Simple broadcasting network in NS2

I am new to NS2 and I am implementing a simple broadcasting network in NS2. The tcl file is given below. Please tell me what is the work of these two variables MESSAGE_PORT and BROADCAST_ADDR?

And please tell me how these methods work :

• Agent/MessagePassing/Flooding instproc recv
• Agent/MessagePassing/Flooding instproc send_message {size message_id data port}

Here is my tcl file :

#the tcl file
Mac/Simple set bandwidth_ 1Mb

set MESSAGE_PORT 42
set BROADCAST_ADDR -1
set group_size 4
set num_groups 6
set num_nodes [expr $group_size * $num_groups]
set val(chan)           Channel/WirelessChannel    ;#Channel Type
set val(prop)           Propagation/TwoRayGround   ;# radio-propagation model
set val(netif)          Phy/WirelessPhy            ;# network interface type
set val(mac)            Mac/Simple
set val(ifq)            Queue/DropTail/PriQueue    ;# interface queue type
set val(ll)             LL                         ;# link layer type
set val(ant)            Antenna/OmniAntenna        ;# antenna model
set val(ifqlen)         50                         ;# max packet in ifq
set val(rp)             AODV
set val(x)              [expr 120*$group_size + 500]
set val(y)              [expr 240*$num_groups + 200]
set ns [new Simulator]
set f [open wireless-flooding-$val(rp).tr w]

$ns trace-all $f
set nf [open wireless-flooding-$val(rp).nam w]

$ns namtrace-all-wireless $nf $val(x) $val(y)
$ns use-newtrace
set topo       [new Topography]

$topo load_flatgrid $val(x) $val(y)
create-god $num_nodes
set chan_1_ [new $val(chan)]

$ns node-config -adhocRouting $val(rp) \
                -llType $val(ll) \
                -macType $val(mac) \
                -ifqType $val(ifq) \
                -ifqLen $val(ifqlen) \
                -antType $val(ant) \
                -propType $val(prop) \
                -phyType $val(netif) \
                -topoInstance $topo \
                -agentTrace ON \
                -routerTrace OFF \
                -macTrace ON \
                -movementTrace OFF \
                -channel $chan_1_

Class Agent/MessagePassing/Flooding -superclass Agent/MessagePassing

Agent/MessagePassing/Flooding instproc recv {source sport size data} {

    $self instvar messages_seen node_
    global ns BROADCAST_ADDR
    set message_id [lindex [split $data ":"] 0]
    puts "\nNode [$node_ node-addr] got message $message_id\n"

    if {[lsearch $messages_seen $message_id] == -1} {
          lappend messages_seen $message_id
        $ns trace-annotate "[$node_ node-addr] received {$data} from $source"
        $ns trace-annotate "[$node_ node-addr] sending message $message_id"
          $self sendto $size $data $BROADCAST_ADDR $sport
    } else {
        $ns trace-annotate "[$node_ node-addr] received redundant message $message_id from $source"
    }
}

Agent/MessagePassing/Flooding instproc send_message {size message_id data port} {
    $self instvar messages_seen node_
    global ns MESSAGE_PORT BROADCAST_ADDR
    lappend messages_seen $message_id
    $ns trace-annotate "[$node_ node-addr] sending message $message_id"
    $self sendto $size "$message_id:$data" $BROADCAST_ADDR $port
}
for {set i 0} {$i < $num_nodes} {incr i} {
    set n($i) [$ns node]
    $n($i) set Y_ [expr 230*floor($i/$group_size) + 160*(($i%$group_size)>=($group_size/2))]
    $n($i) set X_ [expr (90*$group_size)*($i/$group_size%2) + 200*($i%($group_size/2))]
    $n($i) set Z_ 0.0
    $ns initial_node_pos $n($i) 20
}

#attach a new Agent/MessagePassing/Flooding to each node on port $MESSAGE_PORT

for {set i 0} {$i < $num_nodes} {incr i} {
    set a($i) [new Agent/MessagePassing/Flooding]
    $n($i) attach  $a($i) $MESSAGE_PORT
    $a($i) set messages_seen {}
}
# now set up some events

$ns at 0.2 "$a(1) send_message 200 1 {first message}  $MESSAGE_PORT"
$ns at 0.4 "$a([expr $num_nodes/2]) send_message 600 2 {some big message} $MESSAGE_PORT"
$ns at 0.7 "$a([expr $num_nodes-2]) send_message 200 3 {another one} $MESSAGE_PORT"

$ns at 1.0 "finish"
proc finish {} {
        global ns f nf val
        $ns flush-trace
        close $f
        close $nf

#        puts "running nam..."
        exec nam wireless-flooding-$val(rp).nam &
        exit 0
}
$ns run