Communication Architectures
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2/26/2016 Outline • Network Games – Architectures Distributed Computing Systems – Compensation techniques – Cheating – Cloud games (Slides for Final Class) • Peer-to-Peer Systems – Overview – P2P file sharing Communication Architectures Data and Control Architectures Split-screen All peers equal - Limited players -Easy to extend -Doesn’t scale (LAN only) • Want consistency – Same state on each node – Needs tightly coupled, low latency, small nodes • Want responsiveness – More computation locally to reduce network Central server Server pool – Loosely coupled (asynchronous) - Clients only to -Improved server scalability -Server may be -More • In general, cannot do both Tradeoffs bottleneck complex “Relay” Architecture Abstraction Relay Architecture Choices (Example: Dumb terminal, send and wait for response) • Want control to propagate quickly so can update data (responsiveness) • Want to reflect same data on all nodes (consistency) (Example: Smart terminal, send and echo) 1 2/26/2016 Network Game Architectures Outline • Centralized • Network Games – Use only two-way relay (no short-circuit) – Architectures (done) – One node holds data so view is consistent at all times – Lacks responsiveness – Compensation techniques (next) • Distributed and Replicated – Cheating – Allow short-circuit relay, provides responsiveness – Cloud games – What about consistency? Make design decisions • Replicated has copies, used when predictable (e.g., behavior • Peer-to-Peer Systems of non-player characters) • Distributed has local node only, used when unpredictable – Overview (e.g., behavior of players) – P2P file sharing Interest Management – Auras Dead Reckoning • Based on ocean navigation techniques (“dead” == “deduced (ded.)”) • Predict position based on last known position plus direction • Nodes express area of interest to them – Only send updates when deviates past threshold – Do not get messages for outside areas (predicted position) (“warp”) - Only world information in circle/sent sent even if world is larger (actual position) - Side benefit can • When prediction differs and adjust, get “warping” or prevent cheating (later) “rubber-banding” effect – Some techniques move smoothly to place over short time Time Delay Time Warp • Server delays processing of events • With network latency, must lead opponent to hit (even with – Wait until all messages from clients arrive “instant” weapon!) – (Note, game plays at highest round-trip time) • Instead, knowing latency roll-back (warp) to when action took place • Server sends messages to more distant client first, – Usually, estimate latency as ½ round-trip time delays messages to closer – Needs accurate estimate of round-trip time • Client 100 ms behind Client 1 Client 2 Server processes • Still hits (note command arrives command arrives both client commands blood) • (Boxes are bounding Time boxes) Time Delay https://developer.valvesoftware.com/wiki/Source_Multiplayer_Networking 2 2/26/2016 Time Warp Notes Outline • Inconsistency – Player target • Network Games – Move around corner – Architectures (done) – Warp back hit – Compensation techniques (done) – Bullets seem to “bend” around corner! – Cheating (next) “Magic” bullets – Cloud games • Fortunately, player often does not notice – Doesn’t see opponent • Peer-to-Peer Systems – May be just wounded – Overview – P2P file sharing Cheating Packet and Traffic Tampering • Unique to games • Packet interception – prevent some packets from – Other multi-person applications don’t have reaching cheater – e.g., suppress damage packets, so cheater is – e.g, Distributed Interactive Simulation (DIS), not invulnerable public, “employees” so considered trustworthy • Packet replay – repeat event over for added • Cheaters want: advantage – Vandalism – create havoc (relatively few). – e.g., multiple bullets or rockets if otherwise limited • Mostly, game design to prevent (e.g., no friendly fire) • Solutions: – Dominance – gain advantage (more) – MD5 Checksum or Encrypt packets • Next slides – Authoritative host keeps within bounds Packet Tampering Information Exposure • Allows cheater to gain access to • Reflex augmentation - enhance replicated, hidden game data (e.g. status of other players) cheater’s reactions – Passive, since does not alter traffic – e.g., aiming proxy monitors – e.g., ignore “fog of war” in RTS, or “wall opponents movement packets, hack” to see through walls in FPS when cheater fires, improve aim • Cannot be defeated by network alone • Tough to detect • Instead: – e.g., PunkBuster – scan for – Sensitive data should be encoded “known” hacks – Kept in hard-to-detect memory location – Centralized server may detect cheating – False positives? (e.g., attack enemy could not have seen) S. Yeung and