Date: Mon, 12 Aug 2002 12:25:49 -0700 (PDT) From: Oleg Polyakov <opolyakov@yahoo.com> To: Mike Silbersack <silby@silby.com> Cc: freebsd-net@freebsd.org Subject: Re: Initial congestion window increase Message-ID: <20020812192549.24783.qmail@web10409.mail.yahoo.com> In-Reply-To: <20020810215044.O62906-100000@patrocles.silby.com>
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--- Mike Silbersack <silby@silby.com> wrote: > > On Fri, 9 Aug 2002, Oleg Polyakov wrote: > > > Here is a patch allowing to increase TCP's initial > congestion > > window up to 4 mss but less then 4380 bytes as specified in > > experimental RFC 2414 and draft-ietf-tsvwg-initwin-04.txt. > > It doesn't touch idle congestion window as per draft. > > Despite this change being in an RFC, I'm not sure that it's > really worth > implementing. While increasing the slowstart flightsize might > do wonders > for benchmarks of short connections, the actual effect on real > world tests > seems much more murky. 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 believe that there _is_ an argument for using mss * 2 as the > default > flightsize, however. Supposedly, some OSes using delayed ACKs > will delay > the first ack, causing a 200ms delay which can really slow > down the > transfer of small web pages / etc. If you can find (tcpdump) > evidence to > back this up, I could agree with raising the value to 2 * mss. > Beyond > that, however, seems like a cheap way to inflate benchmarks > and cause > congestion. Well, if we have congestion with cwnd = 2 * mss, we have it regardless of initial window. Just in case of initial window = 2 * mss we have congestion 1 rtt earlier... 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... > Mike "Silby" Silbersack > > __________________________________________________ Do You Yahoo!? HotJobs - Search Thousands of New Jobs http://www.hotjobs.com To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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