set ns [new Simulator] : generates an NS simulator object instance, and assigns it to variable ns (italics is used for variables and values in this section).
What this line does is the following:
Initialize the packet format (ignore this for now)
Create a scheduler (default is calendar scheduler)
Select the default address format (ignore this for now) The "Simulator" object has member functions that do the following:
Create compound objects such as nodes and links
Connect network component objects created (ex. attach-agent)
Set network component parameters (mostly for compound objects)
Create connections between agents (ex. between a "tcp" and "sink")
Specify NAM display options
Etc.
Most of member functions are for simulation setup (referred to as plumbing functions in the Overview section) and scheduling, however some of them are for the NAM display. The "Simulator" object member function implementations are located in the "usr/local/ns-2/ns-2.31/tcl/lib/ns-lib.tcl" file.
#Define different colors for data flows (for NAM) $ns color 1 Blue $ns color 2 Red
$ns color fid color : is to set color of the packets for a flow specified by the flow id (fid). This member function of "Simulator" object is for the NAM display, and has no effect on the actual simulation.
#Open the NAM trace file set nf [open out.nam w] $ns namtrace-all $nf
$ns namtrace-all [file-descriptor] : This member function tells the simulator to record simulation traces in NAM input format. It also gives the file name that the trace will be written to later by the command $ns flush-trace. Similarly, the member function trace-all is for recording the simulation trace in a general format.
#Define a 'finish' procedure proc finish {} { global ns nf $ns flush-trace #Close the NAM trace file close $nf #Execute NAM on the trace file exec nam out.nam & exit 0 }
proc finish {}: is called after this simulation is over by the command $ns at 5.0 "finish". In this function, post-simulation processes are specified.
#Create four nodes set n0 [$ns node] set n1 [$ns node] set n2 [$ns node] set n3 [$ns node]
set n0 [$ns node]: The member function node creates a node. A node in NS is compound object made of address and port classifiers (described in a later section). Users can create a node by separately creating an address and a port classifier objects and connecting them together. However, this member function of Simulator object makes the job easier. To see how a node is created, look at the files: "usr/local/ns-2/ns-2.31/tcl/lib/ns-lib.tcl" and "usr/local/ns-2/ns-2.31/tcl/lib/ns-node.tcl".
$ns duplex-link [node1] [node2] [bandwidth] [delay] [queue-type] : creates two simplex links of specified bandwidth and delay, and connects the two specified nodes. In NS, the output queue of a node is implemented as a part of a link, therefore users should specify the queue-type when creating links. In the above simulation script, DropTail queue is used. If the reader wants to use a RED queue, simply replace the word DropTail with RED. The NS implementation of a link is shown in a later section. Like a node, a link is a compound object, and users can create its sub-objects and connect them and the nodes. Link source codes can be found in "usr/local/ns-2/ns-2.31/tcl/libs/ns-lib.tcl" and "usr/local/ns-2/ns-2.31//tcl/libs/ns-link.tcl" files. One thing to note is that you can insert error modules in a link component to simulate a lossy link (actually users can make and insert any network objects). Refer to the NS documentation to find out how to do this.
#Set Queue Size of link (n2-n3) to 10 $ns queue-limit $n2 $n3 10
$ns queue-limit [node1] [node2] [number] : This line sets the queue limit of the two simplex links that connect node1 and node2 to the number specified. At this point, the authors do not know how many of these kinds of member functions of Simulator objects are available and what they are. Please take a look at "usr/local/ns-2/ns-2.31/tcl/libs/ns-lib.tcl" and "usr/local/ns-2/ns-2.31/tcl/libs/ns-link.tcl", or NS documentation for more information.
$ns duplex-link-op [node1] [node2] ...: The next couple of lines are used for the NAM display. To see the effects of these lines, users can comment these lines out and try the simulation.
#Monitor the queue for link (n2-n3). (for NAM) $ns duplex-link-op $n2 $n3 queuePos 0.5
#Setup a TCP connection set tcp [new Agent/TCP] $tcp set class_ 2 $ns attach-agent $n0 $tcp set sink [new Agent/TCPSink] $ns attach-agent $n3 $sink $ns connect $tcp $sink $tcp set fid_ 1
set tcp [new Agent/TCP]: This line shows how to create a TCP agent. But in general, users can create any agent or traffic sources in this way. Agents and traffic sources are in fact basic objects (not compound objects), mostly implemented in C++ and linked to OTcl. Therefore, there are no specific Simulator object member functions that create these object instances. To create agents or traffic sources, a user should know the class names these objects (Agent/TCP, Agnet/TCPSink, Application/FTP and so on). This information can be found in the NS documentation or partly in this documentation. But one shortcut is to look at the "usr/local/ns-2/ns-2.31/tcl/libs/ns-default.tcl" file. This file contains the default configurable parameter value settings for available network objects. Therefore, it works as a good indicator of what kind of network objects are available in NS and what are the configurable parameters.
