CS242 Computer Networks Handout # 18Randy Shull November 27, 2017Wellesley College

Problem Set 9Due: 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 aset of Ethernet frames to study. Make sure your browser’s cache is emptyand start up the Wireshark packet sniffer Enter the following URL into yourbrowser

http://gaia.cs.umass.edu/wireshark-labs/HTTP-ethereal-lab-file3.html

Your browser should display the rather lengthy US Bill of Rights. StopWireshark packet capture. First, find the packet numbers of the HTTPGET message that was sent from your computer to gaia.cs.umass.edu, aswell as the beginning of the HTTP response message sent to your computerby gaia.cs.umass.edu. You should see a screen that looks something likewhat is shown in Figure 1 (where packet 48 in the screen shot contains theHTTP GET message).

Since this exercise is about Ethernet, we’re not interested in IP or higher-layer protocols. So let’s change Wireshark’s “listing of captured packets”window so that it shows information only about protocols below IP. Tohave Wireshark do this, select Analyze->Enabled Protocols. Then uncheckthe IPv4 box and select OK. You should now see an Wireshark windowthat looks like what is shown in Figure 2. In order to answer the followingquestions, you’ll need to look into the packet details and packet contentswindows (the middle and lower display windows in Wireshark).

Exercise 1 [8]: Analyzing Ethernet frames Select the Ethernet framecontaining the HTTP GET message. (Recall that the HTTP GET messageis carried inside of a TCP segment, which is carried inside of an IP data-gram, which is carried inside of an Ethernet frame. Expand the Ethernet IIinformation in the packet details window. Note that the contents of the Eth-ernet frame (header as well as payload) are displayed in the packet contentswindow.

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Figure 1: Packet capture of the bill of rights

Answer the following questions, based on the contents of the Ethernetframe containing the HTTP GET message. Whenever possible, when an-swering a question you should hand in a printout of the packet(s) within thetrace that you used to answer the question asked. Annotate the printout toexplain your answer.

a [1] What is the 48-bit Ethernet address of your computer? You verifyyour answer either by using nfconfig or by going to the Network Prefer-ences on your Mac, clicking on Advanced and then the Ethernet tab.

b [2] What is the 48-bit destination address in the Ethernet frame? Isthis the Ethernet address of gaia.cs.umass.edu? Hint: the answer isNO. What device has this as its Ethernet address?

c [2] Give the hexadecimal value for the two-byte Frame type field. Whatupper layer protocol does this correspond to?

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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 theASCII ‘G’ in “GET” appear in the Ethernet frame? The HTTP GETrequest is encapsulated in a TCP packet, which in turn is encapsulated inan IP packet, which is encapsulated in an Ethernet packet. Each encapsu-lation 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 fol-lowing questions, based on the contents of the Ethernet frame containingthe first byte of the HTTP response message.

a [2] What is the value of the Ethernet source address? Is this the addressof your computer, or of gaia.cs.umass.edu. What device has this as itsEthernet address?

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b [1] What is the destination address in the Ethernet frame? Is this theEthernet address of your computer?

c [2] Give the hexadecimal value for the two-byte Frame type field. Whatupper layer protocol does this correspond to?

d [3] How many bytes from the very start of the Ethernet frame does theASCII ‘0’ in ‘OK’ (i.e., the HTTP response code) appear in the Ethernetframe?

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Problems

Problem 1 [9]: Link Layer Answer each of the following questions aboutthe link layer. Briefly justify your answers.

a [3]: Reliable service If all the links in the Internet were to providereliable service, would the TCP reliable delivery service be completelyredundant? Why or why not?

b [3]: Collisions Suppose two nodes start to transmit at the same timea packet of length L over a broadcast channel of rate R. Denote thepropagation delay between the two nodes as tprop. Will there be a collisionif tprop < L/R? Why or why not?

c [3]: Token-ring protocols Would the token-ring protocol be inefficientif a LAN had a very large perimeter?

Problem 2 [4]: Two-dimensional parity checks Show (give an exampleother than the one in Figure 6.5 in the text, seventh edition) that two-dimensional 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 thatD has the value 10101010. What is the value of R?

Problem 4 [10]: CDMA

a [3]: Single-sender CDMA Consider the single-sender CDMA examplein the notes. What would be the senders output (for the two data bitsshown) 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 ex-ample of the notes. What is the senders output to the channel (before itis added to the signal from sender 1)?

c [4]: Mixed signals Suppose that the receiver shown the two-senderexample 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 2sdata from the aggregate channel signal by using sender 2s code.

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Problem 5 [15]: ALOHA In Section 5.3, we provided an outline of thederivation of the efficiency of slotted ALOHA. In this problem well completethe derivation.

a [5]: Maximizing pfor Slotted ALOHA Recall that when there areN active nodes the efficiency of slotted ALOHA is Np(1 − p)N−1. Findthe value of p that maximizes this expression.

b [5]: Efficiency of Slotted ALOHA Using the value p found in Parta, 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 ofpure ALOHA is 1/(2e). Note: This problem is easy if you have completedthe problem above.

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