netdev - Re: [PATCH 1/2] [TCP] FRTO: Fix fallback to conventional recovery

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Date:	Tue, 13 May 2008 02:53:50 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	ilpo.jarvinen@...sinki.fi
Cc:	netdev@...r.kernel.org, damon@...tek.com
Subject: Re: [PATCH 1/2] [TCP] FRTO: Fix fallback to conventional recovery

From: "Ilpo_Järvinen" <ilpo.jarvinen@...sinki.fi>
Date: Tue, 13 May 2008 12:43:53 +0300

> It seems that commit 009a2e3e4ec ("[TCP] FRTO: Improve
> interoperability with other undo_marker users") run into
> another land-mine which caused fallback to conventional
> recovery to break:
> 
> 1. Cumulative ACK arrives after FRTO retransmission
> 2. tcp_try_to_open sees zero retrans_out, clears retrans_stamp
>    which should be kept like in CA_Loss state it would be
> 3. undo_marker change allowed tcp_packet_delayed to return
>    true because of the cleared retrans_stamp once FRTO is
>    terminated causing LossUndo to occur, which means all loss
>    markings FRTO made are reverted.
> 
> This means that the conventional recovery basically recovered
> one loss per RTT, which is not that efficient. It was quite
> unobvious that the undo_marker change broken something like
> this, I had a quite long session to track it down because of
> the non-intuitiviness of the bug (luckily I had a trivial
> reproducer at hand and I was also able to learn to use kprobes
> in the process as well :-)).
> 
> This together with the NewReno+FRTO fix and FRTO in-order
> workaround this fixes Damon's problems, this and the first
> mentioned are enough to fix Bugzilla #10063.
> 
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
> Reported-by: Damon L. Chesser <damon@...tek.com>
> Tested-by: Damon L. Chesser <damon@...tek.com>
> Tested-by: Sebastian Hyrwall <zibbe@...ko.org>

Applied.
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