[ns] How to use "create-trace"? -- What's wrong with my program?

[Lao Yu <laoyusg@yahoo.com.sg>]

I wrote a program trying to trace the traffic between
two nodes. I want to use the method "create-trace" to
capture only the "Recv" trace. The program can run,
but nothing is written into the output file "out.tr".
Can anyone help me to find of what the problem is?
(BTW, I am using ns-allinone-2.1b8a under Red Hat
Linux 7.1).
This is my program:

#*******************************************
set ns [new Simulator]
set tracefd [open out.tr w]

# Create the nodes 
set n1 [$ns node]
set n2 [$ns node]

# Setup connections between the nodes 
$ns duplex-link $n1 $n2 10Mb 5ms DropTail 

set newtrc [$ns create-trace Recv $tracefd $n1 $n2]
$newtrc set show_tcphdr_ 1

# Configure the nodes
set tcp1 [new Agent/TCP]
$ns attach-agent $n1 $tcp1
set cbr1 [new Application/Traffic/CBR]
$cbr1 set rate_ 64Kb
$cbr1 attach-agent $tcp1

set sink1 [new Agent/TCPSink]
$ns attach-agent $n2 $sink1

# Connect the agents.
$ns connect $tcp1 $sink1

$ns at 1.0 "$cbr1 start"
$ns at 2.0 "$cbr1 stop"
$ns at 10.0 "finish"

proc finish {} {
    global ns tracefd newtrc
    $newtrc flush
    close $tracefd
    exit 0
}

$ns run

#************************************************

Best Regards,
Lao Yu


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