Problem Set 9 Due: Start of class, December 4

Transcription

1 CS242 Computer Networks Handout # 18 Randy Shull November 27, 2017 Wellesley College Problem Set 9 Due: Start of class, December 4 Reading: Kurose & Ross, Sections 6.1 through 6.5 Wireshark Lab [16] In this lab we investigate the Ethernet protocol. Begin by capturing a set of Ethernet frames to study. Make sure your browser s cache is empty and start up the Wireshark packet sniffer Enter the following URL into your browser Your browser should display the rather lengthy US Bill of Rights. Stop Wireshark packet capture. First, find the packet numbers of the HTTP GET message that was sent from your computer to gaia.cs.umass.edu, as well as the beginning of the HTTP response message sent to your computer by gaia.cs.umass.edu. You should see a screen that looks something like what is shown in Figure 1 (where packet 48 in the screen shot contains the HTTP GET message). Since this exercise is about Ethernet, we re not interested in IP or higherlayer protocols. So let s change Wireshark s listing of captured packets window so that it shows information only about protocols below IP. To have Wireshark do this, select Analyze->Enabled Protocols. Then uncheck the IPv4 box and select OK. You should now see an Wireshark window that looks like what is shown in Figure 2. In order to answer the following questions, you ll need to look into the packet details and packet contents windows (the middle and lower display windows in Wireshark). Exercise 1 [8]: Analyzing Ethernet frames Select the Ethernet frame containing the HTTP GET message. (Recall that the HTTP GET message is carried inside of a TCP segment, which is carried inside of an IP datagram, which is carried inside of an Ethernet frame. Expand the Ethernet II information in the packet details window. Note that the contents of the Ethernet frame (header as well as payload) are displayed in the packet contents window. 1

2 Figure 1: Packet capture of the bill of rights Answer the following questions, based on the contents of the Ethernet frame containing the HTTP GET message. Whenever possible, when answering a question you should hand in a printout of the packet(s) within the trace that you used to answer the question asked. Annotate the printout to explain your answer. a [1] What is the 48-bit Ethernet address of your computer? You verify your answer either by using nfconfig or by going to the Network Preferences on your Mac, clicking on Advanced and then the Ethernet tab. b [2] What is the 48-bit destination address in the Ethernet frame? Is this the Ethernet address of gaia.cs.umass.edu? Hint: the answer is NO. What device has this as its Ethernet address? c [2] Give the hexadecimal value for the two-byte Frame type field. What upper layer protocol does this correspond to? 2

3 Figure 2: Packet capture of the bill of rights after unchecking IP box d [3] How many bytes from the very start of the Ethernet frame does the ASCII G in GET appear in the Ethernet frame? The HTTP GET request is encapsulated in a TCP packet, which in turn is encapsulated in an IP packet, which is encapsulated in an Ethernet packet. Each encapsulation has its own header. What are the lengths of each of these headers? Does it make sense that the HTTP GET request starts where it does? Exercise 2 [8]: Ethernet frame of HTTP response Answer the following questions, based on the contents of the Ethernet frame containing the first byte of the HTTP response message. a [2] What is the value of the Ethernet source address? Is this the address of your computer, or of gaia.cs.umass.edu. What device has this as its Ethernet address? 3

4 b [1] What is the destination address in the Ethernet frame? Is this the Ethernet address of your computer? c [2] Give the hexadecimal value for the two-byte Frame type field. What upper layer protocol does this correspond to? d [3] How many bytes from the very start of the Ethernet frame does the ASCII 0 in OK (i.e., the HTTP response code) appear in the Ethernet frame? 4

5 Problems Problem 1 [9]: Link Layer Answer each of the following questions about the link layer. Briefly justify your answers. a [3]: Reliable service If all the links in the Internet were to provide reliable service, would the TCP reliable delivery service be completely redundant? Why or why not? b [3]: Collisions Suppose two nodes start to transmit at the same time a packet of length L over a broadcast channel of rate R. Denote the propagation delay between the two nodes as t prop. Will there be a collision if t prop < L/R? Why or why not? c [3]: Token-ring protocols Would the token-ring protocol be inefficient if a LAN had a very large perimeter? Problem 2 [4]: Two-dimensional parity checks Show (give an example other than the one in Figure 6.5 in the text, seventh edition) that twodimensional parity checks can correct and detect a single bit error. Show (give an example of) a double-bit error that can be detected by not corrected. Problem 3 [3]: CRC Calculation Consider the four-bit generator, G = 1001 as shown in Figure 6.7 of the text (seventh edition) and suppose that D has the value What is the value of R? Problem 4 [10]: CDMA a [3]: Single-sender CDMA Consider the single-sender CDMA example in the notes. What would be the senders output (for the two data bits shown) if the senders CDMA code was (1, -1, 1, -1, 1, -1, 1, -1)? b [3]: Two senders Consider sender 2 shown in two sender CDMA example of the notes. What is the senders output to the channel (before it is added to the signal from sender 1)? c [4]: Mixed signals Suppose that the receiver shown the two-sender example of the notes wanted to receive the data being sent by sender 2. Show (by calculation) that the receiver is indeed able to recover sender 2s data from the aggregate channel signal by using sender 2s code. 5

6 Problem 5 [15]: ALOHA In Section 5.3, we provided an outline of the derivation of the efficiency of slotted ALOHA. In this problem well complete the derivation. a [5]: Maximizing pfor Slotted ALOHA Recall that when there are N active nodes the efficiency of slotted ALOHA is Np(1 p) N 1. Find the value of p that maximizes this expression. b [5]: Efficiency of Slotted ALOHA Using the value p found in Part a, find the efficiency of slotted ALOHA by letting N approach infinity. Hint: (1 1/N) N approaches 1/e as N approaches infinity. c [5]: Efficiency of Pure ALOHA Show that the maximum efficiency of pure ALOHA is 1/(2e). Note: This problem is easy if you have completed the problem above. 6