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Reliable & unreliable messages over UDP. Robust message fragmentation & reassembly. Encryption.
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GameNetworkingSockets is a basic transport layer for games. The features are:

  • Connection-oriented API (like TCP)
  • ... but message-oriented (like UDP), not stream-oriented.
  • Supports both reliable and unreliable message types
  • Messages can be larger than underlying MTU. The protocol performs fragmentation, reassembly, and retransmission for reliable messages.
  • An ack model significantly more sophisticated than a basic TCP-style sliding window. It is based on the "ack vector" model from DCCP (RFC 4340, section 11.4) and Google QUIC and discussed in the context of games by Glenn Fiedler. The basic idea is for the receiver to efficiently communicate to the sender the status of every packet number (whether or not a packet was received with that number). By remembering what segments were sent in each packet, the sender can deduce which individual segments need to be retransmitted.
  • Encryption. AES-GCM-256 per packet, Ed25519 crypto for key exchange and cert signatures. The details for shared key derivation and per-packet IV are based on the design used by Google's QUIC protocol.
  • Tools for simulating loss and detailed stats measurement
  • IPv6

What it does not do:

  • Higher level serialization of entities, delta encoding of changed state variables, etc
  • Compression

Why do I see "Steam" everywhere?

The main interface class is named SteamNetworkingSockets, and many files have "steam" in their name. But Steam is not needed. If you don't make games or aren't on Steam, feel free to use this code for whatever purpose you want.

The reason for "Steam" in the names is that this provides a subset of the functionality of the API with the same name in the Steamworks SDK. Our main reason for releasing this code is so that developers won't have any hesitation coding to the API in the Steamworks SDK. On Steam, you will link against the Steamworks version, and you can get the additional features there (access to the relay network). And on other platforms, you can use this version, which has the same names for everything, the same semantics, the same behavioural quirks. We want you to take maximum advantage of the features in the Steamworks version, and that won't happen if the Steam code is a weird "wart" that's hidden behind #ifdef STEAM.

The desire to match the Steamworks SDK also explains a somewhat anachronistic coding style and weird directory layout. This project is kept in sync with the Steam code here at Valve. When we extracted the code from the much larger codebase, we had to do some relatively gross hackery. The files in folders named tier0, tier1, vstdlib, common, etc have especially suffered trauma. Also if you see code that appears to have unnecessary layers of abstraction, it's probably because those layers are needed to support relayed connection types or some part of the Steamworks SDK.

Language bindings

The library was written in C++, but there is also a plain C interface to facilitate binding to other languages.

Third party language bindings:



  • CMake or Meson, and build tool like Ninja, GNU Make or Visual Studio
  • A C++11-compliant compiler, such as:
    • GCC 7.3 or later
    • Clang 3.3 or later
    • Visual Studio 2017 or later
  • One of the following crypto solutions:
    • OpenSSL 1.1.1 or later
    • OpenSSL 1.1.x, plus ed25519-donna and curve25519-donna. (We've made some minor changes, so the source is included in this project.)
    • bcrypt (windows only)
  • Google protobuf 2.6.1+


If you're building on Linux or Mac, just install the appropriate packages from your package manager.


# apt install libssl-dev

Arch Linux:

# pacman -S openssl

Mac OS X, using Homebrew:

$ brew install openssl
$ export PKG_CONFIG_PATH=$PKG_CONFIG_PATH:/usr/local/opt/openssl/lib/pkgconfig

For MSYS2, see the MSYS2 section. There are packages available in the MinGW repositories for i686 and x86_64.

For Visual Studio, you can install the OpenSSL binaries provided by Shining Light Productions. The Windows CMake distribution understands how to find the OpenSSL binaries from these installers, which makes building a lot easier. Be sure to pick the installers without the "Light"suffix. In this instance, "Light" means no development libraries or headers.


If you're building on Linux or Mac, just install the appropriate packages from your package manager.


# apt install libprotobuf-dev protobuf-compiler

Arch Linux:

# pacman -S protobuf

Mac OS X, using Homebrew:

$ brew install protobuf

For MSYS2, see the MSYS2 section. There are packages available in the MinGW repositories for i686 and x86_64.

