package congestion import ( "math" "time" "github.com/sagernet/quic-go/congestion" ) // This cubic implementation is based on the one found in Chromiums's QUIC // implementation, in the files net/quic/congestion_control/cubic.{hh,cc}. // Constants based on TCP defaults. // The following constants are in 2^10 fractions of a second instead of ms to // allow a 10 shift right to divide. // 1024*1024^3 (first 1024 is from 0.100^3) // where 0.100 is 100 ms which is the scaling round trip time. const ( cubeScale = 40 cubeCongestionWindowScale = 410 cubeFactor congestion.ByteCount = 1 << cubeScale / cubeCongestionWindowScale / maxDatagramSize // TODO: when re-enabling cubic, make sure to use the actual packet size here maxDatagramSize = congestion.ByteCount(InitialPacketSizeIPv4) ) const defaultNumConnections = 1 // Default Cubic backoff factor const beta float32 = 0.7 // Additional backoff factor when loss occurs in the concave part of the Cubic // curve. This additional backoff factor is expected to give up bandwidth to // new concurrent flows and speed up convergence. const betaLastMax float32 = 0.85 // Cubic implements the cubic algorithm from TCP type Cubic struct { clock Clock // Number of connections to simulate. numConnections int // Time when this cycle started, after last loss event. epoch time.Time // Max congestion window used just before last loss event. // Note: to improve fairness to other streams an additional back off is // applied to this value if the new value is below our latest value. lastMaxCongestionWindow congestion.ByteCount // Number of acked bytes since the cycle started (epoch). ackedBytesCount congestion.ByteCount // TCP Reno equivalent congestion window in packets. estimatedTCPcongestionWindow congestion.ByteCount // Origin point of cubic function. originPointCongestionWindow congestion.ByteCount // Time to origin point of cubic function in 2^10 fractions of a second. timeToOriginPoint uint32 // Last congestion window in packets computed by cubic function. lastTargetCongestionWindow congestion.ByteCount } // NewCubic returns a new Cubic instance func NewCubic(clock Clock) *Cubic { c := &Cubic{ clock: clock, numConnections: defaultNumConnections, } c.Reset() return c } // Reset is called after a timeout to reset the cubic state func (c *Cubic) Reset() { c.epoch = time.Time{} c.lastMaxCongestionWindow = 0 c.ackedBytesCount = 0 c.estimatedTCPcongestionWindow = 0 c.originPointCongestionWindow = 0 c.timeToOriginPoint = 0 c.lastTargetCongestionWindow = 0 } func (c *Cubic) alpha() float32 { // TCPFriendly alpha is described in Section 3.3 of the CUBIC paper. Note that // beta here is a cwnd multiplier, and is equal to 1-beta from the paper. // We derive the equivalent alpha for an N-connection emulation as: b := c.beta() return 3 * float32(c.numConnections) * float32(c.numConnections) * (1 - b) / (1 + b) } func (c *Cubic) beta() float32 { // kNConnectionBeta is the backoff factor after loss for our N-connection // emulation, which emulates the effective backoff of an ensemble of N // TCP-Reno connections on a single loss event. The effective multiplier is // computed as: return (float32(c.numConnections) - 1 + beta) / float32(c.numConnections) } func (c *Cubic) betaLastMax() float32 { // betaLastMax is the additional backoff factor after loss for our // N-connection emulation, which emulates the additional backoff of // an ensemble of N TCP-Reno connections on a single loss event. The // effective multiplier is computed as: return (float32(c.numConnections) - 1 + betaLastMax) / float32(c.numConnections) } // OnApplicationLimited is called on ack arrival when sender is unable to use // the available congestion window. Resets Cubic state during quiescence. func (c *Cubic) OnApplicationLimited() { // When sender is not using the available congestion window, the window does // not grow. But to be RTT-independent, Cubic assumes that the sender has been // using the entire window during the time since the beginning of the current // "epoch" (the end of the last loss recovery period). Since // application-limited periods break this assumption, we