Multiplayer

Documentation Unreal Engine Player Combat Multiplayer Replication

Play online on a listen or dedicated server: a player's moves answer the button at once, the server has the final word on what they cost and hit, and the editor can add lag to test it.

Set it up

2 minutes

1
Play as two clients
In the Play dropdown, set Net Mode to Play As Client and Number of Players to 2: a dedicated server with two players.
2
Add lag
In Editor Preferences, Level Editor, Play, Multiplayer Options, tick Enable Network Emulation, set Emulation Target to Everyone, and pick the Network Emulation Profile Average, or Bad for a poor connection. To lag one machine, use its console:
```
NetEmulation.PktLag 80            // 80 ms on each packet it sends
NetEmulation.PktLagVariance 20    // Up to 20 ms either way
NetEmulation.PktLoss 5            // 5% dropped
```
NetEmulation.Off turns it off again.
3
Try it under lag
Swing, hit an enemy, draw a weapon from the belt, open a chest and lock on. The swing, the lock and the draw animation answer the press at once; the enemy's health, the drawn weapon and the chest's walk follow a round trip (a packet to the server and back) later.

What runs where

What	Who decides
Moves	The player's machine, on the press; the server confirms the move or ends it.
Hits	The attacking player's machine, at contact; the server checks each before damage, and finds an AI's hits itself.
Damage, health, defense	The server.
Costs and cooldowns	Both machines check, the server spends. A cooldown blocks at once; its countdown and the spend show a round trip later.
Equipment, belt, inventory	The server, a round trip after the press; the reach plays at once. Each player receives only their own inventory, and a loot chest's inventory reaches every player near it.
Interaction, paired actions	The server, checking the option and reach again; they start a round trip after the press.
Targeting and lock-on	The player's machine, at once.
Camera, HUD, telegraphs, prompts	Each machine on its own, sending nothing.

Before you test online

Check	Why
A team on the pawn, not its controller	Other machines read a controller's team as none (Targeting).
A combat style's Rotation, Targeting, Approach and Movement fragments on Every Machine	The server steers attacks by its own values. Read each fragment's Scope in the style, such as `CombatStyle_UPC_Soulslike`; Every Machine is the default (Combat Styles).
Steering Mode, Attack Facing and the Targeting Config, set at runtime, on every machine	They do not replicate, so the server and each watcher need the same values. Call Set Steering Mode and Set Attack Facing on the UPC Attack Approach, and Set Targeting Config on the UPC Targeting component, on the server and on each player's machine (Attack Steering).
A fast dash holds the movement's Grace Tag	Without it a lagging server pulls a long dash back. Put `UPC.State.Dashing`, the demo's tag, in the dodge's State Tag Window beside `UPC.State.Intangible` (Patterns).
Replication Mode Mixed or Full on an ability system the PlayerState carries	Under Minimal the player's machine receives no cooldown timers. Set it in C++ with `SetReplicationMode`; the UPC Character Base's own runs Mixed.

Cheating and fairness

By default a hit counts once it lands on the attacker's screen, suiting co-op and PvE.

For competitive PvP, set Hit Detection Authority to Server Authoritative: the server finds every hit itself and refuses reported ones, and the attacker sees contact a round trip late. Set it on a character's UPC Melee Trace, or for every character left on Project Default with Default Hit Detection Authority in Project Settings, Plugins, Universal Player Combat (Melee Trace).

AdvancedWhat the server checks

A reported hit must come from a swing window the server's copy of the attack plays, land within the weapon's reach plus what latency explains, and stay within the hits and targets that window allows. One that fails deals nothing.
Moves: which may follow which, and how soon. Costs and cooldowns, equipment changes, interaction reach, and how far a dash carries the player.