to Using the Network Simulator NS

1

• Introduction
• to Using the Network Simulator NS
• by Chung Ling Chi, Gigi

2
Outline

• Introduction
• Interface to the Interpreter (The duality of OTcl
  and C)
• Topology Generation, the nodes and the links
• Agents
• Example
• Conclusion

3
Introduction

• NS is a discrete event simulator targeted at
  networking research.
• Network researchers can study the complex
  interactions between network protocols in complex
  topologies and with a rich set of traffic
  generators.
• extend its functionality

4
Otcl and C The Duality
Interface to the Interpreter

• NS is an object oriented simulator, written in
  C, with an OTcl interpreter as a frontend.

5
OTcl Linkage
Interface to the Interpreter

• C ltgt OTcl
• class Tcl C methods to access the interpreter
• class TclObject Root of basic object hierarchy in
  ns
• class TclClass C class to set up the TclObject
• hierarchy
• class TclCommand Basic Script to provide limited
  global
• commands to the interpreter
• class EmbeddedTcl Container for Tcl scripts that
  are
• pre-loaded at startup
• class InstVar Internal class to bind C member
• variables to OTcl instance variables

6
Class Tcl
Interface to the Interpreter

• The class Tcl
• Obtain a handle
• Invoke Otcl procedures
• Retrieve the result from the interpreter
• On invocation, pass a result string to the
  interpreter
• Return Success/Failure return codes

7
Class Tcl C Methods to access OTcl
Interface to the Interpreter
8
Class TclObject
Interface to the Interpreter

• Base class for most classes in the interpreted
  and compiled hierarchies of ns
• Hierarchy mirrored in C and OTcl
• For example

9
Class TclObject Hierarchy and Shadowing
Interface to the Interpreter
10
Class TclObject Creation/Deletion Mechanisms
Interface to the Interpreter

• Global procedure new
• Global procedure delete

11
Class TclObject Binding Variables
Interface to the Interpreter

• Makes identical C member variables to Otcl
  instance variables
• Syntax
• Initialisation through OTcl class variables
• Other methods bind() (integer), bind_time()
  (time variables), bind_bw() (bandwidth
  variables), bind_bool() (boolean variables)

12
Examples of Specify Bound Variables
Interface to the Interpreter
13
Class TclObject command() methods
Interface to the Interpreter

• Shadow object is accessed by a cmd procedure,
  called instproclike
• For example, distance? is an instance
  procedure of an Adaptive SRM agent

14
command() methods call sequence
Interface to the Interpreter

• Usage

15
Class TclClass
Interface to the Interpreter

• Programmer defines C hierarchy that is mirrored
  in Otcl
• not all classes are mirrored exactly

16
Class TclClass Definition
Interface to the Interpreter

• For example, Adaptive SRM agent class in C is
  mirrored into Agent/SRM/Adaptive

17
Class TclClass Mechanism
Interface to the Interpreter

• Static initialisation by compiler
• Run time activation at startup

18
Class InstVar
Interface to the Interpreter

• One object per bound variable
• Created by TclObjectbind() call
• Constructor activity
• 1. Point to C member variable
• 2. Create instance variable for interpreted
  object
• 3. Enable trap read/writes to instance variable
  
• using Tcl_TraceVar()

19
OTcl Linkage Summary
Interface to the Interpreter

• Class Tcl
• primitives to access the interpreter
• Class TclObject root object for mirrored
  hierarchies
• Unifies interpreted and compiled hierarchy
• Provide seamless access to ns objects in compiled
  code and interpreted scripts
• Class TclClass class that sets up the
  interpreted hierarchy
• establish interpreted hierarchy
• shadowing methods

20
Nodes
Topology Generation

• Configuring the node
• -- Control function
• -- Address and port number
• management,
• unicast routing functions
• -- Agent management
• -- Adding neighbors

21
Multicast Node
Topology Generation
22
Classifiers
Topology Generation

• Table of slots
• Each slot can point to a TclObject
• When a packet is received
• classify() identifies the slot to forward the
  packet to
• If slot is invalid, the classifier calls
  no-slot()
• Many different types of classifiers
• Address Classifier parse address in packet
• Multipath Classifier returns next slot number to
  use
• Replicator uses classifier as a table

23
Classifier Methods
Topology Generation

• Install entries into classifier
• install
• installNext
• Query entries in classifier
• elements returns current list of elements
  inserted
• slot returns handle of object in the specified
  slot
• Delete entry in a particular slot
• clear
• classify internal method receives a packet,
  and returns a slot number for that packet

