send immediate ack request if no packet being sent

libi2pd/Streaming.cpp

@@ -877,13 +877,18 @@ namespace stream
 		}
 		packet[size] = 0;
 		size++; // resend delay	
-		htobuf16 (packet + size, choking ? PACKET_FLAG_DELAY_REQUESTED : 0); // no flags set or delay
+		bool requestImmediateAck = false;
+		if (!choking)
+			requestImmediateAck = m_LastSendTime && ts > m_LastSendTime + REQUEST_IMMEDIATE_ACK_INTERVAL &&
+				ts > m_LastSendTime + REQUEST_IMMEDIATE_ACK_INTERVAL + m_LocalDestination.GetRandom () % REQUEST_IMMEDIATE_ACK_INTERVAL_VARIANCE;
+		htobuf16 (packet + size, (choking || requestImmediateAck) ? PACKET_FLAG_DELAY_REQUESTED : 0); // no flags set or delay requested
 		size += 2; // flags
-		if (choking)
+		if (choking || requestImmediateAck)
 		{
 			htobuf16 (packet + size, 2); // 2 bytes delay interval
-			htobuf16 (packet + size + 2, DELAY_CHOKING); // set choking interval
+			htobuf16 (packet + size + 2, choking ? DELAY_CHOKING : 0); // set choking or immediated ack interval
 			size += 2;
+			if (requestImmediateAck) m_LastSendTime = ts; // ack request sent
 		}	
 		else	
 			htobuf16 (packet + size, 0); // no options

libi2pd/Streaming.h

@@ -70,6 +70,8 @@ namespace stream
 	const int MAX_RECEIVE_TIMEOUT = 20; // in seconds
 	const uint16_t DELAY_CHOKING = 60000; // in milliseconds
 	const uint64_t SEND_INTERVAL = 1000; // in microseconds
+	const uint64_t REQUEST_IMMEDIATE_ACK_INTERVAL = 7500; // in milliseconds 
+	const uint64_t REQUEST_IMMEDIATE_ACK_INTERVAL_VARIANCE = 3200; // in milliseconds 	
 	
 	struct Packet
 	{
@@ -283,7 +285,8 @@ namespace stream
 			float m_WindowSize, m_LastWindowDropSize, m_WindowDropTargetSize;
 			int m_WindowIncCounter, m_RTO, m_AckDelay, m_PrevRTTSample;
 			double m_Jitter;
-			uint64_t m_MinPacingTime, m_PacingTime, m_PacingTimeRem, m_LastSendTime; // microseconds
+			uint64_t m_MinPacingTime, m_PacingTime, m_PacingTimeRem, // microseconds
+				m_LastSendTime; // miliseconds
 			uint64_t m_LastACKSendTime, m_PacketACKInterval, m_PacketACKIntervalRem; // for limit inbound speed
 			int m_NumResendAttempts, m_NumPacketsToSend;
 			size_t m_MTU;