Chapter 2 : The ns-3 Network Simulator George F. Riley Thomas R. Henderson
Apr 01, 2015
Chapter 2 : The ns-3 Network Simulator
George F. RileyThomas R. Henderson
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ns-3 Introduction
ns-3 (http://www.nsnam.org) is a free, open source software project building and maintaining a discrete-event network simulator for research and education
Technical goals: Build and maintain a simulation core
aligned with the needs of the research community
Help to improve the technical rigor of network simulation practice
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ns-3 themes
Research and education focus Build and maintain simulation core,
integrate models developed by other researchers
Support research-driven workflows Open source development model
Research community maintains the models Leverage available tools and models
Write programs to work together Enforce core coding/testing standards
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ns-3 software overview
ns-3 is written in C++, with bindings available for Python simulation programs are C++ executables or
Python programs Python is often a glue language, in practice
ns-3 is a GNU GPLv2-licensed project
ns-3 is not backwards-compatible with ns-2
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ns simulators: a brief history
1988: REAL (Keshav)
1997-2000: DARPA VINT
1990s: ns-1
1996: ns-2
2001-04: DARPA SAMAN, NSF CONSER
2006: ns-3 funded (NSF, INRIA)
June 2008: ns-3.1 release
ns-3 core development (2006-08)ns-3 features anew simulationcore, with modelsdrawn from GTNetS,ns-2, and the "yans" network simulator
1990 2000 2010
quarterlyreleasessince ns-3.1
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Acknowledgment of support
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2.1 Introduction
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Why ns-3?
Despite huge success with ns-2, the ns-3 developers had project goals that motivated the development of a new tool
1) Improve the realism of the models2) Software reuse3) Improved debugging4) Consider long-term software maintenance
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1) attention to realism
Problem: Research often involves a mix of simulations and testbed or live experiments
If the simulator cannot be made to closely model a real system: hard to compare results or validate the model hard to reuse software between the two
domains
ns-3 solution: model nodes more like a real computer support key interfaces such as sockets API and
IP/device driver interface (in Linux) reuse of kernel and application code
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1) attention to realism (example)
An ns-3 Node is a husk of a computer to which applications, stacks, and NICs are added
ApplicationApplicationApplication
(operatingsystems)
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2) software integration
Problem: why reimplement models and tools for which open-source implementations abound?
ns-3 solution: ns-3 conforms to standard input/output
formats so that other tools can be reused. e.g., pcap trace output, ns-2 mobility scripts
ns-3 is adding support for running implementation code Network Simulation Cradle (Jansen) integration
has met with success: Linux TCP code ns-3 “direct code execution” environment
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2) software reuse (cont.)
Example: ns-3 trace viewed with Wireshark:
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3) Improved debugging
Avoid split-language object implementations that proved to be hard to debug in ns-2 (OTcl/C++)
ns-3 uses a C++ core with Python API bindings
Layer a "helper" API on top of a powerful but complex low-layer API
Different APIs for different user expertise
Reduce memory errors via use of smart pointers
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4) Software maintenance
Open source projects are challenged to maintain the software over time
Developers contribute models, then graduate Maintainers change over time
ns-3 chose to use a more rigorous coding standard, detailed code reviews, and regression testing infrastructure
ns-3 project decided not to devote scarce maintenance resources to develop and maintain ns-2 backward compatibility
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2.2 Modeling the Network Elements in ns-3
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2.2 Modeling the Network Elements in ns-3
Key objects in ns-3: nodes, devices, channels, protocols, packets, headers
ApplicationApplication
Protocolstack
Node
NetDeviceNetDevice
ApplicationApplication
Protocolstack
Node
NetDevice
Channel
Packet
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Random Variables
Currently implemented distributions Uniform: values uniformly distributed in an interval Constant: value is always the same (not really random) Sequential: return a sequential list of predefined values Exponential: exponential distribution (poisson process) Normal (gaussian) Log-normal pareto, weibull, triangular, …
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ns-3 random number generator
Uses the MRG32k3a generator from Pierre L'Ecuyer http://www.iro.umontreal.ca/~lecuyer/
myftp/papers/streams00.pdf Period of PRNG is 3.1x10^57
