Changes TCP packet dispatch to use a pool of goroutines.

All inbound segments for connections in ESTABLISHED state are delivered to the
endpoint's queue but for every segment delivered we also queue the endpoint for
processing to a selected processor. This ensures that when there are a large
number of connections in ESTABLISHED state the inbound packets are all handled
by a small number of goroutines and significantly reduces the amount of work the
goscheduler has to perform.

We let connections in other states follow the current path where the
endpoint's goroutine directly handles the segments.

Updates #231

PiperOrigin-RevId: 289728325

10 files changed, 673 insertions, 269 deletions

diff --git a/pkg/tcpip/transport/tcp/BUILD b/pkg/tcpip/transport/tcp/BUILD
index 353bd06f4..0e3ab05ad 100644
--- a/pkg/tcpip/transport/tcp/BUILD
+++ b/pkg/tcpip/transport/tcp/BUILD
@@ -16,6 +16,18 @@ go_template_instance(
     },
 )
 
+go_template_instance(
+    name = "tcp_endpoint_list",
+    out = "tcp_endpoint_list.go",
+    package = "tcp",
+    prefix = "endpoint",
+    template = "//pkg/ilist:generic_list",
+    types = {
+        "Element": "*endpoint",
+        "Linker": "*endpoint",
+    },
+)
+
 go_library(
     name = "tcp",
     srcs = [
@@ -23,6 +35,7 @@ go_library(
         "connect.go",
         "cubic.go",
         "cubic_state.go",
+        "dispatcher.go",
         "endpoint.go",
         "endpoint_state.go",
         "forwarder.go",
@@ -38,6 +51,7 @@ go_library(
         "segment_state.go",
         "snd.go",
         "snd_state.go",
+        "tcp_endpoint_list.go",
         "tcp_segment_list.go",
         "timer.go",
     ],
@@ -45,7 +59,6 @@ go_library(
     imports = ["gvisor.dev/gvisor/pkg/tcpip/buffer"],
     visibility = ["//visibility:public"],
     deps = [
-        "//pkg/log",
         "//pkg/rand",
         "//pkg/sleep",
         "//pkg/sync",
diff --git a/pkg/tcpip/transport/tcp/accept.go b/pkg/tcpip/transport/tcp/accept.go
index 1ea996936..1a2e3efa9 100644
--- a/pkg/tcpip/transport/tcp/accept.go
+++ b/pkg/tcpip/transport/tcp/accept.go
@@ -285,7 +285,7 @@ func (l *listenContext) createEndpointAndPerformHandshake(s *segment, opts *head
 	// listenEP is nil when listenContext is used by tcp.Forwarder.
 	if l.listenEP != nil {
 		l.listenEP.mu.Lock()
-		if l.listenEP.state != StateListen {
+		if l.listenEP.EndpointState() != StateListen {
 			l.listenEP.mu.Unlock()
 			return nil, tcpip.ErrConnectionAborted
 		}
@@ -344,11 +344,12 @@ func (l *listenContext) closeAllPendingEndpoints() {
 // instead.
 func (e *endpoint) deliverAccepted(n *endpoint) {
 	e.mu.Lock()
-	state := e.state
+	state := e.EndpointState()
 	e.pendingAccepted.Add(1)
 	defer e.pendingAccepted.Done()
 	acceptedChan := e.acceptedChan
 	e.mu.Unlock()
+
 	if state == StateListen {
 		acceptedChan <- n
 		e.waiterQueue.Notify(waiter.EventIn)
@@ -562,8 +563,8 @@ func (e *endpoint) handleListenSegment(ctx *listenContext, s *segment) {
 		// We do not use transitionToStateEstablishedLocked here as there is
 		// no handshake state available when doing a SYN cookie based accept.
 		n.stack.Stats().TCP.CurrentEstablished.Increment()
-		n.state = StateEstablished
 		n.isConnectNotified = true
+		n.setEndpointState(StateEstablished)
 
 		// Do the delivery in a separate goroutine so
 		// that we don't block the listen loop in case
@@ -596,7 +597,7 @@ func (e *endpoint) protocolListenLoop(rcvWnd seqnum.Size) *tcpip.Error {
 		// handleSynSegment() from attempting to queue new connections
 		// to the endpoint.
 		e.mu.Lock()
-		e.state = StateClose
+		e.setEndpointState(StateClose)
 
 		// close any endpoints in SYN-RCVD state.
 		ctx.closeAllPendingEndpoints()
diff --git a/pkg/tcpip/transport/tcp/connect.go b/pkg/tcpip/transport/tcp/connect.go
index 613ec1775..f3896715b 100644
--- a/pkg/tcpip/transport/tcp/connect.go
+++ b/pkg/tcpip/transport/tcp/connect.go
@@ -190,7 +190,7 @@ func (h *handshake) resetToSynRcvd(iss seqnum.Value, irs seqnum.Value, opts *hea
 	h.mss = opts.MSS
 	h.sndWndScale = opts.WS
 	h.ep.mu.Lock()
-	h.ep.state = StateSynRecv
+	h.ep.setEndpointState(StateSynRecv)
 	h.ep.mu.Unlock()
 }
 
@@ -274,14 +274,14 @@ func (h *handshake) synSentState(s *segment) *tcpip.Error {
 	// SYN-RCVD state.
 	h.state = handshakeSynRcvd
 	h.ep.mu.Lock()
-	h.ep.state = StateSynRecv
 	ttl := h.ep.ttl
+	h.ep.setEndpointState(StateSynRecv)
 	h.ep.mu.Unlock()
 	synOpts := header.TCPSynOptions{
 		WS:    int(h.effectiveRcvWndScale()),
 		TS:    rcvSynOpts.TS,
 		TSVal: h.ep.timestamp(),
-		TSEcr: h.ep.recentTS,
+		TSEcr: h.ep.recentTimestamp(),
 
 		// We only send SACKPermitted if the other side indicated it
 		// permits SACK. This is not explicitly defined in the RFC but
@@ -341,7 +341,7 @@ func (h *handshake) synRcvdState(s *segment) *tcpip.Error {
 			WS:            h.rcvWndScale,
 			TS:            h.ep.sendTSOk,
 			TSVal:         h.ep.timestamp(),
-			TSEcr:         h.ep.recentTS,
+			TSEcr:         h.ep.recentTimestamp(),
 			SACKPermitted: h.ep.sackPermitted,
 			MSS:           h.ep.amss,
 		}
@@ -501,7 +501,7 @@ func (h *handshake) execute() *tcpip.Error {
 		WS:            h.rcvWndScale,
 		TS:            true,
 		TSVal:         h.ep.timestamp(),
-		TSEcr:         h.ep.recentTS,
+		TSEcr:         h.ep.recentTimestamp(),
 		SACKPermitted: bool(sackEnabled),
 		MSS:           h.ep.amss,
 	}
@@ -792,7 +792,7 @@ func (e *endpoint) makeOptions(sackBlocks []header.SACKBlock) []byte {
 		// Ref: https://tools.ietf.org/html/rfc7323#section-5.4.
 		offset += header.EncodeNOP(options[offset:])
 		offset += header.EncodeNOP(options[offset:])
-		offset += header.EncodeTSOption(e.timestamp(), uint32(e.recentTS), options[offset:])
+		offset += header.EncodeTSOption(e.timestamp(), e.recentTimestamp(), options[offset:])
 	}
 	if e.sackPermitted && len(sackBlocks) > 0 {
 		offset += header.EncodeNOP(options[offset:])
@@ -811,7 +811,7 @@ func (e *endpoint) makeOptions(sackBlocks []header.SACKBlock) []byte {
 // sendRaw sends a TCP segment to the endpoint's peer.
 func (e *endpoint) sendRaw(data buffer.VectorisedView, flags byte, seq, ack seqnum.Value, rcvWnd seqnum.Size) *tcpip.Error {
 	var sackBlocks []header.SACKBlock
-	if e.state == StateEstablished && e.rcv.pendingBufSize > 0 && (flags&header.TCPFlagAck != 0) {
+	if e.EndpointState() == StateEstablished && e.rcv.pendingBufSize > 0 && (flags&header.TCPFlagAck != 0) {
 		sackBlocks = e.sack.Blocks[:e.sack.NumBlocks]
 	}
 	options := e.makeOptions(sackBlocks)
@@ -848,6 +848,9 @@ func (e *endpoint) handleWrite() *tcpip.Error {
 }
 
 func (e *endpoint) handleClose() *tcpip.Error {
+	if !e.EndpointState().connected() {
+		return nil
+	}
 	// Drain the send queue.
 	e.handleWrite()
 
@@ -864,11 +867,7 @@ func (e *endpoint) handleClose() *tcpip.Error {
 func (e *endpoint) resetConnectionLocked(err *tcpip.Error) {
 	// Only send a reset if the connection is being aborted for a reason
 	// other than receiving a reset.
-	if e.state == StateEstablished || e.state == StateCloseWait {
-		e.stack.Stats().TCP.EstablishedResets.Increment()
-		e.stack.Stats().TCP.CurrentEstablished.Decrement()
-	}
-	e.state = StateError
+	e.setEndpointState(StateError)
 	e.HardError = err
 	if err != tcpip.ErrConnectionReset && err != tcpip.ErrTimeout {
 		// The exact sequence number to be used for the RST is the same as the
@@ -888,9 +887,12 @@ func (e *endpoint) resetConnectionLocked(err *tcpip.Error) {
 }
 
 // completeWorkerLocked is called by the worker goroutine when it's about to
-// exit. It marks the worker as completed and performs cleanup work if requested
-// by Close().
+// exit.
 func (e *endpoint) completeWorkerLocked() {
+	// Worker is terminating(either due to moving to
+	// CLOSED or ERROR state, ensure we release all
+	// registrations port reservations even if the socket
+	// itself is not yet closed by the application.
 	e.workerRunning = false
 	if e.workerCleanup {
 		e.cleanupLocked()
@@ -917,8 +919,7 @@ func (e *endpoint) transitionToStateEstablishedLocked(h *handshake) {
 		e.rcvAutoParams.prevCopied = int(h.rcvWnd)
 		e.rcvListMu.Unlock()
 	}
-	h.ep.stack.Stats().TCP.CurrentEstablished.Increment()
-	e.state = StateEstablished
+	e.setEndpointState(StateEstablished)
 }
 
