// SPDX-License-Identifier: GPL-2.0
// Test ECN: verify that Linux TCP ECN sending code uses ECT0 (not ECT1).
//
`./defaults.sh
sysctl -q net.ipv4.tcp_ecn=1  # fully enabled
`

// Initialize connection
    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 4

// ECN handshake: send EW flags in SYN packet, E flag in SYN-ACK response
+.002 ... 0.004 connect(4, ..., ...) = 0

   +0 > SEW 0:0(0) <mss 1460,sackOK,TS val 100 ecr 0,nop,wscale 8>
+.002 < SE. 0:0(0) ack 1 win 32767 <mss 1000,nop,wscale 6,nop,nop,sackOK>
   +0 > . 1:1(0) ack 1

// Write 1 MSS.
+.002 write(4, ..., 1000) = 1000
// Send 1 MSS with ect0.
   +0 > [ect0] P. 1:1001(1000) ack 1