Re: [PATCH net-next] tcp: Fix CWV being too strict on thin streams

On Tue, 2015-09-22 at 09:09 -0700, Eric Dumazet wrote:

On Tue, 2015-09-22 at 07:46 -0700, Eric Dumazet wrote:

quoted

Ahem.

packetdrill can make this in one script, as you can exactly control the
packets that the 'remote' peer would answer.

No need for complex setup. You should try it, and as a bonus we could
easily reproduce the problem and check the fix.

Let see if we can cook a packetdrill scenario.

Following packetdrill skeleton will provide you a connexion with cwnd=1
sshtresh=2  (without patching kernel)

A more complete packetdrill scenario would be

# cat cwv.pkt
// Establish a connection and send 1 MSS.
0     socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
   +0 setsockopt(3, SOL_TCP, TCP_NODELAY, [1], 4) = 0
   +0 bind(3, ..., ...) = 0
   +0 listen(3, 1) = 0

   +0 < S 0:0(0) win 65535 <mss 1000,sackOK,nop,nop>
   +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK>
+.200 < . 1:1(0) ack 1 win 65535
   +0 accept(3, ..., ...) = 4

   +0 write(4, ..., 100) = 100
   +0 > P. 1:101(100) ack 1
+.000 %{ print  tcpi_rto }%

// TLP
+.500~+.505 > P. 1:101(100) ack 1
// RTO
+.600~+.605 > P. 1:101(100) ack 1

+.200 < . 1:1(0) ack 101 win 65535
// cwnd should be 2, ssthresh should be 7
+0 %{ print "tcpi_snd_cwnd=%d tcpi_snd_ssthresh=%d" % (tcpi_snd_cwnd, tcpi_snd_ssthresh) }%
2.000 write(4, ..., 100) = 100
   +0  > P. 101:201(100) ack 1

// TLP
+.500~+.505 > P. 101:201(100) ack 1
+0 %{ print "tcpi_snd_cwnd=%d tcpi_snd_ssthresh=%d" % (tcpi_snd_cwnd, tcpi_snd_ssthresh) }%
// RTO
+1.200~+1.210 > P. 101:201(100) ack 1

+0 %{ print "tcpi_snd_cwnd=%d tcpi_snd_ssthresh=%d" % (tcpi_snd_cwnd, tcpi_snd_ssthresh) }%
+.200 < . 1:1(0) ack 201 win 65535

4.00 write(4, ..., 100) = 100
    +0  > P. 201:301(100) ack 1
4.01 write(4, ..., 100) = 100
	+0 > P. 301:401(100) ack 1
4.02 write(4, ..., 100) = 100
4.03 write(4, ..., 100) = 100
4.04 write(4, ..., 100) = 100
4.05 write(4, ..., 100) = 100
4.06 write(4, ..., 100) = 100
4.07 write(4, ..., 100) = 100
4.08 write(4, ..., 100) = 100
4.09 write(4, ..., 100) = 100
4.10 write(4, ..., 100) = 100
4.11 write(4, ..., 100) = 100
4.12 write(4, ..., 100) = 100
4.13 write(4, ..., 100) = 100
4.14 write(4, ..., 100) = 100
4.15 write(4, ..., 100) = 100
4.16 write(4, ..., 100) = 100
4.17 write(4, ..., 100) = 100
4.18 write(4, ..., 100) = 100
4.19 write(4, ..., 100) = 100

4.20 write(4, ..., 100) = 100
4.20 < . 1:1(0) ack 301 win 65535
4.20 > . 401:1401(1000) ack 1

4.21 write(4, ..., 100) = 100
4.21 < . 1:1(0) ack 401 win 65535
4.21 > P. 1401:2401(1000) ack 1
   +0 %{ print "tcpi_snd_cwnd=%d tcpi_snd_ssthresh=%d" % (tcpi_snd_cwnd, tcpi_snd_ssthresh) }%
4.22 write(4, ..., 100) = 100
4.23 write(4, ..., 100) = 100
4.24 write(4, ..., 100) = 100
4.25 write(4, ..., 100) = 100
4.26 write(4, ..., 100) = 100
4.27 write(4, ..., 100) = 100
4.28 write(4, ..., 100) = 100
4.29 write(4, ..., 100) = 100
4.31 write(4, ..., 100) = 100
4.32 write(4, ..., 100) = 100
4.33 write(4, ..., 100) = 100
4.34 write(4, ..., 100) = 100
4.35 write(4, ..., 100) = 100
4.36 write(4, ..., 100) = 100
4.37 write(4, ..., 100) = 100
4.38 write(4, ..., 100) = 100
4.39 write(4, ..., 100) = 100

