Networks

1
Networks

• Network Protocols
• Peer-to-peer
• Client-Server Configurations
• Trust

2
Topics

• AI
• Flocking coordinated movement
• Planning
• Pathfinding and search
• Networking
• Multiplayer/things not covered today
• Audio

3
Other topics

• Advanced graphics
• Animation
• Deformation, inverse kinematics
• Motion capture
• Particle effects
• Special lighting
• Ethics
• Cheating, justice

4
Other topics

• Fake terrain
• Industry
• Playtesting
• AIwisdom.com

5
Networks

• Required for multiplayer games
• 3 Standard technologies
• Modems
• Ethernet
• Internet

6
Internet

• The greatest thing since sliced bread
• The savior of humanity
• Will increase freedom and democracy
• Around the world
• In your neighborhood

7
Internet User Protocols

• TCP
• Connection
• Reliable
• Bytes arrive in order they were sent
• Collects small packets and transmits them
  together
• Stream of bytes

• UDP
• Connectionless
• Unreliable
• Arbitrary arrival order

8
TCP

• Reliable stream of bytes
• Implies the need for a connection
• Connection sets up data structures
• Hold incoming packets
• Hold outgoing packets
• Handle retransmits

9
TCP Reliability

• Each packet does Send-Receive-Acknowledge
• Sender holds sent packet until ACK is received
• Sender retransmits if ACK takes too long

10
TCP

• One Send-Receive-Ack takes time
• Overlay Sends and Acks
• Maintain a queue in sender and receiver

11
TCP Circular Queue -- Sender

• Sends data and Puts it in send queue
• Sets timer on this queue item
• If timer expires, and no ACK, re-send data
• Set another, longer timer
• Exponentially increasing time
• When ACK received
• If this queue slot is the oldest,
• Free the slot for new data
• If no queue space avail, sender app waits!

12
TCP Receive Queue

• Receiver maintains a queue the same size as the
  senders
• When a packet arrives, send ACK
• If the packet is next in sequence
• Give it to application
• Else Keep it in queue
• Another, earlier packet is on its way

13
TCP

• If no ACKS arrive for a long enough time
• Temporarily gives up
• Sends test packets
• When test packets get through
• Starts slow, builds up

14
TCP Wrap-up

• Connection sets up sequencing and queues
• Reliable arrival Retransmit
• Reliable order Sequence numbers
• TCP bunches up data on 200ms intervals
• Minimizes overhead for small chunks of data
• This option can be turned off
• TCP Has an emergency channel
• OOB Out Of Band

15
UDP

• Connectionless!
• No underlying data to maintain
• Unreliable transmission
• If you lose a packet, its gone
• Network software must handle this
• Out-of-order arrival
• Network software must handle that, too!
• Fast
• When the port gets the data, the app gets it

16
UDP

• Packets will drop!
• 1 in 5 over non-local connection
• Have to do your own re-send
• Some packets are time sensitive
• Care little about the past ship location
• No header compression
• May end up with greater overhead than TCP with PPP

17
Game Architectures

• Peer-to-peer
• Client/Server
• One server per game
• Floating server
• One client is also a server
• Distributed server
• Multiple servers for large world

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Peer-to-Peer

• Simple version Lockstep
• eg. Doom
• Each client transmits to other
• Wait for everyone to get data
• Proceed to next step

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Peer-to-Peer

• Advantages
• Simple
• Nobody has to provide a server
• Including the Games authors!
• Good for turn-based games with low bandwidth
• TCP

• Disadvantages
• Frame rate is that of
• Slowest machine
• Worst connection
• Hackable
• Not good for real-time games

20
Client/Server

• Server per game
• MUDs, Fireteam, NetTrek
• Someone must provide server ()
• Possibly the games authors
• Less hackable
• Single point of failure
• Server must be big well-connected

21
Floating Server

• Peer-to-peer
• Server resolves the action
• One peer is the server
• Unreal
• One player elects to be the server
• X-Wing vs Tie-Fighter
• First player to enter session
• Starcraft
• Player with the CD

22
Multiple Server

• Many machines coordinate service
• Ultima Online, Everquest, AOL
• Used for large virtual worlds
• Everquest
• One server per game-geographic region
• Freeze on handoff affects game play

23
What Data to Send?

• Sending entire world state is usually too much
• Can send just user actions
• Simulation engine does the same thing at each
  client
• Pseudo-random numbers from same seed

