Remove time constraint on first retransmit of a packet.

We don't allow more than one retransmission within one RTT, but the RTT
estimate might be off. Reasonably, the remote end will not send a NACK
until the packet after has been received - so always resend on first
request.

Review URL: https://codereview.webrtc.org/1414563003

Cr-Commit-Position: refs/heads/master@{#10362}

webrtc/modules/rtp_rtcp/source/rtp_packet_history.cc

@@ -16,6 +16,7 @@
 #include <limits>
 #include <set>
 
+#include "webrtc/base/checks.h"
 #include "webrtc/modules/rtp_rtcp/source/rtp_utility.h"
 #include "webrtc/system_wrappers/interface/critical_section_wrapper.h"
 #include "webrtc/system_wrappers/interface/logging.h"
@@ -118,6 +119,7 @@ int32_t RTPPacketHistory::PutRTPPacket(const uint8_t* packet,
       (capture_time_ms > 0) ? capture_time_ms : clock_->TimeInMilliseconds();
   stored_packets_[prev_index_].send_time = 0;  // Packet not sent.
   stored_packets_[prev_index_].storage_type = type;
+  stored_packets_[prev_index_].has_been_retransmitted = false;
 
   ++prev_index_;
   if (prev_index_ >= stored_packets_.size()) {
@@ -173,10 +175,9 @@ bool RTPPacketHistory::GetPacketAndSetSendTime(uint16_t sequence_number,
                                                size_t* packet_length,
                                                int64_t* stored_time_ms) {
   CriticalSectionScoped cs(critsect_.get());
-  assert(*packet_length >= IP_PACKET_SIZE);
+  RTC_CHECK_GE(*packet_length, static_cast<size_t>(IP_PACKET_SIZE));
-  if (!store_) {
+  if (!store_)
     return false;
-  }
 
   int32_t index = 0;
   bool found = FindSeqNum(sequence_number, &index);
@@ -193,18 +194,23 @@ bool RTPPacketHistory::GetPacketAndSetSendTime(uint16_t sequence_number,
     return false;
   }
 
-  // Verify elapsed time since last retrieve.
+  // Verify elapsed time since last retrieve, but only for retransmissions and
+  // always send packet upon first retransmission request.
   int64_t now = clock_->TimeInMilliseconds();
-  if (min_elapsed_time_ms > 0 &&
+  if (min_elapsed_time_ms > 0 && retransmit &&
+      stored_packets_[index].has_been_retransmitted &&
       ((now - stored_packets_[index].send_time) < min_elapsed_time_ms)) {
     return false;
   }
 
-  if (retransmit && stored_packets_[index].storage_type == kDontRetransmit) {
+  if (retransmit) {
+    if (stored_packets_[index].storage_type == kDontRetransmit) {
       // No bytes copied since this packet shouldn't be retransmitted or is
       // of zero size.
       return false;
     }
+    stored_packets_[index].has_been_retransmitted = true;
+  }
   stored_packets_[index].send_time = clock_->TimeInMilliseconds();
   GetPacket(index, packet, packet_length, stored_time_ms);
   return true;

webrtc/modules/rtp_rtcp/source/rtp_packet_history.h

@@ -46,13 +46,12 @@ class RTPPacketHistory {
   // The packet is copied to the buffer pointed to by ptr_rtp_packet.
   // The rtp_packet_length should show the available buffer size.
   // Returns true if packet is found.
-  // rtp_packet_length: returns the copied packet length on success.
+  // packet_length: returns the copied packet length on success.
   // min_elapsed_time_ms: the minimum time that must have elapsed since the last
   // time the packet was resent (parameter is ignored if set to zero).
-  // If the packet is found but the minimum time has not elaped, no bytes are
+  // If the packet is found but the minimum time has not elapsed, no bytes are
   // copied.
   // stored_time_ms: returns the time when the packet was stored.
-  // type: returns the storage type set in PutRTPPacket.
   bool GetPacketAndSetSendTime(uint16_t sequence_number,
                                int64_t min_elapsed_time_ms,
                                bool retransmit,
@@ -94,6 +93,7 @@ class RTPPacketHistory {
     int64_t time_ms = 0;
     int64_t send_time = 0;
     StorageType storage_type = kDontRetransmit;
+    bool has_been_retransmitted = false;
 
