Assignment 1 – Mazewar
Review Session
CS244B Distributed Systems
April 4th, 2014
Wei Shi, Stanford University
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Agenda
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Logistics
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Mazewar
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Game rules
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Protocol design
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Implementation
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Logistics
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0% Teams: due April 4, 2014, 11:59pm - Jointly
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40% Design: due April 10, 2014, 11:59pm - Jointly
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40% Program: due April 21, 2014, 11:59pm - Individually
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20% Questions: due April 21, 2014, 11:59pm – Individually
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Demo tentatively the week of April 21
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Will post sign-up sheet on Piazza
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Local students: in myth clusters(Gates basement)
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SCPD: at least one member comes to campus; others join remotely
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Game Rules
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Rat control
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(Done) Movement with a,s,d,f
 (Done) Peek left/right with Mouse Left/Right button
 Launch missiles with Mouse Middle button
Scoring
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Shoot projectile: -1
 Hit opponent: +11
 Opponent hits you: -5
Player’s view
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(done) Map view that shows other players
Rat’s view that shows missile
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Game Rules
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Only one rat can occupy a square at a time
Rats hit by others should respawn at an unoccupied random
location instantly
Cool-down time for firing (time up to you)
Multiple missiles can occupy the same space (speed up to
you)
One projectile can hit at most one rat
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What You are Given
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Implementation of the graphics
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Implementation of the basic control
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Stub code for setting up UDP multicast
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Event loop (void play())
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Receives Packets
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Keyboard events
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Timer events (EVENT_TIMEOUT)
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Default is 500ms, defined in winsys.cpp
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What You Need to Edit
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netInit()
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Stub networking code in place
Most of your work will be editing play()
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Main event loop
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Receive packets and update local state
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Change local state and send packets
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Protocol Design

Team creates protocol, members implement
individually
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At most three team members
Must be distributed
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Can't have central point of failure
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Over UDP Multicast
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Must deal with lost packets in a reasonable way
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Protocol Assumptions
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Players know the UDP multicast group for the
game
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No cheating
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One standard maze
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One game per UDP multicast group
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Defined as MAZEPORT in mazewar.h
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To be posted on Piazza after team registration
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Protocol Hints
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Determine shared state
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Which state is shared by which clients?
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How will sufficient consistency be maintained?
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Who are the readers/writers of each piece of state?
Create the protocol which will define
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Syntax - clients agree on format of messages
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Semantics - clients agree on meaning of messages
Timing - clients agree on timing semantics
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Protocol Hints
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Use network byte order(use htonl(), etc.)
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When are ACKs needed?
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As opposed to "fire and forget" packets
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A believes he shot B. Do they need to agree?
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A, B both believe they shot C. What do they do?
Joining / Exiting game
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How do players discover each other?
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How should failures / network congestions be handled?
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Protocol Hints
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There’s absolutely no globally synchronous clock
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Ensure inconsistencies don't accumulate
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Think about the tradeoff of the size or frequency of
the packets
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Protocol Example
Latex: \usepackage{bytefield}
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Implementation Hints
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Look at the code early
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Implement some local functionality, e.g. launch missile
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Will help you think about your protocol
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Starter code is heavily influenced by CS249 framework
Start testing interoperability ASAP
Test with simulated packet loss (commented out in
starter code)
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Implementation Hints
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Develop/test/grade on myth clusters
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ssh –X myth.stanford.edu (enable X forwarding)
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If you wish to develop locally, you might encounter
errors.
Try sudo apt-get install
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libxaw7-dev (if missing X11/Xaw/Cardinals.h)
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libx11-dev (if missing X11/stringdefs.h or X11/xlib.h)
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Starter Code
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ssh –X myth.stanford.edu
cd /afs/ir.stanford.edu/class/cs244b/mazewar
cp MazewarSpring2014.tar.gz ~/<your directory>
cd ~/<your directory>
tar –zxvf MazewarSpring2014.tar.gz
Develop!
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Mazewar
Demo
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Questions?
Post your questions on Piazza
https://piazza.com/class#spring2014/cs244b
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