Exam in Computer Networks

Transcription

1 Exam in Computer Networks Date Course code 1DT633 Exam time 3/5 hours Problems 5/8, Total 50/80 points Teacher Lars-Åke Larzon Phone Aid English dictionary Please read the instructions below carefully. You must answer the questions instructively. Only one problem/sheet. If details needed to solve a problem have been left out, make reasonable assumptions and present them in your solution. You may answer in English or Swedish. You can not hand in solutions to all problems, but must chose one or two to exclude If you have not participated in the continuous examination, or if you have participated but failed, the following applies: Your exam time is 5 hours and you can not hand in solutions to more than 8 problems. If you have participated in, and passed, the continuous examination throughout this instance of the course, the following applies: Your exam time is 3 hours and you can only hand in solutions to at most 5 problems out of problems 5-10 in the exam. Indicate that you are writing the short exam in the checkbox on the front page. You must hand in your solutions by 11:00 Good luck!

2 If you already have a G grade from the continuous examination, skip to problem 5 1. Architecture a) In the design of the Internet, several architectural principles have been used. One of them is to be conservative about what to send and liberal about what to receive. Another is the notation about fate sharing. Explain the motivation for the first principle, what the second principle means and how these two principles can be considered to be in conflict with each other. b) Draw a figure of the Internet and the OSI protocol stacks. For each layer in the Internet stack, show what layer(s) in the OSI stack it corresponds to. For the Internet stack, briefly specify what functionality resides in each layer. 2. Protocols a) What are the differences between a connection-oriented and a connectionless protocol? b) What are the advantages and disadvantages of connection-oriented and connectionless protocols respectively? c) Name 4 protocols covered in the course and specify their purpose and whether they are connectionoriented or connectionless. At least one must be connection-oriented and at least one must be connectionless. 3. Data transport a) Describe how the fast retransmission mechanism in TCP works. b) Why are random numbers used as initial sequence numbers for new TCP sessions instead of just starting at 1 every time? c) Consider a long lived TCP connection (e.g. a large file transfer) over a point-to-point connection with a constant bandwidth B between two computers. Assuming that no timeouts occur after the slow-start and that the receiver reads its data fast enough to avoid running out of buffer space at the receiving side, how much throughput will you get from this connection on average? Motivate your answer. (4p)

3 Figure 1: DNS zones (each grey area is a zone) 4. DNS a) Explain how the naming system maintained by the Network Information Center (NIC) worked and why it had to be replaced in the mid 1980s. b) Figure 1 shows an example of a domain hierarchy with a few zones. Explain the purpose of zones and the network component that implements them. c) Describe how a name is resolved when a host in zone it wants to find the IP address of a host in the zone cs, Assume that hosts belonging to the it zone only know the local name server and that the local name server does not have any information about the cs zone cached. Use a figure to illustrate your answer. 5. Networks at home a) To configure a new computer to be attached to the Internet without using autoconfiguration services such as DHCP, you need to specify four parameters in order to make the Internetworking part work properly. Name these parameters, and for each parameter describe why it is important and how it is used. b) It is common to use a NAT box between a home network and the rest of the Internet. Explain what a NAT box is and briefly outline how it operates. c) It can also be a good idea to have some sort of firewall. What two types of firewalls are there and on what levels in the protocol stack do they operate?

4 C A B Figure 2: Computers interconnected via a hub 6. Ethernet Figure 2 shows three computers connected to a 10 Mbit/s Ethernet hub. Each port on the hub forwards the input signal to the output wire of all ports in the hub. Each cable is 100 m long and the propagation speed in the cables is m/s. Computer A has been trying to send a frame and already detected colllisions two times. At time t A = 0s, it has waited for 1,024 ms since it last finished transmitting a jam sequence and now begins its second retransmission sequence of a maximum-sized Ethernet frame. Computer B has a maximum-sized Ethernet frame to send and begins its transmission at t B = 1µs. At exactly the same time, computer C begins its transmission of a minimum-sized Ethernet frame. Both B and C have already waited for 96 bit times at t = t B. When detecting a collision, computer A and B always pick the largest K while computer C always choose K=0. At what times will each frame be delivered to its destination? For full points, you need to present a detailed timeline showing all the events that take place at each node.

5 7. Network Programming The program below is assumed to work as follows. After a TCP server has been set up it waits for a client to connect. After a client has connected, the server reads data from the client, convert it to upper case and then sends it on to another machine it has connected to. After this, the server terminates. (Yes, it is a somewhat stupid service) The code is syntactically correct, but there are some errors. Some of these may cause the program to crash, others may not but are rather bad programming practise. Locate the errors and describe how they are corrected. (10p) #include <string.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #define MYPORT 3490 #define DEST_PORT 3490 int main(int argc, char** argv){ int serverfd, outfd, infd, i, sin_size; struct sockaddr_in my_addr; struct sockaddr_in dest_addr; struct sockaddr_in their_addr; char read_buf[100]; } serverfd = socket(af_inet, SOCK_STREAM, 0); my_addr.sin_family = AF_INET; my_addr.sin_port = MYPORT; my_addr.sin_addr.s_addr = inet_addr(" "); memset(&(my_addr.sin_zero), \0, 8); bind(serverfd, (struct sockaddr *)&my_addr, sizeof(struct sockaddr)); outfd = socket(af_inet, SOCK_STREAM, 0); dest_addr.sin_family = AF_INET; dest_addr.sin_port = DEST_PORT; dest_addr.sin_addr.s_addr = inet_addr(" "); memset(&(dest_addr.sin_zero), \0, 8); // connecting to the node we want to pass data to connect(outfd, (struct sockaddr *)&dest_addr, sizeof(struct sockaddr)); // waiting to accept connection from the server we will get data from infd = accept(serverfd, (struct sockaddr *)&their_addr, &sin_size); recv(serverfd, read_buf, 100, 0); for(i=0;i<100;i++){ // process data read_buf[i] = toupper(read_buf[i]); } send(outfd,read_buf,100,0); return 0;

6 8. Security a) What three types of cryptographic algorithms are there and what are they used for? b) You want to encrypt a message containing a bits using RSA. How must the two primes p and q used to create the public key pair be chosen in order for the encryption and decryption to work properly? c) What trusted intermediaries are used for symmetric and asymmetric encryption mechanisms respectively? d) Alice works in a company that uses Kerberos for authentication. Now, Alice wants to contact the server Bob and access a service on it. Describe the steps involved to set up an encrypted session between Alice and Bob. 9. Routing and Addressing a) How do distance-vector routing algorithms differ from link-state routing algorithms in terms of how often, and to whom, routing updates are transmitted from a node? b) What is the fundamental difference between ibgp and ebgp sessions? c) IPv4 supports two different forms of source routing. What are these called, and how do they differ? d) Consider a network in a building with its computers spread out over 4 floors where all computers have public IP addresses. On each floor, all computers on that floor is connected to a switch which is then connected to the building router. The building router advertises the network prefix /24 to the rest of the Internet. Draw a picture of the network and indicate what network addresses and netmasks are used for each interface in the building router. (4p) 10. Wireless a) Describe the hidden node problem using an illustrative figure. b) How is the hidden node problem dealt with in networks? c) What is an ad hoc network? d) Bluetooth and networks use different techniques to spread out their signal over the available radio spectrum. Name and briefly describe these two techniques.