blob: edbe18dbea596e8610bea8464dea999b78aabdfc [file] [log] [blame]
// Copyright (c) 2015 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "quic/core/congestion_control/cubic_bytes.h"
#include <cstdint>
#include "quic/platform/api/quic_flags.h"
#include "quic/platform/api/quic_test.h"
#include "quic/test_tools/mock_clock.h"
namespace quic {
namespace test {
namespace {
const float kBeta = 0.7f; // Default Cubic backoff factor.
const float kBetaLastMax = 0.85f; // Default Cubic backoff factor.
const uint32_t kNumConnections = 2;
const float kNConnectionBeta = (kNumConnections - 1 + kBeta) / kNumConnections;
const float kNConnectionBetaLastMax =
(kNumConnections - 1 + kBetaLastMax) / kNumConnections;
const float kNConnectionAlpha = 3 * kNumConnections * kNumConnections *
(1 - kNConnectionBeta) / (1 + kNConnectionBeta);
} // namespace
class CubicBytesTest : public QuicTest {
protected:
CubicBytesTest()
: one_ms_(QuicTime::Delta::FromMilliseconds(1)),
hundred_ms_(QuicTime::Delta::FromMilliseconds(100)),
cubic_(&clock_) {}
QuicByteCount RenoCwndInBytes(QuicByteCount current_cwnd) {
QuicByteCount reno_estimated_cwnd =
current_cwnd +
kDefaultTCPMSS * (kNConnectionAlpha * kDefaultTCPMSS) / current_cwnd;
return reno_estimated_cwnd;
}
QuicByteCount ConservativeCwndInBytes(QuicByteCount current_cwnd) {
QuicByteCount conservative_cwnd = current_cwnd + kDefaultTCPMSS / 2;
return conservative_cwnd;
}
QuicByteCount CubicConvexCwndInBytes(QuicByteCount initial_cwnd,
QuicTime::Delta rtt,
QuicTime::Delta elapsed_time) {
const int64_t offset =
((elapsed_time + rtt).ToMicroseconds() << 10) / 1000000;
const QuicByteCount delta_congestion_window =
((410 * offset * offset * offset) * kDefaultTCPMSS >> 40);
const QuicByteCount cubic_cwnd = initial_cwnd + delta_congestion_window;
return cubic_cwnd;
}
QuicByteCount LastMaxCongestionWindow() {
return cubic_.last_max_congestion_window();
}
QuicTime::Delta MaxCubicTimeInterval() {
return cubic_.MaxCubicTimeInterval();
}
const QuicTime::Delta one_ms_;
const QuicTime::Delta hundred_ms_;
MockClock clock_;
CubicBytes cubic_;
};
// TODO(jokulik): The original "AboveOrigin" test, below, is very
// loose. It's nearly impossible to make the test tighter without
// deploying the fix for convex mode. Once cubic convex is deployed,
// replace "AboveOrigin" with this test.
TEST_F(CubicBytesTest, AboveOriginWithTighterBounds) {
// Convex growth.
const QuicTime::Delta rtt_min = hundred_ms_;
int64_t rtt_min_ms = rtt_min.ToMilliseconds();
float rtt_min_s = rtt_min_ms / 1000.0;
QuicByteCount current_cwnd = 10 * kDefaultTCPMSS;
const QuicByteCount initial_cwnd = current_cwnd;
clock_.AdvanceTime(one_ms_);
const QuicTime initial_time = clock_.ApproximateNow();
const QuicByteCount expected_first_cwnd = RenoCwndInBytes(current_cwnd);
current_cwnd = cubic_.CongestionWindowAfterAck(kDefaultTCPMSS, current_cwnd,
rtt_min, initial_time);
ASSERT_EQ(expected_first_cwnd, current_cwnd);
// Normal TCP phase.
