netdev - Re: [PATCH] tcp: TCP_USER_TIMEOUT can not work in tcp_probe

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Date:   Mon, 11 Sep 2017 08:22:28 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     liujian56@...wei.com
Cc:     davem@...emloft.net, kuznet@....inr.ac.ru, yoshfuji@...ux-ipv6.org,
        edumazet@...gle.com, ycheng@...gle.com, hkchu@...gle.com,
        netdev@...r.kernel.org, weiyongjun1@...wei.com
Subject: Re: [PATCH] tcp: TCP_USER_TIMEOUT can not work in tcp_probe_timer()

On Mon, 2017-09-11 at 08:13 -0700, Eric Dumazet wrote:

> You can see we got only 3 probes, not 4.

Here is complete packetdrill test showing that code behaves as expected.

    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
   +0 bind(3, ..., ...) = 0
   +0 listen(3, 1) = 0

   +0 < S 0:0(0) win 0 <mss 1460,sackOK,nop,nop,nop,wscale 7>
   +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8>

// Client advertises a zero receive window, so we can't send.
  +.1 < . 1:1(0) ack 1 win 0
   +0 accept(3, ..., ...) = 4

   +0 setsockopt(4, SOL_TCP, TCP_USER_TIMEOUT, [3000], 4) = 0
   +0 write(4, ..., 2920) = 2920

// Window probes are scheduled just like RTOs.
  +.3~+.31 > . 0:0(0) ack 1
  +.6~+.62 > . 0:0(0) ack 1
 +1.2~+1.24 > . 0:0(0) ack 1

// Peer opens its window too late !
   +3 < . 1:1(0) ack 1 win 1000
   +0 > R 1:1(0)