Last couple of weeks I’m trying to build high-performance consumption of binary protocol through the browser. The protocol is running over TCP. In the perfect world I’d be talking with the remote TCP server, through TCP sockets, connecting directly from the client-side JavaScript and consuming the received binary data.

Because of limitations of the client-side API, JavaScript doesn’t has access to plain TCP sockets, there’s no full happiness. The second best option would be to use intermediate proxy, which:

• Establishes connection with the client (our client-side JavaScript)
• Establishes TCP connection with the remote TCP server
• Forwards each message from the client to the TCP server and vice versa

History

Dark Ages

Before the era of the duplex network communication in JavaScript (i.e. before WebSockets, I don’t consider long polling as solution), the flow would be something like:

• Make XMLHttpRequest to a back-end service (our proxy)
• Translate the request in the back-end service from textual data to binary data, if required
• Forward the request to the target TCP server
• Receive a response by the target server
• Translates the response to textual data, if required
• Response to the XMLHttpRequest

Which wasn’t fun, at all. You initiate new POST/GET request for each client request and in case “Keep-Alive” is not set you might need to open TCP socket for each HTTP connection.

Renaissance

After the era of the WebSockets, but without binary data support, we would:

• Send textual data to the remote WebSocket
• Translate the received data by the proxy
• Forward the data to the TCP server
• Receive a response by the remote TCP server
• Translate the response to textual data, if required
• Response to the client’s message

I haven’t included the initial WebSocket handshake, since there isn’t any significant overhead by it, because it is initiated only a single time.

The performance of the second case is much better by a few reasons:

• No need to send additional HTTP headers, which in some cases are huge
• Reuse a single TCP connection for each message by the client
• Have duplex connection, which allows you to receive push notifications

Anyway, there’s still place of improvement. In the perfect case we want to talk directly to the remote TCP server, without translation of the protocol.

Nowadays, most browsers (even IE10), support transfer of binary data over WebSockets. This allows us to skip two more steps (translate the message sent by the client and response received by the TCP server) and reduce the bandwidth usage (since encoding to base64 will increase the size of the message with around 30%).

Using binary WebSockets, we got something like:

• Send binary data to the proxy’s WebSocket
• Forward the data to the TCP server
• Receive response by the TCP server
• Forward the data to our client

In the last case our proxy will look something like:

'use strict';

var WebSocketServer = require('ws').Server;
var wss = new WebSocketServer({ port: 8081 });
var net = require('net');

wss.on('connection', function (ws) {
  var client;
  ws.on('message', function (msg) {
    console.log('Message received');
    try {
      msg = JSON.parse(msg);
    } catch (e) {
      if (client) {
        client.write(msg);
      }
    }
    if (msg.type === 'handshake') {
      client = net.createConnection(msg.port, msg.host, function () {
        console.log('Connected');
        ws.send(JSON.stringify({
          type: 'handshake',
          status: 'success'
        }));
      });
      client.on('data', function (data) {
        console.log('data');
        ws.send(data, { binary: true });
      });
      client.on('end', function () {
        console.log('end');
        ws.close();
      });
    }
  });
  ws.on('close', function () {
    if (client) {
      client.end();
    }
  });
});

console.log('Listening on', 8081);

And our high-level architecture will look like:

There’s a place for improvements in the code above but you get the basic idea - receive binary data and forward it to the remote TCP server, after the handshake was initiated.

Processing binary data in JavaScript

There are a few primitives we’re going to use: ArrayBuffer, TypedArray and Blob.

The ArrayBuffer object is used to represent a generic, fixed-length raw binary data buffer. You can not directly manipulate the contents of an ArrayBuffer; instead, you create one of the typed array objects or a DataView object which represents the buffer in a specific format, and use that to read and write the contents of the buffer.

The TypedArrays allow us to process words (with length 8, 16, 32 bits) in order to handle the binary messages received by the proxy. In order to tell the WebSockets connection, we want to talk in binary with the proxy we need to:

var socket = new WebSocket('ws://127.0.0.1:8081');
socket.binaryType = 'arraybuffer';

The two possible values for binaryType, of the WebSocket, are arraybuffer and blob. In most cases arraybuffer will be the one, which allows faster processing since it can be used with the synchronous API of the DataView. In case of large pieces of binary data preferable is the blob binary type.

So how would we process the following example, using DataView and arraybuffer:

2      U16      framebuffer-width
2      U16      framebuffer-height
16 PIXEL_FORMAT server-pixel-format
4      U32      name-length

connection.onmessage = function (e) {
  var data = e.data;
  var dv = new DataView(data);
  var width = dv.getUint16(0);
  var height = dv.getUint16(2);
  var format = getPixelFormat(dv);
  var len = dv.getUint32(20);
  console.log('We have width: ' + width +
      'px, height: ' +
      height + 'px, name length: ' + len);
};

DataView provides interface, which allows us to read specific data type by providing given offset. For example dv.getUint32(2) will return unsigned 32 bit integer with offset 2 bytes from the beginning of the buffer.

