CU-MCA-SEM-I-Advanced Computer Networks-Second Draft

Figure 5.2Function of Transport Layer The data is kept in the correct order. This protocol assures that data is sent in same order to the intended recipient. It numbers and organises each segment so that the TCP layer on the other end may reassemble them in the correct sequence. The TCP's primary job is to collect data from the application layer. It then separates the data into multiple packets, assigns a number to each packet, and sends the packets to their destination. TCP, on the other hand, reassembles the packets before sending these to the application layer. Given that TCP is a connection-oriented protocol, the connection will be maintained until the sender and receiver have completed their exchange. Connection-oriented This is a connection-oriented service, which implies that data is only exchanged when the connection is established. The connection will be ended after the data transfer is complete. Duplex (full) It's a full-duplex connection, which means data may flow in both ways at the same time. Stream-oriented TCP is a stream-oriented protocol in that it allows the transmitter to send data in form of a stream of bytes, and the receiver to accept data in the same format. TCP establishes an environment in which the sender and receiver are connected by a virtual circuit, which is an imaginary tube. The stream of bytes is carried through the internet by this virtual circuit. Transport Control Protocol is Required 101 CU IDOL SELF LEARNING MATERIAL (SLM)

The entire task is separated into smaller jobs throughout the layered architecture of a network model. Each task is allocated to a layer that performs the processing. Application layer, transport layer, network layer, data link layer, and physical layer are the five layers in the TCP/IP model. The transport layer is essential for delivering end-to-end communication to application operations. It produces 65,000 ports, allowing numerous programmes to be accessed simultaneously. It receives data from the higher layer, breaks it into tiny packets, and then sends it to the network layer. TCP's Operation Three-way handshaking is used in TCP to establish the connection. The segment is sent by the client including its sequence number. In turn, the server sends a segment from its own sequence number as well as an acknowledgement sequence that is one higher than the client sequence number. Because when client receives the acknowledgment of own segment, it sends it to the server. The connection between the client and the server is established in this manner. Figure 5.3Working of TCP TCP's Advantages • It provides a connection-oriented reliable service that means it ensures data packet delivery. TCP will resend any lost packets if indeed the data packet is lost throughout the network. • It uses a sliding window protocol to provide a flow control mechanism. 102 CU IDOL SELF LEARNING MATERIAL (SLM)

• It uses a checksum for error detection and the Go Back or ARP protocol for error control. • It uses a network congestion avoidance algorithm that combines strategies like additive increase/multiplicative decline (AIMD), gradual start, and congestion window to remove congestion. TCP's disadvantage • Because each segment receives its own TCP header, fragmentation even by router adds a significant amount of overhead. Format of the TCP Header Source port: This is the port where the data is being sent from the programme. As a result, this field contains the 16-bit source port address. The destination port: Specifies the application's port on the receiving end. As a result, this field contains the 16-bit destination port address. Figure 5.4TCP Header The series number of data bytes in such a particular session is stored in this field. Number of acknowledgements: When the ACK flag is set, this carries the following data byte's sequence number and serves as an acknowledgement for the previous data received. If 103 CU IDOL SELF LEARNING MATERIAL (SLM)

such receiver receives the section number 'x,' it will react with the number 'x+1' as an acknowledgment number. HLEN: This determines the header length as indicated by the 4-byte words in the header. The header is between 20 and 60 bytes in size. As a result, the field's value would be between 5 and 15. Reserved: This is a 4-bit field that is set to zero by default and is reserved for future usage. Flags Six control bits or flags are present: URG is an abbreviation for \"urgent pointer.\" If it is set, the information is analysed as soon as possible. If the ACK parameter is set to 0, the data packet may not contain an acknowledgment. If this field is set, the receiving device is asked to push the data directly to the receiving application before buffering it. RST: If set, it demands that a connection be restarted. SYN: This command can be used to establish a connection between hosts. FIN: It's used to close a connection so that no more data may be exchanged. Dimensions of the window It's a 16-bit value. It specifies the maximum data size that the receiver will take. This parameter determines the amount of buffer allotted by the receiver for a segment and is used for flow control between the sender and receiver. The receiver determines the value of this field. Checksum It's a 16-bit value. In UDP, this parameter is optional; however, in TCP/IP, this field is required. Pointer with a deadline If the URG flag is set to 1, it is a pointer to the urgent data byte. It specifies a value which will be applied to the sequence number that obtain the last urgent byte's sequence number. Options It adds to the possibilities. The optional field has a 32-bit representation. If the data in this field is less than 32 bits, padding is necessary to access the remaining bits. 5.4 UDP 104 CU IDOL SELF LEARNING MATERIAL (SLM)

A Transport Layer protocol is User Datagram Protocol (UDP). UDP is a component of the UDP/IP family of Internet Protocols. It is a connectionless and unreliable protocol, unlike TCP. As a result, data transfer does not require the establishment of a link. Despite the fact that Transmission Control Protocol (TCP) has become the most widely used transport layer protocol for most Internet services, it provides guaranteed delivery, reliability, and other benefits, all of these services come at a cost in terms of overhead and delay. UDP is used in this scenario. UDP is required for real-time services such as computer gaming, phone or video conferencing, and live meetings. UDP allows packets to also be dropped rather than processed because high performance is required. Because UDP does not do error checking, it saves bandwidth as well. In terms of latency and capacity, the User Datagram Protocol (UDP) is more effective. Figure 5.5UDP Header UDP Header is a protocol that allows you to send and receive data over the internet TCP headers can range from 20 bytes to 60 bytes, whereas UDP headers are fixed and simple at 8 bytes. The first 8 bytes contain the necessary header information, while the rest of the data is contained in the remaining bytes. Because UDP port numbers are 16 bits long, the range for port numbers is 0 to 65535; port number 0 is allocated. Differentiating between distinct user requests or processes is made easier by port numbers. Source Port is a two-byte parameter that identifies the source's port number. Destination Port is a two-byte field that identifies the port to which the packet is being sent. The length of UDP, including the header and data, is defined as length. It's a field with a resolution of 16 bits. Calculate the checksum: The checksum field has a length of two bytes. It is the 16-bit one's complement sum of the UDP header, pseudo-IP header information, and data, padded by zero octets at the end (if necessary) to create a multiple of two octets. 105 CU IDOL SELF LEARNING MATERIAL (SLM)

Figure 5.6UDP format Notes – In contrast to TCP, UDP does not require checksum calculation. UDP does not give any error or flow control. As a result, UDP error reporting relies on IP and ICMP. UDP's Use Cases: When the amount of data is little, there is less concern about flow and error control, hence it is used for simple request-response communication. Because UDP supports packet switching, it's a good choice for multicasting. Some routing update protocols, such as RIP, use UDP to communicate (Routing Information Protocol). Typically employed in real-time applications when unequal delays among sections of a received message are not tolerated. The transport layer protocol for the following implementations is UDP: DNS (Domain Name Service), BOOTP, and DHCP are examples of network time protocols. NNP is a non-profit organisation dedicated to (Network News Protocol) TFTP, RTSP, and RIP are the day's quote protocols. Some operations can be accomplished by the application layer via UDP, such as route tracing, route recording, and route time stamping. UDP receives datagrams from the Network Layer, adds a header, and sends them to the user. As a result, it's quick. If you delete the checksum field, UDP becomes a null protocol. Reduce your computer's resource requirements. When transferring files using Multicast or Broadcast. Real-time packet transmission, commonly used in multimedia applications. 106 CU IDOL SELF LEARNING MATERIAL (SLM)