// The maximum number of expected Reno RTTs is calculated by
// finding the point where the cubic curve and the reno curve meet.
const int max_reno_rtts =
std::sqrt(kNConnectionAlpha / (.4 * rtt_min_s * rtt_min_s * rtt_min_s)) -
2;
for (int i = 0; i < max_reno_rtts; ++i) {
// Alternatively, we expect it to increase by one, every time we
// receive current_cwnd/Alpha acks back. (This is another way of
// saying we expect cwnd to increase by approximately Alpha once
// we receive current_cwnd number ofacks back).
const uint64_t num_acks_this_epoch =
current_cwnd / kDefaultTCPMSS / kNConnectionAlpha;
const QuicByteCount initial_cwnd_this_epoch = current_cwnd;
for (QuicPacketCount n = 0; n < num_acks_this_epoch; ++n) {
// Call once per ACK.
const QuicByteCount expected_next_cwnd = RenoCwndInBytes(current_cwnd);
current_cwnd = cubic_.CongestionWindowAfterAck(
kDefaultTCPMSS, current_cwnd, rtt_min, clock_.ApproximateNow());
ASSERT_EQ(expected_next_cwnd, current_cwnd);
}
// Our byte-wise Reno implementation is an estimate. We expect
// the cwnd to increase by approximately one MSS every
// cwnd/kDefaultTCPMSS/Alpha acks, but it may be off by as much as
// half a packet for smaller values of current_cwnd.
const QuicByteCount cwnd_change_this_epoch =
current_cwnd - initial_cwnd_this_epoch;
ASSERT_NEAR(kDefaultTCPMSS, cwnd_change_this_epoch, kDefaultTCPMSS / 2);
clock_.AdvanceTime(hundred_ms_);
}
for (int i = 0; i < 54; ++i) {
const uint64_t max_acks_this_epoch = current_cwnd / kDefaultTCPMSS;
const QuicTime::Delta interval = QuicTime::Delta::FromMicroseconds(
hundred_ms_.ToMicroseconds() / max_acks_this_epoch);
for (QuicPacketCount n = 0; n < max_acks_this_epoch; ++n) {
clock_.AdvanceTime(interval);
current_cwnd = cubic_.CongestionWindowAfterAck(
kDefaultTCPMSS, current_cwnd, rtt_min, clock_.ApproximateNow());
const QuicByteCount expected_cwnd = CubicConvexCwndInBytes(
initial_cwnd, rtt_min, (clock_.ApproximateNow() - initial_time));
// If we allow per-ack updates, every update is a small cubic update.
ASSERT_EQ(expected_cwnd, current_cwnd);
}
}
const QuicByteCount expected_cwnd = CubicConvexCwndInBytes(
initial_cwnd, rtt_min, (clock_.ApproximateNow() - initial_time));
current_cwnd = cubic_.CongestionWindowAfterAck(
kDefaultTCPMSS, current_cwnd, rtt_min, clock_.ApproximateNow());
ASSERT_EQ(expected_cwnd, current_cwnd);
}
// TODO(ianswett): This test was disabled when all fixes were enabled, but it
// may be worth fixing.
TEST_F(CubicBytesTest, DISABLED_AboveOrigin) {
// Convex growth.
const QuicTime::Delta rtt_min = hundred_ms_;
QuicByteCount current_cwnd = 10 * kDefaultTCPMSS;
// Without the signed-integer, cubic-convex fix, we start out in the
// wrong mode.
QuicPacketCount expected_cwnd = RenoCwndInBytes(current_cwnd);
// Initialize the state.
clock_.AdvanceTime(one_ms_);
ASSERT_EQ(expected_cwnd,
cubic_.CongestionWindowAfterAck(kDefaultTCPMSS, current_cwnd,
rtt_min, clock_.ApproximateNow()));
current_cwnd = expected_cwnd;
const QuicPacketCount initial_cwnd = expected_cwnd;
// Normal TCP phase.
for (int i = 0; i < 48; ++i) {
for (QuicPacketCount n = 1;
n < current_cwnd / kDefaultTCPMSS / kNConnectionAlpha; ++n) {
// Call once per ACK.
