redapricot/docs/architecture.md

redapricot architecture

This document explains how redapricot is built and why. For the exact bytes on the wire, read PROTOCOL.md.

1. Roles and topology

        ┌───────────────────────── public internet ─────────────────────────┐
        │                                                                     │
  ┌──────────┐   MC handshake (Intent 2/…)     ┌───────────────┐             │
  │  Player  │ ───────────────────────────────▶│               │             │
  └──────────┘   raw Minecraft bytes           │      Hub      │             │
                                               │   (Java/Vert.x)│             │
  ┌──────────┐   Intent 17, magic 0x01         │               │             │
  │  Client  │ ◀──────── control session ──────│  • pattern reg │             │
  │  (Go)    │ ────────────────────────────────│  • CID table   │             │
  │          │   Intent 17, magic 0x02         │  • mux demux   │             │
  │          │ ═════════ worker conns ═════════│               │             │
  └──────────┘   multiplexed player streams    └───────────────┘             │
        │                                                                     │
        ▼ MC bytes (+ optional HAProxy v2)                                    │
  ┌───────────────┐                                                           │
  │ Real MC server│  (behind NAT, next to the client)                         │
  └───────────────┘                                                           │

Everything reaches the hub on one TCP port. The hub distinguishes three kinds of inbound connection purely from the first Minecraft Handshake:

Handshake Intent	Handled as
17 + magic 0x01	a control session from a client
17 + magic 0x02	a worker connection from a client
18	reserved (management/status) — never treated as a player
anything else	a player to be pattern-matched and tunneled

Because players use ordinary intents (1 status, 2 login, 3 transfer), vanilla clients need no changes.

2. Connection lifecycle

2.1 Client establishes a control session

Client                                   Hub
  │  TCP connect                           │
  │─ Handshake(Intent=17, addr=hex(SHA3-224(PSK))) ─▶  verify addr == expected
  │                                        │
  │  (both derive Phase-A keys = ChaCha20(SHA3-256(PSK ‖ dir)))
  │─ Frame#1 [magic=0x01, rand, ts] ──────▶  check |now-ts| ≤ window
  │  (both switch to Phase-B keys = ChaCha20(SHA3-256(rand‖ts ‖ dir)))
  │◀──────────── Frame [SessionReady] ─────│
  │─ Register("mc\.example\.com") ────────▶  patterns["mc\.example\.com"] = (regex, session)
  │◀──────────── RegisterAck ──────────────│
  │  ... periodic Ping/Pong ...            │

Only frame #1 is encrypted with the PSK-derived key; a fresh random rand‖ts becomes the per-connection key for everything after, so two connections never share a keystream beyond that first frame.

2.2 A player arrives and is tunneled

Player            Hub                          Client                 Destination
  │─ Handshake(addr="mc.example.com", Intent=2)─▶ normalize + regex-match
  │                                       │  pause player socket,
  │                                       │  buffer bytes, mint CID
  │                                       │─ ControlRequest(CID, pattern, ip:port) ─▶
  │                                       │                          allocate worker+stream
  │                                       │◀──────── SYN(streamId, CID) ────────────│
  │                                       │  takePending(CID) → bind        dial destination,
  │                                       │  forward buffered bytes          write HAProxy v2 hdr
  │                                       │─ DATA(streamId, handshake…) ───▶ ── handshake ──▶│
  │  resume ─────────────────────────────│                          bridge stream ⇄ dest
  │══════════════ player bytes ══ DATA ══▶│════ DATA ═══▶ dest.write                 │
  │◀═══════════ dest bytes ═══ DATA ══════│◀═══ DATA ════ dest.read                  │
  │  player closes ──────────────────────│─ FIN(streamId) ────────▶ close dest       │

Key points:

• Patterns are regexes. Each registered pattern is a case-insensitive regular expression, matched against the whole normalized hostname (anchored, first match wins). The hub echoes the matched pattern string — not the player's hostname — in ControlRequest, so the client can look it straight up in its own route table. Invalid patterns are rejected at registration with a non-zero RegisterAck status.
• The hub pauses the player socket the instant it matches, so no player bytes are lost while the takeover is arranged; the buffered handshake is forwarded verbatim, so the real server sees exactly what the player sent (including the original hostname — used for virtual-host routing there).
• CID is 16 random bytes minted by the hub and delivered only over the encrypted control session, so only the intended client learns it. Any worker connection presenting the correct CID is allowed to take over — that secrecy is what binds a worker stream to the right pending player without any explicit client identity.
• Disconnects are symmetric: player-close → hub sends FIN → client closes the destination; destination-close → client sends FIN → hub closes the player.

3. Multiplexing (worker connections)

A worker connection is one encrypted TCP link carrying many streams. The frame is intentionally tiny (PROTOCOL.md §7):

[plaintext VarInt length][ FrameType u8 | StreamID VarInt | Data… ]  (payload encrypted)

Only the client opens streams (SYN), so stream-id allocation is a simple per-connection counter with no coordination.

