This document provides an overview of basic packet forwarding in the NS2 network simulator. It discusses NSObjects, which inherit functionality for interfacing with OTcl and handling default actions. NSObjects have a new recv function for receiving packets. Packet forwarding involves an object sending a packet to another object by calling its recv function. Examples of specific NSObject subclasses like Connector and Queue are also presented, which inherit from NSObject and implement packet forwarding using recv.

1. Basic packet
forwarding in
NS2
N S 2 U l t i m a t e . c o m
b y
T e e r a w a t I s s a r i y a k u l

2. OUTLINE
Before you begin...
NsObject
Packet forwarding
Example
Class Connector
Class Queue
Questions?
www.ns2ultimate.com

3. Before you Begin ...
In NS2:
• NO such thing as transmission
• RECEIVE ONLY
www.ns2ultimate.com

4. What we are going to
do here
Suppose you’d like to send a packet from one
NsObject to another
What’s NsObject? ➠ See [ this link ]
packet
NsObject NsObject
www.ns2ultimate.com

5. NSObjects
Inherited Functionalities:
Class TclObject ➠ OTcl interface
Class Handler ➠ Default actions
New Functionalities:
Receive packet ➠ function recv(p,h)
www.ns2ultimate.com

6. This section gives an overview of C++ class hierarchies. The entire hierarchy
NSObjects: Inherited
consists of over 100 C++ classes and struct data types. Here, we only show
a part of the hierarchy (in Fig. 5.1). The readers are referred to [18] for the
functionalities
complete class hierarchy.
OTcl Interface Default Action
TclObject Handler
Simulator PacketQueue NsObject AtHandler QueueHandler
RoutingModule
Network Component
Classifier Connector LanRouter
Uni-directional Point-to-
point Object Connector
Queue Agent ErrorModel LinkDelay Trace
www.ns2ultimate.com
Fig. 5.1. A part of NS2 C++ class hierarchy (this chapter emphasizes on classes

7. NSObject: New
functionality
class NsObject : public TclObject, public Handler {
public:
NsObject();
virtual void recv(Packet*, Handler*) = 0;
};
www.ns2ultimate.com

8. NSObject: New
functionality
class NsObject : public TclObject, public Handler {
public:
NsObject();
virtual void recv(Packet*, Handler*) = 0;
};
TclObject
NsObject
Handler
www.ns2ultimate.com

9. NSObject: New
functionality
class NsObject : public TclObject, public Handler {
public:
NsObject();
virtual void recv(Packet*, Handler*) = 0;
};
packet reception function
Input = a pointer to a Packet object
Input = a pointer to a Handler object
abstract function
www.ns2ultimate.com

10. packet forwarding
NS2 refers to most objects using pointers
Including NsObjects and Packets
Example
“p” = a pointer
“*p” = a place where the pointer “p” pointer
to
www.ns2ultimate.com

11. packet forwarding
Task: An object “*s” sends packet “*p” to an
object “*d”
s p d
packet
NsObject NsObject
www.ns2ultimate.com

12. C++ Statement
From within “*s”, execute one of the following
two C++ Statements:
Given a handler *h: “d->recv(p,h)”
Handler does not exists: “d->recv(p)”
s p d
packet
NsObject NsObject
www.ns2ultimate.com

13. Chapter 15.
Examples
5.1.2 C++ Class Hierarchy
This section gives an overview of C++ class hierarchies. The entire hierarchy
consists of over 100 C++ classes and struct data types. Here, we only show
a part of the hierarchy (in Fig. 5.1). The readers are referred to [18] for the
Six main categories
complete class hierarchy.
OTcl Interface Default Action
TclObject Handler
Simulator PacketQueue NsObject AtHandler QueueHandler
RoutingModule
Network Component
Classifier Connector LanRouter
Uni-directional Point-to-
point Object Connector
Queue Agent ErrorModel LinkDelay Trace
Fig. 5.1. A part of NS2 C++ class hierarchy (this chapter emphasizes on classes
in boxes with thick solid lines). www.ns2ultimate.com

14. Example:
Class Connector
class Connector : public NsObject {
public:
Connector();
NsObject* target_;
NsObject* drop_;
}; • Class variable
• A pointer to NsObject
www.ns2ultimate.com

15. Class Connector
Example configuration
target_ points to the next NsObject
drop_ points to an NsObject responsible for
dropping a packet
100 5 Network Objects
Connector
NsObject NsObject
target_
Upstream Packet forwarding path Downstream
NsObject NsObject
drop_
Packet
dropping
path
NsObject
Packet Dropping
NsObject www.ns2ultimate.com

16. Class Connector
To drop a packet, “send the packet to the
dropping NsObject”
void Connector::drop(Packet* p)
{
if (drop_ != 0)
drop_->recv(p);
else
Packet::free(p);
}
Send a packet *p to the
NsObject *drop_
www.ns2ultimate.com

17. 5.1.2 C++ Class Hierarchy
Another example:
This section gives an overview of C++ class hierarchies. The entire hierarchy
consists of over 100 C++ classes and struct data types. Here, we only show
Class Queue
a part of the hierarchy (in Fig. 5.1). The readers are referred to [18] for the
complete class hierarchy.
OTcl Interface Default Action
TclObject Handler
Simulator PacketQueue NsObject AtHandler QueueHandler
RoutingModule
Network Component
Classifier Connector LanRouter
Uni-directional Point-to-
point Object Connector
Derive from class Connector
Queue Agent ErrorModel LinkDelay Trace
Fig. 5.1. A part of NS2 C++ class hierarchy (this chapter emphasizes on classes
Contain pointers target_ and drop_.
in boxes with thick solid lines).
www.ns2ultimate.com

18. Another example:
Class Queue
void Queue::recv(Packet* p, Handler*)
{
double now = Scheduler::instance().clock();
enque(p);
if (!blocked_) {
p = deque();
if (p != 0) {
blocked_ = 1;
target_->recv(p, &qh_);
}
}
}
Send a packet *p to the NsObject
*target_ with a handler qh_ www.ns2ultimate.com

19. Questions?
How doNetwork Objectsa topology like this?
100 5 we setup
Connector
NsObject NsObject
target_
Upstream Packet forwarding path Downstream
NsObject NsObject
drop_
Packet
dropping
path
NsObject
Packet Dropping
NsObject
Fig. 5.2. Diagram of a connector: The solid arrows represent pointers, while the
What about handler? What is it? What are its
dotted arrows show packet forwarding and dropping paths.
implications?
Program 5.3 Declaration and function recv(p,h) of class Connector
//~/ns/common/connector.h
1 class Connector : public NsObject { www.ns2ultimate.com

20. Stay Tune!
I’ll discuss these
in the following
posts!

21. For more
information
about NS2
P l e a s e s e e
t h i s b o o k
f r o m
S p r i n g e r
T. Issaraiyakul and E. Hossain, “Introduction to Network Simulator NS2”, Springer 2009
or visit www.ns2ultimate.com