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computer networking

Questions and Answers of Computer Networking

In the network of Figure 21.17, find the shortest path trees for router R if the network is using MOSPF with the source connected to the router marked as S. Assume that all routers have interest in
We say that a router in DVMRP creates a shortest-path tree on demand. What is the meaning of this statement? What is the advantage of creating shortest path trees only on demand?
Assume that m is much less than n and that router R is connected to n networks in which only m of these networks are interested in receiving packets related to group G. How can router R manage to
Assume we have 20 hosts in a small AS. There are only four groups in this AS. Find the number of spanning trees in each of the following approaches:a. Source-based treeb. Group-shared tree
It is obvious that we need to have spanning trees for both unicasting and multicasting. How many leaves of the tree are involved in a transmission in each case?a. A unicast transmissionb. A multicast
A router using DVMRP receives a packet with source address 10.14.17.2 from interface 2. If the router forwards the packet, what are the contents of the entry related to this address in the unicast
A multicast router is connected to four networks. The interest of each network is shown below. What is the group list that should be advertised by the router?a. N1: {G1, G2, G3}b. N2: {G1, G3}c. N3:
Why is there no need for the IGMP message to travel outside its own network?
Can a host have more than one multicast address? Explain.
Change the following IP multicast addresses to Ethernet multicast addresses. How many of them specify the same Ethernet address?a. 224.18.72.8b. 235.18.72.8c. 237.18.6.88d. 224.88.12.8
Define the group of each of the following multicast addresses (local network control block, internetwork control block, SSM block, Glop block, or administratively scoped block):a. 224.0.1.7b.
A multicast address for a group is 232.24.60.9. What is its 48-bit Ethernet address for a LAN using TCP/IP?
Define which of the following addresses are multicast addresses:a. 224.8.70.14b. 226.17.3.53c. 240.3.6.25
The AS number in an organization is 24101. Find the range of multicast addresses that the organization can use in the GLOP block.
When we send an e-mail to multiple recipients, are we are using multicasting or multiple unicasting? Give the reason for your answer.
Exactly describe why we cannot use the CIDR notation for the following blocks in Table 21.1:a. AD HOC block with the range 224.0.2.0 to 224.0.255.255.b. The first reserved block with the range
Distinguish between multicasting and multiple unicasting.
Write a program to simulate the path-vector algorithm (Table 20.3). Table 20.3 Path-vector algorithm for a node 1 Path_Vector_Routing ( ) 2 { // Initialization for (y = 1 to N) 3 { if (y is
Write a program to simulate the link-state algorithm (Table 20.2). Dijkstra's Algorithm ( ) // Initialization Tree = {root} 3 I/ Tree is made only of the root 4. for (y = 1 to N) II N is the
Write a program to simulate the distance-vector algorithm (Table 20.1). for (y = 1 to N) { if (y is a neighbor) D[y] = c[myself][y] else 10 D[y] = 0 11 } send vector {D[1], D[2], ..., D[N]} to
In a graph, if we know that the shortest path from node A to node G is (A → B → E → G), what is the shortest path from node G to node A?
Assume a router using RIP has 10 entries in its forwarding table at time t1. Six of these entries are still valid at time t2. Four of these entries have been expired 70, 90, 110, and 210 seconds
Assume the shortest path in a graph from node A to node H is A → B → H.Also assume that the shortest path from node H to node N is H → G → N.What is the shortest path from node A to node N?
When does an OSPF router send each of the following messages?a. Hellob. Data descriptionc. Link-state requestd. Link-state updatee. Link-state acknowledgment
Explain why a router using link-state routing needs to receive the whole LSDB before creating and using its forwarding table. In other words, why can’t the router create its forwarding table with a
To understand how the distance vector algorithm in Table 20.1 works, let us apply it to a four-node internet as shown in Figure 20.32.Table 20.1Assume that all nodes are initialized first. Also
Is the path-vector routing algorithm closer to the distance-vector routing algorithm or to the link-state routing algorithm? Explain.
In distance-vector routing, good news (decrease in a link metric) will propagate fast. In other words, if a link distance decreases, all nodes quickly learn about it and update their vectors. In
List three types of autonomous systems (ASs) described in the text, and make a comparison between them.
