Promul is a simple, lightweight, and easy-to-use relay server and session management implementation for Unity Netcode for GameObjects.

Promul is designed to quickly allow independent and small developers to have fully functional peer-to-peer (P2P) gameplay over top of an open-source relay server, allowing players to play together without complicated port-forwarding configurations, and with minimal infrastructure investment and expenditure for the development team.

It works by providing a relay server and wire protocol for clients, allowing players to both host and join games using Unity's Netcode for GameObjects library. It ships with its own UDP-based networking solution, which can be used independently of the relay protocol.

Promul uses a heavily-modified version of the LibNetLib library for its internal networking, converting the library from a thread-based non-async model to a Task-based Asynchronous Programming (TAP)-based implementation. Technical details of the networking are explained in engineering section below.

Promul is intended to be a free and open-source alternative to Unity Relay. It supports join secret-based session management, with a REST API for creating and deleting sessions, and an admin UI for managing active sessions.

Clone this repository and build and run the program under /Server. The relay server will bind on UDP port 4098 while the API server will bind on TCP port 3000.

To create a new session, call PUT /session/create. The API will respond with the join code.

Install the transport by adding it in the Unity Package Manager by git URL:
https://github.com/jacksonrakena/Promul.git

Next, add PromulTransport to your NetworkManager GameObject. Set the Address and Port to the address and port of your relay server.

Example:

You should now be ready to use Promul as a relay server for your Unity project. You can call NetworkManager's StartHost/StartClient/StartServer methods as normal.

flowchart TD
    subgraph Unity
    Code[Your Unity code] -->|Unity Netcode| Runtime[Promul.Runtime.Unity]
    end

    subgraph Unreal Engine
    UECode[Your UE code] --> RuntimeUE["Promul.Runtime.Unreal\n(planned)"]
    end

    subgraph "Protocol layer"
    Protocol[Promul.Relay.Protocol]
    end
    Protocol-->Core
    Runtime-->Protocol
        RuntimeUE-->Protocol
    
    subgraph Relay server
    API[Promul.Server.API\nFront-facing]-->Server[Promul.Server]
    Server-->Protocol
    end

    subgraph Transit layer
    Core[Promul\nKey networking capabilities]
    Core-->Native[Native sockets]
    Core-->Managed[Managed sockets]
    end

Promul's included networking solution is a very heavily modified version of the amazing LibNetLib by Ruslan Pyrch.

Promul is probably compatible with LibNetLib clients and servers. I don't know, and I don't check.
Compatibility with LibNetLib is not a goal of Promul.

Among other differences:

• The library requires .NET Standard 2.1.
  • This limits the Unity versions that can use the library, but also allows the library to use newer constructs such as ArraySegment<T>.
• The NetDataReader and NetDataWriter classes have been removed in favour of BinaryReader and BinaryWriter from System.IO.
  • This means support for INetSerializable has been removed. Clients are expected to create extension overloads for BinaryWriter.Write and BinaryReader.Read{Type}.
• The logic and receive threads have been removed, in favour of a long-running Task spawned by the NetManager.
• Most of the library's blocking methods have been replaced with Task-based Asynchronous Programming (TAP) methods, using Task. This requires a significant rewrite of most parts of the library.
• NAT punch-through and manual mode have both been removed.
  • The transport will now put transport events onto a queue, and NetworkTransport#PollEvents will dequeue events and pass them to Unity Netcode.
• NetManager#Start has been replaced by PromulManager#ListenAsync, which will begin infinitely looping, waiting for incoming messages.
  • This method will block, so in the Unity transport, it is spawned on an un-awaited Task.
  • Servers are expected to use a paradigm such as BackgroundService to run the long-running Task.
• Documentation has been improved.
• NetPacket no longer has jurisdiction over its data. It is a wrapper around an ArraySegment<byte>.
• Most (if not all) usages of byte[] data, int offset, int count have been replaced with ArraySegment<byte>.
• INetBasedListener and related classes have been removed. All events are invoked on the PromulManager class as regular CLR event types.
• In many classes, extraneous overloads have been removed in favour of default parameters.

Promul's Unity relay uses its own wire protocol over the transit layer. The protocol is shown in the following diagram:

sequenceDiagram
    autonumber
    Host (ID 0)->> Relay API: PUT /session/create
    Relay API->>Relay: Creates session
    Relay API->>-Host (ID 0): Sends join code "KDS92F"   connection info
    
    Host (ID 0)->>Relay: UDP Connect (KDS92F)
    Client (ID 1)->> Relay API: PUT /session/join KDS92F
    note right of Relay API: Join code is transferred out-of-band
    Relay API->>-Client (ID 1): Sends connection address and port

    Client (ID 1)->> Relay: UDP Connect (KDS92F)
    Relay->>Host (ID 0): 0x11 CLIENT_CONNECTED
    Relay->>-Client (ID 1): 0x10 CONNECTED

    rect rgba(0, 0, 255, .1)
            note right of Relay: Data is now relayed by 0x00 DATA messages
    Client (ID 1)->> Relay: 0x00 DATA, target=0
    Relay-->>-Host (ID 0): 0x00 DATA, author=1
    Host (ID 0)->> Relay: 0x00 DATA, target=1,
    Relay->>-Client (ID 1): 0x00 DATA, author=0
    note right of Host (ID 0): author field becomes target when sending to relay
    end

    Client (ID 1)--xRelay: Connection lost
    Relay->> Host (ID 0): 0x12 CLIENT_DISCONNECTED
    note right of Relay: Relay notifies host of disconnection
    
    Host (ID 0)--xRelay: Connection lost
    Relay->>Relay API: Destroys session

• Task-based asynchronous programming rewrite
• Authentication and binding tests
• MTU negotation tests
• Replace logging with a better solution
• Restore merged sending support
• Restore NAT punch-through(?)
• Improve memory and throughput performance
• More real-world replications in tests
• More uses of ValueTask<T> over regular tasks

• Basic create/join session by join code
  • Update session manager
• Basic web interface
  • Further development of web interface
• Multicast(?)

© 2023 Firework Eyes Studio (NZBN 9429048922678) under the MIT License.

This project uses a heavily modified version of LibNetLib.
LiteNetLib © 2020 Ruslan Pyrch, under the following license:

MIT License

Copyright (c) 2020 Ruslan Pyrch

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