Add support for TCP send buffer auto tuning.

Send buffer size in TCP indicates the amount of bytes available for the sender
to transmit. This change will allow TCP to update the send buffer size when
- TCP enters established state.
- ACK is received.

The auto tuning is disabled when the send buffer size is set with the
SO_SNDBUF option.

PiperOrigin-RevId: 390312274

3 files changed, 160 insertions, 4 deletions

diff --git a/pkg/tcpip/transport/tcp/endpoint.go b/pkg/tcpip/transport/tcp/endpoint.go
index 044123185..355719beb 100644
--- a/pkg/tcpip/transport/tcp/endpoint.go
+++ b/pkg/tcpip/transport/tcp/endpoint.go
@@ -1717,6 +1717,27 @@ func (e *endpoint) OnSetReceiveBufferSize(rcvBufSz, oldSz int64) (newSz int64) {
 	return rcvBufSz
 }
 
+// OnSetSendBufferSize implements tcpip.SocketOptionsHandler.OnSetSendBufferSize.
+func (e *endpoint) OnSetSendBufferSize(sz int64) int64 {
+	atomic.StoreUint32(&e.sndQueueInfo.TCPSndBufState.AutoTuneSndBufDisabled, 1)
+	return sz
+}
+
+// WakeupWriters implements tcpip.SocketOptionsHandler.WakeupWriters.
+func (e *endpoint) WakeupWriters() {
+	e.LockUser()
+	defer e.UnlockUser()
+
+	sendBufferSize := e.getSendBufferSize()
+	e.sndQueueInfo.sndQueueMu.Lock()
+	notify := (sendBufferSize - e.sndQueueInfo.SndBufUsed) >= e.sndQueueInfo.SndBufUsed>>1
+	e.sndQueueInfo.sndQueueMu.Unlock()
+
+	if notify {
+		e.waiterQueue.Notify(waiter.WritableEvents)
+	}
+}
+
 // SetSockOptInt sets a socket option.
 func (e *endpoint) SetSockOptInt(opt tcpip.SockOptInt, v int) tcpip.Error {
 	// Lower 2 bits represents ECN bits. RFC 3168, section 23.1
@@ -2329,6 +2350,9 @@ func (e *endpoint) connect(addr tcpip.FullAddress, handshake bool, run bool) tcp
 		e.segmentQueue.mu.Unlock()
 		e.snd.updateMaxPayloadSize(int(e.route.MTU()), 0)
 		e.setEndpointState(StateEstablished)
+		// Set the new auto tuned send buffer size after entering
+		// established state.
+		e.ops.SetSendBufferSize(e.computeTCPSendBufferSize(), false /* notify */)
 	}
 
 	if run {
@@ -2763,13 +2787,20 @@ func (e *endpoint) updateSndBufferUsage(v int) {
 	e.sndQueueInfo.sndQueueMu.Lock()
 	notify := e.sndQueueInfo.SndBufUsed >= sendBufferSize>>1
 	e.sndQueueInfo.SndBufUsed -= v
+
+	// Get the new send buffer size with auto tuning, but do not set it
+	// unless we decide to notify the writers.
+	newSndBufSz := e.computeTCPSendBufferSize()
+
 	// We only notify when there is half the sendBufferSize available after
 	// a full buffer event occurs. This ensures that we don't wake up
 	// writers to queue just 1-2 segments and go back to sleep.
-	notify = notify && e.sndQueueInfo.SndBufUsed < sendBufferSize>>1
+	notify = notify && e.sndQueueInfo.SndBufUsed < int(newSndBufSz)>>1
 	e.sndQueueInfo.sndQueueMu.Unlock()
 
 	if notify {
+		// Set the new send buffer size calculated from auto tuning.
+		e.ops.SetSendBufferSize(newSndBufSz, false /* notify */)
 		e.waiterQueue.Notify(waiter.WritableEvents)
 	}
 }
@@ -3091,3 +3122,36 @@ func GetTCPReceiveBufferLimits(s tcpip.StackHandler) tcpip.ReceiveBufferSizeOpti
 		Max:     ss.Max,
 	}
 }
+
+// computeTCPSendBufferSize implements auto tuning of send buffer size and
+// returns the new send buffer size.
+func (e *endpoint) computeTCPSendBufferSize() int64 {
+	curSndBufSz := int64(e.getSendBufferSize())
+
+	// Auto tuning is disabled when the user explicitly sets the send
+	// buffer size with SO_SNDBUF option.
+	if disabled := atomic.LoadUint32(&e.sndQueueInfo.TCPSndBufState.AutoTuneSndBufDisabled); disabled == 1 {
+		return curSndBufSz
+	}
+
+	const packetOverheadFactor = 2
+	curMSS := e.snd.MaxPayloadSize
+	numSeg := InitialCwnd
+	if numSeg < e.snd.SndCwnd {
+		numSeg = e.snd.SndCwnd
+	}
+
+	// SndCwnd indicates the number of segments that can be sent. This means
+	// that the sender can send upto #SndCwnd segments and the send buffer
+	// size should be set to SndCwnd*MSS to accommodate sending of all the
+	// segments.
+	newSndBufSz := int64(numSeg * curMSS * packetOverheadFactor)
+	if newSndBufSz < curSndBufSz {
+		return curSndBufSz
+	}
+	if ss := GetTCPSendBufferLimits(e.stack); int64(ss.Max) < newSndBufSz {
+		newSndBufSz = int64(ss.Max)
+	}
+
+	return newSndBufSz
+}
diff --git a/pkg/tcpip/transport/tcp/snd.go b/pkg/tcpip/transport/tcp/snd.go
index 92a66f17e..64302f576 100644
--- a/pkg/tcpip/transport/tcp/snd.go
+++ b/pkg/tcpip/transport/tcp/snd.go
@@ -1415,9 +1415,6 @@ func (s *sender) handleRcvdSegment(rcvdSeg *segment) {
 			ackLeft -= datalen
 		}
 