Partitions a pseudo-random number generator into uncorrelated streams and substreams Each RandomVariable gets its own stream This stream partitioned into substreams
Independent replications are handled by incrementing the run number to obtain uncorrelated substreams
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Tracing and statistics
Tracing is a structured form of simulation output Example (from ns-2):+ 1.84375 0 2 cbr 210 ------- 0 0.0 3.1 225 610- 1.84375 0 2 cbr 210 ------- 0 0.0 3.1 225 610r 1.84471 2 1 cbr 210 ------- 1 3.0 1.0 195 600r 1.84566 2 0 ack 40 ------- 2 3.2 0.1 82 602+ 1.84566 0 2 tcp 1000 ------- 2 0.1 3.2 102 611
Problem: Tracing needs vary widely would like to change tracing output without editing the core would like to support multiple outputs
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ns-3 has a new tracing model
ns-3 solution: decouple trace sources from trace sinks
Benefit: Customizable trace sinks
Trace source
Trace source
Trace source
Trace sink
unchangingconfigurable byuser
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ns-3 tracing
various trace sources (e.g., packet receptions, state machine transitions) are plumbed through the system
Documented in a central place
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Basic tracing
Helper classes hide the tracing details from the user, for simple trace types
ascii or pcap traces of devices
std::ofstream ascii; ascii.open ("wns3-helper.tr"); CsmaHelper::EnableAsciiAll (ascii); CsmaHelper::EnablePcapAll ("wns3-helper"); YansWifiPhyHelper::EnablePcapAll ("wsn3-helper");
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Multiple levels of tracing
Highest-level: Use built-in trace sources and sinks and hook a trace file to them
Mid-level: Customize trace source/sink behavior using the tracing namespace
Low-level: Add trace sources to the tracing namespace
Or expose trace source explicitly
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Highest-level of tracing
Use built-in trace sources/sinks, and hook a trace file to them
// Also configure some tcpdump traces; each interface will be traced // The output files will be named // simple-point-to-point.pcap-<nodeId>-<interfaceId> // and can be read by the "tcpdump -r" command (use "-tt" option to // display timestamps correctly) PcapTrace pcaptrace ("simple-point-to-point.pcap"); pcaptrace.TraceAllIp ();
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Mid-level of tracing
Mid-level:Customize trace source/sink behaviour with tracing
namespace
voidPcapTrace::TraceAllIp (void){ NodeList::Connect ("/nodes/*/ipv4/(tx|rx)", MakeCallback (&PcapTrace::LogIp, this));}
Regular expression editing
Hook in a different trace sink
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ns-3 attribute system
Problem: Researchers want to identify all of the values affecting the results of their simulations and configure them easily
ns-3 solution: Each ns-3 object has a set of attributes: A name, help text A type An initial value
Control all simulation parameters for static objects
Dump and read them all in configuration files Visualize them in a GUI Makes it easy to verify the parameters of a
simulation
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Use cases for attributes
An Attribute represents a value in our system An Attribute can be connected to an underlying
variable or function e.g. TcpSocket::m_cwnd; or a trace source
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Use cases for attributes (cont.)
What would users like to do? Know what are all the attributes that affect the simulation at
run time Set a default initial value for a variable Set or get the current value of a variable Initialize the value of a variable when a constructor is called
The attribute system is a unified way of handling these functions
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How to handle attributes
The traditional C++ way: export attributes as part of a class's public API walk pointer chains (and iterators, when needed) to find what
you need use static variables for defaults
The attribute system provides a more convenient API to the user to do these things
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Navigating the attributes
Attributes are exported into a string-based namespace, with filesystem-like paths
namespace supports regular expressions This allows individual instances of attributes to be manipulated
Attributes also can be referenced by name without specifying a path through the object namespace
e.g. “ns3::WifiPhy::TxGain” This allows users to manipulate a global (default) value of the
attribute
A Config class allows users to manipulate the attributes
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Attribute namespace
strings are used to describe paths through the namespace
Config::Set ("/NodeList/1/$ns3::Ns3NscStack<linux2.6.26>/net.ipv4.tcp_sack", StringValue ("0"));
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Navigating the attributes using paths
Examples: Nodes with NodeIds 1, 3, 4, 5, 8, 9, 10, 11:
“/NodeList/[3-5]|[8-11]|1” UdpL4Protocol object instance aggregated to matching nodes:
“/$ns3::UdpL4Protocol”
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What users will do
e.g.: Set a default initial value for a variableConfig::Set (“ns3::WifiPhy::TxGain”, DoubleValue (1.0));
Syntax also supports string values:Config::Set (“WifiPhy::TxGain”, StringValue (“1.0”));
