diff --git a/src/Microsoft.Net.WebSockets/CommonWebSocket.cs b/src/Microsoft.Net.WebSockets/CommonWebSocket.cs
index e1795dbef2..2edd8f6e47 100644
--- a/src/Microsoft.Net.WebSockets/CommonWebSocket.cs
+++ b/src/Microsoft.Net.WebSockets/CommonWebSocket.cs
@@ -36,6 +36,7 @@ namespace Microsoft.Net.WebSockets
         private FrameHeader _frameInProgress;
         private long _frameBytesRemaining;
         private int? _firstDataOpCode;
+        private int _dataUnmaskOffset;
 
         public CommonWebSocket(Stream stream, string subProtocol, TimeSpan keepAliveInterval, int receiveBufferSize, bool maskOutput, bool useZeroMask, bool unmaskInput)
         {
@@ -242,9 +243,8 @@ namespace Microsoft.Net.WebSockets
 
             if (_unmaskInput)
             {
-                // TODO: mask alignment may be off between reads.
                 // _frameInProgress.Masked == _unmaskInput already verified
-                Utilities.MaskInPlace(_frameInProgress.MaskKey, new ArraySegment<byte>(buffer.Array, buffer.Offset, bytesToCopy));
+                Utilities.MaskInPlace(_frameInProgress.MaskKey, ref _dataUnmaskOffset, new ArraySegment<byte>(buffer.Array, buffer.Offset, bytesToCopy));
             }
 
             WebSocketReceiveResult result;
@@ -257,6 +257,7 @@ namespace Microsoft.Net.WebSockets
                     _firstDataOpCode = null;
                 }
                 _frameInProgress = null;
+                _dataUnmaskOffset = 0;
             }
             else
             {
diff --git a/src/Microsoft.Net.WebSockets/Utilities.cs b/src/Microsoft.Net.WebSockets/Utilities.cs
index a4fdcfda4b..c478b4140e 100644
--- a/src/Microsoft.Net.WebSockets/Utilities.cs
+++ b/src/Microsoft.Net.WebSockets/Utilities.cs
@@ -16,8 +16,13 @@ namespace Microsoft.Net.WebSockets
             return frame;
         }
 
-        // Un/Masks the data in place
         public static void MaskInPlace(int mask, ArraySegment<byte> data)
+        {
+            int maskOffset = 0;
+            MaskInPlace(mask, ref maskOffset, data);
+        }
+
+        public static void MaskInPlace(int mask, ref int maskOffset, ArraySegment<byte> data)
         {
             if (mask == 0)
             {
@@ -31,7 +36,6 @@ namespace Microsoft.Net.WebSockets
                 (byte)(mask >> 8),
                 (byte)mask,
             };
-            int maskOffset = 0;
 
             int end = data.Offset + data.Count;
             for (int i = data.Offset; i < end; i++)
diff --git a/test/Microsoft.Net.WebSockets.Test/BufferStream.cs b/test/Microsoft.Net.WebSockets.Test/BufferStream.cs
new file mode 100644
index 0000000000..de2c308f14
--- /dev/null
+++ b/test/Microsoft.Net.WebSockets.Test/BufferStream.cs
@@ -0,0 +1,337 @@
+// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Concurrent;
+using System.Diagnostics.CodeAnalysis;
+using System.Diagnostics.Contracts;
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Microsoft.Net.WebSockets.Test
+{
+    // This steam accepts writes from one side, buffers them internally, and returns the data via Reads
+    // when requested on the other side.
+    public class BufferStream : Stream
+    {
+        private bool _disposed;
+        private bool _aborted;
+        private Exception _abortException;
+        private ConcurrentQueue<byte[]> _bufferedData;
+        private ArraySegment<byte> _topBuffer;
+        private SemaphoreSlim _readLock;
+        private SemaphoreSlim _writeLock;
+        private TaskCompletionSource<object> _readWaitingForData;
+        
+        internal BufferStream()
+        {
+            _readLock = new SemaphoreSlim(1, 1);
+            _writeLock = new SemaphoreSlim(1, 1);
+            _bufferedData = new ConcurrentQueue<byte[]>();
+            _readWaitingForData = new TaskCompletionSource<object>();
+        }
+
+        public override bool CanRead
+        {
+            get { return true; }
+        }
+
+        public override bool CanSeek
+        {
+            get { return false; }
+        }
+
+        public override bool CanWrite
+        {
+            get { return true; }
+        }
+
+        #region NotSupported
+
+        public override long Length
+        {
+            get { throw new NotSupportedException(); }
+        }
+
+        public override long Position
+        {
+            get { throw new NotSupportedException(); }
+            set { throw new NotSupportedException(); }
+        }
+
+        public override long Seek(long offset, SeekOrigin origin)
+        {
+            throw new NotSupportedException();
+        }
+
+        public override void SetLength(long value)
+        {
+            throw new NotSupportedException();
+        }
+
+        #endregion NotSupported
+
+        public override void Flush()
+        {
+            CheckDisposed();
+            // TODO: Wait for data to drain?
