ns2学习，入门示例的理解

最近在学习ns2--network simulator。

以下是对教材《ns与网络模拟》入门案例的一个理解：

程序：

set ns [new Simulator]

set tracefd [open example1.tr w]
$ns trace-all $tracefd
set namtracefd [open example1.nam w]
$ns namtrace-all $namtracefd

proc finish {} {
        global ns tracefd namtracefd
        $ns flush-trace
        close $tracefd
        close $namtracefd
        exec nam example1.nam &
        exit 0
}

set n0 [$ns node]
set n1 [$ns node]

$ns duplex-link $n0 $n1 1Mb 10ms DropTail

set udp0 [new Agent/UDP]
$ns attach-agent $n0 $udp0

set cbr0 [new Application/Traffic/CBR]
$cbr0 set packetSize_ 500
$cbr0 set interval_ 0.005
$cbr0 attach-agent $udp0
set null0 [new Agent/Null]
$ns attach-agent $n1 $null0
$ns connect $udp0 $null0
$ns at 0.5 "$cbr0 start"
$ns at 4.5 "$cbr0 stop"
$ns at 5.0 "finish"

$ns run

整个这段代码描述了如下场景：