--- name: multiplayer-and-network-sync description: "Multiplayer networking -- LiveKit rooms, movement encoding, interpolation, profile sync, entity-participant mapping. Use when working with RoomHub, movement systems, EntityParticipantTable, or remote player sync." user-invocable: false --- # Multiplayer & Network Sync ## Sources - `docs/livekit-networking.md` -- Entity sync via LiveKit rooms, host vs remote entity semantics --- ## Dual-Room Architecture `RoomHub` orchestrates four LiveKit rooms: **Island**, **Scene** (GateKeeper), **Chat**, and **VoiceChat**. Island + Scene are "local rooms" for entity synchronization; Chat + VoiceChat are independent. `RoomHub.StartAsync()` starts Island + Scene + Chat in parallel; VoiceChat starts on demand. `AllLocalRoomsRemoteParticipantIdentities()` merges participants from Island + Scene (cached per frame). ### IConnectiveRoom Lifecycle `ConnectiveRoom` runs an infinite reconnection loop: `PrewarmAsync` -> repeated `CycleStepAsync` with 1s heartbeat. State machine: `Stopped -> Starting -> Running -> Stopping -> Stopped`. Uses `Atomic` for thread-safe state transitions (LiveKit callbacks arrive off main thread). Recovery delay on failure is 5 seconds. **Why two rooms for entities:** The Scene room connects only to the scene the host is standing in, so only co-located players sync scene-specific CRDT state. The Island room provides global visibility -- remote avatars appear even when the host is in a different scene, but Island data is NOT used for scene CRDT synchronization (bandwidth concern). --- ## Host Entity Semantics (CRITICAL GOTCHA) > See `docs/livekit-networking.md` **Host components (`PBIdentityData`, `PBAvatarEquippedData`, `PBAvatarBase`) are ALWAYS present** on the scene from startup through the entire lifecycle -- regardless of whether the host is inside or outside the scene. **Remote entities behave completely differently:** - If the scene is NOT the current scene, remote entities cannot be present in CRDT state (even if visually nearby via Island room) - When the host enters a scene, all remote participants within bounds are propagated as if they just entered - When the host is within the scene and others enter/leave, it works as expected - Remote entities themselves are never explicitly deleted; their CRDT entity range is reserved **Implication for JS scenes:** Host identity components are always queryable. Do not assume their presence means the host is physically inside the scene. Remote player presence accurately reflects co-location only when the host is current. --- ## Movement Encoding & Transmission ### NetworkMovementMessage Fields `timestamp`, `position`, `velocity`, `velocitySqrMagnitude`, `rotationY`, `headIKYawEnabled`, `headIKPitchEnabled`, `headYawAndPitch`, `movementKind`, `animState`, `velocityTier`, `isSliding`, `isStunned`, `isInstant`, `isEmoting`. ### Encoding Pipeline - **`FloatQuantizer`** -- Fixed-size quantization via scaled integers. `Compress(value, min, max, bits)` / `Decompress`. - **`ParcelEncoder`** -- Flattens 2D parcel coordinates `(x, y)` into a 1D index relative to the Genesis City grid. - **`TimestampEncoder`** -- Circular buffer encoding with wraparound detection on decompression. ### Send Throttling `PlayerMovementNetSendSystem` caps at **10 messages/sec**. Adaptive send rate: drops to `MoveSendRate` on movement, doubles (up to `StandSendRate`) when idle. Immediate sends on grounded/jump state changes. Messages are sent to both Island and Scene pipes via `MultiplayerMovementMessageBus.Send()`, supporting both compressed and uncompressed schemas. --- ## Movement Interpolation ### InterpolationSpline Types | Type | Description | |------|-------------| | `Linear` | `Vector3.Lerp` wrapper | | `Hermite` | Cubic Hermite spline matching start/end positions and velocities | | `MonotoneYHermite` | Hermite with Y-axis monotonicity (prevents vertical overshoot) | | `FullMonotonicHermite` | Monotonic on all axes | | `Bezier` | Cubic Bezier with velocity-derived control points | | `VelocityBlending` | Projective velocity blending (Murphy & Lengyel 2011) | | `PositionBlending` | Projective position blending (Murphy & Lengyel 2011) | ### RemotePlayersMovementSystem Runs in `PresentationSystemGroup`. Processes a priority queue