Computer  Science 881
                         Quiz A           Name _______________________

1. In the "slow start" congestion avoidance algorithm the size of the
   congestion window "cwnd" is initially set to

   a. 1 byte.                           b. 1 * the offered window

   b. 1 * the max segment length.       d. 64K.

2. 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

3. When slow-start and congestion-avoidance are performed together
   the size of cwnd typically

   a. Increases at the same rate in     b. Increases faster in slow start
      both the slow start and the          than in congestion avoidance.
      congestion avoidance phases.

   c. Increases faster in congestion    d. Stays the same in both
      avoidance than in slow start.

4. After a packet timeout and retransmission has occurred, the value
    "ssthresh"  signifies:

   a. The cwnd size at which the        b. The cwnd size at which the
      changeover from slow start to        changeover from congestion
      congestion avoidance will occur.     avoidance to slow start will
                                           occur.

   c. The maximum size of cwnd that     d. The minimum cwnd size allowed
      will be allowed on this connection   for the next hour.
      for the next hour.

5. The function of the TCP persist timer is to:

    a. Ensure that the TCP at the other  b. Ensure that a packet that
       end has not crashed when no data     has timed out is eventually
       have been exchanged for a long       delivered.
       time.

    c. Recover from lost window size     d. Recover from half-closed
       update packets.                      sessions.

6. The function of the TCP Keepalive timer is to:

    a. Prevent the TCP at the other     b. Ensure that a packet that
       end from crashing when no data      has timed out is eventually
       have been exchanged for a long      delivered.
       time.

    c. Recover from lost window size    d. Recover from half-closed
       update packets.                     sessions.

7. Suppose that two TCP's are connected by a transcontinental link
    that has supports a transmission rate of 100 MegaBytes/second
    but has a two way round trip transit time of 50 millisecs.

    a. What is the maxium effective throughput (in bytes per second)
       on this channel if the receiver offers a window of 50KBytes.




    b. What window size is necessary to achieve 100 MB througput.




12. Suppose MSS = 256 bytes, cwnd = 2048 bytes and an ack is received for a
    new 256 bytes during congestion avoidance.  What will be the new value
    of cwnd. (Ignore the 1/8 BSD bug/hack).





13. Suppose MSS = 256 bytes, cwnd = 1536, snd.seq = 4096, snd.ack = 3072

    a. If the segment that has sequence # 3072 was actually lost, how
       many more segments can the sender send before having to stop.
       (Assume NO timeout occurs).



    b. Suppose that the sender has stopped when the 3rd duplicate ack
       arrives. What the receipt of the third duplicate ack cause
       cwnd and ssthresh be set to.

       cwnd -



       ssthresh -



    c. How many additional duplicate acks will have to be received
       before a NEW segment can be sent.





    d. Assuming that ONLY the 3072 segment was lost what will be the
       first ACK for new data.




    e. What will cwnd be set to when the first ack for new data arrives