Networking 201

Copyright Priscilla Oppenheimer

Lab 7 Troubleshoot an FTP Problem

Scenario

A user is complaining that she can't FTP a file to the server at address 11.107.238.3. Her FTP software seems to connect to the server OK, but when she types dir to get a directory listing, nothing happens. She never sees a directory listing and she's never given a chance to store or retrieve a file.

She has called you to help. What do you think the problem is?

Download the Trace File and Open It with Ethereal

  1. Download the Lab7FTP.enc trace file.
  2. Start the Ethereal software.
  3. Open the Lab7FTP.enc trace file in Ethereal.

Answer the Following Questions

  1. Which packets in the trace file represent the TCP 3-way handshake for the FTP control process?
  2. What sequence number does the client use in its SYN packet? Don't say zero! If you see zero in Ethereal and don't know why, ask for help.
  3. What window size does the client advertise in its SYN packet?
  4. Does the client slide its window open or closed in the next packet that it sends?
  5. In the first packet of the 3-way handshake, the client says that it supports SACK. What does that stand for? (See RFC 2018.)
  6. Does the server also support SACK?
  7. Will there be a problem if one side supports SACK and the other side does not?
  8. What is the client's FTP user name?
  9. What is the client's FTP password?
  10. Does the client use Active or Passive FTP?
  11. Does the server support the mode (active or passive) that the client requests?
  12. In packet 17, what is odd about the server saying, "Entering passive mode, 192,168,192,55,4,89?" (What kind of address is 192.168.192.55? See RFC 1918 if you don't know.)
  13. What does the 4,89 mean in packet 17? (What port number does it equate to?)
  14. What command did the user issue to cause packet 18?
  15. Explain packet 19.
  16. Explain packet 20.
  17. How many times does the client resend the TCP segment in packet 19?
  18. At approximately what time intervals does the client resend the TCP segment in packet 19?
  19. What do you think the user is doing at this point and what is she seeing on her screen?
  20. Given the information that we can gather from this trace file, what do you think went wrong and why? If you don't have any ideas, ask for help.

Submission

Submit your answers to me. It's that easy this week! :-)