+        }
+
+        public override Task FlushAsync(CancellationToken cancellationToken)
+        {
+            if (cancellationToken.IsCancellationRequested)
+            {
+                TaskCompletionSource<object> tcs = new TaskCompletionSource<object>();
+                tcs.TrySetCanceled();
+                return tcs.Task;
+            }
+
+            Flush();
+
+            // TODO: Wait for data to drain?
+
+            return Task.FromResult(0);
+        }
+
+        public override int Read(byte[] buffer, int offset, int count)
+        {
+            VerifyBuffer(buffer, offset, count, allowEmpty: false);
+            _readLock.Wait();
+            try
+            {
+                int totalRead = 0;
+                do
+                {
+                    // Don't drain buffered data when signaling an abort.
+                    CheckAborted();
+                    if (_topBuffer.Count <= 0)
+                    {
+                        byte[] topBuffer = null;
+                        while (!_bufferedData.TryDequeue(out topBuffer))
+                        {
+                            if (_disposed)
+                            {
+                                CheckAborted();
+                                // Graceful close
+                                return totalRead;
+                            }
+                            WaitForDataAsync().Wait();
+                        }
+                        _topBuffer = new ArraySegment<byte>(topBuffer);
+                    }
+                    int actualCount = Math.Min(count, _topBuffer.Count);
+                    Buffer.BlockCopy(_topBuffer.Array, _topBuffer.Offset, buffer, offset, actualCount);
+                    _topBuffer = new ArraySegment<byte>(_topBuffer.Array,
+                        _topBuffer.Offset + actualCount,
+                        _topBuffer.Count - actualCount);
+                    totalRead += actualCount;
+                    offset += actualCount;
+                    count -= actualCount;
+                }
+                while (count > 0 && (_topBuffer.Count > 0 || _bufferedData.Count > 0));
+                // Keep reading while there is more data available and we have more space to put it in.
+                return totalRead;
+            }
+            finally
+            {
+                _readLock.Release();
+            }
+        }
+
+        public override IAsyncResult BeginRead(byte[] buffer, int offset, int count, AsyncCallback callback, object state)
+        {
+            // TODO: This option doesn't preserve the state object.
+            // return ReadAsync(buffer, offset, count);
+            return base.BeginRead(buffer, offset, count, callback, state);
+        }
+
+        public override int EndRead(IAsyncResult asyncResult)
+        {
+            // return ((Task<int>)asyncResult).Result;
+            return base.EndRead(asyncResult);
+        }
+
+        public async override Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+        {
+            VerifyBuffer(buffer, offset, count, allowEmpty: false);
+            CancellationTokenRegistration registration = cancellationToken.Register(Abort);
+            await _readLock.WaitAsync(cancellationToken);
+            try
+            {
+                int totalRead = 0;
+                do
+                {
+                    // Don't drained buffered data on abort.
+                    CheckAborted();
+                    if (_topBuffer.Count <= 0)
+                    {
+                        byte[] topBuffer = null;
+                        while (!_bufferedData.TryDequeue(out topBuffer))
+                        {
+                            if (_disposed)
+                            {
+                                CheckAborted();
+                                // Graceful close
+                                return totalRead;
+                            }
+                            await WaitForDataAsync();
+                        }
+                        _topBuffer = new ArraySegment<byte>(topBuffer);
+                    }
+                    int actualCount = Math.Min(count, _topBuffer.Count);
+                    Buffer.BlockCopy(_topBuffer.Array, _topBuffer.Offset, buffer, offset, actualCount);
+                    _topBuffer = new ArraySegment<byte>(_topBuffer.Array,
+                        _topBuffer.Offset + actualCount,
+                        _topBuffer.Count - actualCount);
+                    totalRead += actualCount;
+                    offset += actualCount;
+                    count -= actualCount;
+                }
+                while (count > 0 && (_topBuffer.Count > 0 || _bufferedData.Count > 0));
+                // Keep reading while there is more data available and we have more space to put it in.
