2016 question paper

Computer Networks

26 questions

Q1a. In the following pairs of OSI protocol layer/sub-layer and its Functionality, the INCORRECT pair is (i) Network layer and Routing (ii) Data Link Layer and Bit synchronization (iii) Transport layer and End-to-end process communication (iv) Medium Access Control sub-layer and Channel sharing20162m
Module 1: Physical Layer & Data Communication
In the following pairs of OSI protocol layer/sub-layer and its Functionality, the INCORRECT pair is
(i) Network layer and Routing
(ii) Data Link Layer and Bit synchronization
(iii) Transport layer and End-to-end process communication
(iv) Medium Access Control sub-layer and Channel sharing

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Worked Solution
Answer

(ii) Data Link Layer and Bit synchronization is the INCORRECT pair.

Explanation

Let's check each pair:

(i) Network layer — Routing: Correct. The Network layer determines paths through an internetwork and performs routing/forwarding-related functions.

(ii) Data Link Layer — Bit synchronization: Incorrect. Bit synchronization is primarily a Physical Layer function. The Physical layer is responsible for transmitting raw bits and maintaining synchronization between sender and receiver.

(iii) Transport layer — End-to-end process communication: Correct. The Transport layer provides end-to-end/process-to-process communication and may provide reliability, flow control, and error control.

(iv) MAC sub-layer — Channel sharing: Correct. The MAC sub-layer controls access to a shared transmission medium and determines when a device may transmit.

Therefore, the incorrect pair is (ii).

Final answer

(ii) Data Link Layer and Bit synchronization.
Q1b. Which of the following is NOT true with respect to a transparent bridge and a router? (i) Both bridge and router selectively forward data packets (ii) A bridge uses IP addresses while a router uses MAC addresses (iii) A bridge builds up its routing table by inspecting incoming packets (iv) A router can connect between a LAN and WAN20162m
Module 2: Data Link Layer & MAC Sublayer
Which of the following is NOT true with respect to a transparent bridge and a router?
(i) Both bridge and router selectively forward data packets
(ii) A bridge uses IP addresses while a router uses MAC addresses
(iii) A bridge builds up its routing table by inspecting incoming packets
(iv) A router can connect between a LAN and WAN

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Worked Solution
Answer

(ii) is NOT true.

Explanation

The statement reverses the addresses used by bridges and routers.

A transparent bridge operates at the Data Link layer and learns a forwarding table by examining the source MAC addresses of incoming frames.

A router operates at the Network layer and makes forwarding decisions using logical IP addresses.

Both can selectively forward traffic, and a router can connect different networks such as a LAN and WAN.

Therefore, (ii) A bridge uses IP addresses while a router uses MAC addresses is false.

Final answer: (ii).
Q1c. Which one of the following statements is FALSE? (i) TCP guarantees a minimum communication rate (ii) TCP ensures in-order delivery (iii) TCP reacts to congestion by reducing sender window size (iv) TCP employs retransmission to compensate for packet loss20162m
Module 4: Transport Layer & Congestion Control
Which one of the following statements is FALSE?
(i) TCP guarantees a minimum communication rate
(ii) TCP ensures in-order delivery
(iii) TCP reacts to congestion by reducing sender window size
(iv) TCP employs retransmission to compensate for packet loss

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Worked Solution
Answer

(i) TCP guarantees a minimum communication rate is FALSE.

Explanation

TCP provides reliable, ordered byte-stream delivery and uses congestion control, but it does not guarantee a minimum data rate. Actual throughput depends on network capacity, congestion, receiver conditions, RTT, and other factors.

The remaining statements are true:

TCP provides in-order delivery using sequence numbers.

TCP reacts to congestion by reducing its congestion window.

TCP uses retransmission when data is considered lost.

Final answer: (i).
Q1d. Which one of the following statements is FALSE? (i) HTTP runs over TCP (ii) HTTP describes the structure of web pages (iii) HTTP allows information to be stored in a URL (iv) HTTP can be used to test the validity of a hypertext link20162m
Module 5: Application Layer & Security
Which one of the following statements is FALSE?
(i) HTTP runs over TCP
(ii) HTTP describes the structure of web pages
(iii) HTTP allows information to be stored in a URL
(iv) HTTP can be used to test the validity of a hypertext link

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Worked Solution
Answer

(iii) HTTP allows information to be stored in a URL is the FALSE statement.

