Sunday, 9 December 2012

Sharing a file from WP8 using Bluetooth OBEX Object Push Profile

A WP8 app I’m working on needs Bluetooth image transfer capabilities built in, so I started looking at the Microsoft examples and samples as a starting point. For starters I used the following MSDN article:

The article suggests that you can connect to a device with an empty service name, since the selected device returns an empty string:

// Make sure ID_CAP_NETWORKING is enabled in your WMAppManifest.xml, or the next
// line will throw an Access Denied exception.
await socket.ConnectAsync(selectedDevice.HostName, selectedDevice.ServiceName);

This is not the case as a GUID format service address must be supplied which matches one of the listed device Bluetooth capabilities:
  • Advanced Audio Distribution Profile (A2DP 1.2)
  • Audio/Video Remote Control Profile (AVRCP 1.4
  • Hands Free Profile (HFP 1.5)
  • Phone Book Access Profile (PBAP 1.1)
  • Object Push Profile (OPP 1.1)
  • Out of Band (OOB) and Near Field Communications (NFC)
There are a couple of other examples but they aren't particularly helpful either.

I’ve never had any dealing with Bluetooth before, but done lots of TCP and serial comms and got a few pointers from Mike Hole to get me started. At first I looked at the various Bluetooth specs from here (I always like to see protocol specs when working on this kind of thing):

Some of these make good background reading, in particular the following:
General Object Exchange Profile (GOEP) 

Object Push Profile:

However these don’t provide the level of detail required as we need byte-level  description of the protocol. For this you need IrDA Object Exchange Protocol (OBEX) spec which is not available without being a member of IrDA but I found a copy after a bit of googling. There are two other profiles which looked applicable to transferring an image, FTP and BIP (Basic Imaging Profile) but the only applicable profile the phone supports is OPP, which can be used to transfer objects such as files.

Anyway, down to the code. These are a few methods which connect to a peer device, then send bytes from a file (for me this was an image) in packets and then disconnect.

Peer Search
This bit is the same as the examples:

private async void Search()
  this.IsLoading = true;
  this.IsShareExecutable = false;

  this.Message = Resources.StringTable.BTSearching;

  // Note: You can only browse and connect to paired devices!
  // Configure PeerFinder to search for all paired devices.
  PeerFinder.AlternateIdentities["Bluetooth:Paired"] = "";

  this.pairedDevices = await PeerFinder.FindAllPeersAsync();

  if (pairedDevices.Count == 0)
    this.Message = Resources.StringTable.BTNoDevices;
    await this.Share();

Socket Connect
In my test, once a device was found, I went straight in to share a file from the first device:

StreamSocket _stream = null;

DataWriter _dataWriter;
DataReader _dataReader;

private async Task Share()

  // Select a paired device. In this example, just pick the first one.
  PeerInformation selectedDevice = pairedDevices[0];
  int count = pairedDevices.Count;

  // Attempt a connection
  _stream = new StreamSocket();

    // Make sure ID_CAP_NETWORKING is enabled in your WMAppManifest.xml, or the next
    // line will throw an Access Denied exception.
    string oopUUID = "{00001105-0000-1000-8000-00805f9b34fb}";

    await _stream.ConnectAsync(selectedDevice.HostName, oopUUID);
    _dataWriter = new DataWriter(_stream.OutputStream);
    _dataReader = new DataReader(_stream.InputStream);

    // Send data
    int maxServerPacket = await this.ObexConnect();

    if (maxServerPacket > 0)
      if (await ObexPushRequest())
        // Success
        // Failed

  catch (Exception ex)
    this.IsLoading = false;

    this.Message = Resources.StringTable.BTFailed;

This method opens a connection using the OOP UUID GUID, creates data packets, then performs an OBEX connect, followed by an object push and finally a disconnect.

Obex Connect

private async Task<int> ObexConnect()
  //send client request
  byte[] ConnectPacket = new byte[7];

  ConnectPacket[0] = 0x80;                       // Connect
  ConnectPacket[1] = (7 & 0xFF00) >> 8;       // Packetlength Hi Byte
  ConnectPacket[2] = (7 & 0xFF);                    // Packetlength Lo Byte
  ConnectPacket[3] = 0x10;                       // Obex v1
  ConnectPacket[4] = 0x00;                       // No flags
  ConnectPacket[5] = (2048 & 0xFF00) >> 8;    // 2048 byte client max packet size Hi Byte
  ConnectPacket[6] = (2048 & 0xFF);                    // 2048 byte max packet size Lo Byte

  await _dataWriter.StoreAsync();

