Advance Computer Network & Security, Homework Problems with Solutions

By | February 9, 2016

Computer Network

A computer network is a group of computer systems and other computing hardware devices that are linked together through communication channels to facilitate communication and resource-sharing among a wide range of users. Networks are commonly categorized based on their characteristics.

R11. Why do HTTP, FTP, SMTP, and POP3 run on top of TCP rather than on UDP?

  • The applications associated with those protocols require that all application data be received in the correct order and without gaps. TCP provides this service whereas UDP does not.
  • TCP is more reliable than UDP, UDP may have failures or data loss, so we can’t afford to have losses in HTTP, SMTP, pop3 and so on. Accurate data is very important is all these protocols
  • TCP is the connection-oriented protocol and UDP is the connectionless protocol. All these required connection oriented services.

R18. From a user’s perspective, what is the difference between the download-and-delete
mode and the download-and-keep mode in POP3?

            With download and delete, after a user retrieves its messages from a POP server, the messages are deleted. This poses a problem for the nomadic user(moving around a lot), who may want to access the messages from many different machines (office PC, home PC, etc).
            In the download and keep configuration, messages are not deleted after the user retrieves the messages. This can also be inconvenient, as each time the user retrieves the stored messages from a new machine, all of non-deleted messages will be transferred to the new machine (including very old messages).
            Download and delete means the e-mail message is deleted from the server once you have it on your computer. If you delete the message on your computer, the message is gone.
Download and keep mean a copy of the e-mail message remains on the server after it is downloaded to your computer. If you delete the message on your computer, the copy remains on the server.

R19. Is it possible for an organization’s Web server and mail server to have exactly the same alias for a host name (for example, What would be the type for the RR that contains the hostname of the mail server?

           Yes an organization’s mail server and the Web server can have the same alias for a host name.
The MX record is used to map the mail server’s hostname to its IP address.

Terms to Understand

What is Resource Record(RR)
Is a four-tuple that contains(Name,Value,Type,TTL)
What is MX?
Records allows the host name of mail servers to have simple aliases

P7. 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 lookup 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 zero transmission time of the object, how much time elapses from when the client clicks on the link until the client receives the object?


 P8. Referring to Problem P7, suppose the HTML file references eight very small objects on the same server. Neglecting transmission times, how much time elapses with
a. Non-persistent HTTP with no parallel TCP connections?
b. Non-persistent HTTP with the browser configured for 5 parallel connections?
c. Persistent HTTP?



P9. Consider Figure 2.12 in your book, for which there is an institutional network connected to the Internet. Suppose that the average object size is 850,000 bits and that the average request rate from institution browsers to the origin servers is 16 requests per second. 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 is three seconds on average. Model the total average response time as the sum of the average access delay (that is the delay from the Internet router to institution router) and the average Internet delay. For the average access delay use Delta/(1 – Delta * Beta). Where Delta is the average time required to send an object over the access link and Beta is the arrival rate of objects to the access link.
a. Find the total average response time.
b. Now suppose a cache installed in the institutional LAN, Suppose the miss rate is 0.4. Find the total response time


P13. What is the difference between MAIL FROM: in SMTP and From: in the mail message itself?

             The MAIL FROM: is part of the SMTP handshaking protocol, it identifies the sender of the message to the SMTP server, whereas the header field From: is simply part of the mail message body itself.
Understand in brief:
                         The difference Between MAIL FROM: and From: is that the MAIL FROM field contains only the email address, which specifies the server and address the message is coming from, while the From field gives a more human readable sender name (i.e. it translates to “Alice Example”)

P19. In this problem, we use the useful dig tool available on Unix and Linux hosts to explore the hierarchy of DNS servers. Recall that in Figure 2.21, a DNS server higher in the DNS hierarchy Delegates a DNS query to a DNS server lower in the hierarchy, by sending back to the DNS client the name of that lower-level DNS server. First, read the man page for dig, and then answer the following questions.
a. Starting with a root DNS server (from one of the root servers [a-m], initiate a sequence of queries for the IP address for your department’s Web server by using dig. Show the list of names of DNS servers in the delegation chain in answering your query.
b. Repeat part a) for several popular Websites, such as,, or


P25. Consider an overlay network with N active peers, with each pair of peers having an active TCP connection. Additionally, suppose that the TCP connections pass through a total of M routers. How many nodes and edges are there in the corresponding overlay network?

         N nodes and N×(N−1) 2 edges. The edges of the overlay network are formed by the individual TCP connections. Routers are not part of the overlay network since they operate at the lower network layer.(No justification = No points.)
Reference: Source from the Internet

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