|
A Discrete-Event Network Simulator
|
Manual |
|
|
A Discrete-Event Network Simulator
|
Manual |
In ns-3 simulations, there are two main aspects to configuration:
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.
Before delving into details of the attribute value system, it will help to review some basic properties of class ns3::Object.
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 ns3::Object base class. These objects have some additional properties that we exploit for organizing the system and improving the memory management of our objects:
ns-3 objects that use the attribute system derive from either ns3::Object or ns3::ObjectBase. Most ns-3 objects we will discuss derive from ns3::Object, but a few that are outside the smart pointer memory management framework derive from ns3::ObjectBase.
Let’s review a couple of properties of these objects.
As introduced in the ns-3 tutorial, ns-3 objects are memory managed by a reference counting smart pointer implementation, class ns3::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.
As we discussed above in Memory management and class Ptr, at the lowest-level API, objects of type ns3::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 ns3::Object must be allocated on the heap using CreateObject(). Those deriving from ns3::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()s for you.
ns-3 classes that derive from class ns3::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:
Putting all of these concepts together, let’s look at a specific example: class ns3::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> ()
.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 an 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 declaration above is used in conjunction with our object aggregation mechanisms to allow safe up- and down-casting in inheritance trees during GetObject.
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.
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.
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:
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.
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 DropTailQueue that has a member variable that is an unsigned integer m_maxPackets; this member variable controls the depth of the queue.
If we look at the declaration of DropTailQueue, we see the following:
class DropTailQueue : public Queue {
public:
static TypeId GetTypeId (void);
...
private:
std::queue<Ptr<Packet> > m_packets;
uint32_t m_maxPackets;
};
Let’s consider things that a user may want to do with the value of m_maxPackets:
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 functions are moved into the TypeId class; e.g.:
NS_OBJECT_ENSURE_REGISTERED (DropTailQueue);
TypeId DropTailQueue::GetTypeId (void)
{
static TypeId tid = TypeId ("ns3::DropTailQueue")
.SetParent<Queue> ()
.AddConstructor<DropTailQueue> ()
.AddAttribute ("MaxPackets",
"The maximum number of packets accepted by this DropTailQueue.",
UintegerValue (100),
MakeUintegerAccessor (&DropTailQueue::m_maxPackets),
MakeUintegerChecker<uint32_t> ())
;
return tid;
}
The AddAttribute() method is performing a number of things with this value:
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 “MaxPackets” and TypeId 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 (DropTailQueue) 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?
Let’s look at how a user script might access these values. This is based on the script found at src/point-to-point/examples/main-attribute-value.cc, with some details stripped out.:
//
// This is a basic example of how to use the attribute system to
// set and get a value in the underlying system; namely, an unsigned
// integer of the maximum number of packets in a queue
//
int
main (int argc, char *argv[])
{
// By default, the MaxPackets attribute has a value of 100 packets
// (this default can be observed in the function DropTailQueue::GetTypeId)
//
// Here, we set it to 80 packets. We could use one of two value types:
// a string-based value or a Uinteger value
Config::SetDefault ("ns3::DropTailQueue::MaxPackets", StringValue ("80"));
// The below function call is redundant
Config::SetDefault ("ns3::DropTailQueue::MaxPackets", UintegerValue (80));
// Allow the user to override any of the defaults and the above
// SetDefaults() at run-time, via command-line arguments
CommandLine cmd;
cmd.Parse (argc, argv);
The main thing to notice in the above are the two 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 UintegerValue class, can be used to assign the value to the attribute named by “ns3::DropTailQueue::MaxPackets”.
Now, we will create a few objects using the low-level API; here, our newly created queues will not have a m_maxPackets initialized to 100 packets 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> q = CreateObject<DropTailQueue> ();
net0->AddQueue(q);
At this point, we have created a single node (Node 0) and a single PointToPointNetDevice (NetDevice 0) and added a DropTailQueue to it.
Now, we can manipulate the MaxPackets value of the already instantiated DropTailQueue. Here are various ways to do that.
