有线观察排队和丢包，三节点（FTP/TCP+CBR/UDP）

通过利用NAM演示功能，可以明显看到TCP和UDP机制的区别： 三次握手、ACK消息、慢启动等！

链接：http://blog.chinaunix.net/u3/105477/showart_2087706.html

# 场景描述：
# 主要用于观察数据包在节点Agent上的排队和丢包情况
# 有线场景，四个节点，FTT+TCP用蓝色表示, CBR+UDP用红色表示
set ns [new Simulator]
#设置颜色
$ns color 1 Blue
$ns color 2 Red
set tracefd [open out.tr w]
$ns trace-all $tracefd
set namtracefd [open out.nam w]
$ns namtrace-all $namtracefd

proc finish {} {
 global ns tracefd namtracefd
 $ns flush-trace
 close $tracefd
    close $namtracefd 
 exec nam out.nam &
 exit 0
}
set n0 [$ns node]
set n1 [$ns node]
set n2 [$ns node]
set n3 [$ns node]

$ns duplex-link $n0 $n2 2Mb 10ms DropTail
$ns duplex-link $n1 $n2 2Mb 10ms DropTail
$ns duplex-link $n2 $n3 1.7Mb 10ms DropTail
$ns queue-limit $n2 $n3 10

#用于设置NAM显示时，各节点的相对位置
$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
$ns duplex-link-op $n2 $n3 queuePos 0.5

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 ftp [new Application/FTP]
$ftp attach-agent $tcp
$ftp set type_ FTP

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

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 

$ns at 0.5 "$cbr start"
$ns at 1.0 "$ftp start"
$ns at 4.0 "$ftp stop"
$ns at 4.5 "$cbr stop"

#一下这一行可有可无！
$ns at 4.5 "$ns detach-agent $n0 $tcp ; $ns detach-agent $n3 $sink"
$ns at 5.0 "finish"

#会在命令窗口中输出， 因为默认的stdout是命令窗口啊！
puts "CBR packet size=[$cbr set packet_size_]"
puts "CBR interval=[$cbr set interval_]"
$ns run