February 2011Bachelor of Science in Information Technology (BScIT) – Semester 2/ Diploma in Information Technology (DIT) – Semester 2 BT0072 – Computer Networks – 4 Credits (Book ID: B0956) Assignment Set – 1 (60 Marks)Answer all questions 10 x 6 = 60 1. Write about ISO OSI model.Ans:- The layered model that dominated data communications and networking literature before1990 was the Open Systems Interconnection (OSI) model. Everyone believed that the OSImodel would become the ultimate standard for data communications, but this did not happen.The TCP / IP protocol suite became the dominant commercial architecture because it was usedand tested extensively in the Internet; the OSI model was never fully implemented.Established in 1947, the International Standards Organization (ISO) is a multinational bodydedicated to worldwide agreement on International standards. An ISO standard that covers allaspects of network communications is the OSI model, which was first introduced in 1970s.Open System: A set of protocols that allows any two different systems to communicateregardless of their underlying architecture.Purpose of OSI Model: It shows how to facilitate communication between different systemswithout requiring changes to the logic of underlying hardware and software.The OSI model is not a protocol; it is a model for understanding and designing a networkarchitecture that is flexible, robust, and interoperable. The OSI Model is a layered frameworkfor the design of network systems that allows communication between all types of computersystems. It consists of seven separate but related layers, each of which defines a part of theprocess of moving information across a network.Layered Architecture:
The OSI Model is composed of seven ordered layers:· Layer 1 – The Physical Layer· Layer 2 – The Data Link Layer· Layer 3 – The Network Layer· Layer 4 – The Transport Layer· Layer 5 – The Session Layer· Layer 6 – The Presentation Layer· Layer 7 – The Application LayerIn modeling the OSI model, the designers distilled the process of transmitting data to its mostfundamental elements. They identified which networking functions had related uses andcollected those functions into discrete groups that became the layers. Each layer defines afamily of functions distinct from those of the other layers. By defining and localizing thefunctionality in this fashion, the designers created an architecture that is both comprehensiveand flexible.Within a single machine, each layer calls upon the services of the layer below it. Betweenmachines, layer x on one machine communicates with layer x on another machine. Thiscommunication is governed by an agreed-upon series of rules and conventions calledprotocols. The processes on each machine that communicates at a given layer are called peer-to-peer processes. Communication between machines is therefore a peer-to-peer process usingthe protocols appropriate to a given layer. 2. Describe the architecture and usage of ISDN.Ans:- ISDN is developed by ITU-T in 1976. It is a set of protocols that combines digitaltelephony and data transport services. The whole idea is to digitize the telephone network topermit the transmission of audio, video, and text over existing telephone lines.ISDN is an effort to standardize subscriber services, provide user/network interfaces, andfacilitate the internetworking capabilities of existing voice and data networks.
