Computer  Science 826
                        Quiz 5  -         Name ________________________

1. Consider a multihop channel with the following bit rates:

      16Kb         8Kb        2Kb         8Kb
   A -------- B -------- C --------- D -------- E

   Suppose packets are 1 Kbit in size and that host A injects
   packets at a rate of 12 packets per second.

   a. At what rate in packets / second will packets accumulate
      (or have to be discarded) at router B.




   b. At what rate in packets / second will packets accumulate
      (or have to be discarded) at router C.




   c. At what rate in acks / second will acks be delivered
      to host A.




2. Suppose the round trip transit time is 20 msec and the output
   bit rate is 8,000,000 bits / second.  How large must the offered
   window be in BYTES for a sender to be able to send continuously
   without having to stop and wait for ACKS.








Suppose a TCP connection has 32,000 bytes of dedicated input
buffer space.  The present value of recv.nxt is 100,000 but
the receiving application has consumed only up to byte 80,000.

3. In the next OUTGOING packet on this connection the value of
   "ack" and "win"  should be:

   a. (80000,32000)                        b. (100000,32000)

   c. (80000,12000)                        d. (100000,12000)

4. Suppose a TCP packet arrives with ACK=1600, WINDOW=1800 and that
   the recipient has state variable SND.NXT=2800.

   a. What is the size of the offered window?



   b. What is the size of the usable window?



5. Suppose a sender in the state snd.una = 256 and snd.nxt = 1024
   receives an a packet with ack=512,window=256. Did the other end
   just shrink the window? (Assume the sender is acting correctly).

   a. no                       b. yes

   c. impossible to tell from
      data given.

6. For the following examples, answer (Yes) or (No) depending upone
   whether it represents the reprehensible practice of "shrinking
   the window"

   ___ (1)     <--- ack=4096,wnd=2048
                    seq=4096,size=1024 -->
               <--- ack=5120,wnd=512

   ___ (2)     <--- ack=4096,wnd=2048
                    seq=4096,size=2048 -->
               <--- ack=6144,wnd=0

7. TCP's timeout and retransmission procedure

   a. Adds 1 second to the timeout      b. Doubles the timeout delay
      delay each time the same packet      up to a maximum of 64 secs.
      times out up to a limit of 6 sec.

   c. Adds 1 second but with no upper   d. Doubles the interval with
      bound.                               no upper bound.

8. The primary objective the "slow start" mechanism is to

   a. Ensure that the throughput        b. Prevent a fast sender from
      obtained doesn't exceed              overwhelming a slow receiver
      available bandwidth

   c. Prevent a rapid buildup of        d. All of the above apply
      packets at a bottleneck router       equally

9. When slow-start is being performed, each time a full congestion window
   of segments is sent and acked, the size of cwnd typically

   a. Increases by 1 byte               b. Increases by 1 * MSL

   c. Doubles                           d. Stays the same

10. The main objective of Jacobson's new RTO estimator was to

   a. improve the long term estimation  b. improve the mean estimators
      of the mean response time.           ability to track short term
                                           changes in the true mean RTT.

   c. Weight the RTT estimator with
      a reasonable estimate of the
      variance.

11. Karn's improvment to Jacobson's new algorithm for computing RTT and
    RTO was to:

    a. Multiply the variance           b. Only time one outstanding packet
       computed by Jacobson by 4.         at a time rather than trying to
                                          time all of them.
    c. Change the resolution of the
       timer used in the computation.  d. Ignore ACK's for retransmitted
                                          packets in computing RTT and RTO.