cse 381 advanced game programming networked gaming issues
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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 ...

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types of online games
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
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
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
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
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
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
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
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
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
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
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
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 udp14
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
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
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
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
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
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