Creating a new ns-3 model
*************************

This chapter walks through the design process of an ns-3 model. In many research cases, users will not be satisfied to merely adapt existing models, but may want to extend the core of the simulator in a novel way.  We will use the example of adding an ErrorModel to a simple ns-3 link as a motivating example of how one might approach this problem and proceed through a design and implementation.

Design-approach
===============

Consider how you want it to work; what should it do.  Think about these things:

  * functionality:  What functionality should it have?  What attributes or configuration is exposed to the user?

  * reusability:  How much should others be able to reuse my design? Can I reuse code from ns-2 to get started?  How does a user integrate the model with the rest of another simulation?

  * dependencies:  How can I reduce the introduction of outside dependencies on my new code as much as possible (to make it more modular)?  For instance, should I avoid any dependence on IPv4 if I want it to also be used by IPv6?  Should I avoid any dependency on IP at all?

Do not be hesitant to contact the ns-3-users or ns-developers list if you have questions.  In particular, it is important to think about the public API of your new model and ask for feedback.  It also helps to let others know of your work in case you are interested in collaborators.

Example: ErrorModel
-------------------

An error model exists in ns-2.  It allows packets to be passed to a stateful object that determines, based on a random variable, whether the packet is corrupted.  The caller can then decide what to do with the packet (drop it, etc.).

The main API of the error model is a function to pass a packet to, and the return value of this function is a boolean that tells the caller whether any corruption occurred.  Note that depending on the error model, the packet data buffer may or may not be corrupted.  Let's call this function "IsCorrupt()".

So far, in our design, we have:
class ErrorModel
{
public:
 /**
  * \returns true if the Packet is to be considered as errored/corrupted
  * \param pkt Packet to apply error model to
  */
  bool IsCorrupt (Ptr<Packet> pkt);
};
Note that we do not pass a const pointer, thereby allowing the function to modify the packet if IsCorrupt() returns true.  Not all error models will actually modify the packet; whether or not the packet data buffer is corrupted should be documented.

We may also want specialized versions of this, such as in ns-2, so although it is not the only design choice for polymorphism, we assume that we will subclass a base class ErrorModel for specialized classes, such as RateErrorModel, ListErrorModel, etc, such as is done in ns-2.

You may be thinking at this point, "Why not make IsCorrupt() a virtual method?".  That is one approach; the other is to make the public non-virtual function indirect through a private virtual function (this in C++ is known as the non virtual interface idiom and is adopted in the ns-3 ErrorModel class).

Next, should this device have any dependencies on IP or other protocols? We do not want to create dependencies on Internet protocols (the error model should be applicable to non-Internet protocols too), so we'll keep that in mind later.

Another consideration is how objects will include this error model. We envision putting an explicit setter in certain NetDevice implementations, for example.
  /**
   * Attach a receive ErrorModel to the PointToPointNetDevice.
   *
   * The PointToPointNetDevice may optionally include an ErrorModel in
   * the packet receive chain.
   *
   * @see ErrorModel
   * @param em Ptr to the ErrorModel.
   */
  void PointToPointNetDevice::SetReceiveErrorModel(Ptr<ErrorModel> em);
Again, this is not the only choice we have (error models could be aggregated to lots of other objects), but it satisfies our primary use case, which is to allow a user to force errors on otherwise successful packet transmissions, at the NetDevice level.

After some thinking and looking at existing ns-2 code, here is a sample API of a base class and first subclass that could be posted for initial review:
class ErrorModel
{
public:
  ErrorModel ();
  virtual ~ErrorModel ();
  bool IsCorrupt (Ptr<Packet> pkt);
  void Reset (void);
  void Enable (void);
  void Disable (void);
  bool IsEnabled (void) const;
private:
  virtual bool DoCorrupt (Ptr<Packet> pkt) = 0;
  virtual void DoReset (void) = 0;
};

enum ErrorUnit
  {
    EU_BIT,
    EU_BYTE,
    EU_PKT
  };