-		// Update the send buffer usage and notify potential waiters.
-		s.ep.updateSndBufferUsage(int(acked))
-
 		// Clear SACK information for all acked data.
 		s.ep.scoreboard.Delete(s.SndUna)
 
@@ -1437,6 +1434,9 @@ func (s *sender) handleRcvdSegment(rcvdSeg *segment) {
 			}
 		}
 
+		// Update the send buffer usage and notify potential waiters.
+		s.ep.updateSndBufferUsage(int(acked))
+
 		// It is possible for s.outstanding to drop below zero if we get
 		// a retransmit timeout, reset outstanding to zero but later
 		// get an ack that cover previously sent data.
diff --git a/pkg/tcpip/transport/tcp/tcp_test.go b/pkg/tcpip/transport/tcp/tcp_test.go
index 031f01357..db6b0955a 100644
--- a/pkg/tcpip/transport/tcp/tcp_test.go
+++ b/pkg/tcpip/transport/tcp/tcp_test.go
@@ -7964,3 +7964,95 @@ func generateRandomPayload(t *testing.T, n int) []byte {
 	}
 	return buf
 }
+
+func TestSendBufferTuning(t *testing.T) {
+	const maxPayload = 536
+	const mtu = header.TCPMinimumSize + header.IPv4MinimumSize + maxTCPOptionSize + maxPayload
+	const packetOverheadFactor = 2
+
+	testCases := []struct {
+		name               string
+		autoTuningDisabled bool
+	}{
+		{"autoTuningDisabled", true},
+		{"autoTuningEnabled", false},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			c := context.New(t, mtu)
+			defer c.Cleanup()
+
+			// Set the stack option for send buffer size.
+			const defaultSndBufSz = maxPayload * tcp.InitialCwnd
+			const maxSndBufSz = defaultSndBufSz * 10
+			{
+				opt := tcpip.TCPSendBufferSizeRangeOption{Min: 1, Default: defaultSndBufSz, Max: maxSndBufSz}
+				if err := c.Stack().SetTransportProtocolOption(tcp.ProtocolNumber, &opt); err != nil {
+					t.Fatalf("SetTransportProtocolOption(%d, &%#v): %s", tcp.ProtocolNumber, opt, err)
+				}
+			}
+
+			c.CreateConnected(context.TestInitialSequenceNumber, 30000, -1 /* epRcvBuf */)
+
+			oldSz := c.EP.SocketOptions().GetSendBufferSize()
+			if oldSz != defaultSndBufSz {
+				t.Fatalf("Wrong send buffer size got %d want %d", oldSz, defaultSndBufSz)
+			}
+
+			if tc.autoTuningDisabled {
+				c.EP.SocketOptions().SetSendBufferSize(defaultSndBufSz, true /* notify */)
+			}
+
+			data := make([]byte, maxPayload)
+			for i := range data {
+				data[i] = byte(i)
+			}
+
+			w, ch := waiter.NewChannelEntry(nil)
+			c.WQ.EventRegister(&w, waiter.WritableEvents)
+			defer c.WQ.EventUnregister(&w)
+
+			bytesRead := 0
+			for {
+				// Packets will be sent till the send buffer
+				// size is reached.
+				var r bytes.Reader
+				r.Reset(data[bytesRead : bytesRead+maxPayload])
+				_, err := c.EP.Write(&r, tcpip.WriteOptions{})
+				if cmp.Equal(&tcpip.ErrWouldBlock{}, err) {
+					break
+				}
+
+				c.ReceiveAndCheckPacketWithOptions(data, bytesRead, maxPayload, 0)
+				bytesRead += maxPayload
+				data = append(data, data...)
+			}
+
+			// Send an ACK and wait for connection to become writable again.
+			c.SendAck(seqnum.Value(context.TestInitialSequenceNumber).Add(1), bytesRead)
+			select {
+			case <-ch:
+				if err := c.EP.LastError(); err != nil {
+					t.Fatalf("Write failed: %s", err)
+				}
+			case <-time.After(1 * time.Second):
+				t.Fatalf("Timed out waiting for connection")
+			}
+
+			outSz := int64(defaultSndBufSz)
+			if !tc.autoTuningDisabled {
+				// Calculate the new auto tuned send buffer.
+				var info tcpip.TCPInfoOption
+				if err := c.EP.GetSockOpt(&info); err != nil {
+					t.Fatalf("GetSockOpt failed: %v", err)
+				}
+				outSz = (int64(info.SndCwnd) * packetOverheadFactor * (maxPayload))
+			}
+
+			if newSz := c.EP.SocketOptions().GetSendBufferSize(); newSz != outSz {
+				t.Fatalf("Wrong send buffer size, got %d want %d", newSz, outSz)
+			}
+		})
+	}
+}