Bittorrent client

This is learning project, where i build a bittorrent taking the lead from Jesse Li's Building a BitTorrent client from the ground up in Go

Built this to learn how backend systems work at a low level. Implements bencode parsing, tracker communication over HTTP, TCP peer connections, the BitTorrent handshake, and concurrent piece downloading using goroutines and channels.

Bencode

Bencode (short for binary encoding) is a very simple data serialization format used mainly by the BitTorrent protocol.

In BitTorrent, bencode is used to store and transmit information such as:

• .torrent file metadata
• tracker responses
• peer communication data

Bencode only supports four types of values.

• integers: Integers are written with i at the start and e at the end. example: i42e is 42 and i-5e is -5, the structure is ie

• strings: they are written as length:string example: 4:spam is "spam", 11:hello world is "hello world"

• lists: Lists start with l and end with e. example: l4:spam4:eggse is ["spam", "eggs"], li1ei2ei3ee is [1, 2, 3], the structure is l ... e

• dictionaries: Dictionaries start with d and end with e example: d3:cow3:moo4:spam4:eggse is { "cow": "moo", "spam": "eggs" }, structure is d e

bencode.Unmarshal(r, &bto)

It reads the raw bencode bytes from the `.torrent` file and fills in your `bencodeTorrent` struct automatically — matching each bencode key to the correct struct field using those struct tags.

Think of it like this:

raw .torrent file bytes → Unmarshal → bencodeTorrent struct

bittorent handshake

• The length of the protocol identifier, which is always 19 (0x13 in hex)
• The protocol identifier, called the pstr which is always BitTorrent protocol
• Eight reserved bytes, all set to 0. We’d flip some of them to 1 to indicate that we support certain extensions. But we don’t, so we’ll keep them at 0.
• The infohash that we calculated earlier to identify which file we want
• The Peer ID that we made up to identify ourselves

ID	Name	Meaning
0	Choke	Peer won't send you data
1	Unchoke	Peer will send you data
2	Interested	You want data
4	Have	Peer has a piece
5	Bitfield	Which pieces peer has
6	Request	Ask for a block
7	Piece	Actual data