J. Lui. “Dynamic Bayesian approach for detecting cheats in multi- player online games”, Springer Multimedia Systems, Vol. 14, No. 4 Sep. 2008. aimbot human 3 2/26/2016 Outline Cloud-based Games • Connectivity and capacity of networks growing • Network Games • Opportunity for cloud-based games – Architectures (done) – Game processing on servers in cloud – Compensation techniques (done) – Stream game video down to client – Cheating (done) – Client displays video, sends player input up to server – Cloud games (next) Game frames • Peer-to-Peer Systems Server – Overview Server Server – P2P file sharing Player input Thin Client Cloud Servers 20 Why Cloud-based Games? Cloud Game - Modules (1 of 2) • Potential elastic scalability • Input (i) – receives – Overcome processing and storage limitations of clients control messages from – Avoid potential upfront costs for servers, while supporting demand • Ease of deployment players – Client “thin”, so inexpensive ($100 for OnLive console vs. $400 for Playstation 4 console) • Game logic – manages – Potentially less frequent client hardware upgrades game content – Games for different platforms (e.g., Xbox and Playstation) on one device • Networking (n) – • Piracy prevention exchanges data with – Since game code is stored in cloud, server controls content and content cannot be copied server – Unlike other solutions (e.g., DRM), still easy to distribute to players • Rendering (r) – renders • Click-to-play game frames – Game can be run without installation • How to put in cloud? Cloud Game - Modules (2 of 2) Application Streams vs. Game Streams “Cuts” • Traditional thin client • Approximate traffic analysis 1. All game logic on player, applications (e.g., x-term, – 70 kb/s traditional network cloud only relay remote login shell): game – Relatively casual interaction – 700 kb/s virtual world information (traditional • e.g., typing or mouse clicking – 2000-7000 kb/s live video network game) – Infrequent display updates (HD) • e.g., character updates or 2. Player only gets input and scrolling text – 1000-7000 kb/s pre-recorded displays frames (remote • Computer games: video rendering) – Intense interaction • e.g., avatar movement and • Cloud-based games? 3. Player gets input and shooting renders frames (local – Frequently changing displays – 7000 kb/s (HD) rendering) • e.g., 360 degree panning Challenge: Latency since player input requires round-trip to server before player sees effects 4 2/26/2016 Outline Definition of Peer-to-Peer (P2P) • Network Games (done) • Significant autonomy from central servers – Architectures (done) • Exploits resources at edges of Internet – Compensation techniques (done) – Storage and content – Cheating (done) – Multicast routing – Cloud games (done) – CPU cycles – Human knowledge (e.g., recommendations, • Peer-to-Peer Systems (next) classification) – Overview • Resources at edge may have intermittent – P2P file sharing connectivity P2P Includes P2P File Sharing – General • P2P communication • Alice runs P2P client on • Asks for “Hey Jude” – Instant messaging her laptop • Application displays – Voice-over-IP (e.g., Skype) • Registers her content in other peers with copy • P2P multicast routing P2P system • Alice choses one, Bob – e.g., Mbone, Yoid, Scattercast • P2P computation • File is copied from Bob’s – e.g., seti@home, folding@home computer to Alice’s • P2P systems built on overlays P2P – e.g., PlanetLab • While Alice downloads, • P2P file sharing others upload – e.g., Napster, gnutella, KaZaA, eDonkey, BitTorrent … Example: Searching P2P File Sharing Capabilities 1000’s of nodes N2 • Allows Alice to show directory in her file Set of nodes may change N1 N3 system – Anyone can retrieve file from it Key=“title” Internet – Like Web server Value=MP3 data… ? Client Publisher • Allows Alice to copy files from other’s Lookup(“title”) – Like Web client N N 4 N 6 • Allows users to search nodes for content 5 based on keyword matches • Needles versus Haystacks – Like search engine (e.g., Google) Searching for top 40 pop song? Or obscure punk track ‘81 nobody’s heard of? • Search expressiveness Whole word? Regular expressions? File names? Attributes? Whole-text search? 5 2/26/2016 P2P File Sharing Systems Napster: Publish Central Flood Super- Route node flood Centralizedinsert (X, 123.2.21.23 ) Whole File Napster Gnutella Freenet (napster.com) ... Chunk BitTorrent KaZaA (DHTs) Based (swarm) (bytes) eDonkey2k Publish New BT I have X, Y, and Z! 123.2.21.23 Napster: Search Napster: Discussion 123.2.0.18 • Pros – Simple Centralizedsearch(A) – Search scope is O(1) (napster.com returns 123.2.0.18) Fetch – Controllable (pro or con?) returns 163.2.1.0 … • Cons Query Reply – Single point of failure – Server maintains O(N) state – Server does all processing Where is file A? – (Napster’s server farm had difficult