 // transitionToStateCloseLocked ensures that the endpoint is
@@ -927,11 +928,12 @@ func (e *endpoint) transitionToStateEstablishedLocked(h *handshake) {
 // delivered to this endpoint from the demuxer when the endpoint
 // is transitioned to StateClose.
 func (e *endpoint) transitionToStateCloseLocked() {
-	if e.state == StateClose {
+	if e.EndpointState() == StateClose {
 		return
 	}
+	// Mark the endpoint as fully closed for reads/writes.
 	e.cleanupLocked()
-	e.state = StateClose
+	e.setEndpointState(StateClose)
 	e.stack.Stats().TCP.EstablishedClosed.Increment()
 }
 
@@ -946,7 +948,9 @@ func (e *endpoint) tryDeliverSegmentFromClosedEndpoint(s *segment) {
 		s.decRef()
 		return
 	}
-	ep.(*endpoint).enqueueSegment(s)
+	if ep.(*endpoint).enqueueSegment(s) {
+		ep.(*endpoint).newSegmentWaker.Assert()
+	}
 }
 
 func (e *endpoint) handleReset(s *segment) (ok bool, err *tcpip.Error) {
@@ -955,9 +959,8 @@ func (e *endpoint) handleReset(s *segment) (ok bool, err *tcpip.Error) {
 		// except SYN-SENT, all reset (RST) segments are
 		// validated by checking their SEQ-fields." So
 		// we only process it if it's acceptable.
-		s.decRef()
 		e.mu.Lock()
-		switch e.state {
+		switch e.EndpointState() {
 		// In case of a RST in CLOSE-WAIT linux moves
 		// the socket to closed state with an error set
 		// to indicate EPIPE.
@@ -981,103 +984,57 @@ func (e *endpoint) handleReset(s *segment) (ok bool, err *tcpip.Error) {
 			e.transitionToStateCloseLocked()
 			e.HardError = tcpip.ErrAborted
 			e.mu.Unlock()
+			e.notifyProtocolGoroutine(notifyTickleWorker)
 			return false, nil
 		default:
 			e.mu.Unlock()
+			// RFC 793, page 37 states that "in all states
+			// except SYN-SENT, all reset (RST) segments are
+			// validated by checking their SEQ-fields." So
+			// we only process it if it's acceptable.
+
+			// Notify protocol goroutine. This is required when
+			// handleSegment is invoked from the processor goroutine
+			// rather than the worker goroutine.
+			e.notifyProtocolGoroutine(notifyResetByPeer)
 			return false, tcpip.ErrConnectionReset
 		}
 	}
 	return true, nil
 }
 
-// handleSegments pulls segments from the queue and processes them. It returns
-// no error if the protocol loop should continue, an error otherwise.
-func (e *endpoint) handleSegments() *tcpip.Error {
+// handleSegments processes all inbound segments.
+func (e *endpoint) handleSegments(fastPath bool) *tcpip.Error {
 	checkRequeue := true
 	for i := 0; i < maxSegmentsPerWake; i++ {
+		if e.EndpointState() == StateClose || e.EndpointState() == StateError {
+			return nil
+		}
 		s := e.segmentQueue.dequeue()
 		if s == nil {
 			checkRequeue = false
 			break
 		}
 
-		// Invoke the tcp probe if installed.
-		if e.probe != nil {
-			e.probe(e.completeState())
+		cont, err := e.handleSegment(s)
+		if err != nil {
+			s.decRef()
+			e.mu.Lock()
+			e.setEndpointState(StateError)
+			e.HardError = err
+			e.mu.Unlock()
+			return err
 		}
-
-		if s.flagIsSet(header.TCPFlagRst) {
-			if ok, err := e.handleReset(s); !ok {
-				return err
-			}
-		} else if s.flagIsSet(header.TCPFlagSyn) {
-			// See: https://tools.ietf.org/html/rfc5961#section-4.1
-			//   1) If the SYN bit is set, irrespective of the sequence number, TCP
-			//    MUST send an ACK (also referred to as challenge ACK) to the remote
-			//    peer:
-			//
-			//    <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
-			//
-			//    After sending the acknowledgment, TCP MUST drop the unacceptable
-			//    segment and stop processing further.
-			//
-			// By sending an ACK, the remote peer is challenged to confirm the loss
-			// of the previous connection and the request to start a new connection.
-			// A legitimate peer, after restart, would not have a TCB in the
-			// synchronized state.  Thus, when the ACK arrives, the peer should send
-			// a RST segment back with the sequence number derived from the ACK
-			// field that caused the RST.
-
-			// This RST will confirm that the remote peer has indeed closed the
-			// previous connection.  Upon receipt of a valid RST, the local TCP
-			// endpoint MUST terminate its connection.  The local TCP endpoint
-			// should then rely on SYN retransmission from the remote end to
-			// re-establish the connection.
-
-			e.snd.sendAck()
-		} else if s.flagIsSet(header.TCPFlagAck) {
-			// Patch the window size in the segment according to the
-			// send window scale.
-			s.window <<= e.snd.sndWndScale
-
-			// RFC 793, page 41 states that "once in the ESTABLISHED
-			// state all segments must carry current acknowledgment
-			// information."
-			drop, err := e.rcv.handleRcvdSegment(s)
-			if err != nil {
-				s.decRef()
-				return err
-			}
-			if drop {
-				s.decRef()
-				continue
-			}
-
-			// Now check if the received segment has caused us to transition
-			// to a CLOSED state, if yes then terminate processing and do
-			// not invoke the sender.
-			e.mu.RLock()
-			state := e.state
-			e.mu.RUnlock()
-			if state == StateClose {
-				// When we get into StateClose while processing from the queue,
-				// return immediately and let the protocolMainloop handle it.
-				//
-				// We can reach StateClose only while processing a previous segment
-				// or a notification from the protocolMainLoop (caller goroutine).
-				// This means that with this return, the segment dequeue below can
-				// never occur on a closed endpoint.
-				s.decRef()
-				return nil
-			}
-			e.snd.handleRcvdSegment(s)
+		if !cont {
+			s.decRef()
+			return nil
 		}
-		s.decRef()
 	}
 
-	// If the queue is not empty, make sure we'll wake up in the next
-	// iteration.
-	if checkRequeue && !e.segmentQueue.empty() {
+	// When fastPath is true we don't want to wake up the worker
+	// goroutine. If the endpoint has more segments to process the
+	// dispatcher will call handleSegments again anyway.
+	if !fastPath && checkRequeue && !e.segmentQueue.empty() {
 		e.newSegmentWaker.Assert()
 	}
 
@@ -1086,11 +1043,88 @@ func (e *endpoint) handleSegments() *tcpip.Error {
 		e.snd.sendAck()
 	}
 
-	e.resetKeepaliveTimer(true)
+	e.resetKeepaliveTimer(true /* receivedData */)
 
 	return nil
 }
 
+// handleSegment handles a given segment and notifies the worker goroutine if
+// if the connection should be terminated.
+func (e *endpoint) handleSegment(s *segment) (cont bool, err *tcpip.Error) {
+	// Invoke the tcp probe if installed.
+	if e.probe != nil {
+		e.probe(e.completeState())
+	}
+
+	if s.flagIsSet(header.TCPFlagRst) {
+		if ok, err := e.handleReset(s); !ok {
+			return false, err
+		}
+	} else if s.flagIsSet(header.TCPFlagSyn) {
+		// See: https://tools.ietf.org/html/rfc5961#section-4.1
+		//   1) If the SYN bit is set, irrespective of the sequence number, TCP
+		//    MUST send an ACK (also referred to as challenge ACK) to the remote
+		//    peer:
+		//
+		//    <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
+		//
+		//    After sending the acknowledgment, TCP MUST drop the unacceptable
+		//    segment and stop processing further.
+		//
+		// By sending an ACK, the remote peer is challenged to confirm the loss
+		// of the previous connection and the request to start a new connection.
+		// A legitimate peer, after restart, would not have a TCB in the
+		// synchronized state.  Thus, when the ACK arrives, the peer should send
+		// a RST segment back with the sequence number derived from the ACK
+		// field that caused the RST.
+
+		// This RST will confirm that the remote peer has indeed closed the
+		// previous connection.  Upon receipt of a valid RST, the local TCP
+		// endpoint MUST terminate its connection.  The local TCP endpoint
+		// should then rely on SYN retransmission from the remote end to
+		// re-establish the connection.
+
+		e.snd.sendAck()
+	} else if s.flagIsSet(header.TCPFlagAck) {
+		// Patch the window size in the segment according to the
+		// send window scale.
+		s.window <<= e.snd.sndWndScale
+
+		// RFC 793, page 41 states that "once in the ESTABLISHED
+		// state all segments must carry current acknowledgment
+		// information."
+		drop, err := e.rcv.handleRcvdSegment(s)
+		if err != nil {
+			return false, err
+		}
+		if drop {
+			return true, nil
+		}
+
+		// Now check if the received segment has caused us to transition
+		// to a CLOSED state, if yes then terminate processing and do
+		// not invoke the sender.
+		e.mu.RLock()
+		state := e.state
+		e.mu.RUnlock()
+		if state == StateClose {
+			// When we get into StateClose while processing from the queue,
+			// return immediately and let the protocolMainloop handle it.
+			//
+			// We can reach StateClose only while processing a previous segment
+			// or a notification from the protocolMainLoop (caller goroutine).
+			// This means that with this return, the segment dequeue below can
+			// never occur on a closed endpoint.
+			s.decRef()
+			return false, nil
+		}
+
+		e.snd.handleRcvdSegment(s)
+	}
+
+	return true, nil
+}
+
 // keepaliveTimerExpired is called when the keepaliveTimer fires. We send TCP
 // keepalive packets periodically when the connection is idle. If we don't hear
 // from the other side after a number of tries, we terminate the connection.
@@ -1160,7 +1194,7 @@ func (e *endpoint) disableKeepaliveTimer() {
 // protocolMainLoop is the main loop of the TCP protocol. It runs in its own
 // goroutine and is responsible for sending segments and handling received
 // segments.
-func (e *endpoint) protocolMainLoop(handshake bool) *tcpip.Error {
+func (e *endpoint) protocolMainLoop(handshake bool, wakerInitDone chan<- struct{}) *tcpip.Error {
 	var closeTimer *time.Timer
 	var closeWaker sleep.Waker
 