+                return totalRead;
+            }
+            finally
+            {
+                registration.Dispose();
+                _readLock.Release();
+            }
+        }
+
+        // Write with count 0 will still trigger OnFirstWrite
+        public override void Write(byte[] buffer, int offset, int count)
+        {
+            VerifyBuffer(buffer, offset, count, allowEmpty: true);
+            CheckDisposed();
+
+            _writeLock.Wait();
+            try
+            {
+                if (count == 0)
+                {
+                    return;
+                }
+                // Copies are necessary because we don't know what the caller is going to do with the buffer afterwards.
+                byte[] internalBuffer = new byte[count];
+                Buffer.BlockCopy(buffer, offset, internalBuffer, 0, count);
+                _bufferedData.Enqueue(internalBuffer);
+
+                SignalDataAvailable();
+            }
+            finally
+            {
+                _writeLock.Release();
+            }
+        }
+
+        public override IAsyncResult BeginWrite(byte[] buffer, int offset, int count, AsyncCallback callback, object state)
+        {
+            Write(buffer, offset, count);
+            TaskCompletionSource<object> tcs = new TaskCompletionSource<object>(state);
+            tcs.TrySetResult(null);
+            IAsyncResult result = tcs.Task;
+            if (callback != null)
+            {
+                callback(result);
+            }
+            return result;
+        }
+
+        public override void EndWrite(IAsyncResult asyncResult)
+        {
+        }
+
+        public override Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+        {
+            VerifyBuffer(buffer, offset, count, allowEmpty: true);
+            if (cancellationToken.IsCancellationRequested)
+            {
+                TaskCompletionSource<object> tcs = new TaskCompletionSource<object>();
+                tcs.TrySetCanceled();
+                return tcs.Task;
+            }
+
+            Write(buffer, offset, count);
+            return Task.FromResult<object>(null);
+        }
+
+        private static void VerifyBuffer(byte[] buffer, int offset, int count, bool allowEmpty)
+        {
+            if (buffer == null)
+            {
+                throw new ArgumentNullException("buffer");
+            }
+            if (offset < 0 || offset > buffer.Length)
+            {
+                throw new ArgumentOutOfRangeException("offset", offset, string.Empty);
+            }
+            if (count < 0 || count > buffer.Length - offset
+                || (!allowEmpty && count == 0))
+            {
+                throw new ArgumentOutOfRangeException("count", count, string.Empty);
+            }
+        }
+
+        private void SignalDataAvailable()
+        {
+            // Dispatch, as TrySetResult will synchronously execute the waiters callback and block our Write.
+            Task.Factory.StartNew(() => _readWaitingForData.TrySetResult(null));
+        }
+
+        private Task WaitForDataAsync()
+        {
+            _readWaitingForData = new TaskCompletionSource<object>();
+
+            if (!_bufferedData.IsEmpty || _disposed)
+            {
+                // Race, data could have arrived before we created the TCS.
+                _readWaitingForData.TrySetResult(null);
+            }
+
+            return _readWaitingForData.Task;
+        }
+
+        internal void Abort()
+        {
+            Abort(new OperationCanceledException());
+        }
+
+        internal void Abort(Exception innerException)
+        {
+            Contract.Requires(innerException != null);
+            _aborted = true;
+            _abortException = innerException;
+            Dispose();
+        }
+
+        private void CheckAborted()
+        {
+            if (_aborted)
+            {
+                throw new IOException(string.Empty, _abortException);
+            }
+        }
+
+        [SuppressMessage("Microsoft.Usage", "CA2213:DisposableFieldsShouldBeDisposed", MessageId = "_writeLock", Justification = "ODEs from the locks would mask IOEs from abort.")]
+        [SuppressMessage("Microsoft.Usage", "CA2213:DisposableFieldsShouldBeDisposed", MessageId = "_readLock", Justification = "Data can still be read unless we get aborted.")]
+        protected override void Dispose(bool disposing)
+        {
+            if (disposing)
+            {
+                // Throw for further writes, but not reads.  Allow reads to drain the buffered data and then return 0 for further reads.
+                _disposed = true;
+                _readWaitingForData.TrySetResult(null);
+            }
+
+            base.Dispose(disposing);
+        }
+
+        private void CheckDisposed()
+        {
+            if (_disposed)
+            {
+                throw new ObjectDisposedException(GetType().FullName);
+            }
+        }
+    }
+}
diff --git a/test/Microsoft.Net.WebSockets.Test/DuplexStream.cs b/test/Microsoft.Net.WebSockets.Test/DuplexStream.cs
new file mode 100644
index 0000000000..7cd19c17f8
--- /dev/null
+++ b/test/Microsoft.Net.WebSockets.Test/DuplexStream.cs
@@ -0,0 +1,172 @@
+// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved. See License.txt in the project root for license information.
