Transports

Unity Netcode for GameObjects (Netcode) uses Unity Transport by default and supports UNet Transport (deprecated) up to Unity 2022.2 version.

So what is a transport layer?

The transport layer establishes communication between your application and different hosts in a network.

A transport layer can provide:

• Connection-oriented communication to ensure a robust connection before exchanging data with a handshake protocol.
• Maintain order delivery for your packets to fix any discrepancies from the network layer in case of packet drops or device interruption.
• Ensure data integrity by requesting retransmission of missing or corrupted data through using checksums.
• Control the data flow in a network connection to avoid buffer overflow or underflow--causing unnecessary network performance issues.
• Manage network congestion by mediating flow rates and node overloads.
• Adjust data streams to transmit as byte streams or packets.

Unity Transport package

Netcode's default transport Unity Transport is an entire transport layer that you can use to add multiplayer and network features to your project with or without Netcode. Refer to the Transport documentation for more information and how to install the Transport package.

SinglePlayerTransport

Netcode for GameObjects provides a SinglePlayerTransport that you can use to create a local single player network session. This simplifies switching between multiplayer and single player sessions within the same project, while still being able to use existing netcode scripts. The SinglePlayerTransport is a effectively a mock transport that ensures full NetworkTransport functionality without any transport dependencies.

Set up a single player session

In addition to your default network transport, you need to add the SinglePlayerTransport to the NetworkManager GameObject (or child of).

To start a single player session, assign the SinglePlayerTransport to the NetworkManager.NetworkConfig.NetworkTransport configuration property using a script.

For example:

using Unity.Netcode;
using Unity.Netcode.Transports.UTP;
using Unity.Netcode.Transports.SinglePlayer;
using UnityEngine;

public class ExtendedNetworkManager : NetworkManager
{
    private UnityTransport m_UnityTransport;
    private SinglePlayerTransport m_SinglePlayerTransport;

    private void Awake()
    {
        m_UnityTransport = GetComponent<UnityTransport>();
        m_SinglePlayerTransport = GetComponent<SinglePlayerTransport>();
    }

    public void StartSinglePlayer()
    {
        // Use the single player transport when starting a single player session.
        NetworkConfig.NetworkTransport = m_SinglePlayerTransport;
        if (!StartHost())
        {
            NetworkLog.LogError("Failed to start single player session!");
        }
    }

    public void StartHostedSession()
    {
        // Use the network transport when starting a multiplayer session.
        NetworkConfig.NetworkTransport = m_UnityTransport;
        if (!StartHost())
        {
            NetworkLog.LogError("Failed to start hosted session!");
        }
    }
}

As shown in the script above, when starting a single player session the SinglePlayerTransport is assigned to the NetworkConfig.NetworkTransport, and when starting a hosted multiplayer session the UnityTransport is assigned.

Single player limitations

When running a single player session, you should take any netcode script into consideration that requires an actual multiplayer session to exist.

RPC considerations

You can invoke any RPC that includes:

• The host's client.
• The host's server.
• SendTo targets that will be invoked locally:
  • SendTo.Me
  • SendTo.Server
  • SendTo.Everyone
  • SendTo.Authority
  • SendTo.Owner
  • ClientsAndHost
  • SpecifiedInParams: As long as it is targeting the NetworkManager.LocalClientId.
• SendTo targets that will not be invoked locally:
  • SendTo.NotOwner
  • SendTo.NotServer
  • SendTo.NotMe
  • SendTo.NotAuthority
  • SpecifiedInParams: Anything not targeting the NetworkManager.LocalClientId will not be invoked locally.

NetworkVariable considerations

NetworkVariables should work as expected because the host will always be both the server and the owner of anything spawned. This means that write permissions, whether server or owner, should not be an issue.

Distributed authority considerations

To start a single player session using the distributed authority network topology you should:

• Keep your network topology setting set to distributed authority.
• Set the NetworkConfig.NetworkTransport to the SinglePlayerTransport component.
• Start as a host.

Unity's UNet transport layer API

UNet is a deprecated solution that is no longer supported after Unity 2022.2. Unity Transport Package is the default transport for Netcode for GameObjects. We recommend transitioning to Unity Transport as soon as possible.

Community transports or writing your own

You can use any of the community contributed custom transport implementations or write your own.

The community transports are interchangeable transport layers for Netcode and can be installed with the Unity Package Manager. After installation, the transport package will appear in the Select Transport dropdown of the NetworkManager. Check out the Netcode community contributed transports for more information.

To start writing your own and contributing to the community, check out the Netcode community contribution repository for starting points and how to add your content.