Computer Network Gate Previous Year Questions

Consider an enterprise network with two Ethernet segments, a web server and a firewall, connected via three routers as shown below.

What is the number of subnets inside the enterprise network?

a) 3
b) 12
c) 6
d) 8

Consider a network with three routers P, Q, R shown in the figure below. All the links have cost of unity.

The routers exchange distance vector routing information and have converged on the routing tables, after which the link Q−R fails. Assume that P and Q send out routing updates at random times, each at the same average rate. The probability of a routing loop formation (rounded off to one decimal place) between P and Q, leading to count-to-infinity problem, is___________

Consider the data transfer using TCP over a 1 Gbps link. Assuming that the maximum segment lifetime (MSL) is set to 60 seconds, the minimum number of bits required for the sequence number field of the TCP header, to prevent the sequence number space from wrapping around during the MSL is____________.

Consider a 100 Mbps link between an earth station (sender) and a satellite (receiver) at an altitude of 2100 km. The signal propagates at a speed of 3x10⁸ m/s. The time taken (in milliseconds, rounded off to two decimal places) for the receiver to completely receive a packet of 1000 bytes transmitted by the sender is_________.

Consider the resolution of the domain name www.gate.org.in by a DNS resolver. Assume that no resource records are cached anywhere across the DNS servers and that an iterative query mechanism is used in the resolution. The number of DNS query-response pairs involved in completely resolving the domain name is________.

Consider the following statements about the functionality of an IP based router.

I. A router does not modify the IP packets during forwarding.
II. It is not necessary for a router to implement any routing protocol.
III. A router should reassemble IP fragments if the MTU of the outgoing link is larger than the size of the incoming IP packet.

Which of the above statements is/are TRUE ?

a) I and II only
b) I only
c) II and III only
d) II only

Consider a TCP connection between a client and a server with the following specifications; the round trip time is 6 ms, the size of the receiver advertised window is 50 KB, slow-start threshold at the client is 32 KB, and the maximum segment size is 2 KB. The connection is established at time t=0. Assume that there are no timeouts and errors during transmission. Then the size of the congestion window (in KB) at time t+60 ms after all acknowledgements are processed is _________ .

Assume that you have made a request for a web page through your web browser to a web server. Initially the browser cache is empty. Further, the browser is configured to send HTTP requests in non-persistent mode. The web page contains text and five very small images. The minimum number of TCP connections required to display the web page completely in your browser is ________ .

Consider that 15 machines need to be connected in a LAN using 8-port Ethernet switches. Assume that these switches do not have any separate up link ports. The minimum number of switches needed is ___________.

Consider three machines M, N and P with IP addresses 100.10.5.2, 100.10.5.5 and 100.10.5.6 respectively. The subnet mask is set to 255.255.255.252 for all the three machines. Which one of the following is true?

a) M, N and P all belong to the same subnet
b) Only N and P belong to the same subnet
c) M, N, and P belong to three different subnets
d) Only M and N belong to the same subnet

Which of the following protocol pairs can be used to send and retrieve e-mails (in that order)?

a) IMAP POP3
b) SMTP POP3
c) SMTP, MIME
d) IMAP SMTP

Consider a long-lived TCP session with an end-to-end bandwidth of 1 Gbps (= 10⁹ bits-per-second). The session starts with a sequence number of 1234. The minimum time (in seconds, rounded to the closest integer) before this sequence number can be used again is _______

Consider a simple communication system where multiple nodes are connected by a shared broadcast medium (like Ethernet or wireless). The nodes in the system use the following carrier-sense the medium access protocol. A node that receives a packet to transmit will carrier-sense the medium for 5 units of time. If the node does not detect any other transmission in this duration, it starts transmitting its packet in the next time unit. If the node detects another transmission, it waits until this other transmission finishes, and then begins to carrier-sense for 5 time units again. Once they start to transmit, nodes do not perform any collision detection and continue transmission even if a collision occurs. All transmissions last for 20 units of time. Assume that the transmission signal travels at the speed of 10 meters per unit time in the medium. Assume that the system has two nodes P and Q, located at a distance d meters from each other. P starts transmitting a packet at time t = 0 after successfully completing its carrier-sense phase. Node Q has a packet to transmit at time t = 0 and begins to carrier-sense the medium. The maximum distance d (in meters, rounded to the closest integer) that allows Q to successfully avoid a collision between its proposed transmission and P’s on going transmission is _______

Consider an IP packet with a length of 4,500 bytes that includes a 20-byte IPv4 header ans 40-byte TCP header. The packet is forwarded to an IPv4 router that supports a Maximum Transmission Unit (MTU) of 600 bytes. Assume that the length of the IP header in all the outgoing fragments of this packet is 20 bytes. Assume that the fragmentation offset value stored in the first fragment is 0. The fragmentation offset value stored in the third fragment is ______ .

