Update variables for implementation of RACK in TCP

RACK (Recent Acknowledgement) is a new loss detection
algorithm in TCP. These are the fields which should be
stored on connections to implement RACK algorithm.

PiperOrigin-RevId: 324948703

pkg/tcpip/stack/stack.go

@@ -73,6 +73,16 @@ type TCPCubicState struct {
 	WEst                    float64
 }
 
+// TCPRACKState is used to hold a copy of the internal RACK state when the
+// TCPProbeFunc is invoked.
+type TCPRACKState struct {
+	XmitTime    time.Time
+	EndSequence seqnum.Value
+	FACK        seqnum.Value
+	RTT         time.Duration
+	Reord       bool
+}
+
 // TCPEndpointID is the unique 4 tuple that identifies a given endpoint.
 type TCPEndpointID struct {
 	// LocalPort is the local port associated with the endpoint.
@@ -212,6 +222,9 @@ type TCPSenderState struct {
 
 	// Cubic holds the state related to CUBIC congestion control.
 	Cubic TCPCubicState
+
+	// RACKState holds the state related to RACK loss detection algorithm.
+	RACKState TCPRACKState
 }
 
 // TCPSACKInfo holds TCP SACK related information for a given TCP endpoint.

pkg/tcpip/transport/tcp/BUILD

@@ -40,6 +40,8 @@ go_library(
         "endpoint_state.go",
         "forwarder.go",
         "protocol.go",
+        "rack.go",
+        "rack_state.go",
         "rcv.go",
         "rcv_state.go",
         "reno.go",
@@ -83,6 +85,7 @@ go_test(
         "dual_stack_test.go",
         "sack_scoreboard_test.go",
         "tcp_noracedetector_test.go",
+        "tcp_rack_test.go",
         "tcp_sack_test.go",
         "tcp_test.go",
         "tcp_timestamp_test.go",

pkg/tcpip/transport/tcp/accept.go

@@ -521,7 +521,7 @@ func (e *endpoint) handleListenSegment(ctx *listenContext, s *segment) {
 			synOpts := header.TCPSynOptions{
 				WS:    -1,
 				TS:    opts.TS,
-				TSVal: tcpTimeStamp(timeStampOffset()),
+				TSVal: tcpTimeStamp(time.Now(), timeStampOffset()),
 				TSEcr: opts.TSVal,
 				MSS:   mssForRoute(&s.route),
 			}

pkg/tcpip/transport/tcp/connect.go

@@ -1166,13 +1166,18 @@ func (e *endpoint) handleSegments(fastPath bool) *tcpip.Error {
 	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.
+func (e *endpoint) probeSegment() {
 	if e.probe != nil {
 		e.probe(e.completeState())
 	}
+}
+
+// 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. The tcp probe function will update
+	// the TCPEndpointState after the segment is processed.
+	defer e.probeSegment()
 
 	if s.flagIsSet(header.TCPFlagRst) {
 		if ok, err := e.handleReset(s); !ok {

pkg/tcpip/transport/tcp/endpoint.go

@@ -2692,15 +2692,14 @@ func (e *endpoint) maybeEnableTimestamp(synOpts *header.TCPSynOptions) {
 // timestamp returns the timestamp value to be used in the TSVal field of the
 // timestamp option for outgoing TCP segments for a given endpoint.
 func (e *endpoint) timestamp() uint32 {
-	return tcpTimeStamp(e.tsOffset)
+	return tcpTimeStamp(time.Now(), e.tsOffset)
 }
 
 // tcpTimeStamp returns a timestamp offset by the provided offset. This is
 // not inlined above as it's used when SYN cookies are in use and endpoint
 // is not created at the time when the SYN cookie is sent.
-func tcpTimeStamp(offset uint32) uint32 {
-	now := time.Now()
-	return uint32(now.Unix()*1000+int64(now.Nanosecond()/1e6)) + offset
+func tcpTimeStamp(curTime time.Time, offset uint32) uint32 {
+	return uint32(curTime.Unix()*1000+int64(curTime.Nanosecond()/1e6)) + offset
 }
 
