Add congestion control parameters to config

The loss-based congestion control get poor
performance under high bandwidth, high rtt
and packet loss case since the congestion
window becomes 1 mtu and increase slowly
after retransmit timeout. And fast recovery
retransmit cause exit slowly in consecutive
packet loss. This change add paramters to
the config then the user can set them to get
higher throughput in such cases.

association.go

@@ -212,6 +212,9 @@ type Association struct {
 	partialBytesAcked    uint32
 	inFastRecovery       bool
 	fastRecoverExitPoint uint32
+	minCwnd              uint32 // Minimum congestion window
+	fastRtxWnd           uint32 // Send window for fast retransmit
+	cwndCAStep           uint32 // Step of congestion window increase at Congestion Avoidance
 
 	// RTX & Ack timer
 	rtoMgr     *rtoManager
@@ -261,8 +264,16 @@ type Config struct {
 	MaxMessageSize       uint32
 	EnableZeroChecksum   bool
 	LoggerFactory        logging.LoggerFactory
+
+	// congestion control configuration
 	// RTOMax is the maximum retransmission timeout in milliseconds
 	RTOMax float64
+	// Minimum congestion window
+	MinCwnd uint32
+	// Send window for fast retransmit
+	FastRtxWnd uint32
+	// Step of congestion window increase at Congestion Avoidance
+	CwndCAStep uint32
 }
 
 // Server accepts a SCTP stream over a conn
@@ -325,6 +336,9 @@ func createAssociation(config Config) *Association {
 		netConn:              config.NetConn,
 		maxReceiveBufferSize: maxReceiveBufferSize,
 		maxMessageSize:       maxMessageSize,
+		minCwnd:              config.MinCwnd,
+		fastRtxWnd:           config.FastRtxWnd,
+		cwndCAStep:           config.CwndCAStep,
 
 		// These two max values have us not need to follow
 		// 5.1.1 where this peer may be incapable of supporting
@@ -512,7 +526,7 @@ func (a *Association) Close() error {
 	a.log.Debugf("[%s] stats nPackets (out) : %d", a.name, a.stats.getNumPacketsSent())
 	a.log.Debugf("[%s] stats nDATAs (in) : %d", a.name, a.stats.getNumDATAs())
 	a.log.Debugf("[%s] stats nSACKs (in) : %d", a.name, a.stats.getNumSACKsReceived())
-	a.log.Debugf("[%s] stats nSACKs (out) : %d\n", a.name, a.stats.getNumSACKsSent())
+	a.log.Debugf("[%s] stats nSACKs (out) : %d", a.name, a.stats.getNumSACKsSent())
 	a.log.Debugf("[%s] stats nT3Timeouts : %d", a.name, a.stats.getNumT3Timeouts())
 	a.log.Debugf("[%s] stats nAckTimeouts: %d", a.name, a.stats.getNumAckTimeouts())
 	a.log.Debugf("[%s] stats nFastRetrans: %d", a.name, a.stats.getNumFastRetrans())
@@ -803,9 +817,13 @@ func (a *Association) gatherOutboundFastRetransmissionPackets(rawPackets [][]byt
 	if a.willRetransmitFast {
 		a.willRetransmitFast = false
 
-		toFastRetrans := []chunk{}
+		toFastRetrans := []*chunkPayloadData{}
 		fastRetransSize := commonHeaderSize
 
+		fastRetransWnd := a.MTU()
+		if fastRetransWnd < a.fastRtxWnd {
+			fastRetransWnd = a.fastRtxWnd
+		}
 		for i := 0; ; i++ {
 			c, ok := a.inflightQueue.get(a.cumulativeTSNAckPoint + uint32(i) + 1)
 			if !ok {
@@ -831,7 +849,7 @@ func (a *Association) gatherOutboundFastRetransmissionPackets(rawPackets [][]byt
 			//		packet.
 
