Network Physics Created by Ruslan Yavdoshak for Nikitova Games, 2008

Index AI Cover Lags, Lag compensation Smoothing Priority channel, QoS

AI Cover Multiplayer = AI (realization) Multiplayer more fun than AI Conflict 2 side hosts [Player + AI], [Player], where AI is processing when lag has been happened? Prediction of the decision of AI on different hosts

LAGS: Plague of networked games Physics of Latency: <10ms LAN <30ms “Regional Lag” <100ms World Lag with fiber optics ms (avr.) Satellite communication >350ms overloaded channel, slow communication, delay filter Speed of transfer for “acknowledge” transport :PacketSize / (2*latency) Additional latency when packet has been drop Example: 1450 byte packets on a link that can handle 2500 bytes / s of data: Next Packet Time = Current Time + (1450 bytes + 28 byte UDP header) / 2500 = Current Time seconds Computer hardware and operating system latency

How to Overcome Latency Decouple the communications from your game: Place blocking communications calls in a separate thread or use asynchronous communications calls. Never await incoming data from another player before allowing game play to continue. Use predicting, interpolating, and reconciling later to improve game play. When using prediction, transmit not just position information, but also velocity and acceleration. Latch or queue user input (for example, keystrokes) until the next time communications data is to be sent. If you use the more traditional method of checking the keyboard at regular intervals during the game loop and perform this checking only at moments before sending data to the network, you will be prone to missing user input.

How to Overcome Latency Hide latencies in other game elements: Schedule events in the future if you want them to happen simultaneously for all users. Require multiple shots to kill someone, and minimize the number of all-or-nothing, deterministic, latency-sensitive events. Make rockets take a long time in the air (and reconcile the outcome while the rocket is flying). Require time to move from one location to another (don't allow any instantaneous teleporters). Make players, ships, and other objects move in ways that facilitate prediction. For example, build inertia into the way entities in your game move. Use your imagination to incorporate latency within the context of your game concept.

Dead Reckoning Algorithms Cubic splines

Lag Compensation Terminology: Server-side prediction Client-side prediction Drawbacks of movement prediction ( ms.) Hit-side prediction Cheat protection

Lag Compensation

Smoothing Extrapolation correction Queue of packets & “live-modifying” Speed limits Active walk system (steps & turn)

Quality-of-Service (IntServ ) Integrated Services (IntServ): architecture for providing QoS guarantees in IP networks for individual flows fundamental changes to the Internet to reserve end-to-end bandwidths components: admission control resource reservation routing classifier and route selection packet scheduling Problems with that approach: scalability/complexity: maintaining per-flow router state difficult with large number of flows flexible service models: IntServ has only two classes (controlled, guaranteed), we would like “qualitative” service classes

Quality-of-Service (DiffServ ) Differentiated Services (DiffServ): simple functions in network core, relatively complex functions at edge routers (or hosts) –edge: packet classification, packet marking, traffic conditioning –core: forwarding flows are aggregated into classes that receive ‘treatment’ depending on class parameters don’t define define service classes, provide functional components to build service classes

Channels/Flow