use a list of pairs of pairs for ACK2

libraries/networking/src/udt/Connection.cpp

@@ -161,8 +161,11 @@ void Connection::sendACK(bool wasCausedBySyncTimeout) {
     // have the socket send off our packet
     _parentSocket->writeBasePacket(*ackPacket, _destination);
     
+    Q_ASSERT_X(_sentACKs.empty() || _sentACKs.back().first + 1 == _currentACKSubSequenceNumber,
+               "Connection::sendACK", "Adding an invalid ACK to _sentACKs");
+    
     // write this ACK to the map of sent ACKs
-    _sentACKs[_currentACKSubSequenceNumber] = { nextACKNumber, high_resolution_clock::now() };
+    _sentACKs.push_back({ _currentACKSubSequenceNumber, { nextACKNumber, high_resolution_clock::now() }});
     
     // reset the number of data packets received since last ACK
     _packetsSinceACK = 0;
@@ -463,14 +466,17 @@ void Connection::processACK2(std::unique_ptr<ControlPacket> controlPacket) {
     controlPacket->readPrimitive(&subSequenceNumber);
 
     // check if we had that subsequence number in our map
-    auto it = _sentACKs.find(subSequenceNumber);
+    auto it = std::find_if_not(_sentACKs.begin(), _sentACKs.end(), [subSequenceNumber](const ACKListPair& pair){
+        return subSequenceNumber < pair.first;
+    });
+    
     if (it != _sentACKs.end()) {
+        if (it->first == subSequenceNumber){
             // update the RTT using the ACK window
-        SequenceNumberTimePair& pair = it->second;
             
             // calculate the RTT (time now - time ACK sent)
             auto now = high_resolution_clock::now();
-        int rtt = duration_cast<microseconds>(now - pair.second).count();
+            int rtt = duration_cast<microseconds>(now - it->second.second).count();
             
             updateRTT(rtt);
             
@@ -478,8 +484,14 @@ void Connection::processACK2(std::unique_ptr<ControlPacket> controlPacket) {
             _congestionControl->setRTT(_rtt);
             
             // update the last ACKed ACK
-        if (pair.first > _lastReceivedAcknowledgedACK) {
+            if (it->second.first > _lastReceivedAcknowledgedACK) {
-            _lastReceivedAcknowledgedACK = pair.first;
+                _lastReceivedAcknowledgedACK = it->second.first;
+            }
+            
+            // erase this sub-sequence number and anything below it now that we've gotten our timing information
+            _sentACKs.erase(_sentACKs.begin(), it);
+        } else {
+            Q_UNREACHABLE();
         }
     }
     

libraries/networking/src/udt/Connection.h

@@ -13,6 +13,7 @@
 #define hifi_Connection_h
 
 #include <chrono>
+#include <list>
 #include <memory>
 
 #include <QtCore/QObject>
@@ -35,7 +36,8 @@ class Connection : public QObject {
     Q_OBJECT
 public:
     using SequenceNumberTimePair = std::pair<SequenceNumber, std::chrono::high_resolution_clock::time_point>;
-    using SentACKMap = std::unordered_map<SequenceNumber, SequenceNumberTimePair>;
+    using ACKListPair = std::pair<SequenceNumber, SequenceNumberTimePair>;
+    using SentACKList = std::list<ACKListPair>;
     
     Connection(Socket* parentSocket, HifiSockAddr destination, std::unique_ptr<CongestionControl> congestionControl);
     ~Connection();
@@ -100,7 +102,7 @@ private:
     int _bandwidth { 1 }; // Exponential moving average for estimated bandwidth, in packets per second
     int _deliveryRate { 16 }; // Exponential moving average for receiver's receive rate, in packets per second
     
-    SentACKMap _sentACKs; // Map of ACK sub-sequence numbers to ACKed sequence number and sent time
+    SentACKList _sentACKs; // Map of ACK sub-sequence numbers to ACKed sequence number and sent time
     
     Socket* _parentSocket { nullptr };
     HifiSockAddr _destination;