diff -r f1f4b8cd0192 -r cd41f34e548c src/java.net.http/share/classes/jdk/internal/net/http/Http2Connection.java
--- a/src/java.net.http/share/classes/jdk/internal/net/http/Http2Connection.java	Wed Jul 04 16:54:56 2018 +0200
+++ b/src/java.net.http/share/classes/jdk/internal/net/http/Http2Connection.java	Wed Jul 04 16:16:24 2018 +0100
@@ -673,7 +673,11 @@
         client2.deleteConnection(this);
         List<Stream<?>> c = new LinkedList<>(streams.values());
         for (Stream<?> s : c) {
-            s.connectionClosing(t);
+            try {
+                s.connectionClosing(t);
+            } catch (Throwable e) {
+                Log.logError("Failed to close stream {0}: {1}", s.streamid, e);
+            }
         }
         connection.close();
     }
@@ -738,6 +742,9 @@
                 }
 
                 if (!(frame instanceof ResetFrame)) {
+                    if (frame instanceof DataFrame) {
+                        dropDataFrame((DataFrame)frame);
+                    }
                     if (isServerInitiatedStream(streamid)) {
                         if (streamid < nextPushStream) {
                             // trailing data on a cancelled push promise stream,
@@ -776,6 +783,27 @@
         }
     }
 
+    final void dropDataFrame(DataFrame df) {
+        if (closed) return;
+        if (debug.on()) {
+            debug.log("Dropping data frame for stream %d (%d payload bytes)",
+                    df.streamid(), df.payloadLength());
+        }
+        ensureWindowUpdated(df);
+    }
+
+    final void ensureWindowUpdated(DataFrame df) {
+        try {
+            if (closed) return;
+            int length = df.payloadLength();
+            if (length > 0) {
+                windowUpdater.update(length);
+            }
+        } catch(Throwable t) {
+            Log.logError("Unexpected exception while updating window: {0}", (Object)t);
+        }
+    }
+
     private <T> void handlePushPromise(Stream<T> parent, PushPromiseFrame pp)
         throws IOException
     {
@@ -984,7 +1012,6 @@
                      connection.channel().getLocalAddress(),
                      connection.address());
         SettingsFrame sf = new SettingsFrame(clientSettings);
-        int initialWindowSize = sf.getParameter(INITIAL_WINDOW_SIZE);
         ByteBuffer buf = framesEncoder.encodeConnectionPreface(PREFACE_BYTES, sf);
         Log.logFrames(sf, "OUT");
         // send preface bytes and SettingsFrame together
@@ -997,9 +1024,20 @@
         Log.logTrace("Settings Frame sent");
 
         // send a Window update for the receive buffer we are using
-        // minus the initial 64 K specified in protocol
-        final int len = windowUpdater.initialWindowSize - initialWindowSize;
-        if (len > 0) {
+        // minus the initial 64 K -1 specified in protocol:
+        // RFC 7540, Section 6.9.2:
+        // "[...] the connection flow-control window is set to the default
+        // initial window size until a WINDOW_UPDATE frame is received."
+        //
+        // Note that the default initial window size, not to be confused
+        // with the initial window size, is defined by RFC 7540 as
+        // 64K -1.
+        final int len = windowUpdater.initialWindowSize - DEFAULT_INITIAL_WINDOW_SIZE;
+        if (len != 0) {
+            if (Log.channel()) {
+                Log.logChannel("Sending initial connection window update frame: {0} ({1} - {2})",
+                        len, windowUpdater.initialWindowSize, DEFAULT_INITIAL_WINDOW_SIZE);
+            }
             windowUpdater.sendWindowUpdate(len);
         }
         // there will be an ACK to the windows update - which should
@@ -1132,6 +1170,7 @@
 
     private Stream<?> registerNewStream(OutgoingHeaders<Stream<?>> oh) {
         Stream<?> stream = oh.getAttachment();
+        assert stream.streamid == 0;
         int streamid = nextstreamid;
         nextstreamid += 2;
         stream.registerStream(streamid);