1. Network Simulation and Simulator Internals 潘仁義 jypan@comm.ccu.edu.tw
Introduction to ns2
Network Simulation and Simulator Internals
潘仁義
Modified from Su Wen’s ns2 ppt
& Padmaparna Haldar’s

2. The Network Simulator - ns-2
The source code and documentation is currently maintained by VINT project at ISI
Sept 3, 2007: ns-2.32 released.
Mar 10, 2007: ns-2.31 released.
July 2, 2006: ns-3 project announced.
NS2 is a discrete event simulator targeted at networking research
NS2 is an object oriented simulator, written in C++, with an OTcl interpreter as a frontend
抽考: ns2安裝

3. ns-2 Overview Collection of various protocols at multiple layers
TCP(reno, tahoe, vegas, sack)
MAC(802.11, 802.3, TDMA)
Ad-hoc Routing (DSDV, DSR, AODV, TORA)
Sensor Network (diffusion, gaf)
Multicast protocols, Satellite protocols, and many others
Codes are contributed from multiple research communities
Good: Large set of simulation modules
Bad: Level of support and documentation varies
The source code and documentation is currently maintained by VINT project at ISI

4. Documentation introductory: Marc Greis's tutorial
reference: Ns Manual (formerly called "ns Notes and Documentation")
ns by Example
Practical Programming in Tcl and Tk (

5. Current Status ns-2 (2.1b6) Simulator Core Other Components
100K lines of C++
70K lines of OTcl
30K lines of test suite
20K lines of documentation
Other Components
Tcl/TK 8.x, OTcl, TclCL, nam-1
Tcl-debug, GT-ITM, xgraph, …

6. ns Directory Structure
ns-allinone
Tcl8.0
TK8.0
OTcl
tclcl
ns-2
nam-1
tcl
...
C++ code ex
test
lib
mcast
...
examples
validation tests
OTcl code
simple.tcl

7. Running simulations with ns
Compile the simulator core (“ns”)
Write a simulation script in Otcl
e.g. my-test.tcl
Running the simulator
e.g. ns my-test.tcl

8. Hello World simple.tcl arches 74% ns simple.tcl Hello World!
set sim [new Simulator]
$sim at 1 “puts \“Hello World!\””
$sim at 1.5 “exit”
$sim run
arches 74% ns simple.tcl
Hello World!
arches 75%

9. Discrete Event Simulation
Model world as events
Simulator has list of events
Process: take next one, run it, until done
Each event happens in an instant of virtual (simulated) time, but takes an arbitrary amount of real time
Ns uses simple model: single thread of control => no locking or race conditions to worry about (very easy)

10. Discrete Event Examples
simple
queuing
model:
Consider two nodes
on an Ethernet:
t=1, A enqueues pkt on LAN
t=1.01, LAN dequeues pkt
and triggers B A B
t=1.0: A sends pkt to NIC
A’s NIC starts carrier sense
t=1.005: A’s NIC concludes cs,
starts tx
t=1.006: B’s NIC begins reciving pkt
t=1.01: B’s NIC concludes pkt
B’s NIC passes pkt to app
detailed
CSMA/CD
model:

11. ns-2 Environment Simulation Scenario Tcl Script C++ Implementation 1 2
set ns_ [new Simulator]
set node_(0) [$ns_ node]
set node_(1) [$ns_ node]
Tcl Script
class MobileNode : public Node {
friend class PositionHandler;
public:
MobileNode(); • }
C++ Implementation

12. Why two languages? (Tcl & C++)
C++: Detailed protocol simulations require systems programming language
byte manipulation, packet processing, algorithm implementation
Run time speed is important
Turn around time (run simulation, find bug, fix bug, recompile, re-run) is slower
Tcl: Simulation of slightly varying parameters or configurations
quickly exploring a number of scenarios
iteration time (change the model and re-run) is more important