ASSERT_NEAR(
current_cwnd,
cubic_.CongestionWindowAfterAck(kDefaultTCPMSS, current_cwnd, rtt_min,
clock_.ApproximateNow()),
kDefaultTCPMSS);
}
clock_.AdvanceTime(hundred_ms_);
current_cwnd = cubic_.CongestionWindowAfterAck(
kDefaultTCPMSS, current_cwnd, rtt_min, clock_.ApproximateNow());
// When we fix convex mode and the uint64 arithmetic, we
// increase the expected_cwnd only after after the first 100ms,
// rather than after the initial 1ms.
expected_cwnd += kDefaultTCPMSS;
ASSERT_NEAR(expected_cwnd, current_cwnd, kDefaultTCPMSS);
}
// Cubic phase.
for (int i = 0; i < 52; ++i) {
for (QuicPacketCount n = 1; n < current_cwnd / kDefaultTCPMSS; ++n) {
// Call once per ACK.
ASSERT_NEAR(
current_cwnd,
cubic_.CongestionWindowAfterAck(kDefaultTCPMSS, current_cwnd, rtt_min,
clock_.ApproximateNow()),
kDefaultTCPMSS);
}
clock_.AdvanceTime(hundred_ms_);
current_cwnd = cubic_.CongestionWindowAfterAck(
kDefaultTCPMSS, current_cwnd, rtt_min, clock_.ApproximateNow());
}
// Total time elapsed so far; add min_rtt (0.1s) here as well.
float elapsed_time_s = 10.0f + 0.1f;
// |expected_cwnd| is initial value of cwnd + K * t^3, where K = 0.4.
expected_cwnd =
initial_cwnd / kDefaultTCPMSS +
(elapsed_time_s * elapsed_time_s * elapsed_time_s * 410) / 1024;
EXPECT_EQ(expected_cwnd, current_cwnd / kDefaultTCPMSS);
}
// Constructs an artificial scenario to ensure that cubic-convex
// increases are truly fine-grained:
//
// - After starting the epoch, this test advances the elapsed time
// sufficiently far that cubic will do small increases at less than
// MaxCubicTimeInterval() intervals.
//
// - Sets an artificially large initial cwnd to prevent Reno from the
// convex increases on every ack.
TEST_F(CubicBytesTest, AboveOriginFineGrainedCubing) {
// Start the test with an artificially large cwnd to prevent Reno
// from over-taking cubic.
QuicByteCount current_cwnd = 1000 * kDefaultTCPMSS;
const QuicByteCount initial_cwnd = current_cwnd;
const QuicTime::Delta rtt_min = hundred_ms_;
clock_.AdvanceTime(one_ms_);
QuicTime initial_time = clock_.ApproximateNow();
// Start the epoch and then artificially advance the time.
current_cwnd = cubic_.CongestionWindowAfterAck(
kDefaultTCPMSS, current_cwnd, rtt_min, clock_.ApproximateNow());
clock_.AdvanceTime(QuicTime::Delta::FromMilliseconds(600));
current_cwnd = cubic_.CongestionWindowAfterAck(
kDefaultTCPMSS, current_cwnd, rtt_min, clock_.ApproximateNow());
// We expect the algorithm to perform only non-zero, fine-grained cubic
// increases on every ack in this case.
for (int i = 0; i < 100; ++i) {
clock_.AdvanceTime(QuicTime::Delta::FromMilliseconds(10));
const QuicByteCount expected_cwnd = CubicConvexCwndInBytes(
initial_cwnd, rtt_min, (clock_.ApproximateNow() - initial_time));
const QuicByteCount next_cwnd = cubic_.CongestionWindowAfterAck(
kDefaultTCPMSS, current_cwnd, rtt_min, clock_.ApproximateNow());
// Make sure we are performing cubic increases.
ASSERT_EQ(expected_cwnd, next_cwnd);
// Make sure that these are non-zero, less-than-packet sized
// increases.