In distance-vector routing, bad news (increase in a link metric) will propagate slowly. In other words, if a link distance increases, sometimes it takes a long time for all nodes to know the bad
Explain the concept of hop count in RIP. Can you explain why no hop is counted between N1 and R1 in Figure 20.15? Figure 20.15 Hop counts in RIP NI N2 N3 N4 Source Destination RI R2 R3 1 hop (N4) 2
In computer science, when we encounter an algorithm, we often need to ask about the complexity of that algorithm (how many computations we need to do). To find the complexity of the distance
Assume that we have an isolated AS running RIP. We can say that we have at least two different kinds of datagram traffic in this AS. The first kind carries the messages exchanged between hosts; the
Assume that the network in Figure 20.34 uses distance-vector routing with the forwarding table as shown for each node.Figure 20.34If each node periodically announces their vectors to the neighbor
Router A sends two RIP messages to two immediate neighboring routers, B and C. Do the two datagrams carrying the messages have the same source IP addresses? Do the two datagrams have the same
Assume that the network in Figure 20.34 (previous problem) uses distancevector routing with the forwarding table as shown for each node. If each node periodically announces their vectors to the
At any moment, a RIP message may arrive at a router that runs RIP as the routing protocol. Does it mean that the RIP process should be running all the time?
Assume that the network in Figure 20.34 (Problem P20-8) uses distance vector routing with the forwarding table as shown for each node. If node E is added to the network with a link of cost 1 to node
Why do you think RIP uses UDP instead of TCP?
Create the forwarding table for node A in Figure 20.10. Figure 20.10 Least-cost tree Initialization Legend Root node A B Node in the path Node not yet in the path E F Potential path Path
We say that OSPF is a hierarchical intradomain protocol, but RIP is not. What is the reason behind this statement?
Create the shortest path tree and the forwarding table for node G in Figure 20.8. Figure 20.8 Example of a link-state database A B C DE F G A 0|2 00 B| 20|5 00 C00 D 3 0 3 00 4 00 B 4 3. 0 | 00 5 3
In a very small AS using OSPF, is it more efficient to use only one single area (backbone) or several areas?
Why do you think we need only one RIP update message, but several OSPF update messages?
Create the shortest path tree and the forwarding table for node B in Figure 20.8. Figure 20.8 Example of a link-state database A B C DE F G A02 00 B| 20 5 00 3 00 4 00 A C00 D 3 0 3 4 3. o0 0 4
OSPF messages are exchanged between routers. Does this mean that we need to have OSPF processes run all the time to be able to receive an OSPF message when it arrives?
Use Dijkstra's algorithm (Table 20.2) to find the shortest path tree and the forwarding table for node A in the Figure 20.35.Table 20.2 4. 4. 3. 3.
OSPF messages and ICMP messages are directly encapsulated in an IP datagram. If we intercept an IP datagram, how can we tell whether the payload belongs to OSPF or ICMP?
In computer science, when we encounter an algorithm, we often need to ask about the complexity of that algorithm (how many computations we need to do). To find the complexity of Dijkstra’s
Explain what type of OSPF link state is advertised in each of the following cases:a. A router needs to advertise the existence of another router at the end of a point-to-point link.b. A router needs
Assume that A, B, C, D, and E in Figure 20.36 are autonomous systems (ASs). Find the path vector for each AS using the algorithm in Table 20.3. Assume that the best path in this case is the path
Can a router combine the advertisement of a link and a network in a single link-state update?
In Figure 20.24, assume that the intra-AS routing protocol used by AS1 is OSPF, but the one used by AS2 is RIP. Explain how R5 can find how to route a packet to N4. Figure 20.24 A sample internet
Explain why we can have different intradomain routing protocols in different ASs, but we need only one interdomain routing protocol in the whole Internet.
In Figure 20.24, assume that the intra-AS routing protocol used by AS4 and AS3 is RIP. Explain how R8 can find how to route a packet to N13. Figure 20.24 A sample internet with four ASs N13 ASI N7 N8
Can you explain why BGP uses the services of TCP instead of UDP?
Explain why policy routing can be implemented on an interdomain routing, but it cannot be implemented on a intradomain routing.
Explain when each of the following attributes can be used in BGP:a. LOCAL-PREFb. AS-PATHc. NEXT-HOP
In Figure 19.4,a. Show how wrapped sum can be calculated from the sum using modular arithmetic.b. Show how checksum can be calculated from the wrapped sum using modular arithmetic. Figure 19.4
Which protocol is the carrier of the agent advertisement and solicitation messages?
Create a foreign agent advertisement message using 1672 as the sequence number and a lifetime of 4 hours. Select your own values for the bits in the code field. Use at least three care-of addresses
Discuss how the ICMPv4 router solicitation message can also be used for agent solicitation. Why are there no extra fields?
We have the information shown below. Show the contents of the IP datagram header sent from the remote host to the home agent.Mobile host home address: 130.45.6.7/16Mobile host care-of address:
Is registration required if the mobile host acts as a foreign agent? Explain your answer.
Create a home agent advertisement message using 1456 as the sequence number and a lifetime of 3 hours. Select your own values for the bits in the code field. Calculate and insert the value for the
Explain why the registration request and reply are not directly encapsulated in an IP datagram. Why is there a need for the UDP user datagram?
