From 7f1e85a97756b42a28d700c3bc5ae73c2d087641 Mon Sep 17 00:00:00 2001
From: Lucas Clemente <lucas@clemente.io>
Date: Thu, 28 Apr 2016 14:32:48 +0200
Subject: [PATCH] implement CubicSender.RetransmissionDelay

---
 congestion/cubic_sender.go      |  8 ++++++++
 congestion/cubic_sender_test.go | 27 +++++++++++++++++++++++++++
 congestion/interface.go         |  1 +
 3 files changed, 36 insertions(+)

diff --git a/congestion/cubic_sender.go b/congestion/cubic_sender.go
index ecf4b2e8..79a5686e 100644
--- a/congestion/cubic_sender.go
+++ b/congestion/cubic_sender.go
@@ -300,3 +300,11 @@ func (c *cubicSender) OnConnectionMigration() {
 func (c *cubicSender) SetSlowStartLargeReduction(enabled bool) {
 	c.slowStartLargeReduction = enabled
 }
+
+// RetransmissionDelay gives the time to retransmission
+func (c *cubicSender) RetransmissionDelay() time.Duration {
+	if c.rttStats.SmoothedRTT() == 0 {
+		return 0
+	}
+	return c.rttStats.SmoothedRTT() + c.rttStats.MeanDeviation()*4
+}
diff --git a/congestion/cubic_sender_test.go b/congestion/cubic_sender_test.go
index 85b51c76..31e3a4f6 100644
--- a/congestion/cubic_sender_test.go
+++ b/congestion/cubic_sender_test.go
@@ -390,6 +390,33 @@ var _ = Describe("Cubic Sender", func() {
 		Expect(sender.GetCongestionWindow()).To(Equal(defaultWindowTCP))
 	})
 
+	It("retransmission delay", func() {
+		const kRttMs = 10 * time.Millisecond
+		const kDeviationMs = 3 * time.Millisecond
+		Expect(sender.RetransmissionDelay()).To(BeZero())
+
+		rttStats.UpdateRTT(kRttMs, 0, clock.Now())
+
+		// Initial value is to set the median deviation to half of the initial
+		// rtt, the median in then multiplied by a factor of 4 and finally the
+		// smoothed rtt is added which is the initial rtt.
+		expected_delay := kRttMs + kRttMs/2*4
+		Expect(sender.RetransmissionDelay()).To(Equal(expected_delay))
+
+		for i := 0; i < 100; i++ {
+			// Run to make sure that we converge.
+			rttStats.UpdateRTT(kRttMs+kDeviationMs, 0, clock.Now())
+			rttStats.UpdateRTT(kRttMs-kDeviationMs, 0, clock.Now())
+		}
+		expected_delay = kRttMs + kDeviationMs*4
+
+		Expect(rttStats.SmoothedRTT()).To(BeNumerically("~", kRttMs, time.Millisecond))
+		Expect(sender.RetransmissionDelay()).To(BeNumerically("~", expected_delay, time.Millisecond))
+		Expect(sender.BandwidthEstimate() / congestion.BytesPerSecond).To(Equal(congestion.Bandwidth(
+			sender.GetCongestionWindow() * uint64(time.Second) / uint64(rttStats.SmoothedRTT()),
+		)))
+	})
+
 	It("slow start max send window", func() {
 		const kMaxCongestionWindowTCP = 50
 		const kNumberOfAcks = 100
diff --git a/congestion/interface.go b/congestion/interface.go
index 24cd7d77..8860a9de 100644
--- a/congestion/interface.go
+++ b/congestion/interface.go
@@ -16,6 +16,7 @@ type SendAlgorithm interface {
 	OnRetransmissionTimeout(packetsRetransmitted bool)
 	InRecovery() bool
 	OnConnectionMigration()
+	RetransmissionDelay() time.Duration
 
 	// Experiments
 	SetSlowStartLargeReduction(enabled bool)