The goal of ISDN is to form a wide area network that provides universal end-to-endconnectivity over digital media. This can be done by integrating all of the separate transmissionservices into one without adding new links or subscriber lines.The purpose of ISDN is to provide fully integrated digital services to users. These services fallinto three categories:-1. Bearer Services: These services provide the means to transfer information (voice, data, andvideo) between users without the network manipulating the content of that information. Thenetwork does not need to process the information and therefore does not change the content.Bearer services belong to the first three layers of OSI model and are well defined in the ISDNstandard.2. Tele Services: In this service, the network may change or process the contents of the data.These services correspond to layers 4-7 of the OSI model. Teleservices rely on the facilities ofthe bearer services and are designed to accommodate complex user needs without the userhaving to be aware of details of the process. Teleservices include telephony, teletex, videotext,telex, and teleconferencing.3. Supplementary Services: These services provide additional functionality to the bearer andteleservices. Example: Reverse charging, call waiting, and message handling.Principles of ISDN:- Standards for ISDN have been defined by ITU-T (formerly CCITT).ISDN relates standards states the principles of ISDN from the point of view of CCITT. 3. Discuss various LAN protocols.Ans:- With slotted ALOHA, the best channel utilization that can be achieved is 1 / e. This ishardly surprising since with stations transmitting at will, without paying attention to what otherstations are doing, there are bound to be many collisions. In LANs, it is possible to detect whatother stations are doing, and adapt their behavior accordingly. These networks can achieve abetter utilization than 1 / e.CSMA Protocols:- Protocols in which stations listen for a carrier (a transmission) and actaccordingly are called Carrier Sense Protocols. "Multiple Access" describes the fact thatmultiple nodes send and receive on the medium. Transmissions by one node are generallyreceived by all other nodes using the medium. Carrier Sense Multiple Access (CSMA) is aprobabilistic Media Access Control (MAC) protocol in which a node verifies the absence of
other traffic before transmitting on a shared physical medium, such as an electrical bus, or aband of electromagnetic spectrum.The following three protocols discuss the various implementations of the above discussedconcepts: i) Protocol 1. 1-persistent CSMA: ii) Protocol 2. Non-persistent CSMA: iii) Protocol 3. p – persistent CSMACSMA/CD Protocol:- In computer networking, Carrier Sense Multiple Access with CollisionDetection is a network control protocol in which a carrier sensing scheme is used. Atransmitting data station that detects another signal while transmitting a frame, stopstransmitting that frame, transmits a jam signal, and then waits for a random time interval. Therandom time interval also known as "backoff delay" is determined using the truncated binaryexponential backoff algorithm. This delay is used before trying to send that frame again.CSMA/CD is a modification of pure Carrier Sense Multiple Access Collision detection is usedto improve CSMA performance by terminating transmission as soon as a collision is detected,and reducing the probability of a second collision on retry. Methods for collision detection aremedia dependent, but on an electrical bus such as Ethernet, collisions can be detected bycomparing transmitted data with received data. If they differ, another transmitter is overlayingthe first transmitter’s signal (a collision), and transmission terminates immediately. Here thecollision recovery algorithm is nothing but an binary exponential algorithm that determines thewaiting time for retransmission. If the number of collisions for the frame hits 16 then the frameis considered as not recoverable.CSMA/CD can be in anyone of the following three states1. Contention period2. transmission period3. Idle period
4. Explain the concept of framing in Data Link Layer and its importance in data communication.Ans:- Data transmission in the physical layer means moving bits in the form of a signal fromthe source to destination. The physical layer provides bit synchronization to ensure that thesender and receiver use the same bit durations and timing.The data link layer on the other hand needs to pack bits into frames, so that each frame isdistinguishable from another.Framing in the data link layer separates a message from one source to a destination, or fromother messages to other destinations, by adding a sender address and a destination address. Thedestination address defines where the packet is to go; the sender address helps the recipientacknowledge the receipt.Although the whole message could be packed into one frame, it is not normally done. When amessage is carried in one very large frame, even a single – bit error would require theretransmission of the whole message. When a message is divided into smaller frames, a single-bit error affects only that small frame.Fixed-Size