We 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 tmp;
net0->GetAttribute ("TxQueue", tmp);
Ptr<Object> txQueue = tmp.GetObject ();
Using the GetObject function, we can perform a safe downcast to a DropTailQueue, where MaxPackets is a member:
Ptr<DropTailQueue> dtq = txQueue->GetObject <DropTailQueue> ();
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 and not disparate types. Here, the attribute value is assigned to a UintegerValue, and the Get() method on this value produces the (unwrapped) uint32_t.:
UintegerValue limit;
dtq->GetAttribute ("MaxPackets", limit);
NS_LOG_INFO ("1. dtq limit: " << limit.Get () << " packets");
Note that the above downcast is not really needed; we could have done the same using the Ptr<Queue> even though the attribute is a member of the subclass:
txQueue->GetAttribute ("MaxPackets", limit);
NS_LOG_INFO ("2. txQueue limit: " << limit.Get () << " packets");
Now, let’s set it to another value (60 packets):
txQueue->SetAttribute("MaxPackets", UintegerValue (60));
txQueue->GetAttribute ("MaxPackets", limit);
NS_LOG_INFO ("3. txQueue limit changed: " << limit.Get () << " packets");
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/MaxPackets", UintegerValue (25));
txQueue->GetAttribute ("MaxPackets", limit);
NS_LOG_INFO ("4. txQueue limit changed through namespace: " <<
limit.Get () << " packets");
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 Set()):
Config::Set ("/NodeList/*/DeviceList/*/TxQueue/MaxPackets", UintegerValue (15));
txQueue->GetAttribute ("MaxPackets", limit);
NS_LOG_INFO ("5. txQueue limit changed through wildcarded namespace: " <<
limit.Get () << " packets");
Another way to get at the attribute is to use the object name service facility. Here, this attribute is found using a 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 namespaced path.
Names::Add ("server", serverNode);
Names::Add ("server/eth0", serverDevice);
...
Config::Set ("/Names/server/eth0/TxQueue/MaxPackets", UintegerValue (25));
See Object names for a fuller treatment of the ns-3 configuration namespace.
Arbitrary combinations of attributes can be set and fetched from the helper and low-level APIs; either from the constructors themselves:
Ptr<Object> p = CreateObject<MyNewObject> ("n1", v1, "n2", v2, ...);
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"));
Readers will note the new FooValue classes which are subclasses of the AttributeValue base class. These can be thought of as an intermediate class that can be used to convert from raw types to the Values that are used by the attribute system. Recall that this database is holding objects of many types with a single generic type. Conversions to this type can either be done using an intermediate class (IntegerValue, DoubleValue for “floating point”) or via strings. Direct implicit conversion of types to Value 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:
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 ns3::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 ns3::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 virtual TypeId GetInstanceTypeId (void) const; method. Otherwise the ObjectBase::ConstructSelf () will not be able to read the 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 this.
Consider this variable in class 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 TypeId declaration 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 the TcpSocket 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.
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 hook it into this system.
We’ve already introduced what a TypeId definition looks like:
TypeId
RandomWalk2dMobilityModel::GetTypeId (void)
{
static TypeId tid = TypeId ("ns3::RandomWalk2dMobilityModel")
.SetParent<MobilityModel> ()
.SetGroupName ("Mobility")
.AddConstructor<RandomWalk2dMobilityModel> ()
.AddAttribute ("Bounds",
"Bounds of the area to cruise.",
RectangleValue (Rectangle (0.0, 0.0, 100.0, 100.0)),
MakeRectangleAccessor (&RandomWalk2dMobilityModel::m_bounds),
MakeRectangleChecker ())
.AddAttribute ("Time",
"Change current direction and speed after moving for this delay.",
TimeValue (Seconds (1.0)),
MakeTimeAccessor (&RandomWalk2dMobilityModel::m_modeTime),
MakeTimeChecker ())
// etc (more parameters).
;
return tid;
}
The declaration for this in the class declaration is one-line public member method:
public:
static TypeId GetTypeId (void);
Typical mistakes here involve:
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.
From the perspective of the user who writes a new class in the system and wants to hook it in to the attribute system, 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 class declaration for Rectangle in the src/mobility/model directory:
/**
* \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);
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, and the modeler must specify these operators and the string syntactical representation of an instance of the new class.
The ConfigStore is a specialized database for attribute values and default values. Although it is a separately maintained module in src/config-store/ directory, we document it here because of its sole dependency on ns-3 core module and attributes.
Values for ns-3 attributes can be stored in an ASCII or XML text file and loaded into a future simulation. This feature is known as the ns-3 ConfigStore. We can explore this system by using an example from src/config-store/examples/config-store-save.cc.
