Network mechanics

Basics

The network code with the GameFrames (GF) and NetWorkFrames (NWF) works like that:

• GF (happens every n ms):
  • Advance Simulation/Game by one step
    * not directly influenced by user action, e.g. no new building added or new flag set
  • Increase GF number
    later (in the GameLoop):
  • Collect GameCommands (GC)
  • Possibly save game
• NWF (happens before every nth GF):
  • Wait for GC-Message from all players (happens only when a player lags)
  • Execute 1 GC-Message from each player, record those GCs to replay and discard that Message (Here new buildings etc. are placed)
  • Send all accumulated GCs as new GC-Message to server (which forwards it to all players)
  • Clear list of accumulated GCs to start new collection

So obviously

1. GCs are executed with a delay of 1-2 NWFs
   (maximum if send right after a NWF -> Wait 1 NWF till it is sent to server and 1 NWF till it is executed)
2. Each client must send an (empty) GC-Message at the start of the game
   (or the game would wait forever on the next NWF)
3. The length of a NWF must be at least as long as the ping (round-trip over network) to the "slowest" player
   (the GC-Message must be received within a NWF to avoid having players to wait for others)

Server

The server is in charge for:

1. Starting the game
2. Distributing Messages (mostly GC-Messages and Chat)
3. Executing the AI
4. Detecting when a player quit or lags to much behind
5. Detecting asyncs (Game differs between players)

Therefore the general algorithm is like this:

• Send StartGame-Message
  • Tells players to prepare the map and which NWF length to use
  • Initiates sending the first (empty) GC-Message from each player
• Send RealStart-Message (actually a special Done-Message)
  • Contains number of first GF to execute
  • Contains length of 1 GF
• Execute the following in a loop till game is closed:
  1. If time for 1 GF has passed:
     • If it is a NWF:
       • Search for lagging players by checking if a GC-Message (from last NWF) is missing
         If one is found, kick or warn him and skip the rest of step i. (try again a bit later)
       • Send GCs of AI-players
       • Send a NWF-Done-Message with next GF number to execute
     • Run AI for 1 Step
     • Increase GF number
  2. Relay messages

Client

On the clients side it is now pretty straight-forward:

• When StartGame-Message is received, load map, send empty GC-Message
• When RealStart-Message is received, set/check first GF# and set GF-Length, then Start the game:
• Repeat the following every n ms according to GF length:
  1. If it is a NWF:
     • Search for lagging players (non GC-Message received)
       Skip the rest if one is found (try again later)
     • Execute the rest of the NWF (see Basics above)