[ns] Question about strange packet drop behavior when packet error rate bigger than 0.01

[=?GB2312?B?w6vNrg==?=]

Hello everyone,

I made a mistake for the last Email, the value of packet error rate should
be 0.01. Here is a description of my question:

I add the link-error module in a simple link from node0 to node1(ns2.28 and
ns2.33 both have this problem):
ftp->tcp->node->link->node->tcpsink

when I set the error rate bigger than 0.01, the first tcp packet(I think is
syn) alsways have error and is dropped, the next transmission happened after
6 sec.
Here is the trace file:

packet error rate: 0.01
0.10000  0  0  1  0  t_seqno_ 1
0.14032  0  0  1  0  t_seqno_ 2
0.14032  0  0  1  0  t_seqno_ 3
0.18464  0  0  1  0  t_seqno_ 4
... ...

packet error rate: 0.02
0.10000  0  0  1  0  t_seqno_ 1
6.10000  0  0  1  0  t_seqno_ 0
6.10000  0  0  1  0  t_seqno_ 1
6.14032  0  0  1  0  t_seqno_ 2

Does anyone know the reason for this strange behavior?

My tcl script is:

set n0 [$ns node]
set n1 [$ns node]

$ns duplex-link $n0 $n1 2Mb 20ms DropTail

set em [new ErrorModel]
$em set rate_ 0.2
$em unit pkt
$em ranvar [new RandomVariable/Uniform]
$em drop-target [new Agent/Null]
$ns link-lossmodel $em $n0 $n1

set tcp [new Agent/TCP]
$ns attach-agent $n0 $tcp
set sink [new Agent/TCPSink]
$ns attach-agent $n1 $sink
$ns connect $tcp $sink
$tcp set fid_ 1

#Open the t_seqno_ trace file
$tcp trace t_seqno_
set trace_ch [open "trace.txt" w]
$tcp attach $trace_ch

... ...

$ns at 0.1 "$ftp start"
$ns at 63.5 "$ftp stop"


Thank you very much for your help.



Best Regards,
Tong Mao