Data Communication and Networks

1
Data Communication and Networks

• Introduction to cnet Simulator
• September 25, 2003
• Joseph Conron
• Computer Science Department
• New York University
• jconron_at_cs.nyu.edu

2
What is cnet Simulator?

• Program that simulates a network environment
• Provides simulated application and physical
  layers.
• You provide the middle layers
• Written in C and requires that you write your
  protocols in C
• Runs on many Unix systems. We will give you a
  running version for Solaris.
• If you want run it on another Unix, get the
  package and compile/install/test yourself.
• youre on your own no help from me

3
Overview of cnet

• Organized around layered model
• cnet application layer generates message intended
  for other nodes
• Calls a function you write to send message
• You use functions in cnet physical layer to send
• cnet physical layer is designed to corrupt and
  lose frames.
• Your job is to write protocols that provide for
  reliable, ordered message delivery

4
cnet Event Interface

• Communications between layers is accomplished
  through event handlers
• cnet generates events when something important
  happens
• reboot (EV_REBOOT)
• application message available to send
  (EV_APPLICATIONREADY)
• physical layer data available to read
  (EV_PHYSICALREADY)
• timer expired (EV_TIMER)

5
cnet Event Handlers

• How does cnet know which function to call when
  one of these events occur?
• You write C functions and tell cnet which of
  these to call when a given event occurs
  (CNET_set_handler())
• cnet requires that each handler function provides
  a pre-defined signature.
• Your code implements whatever is appropriate for
  the event and protocol

6
Basic Steps to Implement a Protocol

• Describe the network with a topology file
• Indentifies nodes and links between them
• Write reboot_node() function to handle event
  EV_REBOOT_NODE
• Write the event handlers
• Run cnet using your topology file as input

7
What cnet does (1)

• Your topology file includes the name of your
  source file(s)
• Cnet compiles the named files
• If compilation is OK, cnet begins the simulation
• Cnet generates events
• EV_REBOOT_NODE for each node in network
• EV_APPLICATION_READY for nodes enabled to send
  messages
• EV_PHYSICAL_READY for received frames
• EV_TIMERn for timer events.

8
What cnet does (2)

• If you specified it in your topolgy file cnet
  will generate errors at the physical layer
• Simulation will run for as long as youve
  specified
• By wall-clock time
• By simulated time
• By number of generated events
• cnet collects spastics during the run these are
  important because they allow you to evaluate the
  protocols performance.

9
Hello World

• A topology file A protocol source file

Compile hello.c Host NewYork link to
Boston south of Boston Host Boston
include Void reboot_node(CnetEvent
event, CnetTimer timer, CnetData data)
printf(Hello from node s.\n,
nodeinfo.nodename)
10
cnet Reboot Event

• Every node must handle a reboot event (EV_REBOOT)
• Handler signature is
• void reboot_node(CnetEvent event, CnetTimer
  timer, CnetData data)
• Event will be EV_REBOOT, timer and data can be
  ignored
• No need to tell cnet about your reboot_node
  function cnet assumes you have one.
• Your reboot handler does system initialization
  for your node.
• You should be asking the question by now how do
  I know which node my handler is being called
  for?

11
cnet Global Information

• Cnet defines some global data for you
• The variable nodeinfo contains a CnetNodeInfo
  structure (see cnet.h)
• The data in nodeinfo is read-only you should
  not try to change it.
• Example to get your node name, use
  nodeinfo.nodename.

12
Running the simulation

• Cnet W E 2 o hello HELLO
• This command tells cnet to compile and run your
  simulation.
• -W says to run in console (not GUI) mode
• -E 2 says run for 2 events
• -o hello says create output files for each node
  named hello.
• Simulation with no options will run for 3 minutes
  of wall-clock time.

13
cnet Timers

• cnet provides 10 timers (named EV_TIMER1
  EVTIMER10)
• You can start as many timers as you wish using
  just one of cnets timers.
• So why does cnet have 10 timers?
• Allows layers to use independent timers
• Convenience
• myTimer CNET_start_timer(EV_TIMER1, 3000, 0)
• Starts a 3 second timer. The ID of this timer is
  saved in myTimer.

14
Adding a timer to HELLO
include void myTimerHndlr(CnetEvent
event, CnetTimer timer, CnetData data)
printf(Hello from timer handler in node s\n,
nodeinfo.nodename) CNET_Start_Timer(EV_TIMER1
, 3000, 0) void reboot_node(CnetEvent event,
CnetTimer timer, CnetData data)
printf(Hello from node s.\n,
nodeinfo.nodename) CNET_set_handler(EV_TIMER1
, myTimerHndlr, 0) CNET_Start_Timer(EV_TIMER1
, 3000, 0)
15
cnet GUI

• Cnet provides a graphical interface via tcl/tk
• (but you dont really need it!)
• You need a Unix X windows environment to use it.
• If you are running an XServer from home, you can
  connect to the NYU Solaris systems using ssh and
  start an xterm window
• ssh X l
• If you are running NT or W2K, get cygwin. Here
  are instructions I put together to help.

16
Excercises

• Take the HELLO example and change it to print the
  seconds value of cnets wall-clock time (its in
  nodeinfo) at each timer event.
• Add node number to the print message in reboot
  handler.
• Add code to send messages from NewYork to Boston
  (Use CNET_write_physical_reliable())