Jacob Steinfort

• If gamers had to choose either a single-player game or a multiplayer game, most people will choose multiplayer
• Why?
– Beating your friends is more fun than beating an
AI
– Get a different experience every time you play the game

Multiplayer
Single-Player

Importance of Getting Multiplayer Right
• If developers provide a bad experience, people won’t buy the game

Part 1
• CS 3830 - Data Communications and
Computer Networks
– Latency = Delay, amount of time it takes a packet to travel from source to destination
• RTT (Round Trip Time)= source -> destination ->source
• Usually measured in milliseconds (ms)
– Bandwidth = amount of data that can be transferred per unit of time
• Usually measured in Megabits / second (Mbps)
RTT
Bandwidth

– Ideally:
• Infinitely small latency (0 ms)
• Unlimited bandwidth (999999999… Mbps)
– Realistically:

Part 2
• Socket = bidirectional communication endpoint for sending and receiving data with another socket.
• Two main types of sockets:
– TCP (Transmission Control Protocol)
– UDP (User Datagram Protocol)

TCP
Connection based (handshake)
Guaranteed reliable and ordered
Automatically breaks up your data into packets for you
UDP
No concept of connection, have to code this yourself
No guarantee of reliability or ordering of packets. They may arrive out of order, be duplicated, or not arrive at all!
You have to manually break up your data into packets and send them
Makes sure it doesn’t send data too fast for the internet connection to handle (flow control)
You have to make sure you don’t send data too fast for your internet connection to handle
Easy to use, you just read and write data like its a file
Slow
If a packet is lost, you need to devise some way to detect this, and resend that data if necessary
Fast

• Real-time requirement
– For most parts of a game, it doesn’t matter what happened a second ago, you only care about the most recent data

• What is it?
– Technology to help multiple players sustain the belief that they are playing a game together
– Is it possible to achieve this?
• Challenges:
– Latency
– Bandwidth
– Dropped Packets
– Cheaters
– Joining/Quitting Games in progress

What type of games will I be talking about?
• Action Games
– Emphasizes physical challenges, including hand– eye coordination and reaction-time
Best Example:
SHOOTERS!

First technique of Gameplay Networking:
Peer-to-Peer Lockstep
• Each computer exchanging information with every other computer
• Process: extract a game into series of turns and a set of command messages
– Example: turn = 50ms; set of commands = {move unit, attack unit, construct building, …}
• What happens during a turn on one machine?
1. Receive other player’s commands
2. Evolve the game state
3. Start recording commands
4. Stop recording commands and send them to other peers

• Was created for RTS (Real Time Strategy) games
– Game state was too large to transmit
• Had to settle for transmitting changes only

• Deterministic
– Will always produce same output when given same input
– Synchronized: each machine is at the exact same state at any given time

Problems with Peer-to-Peer Lockstep
1. Game could become out of sync
– One small change could destroy synchronization
2. Doesn’t support late join
– Everybody needs to start from the same state
3. Everybody’s perceived latency is equal to the slowest latency
– Necessary for consistency

• Does it work for action games?
– Maybe on LAN, definitely not over the Internet
• Problem: input latency
Doom (1993):
First action game to attempt to implement peer-to-peer lockstep

Part 3
• Client/Server model

Solution:
Client/Server
• Each client has only one connection: to the server
• Clients turned into “dumb” terminals
– Input sent to server (real-time) to be evaluated
– Server sends updated game state (their player and all other players) to the client

• Dedicated Server
– Clients are the only players
• Non-dedicated Server
– Server is also a player
– Server player is called “Host”

1. No more turns = Less latency
– Other player’s latency will not slow you down
Process changes

2. No more consistency issues
– Game is only being simulated on the server
(Peer-to-Peer Lockstep: game simulated on all machines)

Client/Server:
Small Problem
• Frame rate on client is limited to how frequently the server sent the game state to the client
• Solution: Entity Interpolation!

Game State 1
Interpolated
Game State /
Rendered
Frame
Game State 2

Entity Interpolation with dropped packet safeguard
?