// Determine which packets are errored corresponding to an underlying
// random variable distribution, an error rate, and unit for the rate.
class RateErrorModel : public ErrorModel
{
public:
  RateErrorModel ();
  virtual ~RateErrorModel ();
  enum ErrorUnit GetUnit (void) const;
  void SetUnit (enum ErrorUnit error_unit);
  double GetRate (void) const;
  void SetRate (double rate);
  void SetRandomVariable (const RandomVariable &ranvar);
private:
  virtual bool DoCorrupt (Ptr<Packet> pkt);
  virtual void DoReset (void);
};

Scaffolding
===========

Let's say that you are ready to start implementing; you have a fairly clear picture of what you want to build, and you may have solicited some initial review or suggestions from the list. One way to approach the next step (implementation) is to create scaffolding and fill in the details as the design matures.

This section walks through many of the steps you should consider to define scaffolding, or a non-functional skeleton of what your model will eventually implement.  It is usually good practice to not wait to get these details integrated at the end, but instead to plumb a skeleton of your model into the system early and then add functions later once the API and integration seems about right.

Note that you will want to modify a few things in the below presentation for your model since if you follow the error model verbatim, the code you produce will collide with the existing error model.  The below is just an outline of how ErrorModel was built that you can adapt to other models.

Review the ns-3 coding style document
-------------------------------------

At this point, you may want to pause and read the ns-3 coding style document, especially if you are considering to contribute your code back to the project.  The coding style document is linked off the main project page: ns-3 coding style.

Decide where in the source tree the model will reside in
--------------------------------------------------------

All of the ns-3 model source code is in the directory src/. You will need to choose which subdirectory it resides in.  If it is new model code of some sort, it makes sense to put it into the src/ directory somewhere, particularly for ease of integrating with the build system.

In the case of the error model, it is very related to the packet class, so it makes sense to implement this in the src/common/ directory where ns-3 packets are implemented.

waf and wscript
---------------

ns-3 uses the Waf build system.  You will want to integrate your new ns-3 uses the Waf build system.  You will want to integrate your new source files into this system.  This requires that you add your files to the wscript file found in each directory.

Let's start with empty files error-model.h and error-model.cc, and add this to src/common/wscript.  It is really just a matter of adding the .cc file to the rest of the source files, and the .h file to the list of the header files.

Now, pop up to the top level directory and type "./waf --check".  You shouldn't have broken anything by this operation.

include guards
--------------

Next, let's add some include guards in our header file.
#ifndef ERROR_MODEL_H
#define ERROR_MODEL_H
...
#endif

namespace ns3
-------------

ns-3 uses the ns3 namespace to isolate its symbols from other namespaces. Typically, a user will next put an ns-3 namespace block in both the cc and h file.
namespace ns3 {
...
}

At this point, we have some skeletal files in which we can start defining our new classes.  The header file looks like this:

#ifndef ERROR_MODEL_H
#define ERROR_MODEL_H

namespace ns3 {

} // namespace ns3
#endif
while the error-model.cc file simply looks like this:
#include "error-model.h"

namespace ns3 {

} // namespace ns3
These files should compile since they don't really have any contents. We're now ready to start adding classes.

Initial Implementation
======================

At this point, we're still working on some scaffolding, but we can begin to define our classes, with the functionality to be added later.

use of class Object?
--------------------

This is an important design step; whether to use class Object as a base class for your new classes.

As described in the chapter on the ns-3 Object model, classes that inherit from class Object get special properties:

  * the ns-3 type and attribute system (see Attributes)

  * an object aggregation system

  * a smart-pointer reference counting system (class Ptr)

Classes that derive from class ObjectBase get the first two properties above, but do not get smart pointers.  Classes that derive from class RefCountBase get only the smart-pointer reference counting system.

In practice, class Object is the variant of the three above that the ns-3 developer will most commonly encounter.

