# GATE Questions & Answers of Computer Networks Computer Science and Information Technology

#### Computer Networks 97 Question(s) | Weightage 09 (Marks)

Match the followin

 Field Length in bits P. UDP Header’s Port Number I. 48 Q. Ethernet MAC Address II. 8 R. IPv6 Next Header III. 32 S. TCP Header’s Sequence Number IV. 16

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Consider the following statements regarding the slow start phase of the TCP congestion control algorithm. Note that cwnd stands for the TCP congestion window and MSS denotes the Maximum Segment Size.
(i) The cwnd increases by 2 MSS on every successful acknowledgment.
(ii) The cwnd approximately doubles on every successful acknowledgement.
(iii) The cwnd increases by 1 MSS every round trip time.
(iv) The cwnd approximately doubles every round trip time.

Which one of the following is correct?

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Consider a long-lived TCP session with an end-to-end bandwidth of 1 Gbps (= 109 bits-persecond). 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 _______.

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Consider an IP packet with a length of 4,500 bytes that includes a 20-byte IPv4 header and a 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 _______.

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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 based 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 ongoing transmission is _____.

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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?

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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.

(I)  S can launch a birthday attack to replace m with a fraudulent message.

(II)  A third party attacker can launch a birthday attack to replace m with a fraudulent message.

(III)  R can launch a birthday attack to replace m with a fraudulent message.

Which of the following are possible security violations?

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A computer network uses polynomials 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

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In a RSA cryptosystem, a participant A uses two prime numbers p=13 and q=17 to genrate her public and private keys. If the public key of A is 35, then the privae key of A is __________.

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

Bit rate of the transmission chnnel = 1Mbps.
Propagation delay from sender to receive-0.75ms.
Time to process a frame = 0.25ms.
Number of bytes in the information frame=1980.
Number of bytes in the acknowlwdge frame=20.
Number of overhead bytes in the information frame =20.

Assume that there are no transmission errors. Then, the transmission efficiency (expressed in percentage) of the stop-and -wait ARQ protocol for the above parameters is ________(correct to 2 decimal places.)

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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 statement above are CORRECT?

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Consider socket API on a Linux machine that supports connected UDP sockets. A connected UDP socket is a UDP socket on which connect function has already benn called. Which of the following statement is/are CORRECT?

I. A coonected 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.

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

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Consider two hosts X and Y, connected by a single direct link of rate 106 bits/sec. The distance between the two hosts is 10,000 km and the propagation speed along the link is $\style{font-family:'Times New Roman'}{2\times10^8}$ m/sec. Host X sends a file of 50,000 bytes as one large message to host Y continuously. Let the transmission and propagation delays be p milliseconds and  milliseconds, respectively. Then the values of p and q are

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A message is made up entirely of characters from the set X={P, Q, R, S, T}. The table of probabilities for each characters is shown below:

 Character Probabilities P 0.22 Q 0.34 R 0.17 S 0.19 T 0.08 Total 1.00

If a message of 100 characters over X is encoded using Huffman coding, then the expected length of the encode message in bits is __________.

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

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

(i) HTTP

(ii) FTP

(iii) TCP

(iv) POP3

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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 $m$ withakey $K_x$ 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?

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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 _________.

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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.

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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.

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

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

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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.

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A network has a data transmission bandwidth of $20\times10^6$ 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 ________ bytes.

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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.

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Consider a $128\times10^3$ 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 acknowledgement. The minimum number of bits required for the sequence number field to achieve 100% utilization is _________.

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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 curse of the TCP connection. IV. The number of unacknowledged bytes at the sender is always less than or equal to the advertised window.

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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

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

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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?

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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 acknowledgement and the processing time at nodes are negligible.Then the minimum frame size in bytes to achieve a link utilization of at least 50% is _________.

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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 probability 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 ________.

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A link has a transmission speed of 106 bits/sec. It uses data packets of size 1000 bytes each. Assume that the acknowledgement 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 _______.

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Consider the following routing table at an IP router.

 Network No. Net Mask Next Hop 128.96.170.0 255.255.254.0 Interface 0 128.96.168.0 255.255.254.0 Interface 1 128.96.166.0 255.255.254.0 R2 128.96.164.0 255.255.252.0 R3 0.0.0.0 Default R4

For each IP address in Group I identify the correct choice of the next hop form Group II using the entries from the routing table above.

