// Test fast retransmit with 4 packets outstanding, receiver sending SACKs.
// In this variant the receiver supports SACK.

// Establish a connection.
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 32792 <mss 1000,sackOK,nop,nop,nop,wscale 7>
+0  > S. 0:0(0) ack 1 <...>

+.1 < . 1:1(0) ack 1 win 32792
+0  accept(3, ..., ...) = 4

// Send 1 data segment and get an ACK, so cwnd is now 4.
+0  write(4, ..., 1000) = 1000
+0  > . 1:1001(1000) ack 1

+.1 < . 1:1(0) ack 1001 win 32792

// Write 4 data segments.
+0  write(4, ..., 4000) = 4000
+0  > . 1001:5001(4000) ack 1

// Get 3 SACKs.
+.1 < . 1:1(0) ack 1001 win 32792 <sack 2001:3001,nop,nop>
+0  < . 1:1(0) ack 1001 win 32792 <sack 2001:4001,nop,nop>
+0  < . 1:1(0) ack 1001 win 32792 <sack 2001:5001,nop,nop>
// We've received 3 duplicate ACKs, so we do a fast retransmit.
+0  > . 1001:3049(2048) ack 1

// Receiver ACKs all data.
+.1 < . 1:1(0) ack 6001 win 32792