A Discrete-Event Network Simulator
Manual

Configuration and Attributes

In ns-3 simulations, there are two main aspects to configuration:

  • The simulation topology and how objects are connected.
  • The values used by the models instantiated in the topology.

This chapter focuses on the second item above: how the many values in use in ns-3 are organized, documented, and modifiable by ns-3 users. The ns-3 attribute system is also the underpinning of how traces and statistics are gathered in the simulator.

In the course of this chapter we will discuss the various ways to set or modify the values used by ns-3 model objects. In increasing order of specificity, these are:

Method Scope
Default Attribute values set when Attributes are defined in GetTypeId (). Affect all instances of the class.
CommandLine
Config::SetDefault()
ConfigStore
Affect all future instances.
ObjectFactory Affects all instances created with the factory.
Helper methods with (string/ AttributeValue) parameter pairs Affects all instances created by the helper.
MyClass::SetX ()
Object::SetAttribute ()
Config::Set()
Alters this particular instance. Generally this is the only form which can be scheduled to alter an instance once the simulation is running.

By “specificity” we mean that methods in later rows in the table override the values set by, and typically affect fewer instances than, earlier methods.

Before delving into details of the attribute value system, it will help to review some basic properties of class Object.

Object Overview

ns-3 is fundamentally a C++ object-based system. By this we mean that new C++ classes (types) can be declared, defined, and subclassed as usual.

Many ns-3 objects inherit from the Object base class. These objects have some additional properties that we exploit for organizing the system and improving the memory management of our objects:

  • “Metadata” system that links the class name to a lot of meta-information about the object, including:
    • The base class of the subclass,
    • The set of accessible constructors in the subclass,
    • The set of “attributes” of the subclass,
    • Whether each attribute can be set, or is read-only,
    • The allowed range of values for each attribute.
  • Reference counting smart pointer implementation, for memory management.

ns-3 objects that use the attribute system derive from either Object or ObjectBase. Most ns-3 objects we will discuss derive from Object, but a few that are outside the smart pointer memory management framework derive from ObjectBase.

Let’s review a couple of properties of these objects.

Smart Pointers

As introduced in the ns-3 tutorial, ns-3 objects are memory managed by a reference counting smart pointer implementation, class Ptr.

Smart pointers are used extensively in the ns-3 APIs, to avoid passing references to heap-allocated objects that may cause memory leaks. For most basic usage (syntax), treat a smart pointer like a regular pointer:

Ptr<WifiNetDevice> nd = ...;
nd->CallSomeFunction ();
// etc.

So how do you get a smart pointer to an object, as in the first line of this example?

CreateObject

As we discussed above in Memory management and class Ptr, at the lowest-level API, objects of type Object are not instantiated using operator new as usual but instead by a templated function called CreateObject ().

A typical way to create such an object is as follows:

Ptr<WifiNetDevice> nd = CreateObject<WifiNetDevice> ();

You can think of this as being functionally equivalent to:

WifiNetDevice* nd = new WifiNetDevice ();

Objects that derive from Object must be allocated on the heap using CreateObject (). Those deriving from ObjectBase, such as ns-3 helper functions and packet headers and trailers, can be allocated on the stack.

In some scripts, you may not see a lot of CreateObject () calls in the code; this is because there are some helper objects in effect that are doing the CreateObject () calls for you.

TypeId

ns-3 classes that derive from class Object can include a metadata class called TypeId that records meta-information about the class, for use in the object aggregation and component manager systems:

  • A unique string identifying the class.
  • The base class of the subclass, within the metadata system.
  • The set of accessible constructors in the subclass.
  • A list of publicly accessible properties (“attributes”) of the class.

Object Summary

Putting all of these concepts together, let’s look at a specific example: class Node.

The public header file node.h has a declaration that includes a static GetTypeId () function call:

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

This is defined in the node.cc file as follows:

TypeId
Node::GetTypeId (void)
{
  static TypeId tid = TypeId ("ns3::Node")
    .SetParent<Object> ()
    .SetGroupName ("Network")
    .AddConstructor<Node> ()
    .AddAttribute ("DeviceList",
                   "The list of devices associated to this Node.",
                   ObjectVectorValue (),
                   MakeObjectVectorAccessor (&Node::m_devices),
                   MakeObjectVectorChecker<NetDevice> ())
    .AddAttribute ("ApplicationList",
                   "The list of applications associated to this Node.",
                   ObjectVectorValue (),
                   MakeObjectVectorAccessor (&Node::m_applications),
                   MakeObjectVectorChecker<Application> ())
    .AddAttribute ("Id",
                   "The id (unique integer) of this Node.",
                   TypeId::ATTR_GET, // allow only getting it.
                   UintegerValue (0),
                   MakeUintegerAccessor (&Node::m_id),
                   MakeUintegerChecker<uint32_t> ())
    ;
  return tid;
}

Consider the TypeId of the ns-3 Object class as an extended form of run time type information (RTTI). The C++ language includes a simple kind of RTTI in order to support dynamic_cast and typeid operators.