For Visual Studio, the process is a bit more involved, as you need to compile protobuf yourself. The process we used is something like this:

C:\dev> git clone
C:\dev> cd protobuf
C:\dev\protobuf> git checkout -t origin/3.5.x
C:\dev\protobuf> mkdir cmake_build
C:\dev\protobuf> cd cmake_build
C:\dev\protobuf\cmake_build> vcvarsall amd64
C:\dev\protobuf\cmake_build> cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -Dprotobuf_BUILD_TESTS=OFF -Dprotobuf_BUILD_SHARED_LIBS=ON -DCMAKE_INSTALL_PREFIX=c:\sdk\protobuf-amd64 ..\cmake
C:\dev\protobuf\cmake_build> ninja
C:\dev\protobuf\cmake_build> ninja install


If you already have the dependencies installed (see above sections), then you should be able to build fairly trivially.

Using Meson:

$ meson . build
$ ninja -C build

Or CMake:

$ mkdir build
$ cd build
$ cmake -G Ninja ..
$ ninja


You can also build this project on MSYS2. First, follow the instructions on the MSYS2 website for updating your MSYS2 install.

Be sure to follow the instructions at the site above to update MSYS2 before you continue. A fresh install is not up to date by default.

Next install the dependencies for building GameNetworkingSockets (if you want a 32-bit build, install the i686 versions of these packages):

$ pacman -S \
    git \
    mingw-w64-x86_64-gcc \
    mingw-w64-x86_64-meson \
    mingw-w64-x86_64-openssl \
    mingw-w64-x86_64-pkg-config \

And finally, clone the repository and build it:

$ git clone
$ cd GameNetworkingSockets
$ meson . build
$ ninja -C build

NOTE: When building with MSYS2, be sure you launch the correct version of the MSYS2 terminal, as the three different Start menu entries will give you different environment variables that will affect the build. You should run the Start menu item named MSYS2 MinGW 64-bit or MSYS2 MinGW 32-bit, depending on the packages you've installed and what architecture you want to build GameNetworkingSockets for.

Visual Studio

When configuring GameNetworkingSockets using CMake, you need to add the protobuf bin directory to your path in order to help CMake figure out the protobuf installation prefix:

C:\dev\GameNetworkingSockets> mkdir build
C:\dev\GameNetworkingSockets> cd build
C:\dev\GameNetworkingSockets\build> set PATH=%PATH%;C:\sdk\protobuf-amd64\bin
C:\dev\GameNetworkingSockets\build> vcvarsall amd64
C:\dev\GameNetworkingSockets\build> cmake -G Ninja ..
C:\dev\GameNetworkingSockets\build> ninja

Visual Studio Code

If you're using Visual Studio Code, we have a few extensions to recommend installing, which will help build the project. Once you have these extensions installed, open up the .code-workspace file in Visual Studio Code.

C/C++ by Microsoft

This extension provides IntelliSense support for C/C++.

VS Marketplace Link:

CMake Tools by vector-of-bool

This extension allows for configuring the CMake project and building it from within the Visual Studio Code IDE.

VS Marketplace Link:

Meson by Ali Sabil

This extension comes in handy if you're editing the Meson build files.

VS Marketplace Link:


The current focus is to get a stable release that matches the first release of this interface in the public Steamworks SDK.

After that, here are some large features that we expect to add to a future release:

Bandwidth estimation

An earlier version of this code implemented TCP-friendly rate control (RFC 5348) But as part of the reliability layer rewrite, bandwidth estimation has been temporarily broken, and a fixed (configurable) rate is used. It's not clear if it's worth the complexity of implementation and testing to get sender-calculated TCP-friendly rate control implemented, or a simpler method would do just as good. Whatever method we use, needs to work even if the app code inspects the state and decides not to send a message. In this case, the bandwidth estimation logic might perceive that the channel is not "data-limited", when it essentially is. We could add an entry point to allow the application to express this, but this is getting complicated, making it more difficult for app code to do the right thing. It'd be better if it mostly "just worked" when app code does the simple thing.

NAT piercing (ICE/STUN/TURN)

The Steamworks code supports a custom protocol for relaying packets through our network of relays and on our backbone. At this time the open-source code does not have any support for piercing NAT or relaying packets. But since the Steamworks code already has those concepts, it should be pretty easy to add support for this. You'd still be responsible for running the STUN/TURN servers and doing the rendezvous/signalling, but the code could use them.

Non-connection-oriented interface

The Steam version has ISteamMessages, which is a UDP-like interface. Messages are addressed by peer identity, not connection handle. (Both reliable and unreliable messages are still supported.) We should open-source this API, too. Previously it was only for P2P, but we've found that it's useful for porting UDP-based code.

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