@node ns-3 Attributes
@chapter ns-3 Attributes 
@anchor{chap:Attributes}

In ns-3 simulations, there are two main aspects to configuration:
@itemize @bullet
@item the simulation topology and how objects are connected 
@item the values used by the models instantiated in the topology
@end itemize

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 @code{class ns3::Object}.

@node Object Overview
@section 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 @code{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:

@itemize @bullet
@item a "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, and the set of 
"attributes" of the subclass
@item a reference counting smart pointer implementation, for memory
management.
@end itemize

ns-3 objects that use the attribute system derive from either
@code{ns3::Object} or @code{ns3::ObjectBase}.  Most ns-3 objects
we will discuss derive from @code{ns3::Object}, but a few that
are outside the smart pointer memory management framework derive
from @code{ns3::ObjectBase}.

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

@node Smart pointers
@subsection Smart pointers

As introduced above in @ref{Smart Pointers 101}, ns-3 objects 
are memory managed by a 
@uref{http://en.wikipedia.org/wiki/Smart_pointer,,reference counting smart pointer implementation}, @code{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:
@verbatim
  Ptr<WifiNetDevice> nd = ...;
  nd->CallSomeFunction ();
  // etc.
@end verbatim

@node CreateObject
@subsection CreateObject

As we discussed above in @ref{Object Creation}, 
at the lowest-level API, objects of type @code{ns3::Object} are 
not instantiated using @code{operator new} as usual but instead by
a templated function called @code{CreateObject()}.

A typical way to create such an object is as follows:
@verbatim
  Ptr<WifiNetDevice> nd = CreateObject<WifiNetDevice> ();
@end verbatim

You can think of this as being functionally equivalent to:
@verbatim
  WifiNetDevice* nd = new WifiNetDevice ();
@end verbatim

Objects that derive from @code{ns3::Object} must be allocated
on the heap using CreateObject().  Those deriving from 
@code{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.

@node TypeId
@subsection TypeId

ns-3 classes that derive from class ns3::Object can include
a metadata class called @code{TypeId} that records meta-information
about the class, for use in the object aggregation and component
manager systems:
@itemize @bullet
 @item a unique string identifying the class
 @item the base class of the subclass, within the metadata system
 @item the set of accessible constructors in the subclass
@end itemize

@node Object Summary
@subsection Object Summary

Putting all of these concepts together, let's look at a specific
example:  @code{class ns3::Node}.

The public header file node.h has a declaration that includes
a static GetTypeId function call:
@verbatim
class Node : public Object
{
public:
  static TypeId GetTypeId (void);
  ...
@end verbatim

This is defined in the node.cc file as follows:
@verbatim
TypeId 
Node::GetTypeId (void)
{
  static TypeId tid = TypeId ("ns3::Node")
    .SetParent<Object> ()
  return tid;
}
@end verbatim
Finally, when users want to create Nodes, they call:
@verbatim
  Ptr<Node> n = CreateObject<Node> n;
@end verbatim

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

@node Attribute Overview
@section Attribute Overview

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:
@itemize @bullet
@item "I want to trace the packets on the wireless interface only on
the first access point"
@item "I want to trace the value of the TCP congestion window (every
time it changes) on a particular TCP socket"
@item "I want a dump of all values that were used in my simulation."
@end itemize 

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.

@node Functional overview
@subsection Functional overview

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 @code{m_maxPackets};
this member variable controls the depth of the queue.  

If we look at the declaration of DropTailQueue, we see the following:
@verbatim
class DropTailQueue : public Queue {
public:
  static TypeId GetTypeId (void);
  ...

private:
  std::queue<Ptr<Packet> > m_packets;
  uint32_t m_maxPackets;
};
@end verbatim

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

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

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.:  
@verbatim
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.",
                   Uinteger (100),
                   MakeUintegerAccessor (&DropTailQueue::m_maxPackets),
                   MakeUintegerChecker<uint32_t> ())
    ;
  
  return tid;
}
@end verbatim

The AddAttribute() method is performing a number of things with this
value:
@itemize @bullet
@item Binding the variable m_maxPackets to a string "MaxPackets"
@item Providing a default value (100 packets)
@item Providing some help text defining the value
@item Providing a "checker" (not used in this example) that can be used to set
bounds on the allowable range of values
@end itemize

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.

@node Basic usage
@subsection Basic usage

Let's look at how a user script might access these values.  
This is based on the script found at @code{samples/main-attribute-value.cc},
with some details stripped out.
@verbatim
//
// 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", String ("80"));
  // The below function call is redundant
  Config::SetDefault ("ns3::DropTailQueue::MaxPackets", Uinteger(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);
@end verbatim

The main thing to notice in the above are the two calls to 
@code{Config::SetDefault}.  This is how we set the default value
for all subsequently instantiated DropTailQueues.  We illustrate
that two types of Value classes, a String and a Uinteger 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.
@verbatim
  Ptr<Node> n0 = CreateObject<Node> ();

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

  Ptr<Queue> q = CreateObject<DropTailQueue> ();
  net0->AddQueue(q);
@end verbatim

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.

