BT412/context/multiplayer.md

---
id: multiplayer
title: "Multiplayer — replication, netcode, the console"
status: provisional
source_sections: "PROGRESS_LOG.md §7 (Phase 7), §8 (P6); docs/HARD_PROBLEMS.md (P6)"
related_topics: [wintesla-port, combat-damage, locomotion, decomp-reference]
key_terms: [master, replicant, EGG, dead-reckon]
open_questions:
  - "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 <port> for a CONSOLE, boots ConsoleOnly; game mesh
= <port>+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).

## Interactive -net aim + drive (task #48, 2026-07-09) — partial; the blocker is NOT the netcode [T2]
Re-measured on a 2-node one-box run (A `BT_AUTODRIVE`/`BT_GOTO`, `BT_AIM_LOG`, `BT_MP_LOG`). **The prior
"aim ray dead / drive dead in -net" claim was STALE — both work now:** A's aim ray is live (`noRay=0`, the
active eye publishes the aim camera), and A DRIVES (`BT_AUTODRIVE`/`BT_GOTO` move it). What actually blocks
a HUMAN cross-pod kill via the real beam path:
- **Boresight inherits the mech BODY pitch.** The aim ray = the active EYE's view direction (with reticle
  at 0,0). On sloped terrain the mech body pitches to conform (ground model), the cockpit eye/boresight
  pitches with it, and the pick ray points ~8° DOWN (`[EYE] up=(-.03,.99,-.14)` == `[pick] ldir.y≈-0.145`,
  every frame). At range that ray hits terrain (`groundPicks`) ~50-100 m short of the enemy → `mechPicks=0`.
  A close enemy (< ~50 m) would still resolve. Real BT aims the TORSO pitch independently of leg/body tilt;
  the reconstruction ties the boresight to the render eye. Fixing = derive the aim ray from a level torso
  boresight (yaw only) OR add torso pitch, not the pitched body eye.
- **Drive/goto stalls.** A repeatedly stops (`gait adv=0`) ~700-750 m from the enemy before closing to
  weapon/pick range — a locomotion/goto reliability issue, not MP.
- **PickRayHit vs a replicant is UNCONFIRMED** (blocked by the two above — A never faces the enemy at close
  range). `localToWorld` on the replicant IS correct (`[pick] lstart` magnitude ≈ true A→B distance; the
  DeadReckon + `localToWorld=localOrigin` block at mech4.cpp:1191 tracks the replicated position).
Test aid added: `BT_GOTO="enemy"` beelines toward the nearest live replicant (spawn coords vary per run, so
a fixed "x z" can't reliably face the peer). Recipe: `scratchpad/mp_kill_test.sh`.

## Remaining (P6 phase 4 / Phase 7)
Human-playable cross-pod aim (torso-pitch / level-boresight aim ray + drive-to-range reliability, above) 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).