reset the epoch when // in such a period. This reset effectively freezes congestion window growth // through application-limited periods and allows Cubic growth to continue // when the entire window is being used. c.epoch = time.Time{} } // CongestionWindowAfterPacketLoss computes a new congestion window to use after // a loss event. Returns the new congestion window in packets. The new // congestion window is a multiplicative decrease of our current window. func (c *Cubic) CongestionWindowAfterPacketLoss(currentCongestionWindow congestion.ByteCount) congestion.ByteCount { if currentCongestionWindow+maxDatagramSize < c.lastMaxCongestionWindow { // We never reached the old max, so assume we are competing with another // flow. Use our extra back off factor to allow the other flow to go up. c.lastMaxCongestionWindow = congestion.ByteCount(c.betaLastMax() * float32(currentCongestionWindow)) } else { c.lastMaxCongestionWindow = currentCongestionWindow } c.epoch = time.Time{} // Reset time. return congestion.ByteCount(float32(currentCongestionWindow) * c.beta()) } // CongestionWindowAfterAck computes a new congestion window to use after a received ACK. // Returns the new congestion window in packets. The new congestion window // follows a cubic function that depends on the time passed since last // packet loss. func (c *Cubic) CongestionWindowAfterAck( ackedBytes congestion.ByteCount, currentCongestionWindow congestion.ByteCount, delayMin time.Duration, eventTime time.Time, ) congestion.ByteCount { c.ackedBytesCount += ackedBytes if c.epoch.IsZero() { // First ACK after a loss event. c.epoch = eventTime // Start of epoch. c.ackedBytesCount = ackedBytes // Reset count. // Reset estimated_tcp_congestion_window_ to be in sync with cubic. c.estimatedTCPcongestionWindow = currentCongestionWindow if c.lastMaxCongestionWindow <= currentCongestionWindow { c.timeToOriginPoint = 0 c.originPointCongestionWindow = currentCongestionWindow } else { c.timeToOriginPoint = uint32(math.Cbrt(float64(cubeFactor * (c.lastMaxCongestionWindow - currentCongestionWindow)))) c.originPointCongestionWindow = c.lastMaxCongestionWindow } } // Change the time unit from microseconds to 2^10 fractions per second. Take // the round trip time in account. This is done to allow us to use shift as a // divide operator. elapsedTime := int64(eventTime.Add(delayMin).Sub(c.epoch)/time.Microsecond) << 10 / (1000 * 1000) // Right-shifts of negative, signed numbers have implementation-dependent // behavior, so force the offset to be positive, as is done in the kernel. offset := int64(c.timeToOriginPoint) - elapsedTime if offset < 0 { offset = -offset } deltaCongestionWindow := congestion.ByteCount(cubeCongestionWindowScale*offset*offset*offset) * maxDatagramSize >> cubeScale var targetCongestionWindow congestion.ByteCount if elapsedTime > int64(c.timeToOriginPoint) { targetCongestionWindow = c.originPointCongestionWindow + deltaCongestionWindow } else { targetCongestionWindow = c.originPointCongestionWindow - deltaCongestionWindow } // Limit the CWND increase to half the acked bytes. targetCongestionWindow = Min(targetCongestionWindow, currentCongestionWindow+c.ackedBytesCount/2) // Increase the window by approximately Alpha * 1 MSS of bytes every // time we ack an estimated tcp window of bytes. For small // congestion windows (less than 25), the formula below will // increase slightly slower than linearly per estimated tcp window // of bytes. c.estimatedTCPcongestionWindow += congestion.ByteCount(float32(c.ackedBytesCount) * c.alpha() * float32(maxDatagramSize) / float32(c.estimatedTCPcongestionWindow)) c.ackedBytesCount = 0 // We have a new cubic congestion window. c.lastTargetCongestionWindow = targetCongestionWindow // Compute target congestion_window based on cubic target and estimated TCP // congestion_window, use highest (fastest). if targetCongestionWindow < c.estimatedTCPcongestionWindow { targetCongestionWindow = c.estimatedTCPcongestionWindow } return targetCongestionWindow } // SetNumConnections sets the number of emulated connections func (c *Cubic) SetNumConnections(n int) { c.numConnections = n }