  // Get response code
  await _dataReader.LoadAsync(1);
  byte[] buffer = new byte[1];

  if (buffer[0] == 0xA0) // Sucess
    // Get length
    await _dataReader.LoadAsync(2);
    buffer = new byte[2];

    int length = buffer[0] << 8;
    length += buffer[1];

    // Get rest of packet
    await _dataReader.LoadAsync((uint)length - 3);
    buffer = new byte[length - 3];

    int obexVersion = buffer[0];
    int flags = buffer[1];
    int maxServerPacket = buffer[2] << 8 + buffer[3];

    return maxServerPacket;
    return -1;

This establishes a connection and lets the server know the clients buffering capabilities. The client buffer can be something like 256 bytes to 64k - 1. The server then responds with it’s details, but on the laptop seemed to send back the client buffer size.

Create Packets
This method chops the file into 1k chunks and adds the necessary headers. The first packet has extra information about the file like it’s name and size and the last packet has a different op-code to indicate that the transmission has finished. The data to be transferred is in the _shareData variable.

private void CreatePackets()
  int bodyLength = 1024;
  this._packets = new List<byte[]>();           

  // Chop data into packets           
  int blocks = (int)Math.Ceiling((decimal)this._shareData.Length / bodyLength);

  System.Text.UnicodeEncoding encoding = new System.Text.UnicodeEncoding(true, false);
  byte[] encodedName = encoding.GetBytes(FILE_NAME + new char());

  for (int i = 0; i < blocks; i++)
    int headerLength = i == 0 ? 14 + encodedName.Length : 6;

    // Chop data into body
    byte[] body = null;
    if (i < blocks - 1)
      body = new byte[bodyLength];
      body = new byte[this._shareData.Length - (i * bodyLength)];

    System.Buffer.BlockCopy(this._shareData, i * bodyLength, body, 0, body.Length);

    // Create packet
    byte[] packet = new byte[headerLength + body.Length];

    // Build packet
    int offset = 0;
    packet[offset++] = i != blocks - 1 ? (byte)0x02 : (byte)0x82; // 0x02 for first blocks, 0x82 for last
    packet[offset++] = (byte)((packet.Length & 0xFF00) >> 8);
    packet[offset++] = (byte)(packet.Length & 0xFF);

    // Payload details on first packet
    if (i == 0)
      packet[offset++] = 0x01; // Name header
      packet[offset++] = (byte)(((encodedName.Length + 3) & 0xFF00) >> 8);
      packet[offset++] = (byte)((encodedName.Length + 3) & 0xFF);
      System.Buffer.BlockCopy(encodedName, 0, packet, offset, encodedName.Length);
      offset += encodedName.Length;
      packet[offset++] = 0xC3; // Length header
      packet[offset++] = (byte)((this._shareData.Length & 0xFF000000) >> 24);
      packet[offset++] = (byte)((this._shareData.Length & 0xFF0000) >> 16);
      packet[offset++] = (byte)((this._shareData.Length & 0xFF00) >> 8);
      packet[offset++] = (byte)(this._shareData.Length & 0xFF);

    packet[offset++] = 0x48; // Object body chunk header
    packet[offset++] = (byte)(((body.Length + 3) & 0xFF00) >> 8);
    packet[offset++] = (byte)((body.Length + 3) & 0xFF);
    System.Buffer.BlockCopy(body, 0, packet, offset, body.Length);

Push Request
Once the packets have been built sending them is very easy to do:

private async Task<bool> ObexPushRequest()
  foreach (var packet in this._packets)
    await _dataWriter.StoreAsync();

    // Get response code
    await _dataReader.LoadAsync(3);
    byte[] buffer = new byte[3];

    // If not success and not continue it's an error
    if (buffer[0] != 0xA0 && buffer[0] != 0x90)
      return false;
    else if (buffer[0] == 0xA0) // Success
      return true;

    return false;

Once the server has received a packet, it issues an 0x90 to say continue and once complete, 0xA0 for success.

Once finished a disconnect message can be sent, but it is not necessary and most devices and clients don’t implement it, so just disposing the IO objects will probably do:

private async void ObexDisconnect()
  byte[] bytes = new byte[3];
  bytes[0] = 0x81;
  bytes[1] = 0;
  bytes[2] = 3;

  await _dataWriter.StoreAsync();

  await _dataReader.LoadAsync(3);
  byte[] response = new byte[3];

  _stream = null;


As long as a device is paired and waiting to accept a file, this should work nicely. In windows to wait for a file, right-click the Bluetooth icon in the system tray and select ‘Receive a File’. If everything works, you’ll see the file name appear and a progress bar as the file transfers.