4.40 write(4, ..., 100) = 100
4.40 < . 1:1(0) ack 1401 win 65535
4.40 > . 2401:3401(1000) ack 1
4.41 write(4, ..., 100) = 100
4.41 < . 1:1(0) ack 2401 win 65535
4.41 > P. 3401:4301(900) ack 1
   +0 %{ print "tcpi_snd_cwnd=%d tcpi_snd_ssthresh=%d" % (tcpi_snd_cwnd, tcpi_snd_ssthresh) }%
4.42 write(4, ..., 100) = 100
4.43 write(4, ..., 100) = 100
4.44 write(4, ..., 100) = 100
4.45 write(4, ..., 100) = 100
4.46 write(4, ..., 100) = 100
4.47 write(4, ..., 100) = 100
4.48 write(4, ..., 100) = 100
4.49 write(4, ..., 100) = 100
4.50 write(4, ..., 100) = 100
4.51 write(4, ..., 100) = 100
4.52 write(4, ..., 100) = 100
4.53 write(4, ..., 100) = 100
4.54 write(4, ..., 100) = 100
4.55 write(4, ..., 100) = 100
4.56 write(4, ..., 100) = 100
4.57 write(4, ..., 100) = 100
4.58 write(4, ..., 100) = 100
4.59 write(4, ..., 100) = 100
4.60 write(4, ..., 100) = 100
4.60 < . 1:1(0) ack 3401 win 65535
4.60 > . 4301:5301(1000) ack 1
4.61 write(4, ..., 100) = 100
4.61 < . 1:1(0) ack 4301 win 65535
4.61 > P. 5301:6301(1000) ack 1
   +0 %{ print "tcpi_snd_cwnd=%d tcpi_snd_ssthresh=%d" % (tcpi_snd_cwnd, tcpi_snd_ssthresh) }%
4.62 write(4, ..., 100) = 100
4.63 write(4, ..., 100) = 100
4.64 write(4, ..., 100) = 100
4.65 write(4, ..., 100) = 100
4.66 write(4, ..., 100) = 100
4.67 write(4, ..., 100) = 100
4.68 write(4, ..., 100) = 100
4.69 write(4, ..., 100) = 100
4.70 write(4, ..., 100) = 100
4.71 write(4, ..., 100) = 100
4.72 write(4, ..., 100) = 100
4.73 write(4, ..., 100) = 100
4.74 write(4, ..., 100) = 100
4.75 write(4, ..., 100) = 100
4.76 write(4, ..., 100) = 100
4.77 write(4, ..., 100) = 100
4.78 write(4, ..., 100) = 100
4.79 write(4, ..., 100) = 100
4.80 write(4, ..., 100) = 100
4.80 < . 1:1(0) ack 5301 win 65535
4.80 > . 6301:7301(1000) ack 1
4.81 write(4, ..., 100) = 100
4.81 < . 1:1(0) ack 6301 win 65535
   +0 %{ print "tcpi_snd_cwnd=%d tcpi_snd_ssthresh=%d" % (tcpi_snd_cwnd, tcpi_snd_ssthresh) }%
4.81 > P. 7301:8301(1000) ack 1
4.82 write(4, ..., 100) = 100
4.83 write(4, ..., 100) = 100
4.84 write(4, ..., 100) = 100
4.85 write(4, ..., 100) = 100
4.86 write(4, ..., 100) = 100
4.87 write(4, ..., 100) = 100
4.88 write(4, ..., 100) = 100
4.89 write(4, ..., 100) = 100
4.90 write(4, ..., 100) = 100
4.91 write(4, ..., 100) = 100
4.92 write(4, ..., 100) = 100
4.93 write(4, ..., 100) = 100
4.94 write(4, ..., 100) = 100
4.95 write(4, ..., 100) = 100
4.96 write(4, ..., 100) = 100
