BT411/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) [T2]
Re-measured on a 2-node one-box run. **The prior "aim ray dead / drive dead in -net" claim was STALE — both
work:** A's aim ray is live (`noRay=0`) and A drives (`BT_AUTODRIVE`/`BT_GOTO` move it).

**FIXED — torso-pitch aim ray (gyro-stabilized boresight).** The aim/pick ray was the active RENDER EYE's
view direction, which (a) inherits the mech BODY pitch — on a slope the body tilts ~8° to conform to terrain
and the boresight pointed that far DOWN, hitting terrain ~50-100 m short of the enemy (`groundPicks`, never
`mechPicks`); and (b) in CHASE view (the DEFAULT) is the elevated behind-the-shoulder camera, so the ray
started over the mech's shoulder. Fix (L4VIDRND `DPLEyeRenderable::Execute`, the `BTSetAimCamera` publish):
LEVEL the boresight (drop the view-dir pitch, rebuild an upright basis with world +Y) AND anchor its ORIGIN
at the mech's torso (`myEntity->localToWorld` translation + ~5) instead of the render eye. The guns now fire
along the mech's horizontal heading from the torso, independent of view mode and terrain tilt. The reticle X
still carries the torso twist (`BTTwistToReticleX`); reticle Y any elevation. VERIFIED end-to-end: solo,
enemy 120 m ahead, **both chase AND cockpit** view → `[target] MECH under boresight ... mechPicks=59
groundPicks=0` → beam → `*** DESTROYED`. Also proved `PickRayHit` resolves a REPLICANT when aimed at it
(`hit=1` at 757 m in a 2-node run); the replicant `localToWorld` is correct (DeadReckon + `localToWorld=
localOrigin`, mech4.cpp:1191). [T2]

**FIXED — drive-to-range (the automated cross-pod kill now runs end-to-end).** The `BT_GOTO` beeline had
three separate faults: (1) it was gated behind `if(gBTDrive.forced)` (mech4.cpp), and `forced` was set ONLY
by `BT_AUTODRIVE` — so `BT_GOTO` alone never drove (chicken-and-egg: the block that would drive was gated on
the drive being on). Fix: `BT_GOTO` now enables forced mode itself + supplies its own forward throttle (a
"beeline" must drive, not just steer), cutting throttle inside a stop radius (weapon range for `enemy`, so it
holds and shoots instead of ramming). (2) The steering used the `gDriveHeading` SCALAR MIRROR (seeded to 0),
but MP pilots spawn facing a non-zero heading, so the beeline steered the WRONG way. Fix: compute the heading
error from the mech's ACTUAL forward (`localToWorld`'s -Z axis), signed angle to the target. (3) The goto turn
was routed through `controlsMapper->turnDemand`, which read 0 in -net — the heading froze and A drove straight
past B. Fix: apply the goto turn to the orientation integration DIRECTLY (`if(gBTGotoActive) turn=gBTGotoTurn`),
after the mapper read. **CAUSE FOUND LATER (task #51): turnDemand zeroed because `controlMode` was pointer
garbage — the `pilotArray[1]` shrunk-span overrun (see [[reconstruction-gotchas]] §5), MP-only because solo's
overrun wrote 0==BasicMode. Fixed at the root (10-slot array); the direct goto-turn stays (harmless, and it
skips the 1-frame mapper latency). KEYBOARD turning in MP was restored by the root fix.**
VERIFIED 2-node: A `BT_GOTO="enemy"` alone (no autodrive) drives `dist 1673→231 m`, holds at range, `mechPicks=59`,
autofire → **B hdlr=192, DESTROYED** — a fully automated human-style cross-pod kill via the real beam path
(not the FORCE_DMG hook). Solo un-regressed (plain autodrive + goto both work). `scratchpad/mp_kill_test.sh`;
`BT_GOTO_LOG` traces the beeline.

