EINDHOVEN UNIVERSITY OF TECHNOLOGY Department of Mathematics and Computer Science

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1 EINDHOVEN UNIVERSITY OF TECHNOLOGY Department of Mathematics and Computer Science Examination Computer Networks (2IC15) on Monday, June 22 nd 2009, 9.00h-12.00h. First read the entire examination. There are 7 exercises in total. Grades are included between parentheses at all parts and sum up to 11 points. Good luck! 1. (2.0 pt) Network Delay - Consider a server sending a 64 MB audio file to a receiver over a 1Mbps connection using packets of size 1 MB. After a packet is sent, the sender waits until an ACK packet of size 8 bytes is received before a new packet can be sent (no pipelining). Find the latency of the connection if the data transfer lasts 10 minutes in total. Assume that the packet processing delays (t proc ) at the sender and the receiver are negligible. Answer M = 512 Mbits; R = 1 Mbps; P = 8 Mbits; T = 600 s; A = 64bits; 1

2 Total transmission time is: T = M P (t tran1 + t prop + t proc rec + t tran2 + t prop + t proc tran ) = M P ( P R + 2 t prop + t proc tran + A R + t proc rec) latency = t tran1 + t prop Neglecting both processing delays, latency = (P A)M+P T R 2RM = s 2. Transmission Control Protocol (TCP)- Consider the following plot of TCP window size as a function of time. Assuming TCP Reno is the protocol experiencing the behavior shown above, answer the following questions. In all cases, you should provide a short discussion justifying your answer. (a) (0.2 pt) Identify the intervals of time when TCP slow start is operating. Answer TCP slowstart is operating in the intervals [1,6] and [23,26] (b) (0.2 pt) Identify the intervals of time when TCP congestion avoidance is operating. Answer TCP congestion advoidance is operating in the intervals [6,16] and [17,22] (c) (0.2 pt) After the 16 th transmission round, is segment loss detected by a triple duplicate ACK or by a timeout? Answer After the 16 th transmission round, packet loss is recognized by a triple duplicate ACK. If there was a timeout, the congestion window size would have dropped to 1. (d) (0.2 pt) After the 22 nd transmission round, is segment loss detected by a triple duplicate ACK or by a timeout? Answer After the 22 nd transmission round, segment loss is detected due to timeout, and hence the congestion window size is set to 1. (e) (0.3 pt) What is the initial value of Threshold at the first transmission round? Answer Threshold is initially 32, since it is at this window size that slowtart stops and congestion avoidance begins. 2

3 (f) (0.3 pt) What is the value of Threshold at the 18 th transmission round? Answer Threshold is set to half the value of the congestion window when packet loss is detected. When loss is detected during transmission round 16, the congestion windows size is 42. Hence the threshold is 21 during the 18 th transmission round. (g) (0.3 pt) During what transmission round is the 70 th segment sent? Answer During the 1 st transmission round, packet 1 is sent; packet 2-3 are sent in the 2 nd transmission round; packets 4-7 are sent in the 3 rd transmission round; packets 8-15 are sent in the 4 th transmission round; packets are sent in the 5 th transmission round; packets are sent in the 6 th transmission round; packets are sent in the 7 th transmission round. Thus packet 70 is sent in the 7 th transmission round. (h) (0.3 pt) Assuming a packet loss is detected after the 26 th round by the receipt of a triple duplicated ACK, what will be the values of the congestion control window size and of Threshold? Answer The congestion window and Threshold will be set to half the current value of the congestion window (8) when the loss occurred. Thus the new values of the threshold and window is Network Routing - Consider the network shown below, which consists of 4 autonomous systems (AS). Suppose AS3 and AS2 are running OSPF for their intra-as routing protocol, and AS1 and AS4 are running RIP for their intra-as routing protocol. Suppose ebgp and ibgp are used for the inter-as routing protocol. (a) (0.15 pt) Router 3c learns about prefix x from which routing protocol: OSPF, RIP, ebgp, or ibgp? (b) (0.15 pt) Router 3a learns about x from which routing protocol? (c) (0.15 pt) Router 1c learns about x from which routing protocol? (d) (0.15 pt) Router 1d learns about x from which routing protocol? 3

