QA (1)

Islamic University of Gaza Faculty of Engineering Computer Department Computer Networks ECOM 4321 Review Questions

Eng. Motaz Murtaja Note : Some of these question is solved and some only give you the final answer and for these questions you must know how these answer come Solve only problems you think that it is covered in our course. Alice and Bob communicate in an alphabet that has L letters. They use a monoalphabetic substitution cipher with key K. Assume that Trudy obtains a plaintext m and the corresponding ciphertext c. Under what conditions can Trudy recover the key K completely? (a) m contains at least L - 1 unique letters (b) |m| _ L. (c) |m| _ |K|. (d) m is not empty. One advantage of a path vector protocol over distance vector and link state is that (a) it requires fewer messages between the routers (b) it is guaranteed to converge to the shortest paths (c) it enables the implementation of policies that may not c orrespond to simple metrics

(d) it enables load balancing across parallel paths (e) None of the above TCP tries to (a) minimize the number of retransmissions of erroneous packets (b) share the bandwidth of bottleneck links among different connections (c) acknowledge packets link by link instead of end to end (d) correct packets so that they do not have to be retransmitted the third time (using the 3-dup ack)

(e) None of the above Two hosts simultaneously send data through a link of capacity 1Mbps. Host A generates data with a rate of 1Mbps and uses TCP. Host B uses UDP and transmits a 100bytes packet every 1ms. Which host will obtain higher throughput? (a) Host A. (b) Host B. (c) They obtain roughly the same throughput. (d) They experience congestion collapse and negligible throughput.

What is the theoretical upper-bound on the number of simultaneous TCP/IPv4 connections that a host with a single IP address can handle? (Ignore memory limitations.) (a) 216 (b) 232 (c) 264 (d) unlimited Host A sends a TCP segment (Seq = 43, ACK = 103), to which host B replies with a TCP segment (Seq = 103, ACK = 57). The payload of the first TCP segment is (a) 14 bytes long. (b) 43 bytes long (c) 46 bytes long. (d) 57 bytes long. (e) 60 bytes long. The Distance-Vector algorithm is not: (a) iterative. (b) asynchronous. (c) distributed. (d) used in RIP. (e) None of the above. Longest prefix matching is used: (a) in routers to know on which link interface to forward packets. (b) in classless addressing to use the address space more efficiently than in classful addressing

(c) by NAT to increase the available address space in home networks. (d) to assign subnet masks. (e) None of the above. An authoritative DNS server knows a top-level domain server via: (a) hostname. (b) IP address. (c) canonical hostname. (d) alias. (e) domain. A user requests aWeb page that consists of some text and 3 images. The browser’s cache is empty. For this page, the client’s browser: (a) sends 1 http request message and receives 1 http response messages. (b) sends 1 http request message and receives 3 http response messages. (c) sends 1 http request message and receives 4 http response messages. (d) sends 3 http request messages and receives 3 http response messages. (e) sends 4 http request messages and receives 4 http response messages.

Solve the following questions : 1- Consider the network below. Show the iterations followed by node A to compute the shortest paths from itself to all the other nodes using the Dijkstra algorithm. Assume A has already collected all link state information in the network. B

3

C

8

1

4 D

 A 7

2

2

1

F

4

E

Solution:

2- (a)For the following topology, fill in the link-state table provided below using Dijkstra algorithm. The ties are broken based on the node ID, where the node with the lower ID wins.

Solution:

(b) Repeat question (a) after add the following two links to the above topology: A—D with a cost of 2, and C—F with a cost of 2.

Token Bucket and Leaky Bucket : 1- Consider a source sending traffic at 1.6Mbps for the first second of each period, and at 0.4Mbps after that. For simplicity, neglect the packet size. a) Calculate the token bucket parameters R, r and b for the flow. R = 1.6Mbps, r= 0.4Mbps, b = (R – r)*1s = 1.2Mb = 150KB b) Assume a router X that can allocate at most 100KB of buffer to this flow. What is the minimum rate ra that X needs to allocate to the flow so that the flow experiences no losses? 0.8Mbps c) Repeat question (b) assuming that router X can allocate 200KB to the fl ow. ra=0.4Mbps 2- Calculate the token bucket parameters rate r and bucket depth b for a flow with the following traffic requirements: the maximum rate is R = 20Mbps, the maximum rate can only be used up to 4 seconds, and in the first 10 seconds up to 140Mb can be transmitted. The data sent in the first 4 seconds at the rate R is 80Mb. Then, in the next 6s another 60Mb need to be sent at the rate r. This means r = 10Mbps. To compute the value for b, we have b/(R-r) = 4s (i.e. b is depleted at the rate R-r up to its exhaustion), thus b = 40Mb. Router X wants the traffic of this flow to experience a delay of at most 1 second. In this case, what is the minimum rate ra that X needs to allocate to the flow? In the worst case, running at rate ra, for time 4s+1s, we need to transmit the maximum received burst (80Mb), and thus, ra = 80 /(4+1)Mbps = 16Mbps.

