Fixed integer signing bug in cubic bytes/packet implementation

net/quic/core/congestion_control/cubic_bytes.cc

 // 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 "net/quic/core/congestion_control/cubic_bytes.h"
 
 #include <stdint.h>
 #include <algorithm>
 #include <cmath>
 
@@ skipping 25 matching lines @@
 // curve. This additional backoff factor is expected to give up bandwidth to
 // new concurrent flows and speed up convergence.
 const float kBetaLastMax = 0.85f;
 
 }  // namespace
 
 CubicBytes::CubicBytes(const QuicClock* clock)
     : clock_(clock),
       num_connections_(kDefaultNumConnections),
       epoch_(QuicTime::Zero()),
-      last_update_time_(QuicTime::Zero()) {
+      last_update_time_(QuicTime::Zero()),
+      fix_convex_mode_(false) {
   Reset();
 }
 
 void CubicBytes::SetNumConnections(int num_connections) {
   num_connections_ = num_connections;
 }
 
 float CubicBytes::Alpha() const {
   // 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.
@@ skipping 13 matching lines @@
 void CubicBytes::Reset() {
   epoch_ = QuicTime::Zero();             // Reset time.
   last_update_time_ = QuicTime::Zero();  // Reset time.
   last_congestion_window_ = 0;
   last_max_congestion_window_ = 0;
   acked_bytes_count_ = 0;
   estimated_tcp_congestion_window_ = 0;
   origin_point_congestion_window_ = 0;
   time_to_origin_point_ = 0;
   last_target_congestion_window_ = 0;
+  fix_convex_mode_ = false;
+}
+
+void CubicBytes::SetFixConvexMode(bool fix_convex_mode) {
+  fix_convex_mode_ = fix_convex_mode;
 }
 
 void CubicBytes::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
@@ skipping 49 matching lines @@
     }
   }
   // 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.
   int64_t elapsed_time =
       ((current_time + delay_min - epoch_).ToMicroseconds() << 10) /
       kNumMicrosPerSecond;
 
   int64_t offset = time_to_origin_point_ - elapsed_time;
+  if (fix_convex_mode_) {
+    // Right-shifts of negative, signed numbers have
+    // implementation-dependent behavior.  In the fix, force the
+    // offset to be positive, as is done in the kernel.
+    const int64_t positive_offset = labs(time_to_origin_point_ - elapsed_time);
+    offset = positive_offset;
+  }
   QuicByteCount delta_congestion_window =
       ((kCubeCongestionWindowScale * offset * offset * offset) >> kCubeScale) *
       kDefaultTCPMSS;
 
+  const bool add_delta = elapsed_time > time_to_origin_point_;
+  DCHECK(add_delta ||
+         (origin_point_congestion_window_ > delta_congestion_window));
   QuicByteCount target_congestion_window =
-      origin_point_congestion_window_ - delta_congestion_window;
+      (fix_convex_mode_ && add_delta)
+          ? origin_point_congestion_window_ + delta_congestion_window
+          : origin_point_congestion_window_ - delta_congestion_window;
   // Limit the CWND increase to half the acked bytes.
   target_congestion_window =
       min(target_congestion_window,
           current_congestion_window + acked_bytes_count_ / 2);
 
   DCHECK_LT(0u, estimated_tcp_congestion_window_);
-  // Increase the window by Alpha * 1 MSS of bytes every time we ack an
-  // estimated tcp window of bytes.
+  // 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.
   estimated_tcp_congestion_window_ += acked_bytes_count_ *
                                       (Alpha() * kDefaultTCPMSS) /
                                       estimated_tcp_congestion_window_;
   acked_bytes_count_ = 0;
 
   // We have a new cubic congestion window.
   last_target_congestion_window_ = target_congestion_window;
 
   // Compute target congestion_window based on cubic target and estimated TCP
   // congestion_window, use highest (fastest).
   if (target_congestion_window < estimated_tcp_congestion_window_) {
     target_congestion_window = estimated_tcp_congestion_window_;
   }
 
   DVLOG(1) << "Final target congestion_window: " << target_congestion_window;
   return target_congestion_window;
 }
 
 }  // namespace net