CSE4232/5232 Spring 2006 Exam #2, open book, open notes. Name _____________

1. How would you find the IP address of the SMTP server for fit.edu?
Show the exact commands that you would type (15 pts).

  ANSWER:

  nslookup
  set type=mx
  fit.edu
  exit         (optional)

or

  nslookup -type=mx fit.edu


2. Suppose the SMTP server for cs.fit.edu is on smtphost.fit.edu.  How would
you send mail to mmahoney@cs.fit.edu using telnet to the SMTP server?  Show
the exact commands that you would type (15 pts).

  ANSWER:

  telnet smtphost.fit.edu 25
  HELO somehost.com           (optional)
  MAIL FROM:yourname@fit.edu  (sender's email address)
  RCPT TO:mmahoney            (or mmahoney@cs.fit.edu)
  DATA
  Subject: test message       (optional)

  This is a test.             (content doesn't matter)
  .
  QUIT                        (optional)


3. The propagation delay of CAT5 Ethernet cable is 200,000 km/sec (2/3 of
the speed of light).  Suppose that two computers 100 meters apart
start sending simultaneously on a 100 Mb/s link. (6 pts each).

a. What is the size of the smallest possible Ethernet packet, in bits?

  ANSWER: 576 bits (8 byte preamble + 6 byte destination address
  + 6 byte source address + 2 byte protocol + 46 byte payload
  + 4 byte checksum = 72 bytes = 576 bits).

b. How many bits will each computer transmit before detecting a collision?

  ANSWER: 50 bits.  Propagation speed is 0.2 m/ns, or 500 ns end to end.
  Each bit takes 10 ns at 100 Mb/s.  500/10 = 50.

c. What is the probability that the computers will successfully transmit
the packets in 3 retries or less, assuming no other computers transmit?

  ANSWER: 63/64.  The chance of collision on the first 3 retries are
  1/2, 1/4, and 1/8 respectively.  The chance that all 3 will collide
  is 1/2 * 1/4 * 1/8 = 1/64.

d. Suppose that the computers are connected through a switch.  How
much delay will this add, in nanoseconds, if the switch behaves optimally?

  ANSWER: 1120 ns.  The switch cannot start forwarding a packet until
  it has read the preamble (8 bytes) and destination address (6 bytes).
  (8 + 6) * (8 bits per byte) * (10 ns per bit) = 1120 ns.

e. How does a switch know where to send a packet?

  It builds a routing table by observing source addresses.  If a destination
  address is not in the table then it is broadcast.


4. What protocol provides each of the following services?
Choose from the following list: HTTP, HTTPS, telnet, FTP, SSH, SMTP,
SFTP, Ethernet, TCP, IP, UDP, ICMP, DHCP, POP3, ARP, XML, HTML, DNS, NFS.
(4 pts each).

                                                   ANSWERS
                                                   -------
a. Packet framing.                             ___ Ethernet
b. Reliable delivery.                          ___ TCP
c. Routing.                                    ___ IP
d. Resolving a host name to IP address.        ___ DNS
e. Resolving IP address to Ethernet address.   ___ ARP
                                                  
f. Assigning an IP address at boot time.       ___ DHCP
g. Encrypted communication with a web server.  ___ HTTPS
h. Encrypted communication with a remote shell.___ SSH
i. Mail delivery.                              ___ SMTP
j. Mail retrieval.                             ___ POP3