3.1 Pool & allocation

The client keeps 1…maxConn worker connections and places each new stream on the least-loaded one. It opens an additional connection only when the least-loaded connection is saturated (more than 8 active streams) and the pool is below maxConn:

pick least-loaded conn
if leastLoaded.streams > 8 and pool.size < maxConn:
    dial a new worker conn and use it
else:
    use leastLoaded

The e2e test TestConcurrentStreamsUseMultipleConns drives 20 simultaneous streams with maxConn=4 and observes them deterministically spread over 3 connections (9 + 9 + 2), confirming the algorithm.

4. Encryption

• Cipher: ChaCha20 (RFC 8439) as a raw stream cipher over frame payloads. The length prefix is plaintext, which makes the cipher phase switch at rekey trivial (a reader always knows exactly how many ciphertext bytes belong to the current frame and never decrypts the next frame with the wrong key).
• Keys: SHA3-256(phaseKey ‖ 0x01) for client→server and SHA3-256(phaseKey ‖ 0x02) for server→client. Distinct per-direction keys with a fixed zero nonce avoid a two-time pad without nonce management.
• Interop: Java's JCE ChaCha20 and Go's x/crypto/chacha20 produce byte- identical keystreams (including across partial-block, arbitrarily-split writes), and both crypto/sha3 implementations agree — verified directly and pinned by unit tests on both sides against a shared SHA3-224 vector.

5. Threading model

• Hub: a single Vert.x verticle instance. All accepted connections are handled on that verticle's one event loop, so the pattern registry, CID table, and per-connection state are touched by a single thread — no locks on the hot path (concurrent maps are used only defensively). Every socket operation is non-blocking; crypto is CPU-cheap. This trades multi-core scaling for simplicity and correctness.
• Client: goroutine-per-concern. One goroutine reads each connection (control or worker); WriteFrame is mutex-serialized so many stream goroutines can share a worker connection safely. Each stream has two goroutines: run pumps destination → hub, and writeLoop is the only writer to the destination, draining a per-stream queue fed by the worker readLoop. The readLoop itself never writes to a destination, so a stalled destination can never block frame dispatch for other streams.

6. Back-pressure & flow control

Two mechanisms operate at different granularities:

• Per-stream credit windows (PROTOCOL.md §7.3; the windows are exchanged at session establishment): each stream direction has an independent byte budget equal to the receiver's advertised window (default 256 KiB). A sender that exhausts a stream's window pauses only that stream's source — the hub pauses the one player socket, the client parks the one destination-reader goroutine. Credit is granted back (WND frames, batched at half-window) as bytes are actually written to the terminal socket. The result: a slow player or slow destination jams its own stream at a bounded buffer size and nothing else. This is what eliminates head-of-line blocking between streams.
• Aggregate TCP back-pressure on each worker connection: when the shared socket itself is congested (total bandwidth, not one stream), the hub parks all sending players until it drains, and the client's WriteFrame blocks. This is fair — when the pipe is genuinely full, everyone should slow down.

The window also bounds memory: a stream can hold at most one window of undelivered data per direction (the client's pre-connect handshake buffer is covered by the same bound).

Per-stream flow control is mandatory: the hub rejects a session whose Rekey lacks the STREAM_FC flag, and the client rejects a hub that does not echo it — peers that predate the mechanism cannot connect at all.

What remains (by design) is TCP-level head-of-line blocking: a lost packet on a worker connection stalls all its streams for one retransmit. That is inherent to mux-over-TCP; the connection pool is the mitigation, and a datagram transport (QUIC) would be the escape hatch if it ever matters.

7. Failure & recovery

• Control session drop: the client reconnects with capped exponential backoff and re-registers all patterns. Existing worker connections and their live streams are unaffected.
• Worker connection drop: every stream on it is torn down (destinations closed); the hub closes the corresponding player sockets; the client removes the connection from the pool and will dial a fresh one on the next allocation.
• Pending timeout: if no worker takes over a matched player within pendingTimeoutMs, the hub drops the pending entry and closes the player.
• Bad PSK / bad timestamp / bad magic: the hub closes the TCP connection; the client's session establishment fails fast.

8. Known limitations

1. No AEAD — payload integrity/authenticity is not cryptographically guaranteed.
2. TCP-level head-of-line blocking within a worker connection (lost packets; see §6) — per-stream flow control removes the application-level variant only.
3. Single-event-loop hub (see §5) bounds throughput to one core.
4. Intent 18 is reserved but only stubbed (the hub logs and closes).
5. Pattern ownership is last-writer-wins; two clients registering the identical pattern string will silently reassign it. Overlapping-but-distinct regexes are both kept, and when several match one hostname the winner is unspecified.

These are deliberate scope choices for a connectivity-focused P2P tool, not oversights; each is a small, well-isolated change away from being hardened.