Drop non-functional TCP window size checks entirely

Previously we checked the window size, but never incremented or
decremented the count of unacked data, so it was a) confusing
and b) open to the risk that something would increment the
count, thereby blocking the connection entirely.

Now, we just ignore it, effectively using an infinite window
size. If the device is overloaded, we assume our upstream
connections will be applying their own backpressure anyway.
Seems to have worked fine so far, might be worth investigating
if we do see issues on some devices under load.

Author: pimterry

app/src/main/java/tech/httptoolkit/android/vpn/Session.java

@@ -56,12 +56,6 @@ public class Session {
 
 	//the next ack to send to client
 	private long sendNext = 0;
-	private int sendWindow = 0; //window = windowsize x windowscale
-	private int sendWindowSize = 0;
-	private int sendWindowScale = 0;
-	
-	//track how many byte of data has been sent since last ACK from client
-	private volatile int sendAmountSinceLastAck = 0;
 
 	//sent by client during SYN inside tcp options
 	private int maxSegmentSize = 0;
@@ -130,27 +124,6 @@ public class Session {
 		this.sessionCloser = sessionCloser;
 	}
 
-	/**
-	 * decrease value of sendAmountSinceLastAck so that client's window is not full
-	 * @param amount Amount
-	 */
-	synchronized void decreaseAmountSentSinceLastAck(long amount){
-		sendAmountSinceLastAck -= amount;
-		if(sendAmountSinceLastAck < 0){
-			Log.e(TAG, "Amount data to be decreased is over than its window.");
-			sendAmountSinceLastAck = 0;
-		}
-	}
-
-	/**
-	 * determine if client's receiving window is full or not.
-	 * @return boolean
-	 */
-	public boolean isClientWindowFull(){
-		return (sendWindow > 0 && sendAmountSinceLastAck >= sendWindow) ||
-				(sendWindow == 0 && sendAmountSinceLastAck > 65535);
-	}
-
 	/**
 	 * append more data
 	 * @param data Data
@@ -244,10 +217,6 @@ public void setSendNext(long sendNext) {
 		this.sendNext = sendNext;
 	}
 
-	int getSendWindow() {
-		return sendWindow;
-	}
-
 	public int getMaxSegmentSize() {
 		return maxSegmentSize;
 	}
@@ -272,16 +241,6 @@ public int getSourcePort() {
 		return sourcePort;
 	}
 
-	void setSendWindowSizeAndScale(int sendWindowSize, int sendWindowScale) {
-		this.sendWindowSize = sendWindowSize;
-		this.sendWindowScale = sendWindowScale;
-		this.sendWindow = sendWindowSize * sendWindowScale;
-	}
-
-	int getSendWindowScale() {
-		return sendWindowScale;
-	}
-
 	void setAcked(boolean isacked) {
 		this.isacked = isacked;
 	}

app/src/main/java/tech/httptoolkit/android/vpn/SessionHandler.java

@@ -139,8 +139,7 @@ private void handleTCPPacket(ByteBuffer clientPacketData, IPv4Header ipHeader) t
 		int destinationPort = tcpheader.getDestinationPort();
 
 		if(tcpheader.isSYN()) {
-			//3-way handshake + create new session
-			//set windows size and scale, set reply time in options
+			// 3-way handshake + create new session
 			replySynAck(ipHeader,tcpheader);
 		} else if(tcpheader.isACK()) {
 			String key = Session.getSessionKey(destinationIP, destinationPort, sourceIP, sourcePort);
@@ -196,7 +195,7 @@ private void handleTCPPacket(ByteBuffer clientPacketData, IPv4Header ipHeader) t
 					resetConnection(ipHeader, tcpheader);
 				}
 
-				if (!session.isClientWindowFull() && !session.isAbortingConnection()) {
+				if (!session.isAbortingConnection()) {
 					manager.keepSessionAlive(session);
 				}
 			}
@@ -323,23 +322,24 @@ private void sendAckForDisorder(IPv4Header ipHeader, TCPHeader tcpheader, int ac
 	}
 