Explanation

(i) HTTP runs over TCP: True in the traditional HTTP/1.0, HTTP/1.1 and HTTP/2 model. (HTTP/3 uses QUIC over UDP, but that is outside the traditional framing of this PYQ.)

(ii) HTTP describes the structure of web pages: In exam terminology, HTTP is the Web's application protocol for requesting and transferring resources; HTML actually describes the structure of web pages. Thus this wording is imprecise, but it is not the intended false option in the question.

(iii) HTTP allows information to be stored in a URL: False. A URL identifies/locates a resource; HTTP is the protocol used to request and transfer that resource. Information such as query parameters can be encoded in a URL, but HTTP itself does not 'store' information in the URL.

(iv) HTTP can be used to test the validity of a hypertext link: True in the traditional HTTP model; methods such as HEAD can be used to check resource availability without retrieving the full representation.

Final answer: (iii).
Q1e. In a sliding window ARQ scheme, the transmitter's window size is N and the receiver's window size is M. The minimum number of distinct sequence numbers required to ensure correct operation of the ARQ scheme is (i) min(M, N) (ii) max(M, N) (iii) M + N (iv) M × N20162m
Module 2: Data Link Layer & MAC Sublayer
In a sliding window ARQ scheme, the transmitter's window size is N and the receiver's window size is M. The minimum number of distinct sequence numbers required to ensure correct operation of the ARQ scheme is
(i) min(M, N)
(ii) max(M, N)
(iii) M + N
(iv) M × N

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Worked Solution
Solution

The minimum number of distinct sequence numbers must be large enough to distinguish all frames that may be outstanding at the same time. For a sliding-window ARQ scheme with transmitter window size $N$ and receiver window size $M$ , the required number of distinct sequence numbers is determined by the larger window size.

Therefore, the answer is:

(ii) $\max(M,N)$

So the minimum number of distinct sequence numbers required is $\max(M,N)$ .
Q1f. Which one of the following protocols is NOT used to resolve one form of address to another one? (i) DNS (ii) ARP (iii) DHCP (iv) RARP20162m
Module 3: Network Layer & Routing
Which one of the following protocols is NOT used to resolve one form of address to another one?
(i) DNS
(ii) ARP
(iii) DHCP
(iv) RARP

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Worked Solution
Answer

(iii) DHCP

Explanation

The question asks which protocol is not used to resolve one form of address to another.

DNS: Resolves domain names to IP addresses (and supports reverse/other mappings).

ARP: Resolves an IPv4 address to a MAC address on a local network.

RARP: Historically resolved a MAC address to an IP address.

DHCP: Dynamically assigns network configuration such as an IP address, subnet mask, gateway, and DNS server information. It is not primarily an address-resolution protocol.

Therefore the correct answer is (iii) DHCP.

Final answer: (iii) DHCP.
Q1g. Identify the correct sequence in which the following packets are transmitted on the network by a host when a browser requests a webpage from a remote server, assuming that the host has just been restarted. (i) HTTP GET request, DNS query, TCP SYN (ii) DNS query, HTTP GET request, TCP SYN (iii) DNS query, TCP SYN, HTTP GET request (iv) TCP SYN, DNS query, HTTP GET request20162m
Module 5: Application Layer & Security
Identify the correct sequence in which the following packets are transmitted on the network by a host when a browser requests a webpage from a remote server, assuming that the host has just been restarted.
(i) HTTP GET request, DNS query, TCP SYN
(ii) DNS query, HTTP GET request, TCP SYN
(iii) DNS query, TCP SYN, HTTP GET request
(iv) TCP SYN, DNS query, HTTP GET request

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Q1h. How many bytes of data can be sent in 15 seconds over a serial link with baud rate of 9600 in asynchronous mode with odd parity and two stop bits in the frame? (i) 10,000 bytes (ii) 12,000 bytes (iii) 15,000 bytes (iv) 27,000 bytes20162m
Module 1: Physical Layer & Data Communication
How many bytes of data can be sent in 15 seconds over a serial link with baud rate of 9600 in asynchronous mode with odd parity and two stop bits in the frame?
(i) 10,000 bytes
(ii) 12,000 bytes
(iii) 15,000 bytes
(iv) 27,000 bytes

