net/quic/congestion_control/prr_sender.cc

   1 // Copyright (c) 2014 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "net/quic/congestion_control/prr_sender.h"
   6
   7 #include "net/quic/quic_protocol.h"
   8
   9 namespace net {
  10
  11 namespace {
  12 // Constant based on TCP defaults.
  13 const QuicByteCount kMaxSegmentSize = kDefaultTCPMSS;
  14 }  // namespace
  15
  16 PrrSender::PrrSender()
  17     : bytes_sent_since_loss_(0),
  18       bytes_delivered_since_loss_(0),
  19       ack_count_since_loss_(0),
  20       bytes_in_flight_before_loss_(0) {
  21 }
  22
  23 void PrrSender::OnPacketSent(QuicByteCount sent_bytes) {
  24     bytes_sent_since_loss_ += sent_bytes;
  25 }
  26
  27 void PrrSender::OnPacketLost(QuicByteCount bytes_in_flight) {
  28   bytes_sent_since_loss_ = 0;
  29   bytes_in_flight_before_loss_ = bytes_in_flight;
  30   bytes_delivered_since_loss_ = 0;
  31   ack_count_since_loss_ = 0;
  32 }
  33
  34 void PrrSender::OnPacketAcked(QuicByteCount acked_bytes) {
  35   bytes_delivered_since_loss_ += acked_bytes;
  36   ++ack_count_since_loss_;
  37 }
  38
  39 QuicTime::Delta PrrSender::TimeUntilSend(
  40     QuicByteCount congestion_window,
  41     QuicByteCount bytes_in_flight,
  42     QuicByteCount slowstart_threshold) const {
  43   // Return QuicTime::Zero In order to ensure limited transmit always works.
  44   if (bytes_sent_since_loss_ == 0 || bytes_in_flight < kMaxSegmentSize) {
  45     return QuicTime::Delta::Zero();
  46   }
  47   if (congestion_window > bytes_in_flight) {
  48     // During PRR-SSRB, limit outgoing packets to 1 extra MSS per ack, instead
  49     // of sending the entire available window. This prevents burst retransmits
  50     // when more packets are lost than the CWND reduction.
  51     //   limit = MAX(prr_delivered - prr_out, DeliveredData) + MSS
  52     if (bytes_delivered_since_loss_ + ack_count_since_loss_ * kMaxSegmentSize <=
  53         bytes_sent_since_loss_) {
  54       return QuicTime::Delta::Infinite();
  55     }
  56     return QuicTime::Delta::Zero();
  57   }
  58   // Implement Proportional Rate Reduction (RFC6937).
  59   // Checks a simplified version of the PRR formula that doesn't use division:
  60   // AvailableSendWindow =
  61   //   CEIL(prr_delivered * ssthresh / BytesInFlightAtLoss) - prr_sent
  62   if (bytes_delivered_since_loss_ * slowstart_threshold >
  63       bytes_sent_since_loss_ * bytes_in_flight_before_loss_) {
  64     return QuicTime::Delta::Zero();
  65   }
  66   return QuicTime::Delta::Infinite();
  67 }
  68
  69 }  // namespace net