congestion window increase Message-ID: <20020814003743.U93223-100000@patrocles.silby.com> In-Reply-To: <20020812192549.24783.qmail@web10409.mail.yahoo.com>
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On Mon, 12 Aug 2002, Oleg Polyakov wrote: > Yes, real world is murky;) > RFC2414 references RFC2415 - Simulation Studies of Increased > Initial TCP Window Size which shows significantly increased > throughput and decreased delays and some increased drop rate - > around 1% more for connections with initial window=3. > It also references RFC2416 - When TCP Starts Up With Four > Packets > Into Only Three Buffers where experiment shows decreased delays > and "no harm" for transmission. I've read those over, and I'm still not sure what to believe. I have a few other TCP things on my plate right now, so I'd rather concentrate on those for the moment. > By the way looking into tcpdump revealed we have initial window > = 2 * mss in case of slowstart_flightsize = 1. I've seen it on > FreeBSD 4.5 and 4.6 . > What is this - an ancient bug in BSD stack? > > If I set slowstart_flightsize to 0 then tcpdump shows we > starting > with initial window 1... Hrm, interesting. If you can figure out how to fix it so that slowstart_flightsize is accurate, that would be appreciated. After that, then maybe we can worry about changing the default. Incidently, I think that the formula min (4*MSS, max (2*MSS, 4380 bytes)) is bogus. We really have three cases: MTU 512 bytes MTU 1500 bytes (and 1480 or whatever PPPoE uses) MTU 9000 bytes (jumbo frames) The formula above would penalize jumbo frames, while sending a lot of packets for the 512 byte case. Just using MSS * X, where X is a constant for all of the above seems like a better idea. 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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