[ns] fix snoop

[Benyuan Liu <benyuan@newworld.cs.umass.edu>]

Hi,	

  The Berkerly Snoop in NS has been broken since version ns-2.1.b4. I
looked into the code recently and was able to make it work except for
a small problem. In the following I will describe how I fixed and
validated the code. Hope this will help other people in the community.

   The reason that Snoop doesn't work in current version is due to the 
change in MAC layer. In Snoop, (Handler *h == 0) is used to
determine the direction of the packet (up or down). But in current
implementation of ns,  MAC class is a subclass of BiConnector. In
BiConnector, ( HDR_CMN(p)->direction()), instead of the (Handler *h==
0) is used to determine whether to send up or down. So one need to
replace all conditions (h==0) with (HDR_CMN(p)->direction() == hdr_cmn::UP)
in Snoop.cc. 

  Also, to send a packet down at the link layer(snoop.cc), one needs to
specify the direction of the packet. For example, in 
void Snoop::recv() and  int Snoop::snoop_rxmit(), add 

			hdr_cmn *ch = HDR_CMN(p);       
			ch->direction() = hdr_cmn::DOWN; 

before calling parent_->sendDown(p);

 For some reason, in snoop.cc, parent_->ifq()->length() always causes
segmentation fault. The reason may be the C++ side doesn't get the correct
pointer to the interface queue (ifq_). For now I just commented out
the condition and make the queue limit to be big so the queue doesn't
overflow.  
 
 After the above changes, the Snoop agent should be able to work. I
validated the functionality of the Snoop agent by manually droping
some packets and studying the detail trace. I validated all the major
operations like :
   (X)  Snoop timeout retransmission  
   (X)  Ack suppress
   (X)  Snoop Buffer operations

and the program works correctly.

 By the way, the original snoop test file (snoop.tcl) doesn't work.
One problem "add-error" procedure uses old interface. The following is
my own test file.

-------------------------------------------------------------------------
puts "sourcing tcl/lan/vlan.tcl..."
source tcl/lan/vlan.tcl
source tcl/lan/ns-mac.tcl


set opt(tr)	out.tr
set opt(namtr)	"MySnoop.nam"
set opt(seed)	0
set opt(stop)	10
set opt(node)	2

set opt(qsize)	100
set opt(bw)	10Mb
set opt(delay)	1ms
set opt(ll)	LL
set opt(ifq)	Queue/DropTail
set opt(mac)	Mac/802_3
set opt(chan)	Channel
set opt(tcp)	TCP/Reno
set opt(sink)	TCPSink

set opt(app)	FTP

set loss_prob 10


proc finish {} {
	global ns opt

	$ns flush-trace

	exec nam $opt(namtr) &

	exit 0
}


proc create-trace {} {
	global ns opt

	if [file exists $opt(tr)] {
		catch "exec rm -f $opt(tr) $opt(tr)-bw [glob $opt(tr).*]"
	}

	set trfd [open $opt(tr) w]
	$ns trace-all $trfd
	if {$opt(namtr) != ""} {
		$ns namtrace-all [open $opt(namtr) w]
	}
	return $trfd
}

proc add-error {LossyLink} {

    global loss_prob
    
    # creating the uniform distribution random variable
    set loss_random_variable [new RandomVariable/Uniform] 
    $loss_random_variable set min_ 0    # set the range of the random variable;
    $loss_random_variable set max_ 100
    
    # create the error model;
    set loss_module [new ErrorModel]  
    $loss_module drop-target [new Agent/Null] 
    $loss_module set rate_ $loss_prob  # set error rate to (0.1 = 10 / (100 - 0));
    # error unit: packets (the default);
    $loss_module unit pkt      
    
    # attach random var. to loss module;
    $loss_module ranvar $loss_random_variable 

    # keep a handle to the loss module;
    #set sessionhelper [$ns create-session $n0 $tcp0] 
    $LossyLink errormodule $loss_module



}

proc create-topology {} {
	global ns opt 
	global lan node s d

	set num $opt(node)
	for {set i 1} {$i < $num} {incr i} {
		set node($i) [$ns node]
		lappend nodelist $node($i)
	}
	

	set lan [$ns make-lan $nodelist $opt(bw) \
			$opt(delay) $opt(ll) $opt(ifq) $opt(mac) $opt(chan)]
	

	set opt(ll) LL/LLSnoop

	set opt(ifq) Queue/DropTail
	$opt(ifq) set limit_ 100

	# set up snoop agent
	set node(0) [$ns node]
	$lan addNode [list $node(0)]  $opt(bw) $opt(delay) $opt(ll) $opt(ifq) $opt(mac)
	
	# set source and connect to node(0)
	set s [$ns node]
	$ns duplex-link $s $node(0) 20Mb 20ms DropTail
	$ns queue-limit $s $node(0) 100000
	$ns duplex-link-op $s $node(0) orient right

	# set dest and connect to node(1)
	set d [$ns node]
	$ns duplex-link $node(1) $d 10Mb 0ms DropTail
	$ns queue-limit $node(1) $d 1000
	$ns duplex-link-op $d $node(1) orient left



	set LossyLink [$ns link $node(1) $d]  

	add-error $LossyLink
}

## MAIN ##

set ns [new Simulator]

set trfd [create-trace]

create-topology

#set tcp0 [$ns create-connection TCP/Reno $s TCPSink $node(1) 0]
#$tcp0 set window_ 30


#Create a infinite source agent (FTP) tcp and attach it to node n0
set tcp0 [new Agent/TCP/Reno]
$tcp0 set backoff_ 2 
$tcp0 set window_ 30
$ns attach-agent $s $tcp0

set tcp_snk0 [new Agent/TCPSink]
$ns attach-agent $d $tcp_snk0

$ns connect $tcp0 $tcp_snk0


set ftp0 [$tcp0 attach-app FTP]

$ns at 0.0 "$ftp0 start"
$ns at $opt(stop) "finish"
$ns run

----------------------------------------------------------------------------

Please note the Mac protocol I used in the script is MAC/802_3. The
Csma/Ca is not offered in ns and MAC/802_11 doesn't work in this case. I
haven't identify the problem, for my case, I am not particularly
looking at MAC issues. If anyone solves this problem, please notify me.



Hope this helps. 


Benyuan

--------------------