13. C++ and OTcl Separation
“data” / control separation
C++ for “data”:
per packet processing, core of ns
fast to run, detailed, complete control
OTcl for control:
Simulation scenario configurations
Periodic or triggered action
Manipulating existing C++ objects
fast to write and change
running vs. writing speed
Learning and debugging (two languages)

14. Using ns Problem Result Simulation Modify analysis model ns Setup/run
with ns

15. Summary: Generic Script Structure
set ns [new Simulator]
# [Turn on tracing]
# Create topology
# Setup packet loss, link dynamics
# Create routing agents
# Create:
# - multicast groups
# - protocol agents
# - application and/or setup traffic sources
# Post-processing procs
# Start simulation

16. Basic Tcl procedures: proc pow {x n} { if {$n == 1} { return $x }
variables:
set x 10
puts “x is $x”
functions and expressions:
pow x 2
expr x*x
control flow:
if {$x > 0} { return $x } else {
return [expr -$x] }
while { $x > 0 } {
puts $x
incr x –1 }
procedures:
proc pow {x n} {
if {$n == 1} { return $x }
set part [pow $x [expr $n-1]]
return [expr $x*$part] }
Also lists, associative arrays, etc.
=> can use a real programming language to build network topologies, traffic models, etc.

17. Basic otcl
Class Person
# constructor:
Person instproc init {age} {
$self instvar age_
set age_ $age }
# method:
Person instproc greet {} {
puts “$age_ years old: How are you doing?”
# subclass:
Class Kid -superclass Person
Kid instproc greet {} {
$self instvar age_
puts “$age_ years old kid: What’s up, dude?” }
set a [new Person 45]
set b [new Kid 15]
$a greet
$b greet
# “new” in ns2 only
Person a 45; Kid b 15
a greet; b greet
Person info instances
a info class; a info vars
=> can easily make variations of existing things (TCP, TCP/Reno)

18. Otcl and C++: The Duality
C++/OTcl
split objects
otcl
OTcl (object variant of Tcl) and C++ share class hierarchy
TclCL is glue library that makes it easy to share functions, variables, etc

19. C++ and OTcl Linkage Class Tcl: instance of OTcl interpreter
Tcl& tcl = Tcl::instance();
tcl.evalc(“puts stdout hello world”);
tcl.result() and tcl.error()
Class TclObject and TclClass
Variable bindings
bind(“rtt_”, &t_rtt_)
Invoking command method in shadow class
$tcp advanceby 10

20. C++ and Otcl linkage II Some important objects: 目前悟性不夠
NsObject: has recv() method
Connector: has target() and drop()
BiConnector: uptarget() & downtarget()
目前悟性不夠

21. TclObject: Hierarchy and Shadowing
OTcl class
hierarchy
C++ class
hierarchy
TclObject
TclObject
static JSTcpClass : public TclClass {
public:
JSTcpClass():TclClass("Agent/TCP/JS"){}
TclObject*
create(int,const char*const*)
{ return (new JSTcpAgent());} };
Agent
Agent
Agent/TCP/JS
JSTcpAgent
_o123
*tcp
Agent/TCP/JS OTcl
shadow object
Agent/TCP/JS C++
object

22. TclObject: Creation and Deletion
Agent/TCP/JS
constructor
Agent/TCP
constructor
invoke parent
TclObject
constructor
create C++
object
invoke parent
constructor
which C++
object to create?
– TclClass
JSTCPAgent
constructor
invoke parent
complete
initialization
complete
initialization
create OTcl
shadow object
OTcl
C++
do nothing,
return
TclObject (C++)
constructor
invoke parent
constructor
parent (Agent)
bind variables
and return
bind variables
and return

23. Class Hierarchy in ns TclObject NsObject Connector Classifier Queue
Delay
Agent
Trace
AddrClassifier
McastClasifier
DropTail
RED
TCP
Enq
Deq
Drop
Reno
SACK JS

