TUN-5593: Read full packet from UDP connection, even if it exceeds MTU of the transport. When packet length is greater than the MTU of the transport, we will silently drop packets (for now).

datagramsession/manager.go

@@ -95,7 +95,7 @@ func (m *manager) RegisterSession(ctx context.Context, sessionID uuid.UUID, orig
 }
 
 func (m *manager) registerSession(ctx context.Context, registration *registerSessionEvent) {
-	session := newSession(registration.sessionID, m.transport, registration.originProxy)
+	session := newSession(registration.sessionID, m.transport, registration.originProxy, m.log)
 	m.sessions[registration.sessionID] = session
 	registration.resultChan <- session
 }

datagramsession/session.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"github.com/google/uuid"
+	"github.com/rs/zerolog"
 )
 
 const (
@@ -17,7 +18,7 @@ func SessionIdleErr(timeout time.Duration) error {
 	return fmt.Errorf("session idle for %v", timeout)
 }
 
-// Each Session is a bidirectional pipe of datagrams between transport and dstConn
+// Session is a bidirectional pipe of datagrams between transport and dstConn
 // Currently the only implementation of transport is quic DatagramMuxer
 // Destination can be a connection with origin or with eyeball
 // When the destination is origin:
@@ -35,9 +36,10 @@ type Session struct {
 	// activeAtChan is used to communicate the last read/write time
 	activeAtChan chan time.Time
 	closeChan    chan error
+	log          *zerolog.Logger
 }
 
-func newSession(id uuid.UUID, transport transport, dstConn io.ReadWriteCloser) *Session {
+func newSession(id uuid.UUID, transport transport, dstConn io.ReadWriteCloser, log *zerolog.Logger) *Session {
 	return &Session{
 		ID:        id,
 		transport: transport,
@@ -47,6 +49,7 @@ func newSession(id uuid.UUID, transport transport, dstConn io.ReadWriteCloser) *
 		activeAtChan: make(chan time.Time, 2),
 		// capacity is 2 because close() and dstToTransport routine in Serve() can write to this channel
 		closeChan: make(chan error, 2),
+		log:       log,
 	}
 }
 
@@ -54,7 +57,8 @@ func (s *Session) Serve(ctx context.Context, closeAfterIdle time.Duration) (clos
 	go func() {
 		// QUIC implementation copies data to another buffer before returning https://github.com/lucas-clemente/quic-go/blob/v0.24.0/session.go#L1967-L1975
 		// This makes it safe to share readBuffer between iterations
-		readBuffer := make([]byte, s.transport.ReceiveMTU())
+		const maxPacketSize = 1500
+		readBuffer := make([]byte, maxPacketSize)
 		for {
 			if err := s.dstToTransport(readBuffer); err != nil {
 				s.closeChan <- err
@@ -103,8 +107,15 @@ func (s *Session) dstToTransport(buffer []byte) error {
 	n, err := s.dstConn.Read(buffer)
 	s.markActive()
 	if n > 0 {
-		if err := s.transport.SendTo(s.ID, buffer[:n]); err != nil {
-			return err
+		if n <= int(s.transport.MTU()) {
+			err = s.transport.SendTo(s.ID, buffer[:n])
+		} else {
+			// drop packet for now, eventually reply with ICMP for PMTUD
+			s.log.Debug().
+				Str("session", s.ID.String()).
+				Int("len", n).
+				Uint("mtu", s.transport.MTU()).
+				Msg("dropped packet exceeding MTU")
 		}
 	}
 	return err

datagramsession/session_test.go

@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/google/uuid"
+	"github.com/rs/zerolog"
 	"github.com/stretchr/testify/require"
 	"golang.org/x/sync/errgroup"
 )
@@ -44,7 +45,8 @@ func testSessionReturns(t *testing.T, closeBy closeMethod, closeAfterIdle time.D
 		reqChan:  newDatagramChannel(1),
 		respChan: newDatagramChannel(1),
 	}
-	session := newSession(sessionID, transport, cfdConn)
+	log := zerolog.Nop()
+	session := newSession(sessionID, transport, cfdConn, &log)
 
 	ctx, cancel := context.WithCancel(context.Background())
 	sessionDone := make(chan struct{})
@@ -119,7 +121,8 @@ func testActiveSessionNotClosed(t *testing.T, readFromDst bool, writeToDst bool)
 		reqChan:  newDatagramChannel(100),
 		respChan: newDatagramChannel(100),
 	}
-	session := newSession(sessionID, transport, cfdConn)
+	log := zerolog.Nop()
+	session := newSession(sessionID, transport, cfdConn, &log)
 
 	startTime := time.Now()
 	activeUntil := startTime.Add(activeTime)
@@ -181,7 +184,7 @@ func testActiveSessionNotClosed(t *testing.T, readFromDst bool, writeToDst bool)
 
 func TestMarkActiveNotBlocking(t *testing.T) {
 	const concurrentCalls = 50
-	session := newSession(uuid.New(), nil, nil)
+	session := newSession(uuid.New(), nil, nil, nil)
 	var wg sync.WaitGroup
 	wg.Add(concurrentCalls)
 	for i := 0; i < concurrentCalls; i++ {

datagramsession/transport.go

@@ -9,5 +9,5 @@ type transport interface {
 	// ReceiveFrom reads the next datagram from the transport
 	ReceiveFrom() (uuid.UUID, []byte, error)
 	// Max transmission unit to receive from the transport
-	ReceiveMTU() uint
+	MTU() uint
 }

datagramsession/transport_test.go

@@ -22,7 +22,7 @@ func (mt *mockQUICTransport) ReceiveFrom() (uuid.UUID, []byte, error) {
 	return mt.reqChan.Receive(context.Background())
 }
 
-func (mt *mockQUICTransport) ReceiveMTU() uint {
+func (mt *mockQUICTransport) MTU() uint {
 	return 1217
 }