Exam in Computer Networks

Transcription

1 Exam in Computer Networks Date Course code 1DT131 / 1DT633 Exam time 3/5 hours Problems 6/10, Total 60/100 points Teacher Lars-Åke Larzon Phone , Aid English dictionary 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. Please read the instructions below carefully. 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 do all problems in the exam. 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 the problems Indicate that you are writing the short exam in the checkbox on the front page. You must hand in your solutions by 17:00 Good luck!

2 If you already have a G grade from the continuous examination, skip to problem 5 1. Basics a) Draw a picture of the Internet protocol stack and name the different layers. For each layer, specify: - What service the layer provides to the adjacent layer above - What assumptions the layer makes about the adjacent layer beneath (8p) b) Explain what statistical multiplexing is and how it is used in the Internet design. 2. Transport protocols a) The TCP sender in this problem is configured to never use more than 64 Kbyte of buffer space for the sending window. It is connected to a network where the bottleneck link capacity is 80 Mbit/s, the average signal propagation speed in the network between the sender and the receiver is m/s and the one-way delay added by intermediate routers, end-node processing etc. is as large as the propagation delay. How far apart can the end-nodes be without having the maximum buffer size limit the throughput of the connection? (5p) b) Illustrate how a sliding window using cumulative ACK:s works by completing figure 1. X indicates lost packets. Solid arrows represent transmission on data, while dotted arrows are the ACK:s sent in the opposite direction. The receiver buffer size is three packets and the sender has approximately infinite window size on the sending side. No piggybacking is used. Numbers in the squares represents sequence numbers of packets sent and/or received. Values of ACK:s are filled in into the dotted squares. The timeout value is set so that 7 packets can be sent between the original transmission and the retransmission caused by the timeout. Fast retransmission occurs after one duplicate ACK. (5p) Sent 3 Received X X ACK:s Figure 1: A sliding window problem

3 3. Data security a) Illustrate with a figure how a man-in-middle attack can compromise a Diffie-Hellman key exchange. (4p) b) In this problem, the notation E key1 (M) represents encryption of the message M using key 1, while D key2 (M) denotes decryption of the message M using key 2. Alice wants to send an authenticated and encrypted message to Bob using public-key cryptography. She therefore takes her message M and performs the combined encryption/authentication operation E publicbob (E privatealice (M)). What operation will Bob have to perform in order to both decrypt the message and ensure it s really from Alice? c) (Using the same notation as in the previous problem) Alice wishes to reduce the amount of data to encrypt by using the one-way cryptographic hash function H(M). Illustrate how she can combine the E and H operations to reduce the amount of data to encrypt while still keeping every message sent both encrypted and authenticated. 4. The network layer a) What two mechanisms help improve the distance-vector algorithm by removing routing loops that only involve two nodes? b) IP can perform two different types of source routing. What are these called, and what are the difference between them? c) Multicast addresses are chosen randomly when a multicast group is created. Given this, how can a router distinguish between multicast and unicast addresses? d) What is reliable flooding used for in link-state routing protocols? 5. Ethernet a) Before each Ethernet frame, there is a 64-bit preamble. What is the purpose of this? b) How does Ethernet react when a collision is detected? You answer must include all actions taken until next retransmission attempt. (4p) c) Ethernet does not use bit stuffing techniques as in PPP to detect the end of a frame. What does it use instead? d) What is the difference between a hub and a switch?

4 6. Network programming a) User tend to prefer fully qualified domain names (FQDN:s) (e.g., rather than IP addresses. If you are programming a communicating application in C or Java, how do you translate between FQDN:s and IP addresses in your code? b) Fill in what is missing in the shaded areas of figure 2. (6p) c) What does the figure 2 illustrate? Server socket() Client blocks until read() data (request) process request data(reply) Figure 2: Network programming figure read()

5 7. The link layer a) What is an Ad-Hoc network? b) In wireless networks, the hidden node problem can occur. Use an illustrating figure to describe what the problem is. c) Given that link layers often include a checksum that is better than the primitive Internet checksum used in upper layers, why is there an Internet checksum at all? d) Bluetooth networks organize themselves into small cells called piconets. How does Bluetooth deal with interference between several different piconets that are within range of each other? 8. Network security a) Worms are becoming more and more common in the Internet. Assuming the risk of being infected is independent of the OS you are using, give two suggestions of how to reduce the risk of being infected. b) What is the difference between Worms and Viruses? c) Compare filter-based firewalls to proxy-based (a.k.a. application gateways) with respect to differences in operation, limitations and impact from IPsec and SSL (5p) 9. Short answers The statements below describe a property of different protocols, terms or mechanisms in the Internet architecture. Identify the corresponding protocols/terms/mechanisms by simply giving their name. Same protocol/term/mechanism can occur multiple times. a) Error detection mechanism that depends on binary polynomial arithmetics b) Framework for encryption and authentication of IP packets c) Link layer technology used in a ring-shaped network topology d) Exponential increase of cwnd at the beginning of a TCP session e) Splitting IP packets into smaller pieces that are reassembled at the receiver f) Transport protocol normally used by the DNS system g) Protocol used to send h) Device used to cope with signal damping in Ethernets i) Allows flexible borders between the network and host part of an IP address j) Mechanism to avoid data loss due to full buffers at the receiver

6 10. Routing a) A routing domain is illustrated in figure 3. The link between router 2 and router 3 goes down. After this event, routing information is exchanged between the routers to reflect the new topology. What will the forwarding tables in each of the routers look like after all update messages have been processed? b) A gnome sneaks into the network and modifies two of the links between the routers so that they become one-way links. The link between router 1 and 3 can only forward packets in the direction towards router 3, and the link between router 2 and 3 can only forward packets in the direction towards router 2. What will the forwarding tables look like in the routers after all update messages have been updated? (4p) c) All of a sudden, the gnome decides to be even more evil. After having changed two of the links to be unidirectional and awaiting the forwarding tables to stabilize as described in the previous subproblem, he now disables the capability to exchange routing messages between routers. After doing this, he restores the forwarding table of router 3 to its original state (as presented in the figure). How will his actions affect the connectivity between the three end-nodes in the figure? (Describe who will be able to connect whom and through what routers) Note: The routing table for router 2 is wrong in the figure.

7 Router 2 Interface Router 1 Interface Router Interface Figure 3: Routing domain