Characteristics of the User Datagram Protocol The User Datagram Protocol offers properties that make it useful for applications that can cope with data loss. As an illustration: It permits packets to be lost and received in a different format than they were sent, making it ideal for real-time applications when latency is an issue. It's suitable for transaction-based protocols like DNS and the Network Time Protocol (NTP). It can be utilised in situations involving a high number of clients and when real-time mistake correction isn't required, such as gaming, audio or video conferencing, including streaming media. When the application is designed to facilitate the change of retransmitting lost packets and properly arranging received packets, UDP will be used in applications which require lossless data delivery. When compared to TCP, this technique can aid in the faster transfer of huge files. UDP, like TCP, is at Layer 4, the transport layer, of the Open Systems Interconnection (OSI) communication paradigm. UDP collaborates with higher-level protocols like TFTP, RTSP, and SNM to help manage data transport services (SNMP). UDP is a great protocol in network applications where perceived latency is important, such as gaming and audio and video communications, because it can handle minor data loss without hurting perceived quality. Forward error correction techniques, despite some loss, are sometimes employed to improve audio and video quality. UDP is also used for applications which require a secure transmission of data but require their own packet response methods. Such services are advantageous since they are not restricted to specific patterns in order to ensure the completeness and accuracy of the packets transmitted. Users have control over how and when they respond to incorrect or sorted data. Because it provides packet switching, UDP are used for multicasting as well. Furthermore, several routing updates protocols, like the Routing Information Protocol, employ UDP (RIP). When speed is more important than dependability, UDP can be employed. For example, in a quick acquisition application where it's acceptable to lose certain data points, UDP might be a good choice. 5.5 DIFFERENCE BETWEEN TCP AND UDP: 107 CU IDOL SELF LEARNING MATERIAL (SLM)

Basis for TCP UDP Comparison Definition Before transmitting data, TCP UDP sends data to the destination creates a virtual circuit. computer without first checking if the receiver is ready to accept it. Connection Type It is a Connection-Oriented It is a Connectionless protocol protocol Speed Slow high Reliability It is a reliable protocol. It is an unreliable protocol. Header size 20 bytes 8 bytes acknowledgement It waits for the acknowledgement It neither takes the acknowledgement, of data and has the ability to nor it retransmits the damaged frame. resend the lost packets. Table 5.1 TCP vs UDP 5.6SUMMARY • Point-to-point communication is often handled by transport layer protocols, which means this code is in charge of managing, establishing, & closing conversation between multiple specific networked devices. This layer is essentially what permits different networking apps that exist above the transport layer to build client–server, point–to–point communication relationships with another device via functionality. • Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), both transport layer protocols, use port numbers to identify applications communicating with one another. The Internet Assigned Numbers Authority (IANA) assigns port numbers 1 to 1023 to standardised services running with root capabilities. Web servers, for example, listen for client requests on TCP port 80. • Between user applications as well as the network are transport layer protocols. Despite the fact that they provide user-oriented services, its design is based on network characteristics assumptions. The User Datagram Protocol (UDP), which is effectively a thin layer over IP, is one option provided by the Internet. Without any flow, congestion, or error management, UDP provides a best-effort message delivery service. • Higher-layer protocols and applications may build such facilities on top of it if necessary. UDP is beneficial for applications which communicate via Local Area 108 CU IDOL SELF LEARNING MATERIAL (SLM)

Networks in addition to providing nearly immediate access to IP (LANs). The lack of error and congestion control in wired LANs is unimportant because they are often quite dependable and have and plenty bandwidth available. • The TCP (Transmission Control Protocol) as well as the (UDP) User Datagram Protocol are the two fundamental protocols in the TCP/IP protocol family. There are a number of other protocols which are governed by these protocols. 5.7 KEYWORDS • Number of acknowledgements: When the ACK flag is set, this carries the following data byte's sequence number and serves as an acknowledgement for the previous data received. If such receiver receives the section number 'x,' it will react with the number 'x+1' as an acknowledgment number. • HLEN: This determines the header length as indicated by the 4-byte words in the header. The header is between 20 and 60 bytes in size. As a result, the field's value would be between 5 and 15. 5.8 LEARNING ACTIVITY 1. Consider the initial sequence number for a TCP connection is calculated using a time-of- day (ToD) clock that runs even while the host is unavailable. The initial sequence numbers will be generated using the low order 32 bits of the ToD clock's counter. The clock counter advances by one millisecond per millisecond. The maximum packet lifetime is specified as 64 seconds. Which will come closest to the lowest rate at which sequence numbers for packets in a connection can increase? ___________________________________________________________________________ ___________________________________________________________________________ 2. When a TCP connection times out, set the congestion window size to 32 KB. The connection's round-trip time is 100 milliseconds, and the maximum segment size is 2 KB. What is the time it takes the TCP connection to return to the 32 KB congestion window (in milliseconds)? ___________________________________________________________________________ ___________________________________________________________________________ 5.9UNIT END QUESTIONS A. Descriptive Questions Short Questions 1. Define UDP 109 CU IDOL SELF LEARNING MATERIAL (SLM)

2. What is RDP? 3. What is the need of TCP? 4. Draw the UDP header format. 5. Write any few characteristics of UDP. Long Questions 1. List the characteristics of TCP in detail. 2. Explain the working of TCP protocol. 3. Discuss about the use cases of UDP. 4. Explain about UDP header format. 5. Explain the difference between UDP and TCP. B. Multiple Choice Questions 1. ____ is a send and forget protocol a. TCP b. MDP c. UDP d. HCP 2. RDP-type protocols are comparable to ____ a. TCP b. UDP c. FDP d. RCP 3. A ____ is delivered from a source host to a destination host via the IP protocol throughout the network layer a. letter b. source c. destination d. datagram 110 CU IDOL SELF LEARNING MATERIAL (SLM)

4. Which is used to deliver data from a sender to a receiver? a. UDP b. HCL c. TCP d. MNCQ 5. Which command is used to establish a connection between hosts? a. SYN b. RST c. FIN d. ACK Answers 1-c, 2- a, 3- d, 4- c, 5-a 5.10 REFERENCES Reference books • Behrouz A Forouzan, “Data Communications and Networking”, McGraw Hill. • Andrew S. Tanenbaum, “Computer Networks”, Pearson Education. • Subir Kumar Sarkar, T.G. Basavaraju, C. Puttaamadappa, “AdHoc Mobile Wireless Network: Principles, Protocols, and Applications, CRC Press. Textbook references • James F. Kurose, Keith W. Ross, “Computer Networking”, Pearson Education. • Michael A. Gallo, William M. Hancock, “Computer Communications and Networking Technologies”, CENGAGE Learning Websites: • https://www.techopedia.com/definition/10062/wireless-communications • https://www.computernetworkingnotes.com/ • https://www.guru99.com 111 CU IDOL SELF LEARNING MATERIAL (SLM)

