linux-kernel - RE: TCP stack behaviour question

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Message-ID: <005a01c6db50$587929c0$294b82ce@stuartm>
Date:	Mon, 18 Sep 2006 14:29:14 -0400
From:	"Stuart MacDonald" <stuartm@...necttech.com>
To:	"'Stuart MacDonald'" <stuartm@...necttech.com>,
	"'Andi Kleen'" <ak@...e.de>
Cc:	<linux-kernel@...r.kernel.org>
Subject: RE: TCP stack behaviour question

From: Stuart MacDonald [mailto:stuartm@...necttech.com] 
> What happened was this: I had a run where I captured output with
> tcpdump. My original post was based on that, and the results of the
> debug output from my app. For whatever reason, it appears the stack
> didn't generate all of the packets it should have. When the log showed
> a second-last to last retransmit time of about 27 seconds, and then a
> gap of about 400 to the very next packet of any kind, I assumed that
> meant the stack had given up on the retransmits when it appears
> something else was going on.

I did another run and confirmed this. The tcpdump capture shows that
seven retransmits are sent, obeying the exponential backoff. Then
something odd happens. Instead of the 8th retransmit at 7th + 26.88
seconds, there is an arp at 7th + 4.159722 seconds. There are three
arps in fact, each one second apart and directed to the MAC of the
powered-off machine. After this there are further arps (in groups of
three one second apart), but they are broadcast and have a backoff
schedule.

The kernel debugging shows that tcp_write_timeout() and
tcp_retransmit_timer() are still being called though, right up to what
would be the 16th retransmit.

I suppose that the TCP retransmits aren't being sent because the
ethernet and/or IP layers don't know what's going on, which is what's
producing the arps. Is that correct? Is that documented anywhere?

I was expecting to see all 15 retransmits, and was confused when I
didn't see them.

..Stu

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