Unity Multiplayer

Unity Multiplayer & Networking

Multiplayer Architecture Overview

Unity's multiplayer ecosystem comprises several layers:

Topology options:

• Client-Server (Dedicated): Server has authority; clients send inputs, server validates
• Client-Server (Listen/Host): One player acts as both server and client
• Distributed Authority: Ownership-based authority distributed among clients

Use the Multiplayer Center (Window > Multiplayer > Multiplayer Center) to get package recommendations based on your game's needs.

Netcode for GameObjects Setup

Installation

Install com.unity.netcode.gameobjects (v2.10+) via Package Manager. This automatically pulls in com.unity.transport.

NetworkManager Configuration

Add a NetworkManager component to a GameObject in your scene. It is the singleton entry point for all networking.

using Unity.Netcode;

public class GameLauncher : MonoBehaviour
{
    public void StartAsHost()
    {
        NetworkManager.Singleton.StartHost();
    }

    public void StartAsServer()
    {
        NetworkManager.Singleton.StartServer();
    }

    public void StartAsClient()
    {
        NetworkManager.Singleton.StartClient();
    }

    public void Shutdown()
    {
        NetworkManager.Singleton.Shutdown();
    }
}

Key NetworkManager properties:

• IsServer, IsClient, IsHost -- execution context
• ConnectedClients -- dictionary of connected clients
• ConnectedClientsIds -- read-only list of client IDs
• LocalClientId -- local client's ID
• SceneManager -- NetworkSceneManager instance
• SpawnManager -- NetworkSpawnManager instance
• NetworkConfig -- project network configuration

Key NetworkManager events: OnClientConnectedCallback, OnClientDisconnectCallback, OnConnectionEvent, OnServerStarted/OnServerStopped, OnClientStarted/OnClientStopped, OnTransportFailure

NetworkObject and NetworkBehaviour

NetworkObject

Every networked GameObject needs a NetworkObject component. It provides identity, ownership, and visibility.

Key properties:

• NetworkObjectId (ulong) -- unique ID synchronized across network
• IsSpawned -- whether spawned on the network
• OwnerClientId -- client ID of current owner
• IsOwner -- true if local player owns this object
• HasAuthority -- true if local instance has authority

Spawning (server-side only):

// Basic spawn
NetworkObject netObj = Instantiate(prefab).GetComponent<NetworkObject>();
netObj.Spawn();

// Spawn with specific owner
netObj.SpawnWithOwnership(clientId);

// Spawn as player object
netObj.SpawnAsPlayerObject(clientId);

// Despawn
netObj.Despawn(destroy: true);

Ownership (server-side only):

netObj.ChangeOwnership(newClientId);
netObj.RemoveOwnership();

Visibility:

netObj.NetworkShow(clientId);
netObj.NetworkHide(clientId);
bool visible = netObj.IsNetworkVisibleTo(clientId);

NetworkBehaviour

All networked scripts inherit from NetworkBehaviour instead of MonoBehaviour.

Lifecycle methods (in order):

1. OnNetworkPreSpawn(ref NetworkManager) -- before any spawning
2. OnNetworkSpawn() -- after NetworkObject spawns; register handlers here
3. OnNetworkPostSpawn() -- after all sibling NetworkBehaviours spawn
4. OnNetworkPreDespawn() -- before despawn
5. OnNetworkDespawn() -- on despawn

Ownership callbacks:

• OnGainedOwnership() / OnLostOwnership()
• OnOwnershipChanged(ulong previous, ulong current) -- fires on all clients

using Unity.Netcode;

public class PlayerController : NetworkBehaviour
{
    public override void OnNetworkSpawn()
    {
        if (IsOwner)
        {
            // Initialize local player controls
            EnableInput();
        }
    }

    void Update()
    {
        if (!IsOwner) return;
        // Only the owner processes input
        HandleMovement();
    }

    public override void OnNetworkDespawn()
    {
        // Cleanup
    }
}

Status checks: IsServer, IsClient, IsHost, IsOwner, IsSpawned, IsLocalPlayer, HasAuthority

NetworkVariables

NetworkVariable<T> synchronizes state from server to all clients automatically. Type T must be unmanaged (primitives, unmanaged structs).

public class PlayerHealth : NetworkBehaviour
{
    public NetworkVariable<int> Health = new NetworkVariable<int>(
        value: 100,
        readPerm: NetworkVariableReadPermission.Everyone,
        writePerm: NetworkVariableWritePermission.Server
    );

    public override void OnNetworkSpawn()
    {
        Health.OnValueChanged += OnHealthChanged;
    }

    public override void OnNetworkDespawn()
    {
        Health.OnValueChanged -= OnHealthChanged;
    }

    private void OnHealthChanged(int oldValue, int newValue)
    {
        Debug.Log($"Health changed: {oldValue} -> {newValue}");
        UpdateHealthUI(newValue);
    }

