DISCUSSION QUESTIONS

IMPORTANT NOTE: Discussion questions this quarter will serve as a study opportunity. Answer the questions in your notebook or electronically. Use the notes to study for "quizzes". You do not need to submit your answers.


DQ #1

  • Be prepared to define the terms in the following list:
    • Host (client or end system)
    • Server
    • Packet Switching
    • Modem
    • Router
    • Transmission Rate
    • Network Protocols (provide three examples of protocols commonly used)
    • DSL
    • HFC (Hybrid Fiber Coax)
    • Dial-Up
    • Ethernet
    • WiFi
    • Guided Media (with examples)
    • Unguided Media (with examples)
    • LEO Satellites (with examples)

    The following link to Network Encyclopedia will help you find many definitions for the concepts above.


    DQ #2

    The following questions are taken from Kurose & Ross's "Computer Networking: A Top-Down Approach":

    1. Suppose there is exactly one packet switch between a sending host and a receiving host. The transmission rates between the sending host and the switch and between the switch and the receiving host are R1 and R2, respectively. Assuming that the switch uses store-and-forward packet switching, what is the total end-to-end delay to send a packet of length L? (Ignore queuing, propagation delay, and processing delay.)
    2. How long does it take a packet of length 1,000 bytes to propagate over a link of distance 2,500 km, propagation speed 2.5•108 m/s, and transmission rate 2 Mbps? More generally, how long does it take a packet of length L to propagate over a link of distance d, propagation speed s, and transmission rate R bps? Does this delay depend on packet length? Does this delay depend on transmission rate?
    3. Suppose Host A wants to send a large file to Host B. The path from Host A to Host B has three links, of rates R1 = 500 kbps, R2 = 2 Mbps, and R3 = 1 Mbps.
      1. Assuming no other traffic in the network, what is the throughput for the file transfer?
      2. Suppose the file is 4 million bytes. Dividing the file size by the throughput, roughly how long will it take to transfer the file to Host B?
      3. Repeat (a) and (b), but now with R2 reduced to 100 kbps.
    4. What are the five layers in the Internet protocol stack? What are the principal responsibilities of each of these layers?


    DQ #3

    From the List of cyberattacks provided in this link to Wikipedia, choose one attack, read about it, and share your findings with the class (prepare 2 slides with your main points). In this Wikipedia article, the network attacks (cyberattacks) are classified under the following categories:

    • Indiscriminate attacks
    • Destructive attacks
    • Cyberwarfare
    • Government espionage
    • Corporate espionage
    • Stolen e-mail addresses and login credentials
    • Stolen credit card and financial data
    • Blockchain and cryptocurrencies
    • Stolen medical-related data
    • Ransomware attacks
    • Notable hacktivist events


    DQ #4

    The following questions are taken from Kurose & Ross's "Computer Networking: A Top-Down Approach". Consider the following string of ASCII characters that were captured by Wireshark when the browser sent an HTTP GET message (i.e., this is the actual content of an HTTP GET message). The characters <cr><lf> are carriage return and line-feed characters (that is, the italized character string <cr> in the text below represents the single carriage-return character that was contained at that point in the HTTP header). Answer the following questions, indicating where in the HTTP GET message below you find the answer.

    GET /cs453/index.html HTTP/1.1<cr><lf>Host: gaia.cs.umass.edu<cr><lf>User-Agent: Mozilla/5.0 (Windows;U; Windows NT 5.1; en-US; rv:1.7.2) Gecko/20040804 Netscape/7.2 (ax) <cr><lf>Accept:ext/xml, application/xml, application/xhtml+xml, text/html;q=0.9, text/plain;q=0.8,image/png,*/*;q=0.5<cr><lf>Accept-Language: en-us,en;q=0.5<cr><lf>Accept-Encoding: zip,deflate<cr><lf>Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7<cr><lf>Keep-Alive: 300<cr><lf>Connection:keep-alive<cr><lf><cr><lf>

    1. What is the URL of the document requested by the browser?
    2. What version of HTTP is the browser running?
    3. Does the browser request a non-persistent or a persistent connection?
    4. What is the IP address of the host on which the browser is running
    5. What type of browser initiates this message?
    6. Why is the browser type needed in an HTTP request message?

    The text below shows the reply sent from the server in response to the HTTP GET message in the question above. Answer the following questions, indicating where in the message below you find the answer.

    HTTP/1.1 200 OK<cr><lf>Date: Tue, 07 Mar 2008 12:39:45GMT<cr><lf>Server: Apache/2.0.52 (Fedora)<cr><lf>Last-Modified: Sat, 10 Dec2005 18:27:46 GMT<cr><lf>ETag: "526c3-f22-a88a4c80"<cr><lf>Accept-Ranges: bytes<cr><lf>Content-Length: 3874<cr><lf>Keep-Alive: timeout=max=100<cr><lf>Connection: Keep-Alive<cr><lf>Content-Type: text/html; charset=ISO-8859-1<cr><lf><cr><lf><!doctype html public "-//w3c//dtd html 4.0 transitional//en"><lf><html><lf><head><lf> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><lf> <meta name="GENERATOR" content="Mozilla/4.79 [en] (Windows NT 5.0; U) Netscape]"><lf> <title>CMPSCI 453 / 591 / NTU-ST550A Spring 2005 homepage</title><lf></head><lf> <much more document text following here (not shown)>
    1. Was the server able to successfully find the document or not?
    2. What time was the document reply provided?
    3. When was the document last modified?
    4. How many bytes are there in the document being returned?
    5. What are the first 5 bytes of the document being returned?
    6. Did the server agree to a persistent connection?


    DQ #5

    The following questions are taken from Kurose & Ross's "Computer Networking: A Top-Down Approach". UDP and TCP use 1s complement for their checksums. Suppose you have the following three 8-bit bytes: 01010011, 01100110, 01110100.

    1. What is the 1s complement of the sum of these 8-bit bytes? (Note that although UDP and TCP use 16-bit words in computing the checksum, for this problem you are being asked to consider 8-bit sums.) Write down your work.
    2. Why is it that UDP takes the 1s complement of the sum; that is, why not just use the sum?
    3. With the 1s complement scheme, how does the receiver detect errors?
    4. Is it possible that a 1-bit error will go undetected? How about a 2-bit error?

    Imagine that the receiver gets the following three 8-bit bytes: 01010011, 01100110, 01100100. Using your checksum (the 1s complement you calculated in 1. above), verify that the segment arrived with integrity.

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