Game Networking 1

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Game Networking 1

• Bryan Duggan

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History of multiplayer gaming

• Tennis For Two
• William Higinbotham, a nuclear power plant
  technician, 1958
• Players had to control the ball through the use
  of knob and a push button
• An analogue computer that was attached to a
  modified oscilloscope

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Spacewar

• Steve Russell, Martin Graetz and Wayne Wiitanen,
  MIT in 1961
• PDP-1
• Missiles and fuel
• Players had to shoot each others, while avoiding
  to crash with stars.
• The ships were controlled by four switches, that
  turned the ship's direction, its speed, firing
  rate and hyperspace.

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Atari

• Formed by Nolan Bushnell in 1972
• Developed pong in early 1970s
• Gauntlet in early 1980s

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Hosted games

• Dow Jones (The Source)
• Compuserv (H R Block)
• Used idle computer cycles
• Cost between 5 and 22.50 per hour
• Client server architectures
• Telnet client
• Puzzles, spells traps

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Doom

• 1993
• First multi-player 3D networked game
• Up to 4 players, co-op or deathmatch
• Used IPX (non routable protocol) on a LAN
• Peer to peer
• Each peer updates all others 35 times a second
• When updates received from all peers, the game
  advances a tick

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Problems with Doom

• All packets received by all nodes on the network
• Hosts had to process the packet, even though they
  were not playing the game!
• Led to lots of companies banning Doom
• More users than OS/2 and WinNT combined!

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Architectures

• Peer-to peer
• No central node controlling game state
• Works well on LANS, does not scale
• Client server
• One server
• Clients communicate with each other through the
  server
• Peer-to peer, client server hybrid
• Some data communicated between peers eg voice chat

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Evolution of online games

• Quake/QuakeWorld (1996)
• Didn't need tunnelling to communicate over the
  internet
• Used IP instead of IPX
• Persistent servers
• Client server prediction (dead reckoning)
• Push latency control on the client (the amount of
  dead reckoning to use)
• Development of clans
• Commercial competitions cyberathletes

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Dead Reckoning

• Dead reckoning uses prediction to move objects
  about even when their positions are not precisely
  known, reducing the appearance of lag
• Each client maintains precise state for some
  objects (e.g. local player)
• Each client receives periodic updates of the
  position of everyone else, along with velocity
  information, and maybe acceleration
• On each frame, the non-local objects are updated
  by extrapolating their most recent position using
  the available information
• With a client-server model, each player runs
  their own version of the game, while the server
  maintains absolute authority

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Fixing Extrapolation Errors

• What do you do when using dead reckoning, and a
  new position arrives for another player?
• The position that just came in will not agree
  with the place you have the object, due to
  extrapolation errors
• Two options
• Jump to the correct position
• Interpolate the two positions over some period
• Path followed will never be exact, but will match
  reasonably well

Target Path
Your Guess
Extrapolations from new data
Actual
New Data
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Massively multiplayer Games

• 1995 Ultima Online (50 players)
• Microsoft releases
• Ashrons call
• Everquest (500,000 subscribers 1993)
• World of Warcraft
• 2004

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Currently...

• All online gaming happens over IP networks The
  Internet and intranets
• MMORPG
• FPS
• RTS
• http//beej.us/guide/bgnet/ for a guide!!

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4 Primary concerns

• Latency How long does it take for state to be
  transmitted
• Reliability How often is data lost or corrupted
• Bandwidth How much data can be transmitted in a
  given time
• Security How is the game-play protected from
  tampering

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Reducing Latency

• Frame rate latency
• Increase the frame rate (faster graphics, faster
  AI, faster physics)
• Network protocol latency
• Send less stuff (less stuff to copy and shift
  around)
• Switch to a protocol with lower latency
• But may have impact on reliability and security
• Transmission latency
• Send less stuff less time between when the
  first bit and the last bit arrive
• Upgrade your physical network (from dial-up to
  DSL, for instance)

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Working With Latency

• If you cant get rid of latency, you can try to
  hide it
• Any technique will introduce errors in some form
  - you cannot provide immediate, accurate
  information
• Option 1 Sacrifice accurate information, and
  show approximate positions
• Ignore the lag and show a given player old
  information about the other players
• Try to improve upon this by guessing where the
  other players are. But if your guess is wrong,
  incorrect information is shown
• Option 2 Sacrifice game-play
• Deliberately introduce lag into the local
  players experience, so that you have enough time
  to deal with the network

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The Internet is short for

• Interconnected Network

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What is a network?

