ITEC 350: Introduction To Computer Networking Midterm Exam #2 Key. Fall 2008

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1 ITEC 350: Introduction To Computer Networking Midterm Exam #2 Key Closed book and closed notes. Fall 2008 No electronic devices allowed, e.g., calculator, laptop, PDA. Show your work. No work, no credit. Ambiguous answers will be marked wrong. Problem Area Points Score 1 General: True/False 20 2 Web Apps 10 3 Sockets 20 4 Distributed Computing 20 5 P2P/Attacks 20 6 Security 10 Total 100 In taking this exam, you will be expected to abide by the Radford University Honor Code. Name: Signature: Date: 1

2 1. General Knowledge: 20 POINTS Clearly circle your answer. An ambiguous answer will be counted as a wrong answer. CGI is a client side technology. JSP uses <% and %> to indicate where java code is interpreted. HTTP is not used with AJAX. AJAX works by using javascript to manipulate the DOM. Servers always have to connect with a client to receive requests. A TCP connection is started with an ACK message. There are 2 layers added on top of your data when sent from an Ethernet lan to the internet. IP headers provide guaranteed in-order delivery. TCP headers provide congestion control. IP packets cannot be fragmented. UDP is more bandwidth intensive than TCP. All TCP traffic must be ACK d before it is stored in a buffer. The UDP protocol guarantees packet delivery. Header information can be used to provide TCP like features for UDP. Threaded code introduces no new challenges to client/server development. Java RMI requires that you write socket code for client/server communication. An interface in RMI is equivalent to a protocol with socket code. Network performance is significantly degraded using RMI. Traditional file sharing applications rely on a centralized server. There are no provably unbreakable methods of encryption. 2

3 2. Web 10 POINTS List three reasons why you would choose to develop a network application using http as the base protocol over writing custom socket programming or a distributed computing application. Installation of the application is not required. It is a widely known and accepted method of application creation. You are concerned with cross platform delivery. Given a website that displays maps, describe how AJAX could be used to dynamically update the map by highlighting a route between two destinations that the user specifies. Note: no code is expected, only a description of all of the technologies used. Whenever the user enters the routes and hits a button, a javascript function is called on the client that sends the information to the server using an XMLHTTPRequest function. Next the server responds with the route and a java script function is called with this information. The javascript function uses the DOM to update the web page with the correct route. Describe the process that most CGI applications use to send information from a client to a server. Make sure you specify how the information is gathered, the formatting of each variable, and the how the server parses the information. CGI applications start by having a form element on a webpage with a button for submitting the information. Once this is pressed a URL with special formatting is used to send the information over to the client. This is in the form of The server then uses a finite state machine to parse the url and to pull off the first variable denoted by a? and each subsequent variable denoted by a &. 3

4 3. Sockets 20 POINTS What are the two different protocols that you can send data with over a network in Java? What is the difference between them? UDP and TCP. UDP is connectionless, and has a smaller header than TCP. TCP requires a connection between a client and a server, provides in order delivery of packets, guaranteed delivery, and congestion control. Explain how the header information for TCP allows for in order delivery, guaranteed delivery, and congestion control. The TCP header allows for in order delivery by having synchronization and acknowledgment numbers as header information. These are also used for guaranteed delivery because if the receiver does not reply back then the sender knows to resend the message. Lastly, congestion control information is also included in the header. What is one strategy for reducing the number of packets being sent over the network? What java class names are used to implement the strategy? Using buffers to condense information sent over the internet is one method to reduce the number of packets being sent. BufferedReader and BufferedWriter are the java class names that can be used to buffer what is sent through the socket streams. Describe a strategy that can be used to add in order delivery to UDP packets. In order delivery can be accomplished by adding header information to the UDP packet saying this is packet X of Z. 4

5 4. Distributed Computing 10 POINTS At a high level describe how distributed computing allows for network applications to be written in a manner that isn t oriented toward data, as is the case with socket communication. Distributed computing uses interfaces to describe the functionality that is possible from a remote machine. This allows for programming to happen at a higher abstraction level than worrying about a stream of bytes. What are three potential problems that developers face when writing distributed applications? Three problems are is the technology secure, what impacts does this have on performance, and is it available for all of the platforms I am targeting. What does the following string mean when used with Java RMI: rmi://localhost/databaseserver This string means that on the local machine an object has either been bound to the name DatabaseServer and can be used by a remote computer, or it is about to be bound and available to be used by remote computers. List three disadvantages of using Java RMI. It only works with Java programs. It sometimes doesn t work across operating systems. It isn t as fast as pure sockets, even though it is close. 5

6 5. P2P / Attacks 20 points What is the difference between a centralized server and a decentralized server in P2P networks? If a centralized server is taken down the P2P network can no longer function. With decentralized servers, it is possible to have multiple instances of the network running at the same time, preventing a single point of failure. Describe the basic process of how BitTorrent works. Make sure you list the three major components that are used with this protocol. Also, name two legitimate uses for this protocol. Two legitimate uses of this protocol are to reduce bandwidth costs for sharing legal content, and for providing faster download speeds when several people download the file at the same time. Describe three different ways used to infect computers with malicious software. Operating system exploits, web browsers, and clients are three ways to infect computers. Describe three different behaviors that machines infected with malicious software exhibit. They provide pop-up ads, perform actions requested by a remote controller, and possibly lose or have data corrupted. 6

7 6. Security 20 POINTS Describe three methods of encrypting information. Three methods of encrypting information are the Caesar cipher, the transposition cipher, and public/private key encryption. Describe three ways to defeat a man in the middle attack. Using any of the following can defeat a man in the middle attack: a gas sealed pipe, certificate authorities, checking to see where traffic came from. Describe the process for encoding and decoding a message using public / private key encryption that guarantees that it comes from a specific sender and can only be read by the recipient. The sender encrypts the message with their private key, then they then encrypt the message with the recipients public key. They then send the message to the recipient. The recipient uses their private key to decode the message. After this, the recipient uses the public key of the sender to decrypt the message. Given a Caesar cipher with a shift of 3 to the right when encoding and a shift of 3 to the left, decode the following message: d fruuhfw ghfrgh a correct decode 7