netdev - Re: [net PATCH] tcp: Set ECT0 bit in tos/tclass for synack when BPF needs ECN

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Date:   Tue, 24 Nov 2020 14:14:55 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org, daniel@...earbox.net,
        kafai@...com, kernel-team@...com, edumazet@...gle.com,
        brakmo@...com, alexanderduyck@...com
Subject: Re: [net PATCH] tcp: Set ECT0 bit in tos/tclass for synack when BPF
 needs ECN

On Fri, 20 Nov 2020 19:47:44 -0800 Alexander Duyck wrote:
> From: Alexander Duyck <alexanderduyck@...com>
> 
> When a BPF program is used to select between a type of TCP congestion
> control algorithm that uses either ECN or not there is a case where the
> synack for the frame was coming up without the ECT0 bit set. A bit of
> research found that this was due to the final socket being configured to
> dctcp while the listener socket was staying in cubic.
> 
> To reproduce it all that is needed is to monitor TCP traffic while running
> the sample bpf program "samples/bpf/tcp_cong_kern.c". What is observed,
> assuming tcp_dctcp module is loaded or compiled in and the traffic matches
> the rules in the sample file, is that for all frames with the exception of
> the synack the ECT0 bit is set.
> 
> To address that it is necessary to make one additional call to
> tcp_bpf_ca_needs_ecn using the request socket and then use the output of
> that to set the ECT0 bit for the tos/tclass of the packet.
> 
> Fixes: 91b5b21c7c16 ("bpf: Add support for changing congestion control")
> Signed-off-by: Alexander Duyck <alexanderduyck@...com>

Applied, thank you!