Framing In this, there is no need for defining the boundaries of the frames; the sizeitself can be used as a delimiter. Example: The ATM WAN, which uses frames of fixed sizecalled cellsVariable Size Framing This type of framing is prevalent in LANs. In this, we need a way todefine the end of the frame and the beginning of the next. Two approaches are sued for thispurpose: a character-oriented approach and a bit oriented approach.Character Oriented Protocols In these protocols, data to be carried out are 8-bit charactersfrom a coding system such as ASCII.he DLL translates the physical layer’s raw bit stream into discrete units (messages) calledframes. That is because the physical layer just accepts a raw bit stream and to deliver it to thedestination. This bit stream is not guaranteed to be error free. The number of bits received maybe less than, equal to, or more than the number of bits transmitted. They may also havedifferent values. It is up to the DLL to detect and if necessary correct errors. If not correct atleast detect errors and take proper action like ask for retransmission etc. The usual approach ofDLL is to break the bit stream up into discrete frames and then for the purpose of error
detection or correction, it uses some coding technique that finds some checksum for eachframe. When a frame arrives at the destination, the checksum is recomputed. If the newlycomputed checksum is different from the one contained in the frame, the DLL knows that anerror has occurred and takes steps to deal with it. All this is importance work of data link layer. 5. Discuss the IEEE 802.11 Standard.Ans:- EEE has defined the specifications for a wireless LAN, called IEEE 802.11, which coversthe physical and data link layers.Architecture: The standard defines two kinds of services: the basic service set (BSS) and theextended service set (ESS).Basic Service Set: It is made of stationary or mobile wireless stations and an optional centralbase station, known as the access point (AP)Extended Service Set: It is made up of two or more BSSs with APs. In this case, the BSSs areconnected through a distribution system, which is usually a wired LAN. The distributionsystem connects the APs in the BSSs. IEEE 802.11 does not restrict the distribution system; itcan be any IEEE LAN such as an Ethernet. The extended service set uses two types of stations:Mobile and Stationary. The mobile stations are normal stations inside a BSS. The stationarystations are AP stations that are part of wired LAN.When BSSs are connected, the stations within reach of one another can communicate withoutthe use of an AP. However, communication between two stations in two different BSSs occurvia two APs. The idea is similar to communication in a cellular network if we consider eachBSS to be a cell and each AP to be a base station. Note that a mobile station can belong to morethan one BSS at the same time. 6. Write about TCP/IP Protocol suite.Ans:- The TCP/IP protocol suite, also referred to as the Internet protocol suite, is the set ofcommunications protocols that implements the protocol stack on which the Internet and mostcommercial networks run. It is named after the two most important protocols in the suite: theTransmission Control Protocol (TCP) and the Internet Protocol (IP). The TCP/IP protocol suite—like the OSI reference model—is defined as a set of layers. Upper layers are logically closerto the user and deal with more abstract data, relying on lower layer protocols to translate data
into forms that are transmitted physically over the network.The TCP/IP protocol is the primaryfocus of this tutorial.TCP/IP Model and the OSI Reference ModelThe TCP/IP protocol suite was developed before the OSI reference model. As such, it does notdirectly map to the 7-layer OSI reference model. The TCP/IP protocol stack has only layers thatcan be loosely mapped to the OSI protocol stack, as shownin Figure 1-2.Application LayerThe application layer of the TCP/IP model corresponds to the application layer of the OSIreference model. Some well known examples of application level entities within theTCP/IP domain are:• FTP/Telnet/SSH• HTTP/Secure HTTP (SHTTP)• POP3/SMTP• SNMPTransport Layer:- The transport layer of the TCP/IP model maps fairly closely to the transportlayer of the OSI model. Two commonly used transport layer entities are TCP and UserDatagram Protocol (UDP)Internet Layer:- The Internet layer of the TCP/IP model maps to the network layer of the OSImodel. Consequently, the Internet layer is sometimes referred to as the network layer. Theprimary component of the Internet layer is the Internet Protocol (IP).Many of the TCP/IProuting protocols are also classified as part of the Internet layer.Network Access Layer:- The lowest layer of the TCP/IP protocol stack is the network accesslayer. The network access layer contains two sublayers, the media access control (MAC)sublayer and the physical sublayer. The MAC sublayer aligns closely with the data link layer ofthe OSI model, and is sometimes referred to by that name. The physical sublayer aligns withthe physical layer of the OSI model.Examples of the network access layer that will be discussed in this tutorial include:• Ethernet• Wireless Fidelity (Wi-FI)/WiMAX• PPP, PPP over Ethernet (PPPoE)• ATM/Frame Relay 7. Discuss various transmission and switching techniques.Ans:- Optical fiber transmits a signal encoded beam of light by means of total internalreflection. Total internal reflection can occur in any transparent medium that has a higher indexof refraction than the surrounding medium. The principle is described in Figure 2.22. In effect,
