[ns] Can't Seen any tcp flow in trace file

[Salman Malik]

Hi All,

I am a newbie to ns2 and trying to simulate a fat-tree like topology of depth 3 
in the simulator. I am using DV routing protocol for routing and the leafs in 
the topology send packets to the root of the tree (which in my case is a sink). 
I have used the following to attach sources to the leaf nodes:

for {set i 1} {$i <= $switches*$hosts} {incr i} {
    #Create a TCP agent and attach it to hosts
        set tcp($i) [new Agent/TCP]
    # Create exponential traffic sources and attach it to tcp agents
    set e($i) [new Application/Traffic/Exponential]
    $e($i) set packetSize_ 500
    #$e($i) set interval 0.005
    $e($i) set burst_time_ 990ms
    $e($i) set idle_time_ 10ms
    $e($i) set rate_ 100k
    $e($i) attach-agent $tcp($i)
}

And I attach all the leaves to the root-sink node as follows:

#Create a Null agent (a traffic sink) and attach it to node n(3)
set null0 [new Agent/Null]
$ns attach-agent $n(0) $null0

#Connect the traffic source with the traffic sink
for {set i 1} {$i <= $switches*$hosts} {incr i} {
    $ns connect $tcp($i) $null0
}

(Other links in the topology also seem fine, as shown in nam. Further, I also 
started all the sources at the end of .tcl file)

But when I inspect the trace file, I don't see any tcp flows in the file. I 
just see flows of type rtprotodv in the file. 
What could be wrong here? 

Thanks,
Salman