The maximum number of IPv4 router address addresses that can be listed in the record route (RR) option field of an IPv4 header is ____.

Consider a socket API on Linux machine that supports UDP socket. A connected UDP socket is a UDP socket on which connect function has already been called. Which of the following statements is/are correct ?

I. A connected UDP socket can be used to communicate with multiple peers simultaneously.
II. A process can successfully call connect function again for an already connected UDP socket.

a) I only
b) II only
c) Both I and II only
d) Neither I nor II

Consider a socket API on Linux machine that supports UDP socket. A connected UDP socket is a UDP socket on which connect function has already been called. Which of the following statements is/are correct ?

I. A connected UDP socket can be used to communicate
with multiple peers simultaneously.

II. A process can successfully call connect function
again for an already connected UDP socket.

a) I only
b) II only
c) Both I and II
d) Neither I nor II

Consider the following statements about the routing protocols, Routing Information Protocol(RIP) and Open Shortest Path First(OSPF) in an IPv4 network.

I. RIP uses distance vector routing
II. RIP packets are sent using UDP
III. OSPF packets are sent using TCP
IV. OSPF operation is based on link-state routing
Which of the following above are CORRECT?

a) I and IV only
b) I, II and III only
c) I, II and IV only
d) II, III and IV only

The values of parameters for the Stop-and – Wait ARQ protocol are as given below.

In a RSA cryptosystem a particular A uses two prime numbers p = 13 and q =17 to generate her public and private keys. If the public key of A is 35. Then the private key of A is ____________.

Consider a TCP client and a TCP server running on two different machines. After completing data transfer, the TCP client calls close to terminate the connection and a FIN segment is sent to the TCP server. Server-side TCP responds by sending an ACK which is received by the client-side TCP. As per the TCP connection state diagram(RFC 793), in which state does the client side TCP connection wait for the FIN from the server-side TCP?

a) LAST-ACK
b) TIME-WAIT
c) FIN-WAIT-1
d) FIN-WAIT-2

A computer network uses polynomial over GF(2) for error checking with 8 bits as information bits and uses x³ + x + 1 as the generator polynomial to generate the check bits. In this network, the message 01011011 is transmitted as

a) 01011011010
b) 01011011011
c) 01011011101
d) 01011011100

A sender S sends a message m to receiver R, which is digitally signed by S with its private key. In this scenario, one or more of the following security violations can take place.

a) (I) only
b) (II) only
c) (I) and (II) only
d) (II) and (III) only

Consider a 128×103 bits/second satellite communication link with one-way propagation delay of 150 milliseconds. Selective retransmission (repeat) protocol is used on this link to send data with a frame size of 1 kilobyte. Neglect the transmission time of acknowledgment. The minimum number of bits required for the sequence number field to achieve 100% utilization is ___________

A network has a data transmission bandwidth of 20 × 106 bits per second. It uses CSMA/CD in the MAC layer. The maximum signal propagation time from one node to another node is 40 microseconds. The minimum size of a frame in the network is _________

For the IEEE 802.11 MAC protocol for wireless communication, which of the following statements is/are TRUE?

I. At least three non-overlapping channels are available for transmissions.
II. The RTS-CTS mechanism is used for collision detection.
III. Unicast frames are ACKed.

a) All I, II, and III
b) I and III only
c) II and III only
d) II only

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.

a) HTTP GET request, DNS query, TCP SYN
b) DNS query, HTTP GET request, TCP SYN
c) DNS query, TCP SYN, HTTP GET request
d) TCP SYN, DNS query, HTTP GET request

In an Ethernet local area network, which one of the following statements is TRUE ?

a) A station stops to sense the channel once it starts transmitting a frame.
b) The purpose of the jamming signal is to pad the frames that are smaller than the minimum frame size.
c) A station continues to transmit the packet even after the collision is detected.
d) The exponential backoff mechanism reduces the probability of collision on retransmissions

Anarkali digitally signs a message and sends it to Salim. Verification of the signature by Salim requires

a) Anarkali’s public key.
b) Salim’s public key.
c) Salim’s private key.
d) Anarkali’s private key.

A sender uses the Stop-and-Wait ARQ protocol for reliable transmission of frames. Frames are of size 1000 bytes and the transmission rate at the sender is 80 Kbps (1Kbps = 1000 bits/second). Size of an acknowledgement is 100 bytes and the transmission rate at the receiver is 8 Kbps. The one-way propagation delay is 100 milliseconds. Assuming no frame is lost, the sender throughput is __________ bytes/second.

An IP datagram of size 1000 bytes arrives at a router. The router has to forward this packet on a link whose MTU (maximum transmission unit) is 100 bytes. Assume that the size of the IP header is 20 bytes. The number of fragments that the IP datagram will be divided into for transmission is________

For a host machine that uses the token bucket algorithm for congestion control, the token bucket has a capacity of 1 megabyte and the maximum output rate is 20 megabytes per second. Tokens arrive at a rate to sustain output at a rate of 10 megabytes per second. The token bucket is currently full and the machine needs to send 12 megabytes of data. The minimum time required to transmit the data is _________________ seconds.

Consider that B wants to send a message m that is digitally signed to A. Let the pair of private and public keys for A and B be denoted by K^-_x and K⁺_x
for x = A,B, respectively. Let K_x(m)
represent the operation of encrypting m with a key K_x and H(m) represent the message digest.
Which one of the following indicates the CORRECT way of sending the message m along with the digital signature to A?

a) {m,K⁺_B (H(m))}
b) {m,K^-_B (H(m))}
c) {m,K^-_A (H(m))}
d) {m,K⁺_A (H(m))}

Which of the following is/are example(s) of stateful application layer protocols?

(i) HTTP
(ii) FTP
(iii) TCP
(iv) POP3

a) (i) and (ii) only
b) (ii) and (iii) only
c) (ii) and (iv) only
d) (iv) only

Which one of the following protocols is NOT used to resolve one form of address to another one?

a) DNS
b) ARP
c) DHCP
d) RARP

Which one of the following protocols is NOT used to resolve one form of address to another one?

a) DNS
b) ARP
c) DHCP
d) RARP

Consider a network connecting two systems located 8000 kilometers apart. The bandwidth of the network is 500 × 106 bits per second. The propagation speed of the media is 4 × 106 meters per second. It is needed to design a Go-Back-N sliding window protocol for this network. The average packet size is 107 bits. The network is to be used to its full capacity. Assume that processing delays at nodes are negligible. Then, the minimum size in bits of he sequence number field has to be ________.

Consider the following statements.

I. TCP connections are full duplex.
II. TCP has no option for selective acknowledgment
III. TCP connections are message streams.

a) Only I is correct
b) Only I and II are correct
c) Only II and III are correct
d) All of I, II and III are correct

Consider a CSMA/CD network that transmits data at a rate of 100 Mbps (108 bits per second) over a 1 km (kilometre) cable with no repeaters. If the minimum frame size required for this network is 1250 bytes, what is the signal speed (km/sec) in the cable?

a) 8000
b) 10000
c) 16000
d) 20000

A link has a transmission speed of 106 bits/sec. It uses data packets of size 1000 bytes each. Assume that the acknowledgment has negligible transmission delay, and that its propagation delay is the same as the data propagation delay. Also assume that the processing delays at nodes are negligible. The efficiency of the stop-and-wait protocol in this setup is exactly 25%. The value of the one-way propagation delay (in milliseconds) is ___________.

Which one of the following statements is NOT correct about HTTP cookies?

a) A cookies is a piece of code that has the potential to compromise the security of an Internet user.
b) A cookie gains entry to the user’s work area through an HTTP header.
c) A cookie has an expiry date and time.
d) Cookies can be used to track the browsing pattern of a user at a particular site.

Consider a LAN with four nodes S₁, S₂, S₃ and S₄. Time is divided into fixed-size slots, and a node can begin its transmission only at the beginning of a slot. A collision is said to have occurred if more than one node transmit in the same slot. The probabilities of generation of a frame in a time slot by S₁, S₂, S₃ and S₄ are 0.1, 0.2, 0.3 and 0.4, respectively. The probability of sending a frame in the first slot without any collision by any of these four stations is_________

Suppose two hosts use a TCP connection to transfer a large file. Which of the following statements is/are False with respect to the TCP connection?

I. If the sequence number of a segment is m, then the sequence number of the subsequent segment is always m+1.
II. If the estimated round trip time at any given point of time is t sec, the value of the retransmission timeout is always set to greater than or equal to t sec.
III. The size of the advertised window never changes during the course of the TCP connection.
IV. The number of unacknowledged bytes at the sender is always less than or equal to the advertised window.

Which one of the following fields of an IP header is NOT modified by a typical IP router?

a) Checksum
b) Source address
c) Time to Live (TTL)
d) Length

Suppose that everyone in a group of N people wants to communicate secretly with the N–1 others using symmetric key cryptographic system. The communication between any two persons should not be decodable by the others in the group. The number of keys required in the system as a whole to satisfy the confidentiality requirement is

a) 2N
b) N(N – 1)
c) N(N – 1)/2
d) (N – 1)²

In one of the pairs of protocols given below, both the protocols can use multiple TCP connections between the same client and the server. Which one is that?

a) HTTP, FTP
b) HTTP, TELNET
c) FTP, SMTP
d) HTTP, SMTP