the optical fiber acts as a waveguide for frequencies in the range of about 1014 to 1015 Hz.This covers portions of infrared and visible spectra.Light from a source enters the cylindrical glass or plastic core. Rays at shallow angles arereflected and propagated along the fiber; other rays are absorbed by the surrounding material.This form of propagation is called Step-index multimode referring to the variety of angles thatwill reflect.With multimedia transmission, multiple propagation paths exist, each with a different pathlength and hence time to traverse the fiber. This causes signal elements (light pulses) to spreadout in time, which limits the rate at which data can be accurately received. In other words, theneed to leave spacing between the pulses limits data rate. This type of fiber is best suited fortransmission over very short distances. When the fiber core radius is reduced, fewer angles willreflect. By reducing the radius of the core to the order of a wavelength, only a single angle ormode can pass: the axial ray. This single mode propagation provides superior performance forthe following reason, because there is a single transmission path with single mode transmission.The distortion found in multimode cannot occur. Single-mode is typically used for longdistance applications, including telephone and cable television.Finally, by varying the index of refraction of the core, a third type of transmission, known asgraded index multimode, is possible. This type is intermediate between the other two incharacteristics. The higher refractive index at the center makes the light rays moving down theaxis advance more slowly than those curves helically because of the graded index, reducing itstravel distance. The shortened path and higher speed allows light at the periphery to arrive at areceiver at about the same time as the straight rays in the core axis. Graded index fibers areoften used in LANs.Long-haul transmission:-It is becoming increasingly common in the telephone network.Long-haul routes average about 1500kms in length and offer higher capacity (typically 20,000to 60,000 voice channels). These systems compete economically with microwave. Underseaoptical fiber cables also enjoy increasing use.Switching:- For transmission of data beyond local area, communication is typically achievedby transmitting data from source to destination through a network of intermediate switchingnodes. The switching nodes are not concerned with the content of data. Rather their purpose isto provide a switching facility that will move the data from node to node until it reaches thedestination.
8. Discuss IEEE 802 standards for LANs.Ans:- IEEE 802 standards for LANs:-IEEE has standardized a number of LAN’s and MAN’sunder the name of IEEE 802. Few of the standards are listed in figure 3.6. The most importantof the survivor’s are 802.3 (Ethernet) and 802.11 (wireless LAN). Both these two standardshave different physical layers and different MAC sub layers but converge on the same logicallink control sub layer so they have same interface to the network layer.Ethernet was originally based on the idea of computers communicating over a shared coaxialcable acting as a broadcast transmission medium. The methods used show some similarities toradio systems, although there are major differences, such as the fact that it is much easier todetect collisions in a cable broadcast system than a radio broadcast. The common cableproviding the communication channel was likened to the ether and it was from this referencethe name "Ethernet" was derived.From this early and comparatively simple concept, Ethernet evolved into the complexnetworking technology that today powers the vast majority of local computer networks. Thecoaxial cable was later replaced with point-to-point links connected together by hubs and/orswitches in order to reduce installation costs, increase reliability, and enable point-to-pointmanagement and troubleshooting. Star LAN was the first step in the evolution of Ethernet froma coaxial cable bus to a hub-managed, twisted-pair network.Above the physical layer, Ethernet stations communicate by sending each other data packets,small blocks of data that are individually sent and delivered. As with other IEEE 802 LANs,each Ethernet station is given a single 48-bit MAC address, which is used both to specify thedestination and the source of each data packet. Network interface cards (NICs) or chipsnormally do not accept packets addressed to other Ethernet stations. Adapters generally comeprogrammed with a globally unique address, but this can be overridden, either to avoid anaddress change when an adapter is replaced, or to use locally administered addresses. 9. Discuss the concept of Error Detection and Correction techniques.