24. TCP Jump Start – Step 1 New file: tcp-js.h
class JSTcpAgent : public TcpAgent {
public:
virtual void set_initial_window() {
cwnd_ = MAXWIN_; }
JSTcpAgent();
private:
int MAXWIN_; };

25. TCP Jump Start – Step 2 New file: tcp-js.cc bind(“MAXWIN_”, &MAXWIN_);
#include “tcp.h”
#include “tcp-js.h”
static class JSTcpClass : public TclClass {
public:
JSTcpClass() : TclClass("Agent/TCP/JS") {}
TclObject* create(int, const char*const*) {
return (new JSTcpAgent()); }
}class_jstcp;
JSTcpAgent::JSTcpAgent() {
bind(“MAXWIN_”, &MAXWIN_);

26. TclObject::bind() Link C++ member variables to OTcl object variables
TcpAgent::TcpAgent() {
bind(“window_”, &wnd_);
… … }
bind_time(), bind_bool(), bind_bw() (set with unit)
OTcl
set tcp [new Agent/TCP]
$tcp set window_ 200

27. Initialization of Bound Variables
Initialization through OTcl class variables
Agent/TCP set window_ 50
Do all initialization of bound variables in ~ns/lib/ns-default.tcl
Otherwise a warning will be issued when the shadow object is created
After modifying, remember to make ns2

28. Calling C++ functions from Otcl
set tcp [new Agent/TCP]
$tcp advance 10
C++
int TcpAgent::command(int argc,
const char*const* argv) {
if (argc == 3) {
if (strcmp(argv[1], “advance”) == 0) {
int newseq = atoi(argv[2]); ……
return(TCL_OK); }
return (Agent::command(argc, argv);

29. TclObject::command()
OTcl space
no such
procedure
$tcp send
TclObject::unknown{}
$tcp cmd send
C++ space
TcpAgent::command()
match
“send”?
Yes No
process and return
Invoke parent:
return Agent::command()

30. Calling Otcl functions from C++
Agent/TCP instproc advance {num} {
set window_ [expr $window_ + $num]
return $window_ }
C++
Tcl& tcl = Tcl::instance();
char *result;
tcl.evalf(“%s advance %d”, name_, size);
result = tcl.result();
wnd_ = atoi(result);

31. EmbeddedTcl How it works Makefile.in:
tcl2c++: provided by TclCL, converts tcl scripts into a C++ static character array
Makefile.in:
tclsh8.0 bin/tcl-expand.tcl tcl/lib/ns-lib.tcl | tcl2c++ et_ns_lib > gen/ns_tcl.cc

32. Summary TclObject TclClass Tcl: primitives to access Tcl interpreter
Unified interpreted (OTcl) and compiled (C++) class hierarchies
Seamless access (procedure call and variable access) between OTcl and C++
TclClass
The mechanism that makes TclObject work
Tcl: primitives to access Tcl interpreter

33. Event Scheduler Create event scheduler Schedule events Start scheduler
set ns [new Simulator]
Schedule events
$ns at <time> <event>
<event>: any legitimate ns/tcl commands
$ns at 0.1 “$ftp start”
$ns at 4.0 “$ftp stop”
$ns at 5.0 “finish”
Start scheduler
$ns run
ns/common/scheduler.cc
Scheduler::command() “now”, “at”
ns/link/delay.cc
LinkDelay::recv() “s.schedule()”

34. Extending ns in OTcl ns-allinone Tcl8.0 TK8.0 OTcl tclcl ns-2 nam-1
...
C++ code ex
test
mysrc
lib
mcast
...
examples
validation tests
msg.tcl
OTcl code

35. Add Your Changes into ns
source your changes in your sim scripts Or
add to tcl/lib/ns-lib.tcl …
source ../mysrc/msg.tcl
Change Makefile
NS_TCL_LIB = \
tcl/mysrc/msg.tcl \
Recompile

