1 files changed, 96 insertions, 187 deletions

diff --git a/pkg/tcpip/transport/tcp/snd.go b/pkg/tcpip/transport/tcp/snd.go
index 0e0fdf14c..5ed9f7ace 100644
--- a/pkg/tcpip/transport/tcp/snd.go
+++ b/pkg/tcpip/transport/tcp/snd.go
@@ -92,6 +92,17 @@ type congestionControl interface {
 	PostRecovery()
 }
 
+// lossRecovery is an interface that must be implemented by any supported
+// loss recovery algorithm.
+type lossRecovery interface {
+	// DoRecovery is invoked when loss is detected and segments need
+	// to be retransmitted. The cumulative or selective ACK is passed along
+	// with the flag which identifies whether the connection entered fast
+	// retransmit with this ACK and to retransmit the first unacknowledged
+	// segment.
+	DoRecovery(rcvdSeg *segment, fastRetransmit bool)
+}
+
 // sender holds the state necessary to send TCP segments.
 //
 // +stateify savable
@@ -108,6 +119,9 @@ type sender struct {
 	// fr holds state related to fast recovery.
 	fr fastRecovery
 
+	// lr is the loss recovery algorithm used by the sender.
+	lr lossRecovery
+
 	// sndCwnd is the congestion window, in packets.
 	sndCwnd int
 
@@ -276,6 +290,8 @@ func newSender(ep *endpoint, iss, irs seqnum.Value, sndWnd seqnum.Size, mss uint
 
 	s.cc = s.initCongestionControl(ep.cc)
 
+	s.lr = s.initLossRecovery()
+
 	// A negative sndWndScale means that no scaling is in use, otherwise we
 	// store the scaling value.
 	if sndWndScale > 0 {
@@ -330,6 +346,14 @@ func (s *sender) initCongestionControl(congestionControlName tcpip.CongestionCon
 	}
 }
 
+// initLossRecovery initiates the loss recovery algorithm for the sender.
+func (s *sender) initLossRecovery() lossRecovery {
+	if s.ep.sackPermitted {
+		return newSACKRecovery(s)
+	}
+	return newRenoRecovery(s)
+}
+
 // updateMaxPayloadSize updates the maximum payload size based on the given
 // MTU. If this is in response to "packet too big" control packets (indicated
 // by the count argument), it also reduces the number of outstanding packets and
@@ -550,7 +574,7 @@ func (s *sender) retransmitTimerExpired() bool {
 		// We were attempting fast recovery but were not successful.
 		// Leave the state. We don't need to update ssthresh because it
 		// has already been updated when entered fast-recovery.
-		s.leaveFastRecovery()
+		s.leaveRecovery()
 	}
 
 	s.state = RTORecovery
@@ -913,79 +937,6 @@ func (s *sender) maybeSendSegment(seg *segment, limit int, end seqnum.Value) (se
 	return true
 }
 