## Replicant gait animation (task #50, 2026-07-09) — DONE [T2]
A peer's replicant used to slide as a frozen statue (DeadReckon moved it; nothing animated it). Now the
replicant block (mech4.cpp, after `DeadReckon`) derives a SIGNED speed demand from the replicated
velocity — the master publishes `worldLinearVelocity` in every update record (`Mover::WriteUpdateRecord`
MOVER.cpp:759), the replicant stores it in `updateVelocity.linearMotion` (:712); sign = the forward dot
against the mech's -Z facing — writes it to `controlsMapper->speedDemand` (the LEG channel's live source)
and advances the LEG state machine for its JOINT writes only. Travel stays DeadReckon (the returned
distance is discarded) so position always follows the master; the clip cadence matches the replicated
speed. ⚠ REPLICANT GAIT-SCALAR PRIMING (same class as the master's per-frame primes, which live in the
PLAYER-only drive block): `globalTimeScale`/`idleStrideScale` read 0 on a replicant (clip frozen at
advance-time dt*0, legState engaged but legCycle stuck 0), and `reverseSpeedMax2`@0x7a0 (the run-cycle
rise-clamp; LoadLocomotionClips does NOT set it) read debug-heap garbage → legCycleSpeed clamped to
-4.3e8 entering run state 13. Both primed in the replicant block; `legCycleSpeed` itself sane-banded once.
VERIFIED 2-node: the observer's replicant runs the full lifecycle — stand→walk(5,7)→run(10,12) with
legCycle 7.5→34 tracking the master's speed, wind-down (8) and back to Standing(0) when the master stops.
Solo un-regressed. `BT_REPL_LOG` traces pos/vel/legState/legCycle; recipe `scratchpad/mp_gait_test.sh`.
NOTE: the update-record velocity is currently the master's ACTUAL travel velocity (worldLinearVelocity
from the leg channel); the binary publishes the BODY channel's smoothed projection (projectedVelocity,
disasm +0x2a0). Swapping the source is a fidelity refinement DEFERRED — it risks a verified working feed
for a smoothing nuance. [T3 on that nuance]

## Remaining (P6 phase 4 / Phase 7)
The pod-LAN config (real IPs, bare-IP pilot entries); update-record velocity sourced from the body-channel
projection (above). 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** — PARTIALLY resolved (user-observed 2026-07-09): the
   wreck SWAP does appear on the killer's screen, but the death SINK/burial does not (the collapse+
   burial runs only in the owning pod's death transition). Fix path: transcribe the recovered
   Mech-level WriteUpdateRecord death/knockdown record types (5/6 — disasm preserved, task #51).
3b. **Respawn cycle** (task #52) — the AUTHENTIC cycle is now reconstructed [T1 @004c05c4 +
   engine PLAYER.cpp]: mech death transition (mech4.cpp) dispatches `VehicleDead` (ctor default
   `deathCount = -1` = the death notification) to `GetPlayerLink()` → `BTPlayer::
   VehicleDeadMessageHandler` (@004c05c4 structure): MissionEnding(state 4) skips all; -1 →
   dedup on deathPending, ++deathCount, debit `scenarioRole` life, sever `playerVehicle`
   (the WRECK entity stays), re-post the same message at **now + 5.0 s** (5.0f @004c0830)
   stamped with deathCount; out of lives → the binary posts msgID 0x18 at +10 s (mission
   review; deferred). deathCount ≥ 0 / -2 with a vehicle in hand = acquired (clears
   deathPending); without one → engine base @0042db80 (PLAYER.cpp:214: closest non-"win"
   DropZone → AssignDropZone → 2s re-post probe) → DropZoneReply → CreatePlayerVehicle (a NEW
   mech; `deathCount = 0`) + viewpoint relink (Verify(viewpointEntity==NULL) is compiled out at
   DEBUG_LEVEL 0 — release overwrites, same as the 1995 build). @0049fe0c = roster-wide
   slot-11 DeathShutdown broadcast (the death transition already runs that pass).
   CORRECTIONS to the earlier scouting: BTL4.RES **type 12 = InternalAudioStream** (the engine's
   own resource-type table) — `*int.scp` are internal-AUDIO scripts, NOT per-mech intro scripts;
   and `adrop` (id 1994) is a **type-14 MakeMessageStream** = the map's four DropZone make
   records (0xE0 each: classID 3, origin, facing, name "two"/"three"/…), i.e. the respawn
   infrastructure, not a whirlwind. The whirlwind/recovery VISUAL remains unfound: no
   whirlwind/tornado/vortex-named asset in BTL4.RES (ModelList/VideoModel/GameModel/Animation
   sweeps) and no vortex-like .PFX; still open. Mech::Reset (@0049fb74) full subsystem-reset
   sweep is still a bring-up TODO.
