BT411/context/multiplayer.md

id, title, status, source_sections, related_topics, key_terms, open_questions

id	title	status	source_sections	related_topics	key_terms	open_questions
multiplayer	Multiplayer — replication, netcode, the console	provisional	PROGRESS_LOG.md §7 (Phase 7), §8 (P6); docs/HARD_PROBLEMS.md (P6)	wintesla-port combat-damage locomotion decomp-reference	master replicant EGG dead-reckon	Cross-pod combat (damage to a replicant's master); replicant gait; pod-LAN config

Multiplayer

Pod-to-pod networked play. The hard infrastructure exists (WinTesla replaced DOS NETNUB with a WinSock2 TCP stack); the work is integrating + smoke-testing it for BT. Full detail: docs/PROGRESS_LOG.md §7, §8; the problem writeup: docs/HARD_PROBLEMS.md (P6).

The stack (already reconstructed)

• WinTesla replaced NETNUB with a 3446-line WinSock2 TCP reimplementation (MUNGA_L4/L4NET.CPP, SOCK_STREAM/ReliableMode, zero SOCK_DGRAM); the master/replicant distributed-sim core (InterestManager/Entity/HostManager) is complete. So MP = integrate + smoke-test that stack (gated on P3 locomotion + P5 entity lifecycle + subsystem WAVEs). The rumored L4NET.CPP:1853 send-size bug is DEAD CODE (#if MESSAGE_BUFFERING==0). [T2]

Master / replicant model

• master = a locally-simulated entity (your mech + dummies); drives itself + EMITS update records.
• replicant = a peer's mech, a local proxy; DeadReckons toward the master's replicated position. MUST SetValidFlag() at creation or every message defers forever (reconstruction-gotchas §9).

How a session forms

-net <port> = networked mode (the pod listens on for a CONSOLE, boots ConsoleOnly; game mesh = +1). Solo (no -net) never touches WinSock. The mission egg's [pilots] section IS the roster (pilot=ip[:port] + a per-address page); each pod self-identifies by local-IP+game-port, connects to earlier pilots, listens for later ones — a deterministic full mesh; the mission loads when all connect. The CONSOLE (operator station — ABSENT from every archive) delivers the egg as chunked ReceiveEggFileMessage packets + the LAUNCH RunMissionMessage; tools/btconsole.py is our console emulator (⚠ NotationFile::ReadText expects NUL-SEPARATED lines). [T2]

Verified milestones (one box, two instances)

• P6 smoke test: two instances share a world — console egg → TCP mesh → synchronized start → bidirectional entity replication (each renders its own mech + the peer's REPLICANT), 0 crashes.
• Movement replication: A's mech WALKS on B's screen (the replicant tracks the master to ~1.5u via dead-reckoning). The authentic mission-start ladder runs (CreatingMission→…→RunningMission via the console LAUNCH). SIX bugs fixed to get here (dead-reckoner install, replicant-motion DeadReckon, master emission threshold, emission gated on RunningMission, the console-must-LAUNCH fact, replicant validity). [T2]
• Wire-format bug class found+fixed: MakeMessages replicate RAW over TCP, so string payload must be INLINE (char[N] at the binary offsets), not a const char* pointer (garbage cross-pod). Check EVERY MakeMessage for pointer payloads. [T2]

Debug tooling (BT_NET_TRACE, permanent)

[net-tx]/[net-rx] (L4NET), [net-upd] (update lookup), [upd-repl], [ent-exec] (state ladder). Per-instance: BT_LOG=<file> + BT_AFFINITY=<mask> (CPU pin). Update stream ≈ 60 Hz × 144-byte records per moving master. [T2]

Remaining (P6 phase 4 / Phase 7)

Cross-pod COMBAT (target a replicant + route damage to the owning master — Entity::Dispatch already reroutes); interactive 2-window driving; replicant GAIT animation (derive from replicated velocity); the pod-LAN config (real IPs, bare-IP pilot entries). See open-questions. [T3]

MP-front scout (task #45, 2026-07-09) — the P6 chain SURVIVES the combat/HUD rework [T2]

Re-ran the one-box smoke test on the current build (world-pick targeting, weapon groups, death transition, HUD all landed since P6): console egg → mesh → RunningMission on BOTH instances, 174+ ×144-byte update records EACH side (msgID=6), 2 mech trees per instance, 0 crashes over ~6 min. Gap map to a first playable LAN fight (in wiring order):

1. Enemy selection: the world-pick tests only gEnemyMech (the solo dummy, btplayer.cpp:791). MP: the pick must test every OTHER mech (the peer replicants) — generalize to a candidate walk (mech entities != viewpoint). Small.
2. Cross-pod damage: dispatch TakeDamage at the REPLICANT → verify the engine reroute carries it to the owning master (the KB claims Dispatch reroutes; not yet exercised with our STEP-6 handler). The master takes real zone damage + runs its own death transition (task #42 placed the death dispatch victim-side — MP-correct by construction).
3. Victim visuals on the shooter's screen: the wreck swap gates on zone/graphic state the REPLICANT copy may never see (zone damage may not replicate — only pose updates confirmed). Check DamageZone/subsystem state in the update stream; else replicate the death as an event.
4. Cross-pod beam visuals: replicant emitters don't run the local trigger sim, so the peer's beams never draw today. ⭐ The authentic path exists: ServiceDischarge/ContinueDischarge SEND beam-keepalive messages (FUN_0041c350, templates @0x511e6c/@0x511e78, emitter.cpp already stubs them) — decode + route those to drive replicant beam draws.
5. 2-window driving: input gates on window focus (alternate windows; BT_KEY_NOFOCUS for harness). DEATHS scoring should light via the existing BTPostKillScore MP branch. Respawn = the P5 teardown debt (deferred until needed).

Key Relationships

• Base: wintesla-port (L4NET). Depends on: locomotion (update writer), combat-damage (entity lifecycle). Detail: docs/HARD_PROBLEMS.md (P6).