 Group I Group II i) 128.96.171.92 a) Interface 0 ii) 128.96.167.151 b) Interface 1 iii) 128.96.163.151 c) R2 iv) 128.96.165.121 d) R3 e) R4

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Host A sends a UDP datagram containing 8880 bytes of user data to host B over an Ethernet LAN. Ethernet frames may carry data up to 1500 bytes (i.e. MTU = 1500 bytes).Size of UDP header is 8 bytes and size of IP header is 20 bytes. There is no option field in IP header. How many total number of IP fragments will be transmitted and what will be the contents of offset field in the last fragment?

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Assume that the bandwidth for a TCP connection is 1048560 bits/sec. Let $\alpha$ be the value of RIT in milliseconds (rounded off to the nearest integer) after which the TCP window scale option is needed. Let $\beta$ be the maximum possible window size with window scale option. Then the values of $\alpha$ and $\beta$ are

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Consider the following statements

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

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Consider a CSMA/CD network that transmits data at a rate of 100 Mbps (108 bits per second) over a 1 km (kilometer) 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?

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In the network 200.10.11.144/27, the fourth octet (in decimal) of the last IP address of the network which can be assigned to a host is ______.

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Consider a network connecting two systems located 8000 kilometers apart. The bandwidth of the network is 500 X 106 bits per second. The propagation speed of the media is 4 X 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 the sequence number field has to be _________.

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Two hosts are connected via a packet switch with 107 bits per second links. Each link has a propagation delay of 20 microseconds. The switch begins forwarding a packet 35 microseconds after it receives the same. If 10000 bits of data are to be transmitted between the two hosts using a packet size of 5000 bits, the time elapsed between the transmission of the first bit of data and the reception of the last bit of the data in microseconds is __________.

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Consider the following three statements about link state and distance vector routing protocols, for a large network with 500 network nodes and 4000 links.
[S1] The computational overhead in link state protocols is higher than in distance vector protocols.
[S2] A distance vector protocol (with split horizon) avoids persistent routing loops, but not a link state protocol.
[S3] After a topology change, a link state protocol will converge faster than a distance vector protocol.

Which one of the following is correct about S1, S2, and S3 ?

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Which of the following are used to generate a message digest by the network security protocols?
(P) RSA (Q) SHA-1 (R) DES (S) MD5

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Identify the correct order in which the following actions take place in an interaction between a web browser and a web server.
1. The web browser requests a webpage using HTTP.
2. The web browser establishes a TCP connection with the web server.
3. The web server sends the requested webpage using HTTP.
4. The web browser resolves the domain name using DNS.

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Consider a token ring network with a length of 2 km having 10 stations including a monitoring station. The propagation speed of the signal is 2 × 108 m/s and the token transmission time is ignored. If each station is allowed to hold the token for 2 μsec, the minimum time for which the monitoring station should wait (in μsec)before assuming that the token is lost is _______.

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Let the size of congestion window of a TCP connection be 32 KB when a timeout occurs. The round trip time of the connection is 100 msec and the maximum segment size used is 2 KB. The time taken (in msec) by the TCP connection to get back to 32 KB congestion window is _________.

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Consider a selective repeat sliding window protocol that uses a frame size of 1 KB to send data on a 1.5 Mbps link with a one-way latency of 50 msec. To achieve a link utilization of 60%, the minimum number of bits required to represent the sequence number field is ________.

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Which of the following is TRUE about the interior gateway routing protocols - Routing Information Protocol(RIP) and Open Shortest Path First(OSPF)?

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Which one of following socket API functions converts an unconnected active TCP socket into a passive socket?

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In the diagram shown below,L1 is an Ethernet LAN and L2 is a token-ring LAN. AN IP packet originates from sender S and travers to R,as shown.the links within each ISP and across the twon ISPs, are all point-to-point optical links. The initial value of the TTL fields is 32.The maximum possible value of the TTL fields when R receives the diagram us _______.

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Consider the store and forward packet switched network given below. Assume that the bandwidth of each link is 106 bytes / sec. A user on host A sends a file of size 103 bytes to host B through routers R1 and R2 in three different ways. In the first case a single packet containing the completea file is transmitted from A to B. In the second case, the file is split into 10 equal parts, and these packets are transmitted from A to B. In the third case, the file is split into 20 equal parts and these packets are sent from A to B. Each packet contains 100 bytes of header information along with the user data. Consider only transmission time and ignore processing, queuing and propagation delays. Also assume that there are no errors during transmission. Let T1, T2 and T3 be the times taken to transmit the file in the first, second and third case respectively. Which one of the following is CORRECT?

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An IP machine Q has a path to another IP machine H via three IP routers R1, R2, and R3.

