Set IPv4 ID on all non-atomic datagrams

RFC 6864 imposes various restrictions on the uniqueness of the IPv4
Identification field for non-atomic datagrams, defined as an IP datagram that
either can be fragmented (DF=0) or is already a fragment (MF=1 or positive
fragment offset). In order to be compliant, the ID field is assigned for all
non-atomic datagrams.

Add a TCP unit test that induces retransmissions and checks that the IPv4
ID field is unique every time. Add basic handling of the IP_MTU_DISCOVER
socket option so that the option can be used to disable PMTU discovery,
effectively setting DF=0. Attempting to set the sockopt to anything other
than disabled will fail because PMTU discovery is currently not implemented,
and the default behavior matches that of disabled.

PiperOrigin-RevId: 320081842

2 files changed, 69 insertions, 0 deletions

diff --git a/pkg/tcpip/transport/tcp/endpoint.go b/pkg/tcpip/transport/tcp/endpoint.go
index bd3ec5a8d..caac6ef57 100644
--- a/pkg/tcpip/transport/tcp/endpoint.go
+++ b/pkg/tcpip/transport/tcp/endpoint.go
@@ -1589,6 +1589,13 @@ func (e *endpoint) SetSockOptInt(opt tcpip.SockOptInt, v int) *tcpip.Error {
 		e.UnlockUser()
 		e.notifyProtocolGoroutine(notifyMSSChanged)
 
+	case tcpip.MTUDiscoverOption:
+		// Return not supported if attempting to set this option to
+		// anything other than path MTU discovery disabled.
+		if v != tcpip.PMTUDiscoveryDont {
+			return tcpip.ErrNotSupported
+		}
+
 	case tcpip.ReceiveBufferSizeOption:
 		// Make sure the receive buffer size is within the min and max
 		// allowed.
@@ -1896,6 +1903,11 @@ func (e *endpoint) GetSockOptInt(opt tcpip.SockOptInt) (int, *tcpip.Error) {
 		v := header.TCPDefaultMSS
 		return v, nil
 
+	case tcpip.MTUDiscoverOption:
+		// Always return the path MTU discovery disabled setting since
+		// it's the only one supported.
+		return tcpip.PMTUDiscoveryDont, nil
+
 	case tcpip.ReceiveQueueSizeOption:
 		return e.readyReceiveSize()
 
diff --git a/pkg/tcpip/transport/tcp/tcp_test.go b/pkg/tcpip/transport/tcp/tcp_test.go
index 169adb16b..e67ec42b1 100644
--- a/pkg/tcpip/transport/tcp/tcp_test.go
+++ b/pkg/tcpip/transport/tcp/tcp_test.go
@@ -3095,6 +3095,63 @@ func TestMaxRTO(t *testing.T) {
 	}
 }
 
+// TestRetransmitIPv4IDUniqueness tests that the IPv4 Identification field is
+// unique on retransmits.
+func TestRetransmitIPv4IDUniqueness(t *testing.T) {
+	for _, tc := range []struct {
+		name string
+		size int
+	}{
+		{"1Byte", 1},
+		{"512Bytes", 512},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			c := context.New(t, defaultMTU)
+			defer c.Cleanup()
+
+			c.CreateConnected(789 /* iss */, 30000 /* rcvWnd */, -1 /* epRcvBuf */)
+
+			// Disabling PMTU discovery causes all packets sent from this socket to
+			// have DF=0. This needs to be done because the IPv4 ID uniqueness
+			// applies only to non-atomic IPv4 datagrams as defined in RFC 6864
+			// Section 4, and datagrams with DF=0 are non-atomic.
+			if err := c.EP.SetSockOptInt(tcpip.MTUDiscoverOption, tcpip.PMTUDiscoveryDont); err != nil {
+				t.Fatalf("disabling PMTU discovery via sockopt to force DF=0 failed: %s", err)
+			}
+
+			if _, _, err := c.EP.Write(tcpip.SlicePayload(buffer.NewView(tc.size)), tcpip.WriteOptions{}); err != nil {
+				t.Fatalf("Write failed: %s", err)
+			}
+			pkt := c.GetPacket()
+			checker.IPv4(t, pkt,
+				checker.FragmentFlags(0),
+				checker.TCP(
+					checker.DstPort(context.TestPort),
+					checker.TCPFlagsMatch(header.TCPFlagAck, ^uint8(header.TCPFlagPsh)),
+				),
+			)
+			idSet := map[uint16]struct{}{header.IPv4(pkt).ID(): struct{}{}}
+			// Expect two retransmitted packets, and that all packets received have
+			// unique IPv4 ID values.
+			for i := 0; i <= 2; i++ {
+				pkt := c.GetPacket()
+				checker.IPv4(t, pkt,
+					checker.FragmentFlags(0),
+					checker.TCP(
+						checker.DstPort(context.TestPort),
+						checker.TCPFlagsMatch(header.TCPFlagAck, ^uint8(header.TCPFlagPsh)),
+					),
+				)
+				id := header.IPv4(pkt).ID()
+				if _, exists := idSet[id]; exists {
+					t.Fatalf("duplicate IPv4 ID=%d found in retransmitted packet", id)
+				}
+				idSet[id] = struct{}{}
+			}
+		})
+	}
+}
+
 func TestFinImmediately(t *testing.T) {
 	c := context.New(t, defaultMTU)
 	defer c.Cleanup()