2023 question paper

Computer Networks

29 questions

1. Q1a. Flow control is the responsibility of? (i) Data Link Layer (ii) Transport Layer (iii) Both (a) and (b) (iv) Application Layer20232m

   Module 2: Data Link Layer & MAC Sublayer

   Flow control is the responsibility of?
   (i) Data Link Layer
   (ii) Transport Layer
   (iii) Both (a) and (b)
   (iv) Application Layer

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   Worked Solution

   Answer

   (iii) Both (a) and (b) — i.e. Data Link Layer and Transport Layer.

   Explanation

   Flow control means controlling the rate at which a sender transmits data so that a receiver is not overwhelmed.

   Flow control can be performed at more than one layer, but the mechanism and scope differ:

   1. Data Link Layer: Provides hop-to-hop flow control between directly connected devices. Protocols at this layer can regulate the amount of data sent over a particular link.

   2. Transport Layer: Provides end-to-end flow control between communicating processes. For example, TCP uses a receiver window (rwnd) so that a sender does not transmit more data than the receiving host can buffer.

   Therefore, both the Data Link and Transport layers can perform flow-control functions, but at different scopes.

   Exam point

   Data Link = hop-to-hop flow control

   Transport = end-to-end flow control

   Final answer: (iii) Both (a) and (b).

2. Q1b. Which one of the following layers of abstraction is not present in the TCP/IP model but is included in the OSI model? (i) Network layer (ii) Transport layer (iii) Session layer (iv) Application layer20232m

   Module 1: Physical Layer & Data Communication

   Which one of the following layers of abstraction is not present in the TCP/IP model but is included in the OSI model?
   (i) Network layer
   (ii) Transport layer
   (iii) Session layer
   (iv) Application layer

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   Worked Solution

   Answer

   (iii) Session layer

   Explanation

   The OSI model has separate Session and Presentation layers, whereas the traditional TCP/IP model combines their functions into its broader Application layer.

   The Network, Transport, and Application functions are represented in both models, although the names and grouping differ. The Session layer is therefore the best answer among the options.

   Final answer: (iii) Session layer.

3. Q1c. How many bits are required to represent IPv4 address in any IPv4 header? (i) 64 bits (ii) 32 bits (iii) 16 bits (iv) 8 bits20232m

   Module 3: Network Layer & Routing

   How many bits are required to represent IPv4 address in any IPv4 header?
   (i) 64 bits
   (ii) 32 bits
   (iii) 16 bits
   (iv) 8 bits

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   Worked Solution

   Answer

   (ii) 32 bits

   Explanation

   An IPv4 address is 32 bits long, normally written as four 8-bit octets in dotted-decimal notation, for example 192.168.1.10.

   The IPv4 header contains two 32-bit address fields:

   • Source address: 32 bits
   • Destination address: 32 bits

   Thus, each individual IPv4 address requires 32 bits.

   Final answer: (ii) 32 bits.

4. Q1d. Which one of the following functionalities do TCP and UDP have in common? (i) In-order delivery (ii) Reliable transfer (iii) Checksum (iv) Connection establishment20232m

   Module 4: Transport Layer & Congestion Control

   Which one of the following functionalities do TCP and UDP have in common?
   (i) In-order delivery
   (ii) Reliable transfer
   (iii) Checksum
   (iv) Connection establishment

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   Worked Solution

   Answer

   (iii) Checksum

   Explanation

   TCP and UDP both use a checksum to detect errors in the transmitted segment/datagram.

   • In-order delivery: TCP provides it; UDP does not guarantee it.
   • Reliable transfer: TCP provides reliability through acknowledgements, retransmissions, etc.; UDP does not.
   • Checksum: Both TCP and UDP include a checksum field for error detection.
   • Connection establishment: TCP is connection-oriented and establishes a connection; UDP is connectionless.

   Therefore, the correct answer is (iii) Checksum.

5. Q1e. What type of application would prefer UDP transport service? (i) Video conferencing (ii) World Wide Web (WWW) (iii) Online chess game (iv) Email20232m

   Module 5: Application Layer & Security

   What type of application would prefer UDP transport service?
   (i) Video conferencing
   (ii) World Wide Web (WWW)
   (iii) Online chess game
   (iv) Email

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   Worked Solution

   Answer

   (i) Video conferencing

   Explanation

   UDP is connectionless and does not provide TCP-style reliable, ordered delivery. This makes it useful for real-time applications where low delay is more important than retransmitting every lost packet.

   In video conferencing, a small amount of packet loss may be preferable to the delay caused by retransmission. Real-time media applications can also use application-level mechanisms to handle loss.

   • WWW: Traditionally uses TCP (modern HTTP/3 uses QUIC over UDP, but this question follows the traditional model).
   • Email: Uses TCP-based protocols such as SMTP.
   • Online chess: Usually prioritizes reliable delivery of moves and can use TCP.

   Final answer: (i) Video conferencing.

6. Q1f. Which one of the following statements is true about Go-Back-N (GBN) protocol? (i) Sequence number in GBN needs to be at least twice as large as the window size (ii) GBN uses multiple timers (iii) GBN retransmits packets upon receiving duplicate acknowledgements (iv) GBN discards packets that are received out of order20232m

   Module 2: Data Link Layer & MAC Sublayer

   Which one of the following statements is true about Go-Back-N (GBN) protocol?
   (i) Sequence number in GBN needs to be at least twice as large as the window size
   (ii) GBN uses multiple timers
   (iii) GBN retransmits packets upon receiving duplicate acknowledgements
   (iv) GBN discards packets that are received out of order

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   Worked Solution

   Answer

   (iv) GBN discards packets that are received out of order.

   Explanation

   In Go-Back-N (GBN) ARQ, the receiver accepts only the next packet in sequence. If a packet arrives out of order, the receiver does not buffer it as an accepted packet; it discards it and typically sends a duplicate cumulative ACK for the last correctly received in-order packet.

   Why the other options are wrong

   • (i) False: GBN does not require the sequence-number space to be at least twice the window size. A common condition is $W \le 2^k-1$, where $W$ is the sender window and $k$ is the number of sequence-number bits.
   • (ii) False: Standard GBN uses one timer, associated with the oldest unacknowledged packet.
   • (iii) False: Duplicate ACKs inform the sender about the missing packet, but standard GBN retransmission is primarily triggered by the timer expiring, after which the outstanding packets are retransmitted from the missing one onward.
   • (iv) True: Out-of-order packets are discarded by the GBN receiver.

   Final answer: (iv) Discards packets received out of order.

7. Q1g. Which one of the following statements is true about Selective Repeat (SR) protocol? (i) SR receiver sends duplicate acknowledgement for all packets that are not expected (ii) SR uses multiple timers (iii) SR retransmits packets upon receiving duplicate acknowledgements (iv) SR uses cumulative acknowledgement20232m

   Module 2: Data Link Layer & MAC Sublayer

   Which one of the following statements is true about Selective Repeat (SR) protocol?
   (i) SR receiver sends duplicate acknowledgement for all packets that are not expected
   (ii) SR uses multiple timers
   (iii) SR retransmits packets upon receiving duplicate acknowledgements
   (iv) SR uses cumulative acknowledgement

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8. Q1h. Which one of the following statements is true about TCP's Slow Start algorithm? (i) TCP's congestion window size increases linearly (ii) TCP's congestion window size decreases linearly (iii) TCP's congestion window size increases exponentially (iv) TCP's congestion window size decreases exponentially20232m

   Module 4: Transport Layer & Congestion Control

   Which one of the following statements is true about TCP's Slow Start algorithm?
   (i) TCP's congestion window size increases linearly
   (ii) TCP's congestion window size decreases linearly
   (iii) TCP's congestion window size increases exponentially
   (iv) TCP's congestion window size decreases exponentially

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9. Q1i. In which computer network topology, a single cable is used, through which all data propagates and to which all computers and networked nodes are connected? (i) Star (ii) Bus (iii) Ring (iv) Mesh20232m

   Module 1: Physical Layer & Data Communication

   In which computer network topology, a single cable is used, through which all data propagates and to which all computers and networked nodes are connected?
   (i) Star
   (ii) Bus
   (iii) Ring
   (iv) Mesh

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10. Q1j. Bluetooth is the wireless technology for (i) Personal area network (ii) Local area network (iii) Metropolitan area network (iv) Wide area network20232m

    Module 1: Physical Layer & Data Communication

    Bluetooth is the wireless technology for
    (i) Personal area network
    (ii) Local area network
    (iii) Metropolitan area network
    (iv) Wide area network

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12. Q2a. List, in order, the layers of the TCP/IP protocol stack, and briefly write the functions provided by each layer. Also give an example of a protocol or networking technology associated with each layer.20237m

    Module 1: Physical Layer & Data Communication

    List, in order, the layers of the TCP/IP protocol stack, and briefly write the functions provided by each layer. Also give an example of a protocol or networking technology associated with each layer.

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13. Q2b. Draw the IPV4 Datagram Header Format and explain it.20237m

    Module 3: Network Layer & Routing

    Draw the IPV4 Datagram Header Format and explain it.

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14. Q3a. What are the differences in the treatment of collisions in CSMA/CD (Carrier Sense Multiple Access, with Collision Detection) and CSMA/CA (Carrier Sense Multiple Access, with Collision Avoidance)?20237m

    Module 2: Data Link Layer & MAC Sublayer

    What are the differences in the treatment of collisions in CSMA/CD (Carrier Sense Multiple Access, with Collision Detection) and CSMA/CA (Carrier Sense Multiple Access, with Collision Avoidance)?

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15. Q3b. Write down the sender-side steps and the receiver-side steps in Cyclic Redundancy Check (CRC) method, with examples.20237m

    Module 2: Data Link Layer & MAC Sublayer

    Write down the sender-side steps and the receiver-side steps in Cyclic Redundancy Check (CRC) method, with examples.

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16. Q4a. Explain why Slotted ALOHA approach performs better than pure ALOHA approach?20237m

    Module 2: Data Link Layer & MAC Sublayer

    Explain why Slotted ALOHA approach performs better than pure ALOHA approach?

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17. Q4b. In a Slotted ALOHA system, suppose there are 5 nodes, in which each node transmits a frame with probability p. What is the probability that in a given time slot, one of the nodes successfully transmits a frame?20237m

    Module 2: Data Link Layer & MAC Sublayer

    In a Slotted ALOHA system, suppose there are 5 nodes, in which each node transmits a frame with probability p. What is the probability that in a given time slot, one of the nodes successfully transmits a frame?

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18. Q4c. Consider a sender S and a receiver R that are connected on a network, where the one-way latency is 250 milliseconds. The data rate of that network is 32000 bits per second, and size of each frame in that network is 1000 bits. Assume that S and R are using sliding windows with Selective Repeat. How large must the sliding window for S be in order to maximize the utilization?20237m

    Module 2: Data Link Layer & MAC Sublayer

    Consider a sender S and a receiver R that are connected on a network, where the one-way latency is 250 milliseconds. The data rate of that network is 32000 bits per second, and size of each frame in that network is 1000 bits. Assume that S and R are using sliding windows with Selective Repeat. How large must the sliding window for S be in order to maximize the utilization?

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19. Q5a. Discuss Link-State routing protocol with an example.20237m

    Module 3: Network Layer & Routing

    Discuss Link-State routing protocol with an example.

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20. Q5b. Explain count-to-infinity problem associated with distance vector routing. Discuss the countermeasures taken in Split Horizon and Poison Reverse techniques.20237m

    Module 3: Network Layer & Routing

    Explain count-to-infinity problem associated with distance vector routing. Discuss the countermeasures taken in Split Horizon and Poison Reverse techniques.

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21. Q6a. An ISP is granted the block 80.70.56.0/21. The ISP needs to allocate addresses for 2 organizations each 500 addresses, 2 organizations each with 250 addresses and 3 organizations each with 50 addresses. Design a subnet and find the range of addresses and unallocated addresses for each organization.20237m

    Module 3: Network Layer & Routing

    An ISP is granted the block 80.70.56.0/21. The ISP needs to allocate addresses for 2 organizations each 500 addresses, 2 organizations each with 250 addresses and 3 organizations each with 50 addresses. Design a subnet and find the range of addresses and unallocated addresses for each organization.

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23. Q6b. Compare any two of the following: (i) TCP and UDP, (ii) Packet switching and Circuit switching, (iii) Bit-stuffing and Byte-stuffing20237m

    Module 4: Transport Layer & Congestion Control

    Compare any two of the following: (i) TCP and UDP, (ii) Packet switching and Circuit switching, (iii) Bit-stuffing and Byte-stuffing

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24. Q7a. Consider a router receiving an IP packet of header size 20 Bytes (having no optional fields) and payload size of 4000 Bytes. Now the router must transmit this packet across an Ethernet network, which has Maximum Transmission Unit (MTU) of 1500 Bytes. If the router attempts to send the maximum payload bits permissible in each attempt, how fragmentation is to be carried out to transmit the IP packet in the Ethernet network?20237m

    Module 3: Network Layer & Routing

    Consider a router receiving an IP packet of header size 20 Bytes (having no optional fields) and payload size of 4000 Bytes. Now the router must transmit this packet across an Ethernet network, which has Maximum Transmission Unit (MTU) of 1500 Bytes. If the router attempts to send the maximum payload bits permissible in each attempt, how fragmentation is to be carried out to transmit the IP packet in the Ethernet network?

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    Worked Solution

    Solution

    The IP packet has:

    • Header size = 20 bytes
    • Payload = 4000 bytes
    • MTU = 1500 bytes

    The maximum payload that can fit in one fragment is:

    $1500 - 20 = 1480$ bytes.

    Since IPv4 fragment offsets are measured in units of 8 bytes, 1480 bytes is valid because $1480/8=185$.

    Therefore the 4000-byte payload is divided as:

    • Fragment 1: 1480 bytes payload
    • Fragment 2: 1480 bytes payload
    • Fragment 3: 1040 bytes payload

    The fragment offsets are:

    • Fragment 1: $0/8=0$
    • Fragment 2: $1480/8=185$
    • Fragment 3: $(1480+1480)/8=370$

    Fragment	Payload	Total Length	Fragment Offset	MF bit
    1	1480 bytes	1500 bytes	0	1
    2	1480 bytes	1500 bytes	185	1
    3	1040 bytes	1060 bytes	370	0

    The MF (More Fragments) bit is 1 for the first two fragments and 0 for the final fragment.

    Thus, the original packet is transmitted as three IPv4 fragments with offsets 0, 185, and 370.

25. Q7b. What are the differences between leaky bucket algorithm and token bucket algorithm?20237m

    Module 4: Transport Layer & Congestion Control

    What are the differences between leaky bucket algorithm and token bucket algorithm?

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26. Q8a. Write down the handshaking steps required to setup a TCP connection between two applications running on two end hosts A and B. Also, write down the purpose of each data unit of TCP that is sent and received for establishing TCP connection.20237m

    Module 4: Transport Layer & Congestion Control

    Write down the handshaking steps required to setup a TCP connection between two applications running on two end hosts A and B. Also, write down the purpose of each data unit of TCP that is sent and received for establishing TCP connection.

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27. Q8b. Write down the similarities and differences between Hyper-Text Transfer Protocol (HTTP) and File Transfer Protocol (FTP)20237m

    Module 5: Application Layer & Security

    Write down the similarities and differences between Hyper-Text Transfer Protocol (HTTP) and File Transfer Protocol (FTP)

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28. Q9a. Write short notes on: Hamming distance20237m

    Module 2: Data Link Layer & MAC Sublayer

    Write short notes on: Hamming distance

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29. Q9b. Write short notes on: DNS20237m

    Module 5: Application Layer & Security

    Write short notes on: DNS

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30. Q9c. Write short notes on: DHCP20237m

    Module 3: Network Layer & Routing

    Write short notes on: DHCP

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31. Q9d. Write short notes on: Telnet20237m

    Module 5: Application Layer & Security

    Write short notes on: Telnet

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