 // timeStampOffset returns a randomized timestamp offset to be used when sending
@@ -2843,6 +2842,14 @@ func (e *endpoint) completeState() stack.TCPEndpointState {
 			WEst:                    cubic.wEst,
 		}
 	}
+
+	rc := e.snd.rc
+	s.Sender.RACKState = stack.TCPRACKState{
+		XmitTime:    rc.xmitTime,
+		EndSequence: rc.endSequence,
+		FACK:        rc.fack,
+		RTT:         rc.rtt,
+	}
 	return s
 }
 

pkg/tcpip/transport/tcp/rack.go

@@ -0,0 +1,82 @@
+// Copyright 2020 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 (
+	"time"
+
+	"gvisor.dev/gvisor/pkg/tcpip/seqnum"
+)
+
+// RACK is a loss detection algorithm used in TCP to detect packet loss and
+// reordering using transmission timestamp of the packets instead of packet or
+// sequence counts. To use RACK, SACK should be enabled on the connection.
+
+// rackControl stores the rack related fields.
+// See: https://tools.ietf.org/html/draft-ietf-tcpm-rack-08#section-6.1
+//
+// +stateify savable
+type rackControl struct {
+	// xmitTime is the latest transmission timestamp of rackControl.seg.
+	xmitTime time.Time `state:".(unixTime)"`
+
+	// endSequence is the ending TCP sequence number of rackControl.seg.
+	endSequence seqnum.Value
+
+	// fack is the highest selectively or cumulatively acknowledged
+	// sequence.
+	fack seqnum.Value
+
+	// rtt is the RTT of the most recently delivered packet on the
+	// connection (either cumulatively acknowledged or selectively
+	// acknowledged) that was not marked invalid as a possible spurious
+	// retransmission.
+	rtt time.Duration
+}
+
+// Update will update the RACK related fields when an ACK has been received.
+// See: https://tools.ietf.org/html/draft-ietf-tcpm-rack-08#section-7.2
+func (rc *rackControl) Update(seg *segment, ackSeg *segment, srtt time.Duration, offset uint32) {
+	rtt := time.Now().Sub(seg.xmitTime)
+
+	// If the ACK is for a retransmitted packet, do not update if it is a
+	// spurious inference which is determined by below checks:
+	// 1. When Timestamping option is available, if the TSVal is less than the
+	// transmit time of the most recent retransmitted packet.
+	// 2. When RTT calculated for the packet is less than the smoothed RTT
+	// for the connection.
+	// See: https://tools.ietf.org/html/draft-ietf-tcpm-rack-08#section-7.2
+	// step 2
+	if seg.xmitCount > 1 {
+		if ackSeg.parsedOptions.TS && ackSeg.parsedOptions.TSEcr != 0 {
+			if ackSeg.parsedOptions.TSEcr < tcpTimeStamp(seg.xmitTime, offset) {
+				return
+			}
+		}
+		if rtt < srtt {
+			return
+		}
+	}
+
+	rc.rtt = rtt
+	// Update rc.xmitTime and rc.endSequence to the transmit time and
+	// ending sequence number of the packet which has been acknowledged
+	// most recently.
+	endSeq := seg.sequenceNumber.Add(seqnum.Size(seg.data.Size()))
+	if rc.xmitTime.Before(seg.xmitTime) || (seg.xmitTime.Equal(rc.xmitTime) && rc.endSequence.LessThan(endSeq)) {
+		rc.xmitTime = seg.xmitTime
+		rc.endSequence = endSeq
+	}
+}

pkg/tcpip/transport/tcp/rack_state.go

@@ -0,0 +1,29 @@
+// Copyright 2020 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 (
+	"time"
+)
+
+// saveXmitTime is invoked by stateify.
+func (rc *rackControl) saveXmitTime() unixTime {
+	return unixTime{rc.xmitTime.Unix(), rc.xmitTime.UnixNano()}
+}
+
+// loadXmitTime is invoked by stateify.
+func (rc *rackControl) loadXmitTime(unix unixTime) {
+	rc.xmitTime = time.Unix(unix.second, unix.nano)
+}

pkg/tcpip/transport/tcp/snd.go

@@ -191,6 +191,10 @@ type sender struct {
 
 	// cc is the congestion control algorithm in use for this sender.
 	cc congestionControl
+
+	// rc has the fields needed for implementing RACK loss detection
+	// algorithm.
+	rc rackControl
 }
 
