openfsd/docs/about.md

About

The FSD protocol functions primarily as a message forwarder. Aside from a few direct client-server interactions, its main purpose is to relay messages between flight simulator clients via a centralized server. This architecture is not peer-to-peer; all communication is routed through the central server.

FSD is a plaintext protocol that can be easily intercepted and analyzed with tools such as Wireshark. By default, it operates on TCP port 6809, and its functionality can be tested using telnet:

telnet <FSD server address> 6809

Various implementations of FSD exist, each with unique protocol nuances. This project specifically replicates VATSIM behavior, which differs from other networks such as IVAO.

• FSD messages consist of plaintext MS-DOS-style lines ending with CR/LF characters.
• The protocol is strictly limited to the ISO/IEC 8859-1 aka. 'Latin alphabet no. 1' character set.
• Each message, or 'line', begins with a 1- or 3-character packet identifier, followed by colon (:) delimited fields.
• All numerical values are encoded as base-10 (or occasionally base-16) ASCII strings, with no raw binary data used.
• Clients are identified by plaintext aviation callsigns (e.g., N7938C).
• Most packets include "From" and "To" fields, which serve as source and recipient identifiers, respectively.
• Depending on the packet type, the "To" field may specify a single recipient or a group of clients.

Example Server Identification Packet

$DISERVER:CLIENT:VATSIM FSD V3.43:d95f57db664f\r\n

Hexadecimal Representation

00000000  24 44 49 53 45 52 56 45  52 3a 43 4c 49 45 4e 54   $DISERVE R:CLIENT
00000010  3a 56 41 54 53 49 4d 20  46 53 44 20 56 33 2e 34   :VATSIM  FSD V3.4
00000020  33 3a 64 39 35 66 35 37  64 62 36 36 34 66 0d 0a   3:d95f57 db664f..

Explanation

• Packet Type Identifier: $DI
• Fields are delimited by colon (:) characters.
• Sender: SERVER (reserved callsign for the server)
  
• Recipient: CLIENT (placeholder used for an unknown client)
  
• Server version identifier: VATSIM FSD V3.43
  
• Random data (see documentation): d95f57db664f
• Packet is terminated with the delimiter sequence: \r\n (0x0d0a)

For technical documentation on packet types, see Protocol.