4.97 write(4, ..., 100) = 100
4.98 write(4, ..., 100) = 100
4.99 write(4, ..., 100) = 100
5.00 write(4, ..., 100) = 100
5.00 < . 1:1(0) ack 7301 win 65535
5.00 > . 8301:9301(1000) ack 1
5.01 write(4, ..., 100) = 100
5.01 < . 1:1(0) ack 8301 win 65535
5.01 > P. 9301:10301(1000) ack 1
  +0 %{ print "tcpi_snd_cwnd=%d tcpi_snd_ssthresh=%d" % (tcpi_snd_cwnd, tcpi_snd_ssthresh) }%
5.02 write(4, ..., 100) = 100
5.03 write(4, ..., 100) = 100
5.04 write(4, ..., 100) = 100
5.05 write(4, ..., 100) = 100
5.06 write(4, ..., 100) = 100
5.07 write(4, ..., 100) = 100
5.08 write(4, ..., 100) = 100
5.09 write(4, ..., 100) = 100
5.10 write(4, ..., 100) = 100
5.11 write(4, ..., 100) = 100
5.12 write(4, ..., 100) = 100
5.13 write(4, ..., 100) = 100
5.14 write(4, ..., 100) = 100
5.15 write(4, ..., 100) = 100
5.16 write(4, ..., 100) = 100
5.17 write(4, ..., 100) = 100
5.18 write(4, ..., 100) = 100
5.19 write(4, ..., 100) = 100
5.20 write(4, ..., 100) = 100
5.20 < . 1:1(0) ack 9301 win 65535
5.20 > . 10301:11301(1000) ack 1
5.21 write(4, ..., 100) = 100
5.21 < . 1:1(0) ack 10301 win 65535
5.21 > P. 11301:12301(1000) ack 1
+0 %{ print "tcpi_snd_cwnd=%d tcpi_snd_ssthresh=%d" % (tcpi_snd_cwnd, tcpi_snd_ssthresh) }%
5.22 write(4, ..., 100) = 100
5.23 write(4, ..., 100) = 100
5.24 write(4, ..., 100) = 100
5.25 write(4, ..., 100) = 100
5.26 write(4, ..., 100) = 100
5.27 write(4, ..., 100) = 100
5.28 write(4, ..., 100) = 100
5.29 write(4, ..., 100) = 100
5.30 write(4, ..., 100) = 100
5.31 write(4, ..., 100) = 100
5.32 write(4, ..., 100) = 100
5.33 write(4, ..., 100) = 100
5.34 write(4, ..., 100) = 100
5.35 write(4, ..., 100) = 100
5.36 write(4, ..., 100) = 100
5.37 write(4, ..., 100) = 100
5.38 write(4, ..., 100) = 100
5.39 write(4, ..., 100) = 100
5.40 write(4, ..., 100) = 100
5.40 < . 1:1(0) ack 11301 win 65535
5.40 > . 12301:13301(1000) ack 1
5.41 write(4, ..., 100) = 100
5.41 < . 1:1(0) ack 12301 win 65535
5.41 > P. 13301:14301(1000) ack 1
+0 %{ print "tcpi_snd_cwnd=%d tcpi_snd_ssthresh=%d" % (tcpi_snd_cwnd, tcpi_snd_ssthresh) }% 


Output is :
tcpi_snd_cwnd=2 tcpi_snd_ssthresh=7
tcpi_snd_cwnd=2 tcpi_snd_ssthresh=7
tcpi_snd_cwnd=1 tcpi_snd_ssthresh=2
tcpi_snd_cwnd=2 tcpi_snd_ssthresh=2
tcpi_snd_cwnd=2 tcpi_snd_ssthresh=2
tcpi_snd_cwnd=2 tcpi_snd_ssthresh=2
tcpi_snd_cwnd=2 tcpi_snd_ssthresh=2
tcpi_snd_cwnd=2 tcpi_snd_ssthresh=2
tcpi_snd_cwnd=2 tcpi_snd_ssthresh=2
tcpi_snd_cwnd=2 tcpi_snd_ssthresh=2