Date: Thu, 18 Feb 2010 08:18:31 -0800 From: Stephen Hurd <shurd@sasktel.net> To: Harald Schmalzbauer <h.schmalzbauer@omnilan.de> Cc: freebsd-stable@freebsd.org, Patrick Mahan <mahan@mahan.org> Subject: Re: RELENG_8 ignoring TCP window size? [Was: Re: Help for TCP understanding wanted, ACK-MSS-Window [Was: Re: best practice to watch TCP parms of established sockets]] Message-ID: <4B7D6857.80902@sasktel.net> In-Reply-To: <4B7D6635.20605@sasktel.net> References: <4B7C1365.9070806@omnilan.de> <70CD649D-7659-4CE2-A16C-49B8C891CB5B@mac.com> <4B7C4066.5040006@omnilan.de> <4B7D3938.1000309@omnilan.de> <4B7D5AC4.9020509@mahan.org> <4B7D61DE.2020906@omnilan.de> <4B7D6635.20605@sasktel.net>
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Stephen Hurd wrote: >> Some experimental results: >> When rsyncing with windows, and FreeBSD is receiver, I see the same >> ACK ever two segemnts, but speed is at 72MB/s. >> When FreeBSD is sender and Windows is receiver, it looks more I >> expected. There are about 20 data segments before a ACK is returned. >> And there are TCP Window Update Segments, reflecting smaller >> receiver buffers on the windows side. But this happens at a >> throughput of 82MB/s!!! So the windows machine is behaving like I >> understand the TCP flow control. >> Any explanation why the FreeBSD machine seems to ignore window size? > > The idea of delayed ACKs is to allow an ACK to be sent with data if > there will be data sent right away, not to combine ACKs... leaving out > ACKs makes calculation of RTT problematical which causes performance > problems all over the place... maybe the dearth of ACKs from the > windows system is causing the problem? If the problem is the ACKs from the Windows system, you should be seeing a large number of retransmits from the FreeBSD system (on the order of one retransmit per five packets). Is this what you're seeing? If you have a capture you could share covering a few seconds, I could take a look and provide a better opinion.
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