# VATSIM FSD Protocol Documentation

> Updated January 2025

FSD is an application-layer TCP/IP protocol facilitating real-time communications between all clients connected to the [VATSIM](https://vatsim.net) Network.

This documentation is reverse-engineered.

See [openfsd](https://github.com/renorris/openfsd) for an alternative, free and open-source FSD server implementation.

For an introduction, see the [About](/about/) page.

*This documentation is an independent work and is not affiliated with, endorsed by, or associated with VATSIM, Inc.*