Q—R1—R2—R3—H

H acts as an HTTP server, and Q connects to H via HTTP and downloads a file. Session layer encryption is used, with DES as the shared key encryption protocol. Consider the following four pieces of information:

[I1] The URL of the file downloaded by Q
[I2] The TCP port numbers at Q and H
[I3] The IP addresses of Q and H
[I4] The link layer addresses of Q and H

Which of I1, I2, I3, and I4 can an intruder learn through sniffing at R2 alone?

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A graphical HTML browser resident at a network client machine Q accesses a static HTML webpage from a HTTP server S. The static HTML page has exactly one static embedded image which is also at S. Assuming no caching, which one of the following is correct about the HTML webpage loading (including the embedded image)?

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In the following pairs of OSI protocol layer/sub-layer and its functionality, the INCORRECT pair is

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A bit-stuffing based framing protocol uses an 8-bit delimiter pattern of 01111110. If the output bit-string after stuffing is 01111100101, then the input bit-string is

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Host A (on TCP/IP v4 network A) sends an IP datagram D to host B (also on TCP/IP v4 network B). Assume that no error occurred during the transmission of D. When D reaches B, which of the following IP header field(s) may be different from that of the original datagram D?

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An IP router implementing Classless Inter-domain Routing (CIDR) receives a packet with address 131.23.151.76. The router’s routing table has the following entries:

 Prefix Output Interface Identifier 131.16.0.0/ 12 3 131.28.0.0/ 14 5 131.19.0.0/ 16 2 131.22.0.0/ 15 1

The identifier of the output interface on which this packet will be forwarded is ______.

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Every host in an IPv4 network has a 1-second resolution real-time clock with battery backup. Each host needs to generate up to 1000 unique identifiers per second. Assume that each host has a globally unique IPv4 address. Design a 50-bit globally unique ID for this purpose. After what period (in seconds) will the identifiers generated by a host wrap around?

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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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The transport layer protocols used for real time multimedia, file transfer, DNS and email,respectively are

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Using public key cryptography, X adds a digital signature σ to message M, encrypts <M, σ>, and sends it to Y, where it is decrypted. Which one of the following sequences of keys is used for the operations?

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Assume that source S and destination D are connected through two intermediate routers labeled 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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Determine the maximum length of the cable (in km) for transmitting data at a rate of 500 Mbps in an Ethernet LAN with frames of size 10,000 bits. Assume the signal speed in the cable to be 2,00,000 km/s.

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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. The position of the datagram, the sequence numbers of the first and the last bytes of the payload, respectively are

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The protocol data unit (PDU) for the application layer in the Internet stack is

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Which of the following transport layer protocols is used to support electronic mail?

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In the IPv4 addressing format, the number of networks allowed under Class C addresses is

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An Internet Service Provider (ISP) has the following chunk of CIDR-based IP addresses available with it: 245.248.128.0/20. The ISP wants to give half of this chunk of addresses to Organization A, and a quarter to Organization B, while retaining the remaining with itself. Which of the following is a valid allocation of addresses to A and B?

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Consider a source computer (S) transmitting a file of size 106 bits to a destination computer (D) over a network of two routers (R1 and R2) and three links (L1, L2, and L3). L1 connects S to R1; L2 connects R1 to R2; and L3 connects R2 to D. Let each link be of length 100 km. Assume signals travel over each link at a speed of 108 meters per second. Assume that the link bandwidth on each link is 1Mbps. Let the file be broken down into 1000 packets each of size 1000 bits. Find the total sum of transmission and propagation delays in transmitting the file from S to D?

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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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A layer-4 firewall (a device that can look at all protocol headers up to the transport layer) CANNOT

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Consider different activities related to email.

m1: Send an email from a mail client to a mail server

m2: Download an email from mailbox server to a mail client

m3: Checking email in a web browser

Which is the application level protocol used in each activity?

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Consider a network with five nodes, N1 to N5, as shown below

The network uses a Distance Vector Routing Protocol.once the Route have stabilized, the distance vectors

at different nodes are as following
N1:(0,1, 7, 8, 4)
N2: (1, 0, 6, 7, 3)
N3: (7, 6, 0, 2, 6)
N4: (8,7, 2,0,4)
N5: (4, 3, 6, 4, 0)

Each distance vector is the distance of best known path at that instance to nodes, N1 to N5, where the distance to itself is 0. Also, all links are symmetric and the cost is identical in both directions. In each round, all nodes exchange their distance vectors with their respective neighbors. Then all nodes update the distance vectors. In between two rounds, any change in cost of a link will cause the two incident nodes to change only that entry in their distance vectors.