of `NetworkMovementMessage` per remote player: 1. **First message** -- teleport to position 2. **Cooldown** -- wait 2x `MoveSendRate` for stability 3. **Interpolation** -- starts between past and new message using selected spline; falls back to `Linear` when velocity is near-zero or grounded state changes 4. **Extrapolation** -- when no messages arrive and speed > threshold, `ExtrapolationComponent` continues movement along last velocity for up to `TotalMoveDuration` 5. **Blend** -- after extrapolation stops, blends back with speed-limited interpolation **Catch-Up:** When inbox has more messages than `CatchUpMessagesMin`, interpolation duration is shortened proportionally to smooth DeltaTime. --- ## Profile & Entity Sync ### EntityParticipantTable Bidirectional mapping between wallet IDs and ECS entities, with room source tracking (`ISLAND`, `GATEKEEPER`, or both). NOT thread-safe -- only accessed from main thread. Key API: `Register(walletId, entity, fromRoom)`, `Release(walletId, fromRoom)` (returns true only when `ConnectedTo == RoomSource.NONE`), `AddRoomSource(walletId, fromRoom)`. `Release` only removes the entity when disconnected from all rooms. This allows a player visible via Island to remain alive when they leave the Scene room. ### ThreadSafe Disconnect Buffering LiveKit participant callbacks fire off the main thread. `LiveKitRemoveIntentions` buffers disconnect events using `MutexSync`. Main thread consumes via `OwnedBunch` which acquires the lock on construction, exposes the collection, and clears + releases on `Dispose()`. ### ProfileBroadcast Sends `AnnounceProfileVersion` to both Island and Scene pipes. Async: fetches self-profile version before sending via `SendAndDisposeAsync` with `KindReliable`. --- ## SDK Propagation Systems - **`PlayerTransformPropagationSystem`** (Global -> Scene) -- Copies `CharacterTransform` into the scene world's `SDKTransform` when transform has changed and player is assigned to scene. - **`WritePlayerTransformSystem`** (Scene -> CRDT) -- Converts `SDKTransform` to scene-relative coordinates and writes via `IECSToCRDTWriter`. - **`WritePlayerIdentityDataSystem`** (Scene -> CRDT) -- Writes `PBPlayerIdentityData` (address + isGuest) on dirty, with force-write on `Initialize()`. Uses static lambda to avoid closures. - **`PlayerProfileDataPropagationSystem`** (Global -> Scene) -- Copies `Profile` data to scene entities via `CharacterDataPropagationUtility` when profile or CRDT entity assignment is dirty. --- ## Detailed Reference For detailed code examples, see [reference.md](reference.md). --- ## Cross-References - **ecs-system-and-component-design** -- System patterns, `ref var` mutation rules, cleanup lifecycle - **async-programming** -- LiveKit async flows, `CancellationTokenSource` management, `UniTaskVoid` exception handling - **scene-runtime-and-crdt** -- CRDT bridge (`IECSToCRDTWriter`), scene state machine, `MultiThreadSync` - **cross-world-ecs-access** -- Global/scene world access, `PlayerCRDTEntity`/`PlayerSceneCRDTEntity` bridge, `ISceneIsCurrentListener` --- Source attribution: decentraland Original project: https://github.com/decentraland/unity-explorer/blob/9a03ce0931a819ca98557660d4d7531009ce100c/.claude/skills/multiplayer-and-network-sync/SKILL.md License: Apache-2.0 License information: https://www.apache.org/licenses/LICENSE-2.0 Original upstream instructions; downloaded from Mega AI Hub. ## Upstream license notice Apache License Version 2.0, January 2004 http://www.apache.org/licenses/ TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION 1. 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See the License for the specific language governing permissions and limitations under the License. # Mega AI Hub resource metadata Creator / publisher: decentraland Original project: https://github.com/decentraland/unity-explorer/blob/9a03ce0931a819ca98557660d4d7531009ce100c/.claude/skills/multiplayer-and-network-sync/SKILL.md Attribution basis: Source repository record Fields of work: Development & Engineering Roles: Backend Developer, Full-Stack Developer, Game Developer Specialties: API, Design, Frontend, PowerPoint Tools: Livekit Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: 9b733ea569e5201e2248fd0bbeb987cfa1066f4012b73bf9b2967fbfe2814c50