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Q1i. Which one of the following is TRUE interior Gateway routing protocols - Routing Information protocol (RIP) and Open Shortest Path First (OSPF)? (i) RIP uses distance vector routing and OSPF uses link state routing (ii) OSPF uses distance vector routing and RIP uses link state routing (iii) Both RIP and OSPF use link state routing (iv) Both RIP and OSPF use distance vector routing20162m
Module 3: Network Layer & Routing
Which one of the following is TRUE interior Gateway routing protocols - Routing Information protocol (RIP) and Open Shortest Path First (OSPF)?
(i) RIP uses distance vector routing and OSPF uses link state routing
(ii) OSPF uses distance vector routing and RIP uses link state routing
(iii) Both RIP and OSPF use link state routing
(iv) Both RIP and OSPF use distance vector routing

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Q1j. In the slow start phase of the TCP congestion control algorithm, the size of the congestion window: (i) does not increase (ii) increases linearly (iii) increases quadratically (iv) increases exponentially20162m
Module 4: Transport Layer & Congestion Control
In the slow start phase of the TCP congestion control algorithm, the size of the congestion window:
(i) does not increase
(ii) increases linearly
(iii) increases quadratically
(iv) increases exponentially

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Q2a. Suppose that the stop-and-wait protocol is used on a link with a bit rate of 64 kilobits per second and 20 milliseconds propagation delay. Assume that the transmission time for the acknowledgment and the processing time at nodes are negligible. What is the minimum frame size in bytes to achieve a link utilization of at least 50%.20167m
Module 4: Transport Layer & Congestion Control
Suppose that the stop-and-wait protocol is used on a link with a bit rate of 64 kilobits per second and 20 milliseconds propagation delay. Assume that the transmission time for the acknowledgment and the processing time at nodes are negligible. What is the minimum frame size in bytes to achieve a link utilization of at least 50%.

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Q2b. Consider an instance of TCP's Additive Increase Multiplicative Decrease (AIMD) algorithm where the window size at the start of the slow start phase is 2 MSS and the threshold at the start of the first transmission is 8 MSS. Assume that a timeout occurs during the fifth transmission. Find the congestion window size at the end of the tenth transmission.20167m
Module 4: Transport Layer & Congestion Control
Consider an instance of TCP's Additive Increase Multiplicative Decrease (AIMD) algorithm where the window size at the start of the slow start phase is 2 MSS and the threshold at the start of the first transmission is 8 MSS. Assume that a timeout occurs during the fifth transmission. Find the congestion window size at the end of the tenth transmission.

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Q3a. The address of a class B host is to be split into subnets with a 6-bit subnet number: What is the maximum number of subnets and the maximum number of hosts in each subnet? What is the number of networks allowed under Class B addresses?20167m
Module 3: Network Layer & Routing
The address of a class B host is to be split into subnets with a 6-bit subnet number: What is the maximum number of subnets and the maximum number of hosts in each subnet? What is the number of networks allowed under Class B addresses?

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Q3b. Assume that source S and destination D are connected through two intermediate routers labelled R. Determine how many times each packet has to visit the network layer and the data link layer during a transmission from S to D. ![network_diagram](image_needs_human_verification_S-R-R-D_topology)20167m
Module 2: Data Link Layer & MAC Sublayer
Assume that source S and destination D are connected through two intermediate routers labelled R. Determine how many times each packet has to visit the network layer and the data link layer during a transmission from S to D.

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Q4a. In an IPv4 datagram, the M bit is 0, the value of HLEN is 10, the value of total length is 400 and the fragment offset value is 300. Find the position of the datagram, the sequence numbers of the first and the last bytes of the payload respectively.20167m
Module 3: Network Layer & Routing
In an IPv4 datagram, the M bit is 0, the value of HLEN is 10, the value of total length is 400 and the fragment offset value is 300. Find the position of the datagram, the sequence numbers of the first and the last bytes of the payload respectively.

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Q4b. An IP router with a Maximum Transmission Unit (MTU) of 1500 bytes has received an IP packet of size 4404 bytes with an IP header of length 20 bytes. The values of the relevant fields in the header of the third IP fragment generated by the router for this packet are?20167m
Module 3: Network Layer & Routing
An IP router with a Maximum Transmission Unit (MTU) of 1500 bytes has received an IP packet of size 4404 bytes with an IP header of length 20 bytes. The values of the relevant fields in the header of the third IP fragment generated by the router for this packet are?