24
Sending User Actions--Problems

• Any error in engine
• Divergence in worlds
• Small error can lead to big divergence
• X-Wing vs Tie Fighter
• Created a resynchronize protocol
• Causes jumps
• Wrote smoothing algorithm for resynchs
• Sim City 2000 Network Edition
• Send checksums for world state each turn

25
Prediction

• Eg. Unreal
• Waiting for user inputs is too slow
• Client does prediction
• Motion prediction
• Server corrects things if client is wrong

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Prediction Dead Reckoning

• Eg. SIMNET (US Army Tank Simulator)
• Each vehicle simulates own tank
• Sends data every 5 seconds, updating
• Position, Speed, Acceleration
• Expected path
• Prediction violation criteria
• Receiver simulates own tank
• AND simulates local copy of other tanks

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Dead Recokoning

• Receiver gets latest 5-second update
• Updates own copy of other tanks
• Predicts other tanks
• Using prediction data
• Until new data arrives
• Each simulator also sends update
• When own prediction violates own criteria
• Assumes latencies

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Dead Reckoning
Sim A
Sim B
Sim B
Sim A
As Predicted Path
As Predicted Path
Bs Predicted Path
Bs Predicted Path
Predict B
Predict A
Predict A
Predict B
Transmit new prediction every 5 seconds
B Exceeds prediction predict again and transmit
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Dead Reckoning Requirements

• Data structures for other entities
• Model of entity behavior
• Vehicle speed, acceleration range, turn radius
• Responsiveness to commands
• Situation parameters
• Following a road
• Precomputed path (NPCs)

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Multiple Copies

• Maintain 2 Data sets
• Now
• Accurate self
• Predicted others
• Zero latency for self
• Ground Truth
• Accurate everybody
• Large latency for almost everybody
• 200-500ms ago

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Latency Issues

• When latencies get high
• Prediction gets worse and worse
• Correcting prediction errors may cause visual
  jumps
• Easy to notice!
• If jumps are large enough
• Temporarily interpolate between wrong prediction
  and the new correction

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Prediction Interpolation
Interpolated Response
Real
Predicted
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Token Ownership

• Some games may allow distributed ownership
• Ballistic simulation
• Shooter fires bullet
• Intended target receives the simulation
• Sports - eg. Tennis
• Player A hits ball
• Player B gets simulation token
• B simulates ball path from As racket

34
Trust

• Never trust the client
• Data on the users hard drive is insecure
• Diablo utility to modify character data
• Wrote patch to prevent hacking
• Throws out your stuff if theres a time
  inconsistency
• Daylight savings nuked my stuff!

35
Trust

• Network communications are insecure
• NetTrek communications are encrypted
• NetTrek also requires blessed client
• Servers have different policies on requiring a
  blessed client
• Prevents robot players or assistants

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Trust -- Checksums

• First line of defense
• Checksum of all packets
• Include header in checksum!
• Stops casual tampering
• Hash function
• Hard to compute source value from result
• MD5

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Checksums

• Not immune to
• Code disassembly
• Packet replay
• Packet replay attack
• Capture a legal packet, and re-send it more
  frequently than allowed
• Client can restrict send frequency
• Server cannot reject high-frequency packets
• Internet bunch-ups are source of OK bunch-ups

38
Combating Replay

• Each new packet client sends is different
• Add a pseudo-random number to each packet
• Not just sequence number!
• Client Server match pseudo-random numbers
• Random numbers
• Seeds must match!
• Dropped packets include sequence number!

39
Combating Replay

• XOR each packet with a pseudo-random bit pattern
• Make sure the bit patterns are in sync!
• Based on previous synchronized pseudo-random
  numbers
• Add junk Confuse length analysis

40
Reverse Engineering

• Remove symbols
• Put encryption code in with rest of network stuff
• Compute magic numbers
• At runtime
• In server
• Encrypt from the start!

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Lists Of Servers

• Denial of service
• Send a packet to server-server saying Im a
  server
• Fake the IP return address with a random IP
• Server-server adds new server to list
• Server may run out of memory storing hundreds of
  thousands of fake servers

42
List of Servers

• Require a dialog
• Server-list server responds with
• Password
• Keepalive interval
• Password must be given by attacker at the correct
  time
• Works OK if client is not better connected!

43
References

• CS4455 course at GA Tech by Chris Shaw
• http//www.cc.gatech.edu/classes/AY2005/cs4455_fal
  l/