$ns attach-agent [node] [agent] : The attach-agent member function attaches an agent object created to a node object. Actually, what this function does is call the attach member function of specified node, which attaches the given agent to itself. Therefore, a user can do the same thing by, for example, $n0 attach $tcp. Similarly, each agent object has a member function attach-agent that attaches a traffic source object to itself.
$ns connect [agent1] [agent2] : After two agents that will communicate with each other are created, the next thing is to establish a logical network connection between them. This line establishes a network connection by setting the destination address to each others' network and port address pair.
#Setup a FTP over TCP connection set ftp [new Application/FTP] $ftp attach-agent $tcp $ftp set type_ FTP
#Setup a UDP connection set udp [new Agent/UDP] $ns attach-agent $n1 $udp set null [new Agent/Null] $ns attach-agent $n3 $null $ns connect $udp $null $udp set fid_ 2
#Setup a CBR over UDP connection set cbr [new Application/Traffic/CBR] $cbr attach-agent $udp $cbr set type_ CBR $cbr set packet_size_ 1000 $cbr set rate_ 1mb $cbr set random_ false
#Schedule events for the CBR and FTP agents $ns at 0.1 "$cbr start" $ns at 1.0 "$ftp start" $ns at 4.0 "$ftp stop" $ns at 4.5 "$cbr stop"
$ns at [time] "string" : This member function of a Simulator object makes the scheduler (scheduler_ is the variable that points the scheduler object created by [new Scheduler] command at the beginning of the script) to schedule the execution of the specified string at given simulation time. For example, $ns at 0.1 "$cbr start" will make the scheduler call a start member function of the CBR traffic source object, which starts the CBR to transmit data. In NS, usually a traffic source does not transmit actual data, but it notifies the underlying agent that it has some amount of data to transmit, and the agent, just knowing how much of the data to transfer, creates packets and sends them.
#Detach tcp and sink agents (not really necessary) $ns at 4.5 "$ns detach-agent $n0 $tcp ; $ns detach-agent $n3 $sink"
#Call the finish procedure after 5 seconds of simulation time $ns at 5.0 "finish"
#Print CBR packet size and interval puts "CBR packet size = [$cbr set packet_size_]" puts "CBR interval = [$cbr set interval_]"
이 네트워크는 위 그림과 같이 4개의 노드(n0, n1, n2, n3)로 구성된다. n0와 n2, n1과 n2 간에는 2Mbps의 bandwidth와 10ms의 delay를 갖는 duplex links로.. n2와 n3 간에는 1.7Mbps의 bandwidth와 20ms의 delay를 갖는 deplex links로 연결된다. 각 노드는 최대사이즈가 10인 DropTail queue를 사용한다.
"tcp" agent는 n0에 붙고, n3에 붙은 tcp "sink" agent와 연결된다. 디폴트로 패킷의 최대사이즈는 tcp agent에서 1KByte로 생성한다. tcp "sink" agent는 ACK 패킷을 sender(tcp agent) 측으로 생성하여 보내며, 받은 패킷을 제거한다(? frees the received packets).
"udp" agent는 n1에 붙고, n3에 붙은 "null" agnet와 연결된다. "null" agent는 단지 받은 패킷을 제거한다(? frees the packets received).
"ftp"와 "cbr" traffic generator(source)는 각각 "tcp" "udp" agent에 붙으며, "cbr"은 1Mbps 속도로 1KByte 패킷을 생성하기위해 구성된다.
"cbr"은 0.1sec에서 시작하고 4.5sec에서 멈춘다. "ftp"는 1.0sec에서 시작하고 4.0sec에서 멈춘다.
Trace Analysis Example
Trace Format Example
event "+" : 큐에 들어오는 패킷 "-" : 큐에서 나가는 패킷 "r" : 패킷이 다른 노드에 도착 "d" : 큐에서 드롭된 패킷 time : 패킷의 이동시간 from / to node : 패킷의 이동 경로 pkt type : 패킷 타입 pkt size : 패킷 사이즈 flag : '---------' 표시는 어떤 플래그도 이용하고 있지 않음을 의미 src_addr / dst_addr : ex. 0.0, 3.0은 각각 node.port를 의미 seq num : 패킷의 순서번호 (udp는 순서번호를 이용하지 않지만, NS2에서는 사용) pkt id : 패킷 유일 id를 나타내는 필드
invalid-file