The SetParent<Object> () call in the definition above is used in conjunction with our object aggregation mechanisms to allow safe up- and down-casting in inheritance trees during GetObject (). It also enables subclasses to inherit the Attributes of their parent class.

The AddConstructor<Node> () call is used in conjunction with our abstract object factory mechanisms to allow us to construct C++ objects without forcing a user to know the concrete class of the object she is building.

The three calls to AddAttribute () associate a given string with a strongly typed value in the class. Notice that you must provide a help string which may be displayed, for example, via command line processors. Each Attribute is associated with mechanisms for accessing the underlying member variable in the object (for example, MakeUintegerAccessor () tells the generic Attribute code how to get to the node ID above). There are also “Checker” methods which are used to validate values against range limitations, such as maximum and minimum allowed values.

When users want to create Nodes, they will usually call some form of CreateObject (),:

Ptr<Node> n = CreateObject<Node> ();

or more abstractly, using an object factory, you can create a Node object without even knowing the concrete C++ type:

ObjectFactory factory;
const std::string typeId = "ns3::Node'';
factory.SetTypeId (typeId);
Ptr<Object> node = factory.Create <Object> ();

Both of these methods result in fully initialized attributes being available in the resulting Object instances.

We next discuss how attributes (values associated with member variables or functions of the class) are plumbed into the above TypeId.

Attributes

The goal of the attribute system is to organize the access of internal member objects of a simulation. This goal arises because, typically in simulation, users will cut and paste/modify existing simulation scripts, or will use higher-level simulation constructs, but often will be interested in studying or tracing particular internal variables. For instance, use cases such as:

  • “I want to trace the packets on the wireless interface only on the first access point.”
  • “I want to trace the value of the TCP congestion window (every time it changes) on a particular TCP socket.”
  • “I want a dump of all values that were used in my simulation.”

Similarly, users may want fine-grained access to internal variables in the simulation, or may want to broadly change the initial value used for a particular parameter in all subsequently created objects. Finally, users may wish to know what variables are settable and retrievable in a simulation configuration. This is not just for direct simulation interaction on the command line; consider also a (future) graphical user interface that would like to be able to provide a feature whereby a user might right-click on an node on the canvas and see a hierarchical, organized list of parameters that are settable on the node and its constituent member objects, and help text and default values for each parameter.

Defining Attributes

We provide a way for users to access values deep in the system, without having to plumb accessors (pointers) through the system and walk pointer chains to get to them. Consider a class QueueBase that has a member variable m_maxSize controlling the depth of the queue.

If we look at the declaration of QueueBase, we see the following:

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

private:
  ...
  QueueSize m_maxSize;                //!< max queue size
  ...
};

QueueSize is a special type in ns-3 that allows size to be represented in different units:

enum QueueSizeUnit
{
  PACKETS,     /**< Use number of packets for queue size */
  BYTES,       /**< Use number of bytes for queue size */
};

class QueueSize
{
  ...
private:
  ...
  QueueSizeUnit m_unit; //!< unit
  uint32_t m_value;     //!< queue size [bytes or packets]
};

Finally, the class DropTailQueue inherits from this base class and provides the semantics that packets that are submitted to a full queue will be dropped from the back of the queue (“drop tail”).

/**
 * \ingroup queue
 *
 * \brief A FIFO packet queue that drops tail-end packets on overflow
 */
template <typename Item>
class DropTailQueue : public Queue<Item>

Let’s consider things that a user may want to do with the value of m_maxSize:

  • Set a default value for the system, such that whenever a new DropTailQueue is created, this member is initialized to that default.
  • Set or get the value on an already instantiated queue.

The above things typically require providing Set () and Get () functions, and some type of global default value.

In the ns-3 attribute system, these value definitions and accessor function registrations are moved into the TypeId class; e.g.:

NS_OBJECT_ENSURE_REGISTERED (QueueBase);

TypeId
QueueBase::GetTypeId (void)
{
  static TypeId tid = TypeId ("ns3::DropTailQueue")
    .SetParent<Queue> ()
    .SetGroupName ("Network")
    ...
    .AddAttribute ("MaxSize",
                   "The max queue size",
                   QueueSizeValue (QueueSize ("100p")),
                   MakeQueueSizeAccessor (&QueueBase::SetMaxSize,
                                          &QueueBase::GetMaxSize),
                   MakeQueueSizeChecker ())
    ...
    ;

  return tid;
}

The AddAttribute () method is performing a number of things for the m_maxSize value:

  • Binding the (usually private) member variable m_maxSize to a public string "MaxSize".
  • Providing a default value (0 packets).
  • Providing some help text defining the meaning of the value.
  • Providing a “Checker” (not used in this example) that can be used to set bounds on the allowable range of values.

The key point is that now the value of this variable and its default value are accessible in the attribute namespace, which is based on strings such as "MaxSize" and TypeId name strings. In the next section, we will provide an example script that shows how users may manipulate these values.

