Computer  Science 826
                              Quiz 7           Name _______________________

1. Suppose that two TCP's are connected by a transcontinental link
    that has supports a transmission rate of 1,000,000,000 bits/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 25K Bytes.







    b. What is the MINIMUM window size in bytes needed to achieve a throughput
       of 1,000,000,000 bits / second.







2. The sequence number wrapping problem can occur in an LFN

    a. Any time a very large window b. Any time a very high transmission
       is used regardless of the       rate is used regardless of the window
       transmission rate.              size

    c. Only when high transmission
       rates and large window sizes
       are used together.

3. Large window sizes are needed

    a. Anytime Gigabyte transmission b. Any time propogation delays are
       speeds are used.                 > 500 ms regardless of bit
                                        transmission rate.

    c. Only when the product of the  d. Whenever Gigabyte transmission
       trasmission rate and the         speeds OR 500 ms propogation
       propogation delay is large.      delays exist.

4. The maximum possible window size allowed in tcp by the window
    scale factor is:

    a. 2^16 bytes                    b. 2^20 bytes

    c. 2^30 bytes                    d. 2^32 bytes

5. Which of the following is NOT a good way to recover from the
   Silly Window Syndrome

   a. Refrain from transmitting      b. Refrain from sending an ack
      until the usable window is        until available buffer space
      at least one MSS.                 is at least one MSS.

   c. Each time "N" bytes are acked
      reduce the offered window by
      "N" bytes until available buffer
      space is at least MSS.


6. a. In a Vegas TCP suppose Base_RTT = 0.100 seconds and cwnd = 10000.
      What is the "expected" throughput





   b. Suppose the ack for segment number 400000 has just been received.
      snd.nxt = 408000 and the RTT is 0.125 seconds. What is the
      actual throughput during the period between the transmission
      of segment 400000 and the receipt of its ACK.




   c. What is N_extra in bytes?




   d. Suppose the MSS is 1000. Should Vegas increase cwnd, decrease
      cwnd, or hold it constant during the next RTT.




7. The TCP persist timer is used to recover from:

   a. Half open sessions               b. Lost window updates

   c. The Silly window syndrome


8. Suppose a TCP connection is sending data at 128 Mbps.  How
   long will it take for sequence number wrap to occur?







9. The TCP keepalive timer is used to recover from:

   a. Half open sessions               b. Lost window updates

   c. The Silly window syndrome