4 files changed, 87 insertions, 5 deletions

diff --git a/pkg/tcpip/transport/tcp/connect.go b/pkg/tcpip/transport/tcp/connect.go
index 965779a68..00cb39560 100644
--- a/pkg/tcpip/transport/tcp/connect.go
+++ b/pkg/tcpip/transport/tcp/connect.go
@@ -596,9 +596,7 @@ func sendTCP(r *stack.Route, id stack.TransportEndpointID, data buffer.Vectorise
 	if r.Capabilities()&stack.CapabilityChecksumOffload == 0 {
 		length := uint16(hdr.UsedLength() + data.Size())
 		xsum := r.PseudoHeaderChecksum(ProtocolNumber)
-		for _, v := range data.Views() {
-			xsum = header.Checksum(v, xsum)
-		}
+		xsum = header.ChecksumVV(data, xsum)
 
 		tcp.SetChecksum(^tcp.CalculateChecksum(xsum, length))
 	}
diff --git a/pkg/tcpip/transport/tcp/endpoint.go b/pkg/tcpip/transport/tcp/endpoint.go
index 673a65c31..0b395b5b0 100644
--- a/pkg/tcpip/transport/tcp/endpoint.go
+++ b/pkg/tcpip/transport/tcp/endpoint.go
@@ -243,6 +243,16 @@ type endpoint struct {
 	connectingAddress tcpip.Address
 }
 
+// StopWork halts packet processing. Only to be used in tests.
+func (e *endpoint) StopWork() {
+	e.workMu.Lock()
+}
+
+// ResumeWork resumes packet processing. Only to be used in tests.
+func (e *endpoint) ResumeWork() {
+	e.workMu.Unlock()
+}
+
 // keepalive is a synchronization wrapper used to appease stateify. See the
 // comment in endpoint, where it is used.
 //
diff --git a/pkg/tcpip/transport/tcp/snd.go b/pkg/tcpip/transport/tcp/snd.go
index b260c0e57..71bfcbd36 100644
--- a/pkg/tcpip/transport/tcp/snd.go
+++ b/pkg/tcpip/transport/tcp/snd.go
@@ -403,15 +403,36 @@ func (s *sender) sendData() {
 	// TODO: We currently don't merge multiple send buffers
 	// into one segment if they happen to fit. We should do that
 	// eventually.
-	var seg *segment
+	seg := s.writeNext
 	end := s.sndUna.Add(s.sndWnd)
 	var dataSent bool
-	for seg = s.writeNext; seg != nil && s.outstanding < s.sndCwnd; seg = seg.Next() {
+	for next := (*segment)(nil); seg != nil && s.outstanding < s.sndCwnd; seg = next {
+		next = seg.Next()
+
 		// We abuse the flags field to determine if we have already
 		// assigned a sequence number to this segment.
 		if seg.flags == 0 {
 			seg.sequenceNumber = s.sndNxt
 			seg.flags = flagAck | flagPsh
+			// Merge segments if allowed.
+			if seg.data.Size() != 0 {
+				available := int(seg.sequenceNumber.Size(end))
+				if available > limit {
+					available = limit
+				}
+
+				for next != nil && next.data.Size() != 0 {
+					if seg.data.Size()+next.data.Size() > available {
+						break
+					}
+
+					seg.data.Append(&next.data)
+
+					// Consume the segment that we just merged in.
+					s.writeList.Remove(next)
+					next = next.Next()
+				}
+			}
 		}
 
 		var segEnd seqnum.Value
@@ -442,6 +463,7 @@ func (s *sender) sendData() {
 				nSeg.data.TrimFront(available)
 				nSeg.sequenceNumber.UpdateForward(seqnum.Size(available))
 				s.writeList.InsertAfter(seg, nSeg)
+				next = nSeg
 				seg.data.CapLength(available)
 			}
 
diff --git a/pkg/tcpip/transport/tcp/tcp_test.go b/pkg/tcpip/transport/tcp/tcp_test.go
index 04e046257..75868c4a2 100644
--- a/pkg/tcpip/transport/tcp/tcp_test.go
+++ b/pkg/tcpip/transport/tcp/tcp_test.go
@@ -1254,6 +1254,58 @@ func TestZeroScaledWindowReceive(t *testing.T) {
 	)
 }
 
+func TestSegmentMerging(t *testing.T) {
+	c := context.New(t, defaultMTU)
+	defer c.Cleanup()
+
+	c.CreateConnected(789, 30000, nil)
+
+	// Prevent the endpoint from processing packets.
+	worker := c.EP.(interface {
+		StopWork()
+		ResumeWork()
+	})
+	worker.StopWork()
+
+	var allData []byte
+	for i, data := range [][]byte{{1, 2, 3, 4}, {5, 6, 7}, {8, 9}, {10}, {11}} {
+		allData = append(allData, data...)
+		view := buffer.NewViewFromBytes(data)
+		if _, _, err := c.EP.Write(tcpip.SlicePayload(view), tcpip.WriteOptions{}); err != nil {
+			t.Fatalf("Write #%d failed: %v", i+1, err)
+		}
+	}
+
+	// Let the endpoint process the segments that we just sent.
+	worker.ResumeWork()
+
+	// Check that data is received.
+	b := c.GetPacket()
+	checker.IPv4(t, b,
+		checker.PayloadLen(len(allData)+header.TCPMinimumSize),
+		checker.TCP(
+			checker.DstPort(context.TestPort),
+			checker.SeqNum(uint32(c.IRS)+1),
+			checker.AckNum(790),
+			checker.TCPFlagsMatch(header.TCPFlagAck, ^uint8(header.TCPFlagPsh)),
+		),
+	)
+
+	if got := b[header.IPv4MinimumSize+header.TCPMinimumSize:]; !bytes.Equal(got, allData) {
+		t.Fatalf("got data = %v, want = %v", got, allData)
+	}
+
+	// Acknowledge the data.
+	c.SendPacket(nil, &context.Headers{
+		SrcPort: context.TestPort,
+		DstPort: c.Port,
+		Flags:   header.TCPFlagAck,
+		SeqNum:  790,
+		AckNum:  c.IRS.Add(1 + seqnum.Size(len(allData))),
+		RcvWnd:  30000,
+	})
+}
+
 func testBrokenUpWrite(t *testing.T, c *context.Context, maxPayload int) {
 	payloadMultiplier := 10
 	dataLen := payloadMultiplier * maxPayload