@@ -1182,6 +1216,7 @@ func (e *endpoint) protocolMainLoop(handshake bool) *tcpip.Error {
 		}
 
 		e.mu.Unlock()
+		e.workMu.Unlock()
 		// When the protocol loop exits we should wake up our waiters.
 		e.waiterQueue.Notify(waiter.EventHUp | waiter.EventErr | waiter.EventIn | waiter.EventOut)
 	}
@@ -1193,7 +1228,7 @@ func (e *endpoint) protocolMainLoop(handshake bool) *tcpip.Error {
 		initialRcvWnd := e.initialReceiveWindow()
 		h := newHandshake(e, seqnum.Size(initialRcvWnd))
 		e.mu.Lock()
-		h.ep.state = StateSynSent
+		h.ep.setEndpointState(StateSynSent)
 		e.mu.Unlock()
 
 		if err := h.execute(); err != nil {
@@ -1202,12 +1237,11 @@ func (e *endpoint) protocolMainLoop(handshake bool) *tcpip.Error {
 			e.lastErrorMu.Unlock()
 
 			e.mu.Lock()
-			e.state = StateError
+			e.setEndpointState(StateError)
 			e.HardError = err
 
 			// Lock released below.
 			epilogue()
-
 			return err
 		}
 	}
@@ -1215,7 +1249,6 @@ func (e *endpoint) protocolMainLoop(handshake bool) *tcpip.Error {
 	e.keepalive.timer.init(&e.keepalive.waker)
 	defer e.keepalive.timer.cleanup()
 
-	// Tell waiters that the endpoint is connected and writable.
 	e.mu.Lock()
 	drained := e.drainDone != nil
 	e.mu.Unlock()
@@ -1224,8 +1257,6 @@ func (e *endpoint) protocolMainLoop(handshake bool) *tcpip.Error {
 		<-e.undrain
 	}
 
-	e.waiterQueue.Notify(waiter.EventOut)
-
 	// Set up the functions that will be called when the main protocol loop
 	// wakes up.
 	funcs := []struct {
@@ -1241,17 +1272,14 @@ func (e *endpoint) protocolMainLoop(handshake bool) *tcpip.Error {
 			f: e.handleClose,
 		},
 		{
-			w: &e.newSegmentWaker,
-			f: e.handleSegments,
-		},
-		{
 			w: &closeWaker,
 			f: func() *tcpip.Error {
 				// This means the socket is being closed due
-				// to the TCP_FIN_WAIT2 timeout was hit. Just
+				// to the TCP-FIN-WAIT2 timeout was hit. Just
 				// mark the socket as closed.
 				e.mu.Lock()
 				e.transitionToStateCloseLocked()
+				e.workerCleanup = true
 				e.mu.Unlock()
 				return nil
 			},
@@ -1267,6 +1295,12 @@ func (e *endpoint) protocolMainLoop(handshake bool) *tcpip.Error {
 			},
 		},
 		{
+			w: &e.newSegmentWaker,
+			f: func() *tcpip.Error {
+				return e.handleSegments(false /* fastPath */)
+			},
+		},
+		{
 			w: &e.keepalive.waker,
 			f: e.keepaliveTimerExpired,
 		},
@@ -1293,14 +1327,16 @@ func (e *endpoint) protocolMainLoop(handshake bool) *tcpip.Error {
 				}
 
 				if n&notifyReset != 0 {
-					e.mu.Lock()
-					e.resetConnectionLocked(tcpip.ErrConnectionAborted)
-					e.mu.Unlock()
+					return tcpip.ErrConnectionAborted
+				}
+
+				if n&notifyResetByPeer != 0 {
+					return tcpip.ErrConnectionReset
 				}
 
 				if n&notifyClose != 0 && closeTimer == nil {
 					e.mu.Lock()
-					if e.state == StateFinWait2 && e.closed {
+					if e.EndpointState() == StateFinWait2 && e.closed {
 						// The socket has been closed and we are in FIN_WAIT2
 						// so start the FIN_WAIT2 timer.
 						closeTimer = time.AfterFunc(e.tcpLingerTimeout, func() {
@@ -1320,11 +1356,11 @@ func (e *endpoint) protocolMainLoop(handshake bool) *tcpip.Error {
 
 				if n&notifyDrain != 0 {
 					for !e.segmentQueue.empty() {
-						if err := e.handleSegments(); err != nil {
+						if err := e.handleSegments(false /* fastPath */); err != nil {
 							return err
 						}
 					}
-					if e.state != StateClose && e.state != StateError {
+					if e.EndpointState() != StateClose && e.EndpointState() != StateError {
 						// Only block the worker if the endpoint
 						// is not in closed state or error state.
 						close(e.drainDone)
@@ -1349,14 +1385,21 @@ func (e *endpoint) protocolMainLoop(handshake bool) *tcpip.Error {
 		s.AddWaker(funcs[i].w, i)
 	}
 
+	// Notify the caller that the waker initialization is complete and the
+	// endpoint is ready.
+	if wakerInitDone != nil {
+		close(wakerInitDone)
+	}
+
+	// Tell waiters that the endpoint is connected and writable.
+	e.waiterQueue.Notify(waiter.EventOut)
+
 	// The following assertions and notifications are needed for restored
 	// endpoints. Fresh newly created endpoints have empty states and should
 	// not invoke any.
-	e.segmentQueue.mu.Lock()
-	if !e.segmentQueue.list.Empty() {
+	if !e.segmentQueue.empty() {
 		e.newSegmentWaker.Assert()
 	}
-	e.segmentQueue.mu.Unlock()
 
 	e.rcvListMu.Lock()
 	if !e.rcvList.Empty() {
@@ -1372,27 +1415,32 @@ func (e *endpoint) protocolMainLoop(handshake bool) *tcpip.Error {
 	// Main loop. Handle segments until both send and receive ends of the
 	// connection have completed.
 
-	for e.state != StateTimeWait && e.state != StateClose && e.state != StateError {
+	for e.EndpointState() != StateTimeWait && e.EndpointState() != StateClose && e.EndpointState() != StateError {
 		e.mu.Unlock()
 		e.workMu.Unlock()
 		v, _ := s.Fetch(true)
 		e.workMu.Lock()
+		// We need to double check here because the notification
+		// maybe stale by the time we got around to processing it.
+		// NOTE: since we now hold the workMu the processors cannot
+		// change the state of the endpoint so it' safe to proceed
+		// after this check.
+		if e.EndpointState() == StateTimeWait || e.EndpointState() == StateClose || e.EndpointState() == StateError {
+			e.mu.Lock()
+			break
+		}
 		if err := funcs[v].f(); err != nil {
 			e.mu.Lock()
-			// Ensure we release all endpoint registration and route
-			// references as the connection is now in an error
-			// state.
 			e.workerCleanup = true
 			e.resetConnectionLocked(err)
 			// Lock released below.
 			epilogue()
-
 			return nil
 		}
 		e.mu.Lock()
 	}
 
-	state := e.state
+	state := e.EndpointState()
 	e.mu.Unlock()
 	var reuseTW func()
 	if state == StateTimeWait {
@@ -1405,13 +1453,15 @@ func (e *endpoint) protocolMainLoop(handshake bool) *tcpip.Error {
 		s.Done()
 		// Wake up any waiters before we enter TIME_WAIT.
 		e.waiterQueue.Notify(waiter.EventHUp | waiter.EventErr | waiter.EventIn | waiter.EventOut)
+		e.mu.Lock()
+		e.workerCleanup = true
+		e.mu.Unlock()
 		reuseTW = e.doTimeWait()
 	}
 
 	// Mark endpoint as closed.
 	e.mu.Lock()
-	if e.state != StateError {
-		e.stack.Stats().TCP.CurrentEstablished.Decrement()
+	if e.EndpointState() != StateError {
 		e.transitionToStateCloseLocked()
 	}
 
