diff --git a/src/main/java/net/schmizz/sshj/connection/channel/ChannelOutputStream.java b/src/main/java/net/schmizz/sshj/connection/channel/ChannelOutputStream.java
index d79d2c12..be6f7e44 100644
--- a/src/main/java/net/schmizz/sshj/connection/channel/ChannelOutputStream.java
+++ b/src/main/java/net/schmizz/sshj/connection/channel/ChannelOutputStream.java
@@ -35,12 +35,14 @@
  */
 package net.schmizz.sshj.connection.channel;
 
+import net.schmizz.sshj.common.Buffer;
 import net.schmizz.sshj.common.ErrorNotifiable;
 import net.schmizz.sshj.common.Message;
 import net.schmizz.sshj.common.SSHException;
 import net.schmizz.sshj.common.SSHPacket;
 import net.schmizz.sshj.connection.ConnectionException;
 import net.schmizz.sshj.transport.Transport;
+import net.schmizz.sshj.transport.TransportException;
 
 import java.io.IOException;
 import java.io.OutputStream;
@@ -56,26 +58,92 @@ public final class ChannelOutputStream
     private final Channel chan;
     private final Transport trans;
     private final Window.Remote win;
-    private final SSHPacket buffer = new SSHPacket();
+
+    private final DataBuffer buffer = new DataBuffer();
     private final byte[] b = new byte[1];
-    private int bufferLength;
+
     private boolean closed;
     private SSHException error;
 
+    private final class DataBuffer {
+
+        private final int headerOffset;
+        private final int dataOffset;
+
+        private final SSHPacket packet = new SSHPacket(Message.CHANNEL_DATA);
+        private final Buffer.PlainBuffer leftOvers = new Buffer.PlainBuffer();
+
+        DataBuffer() {
+            headerOffset = packet.rpos();
+            packet.putUInt32(0); // recipient
+            packet.putUInt32(0); // data length
+            dataOffset = packet.wpos();
+        }
+
+        int write(byte[] data, int off, int len)
+                throws TransportException, ConnectionException {
+            final int bufferSize = packet.wpos() - dataOffset;
+            if (bufferSize >= win.getMaxPacketSize()) {
+                flush(bufferSize);
+                return 0;
+            } else {
+                final int n = Math.min(len - off, win.getMaxPacketSize() - bufferSize);
+                packet.putRawBytes(data, off, n);
+                return n;
+            }
+        }
+
+        void flush()
+                throws TransportException, ConnectionException {
+            flush(packet.wpos() - dataOffset);
+        }
+
+        void flush(int bufferSize)
+                throws TransportException, ConnectionException {
+            while (bufferSize > 0) {
+
+                int remoteWindowSize = win.getSize();
+                if (remoteWindowSize == 0)
+                    remoteWindowSize = win.awaitExpansion(remoteWindowSize);
+
+                // We can only write the min. of
+                // a) how much data we have
+                // b) the max packet size
+                // c) what the current window size will allow
+                final int writeNow = Math.min(bufferSize, Math.min(win.getMaxPacketSize(), remoteWindowSize));
+
+                packet.wpos(headerOffset);
+                packet.putMessageID(Message.CHANNEL_DATA);
+                packet.putUInt32(chan.getRecipient());
+                packet.putUInt32(writeNow);
+                packet.wpos(dataOffset + writeNow);
+
+                final int leftOverBytes = bufferSize - writeNow;
+                if (leftOverBytes > 0) {
+                    leftOvers.putRawBytes(packet.array(), packet.wpos(), leftOverBytes);
+                }
+
+                trans.write(packet);
+                win.consume(writeNow);
+
+                packet.rpos(headerOffset);
+                packet.wpos(dataOffset);
+
+                if (leftOverBytes > 0) {
+                    packet.putBuffer(leftOvers);
+                    leftOvers.clear();
+                }
+
+                bufferSize = leftOverBytes;
+            }
+        }
+
+    }
+
     public ChannelOutputStream(Channel chan, Transport trans, Window.Remote win) {
         this.chan = chan;
         this.trans = trans;
         this.win = win;
-        prepBuffer();
-    }
-
-    private void prepBuffer() {
-        bufferLength = 0;
-        buffer.rpos(5);
-        buffer.wpos(5);
-        buffer.putMessageID(Message.CHANNEL_DATA);
-        buffer.putUInt32(0); // meant to be recipient
-        buffer.putUInt32(0); // meant to be data length
     }
 