Note that initialization of the attribute relies on the macro NS_OBJECT_ENSURE_REGISTERED (QueueBase) being called; if you leave this out of your new class implementation, your attributes will not be initialized correctly.

While we have described how to create attributes, we still haven’t described how to access and manage these values. For instance, there is no globals.h header file where these are stored; attributes are stored with their classes. Questions that naturally arise are how do users easily learn about all of the attributes of their models, and how does a user access these attributes, or document their values as part of the record of their simulation?

Detailed documentation of the actual attributes defined for a type, and a global list of all defined attributes, are available in the API documentation. For the rest of this document we are going to demonstrate the various ways of getting and setting attribute values.

Setting Default Values

Config::SetDefault and CommandLine

Let’s look at how a user script might access a specific attribute value. We’re going to use the src/point-to-point/examples/main-attribute-value.cc script for illustration, with some details stripped out. The main function begins:

// This is a basic example of how to use the attribute system to
// set and get a value in the underlying system; namely, the maximum
// size of the FIFO queue in the PointToPointNetDevice
//

int
main (int argc, char *argv[])
{

  // Queues in ns-3 are objects that hold items (other objects) in
  // a queue structure.  The C++ implementation uses templates to
  // allow queues to hold various types of items, but the most
  // common is a pointer to a packet (Ptr<Packet>).
  //
  // The maximum queue size can either be enforced in bytes ('b') or
  // packets ('p').  A special type called the ns3::QueueSize can
  // hold queue size values in either unit (bytes or packets).  The
  // queue base class ns3::QueueBase has a MaxSize attribute that can
  // be set to a QueueSize.

  // By default, the MaxSize attribute has a value of 100 packets ('100p')
  // (this default can be observed in the function QueueBase::GetTypeId)
  //
  // Here, we set it to 80 packets.  We could use one of two value types:
  // a string-based value or a QueueSizeValue value
  Config::SetDefault ("ns3::QueueBase::MaxSize", StringValue ("80p"));
  // The below function call is redundant
  Config::SetDefault ("ns3::QueueBase::MaxSize", QueueSizeValue (QueueSize (QueueSizeUnit::PACKETS, 80)));

The main thing to notice in the above are the two equivalent calls to Config::SetDefault (). This is how we set the default value for all subsequently instantiated DropTailQueues. We illustrate that two types of Value classes, a StringValue and a QueueSizeValue class, can be used to assign the value to the attribute named by “ns3::QueueBase::MaxSize”.

It is also possible to manipulate Attributes using the CommandLine; we saw some examples early in the ns-3 Tutorial. In particular, it is straightforward to add a shorthand argument name, such as --maxSize, for an Attribute that is particular relevant for your model, in this case "ns3::QueueBase::MaxSize". This has the additional feature that the help string for the Attribute will be printed as part of the usage message for the script. For more information see the CommandLine API documentation.

// Allow the user to override any of the defaults and the above
// SetDefaults() at run-time, via command-line arguments
// For example, via "--ns3::QueueBase::MaxSize=80p"
CommandLine cmd;
// This provides yet another way to set the value from the command line:
cmd.AddValue ("maxSize", "ns3::QueueBase::MaxSize");
cmd.Parse (argc, argv);

Now, we will create a few objects using the low-level API. Our newly created queues will not have m_maxSize initialized to 0 packets, as defined in the QueueBase::GetTypeId () function, but to 80 packets, because of what we did above with default values.:

Ptr<Node> n0 = CreateObject<Node> ();

Ptr<PointToPointNetDevice> net0 = CreateObject<PointToPointNetDevice> ();
n0->AddDevice (net0);

Ptr<Queue<Packet> > q = CreateObject<DropTailQueue<Packet> > ();
net0->AddQueue(q);

At this point, we have created a single Node (n0) and a single PointToPointNetDevice (net0), and added a DropTailQueue (q) to net0.

Constructors, Helpers and ObjectFactory

Arbitrary combinations of attributes can be set and fetched from the helper and low-level APIs; either from the constructors themselves:

Ptr<GridPositionAllocator> p =
  CreateObjectWithAttributes<GridPositionAllocator>
    ("MinX", DoubleValue (-100.0),
     "MinY", DoubleValue (-100.0),
     "DeltaX", DoubleValue (5.0),
     "DeltaY", DoubleValue (20.0),
     "GridWidth", UintegerValue (20),
     "LayoutType", StringValue ("RowFirst"));

or from the higher-level helper APIs, such as:

mobility.SetPositionAllocator
    ("ns3::GridPositionAllocator",
     "MinX", DoubleValue (-100.0),
     "MinY", DoubleValue (-100.0),
     "DeltaX", DoubleValue (5.0),
     "DeltaY", DoubleValue (20.0),
     "GridWidth", UintegerValue (20),
     "LayoutType", StringValue ("RowFirst"));

We don’t illustrate it here, but you can also configure an ObjectFactory with new values for specific attributes. Instances created by the ObjectFactory will have those attributes set during construction. This is very similar to using one of the helper APIs for the class.