@@ -1468,7 +1518,11 @@ func (e *endpoint) handleTimeWaitSegments() (extendTimeWait bool, reuseTW func()
 					tcpEP := listenEP.(*endpoint)
 					if EndpointState(tcpEP.State()) == StateListen {
 						reuseTW = func() {
-							tcpEP.enqueueSegment(s)
+							if !tcpEP.enqueueSegment(s) {
+								s.decRef()
+								return
+							}
+							tcpEP.newSegmentWaker.Assert()
 						}
 						// We explicitly do not decRef
 						// the segment as it's still
diff --git a/pkg/tcpip/transport/tcp/dispatcher.go b/pkg/tcpip/transport/tcp/dispatcher.go
new file mode 100644
index 000000000..a72f0c379
--- /dev/null
+++ b/pkg/tcpip/transport/tcp/dispatcher.go
@@ -0,0 +1,218 @@
+// Copyright 2018 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package tcp
+
+import (
+	"gvisor.dev/gvisor/pkg/rand"
+	"gvisor.dev/gvisor/pkg/sleep"
+	"gvisor.dev/gvisor/pkg/sync"
+	"gvisor.dev/gvisor/pkg/tcpip"
+	"gvisor.dev/gvisor/pkg/tcpip/hash/jenkins"
+	"gvisor.dev/gvisor/pkg/tcpip/header"
+	"gvisor.dev/gvisor/pkg/tcpip/stack"
+)
+
+// epQueue is a queue of endpoints.
+type epQueue struct {
+	mu   sync.Mutex
+	list endpointList
+}
+
+// enqueue adds e to the queue if the endpoint is not already on the queue.
+func (q *epQueue) enqueue(e *endpoint) {
+	q.mu.Lock()
+	if e.pendingProcessing {
+		q.mu.Unlock()
+		return
+	}
+	q.list.PushBack(e)
+	e.pendingProcessing = true
+	q.mu.Unlock()
+}
+
+// dequeue removes and returns the first element from the queue if available,
+// returns nil otherwise.
+func (q *epQueue) dequeue() *endpoint {
+	q.mu.Lock()
+	if e := q.list.Front(); e != nil {
+		q.list.Remove(e)
+		e.pendingProcessing = false
+		q.mu.Unlock()
+		return e
+	}
+	q.mu.Unlock()
+	return nil
+}
+
+// empty returns true if the queue is empty, false otherwise.
+func (q *epQueue) empty() bool {
+	q.mu.Lock()
+	v := q.list.Empty()
+	q.mu.Unlock()
+	return v
+}
+
+// processor is responsible for processing packets queued to a tcp endpoint.
+type processor struct {
+	epQ              epQueue
+	newEndpointWaker sleep.Waker
+	id               int
+}
+
+func newProcessor(id int) *processor {
+	p := &processor{
+		id: id,
+	}
+	go p.handleSegments()
+	return p
+}
+
+func (p *processor) queueEndpoint(ep *endpoint) {
+	// Queue an endpoint for processing by the processor goroutine.
+	p.epQ.enqueue(ep)
+	p.newEndpointWaker.Assert()
+}
+
+func (p *processor) handleSegments() {
+	const newEndpointWaker = 1
+	s := sleep.Sleeper{}
+	s.AddWaker(&p.newEndpointWaker, newEndpointWaker)
+	defer s.Done()
+	for {
+		s.Fetch(true)
+		for ep := p.epQ.dequeue(); ep != nil; ep = p.epQ.dequeue() {
+			if ep.segmentQueue.empty() {
+				continue
+			}
+
+			// If socket has transitioned out of connected state
+			// then just let the worker handle the packet.
+			//
+			// NOTE: We read this outside of e.mu lock which means
+			// that by the time we get to handleSegments the
+			// endpoint may not be in ESTABLISHED. But this should
+			// be fine as all normal shutdown states are handled by
+			// handleSegments and if the endpoint moves to a
+			// CLOSED/ERROR state then handleSegments is a noop.
+			if ep.EndpointState() != StateEstablished {
+				ep.newSegmentWaker.Assert()
+				continue
+			}
+
+			if !ep.workMu.TryLock() {
+				ep.newSegmentWaker.Assert()
+				continue
+			}
+			// If the endpoint is in a connected state then we do
+			// direct delivery to ensure low latency and avoid
+			// scheduler interactions.
+			if err := ep.handleSegments(true /* fastPath */); err != nil || ep.EndpointState() == StateClose {
+				ep.notifyProtocolGoroutine(notifyTickleWorker)
+				ep.workMu.Unlock()
+				continue
+			}
+
+			if !ep.segmentQueue.empty() {
+				p.epQ.enqueue(ep)
+			}
+
+			ep.workMu.Unlock()
+		}
+	}
+}
+
+// dispatcher manages a pool of TCP endpoint processors which are responsible
+// for the processing of inbound segments. This fixed pool of processor
+// goroutines do full tcp processing. The processor is selected based on the
+// hash of the endpoint id to ensure that delivery for the same endpoint happens
+// in-order.
+type dispatcher struct {
+	processors []*processor
+	seed       uint32
+}
+
+func newDispatcher(nProcessors int) *dispatcher {
+	processors := []*processor{}
+	for i := 0; i < nProcessors; i++ {
+		processors = append(processors, newProcessor(i))
+	}
+	return &dispatcher{
+		processors: processors,
+		seed:       generateRandUint32(),
+	}
+}
+
+func (d *dispatcher) queuePacket(r *stack.Route, stackEP stack.TransportEndpoint, id stack.TransportEndpointID, pkt tcpip.PacketBuffer) {
+	ep := stackEP.(*endpoint)
+	s := newSegment(r, id, pkt)
+	if !s.parse() {
+		ep.stack.Stats().MalformedRcvdPackets.Increment()
+		ep.stack.Stats().TCP.InvalidSegmentsReceived.Increment()
+		ep.stats.ReceiveErrors.MalformedPacketsReceived.Increment()
+		s.decRef()
+		return
+	}
+
+	if !s.csumValid {
+		ep.stack.Stats().MalformedRcvdPackets.Increment()
+		ep.stack.Stats().TCP.ChecksumErrors.Increment()
+		ep.stats.ReceiveErrors.ChecksumErrors.Increment()
+		s.decRef()
+		return
+	}
+
+	ep.stack.Stats().TCP.ValidSegmentsReceived.Increment()
+	ep.stats.SegmentsReceived.Increment()
+	if (s.flags & header.TCPFlagRst) != 0 {
+		ep.stack.Stats().TCP.ResetsReceived.Increment()
+	}
+
+	if !ep.enqueueSegment(s) {
+		s.decRef()
+		return
+	}
+
+	// For sockets not in established state let the worker goroutine
+	// handle the packets.
+	if ep.EndpointState() != StateEstablished {
+		ep.newSegmentWaker.Assert()
+		return
+	}
+
+	d.selectProcessor(id).queueEndpoint(ep)
+}
+
+func generateRandUint32() uint32 {
+	b := make([]byte, 4)
+	if _, err := rand.Read(b); err != nil {
+		panic(err)
+	}
+	return uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24
+}
+
+func (d *dispatcher) selectProcessor(id stack.TransportEndpointID) *processor {
+	payload := []byte{
+		byte(id.LocalPort),
+		byte(id.LocalPort >> 8),
+		byte(id.RemotePort),
+		byte(id.RemotePort >> 8)}
+
+	h := jenkins.Sum32(d.seed)
+	h.Write(payload)
+	h.Write([]byte(id.LocalAddress))
+	h.Write([]byte(id.RemoteAddress))
+
+	return d.processors[h.Sum32()%uint32(len(d.processors))]
+}
diff --git a/pkg/tcpip/transport/tcp/endpoint.go b/pkg/tcpip/transport/tcp/endpoint.go
index cc8b533c8..1799c6e10 100644
--- a/pkg/tcpip/transport/tcp/endpoint.go
+++ b/pkg/tcpip/transport/tcp/endpoint.go
@@ -120,6 +120,7 @@ const (
 	notifyMTUChanged
 	notifyDrain
 	notifyReset
+	notifyResetByPeer
 	notifyKeepaliveChanged
 	notifyMSSChanged
 	// notifyTickleWorker is used to tickle the protocol main loop during a
@@ -127,6 +128,7 @@ const (
 	// ensures the loop terminates if the final state of the endpoint is
 	// say TIME_WAIT.
 	notifyTickleWorker
+	notifyError
 )
 
 // SACKInfo holds TCP SACK related information for a given endpoint.
@@ -283,6 +285,18 @@ func (*EndpointInfo) IsEndpointInfo() {}
 type endpoint struct {
 	EndpointInfo
 
+	// endpointEntry is used to queue endpoints for processing to the
+	// a given tcp processor goroutine.
+	//
+	// Precondition: epQueue.mu must be held to read/write this field..
+	endpointEntry `state:"nosave"`
+
+	// pendingProcessing is true if this endpoint is queued for processing
+	// to a TCP processor.
+	//
+	// Precondition: epQueue.mu must be held to read/write this field..
+	pendingProcessing bool `state:"nosave"`
+
 	// workMu is used to arbitrate which goroutine may perform protocol
 	// work. Only the main protocol goroutine is expected to call Lock() on
 	// it, but other goroutines (e.g., send) may call TryLock() to eagerly
@@ -324,6 +338,7 @@ type endpoint struct {
 	// The following fields are protected by the mutex.
 	mu sync.RWMutex `state:"nosave"`
 
+	// state must be read/set using the EndpointState()/setEndpointState() methods.
 	state EndpointState `state:".(EndpointState)"`
 
 	// origEndpointState is only used during a restore phase to save the
@@ -359,7 +374,7 @@ type endpoint struct {
 	workerRunning bool
 
 	// workerCleanup specifies if the worker goroutine must perform cleanup
-	// before exitting. This can only be set to true when workerRunning is
+	// before exiting. This can only be set to true when workerRunning is
 	// also true, and they're both protected by the mutex.
 	workerCleanup bool
 
@@ -371,6 +386,8 @@ type endpoint struct {
 	// recentTS is the timestamp that should be sent in the TSEcr field of
 	// the timestamp for future segments sent by the endpoint. This field is
 	// updated if required when a new segment is received by this endpoint.
+	//
+	// recentTS must be read/written atomically.
 	recentTS uint32
 
 	// tsOffset is a randomized offset added to the value of the
@@ -567,6 +584,47 @@ func (e *endpoint) ResumeWork() {
 	e.workMu.Unlock()
 }
 
+// setEndpointState updates the state of the endpoint to state atomically. This
+// method is unexported as the only place we should update the state is in this
+// package but we allow the state to be read freely without holding e.mu.
+//
+// Precondition: e.mu must be held to call this method.
+func (e *endpoint) setEndpointState(state EndpointState) {
+	oldstate := EndpointState(atomic.LoadUint32((*uint32)(&e.state)))
+	switch state {
+	case StateEstablished:
+		e.stack.Stats().TCP.CurrentEstablished.Increment()
+	case StateError:
+		fallthrough
+	case StateClose:
+		if oldstate == StateCloseWait || oldstate == StateEstablished {
+			e.stack.Stats().TCP.EstablishedResets.Increment()
+		}
+		fallthrough
+	default:
+		if oldstate == StateEstablished {
+			e.stack.Stats().TCP.CurrentEstablished.Decrement()
+		}
+	}
+	atomic.StoreUint32((*uint32)(&e.state), uint32(state))
+}
+
+// EndpointState returns the current state of the endpoint.
+func (e *endpoint) EndpointState() EndpointState {
+	return EndpointState(atomic.LoadUint32((*uint32)(&e.state)))
+}
+
+// setRecentTimestamp atomically sets the recentTS field to the
+// provided value.
+func (e *endpoint) setRecentTimestamp(recentTS uint32) {
+	atomic.StoreUint32(&e.recentTS, recentTS)
+}
+
+// recentTimestamp atomically reads and returns the value of the recentTS field.
+func (e *endpoint) recentTimestamp() uint32 {
+	return atomic.LoadUint32(&e.recentTS)
+}
+
 // keepalive is a synchronization wrapper used to appease stateify. See the
 // comment in endpoint, where it is used.
 //
@@ -656,7 +714,7 @@ func (e *endpoint) Readiness(mask waiter.EventMask) waiter.EventMask {
 	e.mu.RLock()
 	defer e.mu.RUnlock()
 
-	switch e.state {
+	switch e.EndpointState() {
 	case StateInitial, StateBound, StateConnecting, StateSynSent, StateSynRecv:
 		// Ready for nothing.
 