   **Three respawn bugs found + fixed on the way (all [T2], 2-node force-damage verified):**
   (i) **death-latch un-latch** — `IsMechDestroyed` tested `graphicAlarm>=9` alone; a later leg hit
   on the wreck rewrites the alarm to 4/3, un-latching → the death transition RE-RAN (double kill,
   double `VehicleDead(-1)`). Fixed to latch on `movementMode 2||9` (see combat-damage.md
   "Death-latch correction"). (ii) **score into severed vehicle** — `BTPlayer::ScoreMessageHandler`
   / `ScoreInflictedMessageHandler` deref `playerVehicle` unguarded; during the 5s dead window it is
   0 → guard added (apply the award, skip the vehicle response). (iii) **renderer LoadMission
   re-entry** — the KILLER bug: `MakeAndLinkViewpointEntity` (APP.cpp:1265) calls
   `videoRenderer->LoadMission` on EVERY viewpoint-make, so the respawn re-runs
   `DPLReadEnvironment`→`DPLReadINIPage`; its light block `Fail`s ("All lights must be defined on a
   single INI page") because `sceneLightCount` was never reset between reads (ctor-only init). Fixed
   in L4VIDEO.cpp `DPLReadEnvironment`: tear down the prior mission's `sceneLight[]` +
   `sceneLightCount=0` before re-reading (the per-page invariant is intra-read only). This is a
   PORT-layer re-entrancy gap in the 2007 renderer, not BT logic. Caught via cdb `bp ucrtbased!abort`
   — a `Fail`/`Check` is an abort() MessageBox, not an AV, so `sxe av` misses it.
4. ✅ **Cross-pod beam visuals — DONE (task #51, 2026-07-09).** The keepalive theory needed one
   correction: `FUN_0041c350` = (a) queue the LOCAL deferred beam-effect callback (@0x4bac0c) on the
   app+0x34 manager (our per-weapon render walk plays that role) AND (b) set the subsystem DIRTY bit —
   which maps to the 2007 engine's `updateModel`/`ForceUpdate()`. The REPLICATION rides **subsystem
   update records inside the mech's update message**: the roster walk passes the entity's stream to
   every subsystem's `PerformAndWatch`; `Emitter::WriteUpdateRecord` (@004ba65c, corrected: record
   LENGTH not recordID; rec+0x30 = the TARGET's **EntityID** not a colour) serializes
   firingActive/beamEndpoint/beamFlag/targetID; the engine's `Entity::UpdateMessageHandler` routes
   records by `subsystemID` to `Emitter::ReadUpdateRecord` (@004ba568 — was MIStranscribed as
   TakeDamage; the whole weapon-family slot map was swapped: slot 6=Read, 7=Write, 9=TakeDamage — see
   mechweap.hpp). `ServiceDischarge`/`ContinueDischarge` call `ForceUpdate()` per keepalive tick +
   once at beam end. The render walk was extracted to `Mech::DrawWeaponBeams` (per-mech port cache)
   and runs for REPLICANTS. ⚠ two silent-failure traps fixed: the overrides originally used the
   class's own shadowing `UpdateRecord` typedef as the param type → never overrode the engine virtual
   (base ran, nothing serialized); and `recordLength` must be each level's sizeof or the stream
   framing misparses. VERIFIED 2-node: A fires 57 volleys → 225 records → B applies 225 → draws 414
   replicant beams (PPC blue / laser red from A's replicant's gun ports). Solo un-regressed.
   REMAINING (deferred): the MECH-level writer @0x4a0c2c (recovered + preserved:
   `reference/decomp/mech_writeupdate_004a0c2c.disasm.txt`; 9 record types decoded — 0/1/4 pose
   variants, 2 alarm, 3 leg-state+heat with body-channel write-through re-sync, 5 knockdown
   (SetBodyAnimation(0x20) write-through), 6 death, 7 impact, 8 movementMode) — transcribing it
   would replicate remote KNOCKDOWN/death/heat states; beams did not need it.
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).