Attribute Value
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Fine-grained attribute handling
Set or get the current value of a variable Here, one needs the path in the namespace to the right
instance of the object
Config::SetAttribute(“/NodeList/5/DeviceList/3/Phy/TxGain”, DoubleValue(1.0));
DoubleValue d; nodePtr->GetAttribute ( “/NodeList/5/NetDevice/3/Phy/TxGain”, v);
Users can get pointers to instances also, and pointers to trace sources, in the same way
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ns-3 attribute system
Object attributes are organized and documented in the Doxygen
Enables the construction of graphical configuration tools:
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Attribute documentation
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Options to manipulate attributes
Individual object attributes often derive from default values Setting the default value will affect all subsequently created
objects Ability to configure attributes on a per-object basis
Set the default value of an attribute from the command-line:CommandLine cmd;cmd.Parse (argc, argv);
Set the default value of an attribute with NS_ATTRIBUTE_DEFAULT
Set the default value of an attribute in C++:Config::SetDefault ("ns3::Ipv4L3Protocol::CalcChecksum",BooleanValue (true));
Set an attribute directly on a specic object:Ptr<CsmaChannel> csmaChannel = ...;csmaChannel->SetAttribute ("DataRate",StringValue ("5Mbps"));
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2.3 Simulating a Computer Network in ns-3
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Four basic steps to perform
1. Create the network topology
2. Create the data demand on the network
3. Execute the simulation
4. Analyze the results
Program listing 2-1 annotates the file "first.cc", whichis the first tutorial program discussed in the ns-3 tutorial
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2.4: Smart Pointers in ns-3
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Smart Pointers
Memory management in a C++ program is a complex process, and is often done incorrectly or inconsistently.
We have settled on a reference counting design using so-called "smart-pointers"
All objects maintain an internal reference count that allows the object to safely delete itself when the count goes to zero
We provide a class called "Ptr" that is similar to the intrusive_ptr of the Boost C++ library
This implementation allows you to manipulate the smart pointer as if it was a normal pointer: you can compare it with zero, compare it against other pointers, assign zero to it, etc.
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2.5: Representing Packets in ns-3
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2.5: Representing Packets in ns-3
The design of the Packet framework of ns-3 was heavily guided by a few important use-cases:
avoid changing the core of the simulator to introduce new types of packet headers or trailers
maximize the ease of integration with real-world code and systems
make it easy to support fragmentation, defragmentation, and, concatenation which are important, especially in wireless systems.
make memory management of this object efficient allow actual application data or dummy application bytes
for emulated applications
Each network packet contains a byte buffer, a set of byte tags, a set of packet tags, and metadata.
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Packet class organization
class Packet providesthe user API
each Packet has a reference to a Bufferholding packed packetdata
optional PacketMetadataprovides metadataabout the contents ofthe buffer to enable(e.g.) pretty-printing
Packet Tags containsimulation-specificinformation such ascross-layer messages, or simulation flow IDs
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Copy-on-write semantics
Packets implement copy-on-write semantics to reduce the number of deep data buffer copies during a simulation
Packets are reference-counted objects; multiple clients can hold references to the same packet, and the packet is automatically freed when all references are deleted
When more than one reference to a packet buffer exists, the packet buffer can be shared unless a so-called "dirty"operation (such as editing a protocol header) is performed by one of the packet users
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2.6: Object Aggregation in ns-3
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ns-3 object aggregation
Problem: coupling between models hinders software reuse in different configurations must intrusively edit the base class for this or, leads to C++ downcasting, e.g.:
// Channels use Node pointers, but here I really want a // MobileNode pointer, to access the MobileNode API
doubleWirelessChannel::get_pdelay(Node* tnode, Node* rnode){ // Scheduler &s = Scheduler::instance(); MobileNode* tmnode = (MobileNode*)tnode; MobileNode* rmnode = (MobileNode*)rnode;
known as the C++ “weak base class” problem
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ns-3 object aggregation (cont.)
ns-3 solution: an object aggregation model objects can be aggregated to other objects
at run-time a “query interface” is provided to ask
whether an particular object is aggregated similar in spirit to COM or Bonobo objects
// aggregate an optional mobility object to a node:node->AggregateObject (mobility);...