@@ -672,7 +730,7 @@ func (e *endpoint) Readiness(mask waiter.EventMask) waiter.EventMask {
 			}
 		}
 	}
-	if e.state.connected() {
+	if e.EndpointState().connected() {
 		// Determine if the endpoint is writable if requested.
 		if (mask & waiter.EventOut) != 0 {
 			e.sndBufMu.Lock()
@@ -733,14 +791,20 @@ func (e *endpoint) Close() {
 	// Issue a shutdown so that the peer knows we won't send any more data
 	// if we're connected, or stop accepting if we're listening.
 	e.Shutdown(tcpip.ShutdownWrite | tcpip.ShutdownRead)
+	e.closeNoShutdown()
+}
 
+// closeNoShutdown closes the endpoint without doing a full shutdown. This is
+// used when a connection needs to be aborted with a RST and we want to skip
+// a full 4 way TCP shutdown.
+func (e *endpoint) closeNoShutdown() {
 	e.mu.Lock()
 
 	// For listening sockets, we always release ports inline so that they
 	// are immediately available for reuse after Close() is called. If also
 	// registered, we unregister as well otherwise the next user would fail
 	// in Listen() when trying to register.
-	if e.state == StateListen && e.isPortReserved {
+	if e.EndpointState() == StateListen && e.isPortReserved {
 		if e.isRegistered {
 			e.stack.StartTransportEndpointCleanup(e.boundNICID, e.effectiveNetProtos, ProtocolNumber, e.ID, e, e.boundBindToDevice)
 			e.isRegistered = false
@@ -780,6 +844,8 @@ func (e *endpoint) closePendingAcceptableConnectionsLocked() {
 		defer close(done)
 		for n := range e.acceptedChan {
 			n.notifyProtocolGoroutine(notifyReset)
+			// close all connections that have completed but
+			// not accepted by the application.
 			n.Close()
 		}
 	}()
@@ -797,11 +863,13 @@ func (e *endpoint) closePendingAcceptableConnectionsLocked() {
 // after Close() is called and the worker goroutine (if any) is done with its
 // work.
 func (e *endpoint) cleanupLocked() {
+
 	// Close all endpoints that might have been accepted by TCP but not by
 	// the client.
 	if e.acceptedChan != nil {
 		e.closePendingAcceptableConnectionsLocked()
 	}
+
 	e.workerCleanup = false
 
 	if e.isRegistered {
@@ -920,7 +988,7 @@ func (e *endpoint) Read(*tcpip.FullAddress) (buffer.View, tcpip.ControlMessages,
 	// reads to proceed before returning a ECONNRESET.
 	e.rcvListMu.Lock()
 	bufUsed := e.rcvBufUsed
-	if s := e.state; !s.connected() && s != StateClose && bufUsed == 0 {
+	if s := e.EndpointState(); !s.connected() && s != StateClose && bufUsed == 0 {
 		e.rcvListMu.Unlock()
 		he := e.HardError
 		e.mu.RUnlock()
@@ -944,7 +1012,7 @@ func (e *endpoint) Read(*tcpip.FullAddress) (buffer.View, tcpip.ControlMessages,
 
 func (e *endpoint) readLocked() (buffer.View, *tcpip.Error) {
 	if e.rcvBufUsed == 0 {
-		if e.rcvClosed || !e.state.connected() {
+		if e.rcvClosed || !e.EndpointState().connected() {
 			return buffer.View{}, tcpip.ErrClosedForReceive
 		}
 		return buffer.View{}, tcpip.ErrWouldBlock
@@ -980,8 +1048,8 @@ func (e *endpoint) readLocked() (buffer.View, *tcpip.Error) {
 // Caller must hold e.mu and e.sndBufMu
 func (e *endpoint) isEndpointWritableLocked() (int, *tcpip.Error) {
 	// The endpoint cannot be written to if it's not connected.
-	if !e.state.connected() {
-		switch e.state {
+	if !e.EndpointState().connected() {
+		switch e.EndpointState() {
 		case StateError:
 			return 0, e.HardError
 		default:
@@ -1039,42 +1107,86 @@ func (e *endpoint) Write(p tcpip.Payloader, opts tcpip.WriteOptions) (int64, <-c
 		return 0, nil, perr
 	}
 
-	if !opts.Atomic { // See above.
-		e.mu.RLock()
-		e.sndBufMu.Lock()
+	if opts.Atomic {
+		// Add data to the send queue.
+		s := newSegmentFromView(&e.route, e.ID, v)
+		e.sndBufUsed += len(v)
+		e.sndBufInQueue += seqnum.Size(len(v))
+		e.sndQueue.PushBack(s)
+		e.sndBufMu.Unlock()
+		// Release the endpoint lock to prevent deadlocks due to lock
+		// order inversion when acquiring workMu.
+		e.mu.RUnlock()
+	}
 
-		// Because we released the lock before copying, check state again
-		// to make sure the endpoint is still in a valid state for a write.
-		avail, err = e.isEndpointWritableLocked()
-		if err != nil {
+	if e.workMu.TryLock() {
+		// Since we released locks in between it's possible that the
+		// endpoint transitioned to a CLOSED/ERROR states so make
+		// sure endpoint is still writable before trying to write.
+		if !opts.Atomic { // See above.
+			e.mu.RLock()
+			e.sndBufMu.Lock()
+
+			// Because we released the lock before copying, check state again
+			// to make sure the endpoint is still in a valid state for a write.
+			avail, err = e.isEndpointWritableLocked()
+			if err != nil {
+				e.sndBufMu.Unlock()
+				e.mu.RUnlock()
+				e.stats.WriteErrors.WriteClosed.Increment()
+				return 0, nil, err
+			}
+
+			// Discard any excess data copied in due to avail being reduced due
+			// to a simultaneous write call to the socket.
+			if avail < len(v) {
+				v = v[:avail]
+			}
+			// Add data to the send queue.
+			s := newSegmentFromView(&e.route, e.ID, v)
+			e.sndBufUsed += len(v)
+			e.sndBufInQueue += seqnum.Size(len(v))
+			e.sndQueue.PushBack(s)
 			e.sndBufMu.Unlock()
+			// Release the endpoint lock to prevent deadlocks due to lock
+			// order inversion when acquiring workMu.
 			e.mu.RUnlock()
-			e.stats.WriteErrors.WriteClosed.Increment()
-			return 0, nil, err
-		}
 
-		// Discard any excess data copied in due to avail being reduced due
-		// to a simultaneous write call to the socket.
-		if avail < len(v) {
-			v = v[:avail]
 		}
-	}
-
-	// Add data to the send queue.
-	s := newSegmentFromView(&e.route, e.ID, v)
-	e.sndBufUsed += len(v)
-	e.sndBufInQueue += seqnum.Size(len(v))
-	e.sndQueue.PushBack(s)
-	e.sndBufMu.Unlock()
-	// Release the endpoint lock to prevent deadlocks due to lock
-	// order inversion when acquiring workMu.
-	e.mu.RUnlock()
-
-	if e.workMu.TryLock() {
 		// Do the work inline.
 		e.handleWrite()
 		e.workMu.Unlock()
 	} else {
+		if !opts.Atomic { // See above.
+			e.mu.RLock()
+			e.sndBufMu.Lock()
+
+			// Because we released the lock before copying, check state again
+			// to make sure the endpoint is still in a valid state for a write.
+			avail, err = e.isEndpointWritableLocked()
+			if err != nil {
+				e.sndBufMu.Unlock()
+				e.mu.RUnlock()
+				e.stats.WriteErrors.WriteClosed.Increment()
+				return 0, nil, err
+			}
+
+			// Discard any excess data copied in due to avail being reduced due
+			// to a simultaneous write call to the socket.
+			if avail < len(v) {
+				v = v[:avail]
+			}
+			// Add data to the send queue.
+			s := newSegmentFromView(&e.route, e.ID, v)
+			e.sndBufUsed += len(v)
+			e.sndBufInQueue += seqnum.Size(len(v))
+			e.sndQueue.PushBack(s)
+			e.sndBufMu.Unlock()
+			// Release the endpoint lock to prevent deadlocks due to lock
+			// order inversion when acquiring workMu.
+			e.mu.RUnlock()
+
+		}
 		// Let the protocol goroutine do the work.
 		e.sndWaker.Assert()
 	}
@@ -1091,7 +1203,7 @@ func (e *endpoint) Peek(vec [][]byte) (int64, tcpip.ControlMessages, *tcpip.Erro
 