To review, there are several ways to set values for attributes for class instances to be created in the future:

  • Config::SetDefault ()
  • CommandLine::AddValue ()
  • CreateObjectWithAttributes<> ()
  • Various helper APIs

But what if you’ve already created an instance, and you want to change the value of the attribute? In this example, how can we manipulate the m_maxSize value of the already instantiated DropTailQueue? Here are various ways to do that.

Changing Values

SmartPointer

Assume that a smart pointer (Ptr) to a relevant network device is in hand; in the current example, it is the net0 pointer.

One way to change the value is to access a pointer to the underlying queue and modify its attribute.

First, we observe that we can get a pointer to the (base class) Queue via the PointToPointNetDevice attributes, where it is called "TxQueue":

PointerValue ptr;
net0->GetAttribute ("TxQueue", ptr);
Ptr<Queue<Packet> > txQueue = ptr.Get<Queue<Packet> > ();

Using the GetObject () function, we can perform a safe downcast to a DropTailQueue. The NS_ASSERT checks that the pointer is valid.

Ptr<DropTailQueue<Packet> > dtq = txQueue->GetObject <DropTailQueue<Packet> > ();
NS_ASSERT (dtq != 0);

Next, we can get the value of an attribute on this queue. We have introduced wrapper Value classes for the underlying data types, similar to Java wrappers around these types, since the attribute system stores values serialized to strings, and not disparate types. Here, the attribute value is assigned to a QueueSizeValue, and the Get () method on this value produces the (unwrapped) QueueSize. That is, the variable limit is written into by the GetAttribute method.:

QueueSizeValue limit;
dtq->GetAttribute ("MaxSize", limit);
NS_LOG_INFO ("1.  dtq limit: " << limit.Get ());

Note that the above downcast is not really needed; we could have gotten the attribute value directly from txQueue:

txQueue->GetAttribute ("MaxSize", limit);
NS_LOG_INFO ("2.  txQueue limit: " << limit.Get ());

Now, let’s set it to another value (60 packets). Let’s also make use of the StringValue shorthand notation to set the size by passing in a string (the string must be a positive integer suffixed by either the p or b character).

txQueue->SetAttribute ("MaxSize", StringValue ("60p"));
txQueue->GetAttribute ("MaxSize", limit);
NS_LOG_INFO ("3.  txQueue limit changed: " << limit.Get ());

Config Namespace Path

An alternative way to get at the attribute is to use the configuration namespace. Here, this attribute resides on a known path in this namespace; this approach is useful if one doesn’t have access to the underlying pointers and would like to configure a specific attribute with a single statement.

Config::Set ("/NodeList/0/DeviceList/0/TxQueue/MaxSize",
             StringValue ("25p"));
txQueue->GetAttribute ("MaxSize", limit);
NS_LOG_INFO ("4.  txQueue limit changed through namespace: "
             << limit.Get ());

The configuration path often has the form of ".../<container name>/<index>/.../<attribute>/<attribute>" to refer to a specific instance by index of an object in the container. In this case the first container is the list of all Nodes; the second container is the list of all NetDevices on the chosen Node. Finally, the configuration path usually ends with a succession of member attributes, in this case the "MaxSize" attribute of the "TxQueue" of the chosen NetDevice.

We could have also used wildcards to set this value for all nodes and all net devices (which in this simple example has the same effect as the previous Config::Set ()):

Config::Set ("/NodeList/*/DeviceList/*/TxQueue/MaxSize",
             StringValue ("15p"));
txQueue->GetAttribute ("MaxSize", limit);
NS_LOG_INFO ("5.  txQueue limit changed through wildcarded namespace: "
             << limit.Get ());

If you run this program from the command line, you should see the following output corresponding to the steps we took above:

$ ./waf --run main-attribute-value
1.  dtq limit: 80p
2.  txQueue limit: 80p
3.  txQueue limit changed: 60p
4.  txQueue limit changed through namespace: 25p
5.  txQueue limit changed through wildcarded namespace: 15p

Object Name Service

Another way to get at the attribute is to use the object name service facility. The object name service allows us to add items to the configuration namespace under the "/Names/" path with a user-defined name string. This approach is useful if one doesn’t have access to the underlying pointers and it is difficult to determine the required concrete configuration namespace path.

Names::Add ("server", n0);
Names::Add ("server/eth0", net0);

...

Config::Set ("/Names/server/eth0/TxQueue/MaxPackets", UintegerValue (25));

Here we’ve added the path elements "server" and "eth0" under the "/Names/" namespace, then used the resulting configuration path to set the attribute.

See Object names for a fuller treatment of the ns-3 configuration namespace.

