CS337
Midterm 1
Ondich
Due in class on Friday, February 20, 1998
This exam is open-book, open-notes, open-computer, open-Internet,
but closed-other-people. You may, however, ask Jeff Ondich
questions. In particular, you should feel free to ask to use
reference materials he may have in his office.
This exam is part normal exam and part scavenger hunt. Some
of the questions are easy to answer if you have a pen and a
piece of paper, while others will require you to do some
research, probably of the Web browsing variety. Answer the
questions as completely and clearly as you can. Have fun.
- Suppose you want to use the UNIX command telnet
as an FTP client, and your goal is to retrieve a file called
gobackn.c from the directory ~/jondich in
the account networks on turing.mathcs.carleton.edu.
What UNIX command (including command-line arguments) will
you execute? Once connected to turing's FTP server, what
do you need to type to retrieve the file in question?
- What is RSA 129? In whose column was it introduced?
Who solved it, when, and how? What are the magic words, and what
do they mean?
- Suppose N is the 129-digit product of a 65-digit prime and a
64-digit prime. If you try to factor this number by computing
N mod k for each odd number k starting with 3 and stopping when
N mod k = 0, how long will it take? Pretend that your computer
can compute N mod k for a billion different values of k each second.
- In what contexts is Manchester encoding used? Why is it
used instead of a high voltage for 1 and a low voltage for 0?
- If you want to make your Hayes-compatible modem answer the
phone on the fourth ring, what command would you send it?
What's the largest number of rings you could make the modem
wait before answering?
- In the context of data link protocols, we have seen
at least two definitions of "efficiency." Give two definitions
of efficiency.
- Suppose two hosts on an 802.3 network are trying to transmit.
Their first attempt results in a collision. If no other hosts
try to transmit, what is the probability that the two hosts in
question will collide at least two more times?
- If you are sending this exam (an ASCII HTML file) over
a network that uses
the bit stuffing scheme described on page 181 of Tanenbaum,
are there characters or combinations of characters that would
require bit stuffing? Explain.
- If you search for the string "pr-01" in the
RFC
index, you will find some pretty interesting documents. Why?
What's your favorite RFC you found this way?
- Concerning the Go-Back-N algorithm on pages 210-211 of Tanenbaum.
- Suppose you eliminated all timer-related code instructions
from Tanenbaum's code. Under what circumstances could the resulting
code reach deadlock (that is, a condition where both sender and
receiver are waiting for events that can't occur).
- Even if you don't modify Tanenbaum's code, there's a way
to reach a form of deadlock. What is it?
- Last week, I told several of you that you could implement
Tanenbaum's code using just one timer instead of one timer per
entry in the sender's window. Suppose you used a single timer,
and reset it every time you sent a frame. Would the resulting
protocol work? How would it differ in performance from
the one-timer-per-window-entry version?
Jeff Ondich,
Department of Mathematics and Computer Science,
Carleton College, Northfield, MN
55057,
(507) 646-4364,
jondich@carleton.edu