     @Override
@@ -86,19 +154,13 @@ public final class ChannelOutputStream
     }
 
     @Override
-    public synchronized void write(byte[] data, int off, int len)
+    public synchronized void write(final byte[] data, int off, int len)
             throws IOException {
         checkClose();
         while (len > 0) {
-            final int x = Math.min(len, win.getMaxPacketSize() - bufferLength);
-            if (x <= 0) {
-                flush();
-                continue;
-            }
-            buffer.putRawBytes(data, off, x);
-            bufferLength += x;
-            off += x;
-            len -= x;
+            final int n = buffer.write(data, off, len);
+            off += n;
+            len -= n;
         }
     }
 
@@ -107,55 +169,44 @@ public final class ChannelOutputStream
         this.error = error;
     }
 
-    private synchronized void checkClose()
+    private void checkClose()
             throws SSHException {
-        if (closed)
+        if (closed) {
             if (error != null)
                 throw error;
             else
                 throw new ConnectionException("Stream closed");
+        }
     }
 
     @Override
     public synchronized void close()
             throws IOException {
-        if (!closed)
+        if (!closed) {
             try {
-                flush();
+                buffer.flush();
                 chan.sendEOF();
             } finally {
                 setClosed();
             }
+        }
     }
 
     public synchronized void setClosed() {
         closed = true;
     }
 
+    /**
+     * Send all data currently buffered. If window space is exhausted in the process, this will block
+     * until it is expanded by the server.
+     *
+     * @throws IOException
+     */
     @Override
     public synchronized void flush()
             throws IOException {
         checkClose();
-
-        if (bufferLength <= 0) // No data to send
-            return;
-
-        putRecipientAndLength();
-
-        try {
-            win.waitAndConsume(bufferLength);
-            trans.write(buffer);
-        } finally {
-            prepBuffer();
-        }
-    }
-
-    private void putRecipientAndLength() {
-        final int origPos = buffer.wpos();
-        buffer.wpos(6);
-        buffer.putUInt32(chan.getRecipient());
-        buffer.putUInt32(bufferLength);
-        buffer.wpos(origPos);
+        buffer.flush();
     }
 
     @Override
diff --git a/src/main/java/net/schmizz/sshj/connection/channel/Window.java b/src/main/java/net/schmizz/sshj/connection/channel/Window.java
index a967746f..376ee993 100644
--- a/src/main/java/net/schmizz/sshj/connection/channel/Window.java
+++ b/src/main/java/net/schmizz/sshj/connection/channel/Window.java
@@ -48,7 +48,9 @@ public abstract class Window {
     }
 
     public int getSize() {
-        return size;
+        synchronized (lock) {
+            return size;
+        }
     }
 
     public void consume(int dec)
@@ -74,18 +76,18 @@ public abstract class Window {
             super(initialWinSize, maxPacketSize);
         }
 
-        public void waitAndConsume(int howMuch)
+        public int awaitExpansion(int was)
                 throws ConnectionException {
             synchronized (lock) {
-                while (size < howMuch) {
-                    log.debug("Waiting, need window space for {} bytes", howMuch);
+                while (size <= was) {
+                    log.debug("Waiting, need size to grow from {} bytes", was);
                     try {
                         lock.wait();
                     } catch (InterruptedException ie) {
                         throw new ConnectionException(ie);
                     }
                 }
-                consume(howMuch);
+                return size;
             }
         }