Use the user supplied TCP MSS when creating a new active socket

This change supports using a user supplied TCP MSS for new active TCP
connections. Note, the user supplied MSS must be less than or equal to the
maximum possible MSS for a TCP connection's route. If it is greater than the
maximum possible MSS, the maximum possible MSS will be used as the connection's
MSS instead.

This change does not use this user supplied MSS for connections accepted from
listening sockets - that will come in a later change.

Test: Test that outgoing TCP SYN segments contain a TCP MSS option with the user
supplied MSS if it is not greater than the maximum possible MSS for the route.
PiperOrigin-RevId: 277185125

1 files changed, 124 insertions, 0 deletions

diff --git a/pkg/tcpip/transport/tcp/tcp_test.go b/pkg/tcpip/transport/tcp/tcp_test.go
index 6d808328c..126f26ed3 100644
--- a/pkg/tcpip/transport/tcp/tcp_test.go
+++ b/pkg/tcpip/transport/tcp/tcp_test.go
@@ -474,6 +474,130 @@ func TestSimpleReceive(t *testing.T) {
 	)
 }
 
+// TestUserSuppliedMSSOnConnectV4 tests that the user supplied MSS is used when
+// creating a new active IPv4 TCP socket. It should be present in the sent TCP
+// SYN segment.
+func TestUserSuppliedMSSOnConnectV4(t *testing.T) {
+	const mtu = 5000
+	const maxMSS = mtu - header.IPv4MinimumSize - header.TCPMinimumSize
+	tests := []struct {
+		name   string
+		setMSS uint16
+		expMSS uint16
+	}{
+		{
+			"EqualToMaxMSS",
+			maxMSS,
+			maxMSS,
+		},
+		{
+			"LessThanMTU",
+			maxMSS - 1,
+			maxMSS - 1,
+		},
+		{
+			"GreaterThanMTU",
+			maxMSS + 1,
+			maxMSS,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			c := context.New(t, mtu)
+			defer c.Cleanup()
+
+			c.Create(-1)
+
+			// Set the MSS socket option.
+			opt := tcpip.MaxSegOption(test.setMSS)
+			if err := c.EP.SetSockOpt(opt); err != nil {
+				t.Fatalf("SetSockOpt(%#v) failed: %s", opt, err)
+			}
+
+			// Get expected window size.
+			rcvBufSize, err := c.EP.GetSockOptInt(tcpip.ReceiveBufferSizeOption)
+			if err != nil {
+				t.Fatalf("GetSockOpt(%v) failed: %s", tcpip.ReceiveBufferSizeOption, err)
+			}
+			ws := tcp.FindWndScale(seqnum.Size(rcvBufSize))
+
+			// Start connection attempt to IPv4 address.
+			if err := c.EP.Connect(tcpip.FullAddress{Addr: context.TestAddr, Port: context.TestPort}); err != tcpip.ErrConnectStarted {
+				t.Fatalf("Unexpected return value from Connect: %v", err)
+			}
+
+			// Receive SYN packet with our user supplied MSS.
+			checker.IPv4(t, c.GetPacket(), checker.TCP(
+				checker.DstPort(context.TestPort),
+				checker.TCPFlags(header.TCPFlagSyn),
+				checker.TCPSynOptions(header.TCPSynOptions{MSS: test.expMSS, WS: ws})))
+		})
+	}
+}
+
+// TestUserSuppliedMSSOnConnectV6 tests that the user supplied MSS is used when
+// creating a new active IPv6 TCP socket. It should be present in the sent TCP
+// SYN segment.
+func TestUserSuppliedMSSOnConnectV6(t *testing.T) {
+	const mtu = 5000
+	const maxMSS = mtu - header.IPv6MinimumSize - header.TCPMinimumSize
+	tests := []struct {
+		name   string
+		setMSS uint16
+		expMSS uint16
+	}{
+		{
+			"EqualToMaxMSS",
+			maxMSS,
+			maxMSS,
+		},
+		{
+			"LessThanMTU",
+			maxMSS - 1,
+			maxMSS - 1,
+		},
+		{
+			"GreaterThanMTU",
+			maxMSS + 1,
+			maxMSS,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			c := context.New(t, mtu)
+			defer c.Cleanup()
+
+			c.CreateV6Endpoint(true)
+
+			// Set the MSS socket option.
+			opt := tcpip.MaxSegOption(test.setMSS)
+			if err := c.EP.SetSockOpt(opt); err != nil {
+				t.Fatalf("SetSockOpt(%#v) failed: %s", opt, err)
+			}
+
+			// Get expected window size.
+			rcvBufSize, err := c.EP.GetSockOptInt(tcpip.ReceiveBufferSizeOption)
+			if err != nil {
+				t.Fatalf("GetSockOpt(%v) failed: %s", tcpip.ReceiveBufferSizeOption, err)
+			}
+			ws := tcp.FindWndScale(seqnum.Size(rcvBufSize))
+
+			// Start connection attempt to IPv6 address.
+			if err := c.EP.Connect(tcpip.FullAddress{Addr: context.TestV6Addr, Port: context.TestPort}); err != tcpip.ErrConnectStarted {
+				t.Fatalf("Unexpected return value from Connect: %v", err)
+			}
+
+			// Receive SYN packet with our user supplied MSS.
+			checker.IPv6(t, c.GetV6Packet(), checker.TCP(
+				checker.DstPort(context.TestPort),
+				checker.TCPFlags(header.TCPFlagSyn),
+				checker.TCPSynOptions(header.TCPSynOptions{MSS: test.expMSS, WS: ws})))
+		})
+	}
+}
+
 func TestTOSV4(t *testing.T) {
 	c := context.New(t, defaultMTU)
 	defer c.Cleanup()