UNIT 6-APPLICATION LAYER STRUCTURE 6.0 Learning Objectives 6.1 Introduction to Application layer 6.2 WWW 6.3 DNS 6.4 MIME 6.5 HTTP 6.6 SMTP 6.7 POP 6.8 IMAP 6.9 FTP 6.10 Telnet 6.11 Summary 6.12 Keywords 6.13 Learning activity 6.14 Unit End Questions 6.15 References 6.0 LEARNING OBJECTIVES After studying this unit, you will be able to: • Learn the basics of Application Layer • Describe about WWW,DNS and MIME • Explain HTTP and SMTP’s features • List the characteristics of POP,IMAP,FTP and Telnet 6.1 INTRODUCTION TO APPLICATION LAYER In the OSI and TCP/IP layered models, the application layer is at the top. Because of the importance of engaging with users and user applications, this layer is present in both tiered Models. This layer is used by applications that are part of a communication system. 112 CU IDOL SELF LEARNING MATERIAL (SLM)

The applications may or may not be directly interacted with by the user. The real communication is started and reflected at the application layer. This layer does not support any other layers because it is at the top of the layer stack. To communicate or send data to a remote host, the Application layer uses Transport and now all layers below it. When an application layer protocol needs to communicate with a peer application layer protocol on a remote host, the data or information is handed over to the Transport layer. The transport layer, with the support of most of the layers below it, takes care of the remainder. Figure 6.1 Application Layer There is a misunderstanding about the Application Layer and its protocol. Except for apps that interface with the communication system, not every user application could be placed in the Application Layer. Design applications and text editors, for example, are not considered application layer programmes. When we use a Web Browser to interact with the network, we are actually using the Hyper Text Transfer Protocol (HTTP). HTTP stands for Hypertext Transfer Protocol.Another example is the File Transfer Protocol, which allows users to send text or binary files over a network. This protocol can be used in GUI-based software such as FileZilla or CuteFTP, as well as in command-line mode. As a result, regardless of the programme you use, the protocol employed by that software is examined at the Application Layer. DNS is a protocol that assists user application protocols like HTTP in completing their tasks.In the OSI model, the application layer is the layer closest to the end user, which implies the application layer and the end user can directly 113 CU IDOL SELF LEARNING MATERIAL (SLM)

interact with the software application. Client and server programmes make up the application layer. The following functionalities are included in the Application layer: Identifying communication partners entails the following steps: The application layer determines whether or not communication partners are available for an application with information to send. Determining the availability of resources: The application layer determines whether the desired communication requires sufficient network resources. Communication synchronisation: All communication between apps necessitates collaboration, which is managed by an application layer. Application Layer Services Network An application layer controls access to a remote host via a virtual terminal. The application accomplishes this by creating a software emulation of a terminal on the remote system. The computer of the user communicates with the software terminal, that communicates with the host. Because the remote host believes it is connecting with one of its own terminals, the user is allowed to log on. An application that allows a user to view files on a remote computer, retrieve files from a computer, and manage files on a remote computer is known as File Transfer, Access, and Management (FTAM). By terms of file structure, file attributes, and the types of operations done on the files and their attributes, FTAM specifies a hierarchical virtual file. Addressing is required to establish communication between the client and the server. When a client sends a request to the server, it includes the server's address as well as its own. The destination address, i.e., client address, is included in the server response to the client request. DNS is utilised to achieve this type of addressing. Email forwarding and storage are provided by an application layer. Directory Services: A distributed database that allows access to global information about numerous objects and services is contained in an application. Authentication: It verifies whether the communication was sent by the sender or received by the receiver, or both. 6.2 WWW The World Wide Web, sometimes known as a Web, is indeed a group of websites or web pages maintained on web servers and accessible via the internet by local computers. Text pages, digital photos, audios, and videos, among other things, can be found on these websites. Consumers can install the content of these sites through the internet utilising technology such 114 CU IDOL SELF LEARNING MATERIAL (SLM)

as laptops, laptops, and cell phones from anywhere in the globe. The WWW, in conjunction with the internet, allows you to retrieve and display text and media on your device. Figure 6.2 World Wide Web Web pages, that are formatted in HTML and linked by connections known as \"hypertext\" or hyperlinks and accessible over HTTP, are the building blocks of the Web. These are electronic linkages that connect relevant pieces of information so that users can easily get what they need. The advantage of hypertext is that it allows you to choose a word or phrase from a text and then go to other sites that provide further information on that word or phrase. A Uniform Resource Locator (URL) is an Internet address given to a web page (URL). A website is a collection of online pages that belong to a given URL, such as www.facebook.com, www.google.com, and so on. As a result, the World Wide Web functions as a massive electronic book, with pages stored on many servers throughout the world. Small websites keep all of their Webpages on a single server, whereas large websites or organisations store their Webpages on multiple servers situated in different countries so that when people from that location search their site, the information may be rapidly retrieved from the nearest server. As a result, the web serves as a platform enabling users can retrieve and exchange data over the internet. Unlike in a book, wherein we move from one page to the next in a sequential order, on the World Wide Web, we visit a web page and then move from that web page to other network sites by following a web of hypertext links. To access the Internet, you'll need a browser installed on your computer. Difference between the term’s \"internet\" and \"World Wide Web\": Some people confuse the terms \"internet\" and \"World Wide Web.\" They believe they are the same, although this is not the case. The Internet is not the same as the World Wide Web. It's a global network of 115 CU IDOL SELF LEARNING MATERIAL (SLM)

computers, laptops, tablets, and other gadgets. It allows people to transfer emails and interact with other users online. When you send an email or speak with someone online, for example, you are using the internet. Figure 6.3 Internets and WWW However, when you access information from a website such as google.com, you are utilising the World Wide Web, which is a network of servers connected by the internet. When you use a browser to request a site from your computer, the server renders that page for you. Your computer is referred to as a client since it runs a programme (web browser) and requests information from another computer (server). The World Wide Web's History: What Is the World Wide Web? Tim Berners-Lee, a British scientist, established the World Wide Web in 1989. At the time, he was employed at CERN. It was originally created to meet the demand for automatic information sharing between scientists all over the world, allowing them to effortlessly communicate the research outcomes of their experiments and investigations. CERN is a community of over 1700 scientists from over 100 nations, where Tim Berners works. These scientists spend part of their time at CERN and the balance of their time at their universities and national laboratories from their home countries, thus they needed reliable communication technologies to share information. Although the internet and hypertext were accessible at the moment, no one had considered how to use the internet to link or distribute documents. Tim concentrated on three key technologies that could help computers communicate with one another: HTML, URLs, and HTTP. The goal of the WWW's creation was to merge modern computer technology, data networks, and hypertext into such a user-friendly and efficient worldwide information system. 116 CU IDOL SELF LEARNING MATERIAL (SLM)

What is the Function of the World Wide Web? We now know that the World Wide Web (WWWW) is a collection of websites linked to the internet that allow users to search for and share information. Let's take a look at how it works now! Figure 6.4 Servers / Browser Architecture The internet's core client-server format, as represented in the accompanying image, governs how the Web functions. When users request web pages or information, the servers store and transport it to their computers on the network. A web server is a computer programme that serves web pages to people who access them through a browser. A client is the computer of a user who requests documents from a server. Users can access the papers that have been obtained using a browser that has been installed on their machine. Web servers store all of the websites. A website, like a person who rents a house, takes up space on a server and is stored there. When a user requests a website's Webpages, the server hosts it, and the website owner is responsible for paying the hosting fee. 117 CU IDOL SELF LEARNING MATERIAL (SLM)