-// handleSACKRecovery implements the loss recovery phase as described in RFC6675
-// section 5, step C.
-func (s *sender) handleSACKRecovery(limit int, end seqnum.Value) (dataSent bool) {
-	s.SetPipe()
-
-	if smss := int(s.ep.scoreboard.SMSS()); limit > smss {
-		// Cap segment size limit to s.smss as SACK recovery requires
-		// that all retransmissions or new segments send during recovery
-		// be of <= SMSS.
-		limit = smss
-	}
-
-	nextSegHint := s.writeList.Front()
-	for s.outstanding < s.sndCwnd {
-		var nextSeg *segment
-		var rescueRtx bool
-		nextSeg, nextSegHint, rescueRtx = s.NextSeg(nextSegHint)
-		if nextSeg == nil {
-			return dataSent
-		}
-		if !s.isAssignedSequenceNumber(nextSeg) || s.sndNxt.LessThanEq(nextSeg.sequenceNumber) {
-			// New data being sent.
-
-			// Step C.3 described below is handled by
-			// maybeSendSegment which increments sndNxt when
-			// a segment is transmitted.
-			//
-			// Step C.3 "If any of the data octets sent in
-			// (C.1) are above HighData, HighData must be
-			// updated to reflect the transmission of
-			// previously unsent data."
-			//
-			// We pass s.smss as the limit as the Step 2) requires that
-			// new data sent should be of size s.smss or less.
-			if sent := s.maybeSendSegment(nextSeg, limit, end); !sent {
-				return dataSent
-			}
-			dataSent = true
-			s.outstanding++
-			s.writeNext = nextSeg.Next()
-			continue
-		}
-
-		// Now handle the retransmission case where we matched either step 1,3 or 4
-		// of the NextSeg algorithm.
-		// RFC 6675, Step C.4.
-		//
-		// "The estimate of the amount of data outstanding in the network
-		// must be updated by incrementing pipe by the number of octets
-		// transmitted in (C.1)."
-		s.outstanding++
-		dataSent = true
-		s.sendSegment(nextSeg)
-
-		segEnd := nextSeg.sequenceNumber.Add(nextSeg.logicalLen())
-		if rescueRtx {
-			// We do the last part of rule (4) of NextSeg here to update
-			// RescueRxt as until this point we don't know if we are going
-			// to use the rescue transmission.
-			s.fr.rescueRxt = s.fr.last
-		} else {
-			// RFC 6675, Step C.2
-			//
-			// "If any of the data octets sent in (C.1) are below
-			// HighData, HighRxt MUST be set to the highest sequence
-			// number of the retransmitted segment unless NextSeg ()
-			// rule (4) was invoked for this retransmission."
-			s.fr.highRxt = segEnd - 1
-		}
-	}
-	return dataSent
-}
-
 func (s *sender) sendZeroWindowProbe() {
 	ack, win := s.ep.rcv.getSendParams()
 	s.unackZeroWindowProbes++
@@ -1014,6 +965,30 @@ func (s *sender) disableZeroWindowProbing() {
 	s.resendTimer.disable()
 }
 
+func (s *sender) postXmit(dataSent bool) {
+	if dataSent {
+		// We sent data, so we should stop the keepalive timer to ensure
+		// that no keepalives are sent while there is pending data.
+		s.ep.disableKeepaliveTimer()
+	}
+
+	// If the sender has advertized zero receive window and we have
+	// data to be sent out, start zero window probing to query the
+	// the remote for it's receive window size.
+	if s.writeNext != nil && s.sndWnd == 0 {
+		s.enableZeroWindowProbing()
+	}
+
+	// Enable the timer if we have pending data and it's not enabled yet.
+	if !s.resendTimer.enabled() && s.sndUna != s.sndNxt {
+		s.resendTimer.enable(s.rto)
+	}
+	// If we have no more pending data, start the keepalive timer.
+	if s.sndUna == s.sndNxt {
+		s.ep.resetKeepaliveTimer(false)
+	}
+}
+
 // sendData sends new data segments. It is called when data becomes available or
 // when the send window opens up.
 func (s *sender) sendData() {
@@ -1034,55 +1009,29 @@ func (s *sender) sendData() {
 	}
 