36. Scalability vs Flexibility
It’s tempting to write all-OTcl simulation
Benefit: quick prototyping
Cost: memory + runtime
Solution
Control the granularity of your split object by migrating methods from OTcl to C++
Conventional Wisdom:
C++ for “data”
Per packet action
OTcl for control
Periodic or triggered action

37. Program size, complexity
THE Merit of OTcl
Program size, complexity
low
high
C/C++
OTcl
split objects
Smoothly adjust the granularity of scripting to balance extensibility and performance
With complete compatibility with existing simulation scripts

38. Object Granularity Tips
Functionality
Per-packet processing  C++
Hooks, frequently changing code  OTcl
Data management
Complex/large data structure  C++
One-time configuration variables  OTcl

39. Memory Conservation Tips
Avoid trace-all
Use arrays for a sequence of variables
Instead of n$i, say n($i)
Avoid OTcl temporary variables
Use dynamic binding
delay_bind() instead of bind()
See object.{h,cc}

40. Memory Leaks Purify or dmalloc, but be careful about split objects:
for {set i 0} {$i < 500} {incr i} {
set a [new RandomVariable/Constant] }
It leaks memory, but can’t be detected!
Solution
Explicitly delete EVERY split object that was new-ed

41. Backup slide

42. OTcl Linkage 描述 名稱 繼承 C++ object class MyAgent Agent Linkage object
OTcl Linkage是c++和OTcl 的一個介面。 描述 名稱 繼承
C++ object class
MyAgent
Agent
Linkage object
CLass
TCL CLass
OTcl object
Agent/MyAgentOtcl
C++ code
OLcl Linkake
OTcl code

43. OTcl Linkage

44. 如何使用OTcl linkage Export C++ class variables to OTcl
- bind(): real or integer variables - bind_time(): time variable - bind_bw(): bandwidth variable - bind_bool(): boolean variable 請在ns-2/tcl/lib/ns-lib.tcl 設預設值，否則會有警告message
OTcl變數
C++變數

45. 如何使用OTcl linkage Export C++ Object Control Commands to OTcl
This is done by defining a "command" member function of your C++ object, which works as an OTcl command interpreter.
When OTcl object( is created and the user tries to call a member function of that object (i.e. $myagent call-my-priv-func), OTcl searches for the given member function name in the OTcl object. If the given member function name cannot be found, then it invokes the "MyAgent::command" passing the invoked OTcl member function name and arguments in argc/argv format.

46. 如何使用OTcl linkage
Export C++ Object Control Commands to OTcl -

47. 如何使用OTcl linkage Execute an OTcl command from C++.
makes an OTcl interpreter print out the value in "my_var1" and "my_var2" private member variables
To execute an OTcl command from C++, you should get a reference to "Tcl::instance()"

48. 如何使用OTcl linkage Compile, run and test "MyAgent“
put "ex-linkage.cc" file, and save it under the "ns-2" directory.
Open "Makefile", add "ex-linkage.o" at the end of object file list.
Re-compile NS using the "make" command.
Run the OTcl script using command "ns ex-linkage.tcl".

49. 如何使用OTcl linkage

50. 如何使用OTcl linkage

51. C++ Guidelines Decide position in class hierarchy
i.e., which class to derive from?
Create new packet header (if necessary)
Create C++ class, fill in methods
Define OTcl linkage (if any)
Write OTcl code (if any)
Build (and debug)

52. Where to Find What?

53. Where to Find What?
ns-lib.tcl: The simulator class and most of its member function definitions except for LAN, Web, and Multicast related ones are located here.
ns-default.tcl: The default values for configurable parameters for various network components are located here. the parameters are actually C++ variables made available to OTcl via an OTcl linkage function
bind(C++_variable_name, OTcl_variable_name)  

54. Where to Find What?
ns-packet.tcl: The packet header format initialization implementation is located here. When you create a new packet header, you should register the header in this file to make the packet header initialization process and give you the offset of your header in the stack.
other OTcl files: Other OTcl files in this directory, The FTP application is entirely implemented in OTcl and the source code is located in "ns-source.tcl".