Question A B C D E F Points / 28 / 16 / 21 / 15 / 18 / 2

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1 Carnegie Mellon Computer Science Department Spring 2005 Midterm Name: Andrew ID: INSTRUCTIONS: There are 15 pages (numbered at the bottom). Make sure you have all of them. Please write your name on this cover and at the top of each page in this booklet except the last. If you find a question ambiguous, be sure to write down any assumptions you make. It is better to partially answer a question than to not attempt it at all. Be clear and concise. Limit your answers to the space provided. Question A B C D E F Points / 28 / 16 / 21 / 15 / 18 / 2

2 A Multiple Choice 1. Which of the following are true about persistent HTTP connections? Circle all that are correct. A. Persistent HTTP allows a server to track the client s requests through a persistent session. B. Only one connection must be opened for a page with no embedded objects served by other servers. C. Persistent HTTP shows the greatest performance advantage over nonpersistent with large objects. D. When the server has finished sending data for all objects referenced by the initially requested page, the server closes the connection. Solution: 5 pts The only correct answer is (B). -2 pts(a) is false HTTP is still stateless. -3 pts(c) is false persistent HTTP helps best when requesting many small objects. -2 pts(d) is false. The client can close the connection then, or the server can do so after a timeout, but the server doesn t know what else the client wants. 2. Charles, an ex-441 student, is given the task of building a new network link technology. Unfortunately, many of his beta-testers complain that their packets get corrupted when using his technology! He tracks the problem down to time synchronization problems between the sender and receiver on the link. Perhaps you can help Charles solve his problems by telling him a little about different encoding methods: For each of these sub-parts, identify whether the encoding can have problems with: A. Long strings of 0s B. Long strings of 1s C. Both long strings of 1s or long strings of 0s D. None of the above (a) Manchester encoding can have problems with: Solution: 2 pts(d) - None of the above. Manchester encoding encodes a 0 as a low-tohigh transition and a 1 as a high to low transition, so you always have lots of transitions. Unfortunately, it does reduce the effective data rate to half. (b) NRZ encoding can have problems with: Solution: 2 pts(c) - both! NRZ encoding only sychronizes on 0-1 and 1-0 transitions. If packets don t have enough of these, the sender/receiver will get out of sync. (c) NRZI encoding can have problems with: Solution: 2 ptsa long string of 0s. NRZI encodes a 1 as a transition, and a 0 as remaining the same. Page 2

3 (d) Draw the digital waveform corresponding to the Manchester encoded version of this sequence of 0s and 1s. We ve given you the NRZ version as a reference NRZ Manchester Solution: Manchester Grading: 2 pts; 1 point deducted for simple mistakes - e.g. inverted waveform 3. Which of the following statements are true about framing and media access: A. Byte-stuffing is useful when packet lengths must remain the same. B. Clock-based framing depends on multiple observations of a special bit pattern. C. 4B/5B encoding uses a scrambling function to reduce the odds of a long string of 1s or 0s. D. Ethernet uses CSMA/CD to ensure fair media access for multiple parties on a LAN. E. FDDI uses a token-ring based scheme with multiple tokens. Solution: Total: 5 pts (B)(2 pts), (D)(2 pts), and (E)(1 pt) are correct answers. (A) (-3 pts) Byte stuffing will increase the size of packets. (E) (-2 pts) 4B/5B eliminate the chance of a long string of 1s or 0s by encoding to a larger size. 4. Which of the following is true about different unicast routing protocols: A. BGP prevents routing loops common in distance vector protocols by having routers inform their neighbors about the path used for each of their routing table entries. B. The use of split-horizon and poison-reverse in a distance-vector protocol like RIP always prevents the count-to-infinity problem when a network partition occurs. Page 3

4 C. Link-state protocols do not suffer from the count-to-infinity problem. D. BGP always uses the shortest path (in terms of router hops) between two nodes. Solution: Total: 5 pts (A)(3 pts) and (C)(2 pts ) are correct answers (B)(-3 pts) Split horizon and poison reverse fix single hop loops, but multiple hop loops can still remain. (E)(-2 pts) BGP uses the shortest number of AS path hops, not router hops. Page 4

5 5. In the network depicted below, circle all of the paths that packets may take under valley free routing between a pair of clients. ISP A ISP Y ISP Z ISP X Legend Peer Customer Peer Provider Client / Stub network A. Client ISP Z ISP X ISP A Client B. Client ISP X ISP Y ISP A ISP Z Client C. Client ISP Y ISP X Client D. Client ISP Z ISP A ISP Y Client E. Client ISP Z ISP X ISP Y Client Solution: Total: 6 pts (C)(3 pts) and (D)(3 pts) are correct paths. 3 pts deducted for each incorrect answer Page 5

