RFC 793 the first version from DARP

The initial idea for the TCP versions are many but the RFC 793 proposed by DARP (Defense Advanced Research Project) is noteworthy. NS2 provides a class called tcp-RFC-793edu depicting the ideology of the proposed system. Main difference between the rfc793 and present day TCP is that there was no slow start, and the window would raise to constant value and then on remain constant.
This pretty much did provide all the necessary headers except the concept of sliding window.
The complete paper can be found here
The implementation code is as below

set ns [new Simulator]
set nf [open mi.nam w]
$ns namtrace-all $nf

set r [open rfc793.tr w]

set tf [open mi.tr w]
$ns trace-all $tf

proc finish { } {
global ns nf tf
$ns flush-trace
close $nf
close $tf
exec nam mi.nam &
exit 0
}

set n0 [$ns node]
set n1 [$ns node]
set n2 [$ns node]
set n3 [$ns node]
$n3 label "destination"

$ns duplex-link $n0 $n2 10Mb 1ms DropTail
$ns duplex-link $n1 $n2 1Mb 100ms DropTail
$ns duplex-link $n2 $n3 1Mb 100ms DropTail


set tcp0 [new Agent/TCP/RFC793edu]
$ns attach-agent $n0 $tcp0

set ftp0 [new Application/FTP]
$ftp0 set packet_Size_ 500
$ftp0 set interval_ 0.005
$ftp0 attach-agent $tcp0

set tcp1 [new Agent/TCP/RFC793edu]
$ns attach-agent $n1 $tcp1

set ftp1 [new Application/FTP]
$ftp1 set packet_Size_ 1000
$ftp1 set interval_ 0.05
$ftp1 attach-agent $tcp1

set sink0 [new Agent/TCPSink]
$ns attach-agent $n3 $sink0

set sink1 [new Agent/TCPSink]
$ns attach-agent $n3 $sink1

$ns connect $tcp0 $sink0
$ns connect $tcp1 $sink1

$ns at 0.1 "$ftp0 start"
$ns at 0.1 "$ftp1 start"

  proc plotWindow {tcpSource file} {
     global ns

     set time 0.1
     set now [$ns now]
     set cwnd [$tcpSource set cwnd_]
     set wnd [$tcpSource set window_]
     puts $file "$now $cwnd"
     $ns at [expr $now+$time] "plotWindow $tcpSource $file"
  }

 $ns at 0.1 "plotWindow $tcp0 $r"


$ns at 10.0 "finish"
$ns run

To give an idea of the changes both the congestion windows taken at time gap of 0.1 for TCP and RFC793 are plotted in a single graph