Implementation Details

Value Classes

Readers will note the TypeValue classes which are subclasses of the AttributeValue base class. These can be thought of as intermediate classes which are used to convert from raw types to the AttributeValues that are used by the attribute system. Recall that this database is holding objects of many types serialized to strings. Conversions to this type can either be done using an intermediate class (such as IntegerValue, or DoubleValue for floating point numbers) or via strings. Direct implicit conversion of types to AttributeValue is not really practical. So in the above, users have a choice of using strings or values:

p->Set ("cwnd", StringValue ("100")); // string-based setter
p->Set ("cwnd", IntegerValue (100)); // integer-based setter

The system provides some macros that help users declare and define new AttributeValue subclasses for new types that they want to introduce into the attribute system:

  • ATTRIBUTE_HELPER_HEADER
  • ATTRIBUTE_HELPER_CPP

See the API documentation for these constructs for more information.

Initialization Order

Attributes in the system must not depend on the state of any other Attribute in this system. This is because an ordering of Attribute initialization is not specified, nor enforced, by the system. A specific example of this can be seen in automated configuration programs such as ConfigStore. Although a given model may arrange it so that Attributes are initialized in a particular order, another automatic configurator may decide independently to change Attributes in, for example, alphabetic order.

Because of this non-specific ordering, no Attribute in the system may have any dependence on any other Attribute. As a corollary, Attribute setters must never fail due to the state of another Attribute. No Attribute setter may change (set) any other Attribute value as a result of changing its value.

This is a very strong restriction and there are cases where Attributes must set consistently to allow correct operation. To this end we do allow for consistency checking when the attribute is used (cf. NS_ASSERT_MSG or NS_ABORT_MSG).

In general, the attribute code to assign values to the underlying class member variables is executed after an object is constructed. But what if you need the values assigned before the constructor body executes, because you need them in the logic of the constructor? There is a way to do this, used for example in the class ConfigStore: call ObjectBase::ConstructSelf () as follows:

ConfigStore::ConfigStore ()
{
  ObjectBase::ConstructSelf (AttributeConstructionList ());
  // continue on with constructor.
}

Beware that the object and all its derived classes must also implement a GetInstanceTypeId () method. Otherwise the ObjectBase::ConstructSelf () will not be able to read the attributes.

Adding Attributes

The ns-3 system will place a number of internal values under the attribute system, but undoubtedly users will want to extend this to pick up ones we have missed, or to add their own classes to the system.

There are three typical use cases:

  • Making an existing class data member accessible as an Attribute, when it isn’t already.
  • Making a new class able to expose some data members as Attributes by giving it a TypeId.
  • Creating an AttributeValue subclass for a new class so that it can be accessed as an Attribute.

Existing Member Variable

Consider this variable in TcpSocket:

uint32_t m_cWnd;   // Congestion window

Suppose that someone working with TCP wanted to get or set the value of that variable using the metadata system. If it were not already provided by ns-3, the user could declare the following addition in the runtime metadata system (to the GetTypeId() definition for TcpSocket):

.AddAttribute ("Congestion window",
               "Tcp congestion window (bytes)",
               UintegerValue (1),
               MakeUintegerAccessor (&TcpSocket::m_cWnd),
               MakeUintegerChecker<uint16_t> ())

Now, the user with a pointer to a TcpSocket instance can perform operations such as setting and getting the value, without having to add these functions explicitly. Furthermore, access controls can be applied, such as allowing the parameter to be read and not written, or bounds checking on the permissible values can be applied.

New Class TypeId

Here, we discuss the impact on a user who wants to add a new class to ns-3. What additional things must be done to enable it to hold attributes?

Let’s assume our new class, called ns3::MyMobility, is a type of mobility model. First, the class should inherit from its parent class, ns3::MobilityModel. In the my-mobility.h header file:

namespace ns3 {

class MyMobility : public MobilityModel
{

This requires we declare the GetTypeId () function. This is a one-line public function declaration:

public:
  /**
   *  Register this type.
   *  \return The object TypeId.
   */
  static TypeId GetTypeId (void);

We’ve already introduced what a TypeId definition will look like in the my-mobility.cc implementation file:

NS_OBJECT_ENSURE_REGISTERED (MyMobility);

TypeId
MyMobility::GetTypeId (void)
{
  static TypeId tid = TypeId ("ns3::MyMobility")
    .SetParent<MobilityModel> ()
    .SetGroupName ("Mobility")
    .AddConstructor<MyMobility> ()
    .AddAttribute ("Bounds",
                   "Bounds of the area to cruise.",
                   RectangleValue (Rectangle (0.0, 0.0, 100.0, 100.0)),
                   MakeRectangleAccessor (&MyMobility::m_bounds),
                   MakeRectangleChecker ())
    .AddAttribute ("Time",
                   "Change current direction and speed after moving for this delay.",
                   TimeValue (Seconds (1.0)),
                   MakeTimeAccessor (&MyMobility::m_modeTime),
                   MakeTimeChecker ())
    // etc (more parameters).
    ;
  return tid;
}

If we don’t want to subclass from an existing class, in the header file we just inherit from ns3::Object, and in the object file we set the parent class to ns3::Object with .SetParent<Object> ().

