Warcraft III, by Blizzard CSE 381 – Advanced Game Programming Networked Gaming Issues

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

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

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

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”

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

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

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

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

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

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

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

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)

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

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?

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

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

References Unreal Network Architecture – Networking Multiplayer Games by Sugih Jamin – Analysis of Factors Affecting Players’ Performance and Perception in Multiplayer Games by Matthias Dick, Oliver Wellnitz, Lars Wolf John Carmack’s Blog – High Latency Multiplayer Gaming by Edward Hannay