Chapter-1 Q&A Answers

Question & Answer:

SECTION 1.1

R1. What is the difference between a host and an end system? List several different

types of end systems. Is a Web server an end system?

Assignment1>>

End systems are also referred to as hosts because they host (that is, run) application Programs. Throughout this book we will use the terms hosts and end systems interchangeably; that is, host = end system. Hosts are sometimes further divided into two categories: clients and servers.

Therefore, a host is an end system, and vice versa. Types of end systems: desktops, laptops, smartphones, tablets, automobiles, various sensors, etc.Yes, a Web server is an end system.

R2. The word protocol is often used to describe diplomatic relations. How does

Wikipedia describe diplomatic protocol?

>>A protocol is described as the rules of etiquette for heads of state, for example in what order diplomats address representatives of other nations often in order of decreasing importance and power.

R3. Why are standards important for protocols?

Assignment1>>

To achieve useful work is done efficiently and economically. All activities in the Internet that involves two or more communication remote entities is governed by a protocol. Standards are important for protocols so that both communicating computers are sending and interpreting data in the same order and manner.

If end systems run different protocols (for example, if one person has manners but the other does not, or if one understands the concept of time and the other does not) the protocols do not interoperate and no useful work can be accomplished.

SECTION 1.2

R4. List six access technologies. Classify each one as home access, enterprise

access, or wide-area wireless access.

>>

DSL(Digital Subscriber Line) - Home Access
Cable - Home Access
FTTH(Fiber to the Home) - Home Access
Dial-Up - Home Access
Satellite - Home Access
Ethernet - Enterprise (and Home) Access
WiFi - Enterprise (and Home) Access
3G - Wide Area Network Access
LTE - Wide Area Network Access

R5. Is HFC transmission rate dedicated or shared among users? Are collisions

possible in a downstream HFC channel? Why or why not?

Assignment1>> HFC (cable internet) transmission rate is shared among users. There are no collisions in the downstream channel because all packets come from a single source, the head end.

R6. List the available residential access technologies in your city. For each type

of access, provide the advertised downstream rate, upstream rate, and

monthly price.

R7. What is the transmission rate of Ethernet LANs?

Assignment1>>

Users - 100 Mbps , Servers - 1 Gbps or even 10 Gbps
Ethernet LANs have transmission rates of 10 Mbps, 100 Mbps, 1 Gbps and 10 Gbps.

R8. What are some of the physical media that Ethernet can run over?

>>Today, Ethernet most commonly runs over twisted-pair copper wire. It also can run over fibers optic links.

R9. Dial-up modems, HFC, DSL and FTTH are all used for residential access.

For each of these access technologies, provide a range of transmission rates

and comment on whether the transmission rate is shared or dedicated.

Assignment1>>

Dial up modems: up to 56 Kbps, bandwidth is dedicated;

ADSL: up to 24 Mbps downstream and 2.5 Mbps upstream, bandwidth is dedicated;

HFC, rates up to 42.8 Mbps and upstream rates of up to 30.7 Mbps, bandwidth is shared.

FTTH: 2-10Mbps upload; 10-20 Mbps download; bandwidth is not shared.

R10. Describe the most popular wireless Internet access technologies today. Compare

and contrast them.

>> There are two popular wireless Internet access technologies today:

a) Wifi (802.11) In a wireless LAN, wireless users transmit/receive packets to/from an base station (i.e., wireless access point) within a radius of few tens of meters. The base station is typically connected to the wired Internet and thus serves to connect wireless users to the wired network.

b) 3G and 4G wide-area wireless access networks. In these systems, packets are transmitted over the same wireless infrastructure used for cellular telephony, with the base station thus being managed by a telecommunications provider. This provides wireless access to users within a radius of tens of kilometers of the base station. .

SECTION 1.3

R11. Suppose there is exactly one packet switch between a sending host and a receiving host. The transmission rates between the sending host and the switch and between the switch and the receiving host are R1 and R2, respectively.

Assuming that the switch uses store-and-forward packet switching, what is the total end-to-end delay to send a packet of length L? (Ignore queuing, propagation delay, and processing delay.)

Assignment1>>

d(end to end) = N(L/R)  // d – delay , N - Number of links, L - Number of bits in packets, R - rate
d(end to end) = (L/R1)+(L/R2)

R12. What advantage does a circuit-switched network have over a packet-switched

network? What advantages does TDM have over FDM in a circuit-switched

network?

Assignment1>>

A circuit-switched network can guarantee a certain amount of end-to-end bandwidth for the duration of a call. Most packet-switched networks today (including the Internet) cannot make any end-to-end guarantees for bandwidth.
In a packet switched network, the packets from different sources flowing on a link do not follow any fixed, pre-defined pattern. In TDM circuit switching, each host gets the same slot in a revolving TDM frame.

