Computer Networks - TDC 463
November 1, 1999
Homework Set #3
Due November 8, 1999 by the start of class

1) Visit http://www.iana.org/numbers.html and go to the the 'P' in the index
   on the page.  What are the well know ports, the registered ports and the
   dynamic/private ports (the actual numbers used)?  View the port list
   (warning, large download, approximately 311KB).  Skim through the first
   100 or so defined port numbers.  See any you recognize?  Which port is
   reserved for telnet?

   Answer:
   Well known ports:          0 through 1023
   The registered ports:      1024 through 49151
   The dynamic/private ports: 49152 through 65535
   Telnet:                    23

2) Assume station A is sending a TCP segment to station B with sequence
   number 'x' and acknowledgement 'y'.  Assume everything went OK, what
   should the sequence and acknowledge numbers be in the segment from B
   back to A?

   Answer: x+1 and y

3) Answer Tanenbaum 6.14

   Answer: No.  A connection is identified only by its sockets.
           Thus (1,p) - (2,q) is the only possible connection between those
           two ports.

4) Answer Tanenbaum 6.15

   Answer: The entire TCP segment must fit in the 65,535-byte payload field
           of an IP packet.  Since the TCP header is a minimum of 20 bytes,
           only 65,515 bytes are left over for TCP data.

5) Answer Tanenbaum 6.19

   Answer: The next transmission will be 1 maximum segment size.  Then 2, 4
           and 8.  So after four successes, it will be 8 KB.

6) If UDP does not provide error control or flow control, why use it at all?

   Answer: You may still need to provide multiplexing to applications above
           the IP layer.

7) What grade of cabling does DSL require?

   Answer: None really.  Whatever POTS runs over.  We joked and said it was
           category 0.  Typically its a pair of 24 or 26 gauge copper wires,
           usually twisted at least a little.  Shielding is not expected.
           In fact, the pair is usually unpredictable.

8) What problem does a three-way handshake solve?

   Answer: The exchange of sequence of numbers between two hosts.

9) Congestion control (flow control) can be implemented at the end stations,
   such as with TCP, but also in network devices (i.e. routers) to control
   traffic.  Discuss the differences, advantages and disadvantages of doing
   flow control at the end points versus in the network.

   Answer: TCP in end stations uses network heuristics (implicit feedback)
           to adjust to network condiditions.  Congestion control in the
           internetwork devices usually respond to watermarks (buffer and
           queue lengths) and sometimes specific fields in packets such as
           the DE bit in frame relay.

           TCP requires that everyone plays by the rules.  Internetwork
           devices can enforce a "policy" based on network-wide settings.

           Note: Other answers accepted, this is just an idea of what I
                 was looking for.

10) The advertised window field in TCP tells a sender
    a. the number of  bytes the receiver received 
    b. how many more bytes the receiver can receive 
    c. the number of  bytes the receiver acknowledged 
    d. the current receive buffer size at the receiver 

    Answer: b