 	// The endpoint can be read if it's connected, or if it's already closed
 	// but has some pending unread data.
-	if s := e.state; !s.connected() && s != StateClose {
+	if s := e.EndpointState(); !s.connected() && s != StateClose {
 		if s == StateError {
 			return 0, tcpip.ControlMessages{}, e.HardError
 		}
@@ -1103,7 +1215,7 @@ func (e *endpoint) Peek(vec [][]byte) (int64, tcpip.ControlMessages, *tcpip.Erro
 	defer e.rcvListMu.Unlock()
 
 	if e.rcvBufUsed == 0 {
-		if e.rcvClosed || !e.state.connected() {
+		if e.rcvClosed || !e.EndpointState().connected() {
 			e.stats.ReadErrors.ReadClosed.Increment()
 			return 0, tcpip.ControlMessages{}, tcpip.ErrClosedForReceive
 		}
@@ -1187,7 +1299,7 @@ func (e *endpoint) SetSockOptBool(opt tcpip.SockOptBool, v bool) *tcpip.Error {
 		defer e.mu.Unlock()
 
 		// We only allow this to be set when we're in the initial state.
-		if e.state != StateInitial {
+		if e.EndpointState() != StateInitial {
 			return tcpip.ErrInvalidEndpointState
 		}
 
@@ -1402,14 +1514,14 @@ func (e *endpoint) SetSockOpt(opt interface{}) *tcpip.Error {
 				// Acquire the work mutex as we may need to
 				// reinitialize the congestion control state.
 				e.mu.Lock()
-				state := e.state
+				state := e.EndpointState()
 				e.cc = v
 				e.mu.Unlock()
 				switch state {
 				case StateEstablished:
 					e.workMu.Lock()
 					e.mu.Lock()
-					if e.state == state {
+					if e.EndpointState() == state {
 						e.snd.cc = e.snd.initCongestionControl(e.cc)
 					}
 					e.mu.Unlock()
@@ -1472,7 +1584,7 @@ func (e *endpoint) readyReceiveSize() (int, *tcpip.Error) {
 	defer e.mu.RUnlock()
 
 	// The endpoint cannot be in listen state.
-	if e.state == StateListen {
+	if e.EndpointState() == StateListen {
 		return 0, tcpip.ErrInvalidEndpointState
 	}
 
@@ -1731,7 +1843,7 @@ func (e *endpoint) connect(addr tcpip.FullAddress, handshake bool, run bool) *tc
 		return err
 	}
 
-	if e.state.connected() {
+	if e.EndpointState().connected() {
 		// The endpoint is already connected. If caller hasn't been
 		// notified yet, return success.
 		if !e.isConnectNotified {
@@ -1743,7 +1855,7 @@ func (e *endpoint) connect(addr tcpip.FullAddress, handshake bool, run bool) *tc
 	}
 
 	nicID := addr.NIC
-	switch e.state {
+	switch e.EndpointState() {
 	case StateBound:
 		// If we're already bound to a NIC but the caller is requesting
 		// that we use a different one now, we cannot proceed.
@@ -1850,7 +1962,7 @@ func (e *endpoint) connect(addr tcpip.FullAddress, handshake bool, run bool) *tc
 	}
 
 	e.isRegistered = true
-	e.state = StateConnecting
+	e.setEndpointState(StateConnecting)
 	e.route = r.Clone()
 	e.boundNICID = nicID
 	e.effectiveNetProtos = netProtos
@@ -1871,14 +1983,13 @@ func (e *endpoint) connect(addr tcpip.FullAddress, handshake bool, run bool) *tc
 		}
 		e.segmentQueue.mu.Unlock()
 		e.snd.updateMaxPayloadSize(int(e.route.MTU()), 0)
-		e.state = StateEstablished
-		e.stack.Stats().TCP.CurrentEstablished.Increment()
+		e.setEndpointState(StateEstablished)
 	}
 
 	if run {
 		e.workerRunning = true
 		e.stack.Stats().TCP.ActiveConnectionOpenings.Increment()
-		go e.protocolMainLoop(handshake) // S/R-SAFE: will be drained before save.
+		go e.protocolMainLoop(handshake, nil) // S/R-SAFE: will be drained before save.
 	}
 
 	return tcpip.ErrConnectStarted
@@ -1896,7 +2007,7 @@ func (e *endpoint) Shutdown(flags tcpip.ShutdownFlags) *tcpip.Error {
 	e.shutdownFlags |= flags
 	finQueued := false
 	switch {
-	case e.state.connected():
+	case e.EndpointState().connected():
 		// Close for read.
 		if (e.shutdownFlags & tcpip.ShutdownRead) != 0 {
 			// Mark read side as closed.
@@ -1908,8 +2019,23 @@ func (e *endpoint) Shutdown(flags tcpip.ShutdownFlags) *tcpip.Error {
 			// If we're fully closed and we have unread data we need to abort
 			// the connection with a RST.
 			if (e.shutdownFlags&tcpip.ShutdownWrite) != 0 && rcvBufUsed > 0 {
-				e.notifyProtocolGoroutine(notifyReset)
+				// Move the socket to error state immediately.
+				// This is done redundantly because in case of
+				// save/restore on a Shutdown/Close() the socket
+				// state needs to indicate the error otherwise
+				// save file will show the socket in established
+				// state even though snd/rcv are closed.
 				e.mu.Unlock()
+				// Try to send an active reset immediately if the
+				// work mutex is available.
+				if e.workMu.TryLock() {
+					e.mu.Lock()
+					e.resetConnectionLocked(tcpip.ErrConnectionAborted)
+					e.mu.Unlock()
+					e.workMu.Unlock()
+				} else {
+					e.notifyProtocolGoroutine(notifyReset)
+				}
 				return nil
 			}
 		}
@@ -1931,11 +2057,10 @@ func (e *endpoint) Shutdown(flags tcpip.ShutdownFlags) *tcpip.Error {
 			finQueued = true
 			// Mark endpoint as closed.
 			e.sndClosed = true
-
 			e.sndBufMu.Unlock()
 		}
 
-	case e.state == StateListen:
+	case e.EndpointState() == StateListen:
 		// Tell protocolListenLoop to stop.
 		if flags&tcpip.ShutdownRead != 0 {
 			e.notifyProtocolGoroutine(notifyClose)
@@ -1976,7 +2101,7 @@ func (e *endpoint) listen(backlog int) *tcpip.Error {
 	// When the endpoint shuts down, it sets workerCleanup to true, and from
 	// that point onward, acceptedChan is the responsibility of the cleanup()
 	// method (and should not be touched anywhere else, including here).
-	if e.state == StateListen && !e.workerCleanup {
+	if e.EndpointState() == StateListen && !e.workerCleanup {
 		// Adjust the size of the channel iff we can fix existing
 		// pending connections into the new one.
 		if len(e.acceptedChan) > backlog {
@@ -1994,7 +2119,7 @@ func (e *endpoint) listen(backlog int) *tcpip.Error {
 		return nil
 	}
 
-	if e.state == StateInitial {
+	if e.EndpointState() == StateInitial {
 		// The listen is called on an unbound socket, the socket is
 		// automatically bound to a random free port with the local
 		// address set to INADDR_ANY.
@@ -2004,7 +2129,7 @@ func (e *endpoint) listen(backlog int) *tcpip.Error {
 	}
 
 	// Endpoint must be bound before it can transition to listen mode.
-	if e.state != StateBound {
+	if e.EndpointState() != StateBound {
 		e.stats.ReadErrors.InvalidEndpointState.Increment()
 		return tcpip.ErrInvalidEndpointState
 	}
@@ -2015,24 +2140,27 @@ func (e *endpoint) listen(backlog int) *tcpip.Error {
 	}
 
 	e.isRegistered = true
-	e.state = StateListen
+	e.setEndpointState(StateListen)
+
 	if e.acceptedChan == nil {
 		e.acceptedChan = make(chan *endpoint, backlog)
 	}
 	e.workerRunning = true
-
 	go e.protocolListenLoop( // S/R-SAFE: drained on save.
 		seqnum.Size(e.receiveBufferAvailable()))
-
 	return nil
 }
 
 // startAcceptedLoop sets up required state and starts a goroutine with the
 // main loop for accepted connections.
 func (e *endpoint) startAcceptedLoop(waiterQueue *waiter.Queue) {
+	e.mu.Lock()
 	e.waiterQueue = waiterQueue
 	e.workerRunning = true
-	go e.protocolMainLoop(false) // S/R-SAFE: drained on save.
+	e.mu.Unlock()
+	wakerInitDone := make(chan struct{})
+	go e.protocolMainLoop(false, wakerInitDone) // S/R-SAFE: drained on save.
+	<-wakerInitDone
 }
 
 // Accept returns a new endpoint if a peer has established a connection
@@ -2042,7 +2170,7 @@ func (e *endpoint) Accept() (tcpip.Endpoint, *waiter.Queue, *tcpip.Error) {
 	defer e.mu.RUnlock()
 
 	// Endpoint must be in listen state before it can accept connections.
-	if e.state != StateListen {
+	if e.EndpointState() != StateListen {
 		return nil, nil, tcpip.ErrInvalidEndpointState
 	}
 
@@ -2069,7 +2197,7 @@ func (e *endpoint) bindLocked(addr tcpip.FullAddress) (err *tcpip.Error) {
 	// Don't allow binding once endpoint is not in the initial state
 	// anymore. This is because once the endpoint goes into a connected or
 	// listen state, it is already bound.
-	if e.state != StateInitial {
+	if e.EndpointState() != StateInitial {
 		return tcpip.ErrAlreadyBound
 	}
 
@@ -2131,7 +2259,7 @@ func (e *endpoint) bindLocked(addr tcpip.FullAddress) (err *tcpip.Error) {
 	}
 
 	// Mark endpoint as bound.
-	e.state = StateBound
+	e.setEndpointState(StateBound)
 
 	return nil
 }
@@ -2153,7 +2281,7 @@ func (e *endpoint) GetRemoteAddress() (tcpip.FullAddress, *tcpip.Error) {
 	e.mu.RLock()
 	defer e.mu.RUnlock()
 
-	if !e.state.connected() {
+	if !e.EndpointState().connected() {
 		return tcpip.FullAddress{}, tcpip.ErrNotConnected
 	}
 
@@ -2164,45 +2292,22 @@ func (e *endpoint) GetRemoteAddress() (tcpip.FullAddress, *tcpip.Error) {
 	}, nil
 }
 