6 B Short Answer 6. Maggie decides to start a small company. She asks her ISP, Acme Networks, to give her enough addresses for 1500 hosts. ACME allocates a subblock from the * address range that they own and tells Maggie to use the following addresses: * * * * * * (a) Maggie has heard that the size of the Internet routing table has grown to huge proportions, and that to be a good citizen, she should announce the fewest number of routes possible to exactly cover her IP addresses. Under CIDR, what is the smallest set of network numbers that the rest of the world would use to describe Maggie s networks (please use address & prefix format)? Solution: 5 pts /22 and /23 (b) Maggie has a second ISP that she uses, RoadRunner Networks. She announces some of her network addresses to both Acme and RoadRunner. (Maggie hadn t talked to you yet these announcements do not correspond to the answers you provided to the previous question). As a result, some router far away in the network produces a forwarding table with the following entries: Destination Next Hop 192.1/ / / / Which next hop should the router use for a packet destined to ? Solution: 3 pts Using longest prefix match, it should use Page 6

7 7. Frank Fandango runs a 10Mbit/s Ethernet between the CMU campus and the homes of his friends. All of these hosts are in a single broadcast domain. Fortunately, they are located just within the maximum distance of an Ethernet. The total size of an Ethernet can be about 2.5 kilometers, and an Ethernet has a minimum packet size of 512 bits. Frank upgrades his network to 100Mbit/s Ethernet, and notices that when only one person sends at a time, or when he sends very large packets, his network works. But when many people send small packets, things don t work at all. (a) Help Frank out. Compute the threshold of how big packets must be in order for things to work: Solution: 4 pts The ethernet is 10 times faster, and Frank s users were at the boundary. Therefore, the packets must be 10 times larger 5120 bits. (b) One solution that Frank came up with is to raise the minimum packet size to the answer from part (a). Suppose that Frank cannot modify the minimum packet size, move the endpoints, lay new cable, or change the software or configuration on the endpoints. You may add new devices to the network. How could Frank change the topology to fix his problems anyway? Solution: 4 pts full credit Add a bridge or switch to separate the network into multiple collision domains. 1pt partial credit: Add a router. Adding a router would require that he change the IP address / gateway configuration on the hosts. Page 7

8 C Routers and Bridges A small company has the network topology shown below. In this topology: Three Ethernet segments (L1, L2, L3) are interconnected with a transparent bridge B and router R. Nodes E, F, I, and J are endsystems. Endsystems do not forward packets. L1 E B L2 F I R L3 J 8. In this problem, use the following notation: the MAC address of node E is e (lower case), its IP address is E (UPPER CASE). If a node has multiple IP or MAC addresses, use the index of the segment to differentiate between them. For example, host F has two different IP addresses, F[1] and F[3], since it is on two LANs. The first line in the table provides an example entry for a packet from E to F on link L1. Suppose host E opens a connection to the Web server J. In the following table, give the MAC and IP source and destination addresses of the SYN packet of the TCP connection as it is observed traveling on links L1, L2, and L3 (i.e. fill in the next 3 lines). Location MAC Source MAC Dest IP Source IP Dest E to F on link L1 e f[1] E F[1] E to J on L1 E to J on L2 E to J on L3 Solution: The correct answer table looks like this: Location MAC Source MAC Dest IP Source IP Dest E to F on link L1 e f[1] E F[1] E to J on L1 e r(2) E J E to J on L2 e r(2) E J E to J on L3 r(3) j E J Grading: max 6pts, 1 pt was deducted for each incorrect table cell 9. The administrator has allocated class C addresses to each of the routed IP subnets. All the networking parameters of the different nodes in the system are statically configured. At some point, the company wants to move server J from link L3 to link L1. What changes are needed to the configuration of J? Solution: IP address needs to be modified, since the machine is moved to a different subnet. Its default gateway needs to be set to R2 (2pts) Page 8

9 What changes are needed to the configuration of R? (Don t worry about DNS for any of these questions). Solution: No changes are needed. (2pts) What other nodes need to be updated? Solution: No other nodes need be updated. (2pts) (Over) Page 9

10 10. At a later time endsystem E is moved to link L2. What nodes must have their configuration changed and how?: Solution: (3pts) None. Because of high network utilization in L3, the administrator reduces network load in L3 by segmenting it. 11. What kind of device should be used if the administrator wishes to minimize his work? Give a reason for your answer. Solution: (3pts) For segmenting, a router or an (intelligent) bridge can be used. Since work (and cost) should be minimal, a bridge should be used 12. What kind of device should be used if a significant portion of the network traffic is broadcast traffic? Give a reason for your answer. Solution: (3pts) A router, since a bridge cannot filter broadcast traffic Many people misunderstood that the broadcast traffic needed to reach all destinations. We meant to specify that the broadcast traffic was like ARP or IP multicast not like some other broadcast protocol. Everyone was given credit for this question Page 10