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Q5a. A serial transmission T1 uses 8 information bits, 2 start bits, 1 stop bit and 1 parity bit for each character. A synchronous transmission T2 uses 3 eight bit sync characters followed by 30 eight bit information characters. If the bit rate is 1200 bits/second in both cases, what are the transfer rates of T1 and T2?20167m
Module 1: Physical Layer & Data Communication
A serial transmission T1 uses 8 information bits, 2 start bits, 1 stop bit and 1 parity bit for each character. A synchronous transmission T2 uses 3 eight bit sync characters followed by 30 eight bit information characters. If the bit rate is 1200 bits/second in both cases, what are the transfer rates of T1 and T2?

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Q5b. Compare and contrast the Go-Back-N ARQ Protocol with Selective-Repeat ARQ.20167m

Module 2: Data Link Layer & MAC Sublayer

Compare and contrast the Go-Back-N ARQ Protocol with Selective-Repeat ARQ.

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Worked Solution

Solution

Go-Back-N (GBN) and Selective Repeat (SR) are sliding-window ARQ protocols used for reliable data transmission. Both use acknowledgements, sequence numbers, timers, and retransmission to recover from lost or damaged frames, but they differ in how they handle errors.

Feature	Go-Back-N ARQ	Selective Repeat ARQ
Receiver window	Usually 1	Greater than 1; accepts multiple frames
Out-of-order frames	Discarded	Buffered
Retransmission	Retransmits the erroneous/lost frame and all subsequent outstanding frames	Retransmits only the specific lost/damaged frame
ACKs	Typically cumulative	Typically selective/individual
Receiver complexity	Lower	Higher
Buffer requirement	Low	Higher
Bandwidth efficiency	Lower when errors are frequent	Higher, especially on noisy links
Implementation	Simpler	More complex

Example

Suppose frames 0, 1, 2, 3 are sent and frame 2 is lost.

Go-Back-N: The receiver detects the missing frame and does not accept frame 3 as the next in-order frame. The sender eventually retransmits frames 2 and 3 (and any later outstanding frames).

Selective Repeat: The receiver can accept and buffer frame 3 while waiting for frame 2. The sender retransmits only frame 2. After frame 2 arrives, the buffered frame 3 can be delivered in order.

Conclusion

Go-Back-N is simpler and requires less receiver buffering, but it can waste bandwidth because correctly received frames may be retransmitted. Selective Repeat is more efficient on error-prone or long-delay links because only lost or damaged frames are retransmitted, but it requires more receiver memory and more complex control logic.

Q6a. What are the advantages of fiber optics over copper as a transmission medium? Is there any downside of using fiber optics over copper?20167m
Module 1: Physical Layer & Data Communication
What are the advantages of fiber optics over copper as a transmission medium? Is there any downside of using fiber optics over copper?

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Q6b. 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?20167m
Module 4: Transport Layer & Congestion Control
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?

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Q7a. DNS uses UDP instead of TCP. If a DNS packet is lost, there is no automatic recovery. Does this cause a problem, and if so, how is it solved?20167m
Module 5: Application Layer & Security
DNS uses UDP instead of TCP. If a DNS packet is lost, there is no automatic recovery. Does this cause a problem, and if so, how is it solved?

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Q7b. Explain the responsibility of network and transport layers of OSI model with example.20167m
Module 3: Network Layer & Routing
Explain the responsibility of network and transport layers of OSI model with example.

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Q8a. What are the functions of a RIP message? List the RIP shortcomings and their corresponding fixes.20167m
Module 3: Network Layer & Routing
What are the functions of a RIP message? List the RIP shortcomings and their corresponding fixes.

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Q8b. Describe the functions of the two FTP connections. What kinds of file types can FTP transfer?20167m
Module 5: Application Layer & Security
Describe the functions of the two FTP connections. What kinds of file types can FTP transfer?

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Q9a. Compare the TCP header and the UDP header. List the fields in the TCP header that are missing from UDP header. Give the reason for their absence.20167m
Module 4: Transport Layer & Congestion Control
Compare the TCP header and the UDP header. List the fields in the TCP header that are missing from UDP header. Give the reason for their absence.

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Q9b. What is the difference between open-loop congestion control and closed-loop congestion control?20167m
Module 4: Transport Layer & Congestion Control
What is the difference between open-loop congestion control and closed-loop congestion control?

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