Redraw Figure 19.18 for the case where the mobile host acts as a foreign agent. Figure 19.18 Data transfer Home network Mobile host Remote network original home Remote host Home ag ent Internet
What are the source and destination IP addresses in a datagram that carries the ICMPv4 message reported by a router?
Briefly describe how we can defeat the following security attacks:a. Packet sniffingb. Packet modificationc. IP spoofing
Explain why the Internet does not create a report message to report the error in an IP datagram that carries an ICMPv4 message.
Redo the checksum in Figure 19.11 using decimal values and modular arithmetic. Figure 19.11 Example of checksum calculation 8 9 TEST 8 & 0 00001000 00000000 00000000 00000000 00000000 00000001
Assume a destination computer receives several packets from a source. How can it be sure that the fragments belonging to a datagram are not mixed with the fragments belonging to another datagram?
Redo the checksum in Figure 19.11 using hexadecimal values.Figure 19.11 Figure 19.11 Example of checksum calculation 8 9 TEST 8 & 0 00001000 00000000 00000000 00000000 00000000 00000001 Replaces 0 9.
Can each of the following be the value of the offset field in a datagram? Explain your answer.a. 8b. 31c. 73d. 56
A packet has arrived in which the offset value is 300 and the payload size is 100 bytes. What are the number of the first byte and the last byte?
Which fields in the datagram is(are) responsible for gluing together all fragments belonging to an original datagram?
Determine if a datagram with the following information is a first fragment, a middle fragment, a last fragment, or the only fragment (no fragmentation):a. M bit is set to 1 and the value of the
Compare and contrast the protocol field at the network layer with the port numbers at the transport layer. What is their common purpose? Why do we need two port-number fields but only one protocol
Which fields of the IPv4 main header may change from router to router?
Can each of the following be the value of the TTL in a datagram? Explain your answer.a. 23b. 0c. 1d. 301
In Figure 19.4, show how the sum, wrapped sum, and checksum can be calculated when the words are given in decimal numbers (the way the words are stored in a computer memory). Figure 19.4 Example of
In an IPv4 datagram, the value of the header-length (HLEN) field is (6)16. How many bytes of options have been added to the packet?
In Figure 19.4, show how the sum, wrapped sum, and checksum can be calculated when each word (16 bits) is created instead of waiting for the whole packet to be created. Figure 19.4 Example of
Mention the three auxiliary protocols at the network layer of the TCP/IP suite that are designed to help the IPv4 protocol.
An IP fragment has arrived with an offset value of 100. How many bytes of data were originally sent by the source before the data in this fragment?
An IP datagram has arrived with the following partial information in the header (in hexadecimal):45000054 00030000 2006...a. What is the header size?b. Are there any options in the packet?c. What is
A host is sending 100 datagrams to another host. If the identification number of the first datagram is 1024, what is the identification number of the last?
In an IPv4 datagram, the value of total-length field is (00A0)16 and the value of the header-length (HLEN) is (5)16. How many bytes of payload are being carried by the datagram? What is the
Can the value of the header length field in an IPv4 packet be less than 5? When is it exactly 5?
Assume router R2 in Figure 18.35 receives a packet with destination address 140.24.7.42. How is the packet routed to its final destination?Figure 18.35 Figure 18.35 Longest mask matching
Can router R1 in Figure 18.35 receive a packet with destination address 140.24.7.194? What will happen to the packet if this occurs?Figure 18.35 Figure 18.35 Longest mask matching 140.24.7.0/26
An organization is granted the block 130.56.0.0/16. The administrator wants to create 1024 subnets.a. Find the number of addresses in each subnet.b. Find the subnet prefix.c. Find the first and the
An ISP is granted the block 80.70.56.0/21. The ISP needs to allocate addresses for two organizations each with 500 addresses, two organizations each with 250 addresses, and three organizations each
An ISP is granted the block 16.12.64.0/20. The ISP needs to allocate addresses for 8 organizations, each with 256 addresses.a. Find the number and range of addresses in the ISP block.b. Find the
A large organization with a large block address (12.44.184.0/21) is split into one medium-size company using the block address (12.44.184.0/22) and two small organizations. If the first small company
Combine the following three blocks of addresses into a single block:a. 16.27.24.0/26b. 16.27.24.64/26c. 16.27.24.128/25
Assume we have an internet with a 9-bit address space. The addresses are divided between three networks (N0 to N2), with 64, 192, and 256 addresses respectively. The internetwork communication is
Assume we have an internet with a 12-bit address space. The addresses are equally divided between eight networks (N0 to N7). The internetwork communication is done through a router with eight
Assume we have an internet with an 8-bit address space. The addresses are equally divided between four networks (N0 to N3). The internetwork communication is done through a router with four
Compare NAT and DHCP. Both can solve the problem of a shortage of addresses in an organization, but by using different strategies.

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