The cost of link N2-N3 reduces to 2 (in both directions). After the next round updates, what will be the new distance vector at node, N3?

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Consider a network with five nodes, N1 to N5, as shown below

The network uses a Distance Vector Routing Protocol.once the Route have stabilized, the distance vectors

at different nodes are as following
N1:(0,1, 7, 8, 4)
N2: (1, 0, 6, 7, 3)
N3: (7, 6, 0, 2, 6)
N4: (8,7, 2,0,4)
N5: (4, 3, 6, 4, 0)

Each distance vector is the distance of best known path at that instance to nodes, N1 to N5, where the distance to itself is 0. Also, all links are symmetric and the cost is identical in both directions. In each round, all nodes exchange their distance vectors with their respective neighbors. Then all nodes update the distance vectors. In between two rounds, any change in cost of a link will cause the two incident nodes to change only that entry in their distance vectors.

The cost of link N2-N3 reduces to 2 (in both directions). After the next round updates, what will be the new distance vector at node, N3?

At the update in the previous question ,the link N1-N2 goes down. N2 will reflect this change immediately in its distance vector as cost, $\infty$. After the NEXROUND of update, what will be the cost to N1 in the distance vector of N3?

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One of the header fields in an IP datagram is the Time to Live (TTL) field. Which of the following statements best explains the need for this field?

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Suppose computers A and B have IP addresses 10.105.1.113 and 10.105.1.91 respectively and they both use the same net mask N. Which of the values of N given below should not be used if A and B should belong to the same network?

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Consider a network with 6 routers R1 to R6 connected with links having weights as shown in the following diagram

All the routers use the distance vector based routing algorithm to update their routing tables. Each router starts with its routing table initialized to contain an entry for each neighbour with the weight of the respective connecting link. After all the routing tables stabilize, how many links in the network will never be used for carrying any data?

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Consider a network with 6 routers R1 to R6 connected with links having weights as shown in the following diagram

All the routers use the distance vector based routing algorithm to update their routing tables. Each router starts with its routing table initialized to contain an entry for each neighbour with the weight of the respective connecting link. After all the routing tables stabilize, how many links in the network will never be used for carrying any data?

Suppose the weights of all unused links in the previous question are changed to 2 and the distance vector algorithm is used again until all routing tables stabilize. How many links will now remain unused?

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In the RSA public key cryptosystem, the private and public keys are (e,n) and (d,n) respectively, where n=p*q and p and q are large primes. Besides, n is public and p and q are private. Let M be an integer such that 0<M<n and Φ(n) = (p-1)(q-1). Now consider the following equations.

I.   M' = Me mod n
M = (M')d mod n
II.  ed $\equiv$ 1 mod n
III. ed $\equiv$ 1 mod Φ(n)
IV. M' = Me mod Φ(n)
M = (M')d mod Φ(n)

Which of the above equations correctly represent RSA cryptosystem?

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While opening a TCP connection, the initial sequence number is to be derived using a time-of-day (ToD) clock that keeps running even when the host is down. The low order 32 bits of the counter of the ToD clock is to be used for the initial sequence numbers. The clock counter increments once per millisecond. The maximum packet lifetime is given to be 64s.

Which one of the choices given below is closest to the minimum permissible rate at which sequence numbers used for packets of a connection can increase?

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Let G(x) be the generator polynomial used for CRC checking. What is the condition that should be satisfied by G(x) to detect odd number of bits in error?

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Frames of 1000 bits are sent over a 106bps duplex link between two hosts. The propagation time is 25ms. Frames are to be transmitted into this link to maximally pack them in transit (within the link).

What is the minimum number of bits (l) that will be required to represent the sequence numbers distinctly? Assume that no time gap needs to be given between transmissions of two frames.

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Frames of 1000 bits are sent over a 106bps duplex link between two hosts. The propagation time is 25ms. Frames are to be transmitted into this link to maximally pack them in transit (within the link).

Suppose that the sliding window protocol is used with the sender window size of 2l, where l is the number of bits identified in the earlier part and acknowledgements are always piggy backed. Aftersending 2l frames, what is the minimum time the sender will have to wait before starting transmission of the next frame? (Identify the closest choice ignoring the frame processing time.)

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What is the maximum size of data that the application layer can pass on to the TCP layer below?

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In the slow start phase of the TCP congestion control algorithm, the size of the congestion window