@subsubsection Pointer-based access

We assume that a smart pointer (Ptr) to a relevant network device is 
in hand; here, 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 
@verbatim
  Ptr<Queue> txQueue = net0->GetAttribute ("TxQueue");
@end verbatim

Using the GetObject function, we can perform a safe downcast
to a DropTailQueue, where MaxPackets is a member
@verbatim
  Ptr<DropTailQueue> dtq = txQueue->GetObject <DropTailQueue> ();
  NS_ASSERT (dtq);
@end verbatim

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 Uinteger, and
the Get() method on this value produces the (unwrapped) uint32_t.
@verbatim
  Uinteger limit = dtq->GetAttribute ("MaxPackets");
  NS_LOG_INFO ("1.  dtq limit: " << limit.Get () << " packets");
@end verbatim
  
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
@verbatim
  limit = txQueue->GetAttribute ("MaxPackets");
  NS_LOG_INFO ("2.  txQueue limit: " << limit.Get () << " packets");
@end verbatim

Now, let's set it to another value (60 packets)
@verbatim
  txQueue->SetAttribute("MaxPackets", Uinteger (60));
  limit = txQueue->GetAttribute ("MaxPackets");
  NS_LOG_INFO ("3.  txQueue limit changed: " << limit.Get () << " packets");
@end verbatim

@subsubsection Namespace-based access

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.  
@verbatim
  Config::Set ("/NodeList/0/DeviceList/0/TxQueue/MaxPackets", Uinteger (25));
  limit = txQueue->GetAttribute ("MaxPackets"); 
  NS_LOG_INFO ("4.  txQueue limit changed through namespace: " << 
    limit.Get () << " packets");
@end verbatim

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())
@verbatim
  Config::Set ("/NodeList/*/DeviceList/*/TxQueue/MaxPackets", Uinteger (15));
  limit = txQueue->GetAttribute ("MaxPackets"); 
  NS_LOG_INFO ("5.  txQueue limit changed through wildcarded namespace: " << 
    limit.Get () << " packets");
@end verbatim

@node Setting through constructors and helper classes
@subsection Setting through constructors helper classes

Arbitrary combinations of attributes can be set and fetched from
the helper and low-level APIs; either from the constructors themselves:
@verbatim
Ptr<Object> p = CreateObject<MyNewObject> ("n1", v1, "n2", v2, ...);
@end verbatim
or from the higher-level helper APIs, such as:
@verbatim
  mobility.SetPositionAllocator ("GridPositionAllocator",
                                 "MinX", FpValue (-100.0),
                                 "MinY", FpValue (-100.0),
                                 "DeltaX", FpValue (5.0),
                                 "DeltaY", FpValue (20.0),
                                 "GridWidth", UintValue (20),
                                 "LayoutType", "RowFirst");
@end verbatim

@node Value classes
@subsection Value classes
Readers will note the new Value classes.  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 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 (IntValue, FpValue 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: 
@verbatim
p->Set ("cwnd", "100"); // string-based setter
p->Set ("cwnd", IntValue(100)); // value-based setter
@end verbatim

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

@node Extending attributes
@section Extending 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.

@subsection Adding an existing internal variable to the metadata system 

// XXX revise me

Consider this variable in class TcpSocket:
@verbatim
 uint32_t m_cWnd;   // Congestion window
@end verbatim

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 metadata system (to the TypeId declaration for TcpSocket):
@verbatim
    .AddParameter ("Congestion window", 
                   "Tcp congestion window (bytes)",
                   MakeUIntParamSpec (&TcpSocket::m_cWnd, 1));

@end verbatim

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.

@subsection Adding a new 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 hook it into this system.

We've already introduced what a TypeId definition looks like:
@verbatim
TypeId
RandomWalk2dMobilityModel::GetTypeId (void)
{  
  static TypeId tid = TypeId ("RandomWalkMobilityModel")    
    .SetParent<MobilityModel> ()    
    .SetGroupName ("Mobility")    
    .AddConstructor<RandomWalk2dMobilityModel> ()
    // followed by a number of Parameters
    .AddParameter ("bounds",                   
                   "Bounds of the area to cruise.",                   
                    MakeRectangleParamSpec (&RandomWalk2dMobilityModel::m_bounds,                                           Rectangle (0.0, 0.0, 100.0, 100.0)))    
    .AddParameter ("time",                   
                   "Change current direction and speed after moving for this delay.",              
                   MakeTimeParamSpec (&RandomWalk2dMobilityModel::m_modeTime,
                                      Seconds (1.0)))

    // etc (more parameters).
@end verbatim

The declaration for this in the class declaration is one-line public
member method:
@verbatim
 public:
  static TypeId GetTypeId (void);
@end verbatim

@section Adding new class type to the Value system

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 Values.  Most of this can be
copy/pasted with macro-ized code.  For instance, consider class
Rectangle in the @code{src/mobility/} directory:

One line is added to the class declaration:
@verbatim
/**
 * \brief a 2d rectangle
 */
class Rectangle
{
...

  VALUE_HELPER_HEADER_1 (Rectangle);
};
@end verbatim
 
One templatized declaration, and two operators, are added below the 
class declaration:

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

VALUE_HELPER_HEADER_2 (Rectangle);
@end verbatim

In the class definition, the code looks like this:

@verbatim
VALUE_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;
}
@end verbatim

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.