tcp: slow start@rfc5681 : updated rules for congestion window

Signed-off-by: Jessica Hamilton <[email protected]>

src/add-ons/kernel/network/protocols/tcp/TCPEndpoint.cpp

@@ -18,6 +18,7 @@
 #include <signal.h>
 #include <stdlib.h>
 #include <string.h>
+#include <stdint.h>
 
 #include <KernelExport.h>
 #include <Select.h>
@@ -427,6 +428,7 @@ TCPEndpoint::TCPEndpoint(net_socket* socket)
 	fSendWindow(0),
 	fSendMaxWindow(0),
 	fSendMaxSegmentSize(TCP_DEFAULT_MAX_SEGMENT_SIZE),
+	fSendMaxSegments(0),
 	fSendQueue(socket->send.buffer_size),
 	fInitialSendSequence(0),
 	fDuplicateAcknowledgeCount(0),
@@ -1392,7 +1394,14 @@ TCPEndpoint::_PrepareReceivePath(tcp_segment_header& segment)
 			fFlags &= ~FLAG_OPTION_TIMESTAMP;
 	}
 
-	fCongestionWindow = 2 * fSendMaxSegmentSize;
+	if (fSendMaxSegmentSize > 2190)
+		fCongestionWindow = 2 * fSendMaxSegmentSize;
+	else if (fSendMaxSegmentSize > 1095)
+		fCongestionWindow = 3 * fSendMaxSegmentSize;
+	else
+		fCongestionWindow = 4 * fSendMaxSegmentSize;
+
+	fSendMaxSegments = fCongestionWindow / fSendMaxSegmentSize;
 	fSlowStartThreshold = (uint32)segment.advertised_window << fSendWindowShift;
 }
 
@@ -1897,6 +1906,9 @@ inline bool
 TCPEndpoint::_ShouldSendSegment(tcp_segment_header& segment, uint32 length,
 	uint32 segmentMaxSize, uint32 flightSize)
 {
+	if (fState == ESTABLISHED && fSendMaxSegments == 0)
+		return false;
+
 	if (length > 0) {
 		// Avoid the silly window syndrome - we only send a segment in case:
 		// - we have a full segment to send, or
@@ -2114,6 +2126,9 @@ TCPEndpoint::_SendQueued(bool force, uint32 sendWindow)
 		fReceiveMaxAdvertised = fReceiveNext
 			+ ((uint32)segment.advertised_window << fReceiveWindowShift);
 
+		if (segmentLength != 0 && fState == ESTABLISHED)
+			--fSendMaxSegments;
+
 		status = next->module->send_routed_data(next, fRoute, buffer);
 		if (status < B_OK) {
 			gBufferModule->free(buffer);
@@ -2207,6 +2222,26 @@ TCPEndpoint::_Acknowledged(tcp_segment_header& segment)
 
 	if (fSendUnacknowledged < segment.acknowledge) {
 		fSendQueue.RemoveUntil(segment.acknowledge);
+
+		// the acknowledgment of the SYN/ACK MUST NOT increase the size of the congestion window
+		if (fSendUnacknowledged != fInitialSendSequence) {
+			if (fCongestionWindow < fSlowStartThreshold)
+				fCongestionWindow += min_c(segment.acknowledge - fSendUnacknowledged.Number(),
+					fSendMaxSegmentSize);
+			else {
+				uint32 increment = fSendMaxSegmentSize * fSendMaxSegmentSize;
+
+				if (increment < fCongestionWindow)
+					increment = 1;
+				else
+					increment /= fCongestionWindow;
+
+				fCongestionWindow += increment;
+			}
+
+			fSendMaxSegments = UINT32_MAX;
+		}
+
 		fSendUnacknowledged = segment.acknowledge;
 		if (fSendNext < fSendUnacknowledged)
 			fSendNext = fSendUnacknowledged;
@@ -2236,20 +2271,6 @@ TCPEndpoint::_Acknowledged(tcp_segment_header& segment)
 			fSendCondition.NotifyAll();
 			gSocketModule->notify(socket, B_SELECT_WRITE, fSendQueue.Free());
 		}
-
-		if (fCongestionWindow < fSlowStartThreshold)
-			fCongestionWindow += fSendMaxSegmentSize;
-	}
-
-	if (fCongestionWindow >= fSlowStartThreshold) {
-		uint32 increment = fSendMaxSegmentSize * fSendMaxSegmentSize;
-
-		if (increment < fCongestionWindow)
-			increment = 1;
-		else
-			increment /= fCongestionWindow;
-
-		fCongestionWindow += increment;
 	}
 
 	// if there is data left to be sent, send it now
@@ -2261,16 +2282,21 @@ TCPEndpoint::_Acknowledged(tcp_segment_header& segment)
 void
 TCPEndpoint::_Retransmit()
 {
+	if (fState < ESTABLISHED) {
+		fRetransmitTimeout = TCP_SYN_RETRANSMIT_TIMEOUT;
+		fCongestionWindow = fSendMaxSegmentSize;
+	} else {
+		_ResetSlowStart();
+
+		// Do exponential back off of the retransmit timeout
+		fRetransmitTimeout *= 2;
+		if (fRetransmitTimeout > TCP_MAX_RETRANSMIT_TIMEOUT)
+			fRetransmitTimeout = TCP_MAX_RETRANSMIT_TIMEOUT;
+	}
+
 	TRACE("Retransmit()");
 
-	_ResetSlowStart();
 	fSendNext = fSendUnacknowledged;
-
-	// Do exponential back off of the retransmit timeout
-	fRetransmitTimeout *= 2;
-	if (fRetransmitTimeout > TCP_MAX_RETRANSMIT_TIMEOUT)
-		fRetransmitTimeout = TCP_MAX_RETRANSMIT_TIMEOUT;
-
 	_SendQueued();
 }
 

src/add-ons/kernel/network/protocols/tcp/TCPEndpoint.h

@@ -145,6 +145,7 @@ private:
 	uint32			fSendWindow;
 	uint32			fSendMaxWindow;
 	uint32			fSendMaxSegmentSize;
+	uint32			fSendMaxSegments;
 	BufferQueue		fSendQueue;
 	tcp_sequence	fLastAcknowledgeSent;
 	tcp_sequence	fInitialSendSequence;

src/add-ons/kernel/network/protocols/tcp/tcp.h

@@ -193,6 +193,8 @@ operator==(tcp_sequence a, tcp_sequence b)
 #define TCP_MIN_RETRANSMIT_TIMEOUT		200000		// 200 msecs
 // Maximum retransmit timeout (per RFC6298)
 #define TCP_MAX_RETRANSMIT_TIMEOUT		60000000	// 60 secs
+// New value for timeout in case of lost SYN (RFC 6298)
+#define TCP_SYN_RETRANSMIT_TIMEOUT 		3000000		// 3 secs
 
 struct tcp_sack {
 	uint32 left_edge;