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To: Andre Oppermann <andre@freebsd.org>
In-reply-to: Your message of "Sat, 23 Sep 2006 12:20:01 +0200."
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Date: Sat, 23 Sep 2006 15:28:36 +0100
From: David Malone <dwmalone@maths.tcd.ie>
Message-ID: <200609231528.aa12963@walton.maths.tcd.ie>
Cc: Andrew Gallatin <gallatin@cs.duke.edu>, alc@freebsd.org,
	freebsd-net@freebsd.org, freebsd-current@freebsd.org,
	Robert Watson <rwatson@freebsd.org>, tegge@freebsd.org
Subject: Re: Much improved sendfile(2) kernel implementation 
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> The congestion window is increased based on the ACK's received.  TSO
> is only done on the send side and only up to the current congestion
> window.  I have been careful not to get any changes in congestion
> control behavior with TSO.  (Which does not mean that there may be
> other bugs lurking in our congestion control.)

I think the reason this happened in Linux was because thw congestion
window is counted in segments, which were now TSO sized. You'd send
1 TSO sized segment, get back (say) 10 ACKs because of segmentation
and increase the window size by 10*TSO_SEG_SIZE/cwnd insead of
10*REAL_MSS/cwnd. We're unlikely to have exactly the same bug,
because we count cwnd in bytes, but it doesn't rule out haveing
other unexpected/subtle interactions (like higher varience of RTT
esitmation - I guess all packets in a TSO segment are now sent with
the same timestamp?).

	David.