8. ns-3

This chapter describes running CORE with the ns-3 network simulator.

8.1. What is ns-3?

ns-3 is a discrete-event network simulator for Internet systems, targeted primarily for research and educational use. [1] By default, ns-3 simulates entire networks, from applications down to channels, and it does so in simulated time, instead of real (wall-clock) time.

CORE can run in conjunction with ns-3 to simulate some types of networks. CORE network namespace virtual nodes can have virtual TAP interfaces installed using the simulator for communication. The simulator needs to run at wall clock time with the real-time scheduler. In this type of configuration, the CORE namespaces are used to provide packets to the ns-3 devices and channels. This allows, for example, wireless models developed for ns-3 to be used in an emulation context.

Users simulate networks with ns-3 by writing C++ programs or Python scripts that import the ns-3 library. Simulation models are objects instantiated in these scripts. Combining the CORE Python modules with ns-3 Python bindings allow a script to easily set up and manage an emulation + simulation environment.

Footnotes

[1]http://www.nsnam.org

8.2. ns-3 Scripting

Currently, ns-3 is supported by writing Python scripts, but not through drag-and-drop actions within the GUI. If you have a copy of the CORE source, look under core/daemon/ns3/examples/ for example scripts; a CORE installation package puts these under /usr/share/core/examples/corens3.

To run these scripts, install CORE so the CORE Python libraries are accessible, and download and build ns-3. This has been tested using ns-3 releases starting with 3.11 (and through 3.16 as of this writing).

The first step is to open an ns-3 waf shell. waf is the build system for ns-3. Opening a waf shell as root will merely set some environment variables useful for finding python modules and ns-3 executables. The following environment variables are extended or set by issuing waf shell:

PATH
PYTHONPATH
LD_LIBRARY_PATH
NS3_MODULE_PATH
NS3_EXECUTABLE_PATH

Open a waf shell as root, so that network namespaces may be instantiated by the script with root permissions. For an example, run the ns3wifi.py program, which simply instantiates 10 nodes (by default) and places them on an ns-3 WiFi channel. That is, the script will instantiate 10 namespace nodes, and create a special tap device that sends packets between the namespace node and a special ns-3 simulation node, where the tap device is bridged to an ns-3 WiFi network device, and attached to an ns-3 WiFi channel.

> cd ns-allinone-3.16/ns-3.16
> sudo ./waf shell
# # use '/usr/local' below if installed from source
# cd /usr/share/core/examples/corens3/
# python -i ns3wifi.py
running ns-3 simulation for 600 seconds

>>> print session
<corens3.obj.Ns3Session object at 0x1963e50>
>>>

The interactive Python shell allows some interaction with the Python objects for the emulation.

In another terminal, nodes can be accessed using vcmd:

vcmd -c /tmp/pycore.10781/n1 -- bash
root@n1:/tmp/pycore.10781/n1.conf#
root@n1:/tmp/pycore.10781/n1.conf# ping 10.0.0.3
PING 10.0.0.3 (10.0.0.3) 56(84) bytes of data.
64 bytes from 10.0.0.3: icmp_req=1 ttl=64 time=7.99 ms
64 bytes from 10.0.0.3: icmp_req=2 ttl=64 time=3.73 ms
64 bytes from 10.0.0.3: icmp_req=3 ttl=64 time=3.60 ms
^C
--- 10.0.0.3 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2002ms
rtt min/avg/max/mdev = 3.603/5.111/7.993/2.038 ms
root@n1:/tmp/pycore.10781/n1.conf#

The ping packets shown above are traversing an ns-3 ad-hoc Wifi simulated network.

To clean up the session, use the Session.shutdown() method from the Python terminal.

>>> print session
<corens3.obj.Ns3Session object at 0x1963e50>
>>>
>>> session.shutdown()
>>>

A CORE/ns-3 Python script will instantiate an Ns3Session, which is a CORE Session having CoreNs3Nodes, an ns-3 MobilityHelper, and a fixed duration. The CoreNs3Node inherits from both the CoreNode and the ns-3 Node classes – it is a network namespace having an associated simulator object. The CORE TunTap interface is used, represented by a ns-3 TapBridge in CONFIGURE_LOCAL mode, where ns-3 creates and configures the tap device. An event is scheduled to install the taps at time 0.

Note

The GUI can be used to run the ns3wifi.py and ns3wifirandomwalk.py scripts directly. First, core-daemon must be stopped and run within the waf root shell. Then the GUI may be run as a normal user, and the Execute Python Script... option may be used from the File menu. Dragging nodes around in the ns3wifi.py example will cause their ns-3 positions to be updated.

