Note: This is a detailed note for the book, Introduction to Network Simulator NS2. You may have to read chapter 6 of the book for better understanding.

Introduction

This post is a sequel of the previous post, which demonstrated the relationship of classifiers, routing modules, and nodes:

• What are they, their purposes, their relationship,
• How to configure them conceptually, and
• Using $ns node, how a default classifier, namely “classifier_”, is created and put in a node.
• Putting a routing modules rtnotif_ and ptnotif_ in a node using “$ns node“ 
• Details of classifiers
• 2 common ways to implement routing modules: rtnotif_ and ptnotif_
• Attaching agents to a node
  

What’s in This Post

So far, what we have looks like the figure below. In the previous post, I discussed how the blue block (classifier_) is inserted in to a node.

This post focuses on how two yellow blocks are placed into a node.

The purposes of routing modules rtnotif_ and ptnotif_

Again, a routing module acts as a single point of management for all relevant classifiers in a node. It is an entry point which propagates commands for all related classifiers. From the above figure, a node has 2 default routing modules, which have different purposes.

• rtnotif_: propagates routing information for all attached classifiers.
• ptnotif_: propagates transport layer bridging information for all attached classifiers.

By default, you should never directly configure classifiers. If you want to add a routing entry, you should do so through rtnotif_. If you want to attach a (transport layer) agent to a node, you should do so through ptnotif_.

In the following, I will use the notation as indicated in the previous post. Please have a look to refresh your memory.

Structure

Both the routing modules are created using “$ns node”. As in the previous post, I will divide the statement “$ns node” into three parts:

• Part I. $ns node
• Part II. Constructor of Class Node
• Part III. Inserting Routing Modules and Classifiers

In fact, the creation of routing modules happens in Part III only. So I am going straight to Part III.

Part III. Inserting Routing Modules and Classifiers

If you recall, this part carries out the last step in the part II:

$mod register $self

where $mod is the base routing module, and $self is the Node instance that we are working on. This means NS2 executes the following instproc

[f_RT] RtModule/Base::register {node}
-> $self next $node

Here, the only relevant statement is “$self next $node”. This statement invokes the instproc register (same name) of the base class of class RtModule/Base which is class RtModule.

Instproc register of class RtModule

There are 2 related steps within instproc RtModule::register, i.e.,

[f_RT] RtModule::register { node }

->

$node route-notify $self

->

$node port-notify $self

Now, let’s look at each steps in sequence

1. [f_NT] Node::route-notify{module}: Insert a routing module $module in the node we are working on. The name of this routing module is $rtnotif_, which propagates routing information for all related (address) classifiers: 
   ->
   if {$rtnotif_ == “”} {
       set rtnotif_ $module
   }
   
   If
   $rtnotif_
   does not exists, store the input routing module (namely
   $module
   ) in the instvar
   $rtnotif_
   of the node we are working on.
   
   
   
   ->
   else {
       $rtnotif_ route-notify $module
   }
   Otherwise, invoke OTcl commands (defined in C++) which concatenates
   $module
   to the end of the routing module chain whose head routing module is
   $rtnotif_
   .
2. [f_NT] Node::port-notify{module}: Insert a routing module $module in the node we are working on. The name of this routing module is $ptnotif_, which propagates transport layer bridge information for all related (port) classifiers.
   ->
   lappend ptnotif_ $module
   
   which stores the input routing module (namely
   $module
   ) as the last entry of the list of routing module
   $ptnotif_
   . Note, a data type ‘list’ in Tcl is similar to ‘array’ in C++.

Summary: What we have done

Again, another long post, isn’t it. But look at the above figure. We have added two more yellow blocks into a node. There are still few more to go. So stay tune!

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Source: T. Issaraiyakul and E. Hossain, “Introduction to Network Simulator NS2”, Springer 2011. Buy it now from Amazon:

You may also find lecture notes and other resource at the following website:http://www.ece.ubc.ca/~teerawat/NS2.htm