+
+using System;
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Microsoft.Net.WebSockets.Test
+{
+    // A duplex wrapper around a read and write stream.
+    public class DuplexStream : Stream
+    {
+        private readonly Stream _readStream;
+        private readonly Stream _writeStream;
+
+        public DuplexStream()
+            : this (new BufferStream(), new BufferStream())
+        {            
+        }
+
+        public DuplexStream(Stream readStream, Stream writeStream)
+        {
+            _readStream = readStream;
+            _writeStream = writeStream;
+        }
+
+        public DuplexStream CreateReverseDuplexStream()
+        {
+            return new DuplexStream(_writeStream, _readStream);
+        }
+
+#region Properties
+
+        public override bool CanRead
+        {
+            get { return _readStream.CanRead; }
+        }
+
+        public override bool CanSeek
+        {
+            get { return false; }
+        }
+
+        public override bool CanTimeout
+        {
+            get { return _readStream.CanTimeout || _writeStream.CanTimeout; }
+        }
+
+        public override bool CanWrite
+        {
+            get { return _writeStream.CanWrite; }
+        }
+
+        public override long Length
+        {
+            get { throw new NotSupportedException(); }
+        }
+
+        public override long Position
+        {
+            get { throw new NotSupportedException(); }
+            set { throw new NotSupportedException(); }
+        }
+
+        public override int ReadTimeout
+        {
+            get { return _readStream.ReadTimeout; }
+            set { _readStream.ReadTimeout = value; }
+        }
+
+        public override int WriteTimeout
+        {
+            get { return _writeStream.WriteTimeout; }
+            set { _writeStream.WriteTimeout = value; }
+        }
+
+#endregion Properties
+
+        public override long Seek(long offset, SeekOrigin origin)
+        {
+            throw new NotSupportedException();
+        }
+
+        public override void SetLength(long value)
+        {
+            throw new NotSupportedException();
+        }
+
+#region Read
+
+        public override int Read(byte[] buffer, int offset, int count)
+        {
+            return _readStream.Read(buffer, offset, count);
+        }
+
+        public override int ReadByte()
+        {
+            return _readStream.ReadByte();
+        }
+
+        public override IAsyncResult BeginRead(byte[] buffer, int offset, int count, AsyncCallback callback, object state)
+        {
+            return _readStream.BeginRead(buffer, offset, count, callback, state);
+        }
+
+        public override int EndRead(IAsyncResult asyncResult)
+        {
+            return _readStream.EndRead(asyncResult);
+        }
+
+        public override Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+        {
+            return _readStream.ReadAsync(buffer, offset, count, cancellationToken);
+        }
+
+        public override Task CopyToAsync(Stream destination, int bufferSize, CancellationToken cancellationToken)
+        {
+            return _readStream.CopyToAsync(destination, bufferSize, cancellationToken);
+        }
+
+#endregion Read
+
+#region Write
+
+        public override void Write(byte[] buffer, int offset, int count)
+        {
+            _writeStream.Write(buffer, offset, count);
+        }
+
+        public override void WriteByte(byte value)
+        {
+            _writeStream.WriteByte(value);
+        }
+
+        public override IAsyncResult BeginWrite(byte[] buffer, int offset, int count, AsyncCallback callback, object state)
+        {
+            return _writeStream.BeginWrite(buffer, offset, count, callback, state);
+        }
+
+        public override void EndWrite(IAsyncResult asyncResult)
+        {
+            _writeStream.EndWrite(asyncResult);
+        }
+
+        public override Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+        {
+            return _writeStream.WriteAsync(buffer, offset, count, cancellationToken);
+        }
+
+        public override void Flush()
+        {
+            _writeStream.Flush();
+        }
+
+        public override Task FlushAsync(CancellationToken cancellationToken)
+        {
+            return _writeStream.FlushAsync(cancellationToken);
+        }
+
+#endregion Write
+
+        protected override void Dispose(bool disposing)
+        {
