1 / 49

Gameplay Networking

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

moana-christian
Download Presentation

Gameplay Networking

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Gameplay Networking Jacob Steinfort

  2. 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

  3. USA 2011 Top Unit Sales Multiplayer Single-Player

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

  5. How do we get multiplayer right?

  6. Computer Networking ReviewPart 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

  7. What is a perfect connection? • Ideally: • Infinitely small latency (0 ms) • Unlimited bandwidth (999999999… Mbps) • Realistically:

  8. Computer Networking ReviewPart 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)

  9. TCP vs. UDP

  10. 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

  11. 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

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

  13. 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? • Receive other player’s commands • Evolve the game state • Start recording commands • Stop recording commands and send them to other peers

  14. 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

  15. 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

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

  17. 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

  18. Computer Networking ReviewPart 3 • Client/Server model

  19. 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

  20. 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”

  21. Client/Server Benefits • No more turns = Less latency • Other player’s latency will not slow you down Process changes

  22. Client/Server Benefits • No more consistency issues • Game is only being simulated on the server (Peer-to-Peer Lockstep: game simulated on all machines)

  23. 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!

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

  25. Entity Interpolationwith dropped packet safeguard ?

  26. 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)

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

  28. 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)

  29. 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 Server Correction Client Prediction Client’s Modified Prediction

  30. Client-Side PredictionHow 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

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

  32. Another problem with Client/Server • I shoot another player and know it is a hit • Client sends shoot command to server • Contains point of origin, and direction • 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.

  33. 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

  34. Actual Solution:Lag Compensation • Client sends shoot command to server • Contains point of origin, and direction • Server gets command • Estimates when the client executed the command using RTT • Rolls back the game to the time of shot • Simulates shot, checks for collision • 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

  35. 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!

  36. Battlefield 3Getting Gameplay Networking Right

  37. Extra Tricks In Use Now • Game: Halo: Reach • Developer: Bungie Studios

  38. A closer look atClient-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)

  39. Entity Extrapolation

  40. Entity Extrapolation • This is how it looks on a Halo: Reach client

  41. 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

  42. Entity Extrapolation • It’s so complicated, why use it? • To reduce lag

  43. 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

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

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

  46. Sources • 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_Networking>. • Steed, Anthony, and Manuel Fradinho. Oliveira. Networked Graphics: Building Networked Games and Virtual Environments. Burlington, MA: Morgan Kaufmann, 2010. Print.

  47. Sources 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

  48. Questions?

More Related