implement ACK decimation

ACK generation closely follows Chrome's QuicConnection::MaybeQueueAck()
in ACK_DECIMATION_WITH_REORDERING mode.

internal/ackhandler/received_packet_handler.go

@@ -3,7 +3,9 @@ package ackhandler
 import (
 	"time"
 
+	"github.com/lucas-clemente/quic-go/internal/congestion"
 	"github.com/lucas-clemente/quic-go/internal/protocol"
+	"github.com/lucas-clemente/quic-go/internal/utils"
 	"github.com/lucas-clemente/quic-go/internal/wire"
 )
 
@@ -15,6 +17,7 @@ type receivedPacketHandler struct {
 	packetHistory *receivedPacketHistory
 
 	ackSendDelay time.Duration
+	rttStats     *congestion.RTTStats
 
 	packetsReceivedSinceLastAck                int
 	retransmittablePacketsReceivedSinceLastAck int
@@ -26,35 +29,53 @@ type receivedPacketHandler struct {
 }
 
 const (
-	// ackSendDelay is the maximum delay that can be applied to an ACK for a retransmittable packet
+	// maximum delay that can be applied to an ACK for a retransmittable packet
 	ackSendDelay = 25 * time.Millisecond
-	// retransmittablePacketsBeforeAck is the number of retransmittable that an ACK is sent for
+	// initial maximum number of retransmittable packets received before sending an ack.
+	initialRetransmittablePacketsBeforeAck = 2
+	// number of retransmittable that an ACK is sent for
 	retransmittablePacketsBeforeAck = 10
+	// 1/5 RTT delay when doing ack decimation
+	ackDecimationDelay = 1.0 / 4
+	// 1/8 RTT delay when doing ack decimation
+	shortAckDecimationDelay = 1.0 / 8
+	// Minimum number of packets received before ack decimation is enabled.
+	// This intends to avoid the beginning of slow start, when CWNDs may be
+	// rapidly increasing.
+	minReceivedBeforeAckDecimation = 100
+	// Maximum number of packets to ack immediately after a missing packet for
+	// fast retransmission to kick in at the sender.  This limit is created to
+	// reduce the number of acks sent that have no benefit for fast retransmission.
+	// Set to the number of nacks needed for fast retransmit plus one for protection
+	// against an ack loss
+	maxPacketsAfterNewMissing = 4
 )
 
 // NewReceivedPacketHandler creates a new receivedPacketHandler
-func NewReceivedPacketHandler(version protocol.VersionNumber) ReceivedPacketHandler {
+func NewReceivedPacketHandler(rttStats *congestion.RTTStats, version protocol.VersionNumber) ReceivedPacketHandler {
 	return &receivedPacketHandler{
 		packetHistory: newReceivedPacketHistory(),
 		ackSendDelay:  ackSendDelay,
+		rttStats:      rttStats,
 		version:       version,
 	}
 }
 
 func (h *receivedPacketHandler) ReceivedPacket(packetNumber protocol.PacketNumber, rcvTime time.Time, shouldInstigateAck bool) error {
+	if packetNumber < h.ignoreBelow {
+		return nil
+	}
+
+	isMissing := h.isMissing(packetNumber)
 	if packetNumber > h.largestObserved {
 		h.largestObserved = packetNumber
 		h.largestObservedReceivedTime = rcvTime
 	}
 
-	if packetNumber < h.ignoreBelow {
-		return nil
-	}
-
 	if err := h.packetHistory.ReceivedPacket(packetNumber); err != nil {
 		return err
 	}
-	h.maybeQueueAck(packetNumber, rcvTime, shouldInstigateAck)
+	h.maybeQueueAck(packetNumber, rcvTime, shouldInstigateAck, isMissing)
 	return nil
 }
 
@@ -65,35 +86,68 @@ func (h *receivedPacketHandler) IgnoreBelow(p protocol.PacketNumber) {
 	h.packetHistory.DeleteBelow(p)
 }
 