In our case, we want to make use of the attribute system, and we will be passing instances of this object across the ns-3 public API, so class Object is appropriate for us.

initial classes
---------------

One way to proceed is to start by defining the bare minimum functions and see if they will compile.  Let's review what all is needed to implement when we derive from class Object.

#ifndef ERROR_MODEL_H
#define ERROR_MODEL_H

#include "ns3/object.h"

namespace ns3 {

class ErrorModel : public Object
{
public:
  static TypeId GetTypeId (void);

  ErrorModel ();
  virtual ~ErrorModel ();
};

class RateErrorModel : public ErrorModel
{
public:
  static TypeId GetTypeId (void);

  RateErrorModel ();
  virtual ~RateErrorModel ();
};
#endif

A few things to note here.  We need to include object.h.  The convention in ns-3 is that if the header file is co-located in the same directory, it may be included without any path prefix.  Therefore, if we were implementing ErrorModel in src/core directory, we could have just said "#include "object.h"".  But we are in src/common, so we must include it as "#include "ns3/object.h"".  Note also that this goes outside the namespace declaration.

Second, each class must implement a static public member function called GetTypeId (void).

Third, it is a good idea to implement constructors and destructors rather than to let the compiler generate them, and to make the destructor virtual. In C++, note also that copy assignment operator and copy constructors are auto-generated if they are not defined, so if you do not want those, you should implement those as private members.  This aspect of C++ is discussed in Scott Meyers' Effective C++ book. item 45.

Let's now look at some corresponding skeletal implementation code in the .cc file.

#include "error-model.h"

namespace ns3 {

NS_OBJECT_ENSURE_REGISTERED (ErrorModel);

TypeId ErrorModel::GetTypeId (void)
{
  static TypeId tid = TypeId ("ns3::ErrorModel")
    .SetParent<Object> ()
    ;
  return tid;
}

ErrorModel::ErrorModel ()
{
}

ErrorModel::~ErrorModel ()
{
}

NS_OBJECT_ENSURE_REGISTERED (RateErrorModel);

TypeId RateErrorModel::GetTypeId (void)
{
  static TypeId tid = TypeId ("ns3::RateErrorModel")
    .SetParent<ErrorModel> ()
    .AddConstructor<RateErrorModel> ()
    ;
  return tid;
}

RateErrorModel::RateErrorModel ()
{
}

RateErrorModel::~RateErrorModel ()
{
}

What is the GetTypeId (void) function?  This function does a few things.  It registers a unique string into the TypeId system.  It establishes the hierarchy of objects in the attribute system (via SetParent). It also declares that certain objects can be created via the object creation framework (AddConstructor).

The macro NS_OBJECT_ENSURE_REGISTERED (classname) is needed also once for every class that defines a new GetTypeId method, and it does the actual registration of the class into the system. The Object model chapter discusses this in more detail.

how to include files from elsewhere
-----------------------------------

log component
-------------

Here, write a bit about adding ns-3 logging macros.  Note that

LOG_COMPONENT_DEFINE is done outside the namespace ns3

constructor, empty function prototypes
--------------------------------------

key variables (default values, attributes)
------------------------------------------

test program 1
--------------

Object Framework
----------------

            static const ClassId cid;

          const InterfaceId ErrorModel::iid =
            MakeInterfaceId ("ErrorModel", Object::iid);

          const ClassId ErrorModel::cid =
            MakeClassId<ErrorModel> ("ErrorModel", ErrorModel::iid);

Adding a sample script
======================

At this point, one may want to try to take the basic scaffolding defined above and add it into the system.  Performing this step now allows one to use a simpler model when plumbing into the system and may also reveal whether any design or API modifications need to be made.  Once this is done, we will return to building out the functionality of the ErrorModels themselves.

