Gameplay Networking Jacob Steinfort

Importance of Multiplayer Games 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

USA 2011 Top Unit Sales Multiplayer Single-Player

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

How do we get multiplayer right?

Computer Networking Review Part 1 CS 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) Bandwidth RTT

– Ideally: Infinitely small latency (0 ms) Unlimited bandwidth ( … Mbps) – Realistically: What is a perfect connection?

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) Computer Networking Review Part 2

TCP vs. UDP TCPUDP Connection based (handshake)No concept of connection, have to code this yourself Guaranteed reliable and orderedNo guarantee of reliability or ordering of packets. They may arrive out of order, be duplicated, or not arrive at all! Automatically breaks up your data into packets for you 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 If a packet is lost, you need to devise some way to detect this, and resend that data if necessary SlowFast

Why UDP? 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

Gameplay Networking 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

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

Peer-to-Peer Lockstep 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

Peer-to-Peer Lockstep 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

Client/Server model Computer Networking Review Part 3

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

2 Types of Client/Server: Dedicated Server – Clients are the only players Non-dedicated Server – Server is also a player – Server player is called “Host”

Client/Server Benefits 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 Benefits

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!

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

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 Client1 has an unfair advantage If not dedicated server, huge host advantage + + = LAG! (user-perceived latency)

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 Server Correction Client Prediction Client’s Modified Prediction User perceives lag

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

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 Counter Stike: View of Player1’s point of view from the server Player2 is running to the left Player2’s actual position on server Player2’s position seen from Player1’s client

Review of Basic Techniques 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

Extra Tricks In Use Now 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)

Entity Extrapolation

This is how it looks on a Halo: Reach client

Entity Extrapolation 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

Entity Extrapolation 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

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

Internet Protocol Stack Application Transport Network Link Physical

Check out Bungie’s Presentation I Shot You First: Networking the Gameplay of HALO: REACH – Game Developers Conference, 2011

Sources Aldridge, David. "I Shot You First: Networking the Gameplay of HALO: REACH." GDC Vault. Game Developers Conference, 28 Mar Web.. Fiedler, Glenn. "Networking for Game Programmers." Gaffer On Games. N.p., 1 Oct Web. 1 Sept Kurose, James F., and Keith W. Ross. Computer Networking: A Top-down Approach. Boston: Pearson/Addison Wesley, Print. "Source Multiplayer Networking." Valve Developer Community. Valve, n.d. Web. 1 Oct Steed, Anthony, and Manuel Fradinho. Oliveira. Networked Graphics: Building Networked Games and Virtual Environments. Burlington, MA: Morgan Kaufmann, Print.

Sources Videos: Gears of War 2 Host Comparison – Battlefield 3 – Jet vs Sniper –

Questions?