diff --git a/lib/protocol/http2/connection.rb b/lib/protocol/http2/connection.rb
index 69bdaea..88b42f4 100644
--- a/lib/protocol/http2/connection.rb
+++ b/lib/protocol/http2/connection.rb
@@ -143,9 +143,14 @@ def ignore_frame?(frame)
 				end
 			end
 			
+			def synchronize
+				yield
+			end
+			
 			# Reads one frame from the network and processes. Processing the frame updates the state of the connection and related streams. If the frame triggers an error, e.g. a protocol error, the connection will typically emit a goaway frame and re-raise the exception. You should continue processing frames until the underlying connection is closed.
 			def read_frame
 				frame = @framer.read_frame(@local_settings.maximum_frame_size)
+				
 				# puts "#{self.class} #{@state} read_frame: class=#{frame.class} stream_id=#{frame.stream_id} flags=#{frame.flags} length=#{frame.length} (remote_stream_id=#{@remote_stream_id})"
 				# puts "Windows: local_window=#{@local_window.inspect}; remote_window=#{@remote_window.inspect}"
 				
@@ -207,12 +212,18 @@ def receive_goaway(frame)
 			end
 			
 			def write_frame(frame)
-				@framer.write_frame(frame)
+				synchronize do
+					@framer.write_frame(frame)
+					@framer.flush
+				end
 			end
 			
 			def write_frames
 				if @framer
-					yield @framer
+					synchronize do
+						yield @framer
+						@framer.flush
+					end
 				else
 					raise EOFError, "Connection closed!"
 				end
diff --git a/lib/protocol/http2/framer.rb b/lib/protocol/http2/framer.rb
index 6c5bcc7..a0032fd 100644
--- a/lib/protocol/http2/framer.rb
+++ b/lib/protocol/http2/framer.rb
@@ -41,6 +41,10 @@ def initialize(stream, frames = FRAMES)
 				@frames = frames
 			end
 			
+			def flush
+				@stream.flush
+			end
+			
 			def close
 				@stream.close
 			end
@@ -69,7 +73,7 @@ def read_frame(maximum_frame_size = MAXIMUM_ALLOWED_FRAME_SIZE)
 				# Read the header:
 				length, type, flags, stream_id = read_header
 				
-				# Async.logger.debug(self) {"read_frame: length=#{length} type=#{type} flags=#{flags} stream_id=#{stream_id} -> klass=#{@frames[type].inspect}"}
+				# Console.debug(self) {"read_frame: length=#{length} type=#{type} flags=#{flags} stream_id=#{stream_id} -> klass=#{@frames[type].inspect}"}
 				
 				# Allocate the frame:
 				klass = @frames[type] || Frame
@@ -78,19 +82,19 @@ def read_frame(maximum_frame_size = MAXIMUM_ALLOWED_FRAME_SIZE)
 				# Read the payload:
 				frame.read(@stream, maximum_frame_size)
 				
-				# Async.logger.debug(self, name: "read") {frame.inspect}
+				# Console.debug(self, name: "read") {frame.inspect}
 				
 				return frame
 			end
 			
+			# Write a frame to the underlying IO.
+			# After writing one or more frames, you should call flush to ensure the frames are sent to the remote peer.
+			# @parameter frame [Frame] the frame to write.
 			def write_frame(frame)
-				# Async.logger.debug(self, name: "write") {frame.inspect}
+				# Console.debug(self, name: "write") {frame.inspect}
 				
 				frame.write(@stream)
 				
-				# Don't call @stream.flush here because it can cause significant contention if there is a semaphore around this method.
-				# @stream.flush
-				
 				return frame
 			end
 			
diff --git a/lib/protocol/http2/server.rb b/lib/protocol/http2/server.rb
index f231c68..46e3338 100644
--- a/lib/protocol/http2/server.rb
+++ b/lib/protocol/http2/server.rb
@@ -31,7 +31,9 @@ def read_connection_preface(settings = [])
 					send_settings(settings)
 					
 					read_frame do |frame|
-						raise ProtocolError, "First frame must be #{SettingsFrame}, but got #{frame.class}" unless frame.is_a? SettingsFrame
+						unless frame.is_a? SettingsFrame
+							raise ProtocolError, "First frame must be #{SettingsFrame}, but got #{frame.class}"
+						end
 					end
 				else
 					raise ProtocolError, "Cannot read connection preface in state #{@state}"
diff --git a/test/protocol/http2/framer.rb b/test/protocol/http2/framer.rb
index 67c2217..e391759 100644
--- a/test/protocol/http2/framer.rb
+++ b/test/protocol/http2/framer.rb
@@ -9,6 +9,13 @@
 	let(:stream) {StringIO.new}
 	let(:framer) {subject.new(stream)}
 	
+	with "#flush" do
+		it "flushes the underlying stream" do
+			expect(stream).to receive(:flush)
+			framer.flush
+		end
+	end
+	
 	with "#closed?" do
 		it "reports the status of the underlying stream" do
 			expect(stream).to receive(:closed?).and_return(true)
diff --git a/test/protocol/http2/server.rb b/test/protocol/http2/server.rb
index 98ec9b1..72628ab 100644
--- a/test/protocol/http2/server.rb
+++ b/test/protocol/http2/server.rb
@@ -42,6 +42,18 @@
 		expect(server.state).to be == :new
 	end
 	
+	it "fails with protocol error if first frame is not settings frame" do
+		framer.write_connection_preface
+		
+		data_frame = Protocol::HTTP2::DataFrame.new
+		data_frame.pack("Hello, World!")
+		framer.write_frame(data_frame)
+		
+		expect do
+			server.read_connection_preface
+		end.to raise_exception(Protocol::HTTP2::ProtocolError, message: be =~ /First frame must be #{Protocol::HTTP2::SettingsFrame}/)
+	end
+	
 	it "cannot read connection preface in open state" do
 		server.open!
 		expect do