24
Links
Topology Generation
25
Connectors
Topology Generation

• Connectors receive incoming packets, and usually
  transmit them to their target or drop-target_
• Many different types of connectors
• Queue holds a certain number of
  packets. Packets exceeding their queue-size
  are sent to the queues trop-target_.
• DelayLink models delay/bandwidth of the link for
  detailed simulations.
• DynaLink decrements TTLs on each packet, drops
  the packer if the TTL becomes zero.
• TTLChecker transmit packets if the link is up,
  drop packet otherwise

26
Connector methods
Topology Generation

• Add tracing or monitoring
• Library support
• trace
• attach-monitors
• init-monitor
• C Trace class objects are implemented
• Define trace file format
• example of trace file

27
Agents
Agents

• Endpoints where network-layer packets are
  constructed or consumed
• implementation of protocols at various layers
• Agent state

28
Agent Methods
Agents

• Packet generation and reception
• Packet allocpkt() allocate new packet and assign
  its fields
• Packet allocpkt(int) allocate new packet with a
  data payload of n bytes and assign its fields
• Member functions to be over-ridden by classes
• void timeout(timeout number)
• subclass-specific time out method
• void recv(Packet, Handler)
• receiving agent main receive path of packets

29
Protocol Agents
Agents
30
OTcl Linkage
Agents

• Creating and Manipulating Agents
• Default Values
• link bind member variable between OTcl and C
  (in ns/tcl/lib/ns-default)

31
Tcp, TCP Sink Agents
Example

• Agents
• Class TCPAgent a simplified TCP sender to send
  packet
• Class TCPSinkAgent receiving the packets and
  return acknowledgments
• Creating the Agent
• OTcl code

32
Tcp, TCP Sink Agents
Example

• C code

33
Tcp, TCP Sink Agents
Example

• Starting the agent
• OTcl code
• C code

34
Tcp, TCP Sink Agents
Example

• Processing Input at Receiver (class TcpSinkAgent)

35
Tcp, TCP Sink Agents
Example

• Processing Responses at the Sender (class
  TcpAgent)

36
Tcp, TCP Sink Agents
Example

• Implementing Timers
• derived from an abstract base class TimerHandler
• public member functions
• protected members

37
Tcp, TCP Sink Agents
Example

• agent wish to override the Agenttimeout()
  method
• Timers used in Tahoe TCP Agent
• delsnd_timer_ delays sending of packets by a
  small random amount of time to avoid phase
  efforts
• rtx_timer_ retransmission timer
• burstsnd_timer_ helps TCP to stagger the
  transmission of a large window into several
  smaller bursts

38
Tcp, TCP Sink Agents
Example

• Three timer classes (in ns/tcp.h)

39
Tcp, TCP Sink Agents
Example

• Timer initalized with this pointer

40
Tcp, TCP Sink Agents
Example

• Define schedule timer events

41
Tcp, TCP Sink Agents
Example

• Timer expire

42
Tcp, TCP Sink Agents
Example

• simple.tcl

set ns new Simulator set f open out.tr w ns
trace-all f set nf open out.nam w ns
namtrace-all nf set s1 ns node set s2 ns
node set r1 ns node set k1 ns node ns
duplex-link s1 r1 10Mb 0.1ms DropTail ns
duplex-link s2 k1 10Mb 0.1ms DropTail ns
simplex-link r1 k1 3.20Mb 20ms DropTail ns
simplex-link k1 r1 0.032Mb 20ms DropTail ns
queue-limit r1 k1 8 ns queue-limit k1 r1 8
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Tcp, TCP Sink Agents
Example

• simple.tcl (cont.)

set tcp ns create-connection TCP s1 TCPSink
s2 1 tcp set fid_ 1 tcp set flow_id 1 tcp
set window_ 1000 set ftp tcp attach-app
FTP ns at 0.0 "ftp start" ns at 50.0
"finish" proc finish global ns f nf ns
flush-trace close f close nf puts "running
nam..." exec nam out.nam exit 0 ns run
44
Tcp, TCP Sink Agents
Example

• Using Nam
• Tcl/TK based animation tool for viewing network
  simulation traces and real world packet trace
  data.
• The trace file is generated by ns (out.nam)

k1
s1
s2
r1
45
Tcp, TCP Sink Agents
Example

• Packets and acks passing

46
Tcp, TCP Sink Agents
Example

• View the information of packet and ack

47
Tcp, TCP Sink Agents
Example

• Analysis on the trace file (out.tr)

48
Conclusion

• ftp//mash.cs.berkeley.edu/dist/vint/
• download software ns-allinone-2.1b5a
• run in Unix
• http//www-mash.cs.berkeley.edu/ns/
• download documentation