-// HandlePacket is called by the stack when new packets arrive to this transport
-// endpoint.
 func (e *endpoint) HandlePacket(r *stack.Route, id stack.TransportEndpointID, pkt tcpip.PacketBuffer) {
-	s := newSegment(r, id, pkt)
-	if !s.parse() {
-		e.stack.Stats().MalformedRcvdPackets.Increment()
-		e.stack.Stats().TCP.InvalidSegmentsReceived.Increment()
-		e.stats.ReceiveErrors.MalformedPacketsReceived.Increment()
-		s.decRef()
-		return
-	}
-
-	if !s.csumValid {
-		e.stack.Stats().MalformedRcvdPackets.Increment()
-		e.stack.Stats().TCP.ChecksumErrors.Increment()
-		e.stats.ReceiveErrors.ChecksumErrors.Increment()
-		s.decRef()
-		return
-	}
-
-	e.stack.Stats().TCP.ValidSegmentsReceived.Increment()
-	e.stats.SegmentsReceived.Increment()
-	if (s.flags & header.TCPFlagRst) != 0 {
-		e.stack.Stats().TCP.ResetsReceived.Increment()
-	}
-
-	e.enqueueSegment(s)
+	// TCP HandlePacket is not required anymore as inbound packets first
+	// land at the Dispatcher which then can either delivery using the
+	// worker go routine or directly do the invoke the tcp processing inline
+	// based on the state of the endpoint.
 }
 
-func (e *endpoint) enqueueSegment(s *segment) {
+func (e *endpoint) enqueueSegment(s *segment) bool {
 	// Send packet to worker goroutine.
-	if e.segmentQueue.enqueue(s) {
-		e.newSegmentWaker.Assert()
-	} else {
+	if !e.segmentQueue.enqueue(s) {
 		// The queue is full, so we drop the segment.
 		e.stack.Stats().DroppedPackets.Increment()
 		e.stats.ReceiveErrors.SegmentQueueDropped.Increment()
-		s.decRef()
+		return false
 	}
+	return true
 }
 
 // HandleControlPacket implements stack.TransportEndpoint.HandleControlPacket.
@@ -2319,8 +2424,8 @@ func (e *endpoint) rcvWndScaleForHandshake() int {
 // updateRecentTimestamp updates the recent timestamp using the algorithm
 // described in https://tools.ietf.org/html/rfc7323#section-4.3
 func (e *endpoint) updateRecentTimestamp(tsVal uint32, maxSentAck seqnum.Value, segSeq seqnum.Value) {
-	if e.sendTSOk && seqnum.Value(e.recentTS).LessThan(seqnum.Value(tsVal)) && segSeq.LessThanEq(maxSentAck) {
-		e.recentTS = tsVal
+	if e.sendTSOk && seqnum.Value(e.recentTimestamp()).LessThan(seqnum.Value(tsVal)) && segSeq.LessThanEq(maxSentAck) {
+		e.setRecentTimestamp(tsVal)
 	}
 }
 
@@ -2330,7 +2435,7 @@ func (e *endpoint) updateRecentTimestamp(tsVal uint32, maxSentAck seqnum.Value,
 func (e *endpoint) maybeEnableTimestamp(synOpts *header.TCPSynOptions) {
 	if synOpts.TS {
 		e.sendTSOk = true
-		e.recentTS = synOpts.TSVal
+		e.setRecentTimestamp(synOpts.TSVal)
 	}
 }
 
@@ -2419,7 +2524,7 @@ func (e *endpoint) completeState() stack.TCPEndpointState {
 
 	// Endpoint TCP Option state.
 	s.SendTSOk = e.sendTSOk
-	s.RecentTS = e.recentTS
+	s.RecentTS = e.recentTimestamp()
 	s.TSOffset = e.tsOffset
 	s.SACKPermitted = e.sackPermitted
 	s.SACK.Blocks = make([]header.SACKBlock, e.sack.NumBlocks)
@@ -2526,9 +2631,7 @@ func (e *endpoint) initGSO() {
 // State implements tcpip.Endpoint.State. It exports the endpoint's protocol
 // state for diagnostics.
 func (e *endpoint) State() uint32 {
-	e.mu.Lock()
-	defer e.mu.Unlock()
-	return uint32(e.state)
+	return uint32(e.EndpointState())
 }
 
 // Info returns a copy of the endpoint info.
diff --git a/pkg/tcpip/transport/tcp/endpoint_state.go b/pkg/tcpip/transport/tcp/endpoint_state.go
index 4b8d867bc..4a46f0ec5 100644
--- a/pkg/tcpip/transport/tcp/endpoint_state.go
+++ b/pkg/tcpip/transport/tcp/endpoint_state.go
@@ -16,6 +16,7 @@ package tcp
 
 import (
 	"fmt"
+	"sync/atomic"
 	"time"
 
 	"gvisor.dev/gvisor/pkg/sync"
@@ -48,7 +49,7 @@ func (e *endpoint) beforeSave() {
 	e.mu.Lock()
 	defer e.mu.Unlock()
 
-	switch e.state {
+	switch e.EndpointState() {
 	case StateInitial, StateBound:
 		// TODO(b/138137272): this enumeration duplicates
 		// EndpointState.connected. remove it.
@@ -70,31 +71,30 @@ func (e *endpoint) beforeSave() {
 		fallthrough
 	case StateListen, StateConnecting:
 		e.drainSegmentLocked()
-		if e.state != StateClose && e.state != StateError {
+		if e.EndpointState() != StateClose && e.EndpointState() != StateError {
 			if !e.workerRunning {
 				panic("endpoint has no worker running in listen, connecting, or connected state")
 			}
 			break
 		}
-		fallthrough
 	case StateError, StateClose:
-		for (e.state == StateError || e.state == StateClose) && e.workerRunning {
+		for e.workerRunning {
 			e.mu.Unlock()
 			time.Sleep(100 * time.Millisecond)
 			e.mu.Lock()
 		}
 		if e.workerRunning {
-			panic("endpoint still has worker running in closed or error state")
+			panic(fmt.Sprintf("endpoint: %+v still has worker running in closed or error state", e.ID))
 		}
 	default:
-		panic(fmt.Sprintf("endpoint in unknown state %v", e.state))
+		panic(fmt.Sprintf("endpoint in unknown state %v", e.EndpointState()))
 	}
 
 	if e.waiterQueue != nil && !e.waiterQueue.IsEmpty() {
 		panic("endpoint still has waiters upon save")
 	}
 
-	if e.state != StateClose && !((e.state == StateBound || e.state == StateListen) == e.isPortReserved) {
+	if e.EndpointState() != StateClose && !((e.EndpointState() == StateBound || e.EndpointState() == StateListen) == e.isPortReserved) {
 		panic("endpoints which are not in the closed state must have a reserved port IFF they are in bound or listen state")
 	}
 }
@@ -135,7 +135,7 @@ func (e *endpoint) loadAcceptedChan(acceptedEndpoints []*endpoint) {
 
 // saveState is invoked by stateify.
 func (e *endpoint) saveState() EndpointState {
-	return e.state
+	return e.EndpointState()
 }
 
 // Endpoint loading must be done in the following ordering by their state, to
@@ -151,7 +151,8 @@ var connectingLoading sync.WaitGroup
 func (e *endpoint) loadState(state EndpointState) {
 	// This is to ensure that the loading wait groups include all applicable
 	// endpoints before any asynchronous calls to the Wait() methods.
-	if state.connected() {
+	// For restore purposes we treat TimeWait like a connected endpoint.
+	if state.connected() || state == StateTimeWait {
 		connectedLoading.Add(1)
 	}
 	switch state {
@@ -160,13 +161,14 @@ func (e *endpoint) loadState(state EndpointState) {
 	case StateConnecting, StateSynSent, StateSynRecv:
 		connectingLoading.Add(1)
 	}
-	e.state = state
+	// Directly update the state here rather than using e.setEndpointState
+	// as the endpoint is still being loaded and the stack reference to increment
+	// metrics is not yet initialized.
+	atomic.StoreUint32((*uint32)(&e.state), uint32(state))
 }
 
 // afterLoad is invoked by stateify.
 func (e *endpoint) afterLoad() {
-	// Freeze segment queue before registering to prevent any segments
-	// from being delivered while it is being restored.
 	e.origEndpointState = e.state
 	// Restore the endpoint to InitialState as it will be moved to
 	// its origEndpointState during Resume.
@@ -180,7 +182,6 @@ func (e *endpoint) Resume(s *stack.Stack) {
 	e.segmentQueue.setLimit(MaxUnprocessedSegments)
 	e.workMu.Init()
 	state := e.origEndpointState
-
 	switch state {
 	case StateInitial, StateBound, StateListen, StateConnecting, StateEstablished:
 		var ss SendBufferSizeOption
@@ -276,7 +277,7 @@ func (e *endpoint) Resume(s *stack.Stack) {
 				listenLoading.Wait()
 				connectingLoading.Wait()
 				bind()
-				e.state = StateClose
+				e.setEndpointState(StateClose)
 				tcpip.AsyncLoading.Done()
 			}()
 		}
@@ -288,6 +289,7 @@ func (e *endpoint) Resume(s *stack.Stack) {
 		e.stack.CompleteTransportEndpointCleanup(e)
 		tcpip.DeleteDanglingEndpoint(e)
 	}
+
 }
 
 // saveLastError is invoked by stateify.
diff --git a/pkg/tcpip/transport/tcp/protocol.go b/pkg/tcpip/transport/tcp/protocol.go
index 9a8f64aa6..958c06fa7 100644
--- a/pkg/tcpip/transport/tcp/protocol.go
+++ b/pkg/tcpip/transport/tcp/protocol.go
@@ -21,6 +21,7 @@
 package tcp
 
 import (
+	"runtime"
 	"strings"
 	"time"
 
@@ -104,6 +105,7 @@ type protocol struct {
 	moderateReceiveBuffer      bool
 	tcpLingerTimeout           time.Duration
 	tcpTimeWaitTimeout         time.Duration
+	dispatcher                 *dispatcher
 }
 
