Multiplayer and Networking Basics

# CHAPTER 16

Multiplayer and Networking Basics #

1. Introduction #

2. Learning Objectives #

• Understand the Host/Server/Client networking architecture.

• Understand the concept of "Server Authority" to prevent cheating.

• Install Unity's Netcode for GameObjects (NGO).

• Synchronize a Player's Transform across multiple computers.

• Understand RPCs (Remote Procedure Calls) for network communication.

3. Client-Server Architecture #

• The Client: This is the player playing the game on their home PC. They send inputs ("I pressed jump") to the Server.

• The Server: The central dictator. It receives inputs, calculates the physics, and tells all Clients exactly what happened.

• The Host: In indie games, having dedicated servers is expensive. Instead, one player acts as both a Client *and* the Server simultaneously. This is called "Hosting."

4. Server Authority (Anti-Cheat) #

• The Client only says: *"I clicked my mouse."*

• The Server says: *"You clicked, therefore you swung your sword, therefore you hit the monster, therefore the monster died, therefore 1000 gold drops."*

5. Netcode for GameObjects (NGO) #

using Unity.Netcode; // Required for multiplayer!

public class PlayerMultiplayer : NetworkBehaviour
{
    void Update()
    {
        // IsOwner checks if THIS computer is the one controlling this specific character.
        // We do not want Player A's keyboard to move Player B's character!
        if (!IsOwner) return;

        // Normal movement code goes here
        transform.Translate(Vector3.forward * Time.deltaTime);
    }
}

6. Synchronizing Movement (Network Transform) #

1. 1. Attach a NetworkObject component to your Player Prefab.

1. 2. Attach a NetworkTransform component to your Player Prefab.

7. RPCs (Remote Procedure Calls) #

• ServerRpc: A message sent from a Client to the Server. ("I am asking to shoot.")

• ClientRpc: A message sent from the Server to all Clients. ("Everyone, play a gunshot sound!")

void Update()
{
    if (!IsOwner) return;

    if (Input.GetMouseButtonDown(0))
    {
        // 1. Tell the Server we want to shoot!
        RequestShootServerRpc(); 
    }
}

[ServerRpc]
void RequestShootServerRpc()
{
    // 2. The Server receives the request, verifies ammo, and tells ALL clients to play the effect.
    PlayShootEffectClientRpc();
}

[ClientRpc]
void PlayShootEffectClientRpc()
{
    // 3. This runs on EVERY computer in the lobby instantly.
    particleSystem.Play();
}

8. Visual Learning: The Multiplayer Pipeline #

[ Player A's PC (Client) ]       [ Player B's PC (Client) ]
           \                           /
       (Sends inputs)            (Receives visual updates)
             \                       /
              v                     v
              [ THE HOST SERVER ]
     (Calculates hit detection, spawns enemies)

9. Best Practices #

• NetworkManager Setup: Every multiplayer game requires an empty GameObject with a NetworkManager component. You must define a Transport (like Unity Transport) which dictates the underlying UDP/TCP network protocols. You must also register your Player Prefab inside the NetworkManager so it knows what to spawn when someone joins!

10. Common Mistakes #

• Syncing Everything: Beginners try to sync the exact rotation of every blade of grass blowing in the wind. This will instantly crash your network bandwidth. Only synchronize objects that actually affect gameplay (Players, Enemies, Bullets). Visual fluff should be calculated locally on each client's machine.

11. Mini Project: Setup a Networked Player #

1. 1. Create a Capsule. Name it "NetworkPlayer".

1. 2. Add a NetworkObject component.

1. 3. Add a NetworkTransform component.

1. 4. Add the PlayerMultiplayer.cs script (from Section 5).

1. 5. Drag the "NetworkPlayer" into your Project Window to make it a Prefab. Delete it from the scene.

1. 6. Create an Empty GameObject named NetworkManager. Add the NetworkManager component.

1. 7. Set the Transport to Unity Transport.

1. 8. Drag your "NetworkPlayer" prefab into the Player Prefab slot on the NetworkManager.

1. 9. When you click "Start Host" in your UI, Unity will automatically spawn your networked player into the world!

12. Practice Exercises #

1. 1. What specific C# base class must your scripts inherit from to access Unity's Netcode functionality?

1. 2. If you want a visual effect (like an explosion) to happen on every player's screen simultaneously, what type of RPC should the Server call?

13. MCQs with Answers #

14. Interview Questions #

• Q: Explain the mechanical difference between a ServerRpc and a ClientRpc in Unity's Netcode for GameObjects. Walk through a scenario utilizing both to throw a grenade.

• Q: Describe the concept of IsOwner in a multiplayer script. Why is checking if (!IsOwner) return; the absolute most critical line of code in a multiplayer movement script?

• Q: Contrast a dedicated Server architecture with a Host/Client architecture. What are the financial and gameplay tradeoffs of relying on players to Host games?

15. FAQs #

16. Summary #

17. Next Chapter Recommendation #