    // Server-side only (due to WritePerm.Server)
    public void TakeDamage(int amount)
    {
        if (!IsServer) return;
        Health.Value -= amount;
    }
}

Write permissions:

• NetworkVariableWritePermission.Server (default) -- only server can write
• NetworkVariableWritePermission.Owner -- only owner can write

NetworkList -- synchronized list (T must be unmanaged + IEquatable<T>):

public class Inventory : NetworkBehaviour
{
    public NetworkList<int> Items;

    void Awake()
    {
        Items = new NetworkList<int>();
    }

    public override void OnNetworkSpawn()
    {
        Items.OnListChanged += OnItemsChanged;
    }

    private void OnItemsChanged(NetworkListEvent<int> changeEvent)
    {
        Debug.Log($"List changed: {changeEvent.Type}");
    }
}

RPCs (ServerRpc, ClientRpc)

RPCs are remote procedure calls between server and clients. Methods must be in a NetworkBehaviour and use the [Rpc] attribute with a SendTo target.

Unified Rpc Attribute (v2.x)

public class CombatSystem : NetworkBehaviour
{
    // Server executes this when any client calls it
    [Rpc(SendTo.Server)]
    void AttackRpc(int targetId, RpcParams rpcParams = default)
    {
        ulong senderId = rpcParams.Receive.SenderClientId;
        ProcessAttack(senderId, targetId);
    }

    // All clients (and host) execute this
    [Rpc(SendTo.ClientsAndHost)]
    void ShowDamageEffectRpc(Vector3 position, int damage)
    {
        SpawnDamagePopup(position, damage);
    }

    // Only the owner executes this
    [Rpc(SendTo.Owner)]
    void NotifyOwnerRpc(string message)
    {
        Debug.Log(message);
    }

    // Everyone including sender
    [Rpc(SendTo.Everyone)]
    void PlaySoundRpc(int soundId)
    {
        AudioManager.Play(soundId);
    }
}

SendTo Targets

Legacy Attributes (still supported)

[ServerRpc]
void RequestSpawnServerRpc(ServerRpcParams rpcParams = default)
{
    // Runs on server; only owner can call by default
}

[ClientRpc]
void UpdateUIClientRpc(int score)
{
    // Runs on all clients
}

Connection Approval

Use ConnectionApprovalCallback on NetworkManager to validate connecting clients.

// Server-side: register approval callback
NetworkManager.Singleton.ConnectionApprovalCallback = (request, response) =>
{
    string password = System.Text.Encoding.UTF8.GetString(request.Payload);
    response.Approved = (password == "secret");
    response.CreatePlayerObject = response.Approved;
    // Optionally set: response.PlayerPrefabHash, response.Position, response.Rotation
    if (!response.Approved) response.Reason = "Invalid password";
    response.Pending = false; // Signal decision is made
};

// Client-side: set payload before connecting
NetworkManager.Singleton.NetworkConfig.ConnectionData =
    System.Text.Encoding.UTF8.GetBytes("secret");
NetworkManager.Singleton.StartClient();

Scene Management

NetworkSceneManager (accessed via NetworkManager.Singleton.SceneManager) handles synchronized scene loading.

public class GameSceneManager : NetworkBehaviour
{
    public void LoadGameScene()
    {
        if (!IsServer) return;
        // Server-only: loads scene on all clients
        NetworkManager.Singleton.SceneManager.LoadScene("GameScene", LoadSceneMode.Single);
    }

    public void LoadAdditiveScene()
    {
        if (!IsServer) return;
        NetworkManager.Singleton.SceneManager.LoadScene("Arena", LoadSceneMode.Additive);
    }

    public override void OnNetworkSpawn()
    {
        NetworkManager.Singleton.SceneManager.OnSceneEvent += OnSceneEvent;
    }

    void OnSceneEvent(SceneEvent sceneEvent)
    {
        switch (sceneEvent.SceneEventType)
        {
            case SceneEventType.LoadComplete:
                Debug.Log($"Client {sceneEvent.ClientId} loaded {sceneEvent.SceneName}");
                break;
            case SceneEventType.LoadEventCompleted:
                Debug.Log($"All clients loaded {sceneEvent.SceneName}");
                break;
            case SceneEventType.SynchronizeComplete:
                Debug.Log($"Client {sceneEvent.ClientId} fully synchronized");
                break;
        }
    }
}

Scene event types: OnLoad, OnUnload, OnSynchronize, OnLoadComplete, OnUnloadComplete, OnLoadEventCompleted, OnUnloadEventCompleted, OnSynchronizeComplete

Important: Do NOT start new scene events within scene event callbacks.

Unity Multiplayer Services (Relay, Lobby)

Full integration examples in references/transport-layer.md

Unity Relay

Relay provides NAT punchthrough via cloud relay servers (no port forwarding needed). Requires UGS authentication.

