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Tech Topic 5 February 8 2010 Gaming Lots of different kinds of games but the most challenging are the interactive games Typically racing or first person shooter FPS games The most interactive of the interactive games are Online PC games e g World of Warcraft Console games e g Xbox PS3 Wii Most console games have a single player component and an interactive component The interactive component is either a subscription model Xbox or free PS3 and Wii Issues in Gaming Scale Bandwidth The number of players and amount of bandwidth that can be supported Limits are generally driven by minimum available bandwidth With dial up modems hard to have any kind of scale Xbox for example requires broadband connection Delay Opinions on delay vary but 50ms 100ms seems common Some allowances go as high as 200ms Processing Used to be an issue but not really anymore or rather is more of an issue for graphics quality but not networking Scale Scale is not so hard to manage Rather requirements are limited based on strict limits on the number of players Other techniques to reduce bandwidth requirements reduce complexity less frequent updates aggregate updates limited Areas of Interest compression etc Consoles are often limited to 8 16 or 32 players Much beyond that and it is hard to have rich interactions PC games have unlimited players but only a small number can co exist in an area For popular areas replicated servers providing identical areas are used Delay Games are even more sensitive to delay than realtime voice conversations Even with minimal buffering and processing physical distance propagation times are often too slow Consider for example highly accurate first person shooter games Quite a few techniques to avoid dealing with delay Fairly sophisticated efforts to measure delay between players one of the components of matchmaking Other in game techniques e g dead reckoning Precision and Deadline Claypool papers talks about these two ways of describing how important delay is Precision accuracy of action Precise v Imprecise Deadline responsiveness Tight v Loose Game phases But we want to look at the really challenging parts Architecture Types Two different types of architectures p p c c s p p c c Peer to peer Client server Distributed Centralized Architecture Types Impacts of architecture type Where control decisions are made Single point versus distributed fashion Single point of control generally better for security Single point is single point of failure may be security issue Single point means server is likely a traffic hotspot Single point may require more capability to handle updates Where data has to be sent In single point players only have to communicate with server In single point server has to communicate with all players Client Capabilities Distributed architecture All clients have to be able to perform data processing and game status updating Centralized architecture A spectrum of possibilities At one end clients just past actions to server and server responds with updated game state player location status inventory etc At the other end clients just past game state all of the processing was done locally Server still has to verify correctness of actions For either architecture may need to re sync periodically Hybrid Architecture For some types of data p2p distribution is used Player to player voice communication Non critical game state For other types of data central server is used Game critical data is sent to the server and processed there for correctness check Dealing with Delay Given the laws of Physics how to overcome delay that must necessarily exist Some reduction of the amount of data to send will help with delay but these days bandwidth constraints are well understood and can be managed Other technique is a combination of local processing and dead reckoning Dead Reckoning Allow local player to estimate new position of remote player based on previous position and history Tradeoff between delay and data consistency The longer the time without an update the more inconsistent the data becomes Use one node central server to deal with inconsistencies One Possible Solution Use a centralized approach Though each of the players has the potential to be the server Have some local processing Can receive user inputs and display those on the screen Can also do limited dead reckoning for remote player actions Expecting only modest amounts of delay Send game state updates to the server Server responds with whether action was allowed Server runs virtual world to compare possibilities versus realities If action is not possible likely security hack fix it If conflict occurs because of dead reckoning fix it Fixes may result in on screen hitch An appearance of minimal delay and fairly good accuracy can be achieved Server Also Checks for Cheats Packet and traffic tampering reflex augmentation proxy replaces human to produce superior results aiming proxy e g aiming proxy packet interception proxy prevents packets from reaching the cheating player e g suppressing the packets containing damage information packet replay the same packet is sent repeatedly fire command e g the fire command packet Information exposure gives information to player he she is not supposed to have Design defects can create loopholes which the cheaters are apt to exploit Learning About How Games Work Traffic traces as a way of reverse engineering what is happening Typically hard to do because traffic is carefully encrypted Can also inject arbitrary delay and observe how game reacts And now more on tracing traffic


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