Typical mistakes here involve:

  • Not calling NS_OBJECT_ENSURE_REGISTERED ()
  • Not calling the SetParent () method, or calling it with the wrong type.
  • Not calling the AddConstructor () method, or calling it with the wrong type.
  • Introducing a typographical error in the name of the TypeId in its constructor.
  • Not using the fully-qualified C++ typename of the enclosing C++ class as the name of the TypeId. Note that "ns3::" is required.

None of these mistakes can be detected by the ns-3 codebase, so users are advised to check carefully multiple times that they got these right.

New AttributeValue Type

From the perspective of the user who writes a new class in the system and wants it to be accessible as an attribute, there is mainly the matter of writing the conversions to/from strings and attribute values. Most of this can be copy/pasted with macro-ized code. For instance, consider a class declaration for Rectangle in the src/mobility/model directory:

Header File
/**
 * \brief a 2d rectangle
 */
class Rectangle
{
  ...

  double xMin;
  double xMax;
  double yMin;
  double yMax;
};

One macro call and two operators, must be added below the class declaration in order to turn a Rectangle into a value usable by the Attribute system:

std::ostream &operator << (std::ostream &os, const Rectangle &rectangle);
std::istream &operator >> (std::istream &is, Rectangle &rectangle);

ATTRIBUTE_HELPER_HEADER (Rectangle);
Implementation File

In the class definition (.cc file), the code looks like this:

ATTRIBUTE_HELPER_CPP (Rectangle);

std::ostream &
operator << (std::ostream &os, const Rectangle &rectangle)
{
  os << rectangle.xMin << "|" << rectangle.xMax << "|" << rectangle.yMin << "|"
     << rectangle.yMax;
  return os;
}
std::istream &
operator >> (std::istream &is, Rectangle &rectangle)
 {
  char c1, c2, c3;
  is >> rectangle.xMin >> c1 >> rectangle.xMax >> c2 >> rectangle.yMin >> c3
     >> rectangle.yMax;
  if (c1 != '|' ||
      c2 != '|' ||
      c3 != '|')
    {
      is.setstate (std::ios_base::failbit);
    }
  return is;
}

These stream operators simply convert from a string representation of the Rectangle ("xMin|xMax|yMin|yMax") to the underlying Rectangle. The modeler must specify these operators and the string syntactical representation of an instance of the new class.

ConfigStore

Values for ns-3 attributes can be stored in an ASCII or XML text file and loaded into a future simulation run. This feature is known as the ns-3 ConfigStore. The ConfigStore is a specialized database for attribute values and default values.

Although it is a separately maintained module in the src/config-store/ directory, we document it here because of its sole dependency on ns-3 core module and attributes.

We can explore this system by using an example from src/config-store/examples/config-store-save.cc.

First, all users of the ConfigStore must include the following statement:

#include "ns3/config-store-module.h"

Next, this program adds a sample object ConfigExample to show how the system is extended:

class ConfigExample : public Object
{
public:
  static TypeId GetTypeId (void) {
    static TypeId tid = TypeId ("ns3::A")
      .SetParent<Object> ()
      .AddAttribute ("TestInt16", "help text",
                     IntegerValue (-2),
                     MakeIntegerAccessor (&A::m_int16),
                     MakeIntegerChecker<int16_t> ())
      ;
      return tid;
    }
  int16_t m_int16;
};

NS_OBJECT_ENSURE_REGISTERED (ConfigExample);

Next, we use the Config subsystem to override the defaults in a couple of ways:

Config::SetDefault ("ns3::ConfigExample::TestInt16", IntegerValue (-5));

Ptr<ConfigExample> a_obj = CreateObject<ConfigExample> ();
NS_ABORT_MSG_UNLESS (a_obj->m_int16 == -5,
                     "Cannot set ConfigExample's integer attribute via Config::SetDefault");

Ptr<ConfigExample> a2_obj = CreateObject<ConfigExample> ();
a2_obj->SetAttribute ("TestInt16", IntegerValue (-3));
IntegerValue iv;
a2_obj->GetAttribute ("TestInt16", iv);
NS_ABORT_MSG_UNLESS (iv.Get () == -3,
                     "Cannot set ConfigExample's integer attribute via SetAttribute");

The next statement is necessary to make sure that (one of) the objects created is rooted in the configuration namespace as an object instance. This normally happens when you aggregate objects to a ns3::Node or ns3::Channel instance, but here, since we are working at the core level, we need to create a new root namespace object:

Config::RegisterRootNamespaceObject (a2_obj);

Writing

Next, we want to output the configuration store. The examples show how to do it in two formats, XML and raw text. In practice, one should perform this step just before calling Simulator::Run () to save the final configuration just before running the simulation.

