Computer  Science 826
                         Quiz 6           Name _______________________

 1. The sequence number wrapping problem can occur:

    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.

 2. a. Suppose the TCP timestamp option is in use and segments
       768 is received.  The receiver witholds his ACK until segment
       1024 is received.  Should the ack carry the timestamp from
       768 or 1024?


   b. Suppose segment 1280 is then lost and segment 1536 arrives.
      Should the duplicate ACK carry the timestamp from 768, 1024
      or 1536?


   c. Suppose segment 1280 wasn't lost after all but just delivered
      out of order.  It arrives shortly after 1536.  Should the
      new ACK carry the timestamp form 768,1024, 1280, or 1536?



3. Suppose MSS = 200 bytes, cwnd = 1600, snd.seq = 4000, snd.ack = 3000

    a. If the segment that has sequence # 3000 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 3000 segment was lost what will be the
       leading edge of the usable window when the first ack for new
       data is received.




4. 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.





5.  Which of the following "hacks" was added in the NewReno version
    of TCP

    a. Slow start and congestion       b. Immediate retransmission
       avoidance                          in response to a partial ack.

    b. Fast retransmit and fast        d. Window scale factor option
       recovery.

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. What two problems associated with the use of drop-tail
   gateways are early random drop gateways designed to fix?

   a -


   b -