Last time I started looking at how to implement a subset of the MTP specification, in order to read files from Android devices. Let's start by writing some code to talk to the device over WebUSB:

const device = await usb.requestDevice({
  filters: [
    {
      vendorId: 3725,
      productId: 8221,
    }
  ]
});

await device.open();

if (device.configuration === null) {
  console.log('selectConfiguration');
  await device.selectConfiguration(1);
}
await device.claimInterface(0);

MTP uses containers to structure what the various commands and responses look like. Here is how to build a container packet (and note that struct.pack() is part of a convenience library):

const buildContainerPacket = (container, payloadLength) => {
  const packetLength = 12 + payloadLength;
  const buf = new ArrayBuffer(packetLength);
  const bytes = new Uint8Array(buf);
  let ctr = struct.pack(bytes, 0, 'issi', packetLength, container.type, container.code, container.transactionID);
  if (payloadLength > 0) {
    ctr += struct.copyBytes(bytes, ctr, container.payload, payloadLength);
  }

  return buf;
};

To open a new session, we fill the container object with the right info and send it off via the bulk pipe:

  const openSession = {
    type: 1, // command block
    code: 0x1002, // open session
    transactionID: 0,
    payload: 1, // session ID
  };
  let data = buildContainerPacket(openSession, 4);
  let result = await device.transferOut(0x01, data);

To read the response, I'm creating a receiveData() function:

  const receiveData = async () => {
    const timeoutID = setTimeout(async() => {
      console.warn('Device not connected');
    }, 5000);

    console.log('Receiving...');

    let incoming = await device.transferIn(0x01, 1024);

    if (incoming.data.byteLength > 0) {
      clearTimeout(timeoutID);

      const [length] = new Uint32Array(incoming.data.buffer, 0, 1);
      console.log('Length:', length);

      // TODO: if length !== incoming.data.byteLength, read more data
    }
  };

To actually receive some data, we call

await receiveData();

and then parse the response (0c 00 00 00 03 00 01 20 01 00 00 00), which is also in the same container format:

• Length: 0x0C000000 – 12 (in little-endian format)
• Type: 0x0300 – Response block
• 0x0120, or 0x2001 in big-endian – response code OK
• 0x0000000000 – transaction ID

So, we're able to successfully open a session!

PS: Hey, I'm halfway through #100DaysToOffload already! 🎉️

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