There are three Attributes that govern the behavior of the ConfigStore: "Mode", "Filename", and "FileFormat". The Mode (default "None") configures whether ns-3 should load configuration from a previously saved file (specify "Mode=Load") or save it to a file (specify "Mode=Save"). The Filename (default "") is where the ConfigStore should read or write its data. The FileFormat (default "RawText") governs whether the ConfigStore format is plain text or Xml ("FileFormat=Xml")

The example shows:

Config::SetDefault ("ns3::ConfigStore::Filename", StringValue ("output-attributes.xml"));
Config::SetDefault ("ns3::ConfigStore::FileFormat", StringValue ("Xml"));
Config::SetDefault ("ns3::ConfigStore::Mode", StringValue ("Save"));
ConfigStore outputConfig;
outputConfig.ConfigureDefaults ();
outputConfig.ConfigureAttributes ();

// Output config store to txt format
Config::SetDefault ("ns3::ConfigStore::Filename", StringValue ("output-attributes.txt"));
Config::SetDefault ("ns3::ConfigStore::FileFormat", StringValue ("RawText"));
Config::SetDefault ("ns3::ConfigStore::Mode", StringValue ("Save"));
ConfigStore outputConfig2;
outputConfig2.ConfigureDefaults ();
outputConfig2.ConfigureAttributes ();

Simulator::Run ();

Simulator::Destroy ();

Note the placement of these statements just prior to the Simulator::Run () statement. This output logs all of the values in place just prior to starting the simulation (i.e. after all of the configuration has taken place).

After running, you can open the output-attributes.txt file and see:

...
default ns3::ErrorModel::IsEnabled "true"
default ns3::RateErrorModel::ErrorUnit "ERROR_UNIT_BYTE"
default ns3::RateErrorModel::ErrorRate "0"
default ns3::RateErrorModel::RanVar "ns3::UniformRandomVariable[Min=0.0|Max=1.0]"
default ns3::BurstErrorModel::ErrorRate "0"
default ns3::BurstErrorModel::BurstStart "ns3::UniformRandomVariable[Min=0.0|Max=1.0]"
default ns3::BurstErrorModel::BurstSize "ns3::UniformRandomVariable[Min=1|Max=4]"
default ns3::PacketSocket::RcvBufSize "131072"
default ns3::PcapFileWrapper::CaptureSize "65535"
default ns3::PcapFileWrapper::NanosecMode "false"
default ns3::SimpleNetDevice::PointToPointMode "false"
default ns3::SimpleNetDevice::TxQueue "ns3::DropTailQueue<Packet>"
default ns3::SimpleNetDevice::DataRate "0bps"
default ns3::PacketSocketClient::MaxPackets "100"
default ns3::PacketSocketClient::Interval "+1000000000.0ns"
default ns3::PacketSocketClient::PacketSize "1024"
default ns3::PacketSocketClient::Priority "0"
default ns3::ConfigStore::Mode "Save"
default ns3::ConfigStore::Filename "output-attributes.txt"
default ns3::ConfigStore::FileFormat "RawText"
default ns3::ConfigExample::TestInt16 "-5"
global SimulatorImplementationType "ns3::DefaultSimulatorImpl"
global SchedulerType "ns3::MapScheduler"
global RngSeed "1"
global RngRun "1"
global ChecksumEnabled "false"
value /$ns3::ConfigExample/TestInt16 "-3"

In the above, several of the default values for attributes for the core and network modules are shown. Then, all the values for the ns-3 global values are recorded. Finally, the value of the instance of ConfigExample that was rooted in the configuration namespace is shown. In a real ns-3 program, many more models, attributes, and defaults would be shown.

An XML version also exists in output-attributes.xml:

<?xml version="1.0" encoding="UTF-8"?>
<ns3>
 <default name="ns3::ErrorModel::IsEnabled" value="true"/>
 <default name="ns3::RateErrorModel::ErrorUnit" value="ERROR_UNIT_BYTE"/>
 <default name="ns3::RateErrorModel::ErrorRate" value="0"/>
 <default name="ns3::RateErrorModel::RanVar" value="ns3::UniformRandomVariable[Min=0.0|Max=1.0]"/>
 <default name="ns3::BurstErrorModel::ErrorRate" value="0"/>
 <default name="ns3::BurstErrorModel::BurstStart" value="ns3::UniformRandomVariable[Min=0.0|Max=1.0]"/>
 <default name="ns3::BurstErrorModel::BurstSize" value="ns3::UniformRandomVariable[Min=1|Max=4]"/>
 <default name="ns3::PacketSocket::RcvBufSize" value="131072"/>
 <default name="ns3::PcapFileWrapper::CaptureSize" value="65535"/>
 <default name="ns3::PcapFileWrapper::NanosecMode" value="false"/>
 <default name="ns3::SimpleNetDevice::PointToPointMode" value="false"/>
 <default name="ns3::SimpleNetDevice::TxQueue" value="ns3::DropTailQueue&lt;Packet&gt;"/>
 <default name="ns3::SimpleNetDevice::DataRate" value="0bps"/>
 <default name="ns3::PacketSocketClient::MaxPackets" value="100"/>
 <default name="ns3::PacketSocketClient::Interval" value="+1000000000.0ns"/>
 <default name="ns3::PacketSocketClient::PacketSize" value="1024"/>
 <default name="ns3::PacketSocketClient::Priority" value="0"/>
 <default name="ns3::ConfigStore::Mode" value="Save"/>
 <default name="ns3::ConfigStore::Filename" value="output-attributes.xml"/>
 <default name="ns3::ConfigStore::FileFormat" value="Xml"/>
 <default name="ns3::ConfigExample::TestInt16" value="-5"/>
 <global name="SimulatorImplementationType" value="ns3::DefaultSimulatorImpl"/>
 <global name="SchedulerType" value="ns3::MapScheduler"/>
 <global name="RngSeed" value="1"/>
 <global name="RngRun" value="1"/>
 <global name="ChecksumEnabled" value="false"/>
 <value path="/$ns3::ConfigExample/TestInt16" value="-3"/>
</ns3>