 			dataChunkSize := dataChunkHeaderSize + uint32(len(c.userData))
-			if a.MTU() < fastRetransSize+dataChunkSize {
+			if fastRetransWnd < fastRetransSize+dataChunkSize {
 				break
 			}
 
@@ -845,10 +863,12 @@ func (a *Association) gatherOutboundFastRetransmissionPackets(rawPackets [][]byt
 		}
 
 		if len(toFastRetrans) > 0 {
-			raw, err := a.marshalPacket(a.createPacket(toFastRetrans))
-			if err != nil {
-				a.log.Warnf("[%s] failed to serialize a DATA packet to be fast-retransmitted", a.name)
-			} else {
+			for _, p := range a.bundleDataChunksIntoPackets(toFastRetrans) {
+				raw, err := a.marshalPacket(p)
+				if err != nil {
+					a.log.Warnf("[%s] failed to serialize a DATA packet to be fast-retransmitted", a.name)
+					continue
+				}
 				rawPackets = append(rawPackets, raw)
 			}
 		}
@@ -1115,6 +1135,9 @@ func (a *Association) CWND() uint32 {
 }
 
 func (a *Association) setCWND(cwnd uint32) {
+	if cwnd < a.minCwnd {
+		cwnd = a.minCwnd
+	}
 	atomic.StoreUint32(&a.cwnd, cwnd)
 }
 
@@ -1720,7 +1743,11 @@ func (a *Association) onCumulativeTSNAckPointAdvanced(totalBytesAcked int) {
 		//      reset partial_bytes_acked to (partial_bytes_acked - cwnd).
 		if a.partialBytesAcked >= a.CWND() && a.pendingQueue.size() > 0 {
 			a.partialBytesAcked -= a.CWND()
-			a.setCWND(a.CWND() + a.MTU())
+			step := a.MTU()
+			if step < a.cwndCAStep {
+				step = a.cwndCAStep
+			}
+			a.setCWND(a.CWND() + step)
 			a.log.Tracef("[%s] updated cwnd=%d ssthresh=%d acked=%d (CA)",
 				a.name, a.CWND(), a.ssthresh, totalBytesAcked)
 		}

association_test.go

@@ -1839,6 +1839,7 @@ func TestAssocCongestionControl(t *testing.T) {
 		br := test.NewBridge()
 
 		a0, a1, err := createNewAssociationPair(br, ackModeNormal, maxReceiveBufferSize)
+		a0.cwndCAStep = 2800 //2 mtu
 		if !assert.Nil(t, err, "failed to create associations") {
 			assert.FailNow(t, "failed due to earlier error")
 		}
@@ -2735,6 +2736,10 @@ func (d *udpDiscardReader) Read(b []byte) (n int, err error) {
 }
 
 func createAssociationPair(udpConn1 net.Conn, udpConn2 net.Conn) (*Association, *Association, error) {
+	return createAssociationPairWithConfig(udpConn1, udpConn2, Config{})
+}
+
+func createAssociationPairWithConfig(udpConn1 net.Conn, udpConn2 net.Conn, config Config) (*Association, *Association, error) {
 	loggerFactory := logging.NewDefaultLoggerFactory()
 
 	a1Chan := make(chan interface{})
@@ -2744,10 +2749,10 @@ func createAssociationPair(udpConn1 net.Conn, udpConn2 net.Conn) (*Association,
 	defer cancel()
 
 	go func() {
-		a, err2 := createClientWithContext(ctx, Config{
-			NetConn:       udpConn1,
-			LoggerFactory: loggerFactory,
-		})
+		cfg := config
+		cfg.NetConn = udpConn1
+		cfg.LoggerFactory = loggerFactory
+		a, err2 := createClientWithContext(ctx, cfg)
 		if err2 != nil {
 			a1Chan <- err2
 		} else {
@@ -2756,11 +2761,13 @@ func createAssociationPair(udpConn1 net.Conn, udpConn2 net.Conn) (*Association,
 	}()
 