Figure 6.5 Architecture of World Wide Web The WWW starts working the instant you open your browser and type a URL into the address bar or search for something on Google. In order to send information (web pages) from servers to clients, three basic technologies are used (computers of users). Hypertext Markup Language (HTML), Hypertext Transfer Protocol (HTTP), and Web browsers are examples of these technologies. What is HTML (Hypertext Markup Language) and what is the World Wide Web? HTML is a markup language that is commonly used to create web pages. It uses HTML elements or tags to define the structure of online pages. These tags are used to organise material like 'header,' 'paragraph,' 'table,' 'Image,' and others. When you open a webpage, you won't see HTML tags because browsers don't show them and only use them to render the text. HTML is a markup language that allows text, graphics, and other resources to be displayed in a Web browser. Figure 6.6 HTML Header 118 CU IDOL SELF LEARNING MATERIAL (SLM)

Web Browser: Figure 6.7 Web Browsers A web browser, sometimes known as a browser, is a programme that displays text, data, images, videos, animation, and other types of content. It offers an integrated interface that lets you to navigate the Internet by clicking on hyperlinked information. When you double-click the Browser icon on your computer to start it, you'll be sent to the World Wide Web, where you can use Google or type a URL in to address bar. Because of their limited capabilities, browsers were initially solely used for surfing. They are now more complex, and may be used for e-mailing, transferring multimedia files, utilising social networking sites, and engaging in online discussion groups, among other things. Google Chrome, Mozilla Firefox, Internet Explorer, Safari, and others are some of the most popular browsers. Hypertext Transfer Protocol (HTTP) is an application layer protocol that allows the World Wide Web to function smoothly and effectively. It's built on a client-server architecture. A web browser is the client that interacts with the web server that hosts the website. This protocol specifies the structure and transmission of messages, as well as the actions that the Web Server and browser should do in response to various commands. When you type a URL into your browser, it sends an HTTP command to the Web server, which then sends the requested Web page. Figure 6.8 HTTP Request / Response Protocol When we use a browser to access a website, we establish a connection with the web server, and the browser talks with the server using HTTP and sends instructions. TCP/IP is used to interact with the server while using HTTP. The connection is ended once the server executes 119 CU IDOL SELF LEARNING MATERIAL (SLM)

the browser's demand and provides a response. As a result, the browser requests information from the server on behalf of the user. WWW Operation: The World Wide Web is based on a client-server model. The steps below will explain how the internet works: After receiving an IP address, the browser makes a web page request to the web server via the HTTP protocol, which defines how the browser and web server communicate. The web server then checks its search for the specified web page after receiving the request via HTTP protocol. If it is found, it is returned to the web browser and the HTTP connection is closed. Now the web browser gets the web page, processes it, and displays its contents in the browser's window. Figure 6.9 Two Tier Architecture 6.3 DNS An application layer protocol specifies how messages are passed between application processes on various systems. Domain Name System is the abbreviation for Domain Name System. DNS is a directory service which maps a host's name to its numerical address on a network. DNS is necessary for the internet to function. A domain name is a series of symbols specified using dots that is assigned to each node in a tree. The Domain Name System (DNS) is a service that converts domain names into IP addresses. This allows network users to search for other hosts using user-friendly names rather than knowing IP addresses. 120 CU IDOL SELF LEARNING MATERIAL (SLM)

For instance, if Edu Soft’s FTP site had an IP address of 132.147.165.50, most people would access it by typing ftp.EduSoft.com. As a result, the domain name is much more trustworthy than the IP address. Figure 6.10 Types of Domains DNS is a TCP/IP protocol that can be used on a variety of platforms. Generic domains, country domain names, and inverse domains are the three divisions of the domain name space. Domains that are generic It categorises the registered hosts based on their general characteristics. The domain name, that is an index to the DNS database, is defined by each node of a tree. It employs three-character designations that identify the sort of organisation. Label Table 6.1: DNS Suffixes Aero Description Biz Airlines and aerospace companies Businesses or firms 121 CU IDOL SELF LEARNING MATERIAL (SLM)

Com Commercial Organizations Coop Cooperative business Organizations Edu Educational institutions Gov Government institutions Info Information service providers Int International Organizations Mil Military groups Museum Museum & other non-profit organizations Name Personal names Net Network Support centres Org Non-profit Organizations Pro Professional individual Organizations 122 CU IDOL SELF LEARNING MATERIAL (SLM)

Figure 6.11 Lists of Extensions Country Domain The format of a country domain is similar to that of a generic domain, except instead of three- character organisational abbreviations, it utilizes two country abbreviations (e.g., us for the United States). Inverse Domain When mapping an address to a name, the inverse domain is employed. When the server receives a request from a client and only the files of authorised clients are stored on the server. To see if the client is on the approved list, it sends a query towards the DNS server, asking for an address to be mapped to the name. DNS's Operation DNS is a network communication mechanism for clients and servers. DNS clients make requests to DNS servers, and DNS servers respond to the queries. 123 CU IDOL SELF LEARNING MATERIAL (SLM)

Forward DNS lookups are queries that contain a name that is turned into an IP address, whereas reverse DNS lookups are requests that contain an IP address that is transformed into a name. DNS uses a distributed database to keep track of the names of all the hosts on the internet. A software program such as DNS resolver makes a query to the DNS server to acquire the IP address of a hostname when a client such as a web browser submits a request containing a hostname. If a DNS server does not have the IP address for a hostname, the request is sent to another DNS server. If the resolver receives an IP address, it completes its request over the internet protocol. 6.4 MIME Multipurpose Internet Mail Extensions (MIME) is an acronym for Multipurpose Internet Mail Extensions. It's used to boost the functionality of Internet e-mail protocols like SMTP. The MIME protocol enables users to send and receive numerous sorts of digital material in e- mail, including photographs, audio, video, and many sorts of documents and files. MIME was developed in 1991 at Bell Communications by a software engineer named Nathan Borenstein. MIME is an e-mail extension protocol, which means it does not operate on its own but works in conjunction with other protocols like SMTP to enhance the possibilities of e-mail. MIME was able to use the internet to transport only text-based files in a limited size English language. It is now utilised by practically all e-mail service providers, including Gmail, Yahoo Mail, and Hotmail. MIME Protocol is Required For the following reasons, the MIME protocol is often used to transport e-mail over a computer network: Multiple languages, such as Hindi, French, Japanese, and Chinese, are supported by the MIME protocol in e-mail. Simple protocols may reject mail that is too large, but MIME has no such restriction. Simple e-mail protocols including such SMTP cannot send images, audio, or video. This necessitates the use of the MIME protocol. Emails are frequently designed using code including such HTML and CSS, and they are mostly used by businesses to sell their products. This sort of code uses MIME to send HTML and CSS-based email. Header of the MIME Protocol To enhance the features of the standard email protocol, MIME adds five more fields to the header element of the actual e-mail. The following are the fields: 124 CU IDOL SELF LEARNING MATERIAL (SLM)

