HW2F11-key - Homework Assignment#2 Due 11:59pm October 9th...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Homework Assignment #2 Due 11:59pm, October 9th EE122 Fall 2011 Please submit your solutions using BSpace (https://bspace.berkeley.edu/). Log in to BSpace with your CalNet ID, click on the EL ENG 122 Fa11 tab, and click on Assignments under Course Tools. Assignments should be submitted in one of the following formats: .txt, .pdf, or .ps. 1) Layers: [5 points] [One point per item] Berkeley has a highly respected networking research group, which recently announced the following new inventions: i. Internet Recycling Protocol (IRP): when this end-to-end reliability protocol resends packets, it doesn't produce new bytes but instead uses bytes from discarded packets thereby reducing greenhouse gases. ii. Beam-Me-Up-Scotty (BMUS): A new communication technology that enables bits to be magically transported to galaxies far, far away. iii. NetworkEsperanto (NE): A universal protocol specification with the following properties: all networks can translate their packets into the Esperanto format, and can translate Esperanto packets into their own format. iv. Face-in-a-Book (FB): A new social networking site where people read books and ignore each other. v. LawnLAN (LL): a network that can carry packets through a lawn to an individually (but only locally) addressed blade of grass. For each of these, list which layer they belong to in the Internet architecture: (a) Application, (b) Transport, (c) Internetworking, (d) Data-Link, (e) Physical i. b ii. e iii. c iv. a v. d 1 2) End-to-End vs Network-based Reliability [10 points] (an exercise in probability) [4 points for items (i) and (ii), 2 points for (iii)] Consider a network with two links. Node A is trying to send packets to node C. Let p1 be the probability of failure on the first link (A-B), and p2 be the probability of failure on the second link (B-C). i) End-to-End Recovery: Assume failures are detected by the ends (i.e., A finds out that the packet did not reach C, and resends). On average, how many retransmissions from A are needed before the packet arrives at the destination C? (a) (!!! ! ! ) )(!!! ! ! ! (b) (!!! ! ! !) )(!!! ! ! ! ! (c) !!! + !!! ! ! A B C ! ! ! !! !! ! ! ! (d) (!!! (e) !!! ! ! )(!!!! ) !! !! !(!!!! ) ! Answer: (b) The probability of an end-to-end failure is p1+(1-p1)p2, where the first term is the probability there is a failure on the first link, and the second term is the probability of a success on the first link but a failure on the second link. We then use the expression that the average number of retransmissions is f/(1-f) where f is the probability of failure. ii) Hop-by-hop recovery: Now assume failures are detected by each link, and the link nodes initiates a retransmission when the packet transmission on that link fails (i.e., if the packet fails on the A-B link, then A resends; if the packet fails on the B-C link, then B resends). On average, how many retransmissions (from either A or B) does it take to successfully reach the destination C? (a) (!!! ! ! ) )(!!! ! ! ! (b) (!!! ! ! !) )(!!! ! ! ! ! ! !! !! ! 2 ! ! (c) !!! + !!! ! ! ! ! (d) (!!! (e) !!! ! ! )(!!!! ) !! !! !(!!!! ) ! Answer: (c) The average number of retransmissions on link 1 is p1/(1-p1) and the average number of retransmissions on link 2 is p2/(1-p2). iii) Which scheme has more retransmissions? Hint: You can either use the results from (i) and (ii), or you can just think about this directly without doing any calculations (i.e., even if you don't understand the math of parts (i) and (ii), you should be able to figure this out). (a) End-to-end (b) Hop-by-hop Answer: (a) The end-to-end case retransmits when there is a failure on either link 1 or link 2, whereas in the hop-by-hop case there is only a retransmission on a link when that particular link fails. Lesson: what do these results tell you about which design choice is better? (Answer: they tell you about efficiency, but do you architect for efficiency or engineer for efficiency?) 3) Reliable Transport [10 points] [Two points per item] Consider a sliding window protocol with a window size of 5 using cumulative ACKs. Retransmissions: retransmissions occur under two conditions: Reception of three duplicate ACKs (that is, three identical ACKs after the initial ACK) Time out after 100msec (timer starts at the beginning of the packet transmission) Timing: Data packets have a transmission time of 1 msec ACK packets have zero transmission time The link has a latency of 10msec. The source A starts off by sending its first packet at time t=0. i) Assume all packets are successfully delivered except the following: 3 When is data packet #3 first retransmitted (expressed in terms of msec after t=0)? (a) 23 (b) 42 (c) 43 (d) 102 (e) 133 Answer: (c) ii) Consider the same scenario, but with everything successfully delivered except the following: The first transmission of data packet #3 The first transmission of data packet #5 The ACK sent in response to the receipt of data packet #6 When is data packet #3 