ASSERT_GT(next_cwnd, current_cwnd);
const QuicByteCount cwnd_delta = next_cwnd - current_cwnd;
ASSERT_GT(kDefaultTCPMSS * .1, cwnd_delta);
current_cwnd = next_cwnd;
}
}
// Constructs an artificial scenario to show what happens when we
// allow per-ack updates, rather than limititing update freqency. In
// this scenario, the first two acks of the epoch produce the same
// cwnd. When we limit per-ack updates, this would cause the
// cessation of cubic updates for 30ms. When we allow per-ack
// updates, the window continues to grow on every ack.
TEST_F(CubicBytesTest, PerAckUpdates) {
// Start the test with a large cwnd and RTT, to force the first
// increase to be a cubic increase.
QuicPacketCount initial_cwnd_packets = 150;
QuicByteCount current_cwnd = initial_cwnd_packets * kDefaultTCPMSS;
const QuicTime::Delta rtt_min = 350 * one_ms_;
// Initialize the epoch
clock_.AdvanceTime(one_ms_);
// Keep track of the growth of the reno-equivalent cwnd.
QuicByteCount reno_cwnd = RenoCwndInBytes(current_cwnd);
current_cwnd = cubic_.CongestionWindowAfterAck(
kDefaultTCPMSS, current_cwnd, rtt_min, clock_.ApproximateNow());
const QuicByteCount initial_cwnd = current_cwnd;
// Simulate the return of cwnd packets in less than
// MaxCubicInterval() time.
const QuicPacketCount max_acks = initial_cwnd_packets / kNConnectionAlpha;
const QuicTime::Delta interval = QuicTime::Delta::FromMicroseconds(
MaxCubicTimeInterval().ToMicroseconds() / (max_acks + 1));
// In this scenario, the first increase is dictated by the cubic
// equation, but it is less than one byte, so the cwnd doesn't
// change. Normally, without per-ack increases, any cwnd plateau
// will cause the cwnd to be pinned for MaxCubicTimeInterval(). If
// we enable per-ack updates, the cwnd will continue to grow,
// regardless of the temporary plateau.
clock_.AdvanceTime(interval);
reno_cwnd = RenoCwndInBytes(reno_cwnd);
ASSERT_EQ(current_cwnd,
cubic_.CongestionWindowAfterAck(kDefaultTCPMSS, current_cwnd,
rtt_min, clock_.ApproximateNow()));
for (QuicPacketCount i = 1; i < max_acks; ++i) {
clock_.AdvanceTime(interval);
const QuicByteCount next_cwnd = cubic_.CongestionWindowAfterAck(
kDefaultTCPMSS, current_cwnd, rtt_min, clock_.ApproximateNow());
reno_cwnd = RenoCwndInBytes(reno_cwnd);
// The window shoud increase on every ack.
ASSERT_LT(current_cwnd, next_cwnd);
ASSERT_EQ(reno_cwnd, next_cwnd);
current_cwnd = next_cwnd;
}
// After all the acks are returned from the epoch, we expect the
// cwnd to have increased by nearly one packet. (Not exactly one
// packet, because our byte-wise Reno algorithm is always a slight
// under-estimation). Without per-ack updates, the current_cwnd
// would otherwise be unchanged.
const QuicByteCount minimum_expected_increase = kDefaultTCPMSS * .9;
EXPECT_LT(minimum_expected_increase + initial_cwnd, current_cwnd);
}
TEST_F(CubicBytesTest, LossEvents) {
const QuicTime::Delta rtt_min = hundred_ms_;
QuicByteCount current_cwnd = 422 * kDefaultTCPMSS;
// Without the signed-integer, cubic-convex fix, we mistakenly
// increment cwnd after only one_ms_ and a single ack.
QuicPacketCount expected_cwnd = RenoCwndInBytes(current_cwnd);
// Initialize the state.
clock_.AdvanceTime(one_ms_);
EXPECT_EQ(expected_cwnd,
cubic_.CongestionWindowAfterAck(kDefaultTCPMSS, current_cwnd,
rtt_min, clock_.ApproximateNow()));
// On the first loss, the last max congestion window is set to the
// congestion window before the loss.