Add basic support in the class
------------------------------

          point-to-point-net-device.h
            class ErrorModel;

            /**
             * Error model for receive packet events
             */
            Ptr<ErrorModel> m_receiveErrorModel;

Add Accessor
------------

            void
          PointToPointNetDevice::SetReceiveErrorModel (Ptr<ErrorModel> em)
          {
            NS_LOG_FUNCTION (this << em);
            m_receiveErrorModel = em;
          }

             .AddAttribute ("ReceiveErrorModel",
                             "The receiver error model used to simulate packet loss",
                             PointerValue (),
                             MakePointerAccessor (&PointToPointNetDevice::m_receiveErrorModel),
                             MakePointerChecker<ErrorModel> ())

Plumb into the system
---------------------

          void PointToPointNetDevice::Receive (Ptr<Packet> packet)
          {
            NS_LOG_FUNCTION (this << packet);
            uint16_t protocol = 0;

            if (m_receiveErrorModel && m_receiveErrorModel->IsCorrupt (packet) )
              {
          //
          // If we have an error model and it indicates that it is time to lose a
          // corrupted packet, don't forward this packet up, let it go.
          //
                m_dropTrace (packet);
              }
            else
              {
          //
          // Hit the receive trace hook, strip off the point-to-point protocol header
          // and forward this packet up the protocol stack.
          //
                m_rxTrace (packet);
                ProcessHeader(packet, protocol);
                m_rxCallback (this, packet, protocol, GetRemote ());
                if (!m_promiscCallback.IsNull ())
                  {           m_promiscCallback (this, packet, protocol, GetRemote (),
                                GetAddress (), NetDevice::PACKET_HOST);
                  }
              }
          }

Create null functional script
-----------------------------

          simple-error-model.cc

            // Error model
            // We want to add an error model to node 3's NetDevice
            // We can obtain a handle to the NetDevice via the channel and node
            // pointers
            Ptr<PointToPointNetDevice> nd3 = PointToPointTopology::GetNetDevice
              (n3, channel2);
            Ptr<ErrorModel> em = Create<ErrorModel> ();
            nd3->SetReceiveErrorModel (em);

          bool
          ErrorModel::DoCorrupt (Packet& p)
          {
            NS_LOG_FUNCTION;
            NS_LOG_UNCOND("Corrupt!");
            return false;
          }

At this point, we can run the program with our trivial ErrorModel plumbed into the receive path of the PointToPointNetDevice.  It prints out the string "Corrupt!" for each packet received at node n3.  Next, we return to the error model to add in a subclass that performs more interesting error modeling.

Add subclass
============

The trivial base class ErrorModel does not do anything interesting, but it provides a useful base class interface (Corrupt () and Reset ()), forwarded to virtual functions that can be subclassed.  Let's next consider what we call a BasicErrorModel which is based on the ns-2 ErrorModel class (in ns-2/queue/errmodel.{cc,h}).

What properties do we want this to have, from a user interface perspective?  We would like for the user to be able to trivially swap out the type of ErrorModel used in the NetDevice.  We would also like the capability to set configurable parameters.

Here are a few simple requirements we will consider:

  * Ability to set the random variable that governs the losses (default is UniformVariable)

  * Ability to set the unit (bit, byte, packet, time) of granularity over which errors are applied.

  * Ability to set the rate of errors (e.g. 10^-3) corresponding to the above unit of granularity.

  * Ability to enable/disable (default is enabled)

How to subclass
---------------

We declare BasicErrorModel to be a subclass of ErrorModel as follows,

          class BasicErrorModel : public ErrorModel
          {
          public:
            static TypeId GetTypeId (void);
            ...
          private:
            // Implement base class pure virtual functions
            virtual bool DoCorrupt (Ptr<Packet> p);
            virtual bool DoReset (void);
            ...
          }

and configure the subclass GetTypeId function by setting a unique TypeId string and setting the Parent to ErrorModel:

TypeId RateErrorModel::GetTypeId (void)
{
  static TypeId tid = TypeId ("ns3::RateErrorModel")
    .SetParent<ErrorModel> ()
    .AddConstructor<RateErrorModel> ()
  ...

Build core functions and unit tests
===================================

assert macros
-------------

Writing unit tests
------------------