cse 381

1. CSE 381 – Advanced Game ProgrammingNetworked Gaming Issues Warcraft III, by Blizzard

2. Types of Online Games • U.S. games market for 2004 was $ 7.3 billion • roughly 42% of gaming was done online in some capacity • Common online game types: • Deathmatch FPS • direct control over avatar • RTS, RPG • indirect, delayed control over avatar • Casual Games • Mobile Games • Used to rarely be plot driven, that’s changed

3. Concerns for Networked Game Programmers • Throughput • amount of data that can be stuffed into the pipe • usually not a problem • Latency • end-to-end delay • a problem for certain game types • Jitter • variation of latency • Packet loss • data lost along the way

4. Bandwidth • Maximum transfer rate of a communication channel • Size of a pipe • The goal of the networked game programmer: • ensure there is little enough information going through the pipe so as not to take up all available bandwidth • ensure enough data is passed to maintain data properly

5. Bad Bandwidth Assumptions • It’s easy to blame bandwidth for poor network performance • There are other culprits • Ex: • Input buffers get filled faster than they get emptied • To quote John Carmack talking about slowdowns: • “most of the problems are due to having far too many buffers and abstractions between the data producers/consumers and the actual wire”

6. Latency & Gameplay Degradation Research • RTS • Warcraft III • with latency of several seconds, still minimal degradation of gameplay • Sports Games • Madden NFL Football • when latency over ½ second, rapid degradation • Car racing simulation • latency over 50ms affects game results • latency over 100ms affects realism of game • FPS • Unreal Tournament 2003 • latency over 75 ms noticeable to players • latency over 100 ms made gameplay less enjoyable

7. Generally Speaking • Typical latency in a game played over a 56k modem would be 160 – 200 ms • Human tolerance for game delays is 100 – 200ms • or lower for fast paced games as we’ve seen • Too bad for mobile games • GPRS typically has round trip times of 1000 – 2700 ms • For newer 3G networks, 400 – 500 ms • Even longer if you want to use GPS

8. Network Information • Much information is available to a network programmer (and API like DirectPlay) • statistics about network traffic • ex: • peak throughput through pipe (bytes per second) • average throughput ( bytes per second) • round trip latency • number of timed-out messages • Is the server sending too much data? • Can it send more? • Ex: long round trip latencies may mean you are saturating either the client or server bandwidth

9. Some DirectPlay Rules • Will not send messages to a remote computer faster than it can process them • If it detects a remote computer isn’t responding fast enough: • will queue the message on the sender • won’t send it until a more appropriate time • ultimately, packets may be coalesced or timed out • Doesn’t this ruin it for everyone? • only those players’ computers struggling to keep up will be affected • Programmers may also check the queue. Why? • to see if it’s filling up • if so, send less data to a client

10. Decisions to make • Types of data to send • Should prioritizing be used? • If so, what algorithm should be used? • TCP vs UDP • Should prediction and/or simulation be used? • If so, what algorithm should be used? • Network Architecture

11. Minimize Data Sent • Remember, the most important thing in tuning your network performance is to eliminate unnecessary data being passed across the network • Rules of thumb: • don’t send strings unless it’s necessary • always send the smallest data types possible • never send a boolean variable • especially if you have more than one • what if you have 4 booleans to send? • send one byte that is bit-masked • The more data you can get in the smallest packets, the more available network bandwidth you have • Throughput requirement examples: • Counter Strike: approx. 34kbit/s • Warcraft III: approx. 5kbit/s

12. Prioritizing • Deciding what data to send • don’t send all data, FPS in particular. Why not? • minimize the amount of data sent • Send only part of game state • that which is currently relevant to a player • Ex: positions of viewable players only • Another approach: prioritize bandwidth • divide network bandwidth up among clients • provide more bandwidth to players who currently need it more

13. TCP vs. UDP • Both transport layer protocols on top of IP • What’s the difference? • TCP is lossless. What’s that? • What TCP (Transmission Control Protocol) gives you: • reliable delivery • retransmission and reordering • congestion control • What UDP (User Datagram Protocol) gives you: • unreliable delivery • no retransmission, packets not ACKnowleged, no reordering • no congestion control • speed & low overhead (good for streaming audio/video)

14. TCP vs. UDP • Which should you Use? • That depends • Some game data can be interpolated • if packet is lost, the game can continue • Some game data must be guaranteed to be received • some things can’t and shouldn’t be interpolated • Understand TCP is slower, but safer • Both are used, both have virtues, both have drawbacks • Programmers are always looking for UDP opportunities • Alternative: write your own “Reliable UDP” and use for certain types of info sent

15. Prediction & Simulation for Clients & Servers • What does a client do in between receiving network communications? • How can it render the game world for every frame? • Prediction & Simulation • use some algorithm to predict what will happen and simulate the results • obviously not everything should be predicted/simulated • For example, where are the other players located?

16. Mobility Models • Prediction & Simulation Algorithms for actor movement • Dead Reckoning • interpolate where an actor will be based on most recent state • what’s the problem with this? • Informed prediction • what are past patterns of movements • statistical predictions

17. Network Architectures • The reality is that most architectures are hybrids • much processing is done on clients • ex: collision detection • would be too much of a strain on servers • much prediction must therefore be done on clients • ex: players manage their own movements • cost of this: cheating

18. References • Unreal Network Architecture • http://unreal.epicgames.com/Network.htm • Networking Multiplayer Games by Sugih Jamin • http://ai.eecs.umich.edu/soar/Classes/494/talks/lecture-15.pdf • Analysis of Factors Affecting Players’ Performance and Perception in Multiplayer Games by Matthias Dick, Oliver Wellnitz, Lars Wolf • John Carmack’s Blog • http://www.armadilloaerospace.com/n.x/johnc • High Latency Multiplayer Gaming by Edward Hannay