 // rtt is a synchronization wrapper used to appease stateify. See the comment
@@ -1272,21 +1276,21 @@ func (s *sender) checkDuplicateAck(seg *segment) (rtx bool) {
 
 // handleRcvdSegment is called when a segment is received; it is responsible for
 // updating the send-related state.
-func (s *sender) handleRcvdSegment(seg *segment) {
+func (s *sender) handleRcvdSegment(rcvdSeg *segment) {
 	// Check if we can extract an RTT measurement from this ack.
-	if !seg.parsedOptions.TS && s.rttMeasureSeqNum.LessThan(seg.ackNumber) {
+	if !rcvdSeg.parsedOptions.TS && s.rttMeasureSeqNum.LessThan(rcvdSeg.ackNumber) {
 		s.updateRTO(time.Now().Sub(s.rttMeasureTime))
 		s.rttMeasureSeqNum = s.sndNxt
 	}
 
 	// Update Timestamp if required. See RFC7323, section-4.3.
-	if s.ep.sendTSOk && seg.parsedOptions.TS {
-		s.ep.updateRecentTimestamp(seg.parsedOptions.TSVal, s.maxSentAck, seg.sequenceNumber)
+	if s.ep.sendTSOk && rcvdSeg.parsedOptions.TS {
+		s.ep.updateRecentTimestamp(rcvdSeg.parsedOptions.TSVal, s.maxSentAck, rcvdSeg.sequenceNumber)
 	}
 
 	// Insert SACKBlock information into our scoreboard.
 	if s.ep.sackPermitted {
-		for _, sb := range seg.parsedOptions.SACKBlocks {
+		for _, sb := range rcvdSeg.parsedOptions.SACKBlocks {
 			// Only insert the SACK block if the following holds
 			// true:
 			//  * SACK block acks data after the ack number in the
@@ -1299,27 +1303,27 @@ func (s *sender) handleRcvdSegment(seg *segment) {
 			// NOTE: This check specifically excludes DSACK blocks
 			// which have start/end before sndUna and are used to
 			// indicate spurious retransmissions.
-			if seg.ackNumber.LessThan(sb.Start) && s.sndUna.LessThan(sb.Start) && sb.End.LessThanEq(s.sndNxt) && !s.ep.scoreboard.IsSACKED(sb) {
+			if rcvdSeg.ackNumber.LessThan(sb.Start) && s.sndUna.LessThan(sb.Start) && sb.End.LessThanEq(s.sndNxt) && !s.ep.scoreboard.IsSACKED(sb) {
 				s.ep.scoreboard.Insert(sb)
-				seg.hasNewSACKInfo = true
+				rcvdSeg.hasNewSACKInfo = true
 			}
 		}
 		s.SetPipe()
 	}
 
 	// Count the duplicates and do the fast retransmit if needed.
-	rtx := s.checkDuplicateAck(seg)
+	rtx := s.checkDuplicateAck(rcvdSeg)
 
 	// Stash away the current window size.
-	s.sndWnd = seg.window
+	s.sndWnd = rcvdSeg.window
 
-	ack := seg.ackNumber
+	ack := rcvdSeg.ackNumber
 
 	// Disable zero window probing if remote advertizes a non-zero receive
 	// window. This can be with an ACK to the zero window probe (where the
 	// acknumber refers to the already acknowledged byte) OR to any previously
 	// unacknowledged segment.
-	if s.zeroWindowProbing && seg.window > 0 &&
+	if s.zeroWindowProbing && rcvdSeg.window > 0 &&
 		(ack == s.sndUna || (ack-1).InRange(s.sndUna, s.sndNxt)) {
 		s.disableZeroWindowProbing()
 	}
@@ -1344,10 +1348,10 @@ func (s *sender) handleRcvdSegment(seg *segment) {
 		//    averaged RTT measurement only if the segment acknowledges
 		//    some new data, i.e., only if it advances the left edge of
 		//    the send window.
-		if s.ep.sendTSOk && seg.parsedOptions.TSEcr != 0 {
+		if s.ep.sendTSOk && rcvdSeg.parsedOptions.TSEcr != 0 {
 			// TSVal/Ecr values sent by Netstack are at a millisecond
 			// granularity.
-			elapsed := time.Duration(s.ep.timestamp()-seg.parsedOptions.TSEcr) * time.Millisecond
+			elapsed := time.Duration(s.ep.timestamp()-rcvdSeg.parsedOptions.TSEcr) * time.Millisecond
 			s.updateRTO(elapsed)
 		}
 
