1. What is a Computer Network? interconnected collection of autonomous computers connected by a communication technology

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1 Review Questions for exam Preparation ( ) 1. What is a Computer Network? interconnected collection of autonomous computers connected by a communication technology 2. What is the Internet? "network of networks collection of networks interconnected by routers a communication medium used by millions ( , chat, Web surfing, streaming media 3. What is the difference between a host and an end system? There is no difference. Throughout this course (textbook), the words host and end system are used interchangeably. End systems include PCs, workstations, Web servers, mail servers, PDAs, Internet-connected game consoles, etc. 4. Why are standards important for protocols? Standards are important for protocols so that people can create networking systems and products that interoperate 5. What s a protocol? The rules used for communication Proper, accepted, and expected behavior 6. Explain client/server model client host requests, receives service from always-on server e.g. Web browser/server; client/server - Client/server model has well-defined roles for each. 7. What is q peer-to-peer model : No fixed clients or servers Each host can act as both client and server at any time 8. Describe the most popular wireless Internet access technologies today. Compare and contrast them. a) Wifi (802.11) In a wireless LAN, wireless users transmit/receive packets to/from an base station (i.e., wireless access point) within a radius of few tens of meters.

2 The base station is typically connected to the wired Internet and thus serves to connect wireless users to the wired network. b) 3G and 4G wide-area wireless access networks. In these systems, packets are transmitted over the same wireless infrastructure used for cellular telephony, with the base station thus being managed by a telecommunications provider. This provides wireless access to users within a radius of tens of kilometers of the base station. 9. What is the abbreviation for each of the following WAN: Wide Area Network (WAN) LAN: Local Area Network MAN: Metropolitan Area Network (MAN) 10. Define the former concepts and give differences between them; give example for each WAN: Spans a large geographic area, e.g., a country or a continent ; A WAN consists of several transmission lines and routers ; Internet is an example of a WAN MAN: City sized : tens of kilometers; A Cable TV Network is an example of a MAN LAN: company/univ local area network (LAN) connects end system to edge router Ethernet: shared or dedicated link connects end system and router (a few km) 10 Mbps, 100Mbps, Gigabit Ethernet widespread deployment: companies, univ, home LANs 11. The following figure shows the components of data communication systems; explain what is meant by each part

3 12. List the OSI and TCP/IP layers 13. What is Encapsulation, Layering and data 14. You must be able to answer questions related to your HW1, Client server application program. 15. What is a socket and how does it work (clients, servers to receive messages, process must have identifier host device has unique 32-bit IP address

4 Sockets process sends/receives messages to/from its socket socket analogous to door sending process shoves (batches) message out door sending process relies on transport infrastructure on other side of door to deliver message to socket at receiving process application process socket application process controlled by app developer transport transport network link Internet network link controlled by OS physical physical Application Layer Does IP address of host on which process runs suffice for identifying the process? no, many processes can be running on same host identifier includes both IP address and port numbers associated with process on host. example port numbers: HTTP server: 80 mail server: 25 to send HTTP message to gaia.cs.umass.edu web server: IP address: port number: 80 more shortly 17. What transport service does an app need? data integrity some apps (e.g., file transfer, web transactions) require 100% reliable data transfer other apps (e.g., audio) can tolerate some loss timing some apps (e.g., Internet telephony, interactive games) require low delay to be effective

5 throughput some apps (e.g., multimedia) require minimum amount of throughput to be effective other apps ( elastic apps ) make use of whatever throughput they get security encryption, data integrity, 18. What is TCP and UDP; compare between UDP and TCP Internet transport protocols services TCP service: reliable transport between sending and receiving process flow control: sender won t overwhelm receiver congestion control: throttle sender when network overloaded does not provide: timing, minimum throughput guarantee, security connection-oriented: setup required between client and server processes UDP service: unreliable data transfer between sending and receiving process does not provide: reliability, flow control, congestion control, timing, throughput guarantee, security, or connection setup, Q: why bother? Why is there a UDP? Application Layer What is Non-persistent HTTP and persistent, what is the difference between them. Look to slides (ch2-27 up to ch2-30) 20. What IS Web caches (proxy server) and how does is work Refer to slides ch2 (47-49) 21. You must understand how to calculate access link Utilization, delay, with cache Look to slides 50-52

6 Caching example: install local cache Calculating access link utilization, delay with cache: suppose cache hit rate is % requests satisfied at cache, 60% requests satisfied at origin access link utilization: 60% of requests use access link data rate to browsers over access link = 0.6*1.50 Mbps =.9 Mbps utilization = 0.9/1.54 =.58 total delay = 0.6 * (delay from origin servers) +0.4 * (delay when satisfied at cache) = 0.6 (2.01) (~msecs) = ~ 1.2 secs less than with 154 Mbps link (and cheaper too!) institutional network public Internet 1.54 Mbps access link 1 Gbps LAN origin servers local web cache Application Layer You must review and understand Socket programming with UDP (slide 58) 23. Suppose we wanted to do a transaction from a remote client to a server as fast as possible. Would you use UDP or TCP? Why? Answer: we would use UDP. With UDP, the transaction can be completed in one roundtrip time (RTT) - the client sends the transaction request into a UDP socket, and the server sends the reply back to the client's UDP socket. With TCP, a minimum of two RTTs are needed - one to set-up the TCP connection, and another for the client to send the request, and for the server to send back the reply. 24. Recall that TCP can be enhanced with SSL to provide process-toprocess security services, including encryption. Does SSL operate at the transport layer or the application layer? If the application developer wants TCP to be enhanced with SSL, what does the developer have to do? Answer: SSL operates at the application layer. The SSL socket takes unencrypted data from the application layer, encrypts it and then passes it to the TCP socket. If the application developer wants TCP to be enhanced with SSL, she has to include the SSL code in the application. 25. What is meant by a handshaking protocol? Answer: 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.

7 26. Describe how Web caching can reduce the delay in receiving a requested object. Will Web caching reduce the delay for all objects requested by a user or for only some of the objects? Why? Answer: Web caching can bring the desired content closer to the user, possibly to the same LAN to which the user s host is connected. Web caching can reduce the delay for all objects, even objects that are not cached, since caching reduces the traffic on links. 27. Consider an e-commerce site that wants to keep a purchase record for each of its customers. Describe how this can be done with cookies. Answer: 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.