R13. Suppose users share a 2 Mbps link.

Also suppose each user transmits continuously at 1 Mbps when transmitting, but each user transmits only 20 percent of the time. (See the discussion of statistical multiplexing in Section 1.3.)

a. When circuit switching is used, how many users can be supported?

b. For the remainder of this problem, suppose packet switching is used. Why will there be essentially no queuing delay before the link if two or fewer  users transmit at the same time? Why will there be a queuing delay if three users transmit at the same time?

c. Find the probability that a given user is transmitting.

d. Suppose now there are three users. Find the probability that at any given time, all three users are transmitting simultaneously. Find the fraction of time during which the queue grows.

>> 

a) two users can be supported because each user requires half of the bandwidth

b)Since each user requires 1Mbps when transmitting, if two or fewer users transmit simultaneously, a maximum of 2Mbps will be required. Since the available bandwidth of the shared link is 2Mbps, there will be no queuing delay before the link. Whereas, if three users transmit simultaneously, the bandwidth required will be 3Mbps which is more than the available bandwidth of the shared link. In this case, there will be queuing delay before the link.

c) Probability that a given user is transmitting = 0.2

d) Probability that all three users are transmitting simultaneously = 3 33 1 3 3 pp = (0.2)3 = 0.008. Since the queue grows when all the users are transmitting, the fraction of time during which the queue grows (which is equal to the probability that all three users are transmitting simultaneously) is 0.008.

R14. Why will two ISPs at the same level of the hierarchy often peer with each other? How does an IXP earn money?

Assignment2>>

If the two ISPs do not peer with each other, then when they send traffic to each other they have to send the traffic through a provider ISP (intermediary), to which they have to pay for carrying the traffic.

By peering with each other directly, the two ISPs can reduce their payments to their provider ISPs. An Internet Exchange Points (IXP) (typically in a standalone building with its own switches) is a meeting point where multiple ISPs can connect and/or peer together.

An ISP earns its money by charging each of the ISPs that connect to the IXP a relatively small fee, which may depend on the amount of traffic sent to or received from the IXP.

R15. Some content providers have created their own networks. Describe Google’s network. What motivates content providers to create these networks?

>>Google's private network connects together all its data centers, big and small.

Traffic between the Google data centers passes over its private network rather than over the public Internet. Many of these data centers are located in, or close to, lower tier ISPs.

Therefore, when Google delivers content to a user, it often can bypass higher tier ISPs. What motivates content providers to create these networks?

First, the content provider has more control over the user experience, since it has to use few intermediary ISPs.

Second, it can save money by sending less traffic into provider networks. Third, if ISPs decide to charge more money to highly profitable content providers (in countries where net neutrality doesn't apply), the content providers can avoid these extra payments.

SECTION 1.4

R16. Consider sending a packet from a source host to a destination host over a fixed route. List the delay components in the end-to-end delay. Which of these delays are constant and which are variable?

Assignment2>>

The delay components are 1) processing delays, 2) transmission delays, 3) propagation delays, and 4) queuing delays. All of these delays are fixed, except for the queuing delays, which are variable.

R17. Visit the Transmission Versus Propagation Delay applet at the companion Web site.

Among the rates, propagation delay, and packet sizes available, find a combination for which the sender finishes transmitting before the first bit of the packet reaches the receiver.

Find another combination for which the first bit of the packet reaches the receiver before the sender finishes transmitting.

>>
a) 1000 km, 1 Mbps, 100 bytes
b) 100 km, 1 Mbps, 100 bytes

R18. How long does it take a packet of length 1,000 bytes to propagate over a link of distance 2,500 km, propagation speed 2.5 · 108 m/s, and transmission rate 2 Mbps?

More generally, how long does it take a packet of length L to propagate over a link of distance d, propagation speed s, and transmission rate R bps? Does this delay depend on packet length? Does this delay depend on transmission rate?

Assignment2>>

1) 10msec; 

2) d/s; 

3) no; no

R19. Suppose Host A wants to send a large file to Host B. The path from Host A to Host B has three links, of rates R1 = 500 kbps, R2 = 2 Mbps, and R3 = 1 Mbps.

a. Assuming no other traffic in the network, what is the throughput for the file transfer?

b. Suppose the file is 4 million bytes. Dividing the file size by the throughput, roughly how long will it take to transfer the file to Host B?

c. Repeat (a) and (b), but now with R2 reduced to 100 kbps.

