[1506] in linux-net channel archive
Re: double acks?
daemon@ATHENA.MIT.EDU (Olaf Erb)
Fri Dec 8 10:39:13 1995
From: Olaf Erb <erb@inss1.etec.uni-karlsruhe.de>
To: linux-vger@wab-tis.rabobank.nl
Date: Thu, 7 Dec 1995 15:18:19 +0100 (MET)
Cc: linux-net@vger.rutgers.edu
In-Reply-To: <LAA27850@sys3.pe1chl.ampr.org> from "Rob Janssen reading Linux mailinglist" at Dec 7, 95 11:42:18 am
Reply-To: erb@inss1.etec.uni-karlsruhe.de
Rob Janssen reading Linux mailinglist writes:
> According to Olaf Erb:
> > I'm wondering if anyone hasn't noticed that Linux always sends double
> > acks on tcp sessions. Just a little example:
> >
> > 44.130.41.11.1186 > 44.130.29.32.79: P 1:3(2) ack 1 win 432
> > Data (2): ^M^J (DF)
> > 44.130.29.32.79 > 44.130.41.11.1186: . ack 3 win 1024
> > 44.130.29.32.79 > 44.130.41.11.1186: . ack 3 win 1024
> >
> > This is on a very slow radio link. The same happens on slip, and
> > on other slow links (100ms rtt is enough to get 2 acks).
> > (tested on 1.2.13, 1.3.45).
> >
> > On slow links every packet is important, so maybe someone already has a
> > solution? Or is it a bug in my config, since nobody mentioned it here or
> > in the newsgroups?
>
> Yes it has been noticed before.
> Linux sends one ack immediately, and another when the receiving
> process reads the data. Normally you will see a larger "win" value
> on the second ACK. Strange... I don't see this in your example. But
> maybe that was just a reconstruction from memory, and not an actual
> trace.
>
> Anyway, there was a patch posted on the linux-net list a few weeks ago, which
> sends the second ACK only when the first one had sent a very small window.
> This is the same as the KA9Q code does. (it only sends the second ACK
> when the first one had sent window 0)
I supplied a window to the route, because >>14k (default) is too much for
our packet radio links. Maybe that's the reason?
Olaf
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