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]

Cross-pod combat step 1+2 (task #46, 2026-07-09) [T2 findings]

Step 1 — target ANY peer mech: DONE + verified. A live-mech registry (BTRegisterMech/ BTDeregisterMech from the Mech ctor/dtor, btplayer.cpp) collects every Mech — player, dummy, AND peer replicants. The mech4 boresight world-pick now walks BTGetTargetCandidates (all mechs ≠ shooter) and picks the CLOSEST the ray strikes, retargeting the whole fire/damage block from the solo gEnemyMech to the picked hotTarget (missile impact + projectile validation generalised via BTIsRegisteredMech). Verified one-box: instance A correctly enumerates B's mech as a live ReplicantInstance (inst=4), 20 zones, ownerID=3, alive. Solo un-regressed (pick→damage→kill chain clean). Step 2 — cross-pod damage: SOLVED + verified end-to-end (task #47, 2026-07-09). A one-line fix (Mech::Make) yields a full cross-pod KILL: instance A (host 2) shoots B's mech (host 3) dead over the network, B runs the death transition (wreck swap + explosion) on its own screen. [T2]

THE BUG (root cause): B's own MASTER mech was INVALID, so every dispatched network message to it was DEFERRED forever. Entity::Receive(Event*) (ENTITY.cpp:165) does if(!IsValid()) event->Defer(); return;. The reconstructed Mech::Make (mech.cpp) force-set ValidFlag ONLY for replicants (the P6 bring-up hack for the partial MakeReady/CheckLoad handshake); a locally-created master never got validated. So A→B TakeDamage arrived, resolved to B's real master mech, Posted, drained to Entity::Receive — which saw valid=0 and Deferred it. Handler never ran → 0 damage.

THE FIX: Mech::Make now sets ValidFlag for the master too (if (mech != 0) mech->SetValidFlag(); — the reconstructed ctor builds the whole mech synchronously, so it's safe). NOTHING ELSE changed: the targeting, the virtual victim->Dispatch, the reroute, the wire, the receive, and the id resolution were all already correct. Solo un-regressed (the solo enemy was already force-valid via btplayer:839; the change only ADDS ValidFlag).

Verified full path (2-node one-box, BT_MP_FORCE_DMG on A firing at B's replicant once/sec):

1. A: victim->Dispatch(&td) (VIRTUAL — the real beam path) → Entity::Dispatch reads inst=Replicant, stamps entityID=3:22 (B's master id), if(ReplicantInstance) SendMessage(ownerID=3, EntityManagerClientID).
2. Wire: NetworkManager::Send (L4NET.CPP:1062) → send() to host 3 (online).
3. B: ReceiveNetworkPacket (NTTMGR.cpp:107) → GetEntityPointer(3:22) = entityIndexSocket.Find (HOSTMGR.h:270) → resolves to B's master mech (classID 3001=0xBB9) → Post(EntityManagerEventPriority).
4. B: event drains → Entity::Receive (now valid=1) → Mech::TakeDamageMessageHandler (mech.cpp:548) → invalidZone (−1) cylinder lookup → zone damage. ~36 hits × 12 → B's mech DESTROYED + death transition.

Two prior mis-diagnoses corrected — both were the WRONG messageID. Entity::TakeDamageMessageID is 18, NOT 21 (confirmed at runtime: td.msgID=18, so Simulation::NextMessageID=3, not 6). Every earlier probe filtered msgID==21, so it NEVER matched a real TakeDamage — the stray msgID=21 hits it caught were an unrelated message, and correlating their 3:19 / classID 48 against the real 3:22 TakeDamage produced two phantom root causes: (a) the "cross-TU Entity layout divergence" (disproven separately — an offsetof probe showed Entity is IDENTICAL in both TUs: sizeof=444, entityID@380 (0x17C), ownerID@388, simulationFlags@32; distinct from the REAL P5 raw-offset stomps at this+0x2d4..0x2f0 in docs/HARD_PROBLEMS.md), and (b) the "localID drops by hostID on the wire" claim. Both are FALSE. The id is correct end-to-end (3:22 = B's master); nothing on the wire is corrupted. Lesson: when a message probe seems to see a wrong id, first confirm the numeric messageID.

Retained gated instrument: [mp-force] (BT_MP_FORCE_DMG + BT_MP_LOG) — the 2-node cross-pod reproducer, off in solo. [mp-hdlr] (BT_MP_NET, mech.cpp:554) logs each applied cross-pod hit. 2-node launch recipe + scratchpad/mp_test.sh: A -egg MP.EGG -net 1501, B -net 1601, then python tools/btconsole.py MP.EGG 127.0.0.1:1501 127.0.0.1:1601 (MP.EGG [pilots] = game ports 1502/1602).

Remaining (P6 phase 4 / Phase 7)

Interactive 2-window driving + the -net-mode aim projection / drive (A's aim ray was dead — noRay, and BT_AUTODRIVE/BT_GOTO didn't move A in -net mode) so a HUMAN (not just the FORCE_DMG hook) can aim + fire cross-pod; 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).