• From dictionary.com
• An openwork fabric or structure in which cords,
  threads, or wires cross at regular intervals.
• Something resembling an openwork fabric or
  structure in form or concept, especially
• A system of lines or channels that cross or
  interconnect a network of railroads.
• A complex, interconnected group or system an
  espionage network.
• An extended group of people with similar
  interests or concerns who interact and remain in
  informal contact for mutual assistance or support.

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IP

• Layered protocols
• Packet switched network
• Hierarchical routing
• Best effort
• Edges core nodes
• IP Address 32 bit number
• Tells you the exact node you are dealing with
• Tells you where on the network that node is
• Usually written as 4 octets or dotted quads

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TCP

• Uses IP
• Creates bi-directional paths
• Reliable, ordered transfer of bytes
• Explicit acknowledgement of packets received
• Can detect if losses have occurred and resend
• TCP can keep retransmitting for seconds in the
  case of packet loss
• Not suitable for real time messaging (FPS)
• Uses a 16 bit port number (sender receiver)
• Allows multiplexing of messages, multiple peer
  process

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UDP

• Connectionless
• Unreliable (packets can get lost)
• No flow control (packets can arrive out of order)
• Uses a 16 bit port number (sender receiver)
• Allows multiplexing of messages, multiple peer
  process

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Port numbers

• Well known port numbers
• 25 SMTP
• 80 HTTP

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DNS

• Domain Name Service
• Uses UDP
• Converts a 32 bit IP address to a friendly name
  and visa versa
• You can use
• NSLOOKUP on the command line

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DHCP

• Uses UDP
• You send a broadcast message looking for an IP
  address
• DHCP servers respond with offers
• You send a message back to DHCP server with an
  acceptance

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Protocols

• Standard messages that get passed between nodes
  on the internet
• Usually standardised in an RFC

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What do protocols look like?

• Binary
• Fixed-length fields, like C-structures
• Type-length-value (TLV)
• Text
• ftp PUT foo
• GET index.html HTTP/1.1
• Content-Length 100

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HTTP Protocol
RFCs 1945 2068 2616
Parameters
Request
Web Browser
Response
Query
E.g. A user fills in a form, which gets submitted
to a web server Port Numbers (16 bit numbers)
identify the service
Results
Web Server
Database (Optional)
http//www.ietf.org/rfc/rfc1945.txt?number1945
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SMTP Protocol
RFCs 821 974 1869 2821

• Simple Mail Transfer Protocol
• Protocol for sending Email
• Originally designed for a secure network

Mail Client
Mail Server
Mail Server
Mail Server
Issues? Security Non-repudiation
http//www.ietf.org/rfc/rfc0821.txt?number821
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Protocol Stacks
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Which to use?From http//www.gamedev.net/communi
ty/forums/showfaq.asp?forum_id15

• Turn-based
• TCP
• RTS games
• TCP
• For role playing games
• UDP, whereas
• Slower RPGs and MUDs
• TCP.
• Action-based games like first-person shooters or
  racers
• Absolutely use UDP.

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Sockets

• A software interface to the internet
• Create on and you can read and write data to/from
  nodes on the internet
• Supported on Windows
• Version 1.0 1992
• Version 1.1 1993
• Version 2.0 1995
• Sockets for IP v6 2001
• Can be TCP, UDP
• Can be blocking, non-blocking or asynchronous