Type of MIME Version Content Content Id Content Description Content Type Encoding Version 1 of MIME It specifies the MIME protocol version. This header typically has a value of 1.0, indicating that the message is MIME formatted. Type of Content It specifies the kind and subtype of data that will be included in the message. These messages can be of various forms, such as text, image, audio, and video, and they can also be divided into subtypes, such as PNG or jpeg for images. Similarly, the Video subtype can be WEBM, MP4, and so forth. Encoding of Content Types This field specifies the method used to transform mail data to ASCII or Binary numbers, such as 7-bit encoding, 8-bit encoding, and so on. Content Identifier All email messages are given a unique \"Content Id\" number in this field so that they can be identified. Description of the content This section contains a short description of the email's content. This signifies that the \"Content Description\" contains information regarding whatever is being mailed. This field also contains information on the file's name, creation date, and modification date. Figure 6.12 Working of MIMI Protocol 125 MIME Protocol Characteristics • Multiple attachments can be sent in a single e-mail. CU IDOL SELF LEARNING MATERIAL (SLM)

• Non-ASCII characters are supported. • It allows you to send emails of any length. • It supports a variety of languages. The MIME's benefit The following are some of the benefits of the MIME protocol: • It may transfer several sorts of files in a message, including text, audio, and video. • It also allows users to send and receive emails in a variety of languages, including Hindi, French, Japanese, and Chinese. • It also allows people to connect HTML and CSS to email, allowing them to customise email according to their needs and make it nice and elegant. • It can send all of the content in an email, regardless of how long it is. • All e-mails are given a unique id. 6.5 HTTP The Hypertext Transfer Protocol (HTTP) is an application-level protocol for hypermedia information systems that are collaborative and dispersed. It is a data communication protocol that is used to connect a client and a server. HTTP is a TCP/IP-based communication protocol that is used on the World Wide Web (WWWW) to deliver data such as picture files, query results, HTML files, and so on. The default port is TCP 80. It establishes a common method for computers to communicate with one another. HTTP (Hyper Text Transfer Protocol) Basic Characteristics: It's the protocol that lets web servers and browsers communicate across the internet. It's a protocol for sending and receiving requests. By default, it uses TCP port 80 for reliable TCP communications. It is stateless, which means that each request is treated as a new one. In other words, by default, the server does not recognise the user. HTTP (Hyper Text Transfer Protocol) Basic Features: The HTTP protocol has three basic characteristics that make it a simple and powerful communication system: HTTP is media agnostic, meaning that it can send any sort of media content as long since both server and the client can process the data. 126 CU IDOL SELF LEARNING MATERIAL (SLM)

HTTP is connectionless: It is a connectionless strategy in which an HTTP client, such as a browser, initiates an HTTP request, then disconnects from the server and waits for a response. HTTP is a stateless protocol: Only during the current request are the client and server aware of each other. Each one of them forget about each other after that. Because of the protocol's stateless nature, however neither client nor the server can keep track of various requests across web pages. HTTP (Hyper Text Transfer Protocol) Basic Architecture: Figure 6.13 Basic Architecture of Client/Server Model The HTTP protocol is a request/response protocol with a client/server architecture. In this protocol, web browsers, search engines, and other applications act as HTTP clients, while the Web server, such as Servlet, acts as a server. 6.6 SMTP Simple Mail Transfer Protocol (SMTP) is an acronym for Simple Mail Transfer Protocol. Simple Mail Transfer Protocol (SMTP) is a set of communication protocols that allows applications to send electronic mail over the internet. 127 CU IDOL SELF LEARNING MATERIAL (SLM)

It's a programme that uses e-mail addresses to deliver messages to several other computer users. It allows users with the same or other computers to exchange mail, and it also supports: It can transmit one or more recipients a single message. Text, voice, video, and graphics can all be used to send messages. It can also send messages through networks that aren't connected to the internet. SMTP is mostly used to establish communication procedures between servers. The servers have a mechanism for identifying themselves and stating the type of communication they are attempting. They have a system in place to deal with problems like an inaccurate email address. Unless the recipient address is incorrect, the receiving server will respond with an error message. Components of SMTP Figure 6.14 Basic Architecture of SMTP First, we'll separate the SMTP client and server into two parts: the user agent (UA) and the mail transfer agent (MTA) (MTA). The user agent (UA) composes the message, generates the envelope, and inserts the message into it. This mail is transferred across the internet through a mail transfer agent (MTA). 128 CU IDOL SELF LEARNING MATERIAL (SLM)

Figure 6.15 Basic Architecture of MTA Protocol By adding a relaying system to SMTP, a more complicated system can be created. More MTAs, operating as either a client or a server to relay the email, can be added instead of simply one MTA on the sending side and one on the receiving side. Figure 6.16Working of MTA Relay 129 CU IDOL SELF LEARNING MATERIAL (SLM)

The mail gateway can be used to send emails to users without using the TCP/IP protocol, and this is accomplished by using the relaying system. The mail gateway is a message transfer agent (MTA) that could be used to receive email. Figure 6.17 MTA Gateway SMTP's Operation Mail Composition: A user composes an e - mail message using just a Mail User Agent to send an e-mail (MUA). A programme that sends and receives mail is known as a Mail User Agent. The message is divided into two parts: the body and the header. The body of the message is the most important element, whereas the header contains information like the sender and recipient addresses. The header also contains descriptive information, such as the message's subject. The message body is analogous to a letter, and the header is analogous to an envelope containing the recipient's address. Mail Submission: When composing an email, the mail client uses SMTP on TCP port 25 to send the completed message to the SMTP server. E-mail addresses are made up of two parts: the recipient's username and the domain name. [email protected], for example, where \"vivek\" is the recipient's username and \"gmail.com\" is the domain name. If the recipient's email address's domain name differs from the sender's domain name, MSA will forward the message to the Mail Transfer Agent (MTA). The MTA will look for the target domain in order to transport the email. To get the destination domain, it looks up the MX record in the Domain Name System. The MX record contains the recipient's domain's 130 CU IDOL SELF LEARNING MATERIAL (SLM)

domain name and IP address. MTA connects to the exchange server to send the message after the record has been found. Mail Receipt and Processing: When an incoming response is transmitted, the exchange server sends it to the incoming server (Mail Delivery Agent), which keeps it until the user retrieves it. Mail Access and Retrieval: MUA can be used to retrieve email saved in MDA (Mail User Agent). Login and password are required to access MUA. 6.7 POP Post Office Protocol (POP) is an acronym for Post Office Protocol. SMTP is a message transfer agent, as we all know. SMPT is used to convey the message again from client to the server, and from there to the recipient server, when it is sent. However, the Message Access Agent is used to send the message from the receiver server to the actual server. POP3 and IMAP are the two types of protocols supported by the Message Access Agent. Figure 6.18 SMTP Mail server Assume the sender wishes to transmit the message to the recipient. The first message is sent to the sender's mail server. The mail is then sent across the internet again from sender's mail server to the receiver's mail server. When the mail arrives at the receiver's mail server, it is forwarded to the user. The entire procedure is carried out via email protocols. The SMTP protocol is used to deliver mail from sender to the sender's mail server, which is subsequently forwarded to the receiver's mail server. The POP or IMAP protocol receives the data at the receiver's mail server and transfers it to the actual user. Because SMTP is a push protocol, the message is pushed first from client to the server. SMTP pushes the message first from client to the recipient's mail server, as shown in the diagram above. A pull protocol is required for the third stage of email communication, and 131 CU IDOL SELF LEARNING MATERIAL (SLM)

