netdev - Re: Linux ECN Handling

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Date:   Tue, 5 Dec 2017 15:04:57 -0500
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Steve Ibanez <sibanez@...nford.edu>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Netdev <netdev@...r.kernel.org>, Florian Westphal <fw@...len.de>,
        Mohammad Alizadeh <alizadeh@...il.mit.edu>,
        Lawrence Brakmo <brakmo@...com>
Subject: Re: Linux ECN Handling

On Tue, Dec 5, 2017 at 2:36 PM, Steve Ibanez <sibanez@...nford.edu> wrote:
> Hi Neal,
>
> I've included a link to small trace of 13 packets which is different
> from the screenshot I attached in my last email, but shows the same
> sequence of events. It's a bit hard to read the tcptrace due to the
> 300ms timeout, so I figured this was the best approach.
>
> slice.pcap: https://drive.google.com/open?id=1hYXbUClHGbQv1hWG1HZWDO2WYf30N6G8

Thanks for the trace! Attached is a screen shot (first screen shot is
for the arriving packets with CWR; second is after the RTO). The
sender behavior looks reasonable. I don't see why the receiver is not
ACKing. As you say, it does look like a receiver bug. You could try
adding instrumentation to try to isolate why the receiver is not
sending an ACK immediately. You might instrument __tcp_ack_snd_check()
and tcp_send_delayed_ack() so that when the most recent incoming
packet had cwr set they printk to log what they are deciding in this
case. Perhaps the tcp_send_delayed_ack()  code is hitting the max_ato
= HZ / 2 code path?

neal

Download attachment "cwr-no-ack-1.png" of type "image/png" (9725 bytes)

Download attachment "cwr-no-ack-2.png" of type "image/png" (11703 bytes)