move calculation of the PTO to the rttStats

The PTO value is needed at various places in the protocol. Calculating
it on the rttStats struct will allow us to pass around this struct.

internal/congestion/rtt_stats.go

@@ -3,6 +3,7 @@ package congestion
 import (
 	"time"
 
+	"github.com/lucas-clemente/quic-go/internal/protocol"
 	"github.com/lucas-clemente/quic-go/internal/utils"
 )
 
@@ -56,6 +57,10 @@ func (r *RTTStats) MeanDeviation() time.Duration { return r.meanDeviation }
 
 func (r *RTTStats) MaxAckDelay() time.Duration { return r.maxAckDelay }
 
+func (r *RTTStats) PTO() time.Duration {
+	return r.SmoothedOrInitialRTT() + utils.MaxDuration(4*r.MeanDeviation(), protocol.TimerGranularity) + r.MaxAckDelay()
+}
+
 // UpdateRTT updates the RTT based on a new sample.
 func (r *RTTStats) UpdateRTT(sendDelta, ackDelay time.Duration, now time.Time) {
 	if sendDelta == utils.InfDuration || sendDelta <= 0 {

internal/congestion/rtt_stats_test.go

@@ -3,6 +3,7 @@ package congestion
 import (
 	"time"
 
+	"github.com/lucas-clemente/quic-go/internal/protocol"
 	"github.com/lucas-clemente/quic-go/internal/utils"
 	. "github.com/onsi/ginkgo"
 	. "github.com/onsi/gomega"
@@ -64,6 +65,22 @@ var _ = Describe("RTT stats", func() {
 		Expect(rttStats.MaxAckDelay()).To(Equal(42 * time.Minute))
 	})
 
+	It("computes the PTO", func() {
+		maxAckDelay := 42 * time.Minute
+		rttStats.SetMaxAckDelay(maxAckDelay)
+		rtt := time.Second
+		rttStats.UpdateRTT(rtt, 0, time.Time{})
+		Expect(rttStats.SmoothedRTT()).To(Equal(rtt))
+		Expect(rttStats.MeanDeviation()).To(Equal(rtt / 2))
+		Expect(rttStats.PTO()).To(Equal(rtt + 4*(rtt/2) + maxAckDelay))
+	})
+
+	It("uses the granularity for computing the PTO for short RTTs", func() {
+		rtt := time.Microsecond
+		rttStats.UpdateRTT(rtt, 0, time.Time{})
+		Expect(rttStats.PTO()).To(Equal(rtt + protocol.TimerGranularity))
+	})
+
 	It("ExpireSmoothedMetrics", func() {
 		initialRtt := (10 * time.Millisecond)
 		rttStats.UpdateRTT(initialRtt, 0, time.Time{})