Users may find the files ns3wimax.py and ns3lte.py in that example directory; those files were similarly configured, but the underlying ns-3 support is not present as of ns-3.16, so they will not work. Specifically, the ns-3 has to be extended to support bridging the Tap device to an LTE and a WiMax device.

8.3. Integration details

The previous example ns3wifi.py used Python API from the special Python objects Ns3Session and Ns3WifiNet. The example program does not import anything directly from the ns-3 python modules; rather, only the above two objects are used, and the API available to configure the underlying ns-3 objects is constrained. For example, Ns3WifiNet instantiates a constant-rate 802.11a-based ad hoc network, using a lot of ns-3 defaults.

However, programs may be written with a blend of ns-3 API and CORE Python API calls. This section examines some of the fundamental objects in the CORE ns-3 support. Source code can be found in daemon/ns3/corens3/obj.py and example code in daemon/ns3/corens3/examples/.

8.3.1. Ns3Session

The Ns3Session class is a CORE Session that starts an ns-3 simulation thread. ns-3 actually runs as a separate process on the same host as the CORE daemon, and the control of starting and stopping this process is performed by the Ns3Session class.

Example:

session = Ns3Session(persistent=True, duration=opt.duration)

Note the use of the duration attribute to control how long the ns-3 simulation should run. By default, the duration is 600 seconds.

Typically, the session keeps track of the ns-3 nodes (holding a node container for references to the nodes). This is accomplished via the addnode() method, e.g.:

for i in xrange(1, opt.numnodes + 1):
  node = session.addnode(name = "n%d" % i)

addnode() creates instances of a CoreNs3Node, which we’ll cover next.

8.3.2. CoreNs3Node

A CoreNs3Node is both a CoreNode and an ns-3 node:

class CoreNs3Node(CoreNode, ns.network.Node):
    ''' The CoreNs3Node is both a CoreNode backed by a network namespace and
    an ns-3 Node simulator object. When linked to simulated networks, the TunTap
    device will be used.

8.3.3. CoreNs3Net

A CoreNs3Net derives from PyCoreNet. This network exists entirely in simulation, using the TunTap device to interact between the emulated and the simulated realm. Ns3WifiNet is a specialization of this.

As an example, this type of code would be typically used to add a WiFi network to a session:

wifi = session.addobj(cls=Ns3WifiNet, name="wlan1", rate="OfdmRate12Mbps")
wifi.setposition(30, 30, 0)

The above two lines will create a wlan1 object and set its initial canvas position. Later in the code, the newnetif method of the CoreNs3Node can be used to add interfaces on particular nodes to this network; e.g.:

for i in xrange(1, opt.numnodes + 1):
    node = session.addnode(name = "n%d" % i)
    node.newnetif(wifi, ["%s/%s" % (prefix.addr(i), prefix.prefixlen)])

8.4. Mobility

Mobility in ns-3 is handled by an object (a MobilityModel) aggregated to an ns-3 node. The MobilityModel is able to report the position of the object in the ns-3 space. This is a slightly different model from, for instance, EMANE, where location is associated with an interface, and the CORE GUI, where mobility is configured by right-clicking on a WiFi cloud.

The CORE GUI supports the ability to render the underlying ns-3 mobility model, if one is configured, on the CORE canvas. For example, the example program ns3wifirandomwalk.py uses five nodes (by default) in a random walk mobility model. This can be executed by starting the core daemon from an ns-3 waf shell:

# sudo bash
# cd /path/to/ns-3
# ./waf shell
# core-daemon

and in a separate window, starting the CORE GUI (not from a waf shell) and selecting the Execute Python script... option from the File menu, selecting the ns3wifirandomwalk.py script.

The program invokes ns-3 mobility through the following statement:

session.setuprandomwalkmobility(bounds=(1000.0, 750.0, 0))

This can be replaced by a different mode of mobility, in which nodes are placed according to a constant mobility model, and a special API call to the CoreNs3Net object is made to use the CORE canvas positions.

-    session.setuprandomwalkmobility(bounds=(1000.0, 750.0, 0))
+    session.setupconstantmobility()
+    wifi.usecorepositions()

In this mode, the user dragging around the nodes on the canvas will cause CORE to update the position of the underlying ns-3 nodes.

8.5. Under Development

Support for ns-3 is fairly new and still under active development. Improved support may be found in the development snapshots available on the web.

The following limitations will be addressed in future releases:

  • GUI configuration and control - currently ns-3 networks can only be instantiated from a Python script or from the GUI hooks facility.
  • Model support - currently the WiFi model is supported. The WiMAX and 3GPP LTE models have been experimented with, but are not currently working with the TapBridge device.

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