Ans:- Whenever bits flow from one point to another, they are subject to unpredictable changesbecause of interference. This interference can change the shape of the signal. In a single-biterror, a 0 is changed to a 1 or a 1 to a 0. In a burst error, multiple bits are changed.Detection Versus Correction:- The correction of errors is more difficult than detection. Inerror detection, we are looking only to see if any error has occurred. The answer is a simple yesor no. In error correction, we need to know the exact number of bits that are corrupted andmore importantly, their location in the message. The number of errors and the size of messageare important.Forward Error Correction Versus Retransmission:- There are two main methods of errorcorrection. Forward error correction is the process in which the receiver tries to guess themessage by using redundant bits. This is possible if the number of errors is small. Correction byretransmission is a technique in which the receiver detects the occurrence of an error and asksthe sender to resend the message. Resending is repeated until a message arrives that thereceiver believes to be error-free.Coding:- Redundancy is achieved through various coding schemes. The sender adds redundantbits through a process that creates a relationship between the redundant bits and the actual databits. The receiver checks the relationships the two sets of bits to detect or correct the errors.The ratio of redundant bits to the data bits and the robustness of the process are importantfactors in any coding scheme. Coding schemes can be divided into two broad categories: BlockCoding and Convolution Coding. We concentrate on Block Coding; Convolution Coding isbeyond the scope in this book. 10. Write about the Point to Point Protocol.Ans:- The Point-to-Point Protocol (PPP) originally emerged as an encapsulation protocol fortransporting IP traffic over point-to-point links. PPP also established a standard for theassignment and management of IP addresses, asynchronous (start/stop) and bit-orientedsynchronous encapsulation, network protocol multiplexing, link configuration, link qualitytesting, error detection, and option negotiation for such capabilities as network layer addressnegotiation and data-compression negotiation. PPP supports these functions by providing anextensible Link Control Protocol (LCP) and a family of Network Control Protocols (NCPs) tonegotiate optional configuration parameters and facilities. In addition to IP, PPP supports otherprotocols, including Novells Internetwork Packet Exchange (IPX) and DE Cnet.
PPP provides a method for transmitting datagram’s over serial point-to-point links. PPPcontains three main components:• A method for encapsulating datagram’s over serial links. PPP uses the High-Level Data LinkControl(HDLC) protocol as a basis for encapsulating datagram’s over point-to-point links. (SeeChapter 16,“Synchronous Data Link Control and Derivatives,” for more information onHDLC.)• An extensible LCP to establish, configure, and test the data link connection.• A family of NCPs for establishing and configuring different network layer protocols. PPP isdesigned to allow the simultaneous use of multiple network layer protocols.General Operation:- To establish communications over a point-to-point link, the originatingPPP first sends LCP frames to configure and (optionally) test the data link. After the link hasbeen established and optional facilities have been negotiated as needed by the LCP, theoriginating PPP sends NCP frames to choose and configure one or more network layerprotocols. When each of the chosen network layer protocols has been configured, packets fromeach network layer protocol can be sent over the link. The link will remain configured forcommunications until explicit LCP or NCP frames close the link, or until some external eventoccurs.
February 2011Bachelor of Science in Information Technology (BScIT) – Semester 2/ Diploma in Information Technology (DIT) – Semester 2 BT0072 – Computer Networks – 4 Credits (Book ID: B0956) Assignment Set – 2 (60 Marks)Answer all questions 10 x 6 = 60 1. Define the term routing. Differentiate static and dynamic routing.Ans:- Routing:-Routing of packets across the subnet is considered the most important functionof the network layer. Two basic types of routing static and dynamic are discussed in this unit.Static routing is also referred to as non-adaptive and dynamic is also referred to as Adaptiverouting methods. Between the two approaches the former one has an advantage of simplicitybut performs poorly when the traffic or the topology of the subnet changes. Some of the wellknown routing techniques is shortest path routing, flooding, flow based, hierarchical, etc.Congestion control:- An important problem that any subnet faces is the congestion because ofheavy traffic in the subnet than the load that it can handle. The subnet gets increasingly loadedwith packets causing increase in delay in the delivery of packets, which can lead to moreretransmission, and ultimately increasing more and more traffic.Internetworking:- A subnet can be considered as an interconnection of multiple subnets. Thenetwork layer now has to perform a job of internetworking among the subnets. That is a task ofinterconnecting all the subnets with the help of various networking devices like repeaters,bridges, routers and gatewaysStatic:- Routing algorithms do not base their routing decisions on measurements or estimatesof the current traffic and topology. The choice of the route to use to get from source todestination or from one node to other is computed in advance. It is done off-line, anddownloaded to the routers when the network is booted.Dynamic:- or adaptive Routing algorithms in contrast change their routing decisions to reflectchanges in topology and usually the traffic as well. Dynamic routing attempts to solve this
problem by constructing routing tables automatically, based on information carried by routingprotocols, and allowing the network to act nearly autonomously in avoiding network failuresand blockages. 2. Differentiate IPv4 and IPv6 addressing schemes.Ans:- Differentiate between IPv4 and IPv6. 3. Describe the following: a. Congestion b. Congestion ControlAns:- Congestion:- An important issue in packet switched network is congestion. Congestionin a network may occur if the load on the network – the number of packets sent to the network0 is greater than the capacity of the network – the number of packets a network can handle.Congestion Control refers to the mechanisms and techniques to control the congestion and keepthe load below the capacity.
Congestion happens in any system that involves waiting. For example, congestion happens on afreeway because any abnormality in the flow, such an accident in the rush hour, createsblockage. Congestion in a network or internetwork occurs because routers and switches havequeues – buffers that hold packets before and after processing. A router, for example, has aninput queue for each interface. When a packet arrives at the incoming interface, it undergoesthree steps before departing as shown below:1. The packet is put at the end of the input queue while waiting to be checked.2. The processing module of the router removes the packet from the input queue once it reachesthe front of the queue and uses its routing table and the destination address to find its route. Thepacket is put in the appropriate output queue and waits for its turn to be sent. We need to beaware of two issues. First, if the rate of packet arrival is higher than the packet processing rate,the input queues become longer and longer. Second, if the packet departure rate is less than thepacket processing rate, the output queues become longer and longer.Congestion Control:- It refers to the techniques and mechanisms that can either preventcongestion, before it happens, or remove congestion, after it has happened. The congestioncontrol mechanisms can be divided into two categories: Open-Loop congestion control andClosed-Loop congestion control . 1. Open Loop Congestion Control 2. Retransmission Policy 3. Window Policy 4. Acknowledgement Policy 5. Discarding Policy 6. Admission Policy. 4. Discuss the Remote Procedure Calls and their importance in Session Layer.Ans:- Remote Procedure Calls:-The primary concern of the session layer is managing thedialog and dealing with errors occurring above the transport layer. There has been considerableresearch at universities and in industries on a radically different model for dialog and errorcontrol based on the connectionless model. This work, which goes under the name RPC(Remote procedure Call), has been widely used in networks and (especially) distributedsystems. RPC does not fit into the OSI Reference Model especially well. It has been designedto be fast, and therefore does not contain a multilayer structure.The Client – Server Model:- Until now it is assumed that the two processes communicatingover a session or transport connection are symmetric. In practice, this assumption is frequently
violated. A common example is a network of diskless personal computers or workstations,called clients, that are communicating over a network with a file server having a disk on whichall the files are stored. In this system, clients access their data by sending requests to the server,which carries out the work and sends back the replies. Communication always takes place inthe form of request-reply pairs, always initiated by clients, never by the server. This model iscalled the Client-Server Model.The beauty of this scheme is that client-server communication now takes the form of procedurecalls instead of I/O commands. All the details of how the network works can be hidden fromthe application program by putting them in the local procedures such as read. These proceduresare called Stubs. 5. Discuss Simple Mail Transfer Protocol in detail.Ans:- Simple Mail Transfer Protocol (SMTP):- The basic Internet mail protocols providemail (note) and message exchange between TCP/IP hosts, but generally require that data berepresented as 7-bit ASCII text. Because this can be restrictive, facilities have been added forthe transmission of data that cannot be represented in this manner. Originally, there were threestandard protocols that apply to mail of this kind. The term Simple Mail Transfer Protocol(SMTP) is frequently used to refer to the combined set of protocols because they are so closelyinterrelated. Strictly speaking, however, SMTP is just one of the three. Those three standardswere:· A standard for the exchange of mail between two computers (STD 10/RFC 821), whichspecified the protocol used to send mail between TCP/IP hosts. This standard was SMTP itself.· A standard (STD 11) on the format of the mail messages, contained in two RFCs. RFC 822described the syntax of mail header fields and defined a set of header fields and theirinterpretation. RFC 1049 described how a set of document types other than plain text ASCIIcan be used in the mail body. The official protocol name for this standard was MAIL.· A standard for the routing of mail using the Domain Name System, described in RFC 974.The official protocol name for this standard was DNS-MX.Common SMTP header fields:-
6. Explain any two static routing algorithms.Ans:- 7. Discuss IPv4 addressing schemes.Ans:- An IP address is an identifier that is assigned at the Internet layer to an interface or a setof interfaces. Each IP address can identify the source or destination of IP packets. For IPV4,every node on a network has one or more interfaces, and you can enable TCP/IP on each ofthose interfaces. When you enable TCP/IP on an interface, you assign it one or more logicalIPV4 addresses, either automatically or manually. The IPV4 address is a logical addressbecause it is assigned at the Internet layer and has no relation to the addresses that are used atthe Network Interface layer. IPV4 addresses are 32 bits long.IPV4 Address Syntax:-If network administrators expressed IPV4 addresses using binarynotation, each address would appear as a 32-digit string of 1s and 0s. Because such strings arecumbersome to express and remember, administrators use dotted decimal notation, in whichperiods (or dots) separate four decimal numbers (from 0 to 255). Each decimal number, knownas an octet, represents 8 bits (1 byte) of the 32-bit address.When referring to an IPV4 address, use the notation w.x.y.z. shows the IPV4 address structure.
To become adept at moving between binary and decimal formats, you can review the binaryand decimal numbering systems and how to convert between them. Although you can use thecalculator in the Microsoft Windows Server 2003 or Windows XP operating systems to convertbetween decimal and binary, you will better understand the conversions if you can do themmanually. 8. Discuss Congestion Avoidance in Transport Layer.Ans:- Congestion avoidance:-The assumption of the algorithm is that packet loss caused bydamage is very small. Therefore, the loss of a packet signals congestion somewhere in thenetwork between the source and destination. There are two indications of packet loss:· A timeout occurs.· Duplicate ACKs are received.Congestion avoidance and slow start are independent algorithms with different objectives. Butwhen congestion occurs, TCP must slow down its transmission rate of packets into the networkand invoke slow start to get things going again. In practice, they are implemented together.Congestion avoidance and slow start require that two variables be maintained for eachconnection:· A congestion window, cwnd· A slow start threshold size, ssthreshThe combined algorithm operates as follows:
1. Initialization for a given connection sets cwnd to one segment and ssthresh to 65535 bytes.2. The TCP output routine never sends more than the lower value of cwnd or the receiver’sadvertised window.3. When congestion occurs (timeout or duplicate ACK), one-half of the current window size issaved in ssthresh. Additionally, if the congestion is indicated by a timeout, cwnd is set to onesegment.4. When new data is acknowledged by the other end, increase cwnd, but the way it increasesdepends on whether TCP is performing slow start or congestion avoidance. If cwnd is less thanor equal to ssthresh, TCP is in slow start; otherwise, TCP is performing congestion avoidance.Slow start and congestion avoidance behavior in action:Fast retransmit:-Fast retransmit avoids having TCP wait for a timeout to resend lost segments
