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EEC-484/584 Computer Networks


EEC-484/584 Computer Networks Lecture 7 Wenbing Zhao (Part of the s are based on Drs. Kurose & Ross s s for their Computer Networking ... – PowerPoint PPT presentation

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Title: EEC-484/584 Computer Networks

EEC-484/584Computer Networks
  • Lecture 7
  • Wenbing Zhao
  • (Part of the slides are based on Drs. Kurose
    Rosss slides for their Computer Networking book,
    and on materials supplied by Dr. Louise Moser at
    UCSB and Prentice-Hall)

  • DNS Resource Records
  • Optional homework problems

Type A RR
  • Type A DNS records map a host name to an IPv4

Type NS RR
  • Type NS records within the zone file are
    authoritative records for the zone's name servers

  • A Type CNAME record maps an alias or nickname to
    the real or Canonical name

Type MX RR
  • Type MX records Specifies the name and relative
    preference of mail servers for the zone

Layered Protocols
  • Q1. If the unit exchanged at the data link level
    is called a frame and the unit exchanged at the
    network level is called a packet, do frames
    encapsulate packets or do packets encapsulate
    frames? Explain your answer.

Layered Protocols
  • Q2. A system has an n-layer protocol hierarchy.
    Applications generate messages of length M bytes.
    At each of the layers, an h-byte header is added.
    What fraction of the network bandwidth is filled
    with headers?

Reliable Communication
  • Q3. When a file is transferred between two
    computers, two acknowledgement strategies are
    possible. In the first one, the file is chopped
    up into packets, which are individually
    acknowledged by the receiver, but the file
    transfer as a whole is not acknowledged. In the
    second one, the packets are not acknowledged
    individually, but the entire file is acknowledged
    when it arrives. Discuss these two approaches.

  • Q4. Imagine that someone in the CS Department at
    Stanford has just written a new program that he
    wants to distribute by FTP. He puts the program
    in the FTP directory ftp/pub/freebies/newprog.c.
    What is the URL for this program likely to be?

Web and Cookies
  • Q5. In Fig. 7-25, keeps track of
    user preferences in a cookie. A disadvantage of
    this scheme is that cookies are limited to 4 KB,
    so if the preferences are extensive, for example,
    many stocks, sports teams, types of news stories,
    weather for multiple cities, specials in numerous
    product categories, and more, the 4-KB limit may
    be reached. Design an alternative way to keep
    track of preferences that does not have this

Web and Cookies
  • Q6. Sloth Bank wants to make on-line banking easy
    for its lazy customers, so after a customer signs
    up and is authenticated by a password, the bank
    returns a cookie containing a customer ID number.
    In this way, the customer does not have to
    identify himself or type a password on future
    visits to the on-line bank. What do you think of
    this idea? Will it work? Is it a good idea?

HTTP and Caching
  • Q7. The If-Modified-Since header can be used to
    check whether a cached page is still valid.
    Requests can be made for pages containing images,
    sound, video, and so on, as well as HTML. Do you
    think the effectiveness of this technique is
    better or worse for JPEG images ascompared to

Web and Caching
  • Q8. Consider an institutional network connected
    to the internet. Suppose that the average object
    size is 900,000 bits and that the average request
    rate from the institutions browsers to the
    origin servers is 1.5 requests per second. The
    bandwidth of the access link is 1.5 Mbps. Also
    suppose that the amount of time it takes from
    when the router on the Internet side of the
    access link forwards an HTTP request until it
    receives the response in two seconds on average.
    Model the total average response time as the sum
    of the average access delay (that is, the delay
    from Internet router to institution router) and
    the average Internet delay. (continued on next

Web and Caching
  • Q8 (contd) For the average access delay, use
    D/(1-Db), where D is the average time required to
    send an object over the access link and b is the
    arrival rate of objects to the access link.
  • Find the total average response time.
  • Now suppose a cache is installed in the
    institutional LAN. Suppose the high rate is 0.4.
    Find the total response time.

  • Q9. Under what conditions is using a CDN a bad

  • Q10. DNS typically uses UDP instead of TCP. If a
    DNS packet is lost, there is no automatic
    recovery. Does this cause a problem, and if so,
    how is it solved?

  • Q11. Although it was not mentioned in the text,
    an alternative form for a URL is to use the IP
    address instead of its DNS name. An example of
    using an IP address is http//
    html. How does the browser know whether the name
    following the scheme is a DNS name or an IP

  • Q12. Suppose within your Web browser you click on
    a link to obtain a Web page. The IP address for
    the associated URL is not cached in your local
    host, so a DNS look-up is necessary to obtain the
    IP address. Suppose that n DNS servers are
    visited before your host receives the IP address
    from DNS the successive visits incur an RTT of
    RTT1, , RTTn. Further suppose that the Web page
    associated with the link contains exactly one
    object, consisting of a small amount of HTML
    text. Let RTT0 denote the RTT between the local
    host and the server containing the object.
    Assuming 0 transmission time of the object, how
    much time elapses from when the client clicks on
    the link until the client receives the object?

Physical Layer
  • Q13. Television channels are 6 MHz wide. How many
    bits/sec can be sent if four-level digital
    signals are used? Assume a noiseless channel.

Physical Layer
  • Q14. A noiseless 4-kHz channel is sampled every 1
    msec. What is the maximum data rate? (Assuming
    each sample is 16 bits)

Physical Layer
  • Q15. What signal-to-noise ratio is needed to put
    a T1 carrier (1.544 Mbps) on a 50-kHz line?

Physical Layer
  • Q16. If a binary signal is sent over a 3-kHz
    channel whose signal-to-noise ratio is 20 dB,
    what is the maximum achievable data rate?