Client/Server with Entity Interpolation
Benefits:
• Provides a very smooth experience (unlimited framerate) that is much better than Peer-to-
Peer Lockstep
• Clients still run minimal code (no physics/collisions)

Client/Server:
Big Problem
+
= LAG!
(user-perceived latency)
• Client1 has an unfair advantage
• If not dedicated server, huge host advantage

New Solution:
Client-Side Prediction
• As soon as user changes input, their machine predicts where the server is going to put them
– Push forward on keyboard, instantaneously move forward on screen
• Client needs to run more code
– Client needs to be aware of physics and collisions
(don’t want to run through a wall)

Client-Side Prediction
(This is where it gets complicated)
• After RTT has passed, the client gets the updated game state from the server and verifies that its game state WAS equal to what it predicted
– If not, client performs correction
User perceives lag
Client
Prediction
Server
Correction
Client’s
Modified
Prediction

Client-Side Prediction
How to implement?
• Need a circular buffer on the client to store the user’s last few commands
– When correction comes in from server, client performs new predictions based on the saved commands
Client
Prediction
Command Predicted Actual
Forward - 3 seconds
Location
-
Forward - 3 seconds (0,3)
-
-
Location
(0,2)
(0,2)
Right - 4 seconds (4,3) (4,2)
-
-
(4,2)
Forward - 2 seconds (4,5) (4,4) (4,4)
…
…
…
…
-
-

Gears of War 1 (2006) and 2 (2008):
Host Advantage
0:57
Host
1:05
Non-host

Another problem with Client/Server
1. I shoot another player and know it is a hit
2. Client sends shoot command to server
– Contains point of origin, and direction
3. Server gets the command and evaluates the shot
– Server: client did not get the hit
Why?
The position of other players on the client’s machine is always out of date. You only see a “ghost” of other players.

Possible Solution:
Give Client authority over shots
• Client performs collision check right after shot
– If hit, send a “hit confirmed” message to server
• Problem?
– CHEATERS!
– Man-in-the-middle attack

Actual Solution:
Lag Compensation
1. Client sends shoot command to server
– Contains point of origin, and direction
2. Server gets command a) Estimates when the client executed the command using RTT b) Rolls back the game to the time of shot c) Simulates shot, checks for collision d) Server updates current game state
Player2’s position seen from
Player1’s client
Counter Stike:
• View of Player1’s point of view from the server
• Player2 is running to the left
Player2’s actual position on server

• Peer-To-Peer Lockstep
– Simple, works well for RTS games
• Client/Server
– Much better for action games, frame rate limited
• Client/Server + Interpolation
– Unlimited frame rate, still has large input latency
• + Client-Side Prediction
– No more input latency, interacting with other players is lagged though
• + Lag Compensation
– No lag with other players
– Amazing!

Battlefield 3
Getting Gameplay Networking Right

• Game: Halo: Reach
• Developer: Bungie Studios

A closer look at
Client-Side Prediction
• This technique works great for the character that the client is controlling
• What about the other objects in the game
(e.g. a grenade)

• This is how it looks on a Halo: Reach client

• Only makes sense for objects that have predictable paths
– Grenades, rockets, anything not being controlled by a user
• Doesn’t make sense to use on other players
– Unpredictable direction changes
– Have to stick with Interpolation for other players

• It’s so complicated, why use it?
• To reduce lag

Another Trick:
Prioritization
• A Halo game can have hundreds of objects
– Some objects are less important to update on one client and more important to update on another
• Solution: Real-time prioritization
– Each object for each client has an update priority

0.22/0.97/127
0.50/1.00/0
Legend:
Final priority / relevance / desired update period (ms)

Internet
Protocol Stack
Application
Transport
Network
Link
Physical
Layer
Game Runs the game
Game Interface Extract and apply replicated data
Prioritization
Replication
Channel
Manager
Transport
Bungie’s Networking Stack
Purpose
Rate the priority of all possible replication options
Protocols with various reliability guarantees
Flow and congestion control
Send & receive on sockets

• I Shot You First: Networking the Gameplay of
HALO: REACH
– Game Developers Conference, 2011

• Aldridge, David. "I Shot You First: Networking the Gameplay of HALO:
REACH." GDC Vault. Game Developers Conference, 28 Mar. 2011. Web.
<http://www.gdcvault.com/play/1014345/I-Shot-You-First-Networking>.
• Fiedler, Glenn. "Networking for Game Programmers." Gaffer On Games.
N.p., 1 Oct. 2008. Web. 1 Sept. 2012.
<http://gafferongames.com/networking-for-game-programmers/>.
• Kurose, James F., and Keith W. Ross. Computer Networking: A Top-down
Approach. Boston: Pearson/Addison Wesley, 2008. Print.
• "Source Multiplayer Networking." Valve Developer Community. Valve, n.d.
Web. 1 Oct. 2012.
<https://developer.valvesoftware.com/wiki/Source_Multiplayer_Networki ng>.
• Steed, Anthony, and Manuel Fradinho. Oliveira. Networked Graphics:
Building Networked Games and Virtual Environments. Burlington, MA:
Morgan Kaufmann, 2010. Print.

Videos:
• Gears of War 2 Host Comparison
– http://www.youtube.com/watch?v=7eToxVxGO9k
• Battlefield 3 – Jet vs Sniper
– http://www.youtube.com/watch?v=o1s0ED51Tic