Developers Guide

Source Code

• The source for the project is versioned using git
• The repository is hosted at sourceforge.net
  • https://git.code.sf.net/p/fgms

To checkout the source code run

git clone https://git.code.sf.net/p/fgms/src fgms-src

Code Layout

• src/
  • server/ - the source code for fgms
  • flightgear/ - see flightgear below
  • plib/ - see plib below
  • simgear/ - see simgear below
• contrib/
  • fgracker/ - the Tracker Server
  • mpstatus/ - a www page and perl script to query mp statuses
  • plib/ - see plib below
  • tools/ - contains check_fgms.sh, a script to check fgms status
• docs/ - stuff to do with documentation in this page

Other Libs

The fgms repository contains all the libs required to install. However some parts were copied, borrowed etc from other sources:

simgear

• Low level maths, geospatial calculations, etc
• fgms used the constants, logging and some structures
• simgear and flightear source are very closely related
• Info: http://wiki.flightgear.org/SimGear
• Code: https://gitorious.org/fg/simgear
• parts of simgear have been copied into src/simgear

plib

• PLIB - A Suite of Portable Game Libraries
• fgms used the network sockets (see netSocket.hxx and netSocket.cxx)
• Info: http://plib.sourceforge.net
• parts of plib have been copied into src/plib/

flightgear

• fgms uses parts of the mpmessages and XDR code. (see mpmessages.hxx and tiny_xdr.hxx)
• Info: FlightGear page
• parts of flightgear have been copied into src/flightgear/

MultiPlayer Overview

This is an overview about how the communicatinons works between fgfs instances via an fgms.

• The table below shows communications with three players over a network
• Messages are sent a few times a second

fgms - me	packet	fgms	packet	fgms - others
me	> send >	> relay >	> recv >	other 1
			> recv >	other 2
me	< recv <	< relay <	< send <	other 1
		> relay >	> recv >	other 2

Data Interchange

• Send
  • packets are assembled on an fgfs instance, values such as callsign, position, speed etc
  • the values are packaged into an XDR encoded binary blob
  • the packet is transmitted via udp out to an mp server
  • eg –multiplayer=out,15,mpserver14.flightgear.org,5000
• MpServer
  • an fgms server will recieve this as a "LOCAL" connection, ie a direct connection (FG_SERVER::HandlePacket)
  • fgms will then relay the packet to other local players, and to the relay servers (FG_SERVER::SendToRelays)
• Reciever
  • a fgms client the other ends will recive the packet
  • decode the data encoded in XDR
  • then update the sim with this data
• And < vice > versa

Data Packets

• The data encoded into a sequence of bytes using XDR.
• Every packet starts with 'Header' bytes which define whats in the rest of the packet.

  An Mp Message Packet
  Header	Other data

Header

• The T_MsgHdr is the c struct used to define the header.

  Name	Comment
  Magic	The "magic" that must start each packet, see MSG_MAGIC
  ProtoVer	The protocol version which must match - see PROTO_VER
  MsgId	the MsgId is the "type" of message that is sent
  MsgLen	The length of this message
  ReplyAddr	–not-used–
  ReplyPort	–not-used–
  Callsign	Length is MAX_CALLSIGN_LEN abc123

  • T_MsgHdr::MsgId - the MsgId is one of:
    • ::POS_DATA_ID - a position message and the most trafficked
    • ::RESET_DATA_ID - to reset ?
    • CHAT_MSG_ID - for mp chat BUT

      Warning

      CHAT_MSG_ID is not used in current implementation. Instead chat is packaged into properties.

Position Message

• T_PositionMsg is the c struct used to define a position

  Name	Comment
  Model	Length is MAX_MODEL_NAME_LEN eg /model/foo/aero.xml
  time	epoch time
  lag	lag ??
  position	double[3]
  orientation	float[3]
  linearVel	float[3]
  angularVel	float[3]
  linearAccel	float[3]
  angularAccel	float[3]

XDR

• External Data Representation
  • Info: http://en.wikipedia.org/wiki/External_Data_Representation
  • Spec: http://www.ietf.org/rfc/rfc1832.txt

XDR is a standard data serialization format. It allows data to be transferred between different kinds of computer systems. Converting from the local representation to XDR is called encoding. Converting from XDR to the local representation is called decoding. XDR is implemented as a software library of functions which is portable between different operating systems and is also independent of the transport layer.

fgms uses a small subset of xdr necessary for multiplayer. This is defined in tiny_xdr.hxx

Contribute

You are more than welcome to contribute to the project.

• If you have a patch, or a bug then contact one of the Developers or the FG Mailing List
• Looks at the Bugs

Documentation

The documentation is created using doxygen and is generated from source.

• Here for some formating info
  • http://www.stack.nl/~dimitri/doxygen/manual/commands.html
• Private variables/function are documented
• Pages, ie stuff not related to source such as this page are in /doc/pages/
• The doxygen tag file is located at fgms.tag
• Doxygen 1.8+ is required for building the documents.