M.Sc. Computer Science
Computer Systems
Additional Questions on Week 9 Materials – Part II
Question #1: List five non-proprietary Internet applications and the application-layer protocols that
they use.
a) The Web: HTTP
b) file transfer: FTP
c) remote login: Telnet
d) e-mail: SMTP
e) BitTorrent file sharing: BitTorrent protocol
Question #2: What information is used by a process running on one host to identify a process running
on another host?
The IP address of the destination host and the port number of the socket in the destination process.
Question #3: Assume you open a browser and enter http://yourbusiness.com/about.html in the address
bar. What happens until the webpage is displayed? Provide details about the protocol(s) used and a
high-level description of the messages exchanged.
In the first step, the domain name “yourbusiness.com” is translated into its IP address using DNS
servers. This could be a multi-step process involving local DNS server, root servers, TLD servers and
the Authoritative DNS servers.
Once the translation is complete, the client contacts the HTTP server (using the IP address) for
transferring the webpage. For the case of non-persistent HTTP connections, steps are outlined below.
1. The HTTP client process initiates a TCP connection to the server “yourbusiness.com” (using its IP)
on port number 80, which is the default port number for HTTP. Associated with the TCP connection,
there will be a socket at the client and a socket at the server.
2. The HTTP client sends an HTTP request message to the server via its socket.The request message
includes the path /about.html.
3. The HTTP server process receives the request message via its socket, retrieves the object /about.html
from its storage, encapsulates the object in an HTTP response message, and sends the response
message to the client via its socket.
4. The HTTP server process tells TCP to close the TCP connection.
5. The HTTP client receives the response message. The TCP connection terminates. The message
indicates that the encapsulated object is an HTML file. The client extracts the file from the response
message, examines the HTML file, and displays the webpage on the client side.
http://yourbusiness.com/about.html
Application layer protocols: DNS and HTTP
Transport layer protocols: UDP for DNS; TCP for HTTP
Question #4: What is meant by a handshaking protocol? Do HTTP and SMTP use handshaking?
A protocol uses handshaking if the two communicating entities first exchange control packets before
sending data to each other. SMTP uses handshaking at the application layer whereas HTTP does not.
Question #5: How can websites keep track of its users?
When the user first visits the site, the server creates a unique identification number, creates an entry in
its back-end database, and returns this identification number as a cookie number. This cookie number is
stored on the user’s host and is managed by the browser. During each subsequent visit (and purchase),
the browser sends the cookie number back to the site. Thus the site knows when this user (more
precisely, this browser) is visiting the site.
Question #6: What does a stateless protocol mean? Is IMAP stateless? What about SMTP?
Stateless Protocols are the type of network protocols in which Client send request to the server and
server response back according to current state. It does not require the server to retain session
information or a status about each communicating partner for multiple request.
HTTP (Hypertext Transfer Protocol), UDP (User Datagram Protocol), DNS (Domain Name System)
are the example of Stateless Protocols.
IMAP is a stateful protocol, because the IMAP server must maintain a folder hierarchy for each of its
users, this state information persists across a particular user’s successive accesses to the IMAP server.
Note that SMTP is a stateless protocol as the mail server does not maintain any connection with the
client, it does not store any information about the client. If an email is asked to be sent twice, the server
will resend it without saying that the email has been sent.
Question #7: Suppose that your department has a local DNS server for all computers in the
department. You are an ordinary user (i.e. not a network/system administrator). Can you determine if an
external Web site was likely accessed from a computer in your department a couple of seconds ago?
Explain.
Yes, we can use dig to query that Web site in the local DNS server. For example, “dig cnn.com” will
return the query time for finding cnn.com. If cnn.com was just accessed a couple of seconds ago, an
entry for cnn.com is cached in the local DNS cache, so the query time is 0 msec. Otherwise, the query
time is large.
Question #8: Why are MX records needed? Would it not be enough to use CNAME record?
MX Record for domain.com = mail.domain.com
A Record for mail.domain.com = 123.123.123.123
The MX lookup for domain.com would return mail.domain.com (which then given an A record lookup
would return 123.123.123.123)
The A record simply points the host name to the IP address, and it can be used for any host not just
mailservers. The MX record is specific to Mail eXchange.