+            if (disposing)
+            {
+                _readStream.Dispose();
+                _writeStream.Dispose();
+            }
+            base.Dispose(disposing);
+        }
+    }
+}
diff --git a/test/Microsoft.Net.WebSockets.Test/DuplexTests.cs b/test/Microsoft.Net.WebSockets.Test/DuplexTests.cs
new file mode 100644
index 0000000000..117c233e88
--- /dev/null
+++ b/test/Microsoft.Net.WebSockets.Test/DuplexTests.cs
@@ -0,0 +1,63 @@
+using System;
+using System.Net.WebSockets;
+using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
+using Xunit;
+
+namespace Microsoft.Net.WebSockets.Test
+{
+    public class DuplexTests
+    {
+        [Fact]
+        public async Task SendAndReceive()
+        {
+            DuplexStream serverStream = new DuplexStream();
+            DuplexStream clientStream = serverStream.CreateReverseDuplexStream();
+
+            WebSocket serverWebSocket = CommonWebSocket.CreateServerWebSocket(serverStream, null, TimeSpan.FromMinutes(2), 1024);
+            WebSocket clientWebSocket = CommonWebSocket.CreateClientWebSocket(clientStream, null, TimeSpan.FromMinutes(2), 1024, false);
+
+            byte[] clientBuffer = Encoding.ASCII.GetBytes("abcdefghijklmnopqrstuvwxyz");
+            byte[] serverBuffer = new byte[clientBuffer.Length];
+
+            await clientWebSocket.SendAsync(new ArraySegment<byte>(clientBuffer), WebSocketMessageType.Text, true, CancellationToken.None);
+            WebSocketReceiveResult serverResult = await serverWebSocket.ReceiveAsync(new ArraySegment<byte>(serverBuffer), CancellationToken.None);
+            Assert.True(serverResult.EndOfMessage);
+            Assert.Equal(clientBuffer.Length, serverResult.Count);
+            Assert.Equal(WebSocketMessageType.Text, serverResult.MessageType);
+            Assert.Equal(clientBuffer, serverBuffer);
+        }
+
+        [Fact]
+        // Tests server unmasking with offset masks
+        public async Task ServerReceiveOffsetData()
+        {
+            DuplexStream serverStream = new DuplexStream();
+            DuplexStream clientStream = serverStream.CreateReverseDuplexStream();
+
+            WebSocket serverWebSocket = CommonWebSocket.CreateServerWebSocket(serverStream, null, TimeSpan.FromMinutes(2), 1024);
+            WebSocket clientWebSocket = CommonWebSocket.CreateClientWebSocket(clientStream, null, TimeSpan.FromMinutes(2), 1024, false);
+
+            byte[] clientBuffer = Encoding.ASCII.GetBytes("abcdefghijklmnopqrstuvwxyz");
+            byte[] serverBuffer = new byte[clientBuffer.Length];
+
+            await clientWebSocket.SendAsync(new ArraySegment<byte>(clientBuffer), WebSocketMessageType.Text, true, CancellationToken.None);
+            WebSocketReceiveResult serverResult = await serverWebSocket.ReceiveAsync(new ArraySegment<byte>(serverBuffer, 0, 3), CancellationToken.None);
+            Assert.False(serverResult.EndOfMessage);
+            Assert.Equal(3, serverResult.Count);
+            Assert.Equal(WebSocketMessageType.Text, serverResult.MessageType);
+
+            serverResult = await serverWebSocket.ReceiveAsync(new ArraySegment<byte>(serverBuffer, 3, 10), CancellationToken.None);
+            Assert.False(serverResult.EndOfMessage);
+            Assert.Equal(10, serverResult.Count);
+            Assert.Equal(WebSocketMessageType.Text, serverResult.MessageType);
+            
+            serverResult = await serverWebSocket.ReceiveAsync(new ArraySegment<byte>(serverBuffer, 13, 13), CancellationToken.None);
+            Assert.True(serverResult.EndOfMessage);
+            Assert.Equal(13, serverResult.Count);
+            Assert.Equal(WebSocketMessageType.Text, serverResult.MessageType);
+            Assert.Equal(clientBuffer, serverBuffer);
+        }
+    }
+}
diff --git a/test/Microsoft.Net.WebSockets.Test/Microsoft.Net.WebSockets.Test.csproj b/test/Microsoft.Net.WebSockets.Test/Microsoft.Net.WebSockets.Test.csproj
index fcfd99f1a4..25f16b963c 100644
--- a/test/Microsoft.Net.WebSockets.Test/Microsoft.Net.WebSockets.Test.csproj
+++ b/test/Microsoft.Net.WebSockets.Test/Microsoft.Net.WebSockets.Test.csproj
@@ -45,6 +45,9 @@
     </Reference>
   </ItemGroup>
   <ItemGroup>
+    <Compile Include="DuplexTests.cs" />
+    <Compile Include="BufferStream.cs" />
+    <Compile Include="DuplexStream.cs" />
     <Compile Include="UtilitiesTests.cs" />
     <Compile Include="WebSocketClientTests.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />