1401/159.334 ALB
MASSEY UNIVERSITY ALBANY CAMPUS
EXAMINATION FOR 159.334 COMPUTER NETWORKING
Semester One 2014
Copyright By PowCoder代写 加微信 powcoder
Time allowed: THREE (3) hours
Attempt ALL 6 (SIX) questions.
All questions to be answered in the blue answer booklet provided. This examination contributes 60% to the final assessment.
Calculators are permitted with no restrictions on type of calculator used.
1401/159.334 CP ALB
Question 1: Miscellaneous (TOTAL: 12 MARKS)
a) What are the two commands sent by the FTP client to the FTP server (operating in Active mode) whenever a user types a ‘put’ command (using Windows O/S)?
b) Refer to the FTP Server – FTP Client figure. What will be the next 3 succeeding replies/ actions of the FTP Server, assuming that the RETR
c) IP datagram. Suppose a host has a file consisting of 2 million bytes. The host is going to send this file over a link with an MTU of 1,500 bytes. How many datagrams are required to send this file? Assume that both the TCP segment and the link-layer frame require only the minimum header size.
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d) IP Fragmentation. Consider sending a 2,000-byte datagram into a link with an MTU of 980 bytes. Suppose that the original datagram has an identification number of 227. How many fragments are generated? For each fragment, provide the following details (as indicated in the table below). Copy your answer (complete tables) on the blue answer book. Note: Use 1 table per fragment, to show your answers.
ID Size (in bytes) Fragment offset Fragment flag
ID Size (in bytes) Fragment offset Fragment flag
ID Size (in bytes) Fragment offset Fragment flag
e) Suppose you want to write client-server programs involving a single-threaded server that simply echoes back any received client messages. In addition, you would like your server to be able to communicate to 3 clients simultaneously (or at least you would like your 3 clients to think that they can communicate with the server simultaneously), which transport layer protocol would you use for your socket program?
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Question 2: Application Layer: (TOTAL: 10 MARKS)
a) How does SMTP differ from HTTP, in terms of sending multiple objects? Suppose 3 objects need to be sent, how many messages will be used by each protocol to carry the objects?
b) Assuming that persistent HTTP with pipelining is used by a client and a server, and assuming that the client has already received a copy of the html file of the web site, how many round trip times (RTTs) would it take for a web browser (client) to request for 2 objects from the server and receive the objects? Assume that it takes 0.25 RTT to retrieve 1 object. Explain your answer. (Hint: draw a diagram to illustrate the exchange of packets.)
c) TCP socket accept() function. What is the purpose of the connection-oriented welcoming socket, which the server uses to perform an accept()? Once the accept is done, does the server use the same welcoming socket to communicate back to the client? Explain your answer.
d) If you want to know what is the canonical name of www.gmail.com, what network tool (provided by the Windows operating system) would you use?
e) Give 1 service offered by DNS.
[1 mark] [1 mark]
f) DNS queries and DNS reply messages are sent using which transport protocol?
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Question 3: Transport Layer: (TOTAL: 8 MARKS)
a) The following TCP packet (segment) is sent over an established connection: seq=1024 ack=2048 data=”334”
Assuming that the receiver is expected to echo the data received, what are the entries for Seq, ACK and data that the receiver sends to acknowledge the segment above?
b) Refer to the figure depicting TCP Congestion Control. Assume that 1MSS is equal to 500 bytes. If at transmission round #8, TCP receives a Timeout event, calculate the new values for the SSThreshold and Congestion Window size.
c) Refer again to the figure depicting TCP Congestion Control. Assume that 1MSS is equal to 500 bytes. If at transmission round #8, TCP receives 1 new acknowledgement, calculate the new value for the Congestion Window size (in bytes).
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d) Calculate the UDP checksum for a segment containing a sequence of two 16-bit integers:
1110011001100110 11101010101010101
1401/159.334 CP ALB
Question 4: Network Layer (TOTAL: 16 MARKS) Part 1. Routing (DISTANCE VECTOR ALGORITHM):
The figure depicts an autonomous system (AS) comprising of 6 routers connected via certain links, with the numbers indicating the time cost for each link between the routers. The distance vector algorithm is used for routing packets.
a) Write the initial distance table of Router C. Copy your answer (complete table) on the blue answer book.
Destination
via neighbour ADF
b) Suppose that Router C receives three distance vectors (see the details of the routing advertisements given below) from Routers A, D and F. Show the updated distance table of router C after receiving all three advertisements. Show your answer by providing the missing values on the marked cells in the table below. (Marked cells = cells with question mark symbols) Copy your answer (complete table) on the blue answer book.
