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\title{CSc 450/550: Computer Networks (Summer 2006)}

\author{Assignment 2}

\date{To be handed in at the lecture on June 19, 2006}

\maketitle

\begin{enumerate}
  
\item Computer A and Computer B are interconnected by a
  store-and-forward Router C and two 10 Mbps duplex communication
  links (i.e., AC and CB) of 1000 km length each. A binary file of 10
  MB is to be transferred from A to B. Signal is propagated at $10^8$
  m per second in the communication channel.

  \begin{enumerate}
    
  \item if the whole file is transferred as one big message (i.e.,
    message switching), how long does it take for B to receive the
    entire file after A starts to send the file?
    
  \item if the file is transferred in a series of 10 KB packets (i.e.,
    packet switching), how long does it take for B to receive the
    entire file after A starts to send the file?
    
  \item if it takes 10 ms for C to make the forward decision for a
    message or packet, please recalculate the time for B to receive
    the file with message or packet switching.
    
  \item please calculate the minimal buffer size required at C to
    support message or packet switching without or with the 10 ms
    processing time for each message or packet.

  \end{enumerate}
  
\item In a few sentences, please explain and compare frequency
  division multiplexing, wavelength division multiplexing, and time
  division multiplexing.
  
\item In a few sentences, please explain the scalability issue of the
  client-server model over the Internet and compare the different ways
  to improve it.

\item In a few sentences, please explain and compare recursive and
  iterative DNS query.
  
\item A client needs to send a 5-byte request message to a server over
  TCP, and the server will respond with a 10-byte reply message.
  Assume initial sequence number is 0. Please write down the sequence
  of packets exchanged between the client and the server, along with
  TCP flag(s), sequence number, and acknowledgment number in each TCP
  packet.
 
\item Please calculate the TCP/IP-style checksum for a 3-byte message
  in its hexadecimal representation: AB, CD, EF. Please also explain
  when the checksum would fail to detect possible packet errors.
  
\item Please calculate the TCP timeout value (RTO) for the following
  round-trip time (RTT) sequence: 1, 2, 3, 4, 1, 2, 3, 4. The initial
  SRTT is 1 and the initial RTTV is 0.5.
  
  \hfill {\bf (continue on the next page)}
  
\item For a simplified HTML document with URL {\texttt
    http://a.b.com/index.html}
  
\begin{verbatim}
  <html> 
  <a href=``image/fig1-big.jpg''><img src=``image/fig1-small.jpg''></a>
  <img src=``http://x.y.com/image/fig2.gif''>
  <img src=``http://a.b.com/image/fig3.gif''>
  </html>
\end{verbatim}
  
  and for a web browser that loads images (JPEG and GIF files)
  automatically, how long (in round-trip time) does it take to display
  this Web page properly (DNS overhead is omitted), if all files are
  small enough to be accommodated in one packet and:
  
  \begin{enumerate}
  \item non-persistent HTTP connections are used, or
  \item persistent HTTP connections with non-pipelining are used, or
  \item persistent HTTP connections with pipelining are used.
  \end{enumerate}
  
  How about if at most two parallel TCP connections per server can be
  used by the browser with the above three scenarios?

\end{enumerate}

\hfill {\bf --- END ---}

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