Building a BitTorrent Client, Part 3

See the previous 2 parts here:

Part 1 - The start of the journey

Part 2 - Not so buff now

In this part I am going to build the peer protocol, connect to a peer, and get a handshake back. This part took me longer for me to figure out, there is more fiddling with buffers directly and required a lot of trial and error.

First of all, what is a peer protocol? What is a peer? What is a handshake?

Again I am referencing the BitTorrent Spec for this: Spec. This document is dense with information and requires a bit of re-reading, or at least it did for me.

My understanding is this:

A peer protocol is the way that BitTorrent peers exchange data, a handshake is performed and then a stream of messages.

A peer is the end user, both downloader and uploader.

A handshake is the initial contact between peers. The format is set, and ensures both peers are working on the same torrent file.

Building the peer protocol

The main purpose of this is to encode and decode the messages with the correct IDs and provide data if required, e.g. Choke, Unchoke, request etc.

For the most part its just going through the BitTorrent spec one-by-one and creating a way for the buffers to be encoded or decoded into the correct format, and passed onto other parts of the code correctly.

I wont go through each one, but I will link the GitHub code for reference. (This is my attempt, again probably not the best/fastest/strongest way).

Decoding the handshake

export const decodeHandshake = (encoded: Buffer) => {
  const protocolLength = encoded[0];
  const protocol = encoded.subarray(1, 20).toString("utf8");
  const reserved = encoded.subarray(20, 28);
  const infoHash = encoded.subarray(28, 48);
  const peerId = encoded.subarray(48, 68);

  if (protocolLength !== 19) throw new Error("Protocol Length is incorrect");
  if (protocol !== "BitTorrent protocol")
    throw new Error("Protocol is incorrect");

  return {
    first_byte: protocolLength,
    bittorrentProtocol: protocol,
    reserved,
    info_hash: infoHash,
    peer_id: peerId,
  };
};

The information pulled out of the initial Buffer is:

1. Protocol Length (byte 0) - should be 19 according to the spec
2. Protocol String (bytes 1-19) - The string 'BitTorrent protocol'
3. Reserved Bytes (bytes 20-27) - Future protocol extensions
4. Info Hash (bytes 28-47) - 20-byte SHA1 hash identifying the torrent
5. Peer Id (bytes 48-67) - identify the specific peer

Subarray allows us to take parts of the buffer. Like .slice() does in regular ol' Javascript. In this case, I am taking the buffer sent to us, and grabbing certain parts of it to check for the correct information.

Another example, this is for encoding a request to send to the socket connection.

export const encodeRequest = (
  pieceIndex: number,
  offset: number,
  length: number
) => {
	  const buffer = Buffer.alloc(17);
	  buffer.writeUInt32BE(13, 0);
	  buffer[4] = 6;
	
	  buffer.writeUInt32BE(pieceIndex, 5);
	  buffer.writeUInt32BE(offset, 9);
	  buffer.writeUInt32BE(length, 13);
	
	  return buffer;
};

In this function I created the buffer, allocated the correct amount of bytes and then filled the buffer with the byte version of the ID, index, offset, and length.

I followed the spec, and created a function to encode and decode each type of message in the protocol. See the link here for the full file: peer-protocol.ts

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How to use this protocol?

In another file, I used the encode handshake I had just created with the info_hash and the peer_id gained from the previous steps. I then created the socket and sent the handshake, at first I tried to just get 1 peer to respond.

The result was a handshake back - which my decode handshake function can then handle.

const socket = net.createConnection(PEER_PORT, PEER_IP, () => {
	console.log(`Connected to ${PEER_IP}:${PEER_PORT}`);
	socket.write(handshake);
	console.log("Handshake sent.");
});

socket.on("connect", () => {
	console.log("Connected");
});

socket.on("data", (data) => {
	if (!isHandshakeDone) {
		const res = decodeHandshake(data);
		console.log("🤝 Handshake received:", res);
		isHandshakeDone = true;
	} else {
		parseMessage(data);
	}
});

To handle all the different requests I thought I would be best to create a couple of classes.

1. A Coordinator Class - Handles the overall download, to select pieces, assign downloads for each peer.
2. A Peer Class - This will be a class for each peer I connect to, to push through requests and received data.

In doing this I could then handle a peer instance for each peer in my peerlist. The coordinator could keep track of all of these peers and send the various handshakes, and piece requests.

In the end, success - a handshake is made, and so far - no data at all is transferred. So we are still a little ways off from a working torrenting program. But we are getting there.

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I found this part both interesting and challenging, Its definitely pushing me out of my comfort zone dealing with buffers and bytes, so I am glad I took up this project.

The next part will continue with the final piece of the puzzle to get a torrent downloaded: Requesting pieces, assembling pieces, building final files, closing the torrent.

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Helpful Links

• BitTorrent Spec
• Buffer Documentation