POP is a pull protocol. When mail is sent from the recipient's mail server to the client, it signifies the client is retrieving the message from the server. What exactly is POP3? POP3 is a straightforward protocol with limited capabilities. The POP3 client is installed on the recipient's machine, while the POP3 server is deployed on the recipient's mail server in the case of the POP3 protocol. Figure 6.19 POP3 Protocol The POP3 protocol has a long history. The internet technical task force published RFC 918, the first version of the post office protocol, in 1984. The POP3 protocol, that is used to get emails from the server, was created by the developers as a simple and effective email system. This allows you to access your mails rather than your mailbox while you're not connected to the internet. RFC 937 introduced the post office protocol version 2;however, it was replaced by RFC 1081 in 1988, which introduced the post office protocol version 3. POP3 was then refined for the next ten years before being released. It was published in 1996 after it had been thoroughly improved. Although the POP3 protocol has been improved throughout time, the inventors have kept the essential premise of a three-stage process for retrieving messages between the client and server. They sought to keep this protocol as basic as possible, and the result is that it is now highly popular. In order to establish a connection here between POP3 server and the POP3 client, the POP3 server requests the POP3 client's user name. The ok message is sent if the username is found 132 CU IDOL SELF LEARNING MATERIAL (SLM)

on the POP3 server. It then asks the POP3 client for the password, which the POP3 client then sends to the POP3 server. The POP3 server provides the OK message and the connection is created if the password matches. The client can see the array of mails on the POP3 mail server after establishing a connection. The user will receive the email numbers and sizes from the server in the list of mails. From this list, the user can begin retrieving mail. Once the client has retrieved all of the emails from the server, the server's emails are erased. As a result, we may claim that the emails are confined to a single system, and that accessing the same emails on another machine is impossible. Configuring basic email settings to keep a copy of mail on the mail server can help you avoid this problem. The POP3 protocol has a number of advantages. The following are some of the benefits of using the POP3 protocol: It allows users to access their emails while they are not connected to the internet. Only when downloading emails from either the server does it require an internet connection. Once the emails are downloaded from the server, they are stored on our computer's hard drive, which may be viewed without using the internet. As a result, we can conclude that the POP3 protocol doesn't really necessitate continuous internet access. It gives us quick and easy access to our emails because they are already saved on our computer. There are no restrictions on the size of emails we receive or send. Because all of the mails are saved on the local machine, it takes up less server storage space. The mailbox has a maximum size;however, it is restricted by the width of the hard disc. Because it is a simple protocol, it is one of the most widely utilised nowadays. It’s simple to set up and utilise. The POP3 protocol has a number of drawbacks. The following are some of the benefits of using the POP3 protocol: When emails are downloaded from the server, the server deletes all of the emails by default. As a result, unless other machines are configured to leave a copy of the message on the server, mails cannot be read from other machines. It can be tough to transfer the mail folder from one system to another. Because all of the attachments are saved on your local PC, a virus assault is very likely if the virus scanner does not detect them. The computer may be harmed as a result of the virus attack. The email folder that is downloaded first from mail server has the potential to get corrupted as well. Because the emails are kept locally, anyone who uses your computer can view the email folder. 6.8 IMAP Protocol IMAP (Internet Message Access Protocol) 133 CU IDOL SELF LEARNING MATERIAL (SLM)

Internet Message Access Protocol (IMAP) is an acronym for Internet Message Access Protocol. It was an application layer protocol that allows you to receive emails from a mail server. POP3 is one of the most widely used protocols for retrieving emails. Figure 6.20 IMAP Protocol The client/server model is also used. On the one hand, we get an IMAP client, which is a computer programme. On the other hand, we have an IMAP server, which is likewise a computer-based operation. The two computers are linked through a network. Because the IMAP protocol is based on the TCP/IP transport layer, it makes use of the protocol's dependability implicitly. The IMAP server listens to port 143 by default whenever a TCP link is made here between IMAP client and the IMAP server, although this port number can be altered. IMAP is configured to use two ports by default: IMAP port 143 is a non-encrypted port. When an IMAP client wishes to connect securely to IMAP, it uses port 993. Why should we utilise the IMAP protocol rather than the POP3 protocol? POP3 is quickly overtaking IMAP as the most used method of accessing TCP/IP mailboxes. It uses the offline mail access approach, which implies that messages are downloaded from the mail server and then erased from the mail server. The POP3 protocol is now used by millions of people to access their incoming emails. It can't be used as much because of the offline mail access model. In an ideal environment, we would choose the online model. We must continually be linked to the internet in the online model. The major issue with POP3 offline access is that the mails are permanently gone from the server, making it impossible for numerous machines to view them. This problem can be solved by storing emails on a remote server instead of on a local server. Another challenge that the POP3 faces is data security and safety. The disconnected access paradigm, which gives the advantages from both online and offline access, is the solution to this dilemma. The user can retrieve mail for local usage using the detached access model, similar to the POP3 protocol, and the user does not have to be connected to the internet all of the time. Changes to mailboxes, on the other hand, are synced in between client and the server. The email is saved on the server so that it can be accessed by other programmes in the 134 CU IDOL SELF LEARNING MATERIAL (SLM)

future. When developers realised the advantages, they started working on implementing the detached access model. This is accomplished through the use of POP3 commands, which allow you to leave your emails on the server. This works to some extent, but trying to keep track of which messages were new or old becomes a problem since both are retrieved and left on the server. As a result, the POP3 is missing some elements that are essential for a proper detached access model to work. The development of a new protocol to give a more capable manner of accessing user mailboxes began in the mid-1980s at Stanford University. As a result, the interactive mail access protocol, which had been later renamed Internet Message Access Protocol, was created. History and Standards of the IMAP IMAP version 2 was the first version of IMAP to be officially documented as an internet standard in RFC 1064, which was published in July 1988. It was modified in August 1990, with the same version, in RFC 1176. As a result, they created IMAP3, a new version 3 document. In February 1991, the RFC 1203 was published. However, because IMAP3 was never embraced by the market, users continued to use IMAP2. IMAPbis, a protocol extension, was later built to add support for Multipurpose Internet Mail Extensions (MIME) to IMAP. Because of MIME's utility, this was a significant advance. IMAPbis was never published as just an RFC, despite this. This could be due to issues with the IMAP3 protocol. IMAP version 4, or IMAP4, was released in two RFCs in December 1994: RFC 1730, which described the basic protocol, and RFC 1731, which described the authentication mechanism for IMAP 4. IMAP 4 is the most recent and extensively used version of IMAP. It is still being developed, and the most recent version is known as IMAP4rev1 and is defined in RFC 2060. RFC 3501 is the most recent revision. Features of IMAP IMAP was created for a specific purpose: to give users more flexibility in how they access their inbox. It can function from any of the three modes: online, offline, or disconnected. The offline and disconnected states are of particular importance to most protocol users. An IMAP protocol has the following characteristics: Access and retrieve mail from a distant server: The user can access and retrieve mail from a remote server while keeping the messages there. Set message flags: The system can view track of which messages he has already viewed by setting message flags. Manage multiple mailboxes: The user has the ability to manage several mailboxes and transfer messages between them. For those working on numerous projects, the user can categorise them into various categories.Before you download anything, make sure you have the following information: Before downloading the message from the mail server, it determines to either retrieve or not. 135 CU IDOL SELF LEARNING MATERIAL (SLM)

