Backroll is a reimplementation of GGPO in C#, made explicitly for Unity, and retooled to use Hourai Networking as a transport level abstraction layer.

Backroll is currently alpha-quality software. Expect bugs and use at your own risk.

Backroll is most easily installable via Unity Package Manager. In Unity 2018.3+, add the following to your Packages/manifest.json:

{
  "dependencies": {
    "com.houraiteahouse.backroll": "0.1.1"
  },
  "scopedRegistries": [
    {
      "name": "Hourai Teahouse",
      "url": "https://upm.houraiteahouse.net",
      "scopes": [
        "com.houraiteahouse",
        "com.facepunch",
        "com.discord"
      ]
    }
  ]
}

NOTE: Backroll is compiled with specific constants which apply hard limitations on the kind of data it can handle, like the maximum number of players in a given game and in-memory size of a input.Generally speaking, these are common sense limitations, growing outside these limitations may suggest that Backroll is not the correct solution you're looking for. If that does not deter you, it is then suggested to directly embed the package under your project's Packages/ directory by git cloning it there and directly editing the source code.

TODO(james7132): Document this

Networking real-time games is hard. Latency in fast paced games can be a serious issue for players. For example, fast paced fighting games often require reaction windows of less than 100 ms. With the round trip time of many network connections stretching into 200-500 ms of latency, traditional methods of networking (i.e. lockstep simulation, frame delay, etc.) may introduce alterations to gameplay that negatively impact player experience.

Rollback netcode attempts to remedy this with speculative execution: it attempts to predict remote inputs based on prior inputs. This allows games to continue simulation as if the game were entirely local, rolling back and replaying the simulation only if recieved inputs differ from the prediction. Players only notice the network latency if the predictions are wrong, making for a much smoother online experience.

GGPO (Good Game Peace Out) was one of the original implemenations of rollback netcode originally intended for use in fighting games, targetting only

GGPO currently only builds on Windows, being directly dependent on Win32 APIs. Unity offers one of the best options when building cross platform games. Reimplementing GGPO as Backroll in Unity enables much easier cross platform networking solutions.

Unity also added unsafe raw pointer manipulation utilities commonly used in C/C++ (memcpy, mpmset, etc.) commonly used across GGPO's source code. This allows Backroll to maintain the largely the same performance profile as the original GGPO.

Tangentially related: porting to C# allows use of Hourai Networking whch enables the transport layer to be abstracted. The original GGPO used a raw UDP communication layer, that developers could replace with their own by rewriting the UdpProtocol class as needed. This however, requires a complete understanding of both GGPO's source code. Hourai Networking adds one level of indirection and makes Backroll agnostic to the underlying network implemenation. This makes integrations with platform specific networks like Steam or Discord trivial.

Backroll is available under The MIT License. This means Backroll is free for commercial and non-commercial use. Attribution is not required, but appreciated.