3- A computer on a 6-Mbps network is regulated by a token bucket. The token bucket is filled at a rate of 1 Mbps. It is initially filled to capacity with 8 megabits. How long can the computer transmit at the full 6 Mbps? Solution:

Notes:

Token Bucket Parameters : r – average rate, i.e., rate at which tokens fill the bucket b – bucket depth (limits size of burst) R – maximum link capacity or peak rate

4- A host on a network is regulated by a token bucket. The token bucket is filled at a rate of 1M tokens/sec. It is initially filled to capacity with 2M tokens. It takes one token for a data cell to be released into the network. Now suppose that data comes in three 4M-cell bursts (i.e., each burst contains 4M cells). Bursts occur at the beginning of the first, second, and third second, and each lasts for 800 msec. How long does it take the host to release all the cells onto the network? Solution:

Final solution = 10 seconds

5- The table below shows the transmission schedule for a given flow. The table shows the number of packets sent in the first second and in each subsequent second. The flow must stay within the bonds of a token bucket filter. Assume the bucket is initially full of tokens. What bucket depth does the flow need for a token rate of 2 packets per second?

Solution: 9 seconds

6- Consider the network below consisting of four routers. Every link has capacity of 1Mbs, and every flow sends data at 1Mbps. Assume that all flows are UDP and use the same packet size.

In words, Flow 1 uses links AB, BC, CD; Flow 2 uses link AB; Flow 3 uses link BC; and Flow 4 uses link CD. a) What is the throughput of each flow, if all routers implement FIFO scheduling? In this case, assume that in case of congestion each packet is dropped with the same probability. Final Answer : Flow 1: 1/4Mbps, Flow 2: 1/2Mbps, Flow 3: 2/3Mbps, Flow 4: 3/4Mbps. Why ???

b) What is the throughput of each flow if all routers implement Fair Queuing? Final Answer : Every flow gets 1/2Mbps. Why ??? c) What is the throughput of each flow if all routers implement Weighted Fair Queuing, and each Flow i has weight i? Final Answer : Flow 1: 1/5Mbps, Flow 2: 2/3Mbps, Flow 3: 3/4Mbps, Flow 4: 4/5Mbps. Why ???

Answer the following questions about weighted fair queuing. a) For the following packets arriving at a router compute the order in which they will be sent. Both flows have weight 1. In case of ties assume Flow 1 has priority. (ignore packet size and don’t compute finishing time)

b) Now answer the same question except Flow 1 has weight 1 and Flow 2 has weight 2. (ignore packet size and don’t compute finishing time)

a- Figure below shows a system of four queues being serviced according to a WFQ policy. The weight given to queues is show in the figure ( A 4 , B  1 and so on ). They are being serviced by a processor at the rate of 10 Mbps.

Fill in the resultant output rates for each these four queues for the different input traffic rates. A 1 10 6 8 1

INPUT RATES B C 1 1 10 10 6 2 0 0 5 3

D 1 10 2 8 5

A 1 4

OUTPUT RATES B C 1 1 1 3

D 1 2

b- Now consider the same system with four queues , but work as follows :

Each of the two second level processors X and Y have a weight of 5. The policy implemented between these two processors is that of TDM, i.e. each of the processors gets a fixed share of the bandwidth. Processor X implements a WFQ scheduling between its two queues A and B. A has a weight of 3 and B has a weight of 2. But Processor Y implements a strict priority between its two queues C and D. Queue C has strictly higher priority than queue D. Fill in the resultant output rates for each these four queues for the different input traffic rates. A 6 2 6

INPUT RATES B C 6 2 6 2 2 6

D 2 6 2

A

OUTPUT RATES B C

D