first retransmitted (expressed in terms of msec after t=0)? (a) 23 (b) 42 (c) 43 (d) 102 (e) 133 Answer: (d). The packet times out. iii) Assume we can only observe the ACK packets arriving at the sender. The same sliding window algorithm is used, with the same timings and retransmission policies apply. Notation (read carefully): The notation Ax is used to mean that the ACK packet is acknowledging the receipt of all packets up to and including data packet x. That is, A5 is acknowledging the receipt of packet 5; to be clear, the notation does not mean that the receiver is expecting packet 5 as the next data packet. Assume that the following ACK packets arrive (just the ordering is shown, no timing information is provided): A1 A2 A3 A3 A4 A5 A6 Which scenarios (described only by the unusual events that occurred; assume all else functioned normally) would have produced such a series of ACKs? (mark all that apply) (a) Data packet number 4 was dropped. (b) Data packet number 4 was delayed, arrived immediately after data packet 5 (c) Data packet 3 was duplicated by the network (d) ACK packet A3 was duplicated by the network (e) ACK packet A4 was delayed, arriving after A5 4 The first transmission of data packet #3 The ACK sent in response to the receipt of data packet #6 Answer: (c), (d) iv) With the same set up as in the previous problem, consider the following stream of ACK packets A1 A2 A3 A5 A4 A6 Which scenarios (described only by the unusual events that occurred; assume all else functioned normally) would have produced such a series of ACKs? (mark all that apply) (a) Data packet number 4 was dropped. (b) Data packet number 4 was delayed, arrived immediately after data packet 5 (c) Data packet 3 was duplicated by the network (d) ACK packet A3 was duplicated by the network (e) ACK packet A4 was delayed, arriving after A5 Answer: (e) v) With the same set up as in the previous problem, consider the following stream of ACK packets A1 A2 A3 A3 A5 A6 Which scenarios (described only by the unusual events that occurred; assume all else functioned normally) would have produced such a series of ACKs? (mark all that apply) (a) Data packet number 4 was dropped. (b) Data packet number 4 was delayed, arrived immediately after data packet 5 (c) Data packet 3 was duplicated by the network (d) ACK packet A3 was duplicated by the network (e) ACK packet A4 was delayed, arriving after A5 Answer: (b) 4) Distance-Vector Routing [15 points] [Five points per item] 5 B 2 3 A 1 C 1 1 1 2 E F 1 1 D The routing table for node A is expressed in a format where the rows indicate the destination and the columns indicate the first hop. That is, the number in (row C column D) denotes the cost of the best currently known path to C that starts with A sending to D. The initial table for node A, before A has exchanged any routing information with any other node, takes the form: Table for A B C D E F B 3 C 1 D 2 Note that in this initial routing table, A does not know paths to any destinations except its immediate neighbors. i) After A receives a route update from B (with B sending its initial table), what are the entries in the routing table? Table for A B C D E F B 3 4 5 C 1 D 2 6 ii) After A then receives a route update from C (with C sending its initial table), what are the entries in the table? Table for A B C D E F iii) Assume now that all nodes exchange tables in an iterative process for as many times as it takes for the tables to settle into a stable state. What does A's table look like? Table for A B C D E F 5) Link-State Routing [10 points] [2.5 points per item] Consider the network discussed in the previous problem, but now we use a link- state routing algorithm. Assume that all the nodes have successfully flooded their information to all the other nodes, so all nodes have an accurate map of the network. The questions will ask about the path a packet takes; show the path by the series of nodes it traverses, such as: A-D-C-E-F. In each case, either show the path until the packet reaches the destination, or the first eight hops in case it enters a loop. i) If A sends a packet to B, what path does the packet take? Answer: A-C-B ii) If A sends a packet to F, what path does the packet take? Answer: A-C-E-F iii) Now assume that the link costs for the links C-E and E-F both change to 6. E announces these changes, and all nodes but C get the update (that is, C still thinks that C-E and E-F have cost 1; don't worry why C thinks that, just take it as a fact). A now sends a packet to F. What path does it take? Answer: A-C-E-D-C-E-D-C 7 B 3 4 5 5 5 C 2 1 2 2 3 D 4 3 2 3 4 B 3 4 5 C 2 1 2 2 D 2 iv) E now resends its flood message, and C finally knows about the change in link costs. When A sends a packet to F, what path does it take? Answer: A-C-B-F 6) Bonus Question [No points, just a beanbag] What was the first band to perform over the Internet to a large (~100 sites) audience? Answer: Severe Tire Damage (as opening act for the Rolling Stones). 8 ...
View Full Document

{[ snackBarMessage ]}

Ask a homework question - tutors are online