This file can be archived with your simulation script and output data.

Reading

Next, we discuss configuring simulations via a stored input configuration file. There are a couple of key differences compared to writing the final simulation configuration. First, we need to place statements such as these at the beginning of the program, before simulation configuration statements are written (so the values are registered before being used in object construction).

Config::SetDefault ("ns3::ConfigStore::Filename", StringValue ("input-defaults.xml"));
Config::SetDefault ("ns3::ConfigStore::Mode", StringValue ("Load"));
Config::SetDefault ("ns3::ConfigStore::FileFormat", StringValue ("Xml"));
ConfigStore inputConfig;
inputConfig.ConfigureDefaults ();

Next, note that loading of input configuration data is limited to Attribute default (i.e. not instance) values, and global values. Attribute instance values are not supported because at this stage of the simulation, before any objects are constructed, there are no such object instances around. (Note, future enhancements to the config store may change this behavior).

Second, while the output of ConfigStore state will list everything in the database, the input file need only contain the specific values to be overridden. So, one way to use this class for input file configuration is to generate an initial configuration using the output ("Save") "Mode" described above, extract from that configuration file only the elements one wishes to change, and move these minimal elements to a new configuration file which can then safely be edited and loaded in a subsequent simulation run.

When the ConfigStore object is instantiated, its attributes "Filename", "Mode", and "FileFormat" must be set, either via command-line or via program statements.

Reading/Writing Example

As a more complicated example, let’s assume that we want to read in a configuration of defaults from an input file named input-defaults.xml, and write out the resulting attributes to a separate file called output-attributes.xml.:

#include "ns3/config-store-module.h"
...
int main (...)
{

  Config::SetDefault ("ns3::ConfigStore::Filename", StringValue ("input-defaults.xml"));
  Config::SetDefault ("ns3::ConfigStore::Mode", StringValue ("Load"));
  Config::SetDefault ("ns3::ConfigStore::FileFormat", StringValue ("Xml"));
  ConfigStore inputConfig;
  inputConfig.ConfigureDefaults ();

  //
  // Allow the user to override any of the defaults and the above Bind () at
  // run-time, viacommand-line arguments
  //
  CommandLine cmd;
  cmd.Parse (argc, argv);

  // setup topology
  ...

  // Invoke just before entering Simulator::Run ()
  Config::SetDefault ("ns3::ConfigStore::Filename", StringValue ("output-attributes.xml"));
  Config::SetDefault ("ns3::ConfigStore::Mode", StringValue ("Save"));
  ConfigStore outputConfig;
  outputConfig.ConfigureAttributes ();
  Simulator::Run ();
}

ConfigStore GUI

There is a GTK-based front end for the ConfigStore. This allows users to use a GUI to access and change variables. Screenshots of this feature are available in the |ns3| Overview presentation.

To use this feature, one must install libgtk and libgtk-dev; an example Ubuntu installation command is:

$ sudo apt-get install libgtk2.0-0 libgtk2.0-dev

To check whether it is configured or not, check the output of the step:

$ ./waf configure --enable-examples --enable-tests

---- Summary of optional NS-3 features:
Python Bindings               : enabled
Python API Scanning Support   : enabled
NS-3 Click Integration        : enabled
GtkConfigStore                : not enabled (library 'gtk+-2.0 >= 2.12' not found)

In the above example, it was not enabled, so it cannot be used until a suitable version is installed and:

$ ./waf configure --enable-examples --enable-tests
$ ./waf

is rerun.

Usage is almost the same as the non-GTK-based version, but there are no ConfigStore attributes involved:

// Invoke just before entering Simulator::Run ()
GtkConfigStore config;
config.ConfigureDefaults ();
config.ConfigureAttributes ();

Now, when you run the script, a GUI should pop up, allowing you to open menus of attributes on different nodes/objects, and then launch the simulation execution when you are done.