4 (e) (0.15 pt) Router 4b learns about x from which routing protocol? (f) (0.25 pt) Once router 1d learns about x, it will put an entry (x, l) in its forwarding table. Will l be equal to l 1 or l 2 for this entry? Explain in one sentence. (g) (0.25 pt) Now suppose that there is a physical link between AS2 and AS4 (between 2c and 4a). Suppose router 1d learns that x is accessible via AS2 as well as AS3. Will l be set to l 1 or l 2? Explain in one sentence. (h) (0.25 pt) Now suppose that there is another AS, called AS5, which lies on the path between AS2 and AS4 (not shown in diagram). Suppose router 1d learns that x is accessible via AS2 AS5 AS4 as well as via AS3 AS4. Will l be set to l 1 or l 2? Explain in one sentence. Answer a. ebgp b. ibgp c. ebgp d. ibgp e. RIP f. l 1. This interface begins the least cost path from 1d towards the gateway router 1c. g. l 2. Both routes have equal AS-PATH length but l 2 begins the path that has the closest NEXT-HOP router. h. l 1. l 1 begins the path that has the shortest AS-PATH. 4. (0.8 pt) IP Addressing - A router has just received the following new IP address ranges: /21, /21, /21, /21. If all of these IP s refer to the same outgoing line interface, can they be aggregated? If so, to what? If not, why not? Answer They can be aggregated to /19 5. Link Layer Addressing - Consider the network depicted in the figure below. The IP addresses and MAC addresses of individual interfaces are as denoted in the figure. Suppose that the sender host with the IP address wants to send an IP datagram to the receiver host with IP address Answer the following questions: 4

5 (a) (0.3 pt) How many subnets are there in this network? Which IP addresses belong to which subnet? Answer Subnet1: , , Subnet2: , , (b) (0.3 pt) What is the destination IP address of the datagram when it leaves the sender host? What is the destination IP address of the datagram when it leaves the router? Answer In both cases (c) (0.9 pt) What is the destination MAC address of the frame when it leaves the sender host? What is the destination MAC address of the frame when it leaves the router? Which protocol is used to determine the destination MAC address? Answer ARP protocol is used. From sender: E6-E BB-4B, From router: 49-BD-D2-C7-56-2A 6. Quality of Service (QoS) (a) (0.6 pt) List 3 network performance characteristics that have a big impact on the end-to-end network QoS, in the perception of the end user. Answer Throughput, delay, jitter, loss, availability. (b) (0.6 pt) List 3 mechanisms that can be used in order to achieve better network QoS. Answer Overprovision of capacity, reservation of resources, admission control, prioritization of services. 7. (2.0 pt) True/False Questions - Each correct answer is worth 0.2 pt. Each wrong answer costs -0.1 pt. No answer means no points are earned or lost. The lowest mark for this question is 0. Please write down True or False. (a) If stored video is streamed directly from a Web server to a media player, True then TCP is used as the underlying transport protocol. (b) In an RTP/UDP streaming scenario, UDP segments are carried inside RTP False packets. (c) When using RTP, it is possible for a sender to change encoding in the True middle of a session. (d) RTP does not provide mechanisms to ensure timely delivery of data. True (e) For a given digital modulation scheme, the higher the Signal-to-Noise Ratio False (SNR), the higher the Bit Error Rate (BER). (f) The IEEE wireless LAN uses CSMA/CD as the random access protocol. False (g) An frame header contains 4 address fields, each of which can hold a True 6-byte MAC address. (h) Consider a mobile node in the Internet. In case of direct routing, a correspondent True node (sender) needs to first query the home agent of the mobile node to learn the mobile node s care-of-address (COA). (i) Assume Alice and Bob want to communicate securely. Alice can use her True private key to sign a message. (j) Alice can use her public key to encrypt a message such that only Bob can decrypt it. False 5