-func (h *receivedPacketHandler) maybeQueueAck(packetNumber protocol.PacketNumber, rcvTime time.Time, shouldInstigateAck bool) {
-	h.packetsReceivedSinceLastAck++
-
-	if shouldInstigateAck {
-		h.retransmittablePacketsReceivedSinceLastAck++
+// isMissing says if a packet was reported missing in the last ACK.
+func (h *receivedPacketHandler) isMissing(p protocol.PacketNumber) bool {
+	if h.lastAck == nil {
+		return false
 	}
+	return p < h.lastAck.LargestAcked && !h.lastAck.AcksPacket(p)
+}
+
+func (h *receivedPacketHandler) hasNewMissingPackets() bool {
+	if h.lastAck == nil {
+		return false
+	}
+	highestRange := h.packetHistory.GetHighestAckRange()
+	return highestRange.First >= h.lastAck.LargestAcked && highestRange.Len() <= maxPacketsAfterNewMissing
+}
+
+// maybeQueueAck queues an ACK, if necessary.
+// It is implemented analogously to Chrome's QuicConnection::MaybeQueueAck()
+// in ACK_DECIMATION_WITH_REORDERING mode.
+func (h *receivedPacketHandler) maybeQueueAck(packetNumber protocol.PacketNumber, rcvTime time.Time, shouldInstigateAck, wasMissing bool) {
+	h.packetsReceivedSinceLastAck++
 
 	// always ack the first packet
 	if h.lastAck == nil {
 		h.ackQueued = true
+		return
 	}
 
-	// if the packet number is smaller than the largest acked packet, it must have been reported missing with the last ACK
-	// note that it cannot be a duplicate because they're already filtered out by ReceivedPacket()
-	if h.lastAck != nil && packetNumber < h.lastAck.LargestAcked {
-		h.ackQueued = true
-	}
-
-	// check if a new missing range above the previously was created
-	if h.lastAck != nil && h.packetHistory.GetHighestAckRange().First > h.lastAck.LargestAcked {
+	// Send an ACK if this packet was reported missing in an ACK sent before.
+	// Ack decimation with reordering relies on the timer to send an ACK, but if
+	// missing packets we reported in the previous ack, send an ACK immediately.
+	if wasMissing {
 		h.ackQueued = true
 	}
 
 	if !h.ackQueued && shouldInstigateAck {
-		if h.retransmittablePacketsReceivedSinceLastAck >= retransmittablePacketsBeforeAck {
-			h.ackQueued = true
+		h.retransmittablePacketsReceivedSinceLastAck++
+
+		if packetNumber > minReceivedBeforeAckDecimation {
+			// ack up to 10 packets at once
+			if h.retransmittablePacketsReceivedSinceLastAck >= retransmittablePacketsBeforeAck {
+				h.ackQueued = true
+			} else if h.ackAlarm.IsZero() {
+				// wait for the minimum of the ack decimation delay or the delayed ack time before sending an ack
+				ackDelay := utils.MinDuration(ackSendDelay, time.Duration(float64(h.rttStats.MinRTT())*float64(ackDecimationDelay)))
+				h.ackAlarm = rcvTime.Add(ackDelay)
+			}
 		} else {
-			if h.ackAlarm.IsZero() {
-				h.ackAlarm = rcvTime.Add(h.ackSendDelay)
+			// send an ACK every 2 retransmittable packets
+			if h.retransmittablePacketsReceivedSinceLastAck >= initialRetransmittablePacketsBeforeAck {
+				h.ackQueued = true
+			} else if h.ackAlarm.IsZero() {
+				h.ackAlarm = rcvTime.Add(ackSendDelay)
+			}
+		}
+		// If there are new missing packets to report, set a short timer to send an ACK.
+		if h.hasNewMissingPackets() {
+			// wait the minimum of 1/8 min RTT and the existing ack time
+			ackDelay := float64(h.rttStats.MinRTT()) * float64(shortAckDecimationDelay)
+			ackTime := rcvTime.Add(time.Duration(ackDelay))
+			if h.ackAlarm.IsZero() || h.ackAlarm.After(ackTime) {
+				h.ackAlarm = ackTime
 			}
 		}
 	}
@@ -125,7 +179,6 @@ func (h *receivedPacketHandler) GetAckFrame() *wire.AckFrame {
 	h.ackQueued = false
 	h.packetsReceivedSinceLastAck = 0
 	h.retransmittablePacketsReceivedSinceLastAck = 0
-
 	return ack
 }