two questions about ns simulator

["cerberus" <yaocl@locust.csie.ncku.edu.tw>]

Dear Sir:
  I have two questions about using the ns. I don't know if I can get the
answers here.

  1. How to trace a variable in TCP?

     in document 3.2.3 you wrote:
     $tcp trace cwnd_

     I wrote this in my program but nothing happend. I can not get any
report about cwnd.

  2. How to make a network topology like this?

   n1                           n3
    |                               |
   LAN---r1------r2---LAN
    |                               |
   n2                            n4


   Are the transport layer connection(TCP) and the DLL layer
connection(CSMA/CD) independent?


   set tcp1 [$ns create-connection TCP $n1 TCPSink $n3 1]
   $ns make-lan "$n1 $n2" $bw $delay LL Queue/Droptail Mac/Csma/Cd
   $ns duplex-link $r1 $r2 5Mb 2ms DropTail


  Thanks a lot for ur help.

-----------
Chang-Li Yao, Student
Department of Computer Science
National Cheng-Kung University
ROC
e-mail:yaocl@locust.csie.ncku.edu.tw