CPSC 441 COMPUTER COMMUNICATIONS MIDTERM EXAM

Transcription

1 CPSC 441 COMPUTER COMMUNICATIONS MIDTERM EXAM Department of Computer Science University of Calgary Professor: Carey Williamson November 1, 2005 This is a CLOSED BOOK exam. Textbooks, notes, laptops, personal digital assistants, and cellular phones are NOT allowed. However, calculators are permitted. It is a 75 minute exam, with a total of 60 marks. There are 13 questions, and 8 pages (including this cover page). Please read each question carefully, and write your answers legibly in the space provided. You may do the questions in any order you wish, but please USE YOUR TIME WISELY. When you are finished, please hand in your exam paper and sign out. Good luck! Student Name: Student ID: Score: / 60 = % privacy fold here (optional)

2 Multiple Choice Choose the best answer for each of the following 7 questions, for a total of 10 marks The big three network applications in the early days of the Internet were: (a) file transfer, remote login, and network news (b) file transfer, remote login, and electronic mail (c) file transfer, electronic mail, and the Web (d) file transfer, electronic mail, and napster (e) file transfer, network news, and Ethernet 1 2. The Internet Protocol (IPv4) is an example of a: (a) connection-less network layer protocol (b) connection-oriented network layer protocol (c) connection-less transport layer protocol (d) connection-oriented transport layer protocol (e) all of the above 1 3. Which of the following is NOT a version of TCP that is used in the current Internet? (a) TCP Tahoe (b) TCP Reno (c) TCP Vegas (d) TCP Calgary (e) TCP SACK 1 4. Electronic mail ( ) service on the current Internet is commonly provided using: (a) SMTP (b) IMAP (c) POP3 (d) HTTP (e) all of the above 2

3 2 5. The persistent connection feature of HTTP: (a) allows multiple Web objects from the same Web server to be downloaded over the same TCP connection (b) maintains TCP connection state records with all Web servers that you have visited since you initially launched your browser (c) remembers the IP address of every Web server that you have visited since you initially launched your browser (d) all of the above (e) none of the above 2 6. The pipelining feature of HTTP/1.1: (a) allows Web requests to be issued asynchronously with respect to responses (b) allows the client to bundle multiple HTTP requests into the same outgoing TCP data packet (c) allows the server to bundle multiple HTTP responses into the same outgoing TCP data packet (d) all of the above (e) none of the above 2 7. The File Transfer Protocol (FTP) uses: (a) a persistent TCP connection on port 21 for a control session, and a separate nonpersistent TCP connection on port 20 for each data file transfer (b) a non-persistent TCP connection on port 21 for a control session, and a separate nonpersistent TCP connection on port 20 for each data file transfer (c) a persistent TCP connection on port 21 for a control session, and a single persistent TCP connection on port 20 for all of the data file transfers (d) a non-persistent TCP connection on port 21 for a control session, and a single persistent TCP connection on port 20 for all of the data file transfers (e) none of the above 3

4 Definitions For each of the following pairs of terms, define each term, making sure to clarify the key difference(s) between the two terms. (a) (2 marks) circuit switching and packet switching (b) (2 marks) client-server and peer-to-peer (c) (2 marks) protocol and interface (d) (2 marks) pull protocol and push protocol (e) (2 marks) flow control and congestion control (f) (2 marks) stop and wait protocol and sliding window protocol 4

5 Networking Delays 8 9. A packet traversing the Internet typically undergoes several types of delays, including nodal processing delay, transmission delay, propagation delay, and queueing delay. (a) (4 marks) Define each of these four types of delays. (b) (2 marks) Consider a 1500-byte IP packet that is sent from Calgary to Edmonton, traversing a direct point-to-point link with a transmission rate of 10 Megabits per second (Mbps). Using the simple relationship T = L/R, calculate the transmission time T to send this packet of length L on the link with rate R. Make sure to get the units correct in your calculation. Show your work. (c) (2 marks) Suppose that the foregoing link from Calgary to Edmonton is a fiber optic link that is 250 kilometers long. Using the simple relationship t = d/s, calculate the propagation time t for the first bit of the packet to travel the distance d at the speed s = 2.5 x 10 8 meters per second. Make sure to get the units correct in your calculation. 5

6 Domain Name System The Domain Name System (DNS) is used to map between human-readable host names and network-level IP addresses on the Internet. DNS uses a request-response application layer protocol for this purpose. (a) (1 mark) What user-level Unix command can be used to invoke DNS lookups? (b) (1 mark) In client-server socket programming, what system call (in C or C++) is typically used to invoke a DNS lookup? (c) (2 marks) A query to a DNS server may be handled either iteratively or recursively. Explain the difference between these two approaches, using a small diagram if necessary. (d) (4 marks) There are four different Resource Record (RR) types that can be returned in a DNS response: A, CNAME, NS, and MX. Explain each of these types, supplementing each with an example if you so wish. 6

7 Reliable Data Transfer In class, we discussed reliable data transfer (rdt) protocols, and the mechanisms required to offer rdt service in a transport layer protocol. Rdt protocols must overcome a variety of events that can occur at the underlying network layer of the protocol stack. For each of the following network-layer events, list the mechanism(s), if any, used in a transport-layer protocol to achieve reliable data transfer between a sender S and a receiver R across a network. (1 mark each, for a total of 10 marks) (a) an outbound DATA segment from S is lost in the core of the network (b) an incoming ACK segment from R is lost in the core of the network (c) the DATA segments sent from S arrive at R in an order different from that in which they were sent (d) a DATA segment from S is routed to the wrong recipient host T (e) a DATA segment from S is corrupted with some bit errors while in transit (f) an ACK segment from R is corrupted with some bit errors while in transit (g) DATA segments from S arrive more quickly than the receiver R can process them (h) DATA segments from S arrive more quickly than the network can process them (i) a DATA segment from S is too large to transit a specific link in the core of the network (j) two copies of the same DATA segment from S arrive at receiver R 7

8 Transport Layer There is a choice of transport-layer protocol available in the Internet protocol stack. One choice is the Transmission Control Protocol (TCP). Another choice is the User Datagram Protocol (UDP). (a) (2 marks) What are the similarities, if any, between these two protocols? (b) (2 marks) What are the differences, if any, between these two protocols? (c) (2 marks) Give two examples of Internet applications that use TCP. (d) (2 marks) Give two examples of Internet applications that use UDP. TCP Congestion Control In 1988, a congestion control algorithm was added to the TCP protocol to prevent congestion collapse on the Internet. This new congestion control algorithm uses two new TCP state variables called cwnd and ssthresh. (a) (2 marks) What is cwnd? How is it used in TCP s congestion control algorithm? (b) (2 marks) What is ssthresh? How is it used in TCP s congestion control algorithm? *** THE END *** 8