// later, other users of node can query for the optional object: senderMobility = i->first->GetNode ()->GetObject<MobilityModel> ();
// we did not have to edit class Node (base class), or downcast!
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2.7: Events in ns-3
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Events in discrete-event simulation
Simulation time moves discretely from event to event
C++ functions schedule events to occur at specific simulation times
A simulation scheduler orders the event execution
Simulation::Run() gets it all started Simulation stops at specific time or when
events end
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Events in ns-3
An event in ns-3 is simply a function pointer with some state (the technical term is a "functor")
Any object method or static function can be used as an event handler in ns-3
e.g. Simulator::Schedule (function name, object pointer, arguments);
This is different from several other discrete-event simulators, where special Handler() methods provide a centralized place for processing events on an object
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2.7: Events in ns-3
static voidExampleFunction (MyModel *model){ std::cout << "ExampleFunction received event at " << Simulator::Now ().GetSeconds () << "s" << std::endl; model->Start ();}
int main (int argc, char *argv[]){ MyModel model; Simulator::Schedule (Seconds (10.0), &ExampleFunction, &model); Simulator::Run ();
Simulator::Destroy ();} An event: At time 10 seconds,
call ExampleFunction() with the argument &model.
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2.8 Compiling and Running the Simulation
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ns-3 uses the waf build system
Waf is a Python-based framework for configuring, compiling and installing applications.
It is a replacement for other tools such as Autotools, Scons, CMake or Ant
http://code.google.com/p/waf/
For those familiar with autotools:
Autotools -> Waf equivalentconfigure -> ./waf -d [optimized|debug] configuremake -> ./waf
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Running programs
Programs are built as build/<variant>/path/program-name
programs link shared library libns3.so
Using ./waf --shell./waf --shell./build/debug/samples/main-simulator
Using ./waf --run./waf --run examples/csma-bridge./waf --pyrun examples/csma-bridge.py
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2.9: Animating the Simulation
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Animation Overview
ns-3 not directly funding visualizers and configurators no “official” tool; expect multiple to be developed Not integrated (directly) with ns-3
Ns-3 creates “animation” file, visualizers use this as input and create the animation.
netanim, pyviz, and nam for ns-3
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Pyviz: A Python-based animator
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NetVis
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NetVis / XML Interface
<anim lp = "0”><topology minX = "1000" minY = "2382.3" maxX = "10900" maxY = "8617.7"><node lp = "0" id = "0" locX = "4000" locY = "5500” image = "Satellite.png" imageScale = "10.0"/><node lp = "0" id = "1" locX = "7000" locY = "5500” image ="Satellite.png" imageScale = "5.0"/><link fromLp = "0" fromId = "0" toLp = "0" toId = "1"/></topology>
<packet fromLp = "0" fromId = "11" fbTx = "0.66483" lbTx = "0.665264"><rx toLp = "0" toId = "1" fbRx = "0.66583" lbRx = "0.666264"/></packet>
<wpacket fromLp = "0" fromId = "49" fbTx = "0.549245" lbTx = "0.549497” range = "250"><rx toLp = "0" toId = "16" fbRx = "0.549497" lbRx = "0.549749"/><rx toLp = "0" toId = "18" fbRx = "0.549497" lbRx = "0.549749"/><rx toLp = "0" toId = "29" fbRx = "0.549497" lbRx = "0.549749"/><rx toLp = "0" toId = "32" fbRx = "0.549498" lbRx = "0.549749"/><rx toLp = "0" toId = "36" fbRx = "0.549498" lbRx = "0.549749"/><rx toLp = "0" toId = "37" fbRx = "0.549497" lbRx = "0.549749"/></wpacket></anim>
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2.10: Scalability with Distributed Simulation
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Overview
Parallel and distributed discrete event simulation Allows single simulation program to run on multiple interconnected
processors Reduced execution time! Larger topologies!
Terminology Logical process (LP) Rank or system id
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Quick and Easy Example
Figure 1. Simple point-to-point topology
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Quick and Easy Example
Figure 2. Simple point-to-point topology, distributed
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Implementation Details
LP communication Message Passing Interface (MPI) standard Send/Receive time-stamped messages MpiInterface in ns-3
Synchronization Conservative algorithm using lookahead DistributedSimulator in ns-3
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Implementation Details (cont.)