@@ -1361,6 +1365,9 @@ func (s *sender) handleRcvdSegment(seg *segment) {
 
 		ackLeft := acked
 		originalOutstanding := s.outstanding
+		s.rtt.Lock()
+		srtt := s.rtt.srtt
+		s.rtt.Unlock()
 		for ackLeft > 0 {
 			// We use logicalLen here because we can have FIN
 			// segments (which are always at the end of list) that
@@ -1380,6 +1387,11 @@ func (s *sender) handleRcvdSegment(seg *segment) {
 				s.writeNext = seg.Next()
 			}
 
+			// Update the RACK fields if SACK is enabled.
+			if s.ep.sackPermitted {
+				s.rc.Update(seg, rcvdSeg, srtt, s.ep.tsOffset)
+			}
+
 			s.writeList.Remove(seg)
 
 			// if SACK is enabled then Only reduce outstanding if
@@ -1435,7 +1447,7 @@ func (s *sender) handleRcvdSegment(seg *segment) {
 	// that the window opened up, or the congestion window was inflated due
 	// to a duplicate ack during fast recovery. This will also re-enable
 	// the retransmit timer if needed.
-	if !s.ep.sackPermitted || s.fr.active || s.dupAckCount == 0 || seg.hasNewSACKInfo {
+	if !s.ep.sackPermitted || s.fr.active || s.dupAckCount == 0 || rcvdSeg.hasNewSACKInfo {
 		s.sendData()
 	}
 }

pkg/tcpip/transport/tcp/tcp_rack_test.go

@@ -0,0 +1,74 @@
+// Copyright 2020 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_test
+
+import (
+	"testing"
+	"time"
+
+	"gvisor.dev/gvisor/pkg/tcpip"
+	"gvisor.dev/gvisor/pkg/tcpip/buffer"
+	"gvisor.dev/gvisor/pkg/tcpip/header"
+	"gvisor.dev/gvisor/pkg/tcpip/stack"
+	"gvisor.dev/gvisor/pkg/tcpip/transport/tcp/testing/context"
+)
+
+// TestRACKUpdate tests the RACK related fields are updated when an ACK is
+// received on a SACK enabled connection.
+func TestRACKUpdate(t *testing.T) {
+	const maxPayload = 10
+	const tsOptionSize = 12
+	const maxTCPOptionSize = 40
+
+	c := context.New(t, uint32(header.TCPMinimumSize+header.IPv4MinimumSize+maxTCPOptionSize+maxPayload))
+	defer c.Cleanup()
+
+	var xmitTime time.Time
+	c.Stack().AddTCPProbe(func(state stack.TCPEndpointState) {
+		// Validate that the endpoint Sender.RACKState is what we expect.
+		if state.Sender.RACKState.XmitTime.Before(xmitTime) {
+			t.Fatalf("RACK transmit time failed to update when an ACK is received")
+		}
+
+		gotSeq := state.Sender.RACKState.EndSequence
+		wantSeq := state.Sender.SndNxt
+		if !gotSeq.LessThanEq(wantSeq) || gotSeq.LessThan(wantSeq) {
+			t.Fatalf("RACK sequence number failed to update, got: %v, but want: %v", gotSeq, wantSeq)
+		}
+
+		if state.Sender.RACKState.RTT == 0 {
+			t.Fatalf("RACK RTT failed to update when an ACK is received")
+		}
+	})
+	setStackSACKPermitted(t, c, true)
+	createConnectedWithSACKAndTS(c)
+
+	data := buffer.NewView(maxPayload)
+	for i := range data {
+		data[i] = byte(i)
+	}
+
+	// Write the data.
+	xmitTime = time.Now()
+	if _, _, err := c.EP.Write(tcpip.SlicePayload(data), tcpip.WriteOptions{}); err != nil {
+		t.Fatalf("Write failed: %s", err)
+	}
+
+	bytesRead := 0
+	c.ReceiveAndCheckPacketWithOptions(data, bytesRead, maxPayload, tsOptionSize)
+	bytesRead += maxPayload
+	c.SendAck(790, bytesRead)
+	time.Sleep(200 * time.Millisecond)
+}