Handling endianness

Endianness refer to the convention used to interpret the bytes making up a data word when those bytes are stored in computer memory. For the one-byte data types we don’t have any issues handling the byte order, for 16, 32 and 64 we need to do some additional work.

The TypedArray’s standard doesn’t refer to specific endianness used in them, everything depends on the underlaying machine. The usual byte ordering is little endian, but in order to prevent hard-coded values we can use the function getEndianness:

function getEndianness() {
  var a = new ArrayBuffer(4);
  var b = new Uint8Array(a);
  var c = new Uint32Array(a);
  b[0] = 0xa1;
  b[1] = 0xb2;
  b[2] = 0xc3;
  b[3] = 0xd4;
  if (c[0] === 0xd4c3b2a1) {
    return BlobReader.ENDIANNESS.LITTLE_ENDIAN;
  }
  if (c[0] === 0xa1b2c3d4) {
    return BlobReader.ENDIANNESS.BIG_ENDIAN;
  } else {
    throw new Error('Unrecognized endianness');
  }
}

Let’s take a look at the function’s implementation:

• Initially we construct an ArrayBuffer with size 4 bytes
• We create array with unsigned 8bit integers, based on the ArrayBuffer we just created
• We create array with unsigned 32bit integers, based on the same ArrayBuffer
• We assign 8bit values to all indexes in the 8bit array (0xa1, 0xb2, 0xc3, 0xd4). This is going to change the ArrayBuffer used by both Uint8Array and Uint32Array

If the bytes, in the only 32bit word in the second array, keep their initial ordering (i.e. the most significant value is the one we assigned to index 0), the machine has big endian encoding, otherwise it uses little endian.

This snippet is used in my BlobReader implementation, which you can find at GitHub. DataView handles the endianness by specifying the second argument in it’s get-methods. Value true as second argument, indicates that the data which should be read is in little endian encoding, false corresponds to big endian.

So far we can parse short binary strings with the primitives our browser provides (ArrayBuffer, TypedArrays and DataView).

Reading Blobs

As I mentioned above, when you have to deal with huge amount of data, it is much more appropriate to use Blob data instead of ArayBuffer. Blobs could be read using the FileReader API, which is asynchronous by default (always in the main execution thread). Blobs can be read synchronously when used inside Workers with FileReaderSync.

Blob has method in it’s prototype called slice. It accepts interval, as two integers, and returns “sub-blob” composed by the bytes in the interval:

var blob = new Blob([new Uint8Array([1, 2, 3, 4, 5])]);
var subBlob = blob.slice(2, 3);
var fr = new FileReader();
fr.onload = function (e) {
  console.log(new Uint8Array(e.target.result)[0]);
};
fr.readAsArrayBuffer(subBlob);

Each time you want to read specific part of the blob you need to create FileReader instance and eventually slice it. This requires a lot of additional, repetitive work. Also, when you read the code above it is not very semantically clear that you want to read the third element of the array, since there’s a lot of additional code around the construction of the FileReader and handling the onload event.

In order to simplify the process of reading Blobs I created BlobReader, which provides simple interface for reading binary large objects.

BlobReader allows you to read blobs in the following fashion:

// Blob definition
var uint8 = new Uint8Array([1, 2]);
var uint16 = new Uint16Array([3]);
var uint82 = new Uint8Array([4, 3]);
var uint32 = new Uint32Array([8]);
var blob = new Blob([uint8, uint16, uint82, uint32]);

// Reading the blob
BlobReader(blob)
.readUint8('uint8', 2)
.readUint16('uint16')
.readUint8('uint82')
.skip()
.readUint32('uint32')
.commit(function (data) {
  expect(data.uint8[0]).toBe(1);
  expect(data.uint8[1]).toBe(2);
  expect(data.uint16).toBe(3);
  expect(typeof data.uint82).toBe('number');
  expect(data.uint82).toBe(4);
  expect(data.uint32).toBe(8);
});

There are shortcut methods for reading the main data types, each of the methods accept name of the property to be read, number of words of the given size and optionally format (little or big endian). Using the property name you can access the data associated with it, as property of the object passed to the commit callback. skip allows you to skip bytes (like padding).

You can lookup the whole API of the library here.

Reducing the latency

So far, we improved the protocol processing by using WebSockets instead of HTTP, we transfer binary instead of textual data but we can do one more thing. Now the protocol packets are transmitted between the client-side browser application, the proxy and the TCP server, just like the diagram below:

We can reduce the latency by moving the WebSocket proxy to the server machine. This way we will create a thin wrapper around the TCP server, so the data will be transferred locally (we can use something like websockify).

Conclusion

The solutions in this blog post are already used in production in different applications/frameworks. As example you can take a look at FreeRDP WebConnect, which is used by CloudBase.

Although it looks like doing awesome magic and providing magically legacy services in your browser, you should be aware of some issues:

Security

Make sure you use encrypted connection when required (wss)

Performance

Although v8 is extremely fast, you may hit some critical performance issues. In these cases you can move parts of the protocol processing in WebWorkers.

Browser Support

Not all the features we talked about are widely supported, even in the modern browsers.

References

1. BlobReader
2. FileReader API
3. ArrayBuffer
4. Blob