CS 4455 Lecture 24

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CS 4455 Lecture 24

• Networks Latency and Prediction

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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

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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

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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 lt 500ms

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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 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
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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

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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!

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Trust

• Network communications are insecure
• NetTrek communications are encrypted
• NetTrek also requires blessed client
• Servers have different policies on requiring a
  blessed client
• Prevents cyborgs

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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

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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!

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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

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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 saying Im a server
• Fake the IP return address with a random IP
• Server adds new server to list
• Server may run out of memory storing hundreds of
  thousands of servers
• Require a dialog

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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!