netdev - Re: [PATCH next] Revert "dctcp: update cwnd on congestion event"

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Message-Id: <20161206.113442.100496871002228037.davem@davemloft.net>
Date:   Tue, 06 Dec 2016 11:34:42 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     fw@...len.de
Cc:     netdev@...r.kernel.org, ncardwell@...gle.com
Subject: Re: [PATCH next] Revert "dctcp: update cwnd on congestion event"

From: Florian Westphal <fw@...len.de>
Date: Tue,  6 Dec 2016 00:23:00 +0100

> Neal Cardwell says:
>  If I am reading the code correctly, then I would have two concerns:
>  1) Has that been tested? That seems like an extremely dramatic
>     decrease in cwnd. For example, if the cwnd is 80, and there are 40
>     ACKs, and half the ACKs are ECE marked, then my back-of-the-envelope
>     calculations seem to suggest that after just 11 ACKs the cwnd would be
>     down to a minimal value of 2 [..]
>  2) That seems to contradict another passage in the draft [..] where it
>     sazs:
>        Just as specified in [RFC3168], DCTCP does not react to congestion
>        indications more than once for every window of data.
> 
> Neal is right.  Fortunately we don't have to complicate this by testing
> vs. current rtt estimate, we can just revert the patch.
> 
> Normal stack already handles this for us: receiving ACKs with ECE
> set causes a call to tcp_enter_cwr(), from there on the ssthresh gets
> adjusted and prr will take care of cwnd adjustment.
> 
> Fixes: 4780566784b396 ("dctcp: update cwnd on congestion event")
> Cc: Neal Cardwell <ncardwell@...gle.com>
> Signed-off-by: Florian Westphal <fw@...len.de>

Applied.