First, all users must include the following statement:
#include "ns3/config-store-module.h"
Next, this program adds a sample object A to show how the system is extended:
class A : 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 (A);
Next, we use the Config subsystem to override the defaults in a couple of ways:
Config::SetDefault ("ns3::A::TestInt16", IntegerValue (-5));
Ptr<A> a_obj = CreateObject<A> ();
NS_ABORT_MSG_UNLESS (a_obj->m_int16 == -5, "Cannot set A's integer attribute via Config::SetDefault");
Ptr<A> a2_obj = CreateObject<A> ();
a2_obj->SetAttribute ("TestInt16", IntegerValue (-3));
IntegerValue iv;
a2_obj->GetAttribute ("TestInt16", iv);
NS_ABORT_MSG_UNLESS (iv.Get () == -3, "Cannot set A'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 ns3::Node or ns3::Channel but here, since we are working at the core level, we need to create a new root namespace object:
Config::RegisterRootNamespaceObject (a2_obj);
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 (); it will allow the configuration to be saved 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 store its output data. The FileFormat (default “RawText”) governs whether the ConfigStore format is Xml or RawText format.
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::RealtimeSimulatorImpl::SynchronizationMode "BestEffort"
default ns3::RealtimeSimulatorImpl::HardLimit "+100000000.0ns"
default ns3::PcapFileWrapper::CaptureSize "65535"
default ns3::PacketSocket::RcvBufSize "131072"
default ns3::ErrorModel::IsEnabled "true"
default ns3::RateErrorModel::ErrorUnit "EU_BYTE"
default ns3::RateErrorModel::ErrorRate "0"
default ns3::RateErrorModel::RanVar "Uniform:0:1"
default ns3::DropTailQueue::Mode "Packets"
default ns3::DropTailQueue::MaxPackets "100"
default ns3::DropTailQueue::MaxBytes "6553500"
default ns3::Application::StartTime "+0.0ns"
default ns3::Application::StopTime "+0.0ns"
default ns3::ConfigStore::Mode "Save"
default ns3::ConfigStore::Filename "output-attributes.txt"
default ns3::ConfigStore::FileFormat "RawText"
default ns3::A::TestInt16 "-5"
global RngSeed "1"
global RngRun "1"
global SimulatorImplementationType "ns3::DefaultSimulatorImpl"
global SchedulerType "ns3::MapScheduler"
global ChecksumEnabled "false"
value /$ns3::A/TestInt16 "-3"
In the above, all of the default values for attributes for the core module are shown. Then, all the values for the ns-3 global values are recorded. Finally, the value of the instance of A that was rooted in the configuration namespace is shown. In a real ns-3 program, many more models, attributes, and defaults would be shown.
An XML version also exists in output-attributes.xml:
<?xml version="1.0" encoding="UTF-8"?>
<ns3>
<default name="ns3::RealtimeSimulatorImpl::SynchronizationMode" value="BestEffort"/>
<default name="ns3::RealtimeSimulatorImpl::HardLimit" value="+100000000.0ns"/>
<default name="ns3::PcapFileWrapper::CaptureSize" value="65535"/>
<default name="ns3::PacketSocket::RcvBufSize" value="131072"/>
<default name="ns3::ErrorModel::IsEnabled" value="true"/>
<default name="ns3::RateErrorModel::ErrorUnit" value="EU_BYTE"/>
<default name="ns3::RateErrorModel::ErrorRate" value="0"/>
<default name="ns3::RateErrorModel::RanVar" value="Uniform:0:1"/>
<default name="ns3::DropTailQueue::Mode" value="Packets"/>
<default name="ns3::DropTailQueue::MaxPackets" value="100"/>
<default name="ns3::DropTailQueue::MaxBytes" value="6553500"/>
<default name="ns3::Application::StartTime" value="+0.0ns"/>
<default name="ns3::Application::StopTime" value="+0.0ns"/>
<default name="ns3::ConfigStore::Mode" value="Save"/>
<default name="ns3::ConfigStore::Filename" value="output-attributes.xml"/>
<default name="ns3::ConfigStore::FileFormat" value="Xml"/>
<default name="ns3::A::TestInt16" value="-5"/>
<global name="RngSeed" value="1"/>
<global name="RngRun" value="1"/>
<global name="SimulatorImplementationType" value="ns3::DefaultSimulatorImpl"/>
<global name="SchedulerType" value="ns3::MapScheduler"/>
<global name="ChecksumEnabled" value="false"/>
<value path="/$ns3::A/TestInt16" value="-3"/>
</ns3>
This file can be archived with your simulation script and output data.
Next, we discuss using this to configure simulations via an input configuration file. There are a couple of key differences when compared to use for logging 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 config store 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) method 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.
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, via command-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 ();
}
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.
There are a couple of possible improvements: