Network simulations with

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Keefer Rourke

2020.11.24 - Guelph Coding Community

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What we’re going to cover

• The OSI layered network model
• Types of networks to consider
• WANETs and MANETS
• Specifying a simulation
• ns-3 programming with C++
• ns-3 models and classes
• Understanding simulation results
• Your first simulation
• Simulating MANETs
• Pro-tips
• Limitations and aggravations

Computer Networks 101

The OSI network model

• Layers build on top of each other, are sometimes fuzzy

Types of networks

• Networks can be categorized in lots of ways
• Fixed vs fluid infrastructure
• Wired vs wireless
• Physical vs virtual or overlay
• Cellular networks
• LAN vs WAN
• NAT (network address translation) or other
• Topology differences
• Routing strategies
• etc.

Types of routing

Wireless ad hoc networks

Mobile ad hoc networks (MANETs)

• Ad hoc networks refer to networks with:
• Little to no infrastructure
• Nodes that are directly connected to one another without intermediary routers
• Distributed control
• May or may not require prior setup

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• Refer to ad hoc networks with mobile nodes
• Dynamic/changing network topologies
• Self-governance and management of nodes and data

Physical properties of MANETs

• Homogeneous or heterogeneous hardware configurations
• Radio strength, battery capacity, bandwidth availability all real concerns
• Mobile nodes can complicate data transfer
• Dropped connections
• Changing routes
• Data isolation/replication as networks form partitions
• Software solutions needed to make smart decisions given variance of hardware capabilities and node configurations

Simulation programming for beginners

Specifying a simulation

• Simulation area (dimensions and topography)
• Mobility of nodes (static, deterministic, random walk, random waypoint, etc.)
• If something more complicated, can you compose the movement model from other models?
• Network hardware model
• Connection speed
• Radio channels
• Signal propagation loss over area/time
• Formation of network topologies
• How are MAC addresses assigned?
• How are IP addresses assigned?
• RNG seed

ns-3 programming

Some things to keep in mind:

• ns-3 is a “next-generation” simulation under development since 2006
• The architecture is inspired by the Component Object Model (COM) pattern from .NET frameworks
• (We’ll revisit this)
• You’re working with C++
• Beware what’s on the stack and what’s on the heap
• Beware of copy and move semantics

Aside: Modern C++ programming

• Prior to C++11, working with the language sucked
• It didn’t have smart pointers
• It didn’t have range-based for loops
• It didn’t have lambda expressions
• Before C++17, std:: container types didn’t even have uniform interfaces
• In 2020, the standard library is pretty good, and you might even enjoy working with C++ if the codebase is sufficiently new!
• But what about older codebases?

Aside: Modern C++ programming

• Sufficiently old projects developed their own offerings for modern C++ std:: classes
• Firefox (1998), ns-3 (2006), and even QT (2011) each have their own custom implementations for strings, ref-counting pointers, and container classes
• Prepare to learn the “standard library” for these projects if you want to work with them

Getting started with ns-3

Step 1: Get it.

• ns-3 is distributed in a few ways:
• via bake.py
• via git repository
• via tarball download

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Getting started with ns-3

Step 1: Get it.

bake.py:

• Is for “advanced” users
• Can download, build, and install ns-3 and related software and addons
• Is really confusing
• We won’t use it

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Getting started with ns-3

Step 1: Get it.

By git:

• Provides a “basic” installation of ns-3
• Is all we need
• Is not a good way to get it* unless you are planning on hacking on ns-3 itself

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*ns-3 encourages you to write your scripts directly in their source distributions… More on this later.

Getting started with ns-3

Step 1: Get it.

By tarball:

• Just the good stuff

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$ wget https://www.nsnam.org/release/ns-allinone-3.32.tar.bz2

$ tar xjvf ns-allinone-3.32.tar.bz2

$ cd ns-allinone-3.32

$ python3 ./build.py --enable-examples

Getting started with ns-3

Step 2: Understand some weirdnesses

• If you’re used to writing C/C++, you probably expect:
• to link your driver code to compiled library dependencies
• to keep your source code separated from that of your dependencies

# typical c/c++ project

# structure

my_project/

├── build/

├── src/

├── include/

├── dep-1/

├── dep-2/

└── CMakeLists.txt

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# ns-3 project structure

ns-allinone-3.32/

└── ns-3.32/

├── scratch/

│ ├── your-src/

│ └── scratch-simulator.cc

├── …

├── wscript

└── waf

• ns-3 devs want you to forget about how to structure C++ projects
• Your projects now live in their project
• All your code belongs under the scratch/ dir

It is misleading and unhelpful to call ns-3 simulations “scripts”.

… but we’re going to do that anyway.

Getting started with ns-3

Step 2: Understand some weirdnesses

• waf is a build tool <https://waf.io/book/>
• ns-3 devs have extended waf beyond recognition to let you treat your C++ projects as “scripts”
• Always make sure that the top level waf binary is accessible to you (consider adding to your PATH)
• waf will only recognize *.cc and *.h files by default...

# typical c/c++ project

# structure

my_project/

├── build/

├── src/

├── include/

├── dep-1/

├── dep-2/

└── CMakeLists.txt

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# ns-3 project structure

ns-allinone-3.32/

└── ns-3.32/

├── scratch/

│ ├── your-src/

│ └── scratch-simulator.cc

├── …

├── wscript

└── waf

Getting started with ns-3

Step 3: Build and run your first “script”

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$ pwd

/home/user/project/ns-allinone-3.32/ns-3.32/

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$ ./waf --run examples/tutorial/first

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Waf: Entering directory `/home/user/project/ns-allinone-3.32/ns-3.32/build'

Waf: Leaving directory `/home/user/ns-allinone-3.32/ns-3.32/build'

Build commands will be stored in build/compile_commands.json

'build' finished successfully (1.369s)

At time +2s client sent 1024 bytes to 10.1.1.2 port 9

At time +2.00369s server received 1024 bytes from 10.1.1.1 port 49153

Getting started with ns-3

Step 3: Build and run your first “script”

• ns-3 has some tutorial scripts
• But they are really just a random collection of examples of some example network topologies
• Learn what scripts you can run:

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• Some script names are aliases for others…
• Mostly they correspond to source files in subdirs of ns-3.32/

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$ ./waf --run does-not-exist 2>&1 | tr ',' '\n'

So you want to write a simulation?

Aside: COM Programming in ns-3

The Component Object Model is:

• An runtime object oriented model/framework
• Suited to multiple language-bindings and IPC
• Types must be identifiable at runtime, so are each assigned a GUID
• All objects are ref-counted and implement a generic interface that supports this (typically, IUnknown)
• Classes implement one or more interfaces, and are cast by the ->QueryInterface<I>() method
• Enforces separation of interface and implementation*

* except that ns-3 COM doesn’t...

Aside: COM Programming in ns-3

Key ideas:

• Classes are meant to have a prototypical TypeId
• Default class attributes are set through the TypeId
• Attrs are ro/rw, have a string name, value, and help text
• Factories create instances from the TypeId
• Refcounting pointers and factory methods:
• Ptr<Node> node = CreateObject<Node>()
• It’s supposed to be easy to reach into the guts of ns-3 to tweak values

https://www.nsnam.org/docs/manual/html/attributes.html

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Aside: COM Programming in ns-3

Suppose you want to change the packet queue size of particular node in a network to see if there would be a performance impact...

// Set up code. Let’s make a node with a Wifi device and packet queue.

using namespace ns3;

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

Ptr<WifiNetDevice> net0 = CreateObject<WifiNetDevice>();

n->AddDevice (net0);

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

net0->AddQueue(q);

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Aside: COM Programming in ns-3

Suppose you want to change the packet queue size of particular node in a network to see if there would be a performance impact...

// Retrieve the queue and set the attribute.

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PointerValue ptr;

net0->GetAttribute("TxQueue", ptr);

Ptr<Queue<Packet>> txQueue = ptr.Get<Queue<Packet>>();

txQueue->SetAttribute("MaxSize",

QueueSizeValue(QueueSize(QueueSizeUnit::PACKETS, 80));

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Aside: COM Programming in ns-3

Suppose you want to change the packet queue size of particular node in a network to see if there would be a performance impact...

// Retrieve the queue and set the attribute.

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PointerValue ptr;

net0->GetAttribute("TxQueue", ptr);

Ptr<Queue<Packet>> txQueue = ptr.Get<Queue<Packet>>();

txQueue->SetAttribute("MaxSize", StringValue("80p"));

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Aside: COM Programming in ns-3

Suppose you want to change the packet queue size of particular node in a network to see if there would be a performance impact…

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This is made possible by the runtime type-system, so you can save yourself some typing.

// Just set the attribute directly...?

Config::Set("/NodeList/0/DeviceList/0/TxQueue/MaxSize", StringValue("80p")

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// Even for all instances?

Config::Set("ns3::QueueBase::MaxSize", StringValue("80p")

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Aside: COM Programming in ns-3

Suppose you want to mess with something on a particular instance of a broader interface...

// Retrieve the queue and perform a safe downcast.

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PointerValue ptr;

net0->GetAttribute("TxQueue", ptr);

Ptr<Queue<Packet>> txQueue = ptr.Get<Queue<Packet>>();

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

NS_ASSERT(dtq != 0);

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Aside: COM Programming in ns-3

• ns-3 architecture is mostly inspired by COM to address some deficiencies with ns-2’s type system
• A lot of things are done with TypeIds and special strings
• Key patterns to be aware of:
• NS_OBJECT_ENSURE_REGISTERED(Class);
• auto x = ns3::CreateObject<T>();
• x->SetAttribute(“AttrName”, ns3::AttributeValue(...));
• Config::Set(“container/indices/attr/attr/attr”, value);
• Some python bindings are incubating, but as of yet aren’t quite useful

ns-3 models and classes

ns-3 has extensive class reference docs here:

• https://www.nsnam.org/doxygen/index.html

Broadly speaking, every part of the OSI Network Model is represented somehow.

Generally there are “helpers” which make it is bit easier to set up networks with a collection of nodes.

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ns-3 models and classes

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Nodes and NodeContainers

• ns3::Node
• Network entity (e.g. router, modem, repeater, etc.)
• Install and configure network devices on this
• Define how these move around if you want
• ns3::NodeContainer
• Holds a std::vector<Ptr<Node>>
• Can be copied safely
• Nodes can belong to multiple containers

ns-3 models and classes

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Helper classes

• Usually designed to work with NodeContainers
• Configure a factory for instantiating objects
• Network devices
• Routing protocols
• Network applications
• Etc.
• Mostly work the same, but there's no Helper interface

Understanding simulation results

Animation and visualization

Two animation interfaces:

• NetAnim:
• Standalone QT program
• Can visualize simulations after they have run
• Add support by constructing an AnimationInterface just before you start your simulation
• GraphViz:
• Simulations have implicit bindings to PyGraphViz
• Run with the --vis option to see visualizations live as your simulation runs
• Buggy, doesn’t show packet transmissions

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Packet captures

• ns-3 can output pcap files
• Compatible with Wireshark or tcpdump
• Some connection helper classes have methods like:
• EnablePcapAll(std::string)
• EnablePcapIpv4All(std::string)
• Disclaimer: I've never got this to work properly yet in any of my simulations

https://www.nsnam.org/docs/release/3.9/tutorial/tutorial_23.html

Your first simulation

Recipe for a simulation

using namespace ns3;

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

// Get options.

CommandLine cmd;

…

cmd.Parse(argc, argv);

// Set up nodes.

NodeContainer nodes;

...

// Add the animation interface.

AnimationInterface animation("anim-trace-file.xml");

// Run the simulation.

Simulator::Stop(Seconds(10));

Simulator::Run();

Simulator::Destroy();

return EX_OK;

}

Demo time.

Simulating MANETs

Simulating MANETs

• Lots to consider
• Gillis Lab is studying:
• Mobility models
• Data replication and storage
• Requirements for device density
• Cluster formation and identification
• Routing protocols
• Simulations help when you can’t realistically get a bunch of devices operating on a mesh in a community

Mobility

Model is concerned with:

• Defining simulation area
• Setting node velocity (m/s)
• Initial allocation of nodes over area
• Defining patterns of movement over time
• Changing patterns of movement over time

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Some available models:

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• ConstantPosition
• RandomWaypoint
• RandomWalk2d
• GaussMarkov (3D)
• Others…

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For the most part we are only interested in RandomWaypoint and RandomWalk2d.

Mobility models

Some interesting examples.

Random waypoint

Jitter�(RandomWalk2D default)

Walk over area�(RandomWalk2D)

Travellers between partitions on 3x3 grid

Mobility models

Underlying implementation supports 3D movement, but the visualization tools do not appear to.

Routing

Various routing algorithms have been designed for use in MANETs and other distributed networks

• Proactive
• Periodically deliver routing tables through the network
• DSDV, OSLR
• Reactive
• Finds routes based on user demand or other factors
• Uses Route Request or Discovery packets
• AODV
• Hybrid

Recipe for ad hoc network simulations

using namespace ns3;

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

…

YansWifiPhyHelper wifiPhy = YansWifiPhyHelper::Default();

wifiPhy.SetPcapDataLinkType(YansWifiPhyHelper::DLT_IEEE802_11_RADIO);

auto wifiChannel = YansWifiChannelHelper::Default();

wifiPhy.SetChannel(wifiChannel.Create());

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WifiMacHelper wifiMac;

wifiMac.SetType("ns3::AdhocWifiMac");

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WifiHelper wifi;

wifi.SetStandard(WIFI_STANDARD_80211b);

…

return EX_OK;

}

Demo time.

Pro tips

• Everything should be configurable
• Try to provide sane defaults
• Recompiling sucks, so make use of the CommandLine
• Always document your seed
• Use helpers and containers where possible
• Mobility is a good place to start, then add networking
• Start small and build up complexity
• Always use the latest ns-3

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Limitations and aggravations

• ns-3 uses an entirely new/different style of programming to get used to
• Some constructors are very side-effect-y
• Generally terrible default values lead to weird behaviours
• There’s a fair bit of global state to be aware of
• No convenient methods to query information about neighbor nodes in a topology
• Generally slow to develop and run
• Big simulations can really hurt your CPU
• Consider having some capable cloud hardware available?

Buyer beware

Wide open-field

• There's a lot of research out there about MANETs
• Parts are studied together and in isolation
• Higher complexity -> more interesting -> more work

Simulation studies vary greatly

• A lot are difficult or impossible to reproduce
• Simulation parameters and RNG seeds not specified
• Source code not always available :(

Questions?

References

1. https://osi-model.com/
2. https://www.nsnam.org/docs/architecture.pdf
3. https://en.wikipedia.org/wiki/Component_Object_Model
4. https://www.nsnam.org/docs/manual