[ns] Strange behaviour of TCPSink instvar "bytes_"

[Georg Piewald]

Hi everybody,

I experienced a very odd problem. I want to output the number of bytes
received by a TCPSink agent at a specified time. This can be read by the
instance variable $bytes_.
Now if I call a procedure at the required point of time, which outputs
the nr of bytes, everything works fine. Like this:

    ...
    proc testproc {} {
        global tcp1
        puts stdout "Rx bytes (within testproc): [$tcp1 set bytes_]"
    }
    ...
    $ns at 1.0 testproc
    ...

But if I want to output this value directly (without a wrapping
procedure, the output is always 0! Like that:

    $ns at 1.0 "puts stdout \"Rx bytes (directly): [$tcp1 set bytes_]\""

How can that be?
I use ns-2.31. The complete code of a minimal example to reproduce the
problem is here:






######################### Example #############################

set ns [new Simulator]

proc testproc {} {
     global tcp1
     puts stdout "Rx bytes (within testproc): [$tcp1 set bytes_]"
}

set n0 [$ns node]
set n1 [$ns node]
$ns duplex-link $n0 $n1 1mb 50ms DropTail

set tcp0 [new Agent/TCP]
set tcp1 [new Agent/TCPSink]

$ns attach-agent $n0 $tcp0
$ns attach-agent $n1 $tcp1
$ns connect $tcp0 $tcp1

set ftp0 [new Application/FTP]
$ftp0 attach-agent $tcp0

$ns at 0.1 "$ftp0 start"
$ns at 1.0 testproc
$ns at 1.0 "puts stdout \"Rx bytes (directly): [$tcp1 set bytes_]\""
$ns at 1.5 "$ftp0 stop"
$ns at 1.6 "exit 0"
$ns run

######################## /Example #############################


Output:

% ns min_ex.tcl
Received bytes (within testproc): 66600
Received bytes (directly): 0


Thanks for any hint!
Georg