ns2.23——ns-simple.tcl样例解析

Posted 2020-10-21

#Create a simulator object
set ns [new Simulator]

#Define different colors for data flows (for NAM)
$ns color 1 Blue
$ns color 2 Red

#Open the NAM trace file
set nf [open out.nam w]   
$ns namtrace-all $nf

#Define a 'finish' procedure
proc finish {} {
        global ns nf
        $ns flush-trace      
        close $nf            #Close the NAM trace file
        exec nam out.nam &   #Execute NAM on the trace file
        exit 0
}

#Create four nodes
set n0 [$ns node]
set n1 [$ns node]
set n2 [$ns node]
set n3 [$ns node]

#Create links between the nodes （双向链接）
$ns duplex-link $n0 $n2 2Mb 10ms DropTail
$ns duplex-link $n1 $n2 2Mb 10ms DropTail
$ns duplex-link $n2 $n3 1.7Mb 20ms DropTail

Drop Tail(队列长度管理机制,丢尾)

  它有点类似于FIFO(先入先出)的存储方式。Drop Tail最大的优点是原理简单。当路由器队列长度达到最大值时，通过丢包来指示拥塞，先到达路由器的分组首先被传输。由于路由器缓存有限，如果包到达时缓存已满，那么路由器就丢弃该分组。一旦发生丢包，发送端立即被告知网络拥塞，从而调整发送速率。这种做法不考虑被丢弃包的重要程度。

#Set Queue Size of link (n2-n3) to 10
$ns queue-limit $n2 $n3 10

#Give node position (for NAM)
$ns duplex-link-op $n0 $n2 orient right-down # n0在n2的-45°
$ns duplex-link-op $n1 $n2 orient right-up   # n1在n2的45°
$ns duplex-link-op $n2 $n3 orient right      # n2的右边是n3

#队列位置定义了队列与水平的夹角（水平线以上度数为正）
#right-down -45
#right-up    45
#right        0    右

#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]       #创建一个tcp代理
$tcp set class_ 2             #设置tcp流的颜色为红色（n0）
$ns attach-agent $n0 $tcp     
set sink [new Agent/TCPSink]  #Sink：接收器（n3）
$ns attach-agent $n3 $sink
$ns connect $tcp $sink
$tcp set fid_ 1               #set IP-layer flow ID



#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] #创建cbr流量产生器agent
$cbr attach-agent $udp
$cbr set type_ CBR
$cbr set packet_size_ 1000
$cbr set rate_ 1mb
$cbr set random_ false


#Schedule    调度cbr和ftp事件
$ns at 0.1 "$cbr start"
$ns at 1.0 "$ftp start"
$ns at 4.0 "$ftp stop"
$ns at 4.5 "$cbr stop"

#Detach（分离） 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_]"

#Run the simulation
$ns run