Unity Multiplayer

unity-multiplayer

Core Philosophy

Multiplayer game programming is an exercise in distributed systems operating under the constraints of physics and latency. You cannot trust the client; anything calculated on a player's machine—hit detection, movement speed, inventory transactions—will inevitably be hacked. High-performance Unity multiplayer requires a server-authoritative model with Netcode for GameObjects (NGO), Unity Relay & Lobby for NAT traversal, and client-side prediction with server reconciliation to mask network latency for players.

4-Step Unity Multiplayer Architecture

Step 1: Topology & Transport (NGO + Relay + Lobby)

1. Network Topology:
   • Dedicated Game Server (DGS): Absolute authority, zero host advantage, highest server cost. Best for competitive matchmaking.
   • Listen Server / Host-Client: One player acts as both host and server. Zero server cost, but host has 0ms latency advantage.
2. Unity Transport & Relay:
   • Use Unity Transport Package (UTP) combined with Unity Relay service to pierce corporate firewalls and symmetric NATs without requiring players to configure router port forwarding.
3. Unity Lobby:
   • Handles room matchmaking, player slot reservations, and custom lobby metadata before spinning up the network transport.

Step 2: Server Authority & NetworkVariables

1. NetworkBehaviour & Ownership:
   • Code checks IsServer, IsClient, and IsOwner:

     if (!IsOwner) return; // Only local player handles input
     

2. NetworkVariable<T>:
   • Replicates state from Server $\to$ Clients automatically:

     public NetworkVariable<int> Health = new(
         100, 
         NetworkVariableReadPermission.Everyone, 
         NetworkVariableWritePermission.Server // Never allow client writes!
     );
     

Step 3: Remote Procedure Calls (RPCs)

1. ServerRpc: Client calls, Server executes (e.g. RequestFireWeaponServerRpc).
2. ClientRpc: Server calls, all Clients execute (e.g. PlayExplosionFxClientRpc).
3. RPC Rule of Thumb:
   • Use NetworkVariable for persistent state (Health, Gold, Position).
   • Use RPCs for transient one-off events (Sound effects, particle spawns).

Step 4: Lag Compensation & Client Prediction

1. Client-Side Prediction:
   • Client immediately applies local input and updates position locally without waiting for server confirmation round-trip.
2. Server Reconciliation:
   • Server validates movement: $$\Delta \vec{p} \le \vec{v}_{\text{max}} \times \Delta t + \epsilon$$
   • If client position drifts beyond error threshold $\epsilon$, server overwrites client position with authoritative coordinates, and client smoothly blends to the corrected position.
3. Lag Compensated Hitbox History:
   • Server rewinds player hitboxes back in time by the attacker's latency $T_{\text{rtt}} / 2$ to verify if the shot lined up when the client pulled the trigger.

Deliverable Format: Server-Authoritative NGO Weapon Firing Pattern

using Unity.Netcode;
using UnityEngine;

public class NetworkPlayerWeapon : NetworkBehaviour {
    [SerializeField] private GameObject bulletPrefab;
    [SerializeField] private Transform muzzleTransform;

    private void Update() {
        if (!IsOwner) return;
        if (Input.GetButtonDown("Fire1")) {
            // Request firing to the authoritative server
            FireWeaponServerRpc(muzzleTransform.position, muzzleTransform.forward);
        }
    }

    [ServerRpc]
    private void FireWeaponServerRpc(Vector3 origin, Vector3 direction) {
        // Server validation check
        if (Vector3.Distance(origin, muzzleTransform.position) > 1.5f) {
            Debug.LogWarning($"[Security] Player {OwnerClientId} origin spoofing detected!");
            return;
        }

        // Spawn networked bullet
        GameObject bullet = Instantiate(bulletPrefab, origin, Quaternion.LookRotation(direction));
        bullet.GetComponent<NetworkObject>().Spawn();
        
        // Notify all clients to play audio/vfx
        PlayMuzzleFlashClientRpc();
    }

    [ClientRpc]
    private void PlayMuzzleFlashClientRpc() {
        // Play local visual muzzle flash and sound effect
    }
}

Worked Example: Resolving Rubberbanding in a Fast-Paced Arena Shooter

• Problem: In an action arena shooter built on NGO, players with 120ms ping experienced jarring stutter and teleportation (rubberbanding) whenever sprinting.
• Root Cause: The server was transmitting absolute position snapshots at 20Hz; client prediction was lacking, snapping the local player to historical server coordinates.
• Solution: Implemented tick-based input buffering and reconciliation. Client stores a history buffer of unconfirmed input ticks. When receiving a server correction, the client re-simulates all unacknowledged ticks from the corrected state.
• Result: Movement became completely smooth for players with up to 180ms latency.

Verification Checklist

• All NetworkVariable write permissions set strictly to Server.
• Unity Relay service handles NAT punchthrough without manual port forwarding.
• Client movement inputs validated server-side against maximum acceleration limits.
• Weapon hit registration verified on server using historical hitbox rewind.
• Network objects cleanly despawned using NetworkObject.Despawn() on disconnect.

Anti-Patterns

• Client-Authoritative Damage: Allowing clients to send DealDamageServerRpc(targetId, 9999) without server-side weapon range, line-of-sight, or ammo validation.
• Over-Broadcasting RPCs Every Frame: Sending continuous transform updates via RPCs instead of compressed NetworkTransform ticks.
• Ignoring Network Disconnections: Leaving orphaned player GameObjects in the scene when a client forcibly disconnects or loses connection.