It allows you to download a component of a message, such as a single body part from the mime-multi part. This is useful when a message's short-text element contains large multimedia files.Organize mails on the server: POP3 users are not permitted to manage their mails on the server. Users, on the other hand, can manage their emails mostly on server according to personal needs, for as by creating, deleting, or renaming mailboxes. Users can obtain a warrant for the contents of emails.Examine the email header: Before downloading, users can check the email header.User can create folders to organise their emails in a hierarchical order. General Operation of IMAP IMAP, like POP3 and other TCP/IP application protocols, is a client-server protocol. The IMAP4 protocol is only useful when it is installed on the server that hosts the user mailboxes. In c, the POP3 server does not have to be the same physical server as the SMTP server. As a result, in the instance of the IMAP protocol, the mailbox should be available via both SMTP and IMAP for incoming mail retrieval and updates. Figure 6.21 IMAP Protocol The Transmission Control Protocol (TCP) is used by the IMAP to ensure that data is delivered and received in the order. The IMAP4 waits for an incoming connection request from either the IMAP4 client on a well-known port, namely port 143. 136 CU IDOL SELF LEARNING MATERIAL (SLM)

All of the devices are synchronised with both the main server using the IMAP protocol. Assume we have three devices: a desktop, a mobile phone, and a laptop, as indicated in the diagram above. If all of these devices visit the same mailbox, the mailbox will be synchronised across all of them. Whenever mail is opened on one device, it is recorded as open on all other devices; similarly, when mail is deleted on one device, it is deleted on all other devices. As a result, all of the devices are synchronised. We can see all of the folders under IMAP, such as spam, inbox, sent, and so on. We can also make our own folder, referred to as a custom folder, which will be displayed on all other devices. 6.9 FTP File transfer protocol (FTP) is an acronym for File Transfer Protocol. FTP is a TCP/IP-based standard internet protocol for transferring files through one host to another. It is mostly used to transport web page files from their originator to a computer that serves various computers on the internet as a server. It can also be used to download files from other servers to a PC. FTP's Objectives It allows users to share files. It's a tool that encourages people to use distant computers. It transports data in a more secure and efficient manner. What is the purpose of FTP? Transferring data from one machine to another is quite simple and uncomplicated, although it can occasionally cause issues. Two systems, for example, may have distinct file conventions. Text and data may be represented differently in two systems. The directory structures of two systems may differ. By creating two connections between hosts, the FTP protocol solves these issues. One link is utilised for data transfer, while the other is used for control. FTP's Mechanism The FTP's basic model is depicted in the diagram above. The user interface, control procedure, and data transfer process are all parts of the FTP client. The server control process as well as the server data transfer procedure are the two processes that make up the server. 137 CU IDOL SELF LEARNING MATERIAL (SLM)

Figure 6.22 FTP Communication In FTP, there really are two types of connections: Figure 6.23 Types of FTP Connections Control Connection: The control connection communicates using extremely simple rules. We can transfer a single chain of authority or response at a time via control connection. Between the control processes, a control connection is established. During the interactive FTP session, the control connection stays open. Data Connection: Because data types differ, the Data Connection employs a sophisticated set of principles. Between data transfer operations, a data link is established. When a command for transferring files is received, the data connection opens and shuts whenever the file is transferred. Clients for FTP 138 CU IDOL SELF LEARNING MATERIAL (SLM)

A file transfer protocol (FTP) client is an application that allows you to transmit information between multiple hosts on the internet. It acts as an interface to a remote server and upload or download files. It includes a set of commands for connecting to a host, transferring files for both you and your host, and closing the connection. A Web browser can also include an FTP application as a built-in component. This GUI-based FTP client makes file transfers simple and eliminates the need to memorise FTP commands. FTP's benefits include: One of the hugest advantages of FTP is its speed. FTP is one of the quickest methods for transferring messages from one device to another. It is more efficient since we do not have to perform all of the processes to obtain the entire file. Security: We must use the username and password to access the FTP server. As a result, we can conclude that FTP is much more secure. Transferring files back and forth: FTP allows us to transmit files back and forth. Assume you are a firm management who sends information to all employees, who then return information back to the same server. FTP's drawbacks include: All FTP transmissions must be encrypted, according to industry standards. However, not all FTP services are created equal, and not all offer encryption. As a result, we'll have to keep an eye out for FTP services that offer encryption. FTP is used for two purposes: sending and receiving huge files over a network. However, the maximum file size that can be transferred is 2GB. Also, it doesn't allow users to send data to several recipients at the same time. Passwords and file contents are transferred in clear text, allowing for unauthorised listening. As a result, it's possible that attackers will try to guess the FTP password in a brute force assault. It does not work with all operating systems. 6.10 TELNET The internet's primary function should be to provide services to the users. Users may, for example, desire to run separate application programmes at a remote site and then transmit the results to a local site. This necessitates the use of a client-server application such as FTP or SMTP. However, this would prevent us from tailoring a programme to meet each demand. 139 CU IDOL SELF LEARNING MATERIAL (SLM)

A better option would be to include a general client-server programme that allows users to access any application programme on a remote machine. As a result, an application that enables a user to access a remote computer. Telnet, a popular client-server application, is used to address these needs. Terminal Network is abbreviated as Telnet. Telnet establishes a connection to a remote computer by simulating a local terminal on the remote side. There are two different kinds of logins: Local Login Figure 6.24 Basic Telnet Architecture Local login occurs when the user logs into a machine on their local network. The terminal driver accepts the user's keystrokes when the workstation is running a terminal emulator. The terminal driver subsequently sends these characters towards the operating system, which ultimately runs the application programme. Special characters, on the other hand, have a special meaning in the operating system. In UNIX, for example, certain character combinations have unique meanings, such as the control character with \"z\" indicating suspend. Such scenarios are unproblematic since the terminal driver understands the meaning of the characters. However, it may pose issues with remote login. 140 CU IDOL SELF LEARNING MATERIAL (SLM)

Remote Login Figure 6.25 Remote Telnet Connection The user needs complete remote login in able to reach an application programme on a remote computer. How does remote login work? At the local location The terminal driver delivers the characters to the TELNET client after the user provides the keystrokes to it. The TELNET client then converts the characters to a global character set called as network virtual terminal characters & sends them to the TCP/IP stack on the local machine. At the isolated location The commands in NVT forms are sent to the remote system through TCP/IP. The characters are transferred to the operating system and subsequently to the TELNET server at this point. The TELNET server converts the characters into a format that a remote computer may understand. The characters, however, cannot be given directly to the operating system since the TELNET server does not send the characters to the distant operating system. As a result, some software that can take data out from TELNET server is required. These characters are subsequently passed to the relevant application software by the operating system. Virtual Terminal for the Network (NVT) 141 CU IDOL SELF LEARNING MATERIAL (SLM)

