Unreal Multiplayer

unreal-multiplayer

Core Philosophy

Unreal Engine was built from its inception in 1998 as a multiplayer tournament engine. Networking in Unreal is deeply opinionated: it centers around a strict Server-Authoritative Client-Server architecture orchestrated by the GameMode, GameState, PlayerState, and PlayerController classes. The server runs the definitive physical simulation; clients are visual and audio playback terminals that send player intent. Master Unreal multiplayer by understanding the Actor Replication lifecycle, property replication with DOREPLIFETIME, and the strict division between Multicast, Client, and Server RPCs.

4-Step Unreal Multiplayer Architecture

Step 1: The Core Gameplay Framework Classes

1. GameMode (Server Only):
   • Exists exclusively on the authoritative server. Defines match rules, win conditions, player spawning (SpawnDefaultPawnAtTransform), and score tracking. Never replicated to clients.
2. GameState (Server $\to$ All Clients):
   • Replicated to all machines. Tracks match state (Round time, Team scores, Match phase: WaitingToStart, InProgress, PostMatch).
3. PlayerState (Server $\to$ All Clients):
   • Replicated to all machines. Tracks individual player attributes (Player Name, Ping, Kills, Current Score).
4. PlayerController (Server $\leftrightarrow$ Owning Client):
   • Exists on the server and the specific owning client. Handles input translation, UI HUD management, and secure Server RPC communication.

Step 2: Actor & Property Replication (DOREPLIFETIME)

1. Actor Replication Setup:
   • bReplicates = true in constructor.
   • SetReplicateMovement(true) for physical actors.
2. Replicated Properties (ReplicatedUsing):
   • Replicate variables using UPROPERTY(ReplicatedUsing = OnRep_Health).
   • Implement GetLifetimeReplicatedProps:

     void ACharacterBase::GetLifetimeReplicatedProps(TArray<FLifetimeProperty>& OutLifetimeProps) const {
         Super::GetLifetimeReplicatedProps(OutLifetimeProps);
         DOREPLIFETIME(ACharacterBase, Health);
         DOREPLIFETIME_CONDITION(ACharacterBase, SecretInventory, COND_OwnerOnly);
     }
     

3. RepNotify Functions:
   • OnRep_Health() executes automatically on clients whenever the server updates Health.

Step 3: Remote Procedure Call (RPC) Classification

1. Server RPC (Server, Reliable, WithValidation):
   • Called by Owning Client $\to$ Executed on Server.
   • Must implement _Validate (Security gate) and _Implementation:

     bool AWeapon::ServerFire_Validate(FVector AimDir) { return AimDir.IsNormalized(); }
     void AWeapon::ServerFire_Implementation(FVector AimDir) { /* Authoritative Fire */ }
     

2. Client RPC (Client, Reliable):
   • Called by Server $\to$ Executed on Owning Client only (e.g. ClientShowHitMarker).
3. NetMulticast RPC (NetMulticast, Unreliable):
   • Called by Server $\to$ Executed on Server and all simulated clients (e.g. MulticastPlayExplosionFX).

Step 4: Network Relevancy & Dormancy Optimization

1. Network Relevancy (IsNetRelevantFor):
   • Actors beyond view distance or hidden behind terrain are not replicated to distant clients, conserving network bandwidth.
2. Net Dormancy (DORM_DormantAll):
   • Static or rarely changing world actors (chests, switches) are set dormant until an event wakes them up, saving server tick budget.

Deliverable Format: Authoritative Server-Validated Weapon Firing (C++)

// Weapon.h
#pragma once
#include "CoreMinimal.h"
#include "GameFramework/Actor.h"
#include "Weapon.generated.h"

UCLASS()
class AWeapon : public AActor {
    GENERATED_BODY()

public:
    AWeapon();

    UPROPERTY(ReplicatedUsing = OnRep_AmmoCount, BlueprintReadOnly)
    int32 AmmoCount;

    UFUNCTION()
    void OnRep_AmmoCount();

    UFUNCTION(Server, Reliable, WithValidation)
    void ServerFire(FVector_NetQuantize TraceStart, FVector_NetQuantizeNormal AimDir);

    UFUNCTION(NetMulticast, Unreliable)
    void MulticastSpawnMuzzleFX();

protected:
    virtual void GetLifetimeReplicatedProps(TArray<FLifetimeProperty>& OutLifetimeProps) const override;
};

Worked Example: Mitigating Dedicated Server Bandwidth Saturation

• Context: A 32-player Unreal shooter was experiencing severe packet drops and server tick degradation down to 14 Hz during firefights.
• Diagnosis: Using NetProfile and Network Insights, engineers discovered weapon firing was using Reliable NetMulticast RPCs and replicating unquantized 64-bit vector coordinates 30 times per second.
• Optimization:
  1. Converted FVector to FVector_NetQuantize (16-bit integer compression).
  2. Changed cosmetic FX RPCs from Reliable to Unreliable.
  3. Activated Net Dormancy on 800 static interactive loot containers in the map.
• Outcome: Server bandwidth per client decreased by 68%; server tick rate stabilized at 60 Hz.

Verification Checklist

• GameMode contains zero client-side logic and is never accessed from client HUDs.
• All Server RPCs implement _Validate() security checks to prevent packet injection.
• DOREPLIFETIME_CONDITION used to prevent sending private data (e.g. inventory) to non-owners.
• Cosmetic effects transmitted via Unreliable Multicast RPCs.
• Net Relevancy distance and Net Dormancy configured for large maps.

Anti-Patterns

• Calling Server RPCs on Un-Owned Actors: Invoking a Server RPC from a client on an Actor that is not possessed or owned by that client's PlayerController (fails silently).
• Using Reliable RPCs for Frequent Ticks: Marking high-frequency weapon fire or footstep RPCs as Reliable, causing TCP-style packet queues when latency spikes.
• Accessing AGameMode on Clients: Writing GetWorld()->GetAuthGameMode() inside client UI widgets, which returns nullptr on non-server machines.