.. include:: replace.txt

.. highlight:: cpp

Organization

------------

This chapter describes the overall |ns3| software organization and the

corresponding organization of this manual.

|ns3| is a discrete-event network simulator in which the simulation core

and models are implemented in C++. |ns3| is built as a library which may be

statically or dynamically linked to a C++ main program that defines the

simulation topology and starts the simulator. |ns3| also exports nearly all

of its API to Python, allowing Python programs to import an "ns3" module in

much the same way as the |ns3| library is linked by executables in C++.  

.. _software-organization:

.. figure:: figures/software-organization.*

   Software organization of |ns3|

The source code for |ns3| is mostly organized in the ``src`` directory and

can be described by the diagram in :ref:`software-organization`. We will

work our way from the bottom up; in general, modules only have dependencies

on modules beneath them in the figure.

We first describe the core of the simulator; those components that are 

common across all protocol, hardware, and environmental models. 

The simulation core is implemented in ``src/core``. Packets are 

fundamental objects in a network simulator

and are implemented in ``src/network``. These two simulation modules by

themselves are intended to comprise a generic simulation core that can be

used by different kinds of networks, not just Internet-based networks.  The

above modules of |ns3| are independent of specific network and device

models, which are covered in subsequent parts of this manual.

In addition to the above |ns3| core, we introduce, also in the initial 

portion of the manual, two other modules that supplement the core C++-based 

API.  |ns3| programs may access

all of the API directly or may make use of a so-called *helper API* that

provides convenient wrappers or encapsulation of low-level API calls. The

fact that |ns3| programs can be written to two APIs (or a combination

thereof) is a fundamental aspect of the simulator.

We also describe how Python is supported in |ns3| before moving onto

specific models of relevance to network simulation.

The remainder of the manual is focused on documenting the models and

supporting capabilities.  The next part focuses on two fundamental objects in

|ns3|:  the ``Node`` and ``NetDevice``. Two special NetDevice types are

designed to support network emulation use cases, and emulation is described

next.  The following chapter is devoted to Internet-related models, 

including the

sockets API used by Internet applications. The next chapter covers 

applications, and the following chapter describes additional support for 

simulation, such as animators and statistics.

The project maintains a separate manual devoted to testing and validation

of |ns3| code (see the `ns-3 Testing and Validation manual

<http://www.nsnam.org/tutorials.html>`_).