     uint8_t data[IP_PACKET_SIZE];
     size_t length = 0;

webrtc/modules/rtp_rtcp/source/rtp_packet_history_unittest.cc

@@ -161,6 +161,8 @@ TEST_F(RtpPacketHistoryTest, DontRetransmit) {
 }
 
 TEST_F(RtpPacketHistoryTest, MinResendTime) {
+  static const int64_t kMinRetransmitIntervalMs = 100;
+
   hist_->SetStorePacketsStatus(true, 10);
   size_t len = 0;
   int64_t capture_time_ms = fake_clock_.TimeInMilliseconds();
@@ -168,22 +170,57 @@ TEST_F(RtpPacketHistoryTest, MinResendTime) {
   EXPECT_EQ(0, hist_->PutRTPPacket(packet_, len, capture_time_ms,
                                    kAllowRetransmission));
 
+  // First transmission: TimeToSendPacket() call from pacer.
   int64_t time;
   len = kMaxPacketLength;
-  EXPECT_TRUE(hist_->GetPacketAndSetSendTime(kSeqNum, 100, false, packet_, &len,
+  EXPECT_TRUE(
-                                             &time));
+      hist_->GetPacketAndSetSendTime(kSeqNum, 0, false, packet_, &len, &time));
-  fake_clock_.AdvanceTimeMilliseconds(100);
+
+  fake_clock_.AdvanceTimeMilliseconds(kMinRetransmitIntervalMs);
   // Time has elapsed.
   len = kMaxPacketLength;
-  EXPECT_TRUE(hist_->GetPacketAndSetSendTime(kSeqNum, 100, false, packet_, &len,
+  EXPECT_TRUE(hist_->GetPacketAndSetSendTime(kSeqNum, kMinRetransmitIntervalMs,
-                                             &time));
+                                             true, packet_, &len, &time));
   EXPECT_GT(len, 0u);
   EXPECT_EQ(capture_time_ms, time);
 
+  fake_clock_.AdvanceTimeMilliseconds(kMinRetransmitIntervalMs - 1);
   // Time has not elapsed. Packet should be found, but no bytes copied.
   len = kMaxPacketLength;
-  EXPECT_FALSE(hist_->GetPacketAndSetSendTime(kSeqNum, 101, false, packet_,
+  EXPECT_FALSE(hist_->GetPacketAndSetSendTime(kSeqNum, kMinRetransmitIntervalMs,
-                                              &len, &time));
+                                              true, packet_, &len, &time));
+}
+
+TEST_F(RtpPacketHistoryTest, EarlyFirstResend) {
+  static const int64_t kMinRetransmitIntervalMs = 100;
+
+  hist_->SetStorePacketsStatus(true, 10);
+  size_t len = 0;
+  int64_t capture_time_ms = fake_clock_.TimeInMilliseconds();
+  CreateRtpPacket(kSeqNum, kSsrc, kPayload, kTimestamp, packet_, &len);
+  EXPECT_EQ(0, hist_->PutRTPPacket(packet_, len, capture_time_ms,
+                                   kAllowRetransmission));
+
+  // First transmission: TimeToSendPacket() call from pacer.
+  int64_t time;
+  len = kMaxPacketLength;
+  EXPECT_TRUE(
+      hist_->GetPacketAndSetSendTime(kSeqNum, 0, false, packet_, &len, &time));
+
+  fake_clock_.AdvanceTimeMilliseconds(kMinRetransmitIntervalMs - 1);
+  // Time has not elapsed, but this is the first retransmission request so
+  // allow anyway.
+  len = kMaxPacketLength;
+  EXPECT_TRUE(hist_->GetPacketAndSetSendTime(kSeqNum, kMinRetransmitIntervalMs,
+                                             true, packet_, &len, &time));
+  EXPECT_GT(len, 0u);
+  EXPECT_EQ(capture_time_ms, time);
+
+  fake_clock_.AdvanceTimeMilliseconds(kMinRetransmitIntervalMs - 1);
+  // Time has not elapsed. Packet should be found, but no bytes copied.
+  len = kMaxPacketLength;
+  EXPECT_FALSE(hist_->GetPacketAndSetSendTime(kSeqNum, kMinRetransmitIntervalMs,
+                                              true, packet_, &len, &time));
 }
 
 TEST_F(RtpPacketHistoryTest, DynamicExpansion) {