I used to think torrents were magic.
Files downloading from “somewhere”… insanely fast… no server?
So I decided to break that illusion and built a minimal BitTorrent client in Go.
👉 Source code: https://github.com/rohit20001221/ripple
🧠 What is BitTorrent (Really)?
At its core, BitTorrent is simple:
- Split a file into pieces
- Download pieces from multiple peers
- Verify everything using hashes
No central server. Just a swarm of peers.
🏗️ High-Level Architecture
📦 Step 1: Parsing the .torrent File
A .torrent file is bencoded data containing:
- Tracker URL
- File metadata
- Piece hashes
🔑 Info Hash (Critical)
bencode.Marshal(&buf, t.Info)
sha1(buf.Bytes())
This hash uniquely identifies the torrent across the network.
🌐 Step 2: Finding Peers
We contact the tracker with:
info_hashpeer_idport
Tracker returns peers in compact format:
[IP (4 bytes)] [PORT (2 bytes)]
🤝 Step 3: Handshake
Before anything else, peers must agree:
handshake.SetInfoHash(...)
handshake.SetPeerID(...)
If the hash doesn’t match → connection closed.
🧩 Step 4: Bitfield (Who Has What?)
Each peer sends a bitfield.
Each bit represents whether a piece exists:
func (bf BitField) HasIndex(index int) bool {
return bf[byteIndex]>>uint(7-offset)&1 != 0
}
⚡ Step 5: Worker Pool Design
This is where performance comes in.
- Pieces → tasks
- Peers → workers
- Channels → coordination
🧱 Step 6: Breaking Pieces into Blocks
Each piece is split into 16KB blocks:
const MAX_BLOCK_SIZE = 16384
📥 Step 7: Download Flow
🔁 Step 8: Fault Tolerance
Peers are unreliable.
So I requeue failed tasks:
c.TaskQueue <- task
🔐 Step 9: Integrity Check
Every piece is verified:
sha1(piece) == expectedHash
💾 Step 10: Writing the File
Pieces arrive out of order.
So we use random access:
outFile.Seek(offset, 0)
outFile.Write(piece)
🧠 What I Learned
- BitTorrent is simple but powerful
- Concurrency is a game changer
- Real systems must handle failure gracefully
- Protocols are just structured bytes
💡 Future Improvements
- Upload capability
- Resume downloads
🎯 Final Thoughts
Building a BitTorrent client was one of the most educational projects I’ve done.
It forced me to understand:
- Networking
- Concurrency
- Distributed systems
⭐ If you liked this
- Star the repo: https://github.com/rohit20001221/ripple
- Share this post
- Try building your own!
