Putting It All Together

You've built every layer of a BitTorrent client:

• Bencoding — the wire encoding underneath everything
• Metainfo + infohash — what identifies a torrent
• Magnet URIs — bootstrapping without a .torrent file
• HTTP + UDP trackers — centralized peer discovery
• Peer handshake + wire messages — the peer protocol
• Piece verification — the SHA-1 trust boundary
• Rarest-first — the piece-selection strategy
• Choking (tit-for-tat) — the incentive engine
• Endgame mode — finishing without stragglers
• DHT / Kademlia — trackerless peer discovery

What's still out there, beyond this course:

• PEX (Peer Exchange): peers tell each other about other peers they know.
• uTP / LEDBAT: BitTorrent over UDP with friendly congestion control (yields to other TCP traffic).
• Streaming: download pieces in playback order (not rarest-first) for video-on-demand.
• Webseeding (BEP 19): use HTTP servers as additional seeds.
• Encryption (MSE/PE): obfuscate the protocol so ISPs can't trivially throttle BT traffic.

Real implementations:

• libtorrent (Rasterbar) — C++, used by Deluge, qBittorrent.
• transmission — C, lean and fast.
• WebTorrent — JavaScript/WebRTC, runs in browsers.
• Aria2 — multi-protocol downloader supporting BT.

You now understand why BitTorrent scales: every peer is a producer, every peer learns piece availability, every peer cooperates to keep rare pieces alive. It's a beautiful piece of distributed systems design.