 	var dataSent bool
-
-	// RFC 6675 recovery algorithm step C 1-5.
-	if s.fr.active && s.ep.sackPermitted {
-		dataSent = s.handleSACKRecovery(s.maxPayloadSize, end)
-	} else {
-		for seg := s.writeNext; seg != nil && s.outstanding < s.sndCwnd; seg = seg.Next() {
-			cwndLimit := (s.sndCwnd - s.outstanding) * s.maxPayloadSize
-			if cwndLimit < limit {
-				limit = cwndLimit
-			}
-			if s.isAssignedSequenceNumber(seg) && s.ep.sackPermitted && s.ep.scoreboard.IsSACKED(seg.sackBlock()) {
-				// Move writeNext along so that we don't try and scan data that
-				// has already been SACKED.
-				s.writeNext = seg.Next()
-				continue
-			}
-			if sent := s.maybeSendSegment(seg, limit, end); !sent {
-				break
-			}
-			dataSent = true
-			s.outstanding += s.pCount(seg)
+	for seg := s.writeNext; seg != nil && s.outstanding < s.sndCwnd; seg = seg.Next() {
+		cwndLimit := (s.sndCwnd - s.outstanding) * s.maxPayloadSize
+		if cwndLimit < limit {
+			limit = cwndLimit
+		}
+		if s.isAssignedSequenceNumber(seg) && s.ep.sackPermitted && s.ep.scoreboard.IsSACKED(seg.sackBlock()) {
+			// Move writeNext along so that we don't try and scan data that
+			// has already been SACKED.
 			s.writeNext = seg.Next()
+			continue
 		}
+		if sent := s.maybeSendSegment(seg, limit, end); !sent {
+			break
+		}
+		dataSent = true
+		s.outstanding += s.pCount(seg)
+		s.writeNext = seg.Next()
 	}
 
-	if dataSent {
-		// We sent data, so we should stop the keepalive timer to ensure
-		// that no keepalives are sent while there is pending data.
-		s.ep.disableKeepaliveTimer()
-	}
-
-	// If the sender has advertized zero receive window and we have
-	// data to be sent out, start zero window probing to query the
-	// the remote for it's receive window size.
-	if s.writeNext != nil && s.sndWnd == 0 {
-		s.enableZeroWindowProbing()
-	}
-
-	// Enable the timer if we have pending data and it's not enabled yet.
-	if !s.resendTimer.enabled() && s.sndUna != s.sndNxt {
-		s.resendTimer.enable(s.rto)
-	}
-	// If we have no more pending data, start the keepalive timer.
-	if s.sndUna == s.sndNxt {
-		s.ep.resetKeepaliveTimer(false)
-	}
+	s.postXmit(dataSent)
 }
 
-func (s *sender) enterFastRecovery() {
+func (s *sender) enterRecovery() {
 	s.fr.active = true
 	// Save state to reflect we're now in fast recovery.
 	//
@@ -1104,7 +1053,7 @@ func (s *sender) enterFastRecovery() {
 	s.ep.stack.Stats().TCP.FastRecovery.Increment()
 }
 
-func (s *sender) leaveFastRecovery() {
+func (s *sender) leaveRecovery() {
 	s.fr.active = false
 	s.fr.maxCwnd = 0
 	s.dupAckCount = 0
@@ -1115,57 +1064,6 @@ func (s *sender) leaveFastRecovery() {
 	s.cc.PostRecovery()
 }
 
-func (s *sender) handleFastRecovery(seg *segment) (rtx bool) {
-	ack := seg.ackNumber
-	// We are in fast recovery mode. Ignore the ack if it's out of
-	// range.
-	if !ack.InRange(s.sndUna, s.sndNxt+1) {
-		return false
-	}
-
-	// Leave fast recovery if it acknowledges all the data covered by
-	// this fast recovery session.
-	if s.fr.last.LessThan(ack) {
-		s.leaveFastRecovery()
-		return false
-	}
-
-	if s.ep.sackPermitted {
-		// When SACK is enabled we let retransmission be governed by
-		// the SACK logic.
-		return false
-	}
-
-	// Don't count this as a duplicate if it is carrying data or
-	// updating the window.
-	if seg.logicalLen() != 0 || s.sndWnd != seg.window {
-		return false
-	}
-
-	// Inflate the congestion window if we're getting duplicate acks
-	// for the packet we retransmitted.
-	if ack == s.fr.first {
-		// We received a dup, inflate the congestion window by 1 packet
-		// if we're not at the max yet. Only inflate the window if
-		// regular FastRecovery is in use, RFC6675 does not require
-		// inflating cwnd on duplicate ACKs.
-		if s.sndCwnd < s.fr.maxCwnd {
-			s.sndCwnd++
-		}
-		return false
-	}
-
-	// A partial ack was received. Retransmit this packet and
-	// remember it so that we don't retransmit it again. We don't
-	// inflate the window because we're putting the same packet back
-	// onto the wire.
-	//
-	// N.B. The retransmit timer will be reset by the caller.
-	s.fr.first = ack
-	s.dupAckCount = 0
-	return true
-}
-
 // isAssignedSequenceNumber relies on the fact that we only set flags once a
 // sequencenumber is assigned and that is only done right before we send the
 // segment. As a result any segment that has a non-zero flag has a valid
@@ -1228,14 +1126,11 @@ func (s *sender) SetPipe() {
 	s.outstanding = pipe
 }
 
