question on CBQ

#Create a simulator object

set ns [new Simulator]

#Open the nam trace file

set nf [open out.nam w]

$ns namtrace-all $nf

#Create 4 nodes

set n1 [$ns node]

set n2 [$ns node]

set n3 [$ns node]

set n4 [$ns node]

#Connect the nodes

$ns duplex-link $n1 $n3 10Mb 100ms DropTail

$ns duplex-link $n2 $n3 10Mb 100ms DropTail

$ns duplex-link $n3 $n4 1Mb 100ms CBQ

set cbqlink [$ns link $n3 $n4]

$ns duplex-link-op $n1 $n3 orient right-up

$ns duplex-link-op $n2 $n3 orient right

$ns duplex-link-op $n3 $n4 orient right

#Monitor the queue for the link between node 3 and node 4

$ns duplex-link-op $n3 $n4 queuePos 0.5

#Set two leaf classes: lowerClass1 and lowerClass2

set topClass [new CBQClass]

set lowerClass1 [new CBQClass]

set lowerClass2 [new CBQClass]

$topClass setparams none 0 0.80 auto 0 2 0

$lowerClass1 setparams $topClass true 0.35 auto 1 1 100ms

$lowerClass2 setparams $topClass true 0.35 auto 2 1 150ms

set q1 [new Queue/DropTail]

$q1 set limit_ 20

set q2 [new Queue/DropTail]

$q2 set limit_ 30

$lowerClass1 install-queue $q1

$lowerClass2 install-queue $q2

$cbqlink insert $lowerClass1

$cbqlink insert $lowerClass2

$cbqlink bind $lowerClass1 1

$cbqlink bind $lowerClass2 2

#generate traffic from n1 , n2 to n4 via n3

set null1 [new Agent/Null]

$ns attach-agent $n4 $null1

set s1 [new Agent/UDP]

$ns attach-agent $n1 $s1

$ns connect $s1 $null1

set exp1 [new Application/Traffic/Exponential]

$exp1 set packet_size_ 210

$exp1 set burst_time_ 500ms

$exp1 set idle_time_ 500ms

$exp1 set rate_ 100k

$exp1 set fid_ 1

$exp1 attach-agent $s1

set s2 [new Agent/UDP]

$ns attach-agent $n2 $s2

$ns connect $s2 $null1

set exp2 [new Application/Traffic/Exponential]

$exp2 set packet_size_ 210

$exp2 set burst_time_ 500ms

$exp2 set idle_time_ 500ms

$exp2 set rate_ 100k

$exp2 set fid_ 2

$exp2 attach-agent $s2

#Define a 'finish' procedure

proc finish {} {

global ns nf

$ns flush-trace

#Close the trace file

close $nf

#Execute nam on the trace file

exec nam out.nam &

exit 0

}

$ns at 1.0 "$exp1 start"

$ns at 1.0 "$exp2 start"

$ns at 5.0 "$exp1 stop"

$ns at 5.0 "$exp2 stop"

#Run the simulation

$ns run

The error message is: