mc0075 computer networks

MC0075 – Computer Networks

ASSIGNMENT SET – 1

1. Explain in what way computer networks is useful for the people with examples ?

Ans: Computer networking is useful to people in many ways. some of the following points are listed and discussed here to tell my blog readers about the importance of the computer networks in common people’s day-to-day life. They are….

Access to remote information Person-to-person communication Interactive entertainment

Access to remote information will come in many forms. One area in which it is already happening is access to financial institutions. We can pay our utility bills online, can manage our bank accounts, handle our investments electronically, transfer funds online, buying and selling shares online and booking reservations online. Newspapers are going online and can be personalized as per our requirement. We can tell the newspaper that we want everything about corrupt politicians, big fires, scandals involving celebrities, and epidemics, but no foot ball, Thank you. The content of the news will be delivered to our desk as per our interests at night while we sleep every day, the news paper will be downloaded to our computer disk or can be printed on our laser printer. The next step beyond news papers, magazines and scientific journals is the online digital library. We are getting all the news papers content, magazines and scientific journals are available digitally online. Depending on the cost, size and weight of book sized notebook computers, printed books may become obsolete. Another application that falls in this category is access to information systems like the current world wide web, which contains information about the arts, business, cooking, government, health, history, hobbies, recreation, science, sports, travel, and too many other topics.

The second broad category of network use will be person-to-person interactions. Electronic mail or email is widely used by millions of people and it contains also audio and video as well as text. Real time email will allow remote users to communicate with no delay, possible seeing and hearing each other as well. This technology makes it possible to have virtual meetings, called video conference, among far-flung people.

Our third category is entertainment, which is, a huge and growing industry, is video on demand. It is possible to select any movie or television program ever made, in any country, and have it displayed on our screen instantly. New films became interactive, where the user is occasionally prompted for the story direction with alternative scenarios provided for all cases. Live television also became interactive, with the audience participating in quiz shows, choosing among contestants, and so on.

2. Describe the ISO-OSI reference model and discuss the importance of every layer ?

Ans: The OSI, or Open System Interconnection, model defines a networking framework for implementing

protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in

one station, and proceeding to the bottom layer, over the channel to the next station and back up the

hierarchy.

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The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, and proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.

Application (Layer 7) : This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.

Presentation (Layer 6) : This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.

Session (Layer 5) : This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.

Transport (Layer 4) : This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.

Network (Layer 3) : This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.

Data Link (Layer 2) : At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.

Physical (Layer 1) : This layer conveys the bit stream - electrical impulse, light or radio signal -- through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.

3. Explain the following with respect to Data Communications:A) Fourier analysis B) Band limited signals C) Maximum data rate of a channel

Ans: A) Fourier analysisIn 19th century, the French mathematician Fourier proved that any periodic function of time g(t) with period T can be constructed by summing a number of cosines and sines.

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Where f=1/T is the fundamental frequency, and are the sine and cosine amplitudes of thenth harmonics. Such decomposition is called a Fourier series.

B) Band limited signalsConsider the signal given in figure below. Figure shows the signal that is the ASCIIrepresentation of the character ‘b’ which consists of the bit pattern ‘01100010’ along with itsharmonics

Any transmission facility cannot pass all the harmonics and hence few of the harmonics arediminished and distorted. The bandwidth is restricted to low frequencies consisting of 1, 2, 4,and 8 harmonics and then transmitted. Figures show the spectra and reconstructed functionsfor these band-limited signals.

Limiting the bandwidth limits the data rate even for perfect channels. However complexcoding schemes that use several voltage levels do exist and can achieve higher data rates.

C) Maximum data rate of a channelIn 1924, H. Nyquist realized the existence of the fundamental limit and derived the equationexpressing the maximum data for a finite bandwidth noiseless channel. In 1948, ClaudeShannon carried Nyquist work further and extended it to the case of a channel subject torandom noise.

In communications, it is not really the amount of noise that concerns us, but rather the amountof noise compared to the level of the desired signal. That is, it is the ratio of signal to noisepower that is important, rather than the noise power alone. This Signal¬-to-Noise Ratio(SNR), usually expressed in decibel (dB), is one of the most important specifications of anycommunication system. The decibel is a logarithmic unit used for comparisons of power levelsor voltage levels. In order to understand the implication of dB, it is important to know that asound level of zero dB corresponds to the threshold of hearing, which is the smallest soundthat can be heard. A normal speech conversation would measure about 60 dB.If an arbitrary signal is passed through the Low pass filter of bandwidth H, the filtered signalcan be completely reconstructed by making only 2H samples per second. Sampling the linefaster than 2H per second is pointless. If the signal consists of V discrete levels, then Nyquisttheorem states that, for a noiseless channelMaximum data rate = 2H.log2 (V) bits per second. For a noisy channel with bandwidth is again H, knowing signal to noise ratio S/N, themaximum data rate according to Shannon is given asMaximum data rate = H.log2 (1+S/N) bits per second.

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4 . Explain the following concepts of Internetworking:

A) Internet architecture B) Protocols and Significance for internetworking C) Internet layering

model ?

Ans: A) Internet architecture

Internet Architecture: B1-226, B2-56: The Internet is a worldwide, publicly accessiblenetwork of interconnected computer networks that transmit data by packet switching using thestandard Internet Protocol (IP). It is a "network of networks" that consists of millions of smallerdomestic, academic, business, and government networks, which together carry variousinformation and services, such as electronic mail, online chat, file transfer, and the interlinkedweb pages and other documents of the World Wide Web.How are networks interconnected to form an internetwork? The answer has two parts.Physically, two networks can only be connected by a computer that attaches both of them.But just a physical connection cannot provide interconnection where information can beexchanged as there is no guarantee that the computer will cooperate with other machines thatwish to communicate.Internet is not restricted in size. Internets exist that contain a few networks and internets alsoexist that contain thousands of networks. Similarly the number of computers attached to eachnetwork in an internet can vary. Some networks have no computers attached, while othershave hundreds.To have a viable internet, we need a special computer that is willing to transfer packets fromone network to another. Computers that interconnect two networks and pass packets fromone to the other are called internet gateways or internet routers.

B) Protocols and Significance for Internetworking

Protocols for internetworking: Many protocols have been used for use in an internet. Onesuite known as The TCP/IP internet protocol stands out most widely used for internets. Mostnetworking professional simply refer this protocol as TCP/IP. Work on the transmissioncontrol protocol (TCP) began in the 1970’s. The U.S military funded the research in TCP/IPand internetworking through the Advanced Research Projects Agency in short known asARPA.

Significance of internetworking and TCP/IP

Internetworking has become one of the important technique in the modern networking.Internet technology has revolutionized the computer communication. The TCP/IP technologyhas made possible a global Internet, which reaches millions of schools, commercial

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organizations, government and military etc around the world.The worldwide demand for internetworking products has affected most companies sellnetworking technologies. Competition has increased among the companies that sell thehardware and software needed for internetworking. Companies have extended the designs intwo ways

The protocols have adapted to work with many network technologiesAnd new features have been adapted that allow the protocols to transfer data across theInternets

C) Internet Layering Model : Internet uses the TCP/IP reference model. This model is also called as Internet layering model or internet reference model. This model consists of 5 layers as illustrated in figurebelow.

The five layers of TCP/IP reference model

A goal was of continuing the conversation between source and destination even iftransmission went out of operation. The reference model was named after two of its mainprotocols, TCP (Transmission Control Protocol) and IP (Internet Protocol). The purpose ofeach layer of TCP/IP is given below:

Layer 1: Physical layer : This layer corresponds to basic network hardware

Layer 2: Network interface : This layer specifies how to organize data into frames and how a computer transfers frames over a network. It interfaces the TCP/IP protocol stack to the physical network.

Layer 3: Internet : This layer specifies the format of packets sent across an internet. It also specifies themechanism used to forward packets from a computer through one or more routers to the final destination.

Layer 4: Transport : This layer deals with opening and maintaining connections, ensuring that packets are in fact received. The transport layer is the interface between the application layer and the complexhardware of the network. It is designed to allow peer entities on the source and destinationhosts to carry on conversations.

Layer 5: Network interface : Each protocol of this layer specifies how one application uses an internet.

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5 . What is the use of IDENTIFIER and SEQUENCE NUMBER fields of echo request and echo

reply message? Explain.

Ans: The echo request contains an optional data area. The echo reply contains the copy of thedata sent in the request message. The format for the echo request and echo reply is asshown in figure below

echo request and echo reply message format

The field OPTIONALDATA is a variable length that contains data to be returned to the originalsender. An echo reply always returns exactly the same data as ws to receive in the request. Field IDENTIFIER and SEQUENCE NUMBER are used by the sender to match replies to requests. The value of the TYPE field specifies whether it is echo request when equal to 8 or echo reply when equal to 0.

Reports of UnreachabilityWhen a router cannot forward or deliver the datagram to the destination owing to variousproblems, it sends a destination unreachable message back to the original sender and thendrops the datagram.

Destination unreachable message format

The format of destination unreachable is as shown in figure 5.3. The TYPE field in destinationunreachable message contains an integer equal to 3. The CODE field here contains aninteger that describes the problem why the datagram is not reachable. Possible values forCODE field are listed in below figure.

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DE VALUE MEANING

0 Network unreachable1 Host unreachable2 Protocol unreachable3 Port unreachable4 Fragment needed and DF set5 Source route failed6 Destination network unknown7 Destination host unknown8 Source host isolated9 Communication with destination network administratively prohibited

10 Communication with destination host administratively prohibited 11 Network unreachable for type of service 12 Host unreachable for type of service

Possible problems in Destination unreachable message

Network unreachable errors imply routing failures and host unreachable errors imply deliveryfailures. As ICMP error message contains a short prefix of the datagram that caused theproblem, the source will know exactly which address is unreachable.The port is the destination point discussed at the transport layer. If the datagram contains thesource route option with a wrong route, it may report source route failure message. If a routerneeds to fragment a datagram and DF-bit which is don’t fragment bit in IP header is set, therouter sends a Fragment needed and DF set message back to the source. Rests of the errorslisted in figure 5.4 are self explanatory.

Obtaining a subnet maskTo participate in subnet addressing, a host needs to know which bits of the 32-bit internetaddress correspond to physical network and which corresponds to host identifiers. Theinformation needed to interpret the address is represented in 32-bit quantity is called subnetmask. To learn the subnet mask used for local network, a machine can send an addressmask request message to a router and receive address mask reply message.

Address mask request or reply message format

Address mask request or reply message format

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The format address mask request or reply message is as shown in figure 5.10. Hostbroadcasts a request without knowing which specific router will respond. The TYPE fieldvalue is 17 for address mask request and 18 for address mask reply message. A replycontains the networks subnet address mask in the ADDRESS MASK field. IDENTIFIER andSEQUENCE NUMBER fields allow to associate replies with requests.

6 . How does the BGP categorize the networks? Explain.

Ans: BGP uses TCP as the transport protocol, on port 179. Two BGP routers form a TCP connection between one another. These routers are peer routers. The peer routers exchange messages to open and confirm the connection parameters.

BGP routers exchange network reachability information. This information is mainly an indication of the full paths that a route must take in order to reach the destination network. The paths are BGP AS numbers. This information helps in the construction of a graph of ASs that are loop-free. The graph also shows where to apply routing policies in order to enforce some restrictions on the routing behavior.

Any two routers that form a TCP connection in order to exchange BGP routing information are "peers" or "neighbors". BGP peers initially exchange the full BGP routing tables. After this exchange, the peers send incremental updates as the routing table changes. BGP keeps a version number of the BGP table. The version number is the same for all the BGP peers. The version number changes whenever BGP updates the table with routing information changes. The send of keepalive packets ensures that the connection between the BGP peers is alive. Notification packets go out in response to errors or special conditions.

eBGP and iBGP

If an AS has multiple BGP speakers, the AS can serve as a transit service for other ASs. As the diagram in this section shows, AS200 is a transit AS for AS100 and AS300.

In order to send the information to external ASs, there must be an assurance of the reachability for networks. In order to assure network reachability, these processes take place:

Internal BGP (iBGP) peering between routers inside an AS Redistribution of BGP information to IGPs that run in the AS

When BGP runs between routers that belong to two different ASs, this is called exterior BGP (eBGP). When BGP runs between routers in the same AS, this is called iBGP.

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ASSIGNMENT SET – 2

1. List and discuss the service primitives for connection oriented services ?

Ans: A service is formally specified by a set of primitives or operations available to the user to access the service. These primitives tell the service to perform some action or report an action taken by the peer entity. The primitives for the connection oriented service are given in following table.

Primitives Meaning--------------------------------------------------------------------------------------Listen Block waiting for an incoming connectionConnect Establish a connection with waiting peerReceive Block waiting for an incoming messageSend Send a message to the peerDisconnect Terminate a connection

First the server executes LISTEN to indicate that it is ready to accept incoming connections. Theclient executes CONNECT to establish the connection with the server. The server now unblocksthe listener and sends back an acknowledgement. Thus the connection is established.

The next step for a server is to execute RECEIVE to prepare to accept the first request. Thearrival of the request packet unblocks the server so that it can process the request. After it hasdone the work it uses SEND to answer to the client. After all the data transfer is done it can useDISCONNECT for suspending the client. When the server gets this packet, it also issues aDISCONNECT and when it reaches the client, the client process is releases and the connectionis broken. In the process packets may get lost, timings may be wrong; many other complex issues may arise.

2. Describe the following Medium Access Control Sub Layer’s Multiple access protocols:

A) Pure ALOHA or Unslotted ALOHA B) Slotted ALOHA or Impure ALOHA

Ans: A) Pure ALOHA or Unslotted ALOHA

The ALOHA network was created at the University of Hawaii in 1970 under the leadership ofNorman Abramson. The Aloha protocol is an OSI layer 2 protocol for LAN networks with broadcast topology.

The first version of the protocol was basic: If you have data to send, send the data If the message collides with another transmission, try resending later

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B) Slotted ALOHA or Impure ALOHA

An improvement to the original Aloha protocol was Slotted Aloha. It is in 1972, Roberts published a method to double the throughput of an pure ALOHA by uses discrete timeslots. His proposal was to divide the time into discrete slots corresponding to one frame time. This approach requires the users to agree to the frame boundaries. To achieve synchronization one special station emits a pip at the start of each interval similar to a clock. Thus the capacity of slotted ALOHA increased to the maximum throughput of 36.8%.

With Slotted Aloha, a centralized clock sent out small clock tick packets to the outlying stations. Outlying stations were only allowed to send their packets immediately after receiving a clock tick. If there is only one station with a packet to send, this guarantees that there will never be a collision for that packet. On the other hand if there are two stations with packets to send, this algorithm guarantees that there will be a collision, and the whole of the slot period up to the next clock tick is wasted. With some mathematics, it is possible to demonstrate that this protocol does improve the overall channel utilization, by reducing the probability of collisions by a half.

3. Discuss the different types of noise?

Ans: Noise is unwanted electrical or electromagnetic energy that degrades the quality of signals and data. Noise occurs in digital and analog systems, and can affect files and communications of all types, including text, programs, images, audio, and telemetry. On the broadest scale, noise can be classified as either external or internal. Each category consists of several different types.

Internal NoiseInternal noise represents all the different types that arise inside of the communication systemcomponents. It includes thermal noise, shot noise, and flicker noise.

External NoiseExternal noise represents all the different types that arise outside of the communication systemcomponents. It includes atmospheric noise, galactic noise, man-made noise, and interferencefrom other communication sources.

Atmospheric NoiseAtmospheric noise is produced mostly by lightning discharges in thunderstorms. It is usually thedominating external noise source in quite locations at frequencies below about 20 MHz or so.

Extraterrestrial NoiseIt is safe to say that there are almost as many types of space noise as there are sources. Forconvenience, a division into two subgroups will suffice.

Galactic NoiseGalactic noise is caused by disturbances originating outside the earth’s atmosphere. The primary

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sources of galactic noise are the sun, background radiation along the galactic plane, and themany cosmic sources distributed along the galactic plane.

Man-Made NoiseMan-made noise is somewhat obvious from its title and consists of any source of electrical noiseresulting from a manmade device or system.Industrial NoiseBetween the frequencies of 1 to 600 MHz (in urban, suburban and other industrial areas) theintensity of noise made by humans easily outstrips that created by any other source, internal orexternal to the receiver.

InterferenceOne can debate as to whether or not interference from other communication sources should beclassified as “noise.”

4. What is Non-repudiation? Define cryptanalysis ?

Ans : Non-repudiationNetwork security problems can be divided roughly into four categories, Non-repudiation is one of them. It deals with signatures. For example how do one prove that the order was placed by the customer. Nonrepudiation is the assurance that someone cannot deny something. Typically, nonrepudiation refers to the ability to ensure that a party to a contract or a communication cannot deny the authenticity of their signature on a document or the sending of a message that they originated. On the Internet, a digital signature is used not only to ensure that a message or document has been electronically signed by the person that purported to sign the document, but also, since a digital signature can only be created by one person, to ensure that a person cannot later deny that they furnished the signature.

CryptanalysisCryptanalysis refers to the study of ciphers, ciphertext, or cryptosystems (that is, to secret codesystems) with a view to finding weaknesses in them that will permit retrieval of the plaintext from the ciphertext, without necessarily knowing the key or the algorithm. This is known as breaking the cipher, ciphertext, or cryptosystem.

There are numerous techniques for performing cryptanalysis, depending on what access thecryptanalyst has to the plaintext, ciphertext, or other aspects of the cryptosystem.

1) Known-plaintext analysis2) Chosen-plaintext analysis (also known as differential cryptanalysis)3) Ciphertext-only analysis4) Man-in-the-middle attack5) Timing/differential power analysis

5. Explain mask-address pair used in update message. Discuss importance of path attributes ?

Ans: Mask-address pair used in update messageIn the update message many addresses are listed and the

size of the update message goes onincreasing. BGP uses it to store destination address and the associated

mask. A technique,where instead of IP 32-bit address and a 32-bit mask compressed mask-address pair, is

used toreduce the size of the update message. Here BGP encodes the mask into a single octet that precedes

the address.

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Format of compressed mask-address pair

Path AttributePath attributes are factored to reduce the size of the update message.

Importance of path attributes:1. Path information allows a receiver to check for routing loops. The sender can specify exactpath through AS to the destination. If any AS is listed more than once then there is a routing loop.2. Path information allows a receiver to implement policy constraints. A receiver can examine the path so that they should not pass through untrusted AS.3. Path information allows a receiver to know the source of all routes.

6. if we number the input lines of permutaion box given below, as 0,1,2,3,4,5,6,7 from top to bottom,

then find the output of P-box?

Ans:

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