QuicByteCount pre_loss_cwnd = current_cwnd;
ASSERT_EQ(0u, LastMaxCongestionWindow());
expected_cwnd = static_cast<QuicByteCount>(current_cwnd * kNConnectionBeta);
EXPECT_EQ(expected_cwnd,
cubic_.CongestionWindowAfterPacketLoss(current_cwnd));
ASSERT_EQ(pre_loss_cwnd, LastMaxCongestionWindow());
current_cwnd = expected_cwnd;
// On the second loss, the current congestion window has not yet
// reached the last max congestion window. The last max congestion
// window will be reduced by an additional backoff factor to allow
// for competition.
pre_loss_cwnd = current_cwnd;
expected_cwnd = static_cast<QuicByteCount>(current_cwnd * kNConnectionBeta);
ASSERT_EQ(expected_cwnd,
cubic_.CongestionWindowAfterPacketLoss(current_cwnd));
current_cwnd = expected_cwnd;
EXPECT_GT(pre_loss_cwnd, LastMaxCongestionWindow());
QuicByteCount expected_last_max =
static_cast<QuicByteCount>(pre_loss_cwnd * kNConnectionBetaLastMax);
EXPECT_EQ(expected_last_max, LastMaxCongestionWindow());
EXPECT_LT(expected_cwnd, LastMaxCongestionWindow());
// Simulate an increase, and check that we are below the origin.
current_cwnd = cubic_.CongestionWindowAfterAck(
kDefaultTCPMSS, current_cwnd, rtt_min, clock_.ApproximateNow());
EXPECT_GT(LastMaxCongestionWindow(), current_cwnd);
// On the final loss, simulate the condition where the congestion
// window had a chance to grow nearly to the last congestion window.
current_cwnd = LastMaxCongestionWindow() - 1;
pre_loss_cwnd = current_cwnd;
expected_cwnd = static_cast<QuicByteCount>(current_cwnd * kNConnectionBeta);
EXPECT_EQ(expected_cwnd,
cubic_.CongestionWindowAfterPacketLoss(current_cwnd));
expected_last_max = pre_loss_cwnd;
ASSERT_EQ(expected_last_max, LastMaxCongestionWindow());
}
TEST_F(CubicBytesTest, BelowOrigin) {
// Concave growth.
const QuicTime::Delta rtt_min = hundred_ms_;
QuicByteCount current_cwnd = 422 * kDefaultTCPMSS;
// Without the signed-integer, cubic-convex fix, we mistakenly
// increment cwnd after only one_ms_ and a single ack.
QuicPacketCount expected_cwnd = RenoCwndInBytes(current_cwnd);
// Initialize the state.
clock_.AdvanceTime(one_ms_);
EXPECT_EQ(expected_cwnd,
cubic_.CongestionWindowAfterAck(kDefaultTCPMSS, current_cwnd,
rtt_min, clock_.ApproximateNow()));
expected_cwnd = static_cast<QuicPacketCount>(current_cwnd * kNConnectionBeta);
EXPECT_EQ(expected_cwnd,
cubic_.CongestionWindowAfterPacketLoss(current_cwnd));
current_cwnd = expected_cwnd;
// First update after loss to initialize the epoch.
current_cwnd = cubic_.CongestionWindowAfterAck(
kDefaultTCPMSS, current_cwnd, rtt_min, clock_.ApproximateNow());
// Cubic phase.
for (int i = 0; i < 40; ++i) {
clock_.AdvanceTime(hundred_ms_);
current_cwnd = cubic_.CongestionWindowAfterAck(
kDefaultTCPMSS, current_cwnd, rtt_min, clock_.ApproximateNow());
}
expected_cwnd = 553632;
EXPECT_EQ(expected_cwnd, current_cwnd);
}
} // namespace test
} // namespace quic