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Reliable & unreliable messages over UDP. Robust message fragmentation & reassembly. P2P networking / NAT traversal. Encryption.


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GameNetworkingSockets Build Status

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.
  • A reliability layer 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 which segments were sent in each packet, the sender can deduce which segments need to be retransmitted.
  • Encryption. AES-GCM-256 per packet, Curve25519 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 packet latency/loss, and detailed stats measurement
  • Head-of-line blocking control and bandwidth sharing of multiple message streams ("lanes") on the same connection. You can use strict priority values, softer weight values that control how bandwidth is shared, or some combination of the two methods. See ISteamNetworkingSockets::ConfigureConnectionLanes.
  • IPv6 support
  • Peer-to-peer networking:
    • NAT traversal through google WebRTC's ICE implementation.
    • Plug in your own signaling service.
    • Unique "symmetric connect" mode.
    • ISteamNetworkingMessages is an interface designed to make it easy to port UDP-based code to P2P use cases. (By UDP-based, we mean non-connection-oriented code, where each time you send a packet, you specify the recipient's address.)
    • See for more info
  • Cross platform. This library has shipped on consoles, mobile platforms, and non-Steam stores, and has been used to facilitate cross-platform connectivity. Contact us to get access to the code. (We are not allowed to distribute it here.)

What it does not do:

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

Quick API overview

To get an idea of what the API is like, here are a few things to check out:

  • The include/steam folder has the public API headers.
  • The Steamworks SDK documentation offers web-based documentation for these APIs. Note that some features are only available on Steam, such as Steam's authentication service, signaling service, and the SDR relay service.
  • Look at these examples:
    • example_chat.cpp. Very simple client/server program using all reliable messages over ordinary IPv4.
    • test_p2p.cpp. Shows how to get two hosts to connect to each other using P2P connectivity. Also an example of how to write a signaling service plugin.


See BUILDING for more information.

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:

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 access the additional services provided by the Steam Datagram Relay network. On other platforms and stores, as long as you ship a version of your game on Steam, you might be able to take advantage of these services. See the Steamworks documentation for more information. Because this is a live service, and we need to control our security and backward compatibility burden, at this time we are not able to offer access to SDR on other platforms to all partners.

If you aren't a Steam partner, or don't have a version of your game on Steam, then use this opensource version of the API and take advantage of the permissive license to do whatever you want. We want you to take maximum advantage of the features in the Steamworks version. That won't happen if this API is a weird "wart" that's hidden behind #ifdef STEAM, which is why we're making this opensource version available.

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.


Did you find a security vulnerability? Please inform us responsibly; you may be eligible for a bug bounty. See the security policy for more information.