NET ID ___________ 1/15 CS519, Final Exam (May 18, 2004) NAME: _____________________________________________ NETID: ____________ You have 90 minutes to complete the test. Note that there are a lot of questions, so if you get stuck on a question, move on and come back to it later. If you think you can answer a question without looking at your notes, you should probably do so and only go back and check your notes after you have completed the test. It is possible to answer all questions in the space provided. You are free to use more space (margins, whatever), but if you feel you have to then it probably means you are giving a more complex answer than necessary. Zero points will be awarded for any question left blank.NET ID ___________ 2/15 Q1. 1 point Write your NET ID at the top of every odd numbered page of this test. Q2. 5 points Imagine that you are designing a network, and you wish to maximize link utilization on this network. Check all that apply: 1. _______ Configure the queue sizes in your routers to be large 2. _______ Reduce the bandwidth of your links 3. _______ Deploy a traffic-sensitive dynamic routing algorithm 4. _______ Disable TCP congestion control on your hosts 5. _______ Enable fair queuing in your routersNET ID ___________ 3/15 Q3. 5 points Network “Power” is defined as Power = Load / Delay. Imagine that the most critical use of your IP network is high-quality voice telephony. In what region of the power curve shown below should you operate your network. Check one, and state your reasoning (and any assumptions): ____ Region 1 ____ Region 2 ____ Region 3 ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ PowerLoad Region 1 Region 2 Region 3NET ID ___________ 4/15 Q4. 6 points Imagine you have three identical (but separate) wide area IP networks. Each network’s users have broadband access to the network (around 400kbps), and the network itself is composed of high capacity links (several hundred Mbps). Assume that the first network is used for only voice over IP. The second network is used only for web browsing. The third network is used only for file transfer of full-length movies. Assume that the three networks all have the same volume of traffic (though not necessarily the same number of users), and that the network capacity is sufficient to handle the average load. Now imagine that you can configure the queue buffer sizes in the routers. How would you configure the buffer sizes in order to have the same loss rates in all three networks (as measured in bytes dropped)? Check one: 1. _______ Identical buffer sizes across all three networks 2. _______ Different buffer sizes across all three network If you checked number 2 (different queue sizes), which of the three networks (voice, movie, web) should have the largest queue sizes? __________________________________________________ Explain your reasoning, and assumptions (if any), regardless of which number you checked: ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________NET ID ___________ 5/15 Q5. 6 points In the box below, on the left, are listed three TCP header fields. On the right are five functions of TCP. Identify which TCP fields provide which functions (either explicitly or implicitly) by drawing lines between each field and the function or functions it provides. Note that any given field or function may have zero, one, or more than one line attached to it. SequenceNum Flow Control Demultiplexing Acknowledgement Congestion Control Byte Ordering AdvertisedWindow Reliable Byte Delivery Q6. 5 points In the TCP congestion window graph below, label the points where the pipe is most likely empty (i.e., no packets in transit from sender to receiver, and no ACKs in transit from receiver to sender). time cwnd windowNET ID ___________ 6/15 Q7. 6 points Consider two hosts running TCP over a satellite link. The satellite link has a one-way latency of 100ms, and a bandwidth of 100Mbps which is a dedicated channel for the two hosts (by dedicated, I mean that only those two hosts use the channel…they get the full 100Mbps). Assume that the two hosts are using standard TCP slow-start and AIMD. Assume that the MSS (Maximum Segment Size) is 1000 bits (that is, the size of the packets transmitted by the sending host is 1000 bits). (Note that normally packet size is specified as bytes, but I’m using bits here just to make the arithmetic easier.) Finally, assume that the Advertised Window of the receiver is 160 Mbits. From the beginning of transmission, approximately how long does it take for the throughput of the TCP connection to reach its maximum speed? Select one: _______ 100ms _______ 200ms _______ 1.5 sec _______ 3 sec _______ 6 sec Q8. 5 points For the scenario of question Q7, what is the approximate average throughput achieved? _________________________________________________NET ID ___________ 7/15 Q9. 6 points For the scenario of question Q7, imagine that you can change the values of the MSS and the Advertised Window to any legal value (i.e. any value allowed by IP and TCP protocols). What values (if any) would you choose for these two parameters to maximize performance, and what effect would those changes have on performance? New MSS value (if any)______________ Effect: ________________________________________________________ ______________________________________________________________ ______________________________________________________________ New Advertised Window value (if any)______________ Effect: ________________________________________________________ ______________________________________________________________ ______________________________________________________________NET ID ___________ 8/15 Q10. 6 points Assume that a router has three inputs, 1, 2, and 3, and one output. Assume that input 1 is four times faster than inputs 2 and 3, and that the output is considerably slower than any input. Assume three flows, each arriving on a separate