1401/159.334 CP ALB
Router A advertises its minimum costs via the following distance vector:
Destinations ABCDEF
Router D advertises its minimum costs via the following distance vector:
Destinations ABCDEF
Router F advertises its minimum costs via the following distance vector:
Destinations ABCDEF
F836520 Distance Table of Router C before the application of Poisoned Reverse:
Destination
via neighbour ADF
*Note that you only need to fill-in the missing values for 3 cells.
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c) ShowtheupdateddistancetableofrouterCafterapplyingPoisonedReverse.
Distance Table of Router C after the application of Poisoned Reverse:
Destination
via neighbour ADF
*Note that you only need to fill-in the missing values for 3 cells.
Part 2. IP Addressing
d) Consider an IP subnet with prefix 129.17.129.224/27. Provide the range of IP addresses (of form xxx.xxx.xxx.xxx to yyy.yyy.yyy.yyy) that can be assigned to
this subnet.
e) Suppose an organisation owns a block of IP addresses of the form 129.17.129.224/27. Suppose it wants to create four IP subnets from this block, with each block having the same number of IP addresses. What is the range of IP addresses (of form xxx.xxx.xxx.xxx to yyy.yyy.yyy.yyy) that can be assigned to the first subnet?
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Part 3: Packet Forwarding
f) Show the forwarding process if a packet arrives at Router1(R1) in the figure with a destination address of 180.70.65.204. To which interface will the packet be forwarded to? Show your complete solution by showing the network and the host portions of the addresses.
Routing Table for Router 1 (R1)
Mask Network Address
/26 180.70.65.192 /25 180.70.65.128 /24 201.4.22.0
/22 201.4.16.0 Any Any
Next Hop Interface
– M2 – M0 – M3 … M1 180.70.65.200 m2
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Question 5: Link Layer: (TOTAL: 7 MARKS)
a) Calculate the CRC value (in hexadecimal) for the character “1011 1001” considering that the polynomial is the standard 16 bit CRC (0x8005). State any assumptions you make (e.g. initial state of variables, reflected bytes, etc).
b) What multiple access protocol does Ethernet use?
[3 marks] [1 mark]
c) Compare and contrast the given network devices below in terms of traffic isolation, configuration requirements and routing capability. Refer to the given table structure below as a guide in writing your answers. Simply indicate Yes or No in the blank spaces provided corresponding to the feature in question. Copy your answer (complete table) on the blue answer book.
Traffic Isolation Plug-and-Play Optimal Routing
Hubs Routers
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Question 6: Network Security: (TOTAL: 7 MARKS)
a) Alice administers a company network 222.22.0.0/16 and, in general, wants to disallow access to her network from the public internet (R3). However, Alice collaborates with Bob and his colleagues who are at network 111.11/16. Alice wants to let users from Bob’s network access a specific subnet, 222.22.22/24 within her company’s network (R1). The problem is that Trudy belongs to Bob’s network, with subnet 111.11.11/24. Therefore, Alice doesn’t want any traffic from 111.11.11/24 entering anywhere into her network (R2).
Desired Action
222.22.22/24
Let datagram from Bob’s university into a restricted subnet.
111.11.11/24
Don’t let traffic from Trudy’s subnet into anywhere within Alice’s network
R3 0.0.0.0/0 0.0.0.0/0 Deny Don’t let traffic into Alice’s network
Fill-in the blanks provided in the table to reflect what would happen to processes P1, P2, P3 and P4 if the rules were applied in the order of R2, R1 and R3 consecutively, considering that each row corresponds to a process with source and destination IP addresses. Indicate whether or not the process being considered will be denied or permitted, and include also which among the rules causes the action to be taken. Copy your answer (partial table) on the blue answer book.
SOURCE IP P1 111.11.11.1 P2 111.11.11.1
P3 111.11.6.6 P4 111.11.6.6
Sequence of Application of Rules
222.22.6.6 222.22.22.2 222.22.22.2 222.22.6.6
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b) Refer to the figure depicting a Secure e-mail protocol used by Alice. Assume that Trudy can intercept the e-mail. With the protocol provided below, is Alice able to send her email to Bob with confidentiality, authentication and message integrity? Explain your answer briefly as to how the protocol is able to handle the 3 aforementioned security issues. Refer also to the variable descriptions provided below to help you write your explanation.
Note: the variables depict the following information:
• m – message
• dA() – Alice’s private key
• eA() – Alice’s public key
• eB() – Bob’s public key
• KS – random symmetric private key
• H() – hash function known to Bob and Alice
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