Unity Multiplayer & Networking

Multiplayer Architecture Overview

Unity's multiplayer ecosystem comprises several layers:

Layer	Package/Service	Purpose
High-Level	Netcode for GameObjects	GameObject-based networking logic
High-Level	Netcode for Entities	DOTS-based networking
Low-Level	Unity Transport (com.unity.transport)	UDP/WebSocket communication with optional reliability, ordering, fragmentation
Services	Relay	NAT traversal via cloud relay servers
Services	Lobby	Matchmaking and session discovery
Services	Sessions SDK	Player group management
Tools	Multiplayer Play Mode	Simulate up to 4 players in-editor
Tools	Multiplayer Tools	Analysis, debugging, testing utilities

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<T> -- 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

Target	Description
Server	Executes on server; locally if called on server
NotServer	All clients except server (excludes host)
Owner	Object's owner only
NotOwner	Everyone except owner
Authority	Server in client-server; owner in distributed authority
NotAuthority	All non-authority instances
ClientsAndHost	All clients including host
Everyone	All instances on the observer list
Me	Local execution only
NotMe	Everyone except sender
SpecifiedInParams	Target set at runtime via RpcSendParams

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

Class	Key Members
NetworkManager	StartHost(), StartServer(), StartClient(), Shutdown(), Singleton, ConnectedClients, LocalClientId, SceneManager, ConnectionApprovalCallback
NetworkObject	Spawn(), SpawnWithOwnership(), SpawnAsPlayerObject(), Despawn(), ChangeOwnership(), NetworkShow(), NetworkHide(), NetworkObjectId, OwnerClientId, IsOwner, HasAuthority
NetworkBehaviour	OnNetworkSpawn(), OnNetworkDespawn(), OnGainedOwnership(), OnLostOwnership(), IsServer, IsClient, IsHost, IsOwner, IsSpawned, HasAuthority, RpcTarget
NetworkVariable<T>	.Value, OnValueChanged, ReadPerm, WritePerm, CheckDirtyState()
NetworkList<T>	Add(), Remove(), Insert(), Clear(), Count, OnListChanged
NetworkSceneManager	LoadScene(), UnloadScene(), OnSceneEvent
[Rpc(SendTo.X)]	Server, Owner, NotOwner, ClientsAndHost, Everyone, NotMe, Authority, SpecifiedInParams

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