 	/**
-	 * acknowledge a packet and adjust the receiving window to avoid congestion.
+	 * acknowledge a packet.
 	 * @param tcpHeader TCP Header
 	 * @param session Session
 	 */
 	private void acceptAck(TCPHeader tcpHeader, Session session){
 		boolean isCorrupted = PacketUtil.isPacketCorrupted(tcpHeader);
+
 		session.setPacketCorrupted(isCorrupted);
-		if(isCorrupted){
+		if (isCorrupted) {
 			Log.e(TAG,"prev packet was corrupted, last ack# " + tcpHeader.getAckNumber());
 		}
-		if(tcpHeader.getAckNumber() > session.getSendUnack() ||
-				tcpHeader.getAckNumber() == session.getSendNext()){
+
+		if (
+			tcpHeader.getAckNumber() > session.getSendUnack() ||
+			tcpHeader.getAckNumber() == session.getSendNext()
+		) {
 			session.setAcked(true);
 
-			if(tcpHeader.getWindowSize() > 0){
-				session.setSendWindowSizeAndScale(tcpHeader.getWindowSize(), session.getSendWindowScale());
-			}
 			session.setSendUnack(tcpHeader.getAckNumber());
 			session.setRecSequence(tcpHeader.getSequenceNumber());
 			session.setTimestampReplyto(tcpHeader.getTimeStampSender());
@@ -350,6 +350,7 @@ private void acceptAck(TCPHeader tcpHeader, Session session){
 			session.setAcked(false);
 		}
 	}
+
 	/**
 	 * set connection as aborting so that background worker will close it.
 	 * @param ip IP
@@ -386,9 +387,6 @@ private void replySynAck(IPv4Header ip, TCPHeader tcp){
 			return;
 
 		synchronized (session) {
-			int windowScaleFactor = (int) Math.pow(2, tcpheader.getWindowScale());
-			session.setSendWindowSizeAndScale(tcpheader.getWindowSize(), windowScaleFactor);
-			Log.d(TAG, "send-window size: " + session.getSendWindow());
 			session.setMaxSegmentSize(tcpheader.getMaxSegmentSize());
 			session.setSendUnack(tcpheader.getSequenceNumber());
 			session.setSendNext(tcpheader.getSequenceNumber() + 1);

app/src/main/java/tech/httptoolkit/android/vpn/socket/SocketChannelReader.java

@@ -95,22 +95,17 @@ private void readTCP(@NonNull Session session) {
 
 		try {
 			do {
-				if(!session.isClientWindowFull()) {
-					len = channel.read(buffer);
-					if(len > 0) { //-1 mean it reach the end of stream
-						sendToRequester(buffer, len, session);
-						buffer.clear();
-					} else if(len == -1) {
-						Log.d(TAG,"End of data from remote server, will send FIN to client");
-						Log.d(TAG,"send FIN to: " + session);
-						sendFin(session);
-						session.setAbortingConnection(true);
-					}
-				} else {
-					Log.e(TAG,"*** client window is full, now pause for " + session);
-					break;
+				len = channel.read(buffer);
+				if (len > 0) { //-1 mean it reach the end of stream
+					sendToRequester(buffer, len, session);
+					buffer.clear();
+				} else if (len == -1) {
+					Log.d(TAG,"End of data from remote server, will send FIN to client");
+					Log.d(TAG,"send FIN to: " + session);
+					sendFin(session);
+					session.setAbortingConnection(true);
 				}
-			} while(len > 0);
+			} while (len > 0);
 		}catch(NotYetConnectedException e){
 			Log.e(TAG,"socket not connected");
 		}catch(ClosedByInterruptException e){
@@ -148,7 +143,7 @@ private void sendToRequester(ByteBuffer buffer, int dataSize, @NonNull Session s
 	 * @param session Session
 	 */
 	private void pushDataToClient(@NonNull Session session){
-		if(!session.hasReceivedData()){
+		if (!session.hasReceivedData()) {
 			//no data to send
 			Log.d(TAG,"no data for vpn client");
 		}