-// checkDuplicateAck is called when an ack is received. It manages the state
-// related to duplicate acks and determines if a retransmit is needed according
-// to the rules in RFC 6582 (NewReno).
-func (s *sender) checkDuplicateAck(seg *segment) (rtx bool) {
+// detectLoss is called when an ack is received and returns whether a loss is
+// detected. It manages the state related to duplicate acks and determines if
+// a retransmit is needed according to the rules in RFC 6582 (NewReno).
+func (s *sender) detectLoss(seg *segment) (fastRetransmit bool) {
 	ack := seg.ackNumber
-	if s.fr.active {
-		return s.handleFastRecovery(seg)
-	}
 
 	// We're not in fast recovery yet. A segment is considered a duplicate
 	// only if it doesn't carry any data and doesn't update the send window,
@@ -1266,14 +1161,14 @@ func (s *sender) checkDuplicateAck(seg *segment) (rtx bool) {
 	// See: https://tools.ietf.org/html/rfc6582#section-3.2 Step 2
 	//
 	// We only do the check here, the incrementing of last to the highest
-	// sequence number transmitted till now is done when enterFastRecovery
+	// sequence number transmitted till now is done when enterRecovery
 	// is invoked.
 	if !s.fr.last.LessThan(seg.ackNumber) {
 		s.dupAckCount = 0
 		return false
 	}
 	s.cc.HandleNDupAcks()
-	s.enterFastRecovery()
+	s.enterRecovery()
 	s.dupAckCount = 0
 	return true
 }
@@ -1415,14 +1310,23 @@ func (s *sender) handleRcvdSegment(rcvdSeg *segment) {
 		s.SetPipe()
 	}
 
-	// Count the duplicates and do the fast retransmit if needed.
-	rtx := s.checkDuplicateAck(rcvdSeg)
+	ack := rcvdSeg.ackNumber
+	fastRetransmit := false
+	// Do not leave fast recovery, if the ACK is out of range.
+	if s.fr.active {
+		// Leave fast recovery if it acknowledges all the data covered by
+		// this fast recovery session.
+		if ack.InRange(s.sndUna, s.sndNxt+1) && s.fr.last.LessThan(ack) {
+			s.leaveRecovery()
+		}
+	} else {
+		// Detect loss by counting the duplicates and enter recovery.
+		fastRetransmit = s.detectLoss(rcvdSeg)
+	}
 
 	// Stash away the current window size.
 	s.sndWnd = rcvdSeg.window
 
-	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
@@ -1539,19 +1443,24 @@ func (s *sender) handleRcvdSegment(rcvdSeg *segment) {
 			s.resendTimer.disable()
 		}
 	}
+
 	// Now that we've popped all acknowledged data from the retransmit
 	// queue, retransmit if needed.
-	if rtx {
-		s.resendSegment()
+	if s.fr.active {
+		s.lr.DoRecovery(rcvdSeg, fastRetransmit)
+		// When SACK is enabled data sending is governed by steps in
+		// RFC 6675 Section 5 recovery steps  A-C.
+		// See: https://tools.ietf.org/html/rfc6675#section-5.
+		if s.ep.sackPermitted {
+			return
+		}
 	}
 
 	// Send more data now that some of the pending data has been ack'd, or
 	// 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 || rcvdSeg.hasNewSACKInfo {
-		s.sendData()
-	}
+	s.sendData()
 }
 
 // sendSegment sends the specified segment.