Fast recovery:- After fast retransmit sends what appears to be the missing segment, congestionavoidance, but not slow start, is performed. This is the fast recovery algorithm. It is animprovement that allows high throughput under moderate congestion, especially for largewindows. 9. Discuss various design issues of Session Layer.Ans:- Design Issues:- The Session, Presentation and Application layers form the upper layersin the OSI model. These upper layers are concerned with providing user-oriented services.They take the bare-bones, error-free channel provide by the transport layer and add additionalfeatures that are useful to a wide variety of applications, so that the writers of these applicationswill not each have to re-implement these features over and over as part of each separateprogram.The design issues of the session layer include:· Dialog Management· Synchronization· Activity ManagementThe Session layer provides services to the Presentation layer. The service access points arecalled Session Service Access Points (SSAPs) and the protocol data units are called SessionProtocol Data Units (SPDUs). The main function of the session layer is to provide a way for the
session users to establish connections, called sessions, and transfer data over them in an orderlyway. A session might be used for a remote login from a terminal to a distant computer, or for afile transfer, or for any many other purposes. Although connectionless primitives are availablein the session layer, a connectionless session cannot make any use of the user-oriented featuresfor which the session layer was designed. For this reason, we will focus on the connection-oriented model.A session bears a close resemblance to a transport connection, but the two are not identical.Usually, when a request comes into the session layer to establish a session, a transportconnection must be established to carry the connection. When the session is terminated, thetransport connection is released. It is not permitted to multiplex several sessions onto a singletransport connection simultaneously the way the transport layer can multiplex several transportconnections onto a network connection. At any instant of time, each transport connectioncarries at most one session. 10. Describe the Multipurpose Internet Mail Extensions.Ans:- Multipurpose Internet Mail Extensions:- Electronic mail is probably the most widelyused TCP/IP application. However, SMTP is limited to 7-bit ASCII text, with a maximum linelength of 1000 characters. This results in a number of limitations, including:· SMTP cannot transmit executable files or other binary objects. There are ad hoc methods ofencapsulating binary items in SMTP mail items, such as:– Encoding the file as pure hexadecimal– The UNIX uuencode and uudecode utilities, used to encode binary data in the UNIX-to-UNIX Copy (UUCP) mailing systemHowever, none of these can be described as a de facto standard (though uuencode/uudecode isperhaps the most pervasive, due to the pioneering role of UNIX systems in the Internet).· SMTP cannot transmit text data that includes national language characters, because these arerepresented by code points with a value of 128 (decimal) or higher in all character sets based onASCII.· SMTP servers might reject mail messages over a certain size. Any given server can havepermanent or transient limits, or both, on the maximum amount of mail data it can accept froma client at any given time.
· SMTP gateways that translate from ASCII to EBCDIC and vice versa do not use a consistentset of code page mappings, resulting in translation problems.· Some SMTP implementations or other mail transport agents (MTAs) in the Internet do notadhere completely to the SMTP standards defined in RFC 2821. Common problems include:– Removal of trailing white-space characters (tabs and spaces).– Addition of white-space characters to make all lines in a message the same length.– Wrapping of lines longer than 76 characters.– Changing of new line sequences between different conventions. (For example, <CRLF>characters might be converted to <CR> or <LF> sequences.)– Conversion of tab characters to multiple spaces.MIME can be described in five parts:· Protocols for including objects other than US ASCII text mail messages within the bodies ofmessages conforming to RFC 2822. These are described in RFC 2045.· General structure of the MIME media typing system, which defines an initial set of mediatypes. This is described in RFC 2046.· A protocol for encoding non-U.S. ASCII text in the header fields of mail messagesconforming to RFC 2822. This is described in RFC 2047.· Various IANA registration procedures for MIME-related facilities. This is described in RFC2048.· MIME conformance criteria. This is described in RFC 20410.