下面来学习更加复杂一点的TCL脚本的编写
简述：建立有4个节点的拓扑，其中一个节点作为路由器，用来将两个节点发出的数据包转发到第四个节点上面。
在这里将解释将两个节点的数据流区分开来的方法，展示如何去检测一个队列是否是满的，以及一个数据包是如何被丢弃的。
1、建立拓扑
Tcl脚本大多是大同的，我们需要做的是在基础上进行自己需要的修改。基础代码如下：
#Create a simulator object
set ns [new Simulator]#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 the trace fileclose $nf
#Execute nam on the trace fileexec nam out.nam &exit 0
}
# Insert your own code for topology creation
# and agent definitions, etc. here#Call the finish procedure after 5 seconds simulation time
$ns at 5.0 "finish"#Run the simulation
$ns run
做出修改如下：
① 添加四个节点：
set n0 [$ns node]
set n1 [$ns node]
set n2 [$ns node]
set n3 [$ns node]② 添加链接
$ns duplex-link $n0 $n2 1Mb 10ms DropTail
$ns duplex-link $n1 $n2 1Mb 10ms DropTail
$ns duplex-link $n3 $n2 1Mb 10ms DropTail其实上面这样就可以了，但是，为了让它更加美观，在跑nam的时候不要太懵逼，我们需要变一下链接的位置，使用的语句如下：
$ns duplex-link-op $n0 $n2 orient right-down
$ns duplex-link-op $n1 $n2 orient right-up
$ns duplex-link-op $n2 $n3 orient right 在改变的时候我们可以使用right, left, up, down，以及这些单词的结合（结合方式：某-某）
2、添加事件
① 设置代理——建立起两个CBR代理和一个NULL代理。CBR代理给n0和n1，n0、n1用于发送数据，n3用于接收数据。
#Create a UDP agent and attach it to node n0
set udp0 [new Agent/UDP]
$ns attach-agent $n0 $udp0# Create a CBR traffic source and attach it to udp0
set cbr0 [new Application/Traffic/CBR]
$cbr0 set packetSize_ 500
$cbr0 set interval_ 0.005
$cbr0 attach-agent $udp0#Create a UDP agent and attach it to node n1
set udp1 [new Agent/UDP]
$ns attach-agent $n1 $udp1# Create a CBR traffic source and attach it to udp1
set cbr1 [new Application/Traffic/CBR]
$cbr1 set packetSize_ 500
$cbr1 set interval_ 0.005
$cbr1 attach-agent $udp1set null0 [new Agent/Null]
$ns attach-agent $n3 $null0
② CBR代理和NULL代理进行连接
$ns connect $udp0 $null0
$ns connect $udp1 $null0
③ 设置数据流开始的时间和结束的时间
$ns at 0.5 "$cbr0 start"
$ns at 1.0 "$cbr1 start"
$ns at 4.0 "$cbr1 stop"
$ns at 4.5 "$cbr0 stop"
3、区别流
因为有俩发送端，为了区别数据流的情况，我们来给他们加点颜色；
$udp0 set class_ 1
$udp1 set class_ 2
$ns color 1 Blue
$ns color 2 Red然后这样就udp0发出的数据流变蓝色，另外一个变红色。
4、查看链接的队列
添加下面的语句来监视链接的情况
$ns duplex-link-op $n2 $n3 queuePos 0.5为了得到相对公平的情况，我们可以更换使用队列，
$ns duplex-link $n3 $n2 1Mb 10ms SFQ SQF（随机公平排队）这样最后得到的是相对公平的结果
本节完完整代码如下：
#Create a simulator object
set ns [new Simulator]#Define different colors for data flows
$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 the trace fileclose $nf
#Execute nam on the trace fileexec nam out.nam &exit 0
}#Create four nodes
set n0 [$ns node]
set n1 [$ns node]
set n2 [$ns node]
set n3 [$ns node]#Create links between the nodes
$ns duplex-link $n0 $n2 1Mb 10ms DropTail
$ns duplex-link $n1 $n2 1Mb 10ms DropTail
$ns duplex-link $n3 $n2 1Mb 10ms SFQ$ns duplex-link-op $n0 $n2 orient right-down
$ns duplex-link-op $n1 $n2 orient right-up
$ns duplex-link-op $n2 $n3 orient right#Monitor the queue for the link between node 2 and node 3
$ns duplex-link-op $n2 $n3 queuePos 0.5#Create a UDP agent and attach it to node n0
set udp0 [new Agent/UDP]
$udp0 set class_ 1
$ns attach-agent $n0 $udp0# Create a CBR traffic source and attach it to udp0
set cbr0 [new Application/Traffic/CBR]
$cbr0 set packetSize_ 500
$cbr0 set interval_ 0.005
$cbr0 attach-agent $udp0#Create a UDP agent and attach it to node n1
set udp1 [new Agent/UDP]
$udp1 set class_ 2
$ns attach-agent $n1 $udp1# Create a CBR traffic source and attach it to udp1
set cbr1 [new Application/Traffic/CBR]
$cbr1 set packetSize_ 500
$cbr1 set interval_ 0.005
$cbr1 attach-agent $udp1#Create a Null agent (a traffic sink) and attach it to node n3
set null0 [new Agent/Null]
$ns attach-agent $n3 $null0#Connect the traffic sources with the traffic sink
$ns connect $udp0 $null0
$ns connect $udp1 $null0#Schedule events for the CBR agents
$ns at 0.5 "$cbr0 start"
$ns at 1.0 "$cbr1 start"
$ns at 4.0 "$cbr1 stop"
$ns at 4.5 "$cbr0 stop"
#Call the finish procedure after 5 seconds of simulation time
$ns at 5.0 "finish"#Run the simulation
$ns run
)
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