11 D Domain Name Service Harry Bovik is working on a Web site that has multiple replicated servers located throughout the Internet. He plans on using DNS to help direct clients to their nearest server, and comes up with the following hierarchical scheme. Harry has divided his server replicas into three groups (east, west, and central) based on their physical location. A typical query occurs as follows: When a client makes a query for the root.com name server is contacted first. It returns the name server (NS) record for ns1.hb.com (along with a corresponding A record). The TTL of this record is set to 1 day. The ns1.hb.com name server is then queried for the address. It examines the source of the name query and returns an NS record for one of {east-ns, central-ns, west-ns}.distributed.hb.com (along with a corresponding A record). The choice of which name server is based on where ns1 thinks the query came from. Finally, one of {east-ns, central-ns, west-ns}.distributed.hb.com is contacted and it returns an address (A) record for the most lightly loaded web server in its region. The following questions are based on this design. 13. Harry s name server software has only two choices for TTL settings for A and NS records - 1 day and 1 minute. What are reasonable TTLs for the following records? Briefly explain your choice. (a) NS record for {east-ns, central-ns, west-ns}.distributed.hb.com: Solution: 1 day the mapping should remain constant for a long time. (2pts) (b) A record for {east-ns, central-ns, west-ns}.distributed.hb.com: Solution: 1 day the mapping should remain constant for a long time. (2pts) (c) A record returned for the actual Web server: Solution: 1 minute the mapping changes frequently since it depends on the current Web server loads.(2pts) Over Page 11

12 Harry s Web site is especially popular among CMU students. The CMU network administrators estimate that there is one access from CMU every 6 minutes. Each access results in the application resolving the name Assume the following: No other queries are made from CMU All CMU clients use the same local name server Web browsers do not do any caching on their own. 14. How many accesses will be made to the following name servers each HOUR on average to resolve these queries? Use your answers to the previous question, and explain your calculation. (a) ROOT: Solution: 1/24 the mapping needs to be refreshed once per day, so there are 1/24 requests per hour. (3pts) (b) ns1.hb.com: Solution: 1/24 the mapping needs to be refreshed once per day, so there are 1/24 requests per hour. (3pts) (c) one of {east-ns, central-ns, west-ns}.distributed.hb.com: Solution: 10 The time between two requests is 6 minutes. Since the mapping expires after 1 minute, it needs to be refreshed for each request. (3 pts) Page 12

13 E Link Probing 15. Ulysses dreams up a cute new idea for a networking tool. In traceroute, a source sends packets with increasing TTL towards a destination. Traceroute reports the distance and round-trip time (RTT) to each router on the path to the destination. How could Ulysses modify traceroute to report the latency (propagation delay + transmission delay) of a particular link in the network (i.e., not the latency of the entire route to that distance)? You may assume that all network routes are symmetric, and that there are no queuing or processing delays. Solution: To measure the latency of a link at distance k from a source, Ulysses could first use traceroute to determine the endpoints of the link (by sending packets with TTL s of k and k+1), and measure the round-trip time in each case. Assuming routes are symmetric and there are no queuing or processing delays, the difference in times is twice the propagation delay + transmission delay. Here we ll assume that Ulysses is satisfied with knowing this quantity for some specific packet size, as the transmission delay is a function of packet size. (6pts) 16. Ulysses also decides to improve the ping tool as well. He notices that ping can be used to send packets of different sizes. He uses the ping tools on the first hop router for his computer. He notices that the router has a ping time of 10ms for small packets (64byte) and 12 ms for large packets (1500bytes). What is the propagation delay and transmission rate of this network link? (You may again assume that there are no processing or queuing delays.) Solution: Assuming that Ulyssess computer is connected directly to the first-hop router, and that the router responds by sending back an ICMP packet of the same size that it receives (and the speed of the link to the router is the same in both directions), we see that the one-way times across the link are 5ms and 6ms. Let r be the transmission rate of the link in bits/second and d the propagation delay in seconds. We have two equations and two unknowns: d + 512/r = d /r = Subtracting one from the other yields: 11488/r = 1x10 3, or r = 11,488,000 bits/sec. d = / = (6pts) 17. Ulysses is on a roll! He combines his two tools and has traceroute send small and large packets (small - 64 byte and large byte). How could he use the output to measure the bandwidth of a specific link along the path? (Again, assume all routes are symmetric.) Solution: Ulysses has basically reinvented a network tool called pathchar. Using the answer to problem 15, Ulysses can measure the transmission delay + propagation delay for small and large packets across any link along the path. Then using the answer from problem 16, he can calculate the transmission rate (bandwidth) of the link. (6pts) Page 13

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15 The End Have a Great Break! F 2 Free Points for Tearing Off Page: Anonymous Feeback List one thing you liked about the class and would like to see more of or see continued (any topic - lectures, homework, projects, bboards, topics covered or not covered, etc., etc.): List one thing you would like to have changed or have improved about the class: Page 15