Host flow: Create allocation, get join code, configure transport, start host. Client flow: Join allocation with code, configure transport, start client.

// Host: create relay and start
Allocation allocation = await RelayService.Instance.CreateAllocationAsync(maxPlayers);
string joinCode = await RelayService.Instance.GetJoinCodeAsync(allocation.AllocationId);
transport.SetRelayServerData(allocation.ToRelayServerData("dtls")); // "dtls" = encrypted UDP
NetworkManager.Singleton.StartHost();

// Client: join relay and connect
JoinAllocation join = await RelayService.Instance.JoinAllocationAsync(joinCode);
transport.SetRelayServerData(join.ToRelayServerData("dtls"));
NetworkManager.Singleton.StartClient();

Unity Lobby

Lobby provides session discovery and matchmaking. Store the Relay join code in lobby data.

// Create lobby with relay join code
_lobby = await LobbyService.Instance.CreateLobbyAsync(name, maxPlayers, new CreateLobbyOptions {
    Data = new Dictionary<string, DataObject> {
        { "JoinCode", new DataObject(DataObject.VisibilityOptions.Member, relayJoinCode) }
    }
});

// Query available lobbies
QueryResponse response = await Lobbies.Instance.QueryLobbiesAsync(new QueryLobbiesOptions {
    Filters = new List<QueryFilter> {
        new QueryFilter(QueryFilter.FieldOptions.AvailableSlots, "0", QueryFilter.OpOptions.GT)
    }
});

// Join and extract relay code
Lobby lobby = await LobbyService.Instance.JoinLobbyByIdAsync(lobbyId);
string joinCode = lobby.Data["JoinCode"].Value;

// IMPORTANT: Send heartbeats every 15s or lobby expires
await LobbyService.Instance.SendHeartbeatPingAsync(lobby.Id);

Common Patterns

Player Spawning with Custom Prefab (Server-Side)

// In a NetworkBehaviour on the server:
NetworkManager.Singleton.OnClientConnectedCallback += (ulong clientId) => {
    GameObject player = Instantiate(playerPrefab);
    player.GetComponent<NetworkObject>().SpawnAsPlayerObject(clientId);
};

Owner-Authoritative Movement

public class PlayerMovement : NetworkBehaviour
{
    public NetworkVariable<Vector3> Position = new(writePerm: NetworkVariableWritePermission.Owner);
    void Update()
    {
        if (!IsOwner) return;
        // Note: Uses legacy Input for brevity. See unity-input for the new Input System.
        Vector3 move = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
        transform.position += move * Time.deltaTime * 5f;
        Position.Value = transform.position;
    }
}

Server-Authoritative with Input RPCs

public class ServerAuthMovement : NetworkBehaviour
{
    [Rpc(SendTo.Server)]
    void MoveRpc(Vector3 input) { transform.position += input * Time.deltaTime * 5f; }
    void Update()
    {
        if (!IsOwner) return;
        MoveRpc(new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"))); // legacy Input; see unity-input
    }
}

Anti-Patterns

1. Writing to NetworkVariable without authority -- Only the server (or owner with WritePerm.Owner) can write. Client writes are silently ignored.

2. Forgetting IsOwner checks in Update -- Without owner checks, all clients run input logic, causing conflicting state.

3. Using Instantiate without Spawn -- Objects created with Instantiate alone are local-only. Always call Spawn() on the NetworkObject for network visibility.

4. Spawning from client code -- Only the server can spawn NetworkObjects. Clients must send an RPC to request spawning.

5. Heavy data in RPCs instead of NetworkVariables -- RPCs are fire-and-forget; late joiners miss them. Use NetworkVariables for persistent state.

6. Not unsubscribing from OnValueChanged -- Subscribe in OnNetworkSpawn, unsubscribe in OnNetworkDespawn to prevent leaks.

7. Starting scene events inside scene event callbacks -- NetworkSceneManager forbids this; causes undefined behavior.

8. Not sending Lobby heartbeats -- Lobbies expire without periodic SendHeartbeatPingAsync calls (every 15-30 seconds).

9. Using NetworkVariable for frequent small updates -- For high-frequency data (position), prefer NetworkTransform or custom serialization.

10. Calling RPCs before OnNetworkSpawn -- RPCs require the NetworkObject to be spawned. Defer to OnNetworkSpawn.

Key API Quick Reference

Related Skills

• unity-foundations -- Core Unity concepts, GameObjects, components, scene hierarchy
• unity-scripting -- C# scripting, MonoBehaviour lifecycle, coroutines
• unity-physics -- Physics systems; use NetworkRigidbody for synced physics

Additional Resources

• Unity 6.3 Multiplayer Manual
• Netcode for GameObjects 2.10 API
• Unity Transport 2.x
• Unity Relay Documentation
• Unity Lobby Documentation
• Boss Room Sample