 	go func() {
-		a, err2 := createClientWithContext(ctx, Config{
-			NetConn:              udpConn2,
-			LoggerFactory:        loggerFactory,
-			MaxReceiveBufferSize: 100_000,
-		})
+		cfg := config
+		cfg.NetConn = udpConn2
+		cfg.LoggerFactory = loggerFactory
+		if cfg.MaxReceiveBufferSize == 0 {
+			cfg.MaxReceiveBufferSize = 100_000
+		}
+		a, err2 := createClientWithContext(ctx, cfg)
 		if err2 != nil {
 			a2Chan <- err2
 		} else {
@@ -2880,6 +2887,81 @@ func TestAssociationReceiveWindow(t *testing.T) {
 	cancel()
 }
 
+func TestAssociationFastRtxWnd(t *testing.T) {
+	udp1, udp2 := createUDPConnPair()
+	a1, a2, err := createAssociationPairWithConfig(udp1, udp2, Config{MinCwnd: 14000, FastRtxWnd: 14000})
+	require.NoError(t, err)
+	defer noErrorClose(t, a2.Close)
+	defer noErrorClose(t, a1.Close)
+	s1, err := a1.OpenStream(1, PayloadTypeWebRTCBinary)
+	require.NoError(t, err)
+	defer noErrorClose(t, s1.Close)
+	_, err = s1.WriteSCTP([]byte("hello"), PayloadTypeWebRTCBinary)
+	require.NoError(t, err)
+	_, err = a2.AcceptStream()
+	require.NoError(t, err)
+
+	a1.rtoMgr.setRTO(1000, true)
+	// ack the hello packet
+	time.Sleep(1 * time.Second)
+
+	require.Equal(t, a1.minCwnd, a1.CWND())
+
+	var shouldDrop atomic.Bool
+	var dropCounter atomic.Uint32
+	dbConn1, ok := udp1.(*dumbConn2)
+	require.True(t, ok)
+	dbConn2, ok := udp2.(*dumbConn2)
+	require.True(t, ok)
+	dbConn1.remoteInboundHandler = func(packet []byte) {
+		if !shouldDrop.Load() {
+			dbConn2.inboundHandler(packet)
+		} else {
+			dropCounter.Add(1)
+		}
+	}
+
+	shouldDrop.Store(true)
+	// send packets and dropped
+	buf := make([]byte, 1000)
+	for i := 0; i < 10; i++ {
+		_, err = s1.WriteSCTP(buf, PayloadTypeWebRTCBinary)
+		require.NoError(t, err)
+	}
+
+	require.Eventually(t, func() bool { return dropCounter.Load() >= 10 }, 5*time.Second, 10*time.Millisecond, "drop %d", dropCounter.Load())
+	// send packets to trigger fast retransmit
+	shouldDrop.Store(false)
+
+	require.Zero(t, a1.stats.getNumFastRetrans())
+	require.False(t, a1.inFastRecovery)
+
+	// wait SACK
+	sackCh := make(chan []byte, 1)
+	udp2.(*dumbConn2).remoteInboundHandler = func(buf []byte) {
+		p := &packet{}
+		require.NoError(t, p.unmarshal(true, buf))
+		for _, c := range p.chunks {
+			if _, ok := c.(*chunkSelectiveAck); ok {
+				select {
+				case sackCh <- buf:
+				default:
+				}
+				return
+			}
+		}
+	}
+	// wait sack to trigger fast retransmit
+	for i := 0; i < 3; i++ {
+		_, err = s1.WriteSCTP(buf, PayloadTypeWebRTCBinary)
+		require.NoError(t, err)
+		dbConn1.inboundHandler(<-sackCh)
+	}
+	// fast retransmit and new sack sent
+	require.Eventually(t, func() bool { return a1.inFastRecovery }, 5*time.Second, 10*time.Millisecond)
+	require.GreaterOrEqual(t, uint64(10), a1.stats.getNumFastRetrans())
+}
+
 func TestAssociationMaxTSNOffset(t *testing.T) {
 	udp1, udp2 := createUDPConnPair()
 	// a1 is the association used for sending data