1. CSE 381 – Advanced Game Programming Multiplayer Gaming
Doom, by Id

2. Life Before Networked Gaming
Play against your own scores
Play against AI
Play against other players taking turns
Play against other players via a split screen

3. Doom was not the first, but it changed everything
Life After Doom
Doom was not the first, but it changed everything
gaming communities
multiplayer cultures
coincided nicely with Internet boom & graphical computing
Significant Ancestors of DOOM multiplayer:
PLATO games
Empire, based on Star Trek
SPASIM
MUDs

4. John Daleske & Silas Warner of Iowa State Gameplay
Empire (1973)
John Daleske & Silas Warner of Iowa State
Gameplay
supported up to 32 players working in teams
screen updates once every 10 seconds
your view is of space quadrants
you have weapons like phasers
goal is to drop armies on every planet in the universe

5. Originally “Space Sim” Also for PLATO system
SPASIM (1974)
Originally “Space Sim”
Also for PLATO system
Jim Bowery w/ Universities of Iowa & Illinois
He’s offering $500 to anyone who can find an earlier multiplayer FPS:
Used a first person perspective
players flew around in space
others appeared as wireframe ships

6. Star vs. Mesh Topologies (Ref: http://www. webopedia
Star (client/server) Topology
All devices are connected to a central hub (server)
Nodes communicate across the network by passing data through the hub
Mesh (peer to peer) Topology
every node has a connection to every other node in the network.
provides faster communication
burden on clients to enforce application rules
minimize size of messages

7. What is communicated across networks?
What data?
What is communicated across networks?
player locations
player states
player appearance
player orientation
voice communications
Note: send the minimum amount of data necessary
minimize data type sizes (bytes are nice)

8. Networked games must be:
Bottom Line
Networked games must be:
fast
reliable
Which should be used (client/server or peer-to-peer) for MMOs?
it depends, but mostly client/server
Architectures in use:
pure client/server
pure peer to peer
hybrid client/server & peer to peer

9. Multiplayer Issues Latency Cheating Subscriptions
a difficult issue in multiplayer games
How do you handle communication delays?
Where is the delay?
lag is commonly due to overloaded networks
Cheating
How do you ensure fairness?
Subscriptions
How do you ensure only paying customers are playing?

10. What service provider do you want the connection to use?
Decisions to make
What service provider do you want the connection to use?
Will you be “hosting” this connection or will you connect to an existing session?
a first host connection must accept the incoming network data from the other peers
a first host starts the session

11. Other Networking Topics
Questions for a reliable gaming network:
How often should data be sent?
How do you handle lost clients?
How do you handle lost host clients?
How do you handle slow clients?
Peer-to-Peer/Client-Server combinations

12. Server Game State Server maintains database of:
server control objects
“true” game state of various objects (player/object locations, player/object states, etc.)
connections
Continuously, the Server:
Collects input from connections
Updates control objects according to input
Sends updated game state on a need to know basis

13. A networked game is its traffic
Some networked engines have built-in demo recording options
How?
log all network communications. Why?
play back a game for a demo
in effect, a game is its network traffic and the state changes it causes

14. Common Strategies Player data sent to server:
user input (moves, firing, etc) on timed basis (ex: 30 times/sec)
if no input (player idle), little if anything will be sent
Server data sent to player
world game state (other player & enemy locations & states)
on a timed basis
Huge amount of data in the world game state. Many efficiencies required:
Ex: string tagging, game data masking, ghosting, triggers, etc.

15. String data can eat up network bandwidth
String tagging
String data can eat up network bandwidth
Solution? Networked String Table
keeps track of strings sent to/from clients
most strings of data (i.e. player names) are transmitted only once between clients & the game server
if a string that has already been transmitted needs to be sent again:
a tag is sent instead of the full string
tag is a number that identifies the string to be used
so the full string need not be sent again
Obviously this won’t work for chat

16. A network game efficiency
Game Data Masking
A network game efficiency
Servers maintain info about what data has been changed since last broadcast to clients
masks on server keep track of this
Server will then only send data that has changed in next broadcast
Server will then reset

17. Masks Ex: let’s use a 32 bit mask for a control object
What might these numbers represent?
each number represents a piece or pieces of game data
0 means it hasn’t changed, 1 means it has
don’t send the data that’s 0
How many control masks do we need?
one for each game object that clients might need to know about
How many bits should it have?
one bit for each variable the clients might need to know about

18. Example Mask
Control object is evil boss:
first bit is position
second bit is orientation
third bit is state (living or dead)
fourth bit is animation state
etc.

19. Server handling masks Each loop on server:
Reset all masks to all 0’s (0000…000)
Collect all input from clients
Update appropriate variables
player input, physics, AI, etc.
For variables with mask bits, if updated, set bit to 1
Send game state to players, only sending data with 1s in game mask (they’ve changed since last frame)

20. Ghosting
Info sent to clients to be precisely matched to what they need
Why?
so that no excess bandwidth is wasted
Server maintains the authoritative game state for every object in the simulation
Sends objects to individual clients based on "only send the data the client requires"
NOTE: each client will receive their own custom packet

21. How does ghosting work?
Server maintains a list of all objects important to each client
How does it do that?
a scope check
Where’s the player camera?
What might be near it?
During each network update cycle, for each client:
Performs a rapid scope check
Collect the data we need to tell the client about
Builds an update packet
Transmit the packet to the client
Obviously, this can be combined with game data masking

22. For Real Time Strategy Scoping:
Determining Scope
Based upon frustum:
field of view for player
visible distance for the mission
traverse the Scene Graph and mark each object that can be viewed
For Real Time Strategy Scoping:
determine the visible/not visible state of all units in the mission based on friendly unit locations and visibility ranges
construct a list of all visible units per client
scope the entire list of visible units to that client

23. Benefits of Ghosting Minimize the amount of update traffic
Security that clients only have what they should have
clients shouldn’t have “out of game” knowledge
Prevent collusion
a game hacker tactic where two players share the information about the simulation they have
How do we prevent it?
use unique ghost IDs for each player
when a new object comes into scope for a specific client, the server assigns a GhostID to this object, and only tells the client what it's unique GhostID is
clients cannot share GhostIDs without the server’s involvement
the client is never told the actual server ObjectID of the newly scoped object

24. Obviously there is a huge burden on the server
It had better be efficient
Obsess over every last bit sent to clients. Why?
because you may have lots of clients
Packing/unpacking data into/from a bit streams
many compressions techniques available
for strings of 1s and 0s
only use the # of bits necessary for the data sent
ex: 1 bit for a boolean, 6 bits for an integer storing 43
this is a computing trade-off for lower bandwidth

25. Common events & reactions built into a game engine
Triggers
Common events & reactions built into a game engine
Ex:
player walked into room, so close door behind him
Opportunity:
help clients know game state without having to be told by server

26. Common Types of Triggers
area triggers
collide with an area in the world
3D box
animation triggers
used to synchronize environment effects with animations
e.g., footstep sounds with walking animations
weapon state triggers
dictate what to do when a weapon is firing, loading, etc.
player event control triggers
lets client act on certain input before server approval

27. ClassicGaming.com: PLATO Ain’t just Greek
References
ClassicGaming.com: PLATO Ain’t just Greek
GameSpy: The History of MUDs
MSDN: Windows Sockets 2
Webopedia

28. Torque/Networking/Making a Ghostable Object
References
Torque/Networking
Torque/Networking/Making a Ghostable Object
Introduction to Network Programming Concepts