 // Number returns the tcp protocol number.
@@ -134,6 +136,14 @@ func (*protocol) ParsePorts(v buffer.View) (src, dst uint16, err *tcpip.Error) {
 	return h.SourcePort(), h.DestinationPort(), nil
 }
 
+// QueuePacket queues packets targeted at an endpoint after hashing the packet
+// to a specific processing queue. Each queue is serviced by its own processor
+// goroutine which is responsible for dequeuing and doing full TCP dispatch of
+// the packet.
+func (p *protocol) QueuePacket(r *stack.Route, ep stack.TransportEndpoint, id stack.TransportEndpointID, pkt tcpip.PacketBuffer) {
+	p.dispatcher.queuePacket(r, ep, id, pkt)
+}
+
 // HandleUnknownDestinationPacket handles packets targeted at this protocol but
 // that don't match any existing endpoint.
 //
@@ -330,5 +340,6 @@ func NewProtocol() stack.TransportProtocol {
 		availableCongestionControl: []string{ccReno, ccCubic},
 		tcpLingerTimeout:           DefaultTCPLingerTimeout,
 		tcpTimeWaitTimeout:         DefaultTCPTimeWaitTimeout,
+		dispatcher:                 newDispatcher(runtime.GOMAXPROCS(0)),
 	}
 }
diff --git a/pkg/tcpip/transport/tcp/rcv.go b/pkg/tcpip/transport/tcp/rcv.go
index 05c8488f8..958f03ac1 100644
--- a/pkg/tcpip/transport/tcp/rcv.go
+++ b/pkg/tcpip/transport/tcp/rcv.go
@@ -169,19 +169,19 @@ func (r *receiver) consumeSegment(s *segment, segSeq seqnum.Value, segLen seqnum
 		// We just received a FIN, our next state depends on whether we sent a
 		// FIN already or not.
 		r.ep.mu.Lock()
-		switch r.ep.state {
+		switch r.ep.EndpointState() {
 		case StateEstablished:
-			r.ep.state = StateCloseWait
+			r.ep.setEndpointState(StateCloseWait)
 		case StateFinWait1:
 			if s.flagIsSet(header.TCPFlagAck) {
 				// FIN-ACK, transition to TIME-WAIT.
-				r.ep.state = StateTimeWait
+				r.ep.setEndpointState(StateTimeWait)
 			} else {
 				// Simultaneous close, expecting a final ACK.
-				r.ep.state = StateClosing
+				r.ep.setEndpointState(StateClosing)
 			}
 		case StateFinWait2:
-			r.ep.state = StateTimeWait
+			r.ep.setEndpointState(StateTimeWait)
 		}
 		r.ep.mu.Unlock()
 
@@ -205,16 +205,16 @@ func (r *receiver) consumeSegment(s *segment, segSeq seqnum.Value, segLen seqnum
 	// shutdown states.
 	if s.flagIsSet(header.TCPFlagAck) && s.ackNumber == r.ep.snd.sndNxt {
 		r.ep.mu.Lock()
-		switch r.ep.state {
+		switch r.ep.EndpointState() {
 		case StateFinWait1:
-			r.ep.state = StateFinWait2
+			r.ep.setEndpointState(StateFinWait2)
 			// Notify protocol goroutine that we have received an
 			// ACK to our FIN so that it can start the FIN_WAIT2
 			// timer to abort connection if the other side does
 			// not close within 2MSL.
 			r.ep.notifyProtocolGoroutine(notifyClose)
 		case StateClosing:
-			r.ep.state = StateTimeWait
+			r.ep.setEndpointState(StateTimeWait)
 		case StateLastAck:
 			r.ep.transitionToStateCloseLocked()
 		}
@@ -267,7 +267,6 @@ func (r *receiver) handleRcvdSegmentClosing(s *segment, state EndpointState, clo
 	switch state {
 	case StateCloseWait, StateClosing, StateLastAck:
 		if !s.sequenceNumber.LessThanEq(r.rcvNxt) {
-			s.decRef()
 			// Just drop the segment as we have
 			// already received a FIN and this
 			// segment is after the sequence number
@@ -284,7 +283,6 @@ func (r *receiver) handleRcvdSegmentClosing(s *segment, state EndpointState, clo
 		// trigger a RST.
 		endDataSeq := s.sequenceNumber.Add(seqnum.Size(s.data.Size()))
 		if rcvClosed && r.rcvNxt.LessThan(endDataSeq) {
-			s.decRef()
 			return true, tcpip.ErrConnectionAborted
 		}
 		if state == StateFinWait1 {
@@ -314,7 +312,6 @@ func (r *receiver) handleRcvdSegmentClosing(s *segment, state EndpointState, clo
 		// the last actual data octet in a segment in
 		// which it occurs.
 		if closed && (!s.flagIsSet(header.TCPFlagFin) || s.sequenceNumber.Add(s.logicalLen()) != r.rcvNxt+1) {
-			s.decRef()
 			return true, tcpip.ErrConnectionAborted
 		}
 	}
@@ -336,7 +333,7 @@ func (r *receiver) handleRcvdSegmentClosing(s *segment, state EndpointState, clo
 // r as they arrive. It is called by the protocol main loop.
 func (r *receiver) handleRcvdSegment(s *segment) (drop bool, err *tcpip.Error) {
 	r.ep.mu.RLock()
-	state := r.ep.state
+	state := r.ep.EndpointState()
 	closed := r.ep.closed
 	r.ep.mu.RUnlock()
 
diff --git a/pkg/tcpip/transport/tcp/snd.go b/pkg/tcpip/transport/tcp/snd.go
index fdff7ed81..b74b61e7d 100644
--- a/pkg/tcpip/transport/tcp/snd.go
+++ b/pkg/tcpip/transport/tcp/snd.go
@@ -705,17 +705,15 @@ func (s *sender) maybeSendSegment(seg *segment, limit int, end seqnum.Value) (se
 		}
 		seg.flags = header.TCPFlagAck | header.TCPFlagFin
 		segEnd = seg.sequenceNumber.Add(1)
-		// Transition to FIN-WAIT1 state since we're initiating an active close.
-		s.ep.mu.Lock()
-		switch s.ep.state {
+		// Update the state to reflect that we have now
+		// queued a FIN.
+		switch s.ep.EndpointState() {
 		case StateCloseWait:
-			// We've already received a FIN and are now sending our own. The
-			// sender is now awaiting a final ACK for this FIN.
-			s.ep.state = StateLastAck
+			s.ep.setEndpointState(StateLastAck)
 		default:
-			s.ep.state = StateFinWait1
+			s.ep.setEndpointState(StateFinWait1)
 		}
-		s.ep.mu.Unlock()
+
 	} else {
 		// We're sending a non-FIN segment.
 		if seg.flags&header.TCPFlagFin != 0 {
diff --git a/pkg/tcpip/transport/tcp/tcp_test.go b/pkg/tcpip/transport/tcp/tcp_test.go
index 6edfa8dce..a9dfbe857 100644
--- a/pkg/tcpip/transport/tcp/tcp_test.go
+++ b/pkg/tcpip/transport/tcp/tcp_test.go
@@ -293,7 +293,6 @@ func TestTCPResetSentForACKWhenNotUsingSynCookies(t *testing.T) {
 		checker.SeqNum(uint32(c.IRS+1)),
 		checker.AckNum(uint32(iss)+1),
 		checker.TCPFlags(header.TCPFlagFin|header.TCPFlagAck)))
-
 	finHeaders := &context.Headers{
 		SrcPort: context.TestPort,
 		DstPort: context.StackPort,
@@ -459,6 +458,9 @@ func TestConnectResetAfterClose(t *testing.T) {
 		checker.IPv4(t, b,
 			checker.TCP(
 				checker.DstPort(context.TestPort),
+				// RST is always generated with sndNxt which if the FIN
+				// has been sent will be 1 higher than the sequence number
+				// of the FIN itself.
 				checker.SeqNum(uint32(c.IRS)+2),
 				checker.AckNum(0),
 				checker.TCPFlags(header.TCPFlagRst),
@@ -1500,6 +1502,9 @@ func TestRstOnCloseWithUnreadDataFinConvertRst(t *testing.T) {
 		checker.TCP(
 			checker.DstPort(context.TestPort),
 			checker.TCPFlags(header.TCPFlagAck|header.TCPFlagRst),
+			// RST is always generated with sndNxt which if the FIN
+			// has been sent will be 1 higher than the sequence
+			// number of the FIN itself.
 			checker.SeqNum(uint32(c.IRS)+2),
 		))
 	// The RST puts the endpoint into an error state.
@@ -5441,6 +5446,7 @@ func TestReceiveBufferAutoTuningApplicationLimited(t *testing.T) {
 		rawEP.SendPacketWithTS(b[start:start+mss], tsVal)
 		packetsSent++
 	}
+
 	// Resume the worker so that it only sees the packets once all of them
 	// are waiting to be read.
 	worker.ResumeWork()
@@ -5508,7 +5514,7 @@ func TestReceiveBufferAutoTuning(t *testing.T) {
 	stk := c.Stack()
 	// Set lower limits for auto-tuning tests. This is required because the
 	// test stops the worker which can cause packets to be dropped because
-	// the segment queue holding unprocessed packets is limited to 500.
+	// the segment queue holding unprocessed packets is limited to 300.
 	const receiveBufferSize = 80 << 10 // 80KB.
 	const maxReceiveBufferSize = receiveBufferSize * 10
 	if err := stk.SetTransportProtocolOption(tcp.ProtocolNumber, tcp.ReceiveBufferSizeOption{1, receiveBufferSize, maxReceiveBufferSize}); err != nil {
@@ -5563,6 +5569,7 @@ func TestReceiveBufferAutoTuning(t *testing.T) {
 			totalSent += mss
 			packetsSent++
 		}
+
 		// Resume it so that it only sees the packets once all of them
 		// are waiting to be read.
 		worker.ResumeWork()