wgengine/netstack: enable TCP SACK (#6066)

TCP selective acknowledgement can improve throughput by an order
of magnitude in the presence of loss.

Signed-off-by: Jordan Whited <jordan@tailscale.com>
(cherry picked from commit a471681)

wgengine/netstack/netstack.go

@@ -160,6 +160,11 @@ func Create(logf logger.Logf, tundev *tstun.Wrapper, e wgengine.Engine, mc *magi
 		NetworkProtocols:   []stack.NetworkProtocolFactory{ipv4.NewProtocol, ipv6.NewProtocol},
 		TransportProtocols: []stack.TransportProtocolFactory{tcp.NewProtocol, udp.NewProtocol, icmp.NewProtocol4, icmp.NewProtocol6},
 	})
+	sackEnabledOpt := tcpip.TCPSACKEnabled(true) // TCP SACK is disabled by default
+	tcpipErr := ipstack.SetTransportProtocolOption(tcp.ProtocolNumber, &sackEnabledOpt)
+	if tcpipErr != nil {
+		return nil, fmt.Errorf("could not enable TCP SACK: %v", tcpipErr)
+	}
 	linkEP := channel.New(512, mtu, "")
 	if tcpipProblem := ipstack.CreateNIC(nicID, linkEP); tcpipProblem != nil {
 		return nil, fmt.Errorf("could not create netstack NIC: %v", tcpipProblem)