Date: Wed, 6 Dec 2006 15:37:43 -0800 From: Chuck Swiger <cswiger@mac.com> To: freebsd@sopwith.solgatos.com Cc: freebsd-questions@freebsd.org Subject: Re: TCP parameters and interpreting tcpdump output Message-ID: <E7012548-6D08-4DBA-B408-A7F97EBB9DB8@mac.com> In-Reply-To: <200612062246.WAA17265@sopwith.solgatos.com> References: <200612062246.WAA17265@sopwith.solgatos.com>
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On Dec 6, 2006, at 6:46 AM, Dieter wrote:
> I found a couple more things that don't look right.
>
> 000017 IP bsd.63743 > src.65001: . ack 52 win 65535
> 000107 IP bsd.63743 > src.65001: . ack 52 win 65535
> 000012 IP bsd.63743 > src.65001: F 52:52(0) ack 52 win 65535
> 000005 IP bsd.63743 > src.65001: F 52:52(0) ack 52 win 65535
> 000172 IP src.65001 > bsd.63743: . ack 53 win 4096
> 000004 IP src.65001 > bsd.63743: F 52:52(0) ack 53 win 4096
> 000003 IP src.65001 > bsd.63743: . ack 53 win 4096
> 000016 IP bsd.63743 > src.65001: . ack 52 win 65535
> 000011 IP bsd.63743 > src.65001: . ack 53 win 112 <------ why does
> the window suddenly shrink?
I'd guess because both sides have requested that the connection
close...it's a bit hard to say because you're not including complete
data over the life of a connection, or even the sequence numbers.
("tcpdump -nttt" or "tcpdump -ntttv" would be more informative.)
> 002366 IP src.rfe > bsd.12340: P 1:1317(1316) ack 1 win 4096
> 099554 IP bsd.12340 > src.rfe: . ack 1317 win 65535 <------ why
> does it take 99.5 millisec to ack?
>
> The ack time is normally 12 or 13 microseconds, which seems to be
> okay.
> But 99.5 milliseconds is *way* too slow, data will be lost.
>
> Is TCP sitting around waiting for a second packet, so that
> it can be "efficient" and ack two packets at once?
Yup. Coalescing data before sending it results in less overhead.
> What can I do to fix this? Is there a knob I can turn to say
> "ack every packet", or "only wait xxx microseconds for a 2nd packet" ?
You can turn on TCP_NODELAY via setsockopt() to disable the Nagle
algorithm. There are probably sysctl's you can tweak, also...
--
-Chuck
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