Assignment2>>

a) 500 kbps
b) 64 seconds
c) 100kbps; 320 seconds

R20. Suppose end system A wants to send a large file to end system B. At a very high level, describe how end system A creates packets from the file. When one of these packets arrives to a packet switch, what information in the packet does the switch use to determine the link onto which the packet is forwarded? Why is packet switching in the Internet analogous to driving from one city to another and asking directions along the way?

>> End system A breaks the large file into chunks. It adds header to each chunk, thereby generating multiple packets from the file. The header in each packet includes the IP address of the destination (end system B). The packet switch uses the destination IP address in the packet to determine the outgoing link. Asking which road to take is analogous to a packet asking which outgoing link it should be forwarded on, given the packet's destination address.

R21. Visit the Queuing and Loss applet at the companion Web site. What is the maximum emission rate and the minimum transmission rate? With those rates, what is the traffic intensity? Run the applet with these rates and determine how long it takes for packet loss to occur. Then repeat the experiment a second time and determine again how long it takes for packet loss to occur. Are the values different? Why or why not?

>>The maximum emission rate is 500 packets/sec and the maximum transmission rate is 350 packets/sec. The corresponding traffic intensity is 500/350 =1.43 > 1. Loss will eventually occur for each experiment; but the time when loss first occurs will be different from one experiment to the next due to the randomness in the emission process.

SECTION 1.5

R22. List five tasks that a layer can perform. Is it possible that one (or more) of these tasks could be performed by two (or more) layers?

>>Five generic tasks are error control, flow control, segmentation and reassembly, multiplexing, and connection setup. Yes, these tasks can be duplicated at different layers. For example, error control is often provided at more than one layer.

R23. What are the five layers in the Internet protocol stack? What are the principal responsibilities of each of these layers?

>>The five layers in the Internet protocol stack are - from top to bottom - the application layer, the transport layer, the network layer, the link layer, and the physical layer. The principal responsibilities are outlined in Section 1.5.1.

R24. What is an application-layer message? A transport-layer segment? A network layer datagram? A link-layer frame?

>> 

Application-layer message: data which an application wants to send and passed onto the transport layer.

Transport-layer segment: generated by the transport layer and encapsulates application-layer message with transport layer header.

Network-layer datagram: encapsulates transport-layer segment with a network-layer header.

Link-layer frame: encapsulates network- layer datagram with a link-layer header.

R25. Which layers in the Internet protocol stack does a router process? Which layers does a link-layer switch process? Which layers does a host process?

Assignment2>>

1) Routers process network, link and physical layers (layers 1 through 3)…This is a little bit of a white lie, as modern routers sometimes act as firewalls or caching components, and process Transport layer as well.

2) Link layer switches process link and physical layers (layers 1 through2).

3) Hosts process all five layers.

SECTION 1.6

R26. What is the difference between a virus and a worm?

A virus requires some form of user interaction to infect a system. A worm on the other hand, requires no interaction. Instead a hacker may send a piece of malware to a vulnerable network application, and have the application run it, creating the worm.

a) Virus- Require some form of human interaction to spread. Classic example: E-mail viruses.
b) Worms-No user replication needed. Worm in infected host scans IP addresses and port numbers, looking for vulnerable processes to infect.

R27. Describe how a botnet can be created, and how it can be used for a DDoS attack.

Assignment2>>

Creation of a botnet requires an attacker to find vulnerability in some application or system (e.g. exploiting the buffer overflow vulnerability that might exist in an application).

After finding the vulnerability, the attacker needs to scan for hosts that are vulnerable.

The target is basically to compromise a series of systems by exploiting that particular vulnerability.

Any system that is part of the botnet can automatically scan its environment and propagate by exploiting the vulnerability.  An important property of such botnets is that the originator of the botnet can remotely control and issue commands to all the nodes in the botnet.

Hence, it becomes possible for the attacker to issue a command to all the nodes, that target a single node (for example, all nodes in the botnet might be commanded by the attacker to send a TCP SYN message to the target, which might result in a TCP SYN flood attack at the target).

R28. Suppose Alice and Bob are sending packets to each other over a computer network. Suppose Trudy positions herself in the network so that she can capture all the packets sent by Alice and send whatever she wants to Bob; she can also capture all the packets sent by Bob and send whatever she

wants to Alice. List some of the malicious things Trudy can do from this position.

>>Trudy can pretend to be Bob to Alice (and vice-versa) and partially or completely modify the message(s) being sent from Bob to Alice. For example, she can easily change the phrase "Alice, I owe you $1000" to "Alice, I owe you $10,000". Furthermore, Trudy can even drop the packets that are being sent by Bob to Alice (and vise-versa), even if the packets from Bob to Alice are encrypted.

Reference:

https://quizlet.com/89141555/computer-networking-chapter-1-review-questions-flash-cards/