Assigning rank Currently handled manually in simulation script Next step, MpiHelper for easier node/rank mapping
Remote point-to-point links Created automatically between nodes with different ranks through point-
to-point helper Packet sent across using MpiInterface
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Distributing the topology All nodes created on all LPs, regardless of rank Applications are only installed on LPs with target node
Implementation Details (cont.)
Figure 3. Mixed topology, distributed
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Performance Test
DARPA NMS campus network simulation Allows creation of very large topologies Any number of campus networks are created and connected together Different campus networks can be placed on different LPs Tested with 2 CNs, 4 CNs, and 6 CNs
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Campus Network Topology
Figure 4. Single campus network
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2 Campus Networks
Figure 5. Execution time with 2 campus networks Figure 6. Speedup with 2 LPs
0
500
1000
1500
2000
2500
3000
3500
4000
4500
116 164 260 452 836
Tim
e (s
)
Number of nodes
1 LP
2 LPs
1.5
1.75
2
2.25
2.5
116 164 260 452 836
Spee
dup
Number of nodes
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Summary
Distributed simulation in ns-3 allows a user to run a single simulation in parallel on multiple processors
By assigning a different rank to nodes and connecting these nodes with point-to-point links, simulator boundaries are created
Simulator boundaries divide LPs, and each LP can be executed by a different processor
Distributed simulation in ns-3 offers solid performance gains in time of execution for large topologies
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2.11: Emulation Capabilities
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Emulation support
Support moving between simulation and testbeds or live systems
A real-time scheduler, and support for two modes of emulationGlobalValue::Bind (“SimulatorImplementationType”, StringValue (“ns3::RealTimeSimulatorImpl”));
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ns-3 emulation modes
virtualmachin
e ns-3
virtualmachin
e
1) ns-3 interconnects real or virtual machines
realmachin
e
ns-3
Testbed
Testbed
realmachin
e
ns-3
2) testbeds interconnect ns-3 stacks
real machine
Various hybrids of the above are possible
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Example: ns-3 with Linux Network Namespaces
Container
ns-3
Linux (FC 12 or Ubuntu 9.10) machine
tapX/dev/tunX
TapBridge
WiFi
ghost node Wifi
Wifi
ghost node
tapY/dev/tunY
Container
Makes use of a "ghost node" that bridges packets to aLinux namespace container running the protocol stackand applications
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Example: ORBIT and ns-3
• Support for use of Rutgers WINLAB ORBIT radio grid
Image from ORBIT - Wireless Network Testbed website
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Issues in Performing "Co-Simulation"
Ease of use Configuration management and coherence Information coordination (two sets of state)
e.g. IP/MAC address coordination Output data exists in two domains Debugging
Error-free operation (avoidance of misuse) Synchronization, information sharing, exception
handling Checkpoints for execution bring-up Inoperative commands within an execution domain Deal with run-time errors
Soft performance degradation (CPU) and time discontinuities
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2.12: Analyzing the Results
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Frameworks for ns-3
NSF CISE Community Research Infrastructure University of Washington (Tom Henderson),
Georgia Tech (George Riley), Bucknell Univ. (Felipe Perrone)
Project timeline: 2010-14
Figure courtesy of Dr. Felipe Perrone, Bucknell University
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Automation
Inspired by SWAN-Tools and ANSWER frameworks.
User interfaces, description languages, and tools for automation of experiments.
Model composition, structural validation, control of experiments, data processing and storage, and archiving experimental setup.
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Topology generation
Integrate BRITE topology generator (Boston Univ.) into framework.
BRITE is downloaded into distribution and compiled by the ns-3 build system.
The ns-3 simulation script uses a topology helper which reads a BRITE configuration file, receives results from BRITE, and builds the ns-3 topology.
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Higher-level modeling and experiment description
Provide a model and a description of experiment.
Framework generates design of experiment space, distribute simulation runs to machines, collect results and archive in persistent storage.
User mines storage to find, extract, and visualize results.Figure courtesy of Dr. Felipe Perrone, Bucknell University
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General architecture
Figure courtesy of Dr. Felipe Perrone, Bucknell University