Figure 6.26 NVT The network virtual interface is a user interface that specifies how data and commands are conveyed across a network. Systems are heterogeneous in today's environment. For example, a DOS operating system accepts Ctrl+z as an end-of-file token, whereas a UNIX operating system accepts Ctrl+d as an end-of-file token. TELNET solves this problem by defining a network virtual interface, which is a universal interface. The TELNET client converts the characters first from local terminal into NVT format before sending them to the network. The Telnet server then converts the data from NVT form to a format that a distant machine can understand. 6.11SUMMARY • The protocol hierarchy's uppermost layer is the application layer. It is at this layer that genuine communication takes place. To transfer information to a different host, it employs the services of a transport layer, network layer, data link layer, and physical layer. • The Application Layer is the seventh of the OSI model's seven layers. The application layer interface interacts directly with the programme and provides common web app services. The presentation layer receives a request from the application layer. The highest level in open systems is the application layer, which provides services directly to the application process. 142 CU IDOL SELF LEARNING MATERIAL (SLM)

• The application layer protocols are TELNET (TelecommunicationNetwork) and FTP (File Transfer Protocol). Because they establish a link between a remote host and a server, they are connection-oriented protocols. FTP (File Transfer Protocol) is a protocol for transmitting files from one computer to another. TELNET is used to log into a system remotely. • The Domain Name System (DNS) is a decentralised hierarchical naming system between computers, services, and other resources that are connected to an Internet or a private network. It links numerous pieces of information to domain names assigned to each of the participants. • HTTP is a request and response protocol that is supported by both clients and servers (TCP). The end user is the client, while the website is the server. The client sends an HTTP request to the server's specified port and use a web browser, web crawler, and other tools. (Some) resources, such as HTML files and pictures, are stored on the replying server. (We refer to it as) The origin server is this replying server. • Agents, gateways, and tunnels are examples of middleware that can be used between the client device and the origin server. Despite the fact that the TCP/IP protocol is the most widely used on the Internet, the HTTP scheme does not require it and is reliant on the levels it supports. In reality, HTTP can be used in conjunction with any other Network technology or network. HTTP merely expects that the underlying protocol (given by the underlying protocol) is reliable, and that it can utilise any protocol that can provide this promise. 6.12KEYWORDS • FTP- A file transfer protocol (FTP) client is an application that allows you to transmit information between multiple hosts on the internet. • IMAP- The IMAP4 protocol is only useful when it is installed on the server that hosts the user mailboxes. In c, the POP3 server does not have to be the same physical server as the SMTP server. 6.13 LEARNING ACTIVITY 1. Examine the DNS packets to identify the IP address of the server that is being contacted. ___________________________________________________________________________ ___________________________________________________________________________ 2. Determine how to run the telnet program on your computer. At a minimum, telnet takes the name or address of a remote computer to connect to. In this question, specify an integer port number that specifies the application on the remote computer. 143 CU IDOL SELF LEARNING MATERIAL (SLM)

___________________________________________________________________________ ___________________________________________________________________________ 6.14 UNITEND QUESTIONS A. Descriptive Questions Short Questions 1. Define WWW? 2. What is FTAM? 3. Write any two advantages of POP. 4. How does remote login works? 5. Write the benefits of FTP. Long Question 1. Explain the functions of Application layer. 2. Explain the working of MIME. 3. Discuss about the features of DNS. 4. Explain about SMTP 5. Explain the types of logins. B. Multiple Choice Questions 1. Which is a markup language that is commonly used to create web pages? a. HTTP b. MDP c. TCP d. HCP 2. ___ can also be used to download files from other servers to a PC. a. TCP b. FTP c. FDP d. RCP 144 CU IDOL SELF LEARNING MATERIAL (SLM)

3. ___ is a directory service which maps a host's name to its numerical address on a network. a. WWW b. source c. DNS d. FTP 4. Who established the World Wide Web. a. Luther King b. BarrisBoem c. KarlKlinton d. Tim Berners Lee 5. A Uniform Resource Locator (URL) is ____ address given to a web page a. Remote b. Local c. Internet d. Common Answers 1-a, 2- b, 3- c, 4- d, 5-c 6.15 REFERENCES Reference books • Behrouz A Forouzan, “Data Communications and Networking”, McGraw Hill. • Andrew S. Tanenbaum, “Computer Networks”, Pearson Education. • Subir Kumar Sarkar, T.G. Basavaraju, C. Puttaamadappa, “AdHoc Mobile Wireless Network: Principles, Protocols, and Applications, CRC Press. Textbook references • James F. Kurose, Keith W. Ross, “Computer Networking”, Pearson Education. 145 CU IDOL SELF LEARNING MATERIAL (SLM)

• Michael A. Gallo, William M. Hancock, “Computer Communications and Networking Technologies”, CENGAGE Learning Websites: • https://www.techopedia.com/definition/10062/wireless-communications • https://www.computernetworkingnotes.com/ • https://www.guru99.com 146 CU IDOL SELF LEARNING MATERIAL (SLM)

UNIT 7-MOBILE COMPUTING STRUCTURE 7.0 Learning Objective 7.1 Introduction to Mobile Computing 7.2 Mobile Computing Technologies 7.3 Major advantages of Mobile Computing 7.4 Security Issues 7.5 Summary 7.6 Keywords 7.7 Learning activity 7.8 Unit End Questions 7.9 References 7.0 LEARNING OBJECTIVES After studying this unit, you will be able to: • Learn the basics of mobile computing and its need • Explain the importance of mobile computing • Describe the design of mobile computing • List the security Issues in mobile computing 7.1 INTRODUCTION TO MOBILE COMPUTING The fundamental and intermediate principles of mobile computing are covered in this tutorial. This tutorial will begin with an examination of Mobile Computing, its ongoing evolution, and the technology's future tendencies. Our Mobile Computing lesson is appropriate for both beginners and experts. The term \"mobile computing\" refers to a technology which allows data, speech, and video to be transmitted wirelessly from a computer or other device. It does not rely on a fixed physical link for connectivity. It allows users to transition from one specific address to another while communicating. 147 CU IDOL SELF LEARNING MATERIAL (SLM)

Our mobile communication course covers all aspects of mobile computing, including a brief review and origins, evolution, classification, benefits and drawbacks, security concerns, and future trends. Mobile Computing Is Introduced Mobile computing is a software that allows users to send information from one location to another without using a physical link or cords. In other words, mobile computing allows data, voice, and video to be transmitted from a computer or any other electronic network without the need for a fixed physical link. Data is transmitted wirelessly via wireless devices such as mobile phones and laptop computers in this technology. This is only possible because to Mobile Computing technology, which allows you to view and transfer data from every remote location without physically being present. Mobile computer technology allows for a wide range of communication coverage. It is one of the most reliable and fastest-growing areas of computing technology. There are three aspects to the role of mobile computing: • Mobile Communication • Mobile Hardware • Mobile Software Mobile Communication The term \"mobile communication\" refers to a framework that governs the operation of mobile computing technologies. In this application, mobile communication applies to an infrastructure that allows wireless devices to communicate seamlessly and reliably. This architecture ensures that wireless device connectivity is consistent and reliable. The mobile communication framework is made up of communication devices including such protocols, services, bandwidth, and portals that are required to facilitate and support the aforementioned services. The following four forms of mobile communication exist: 148 CU IDOL SELF LEARNING MATERIAL (SLM)

Figure 7.1 Types of Communication • Fixed and Wired • Fixed and Wireless • Wired and mobile • Mobile and Wireless Fixed and Wired: In a Fixed and Wired setup, the devices are fixed in place and are connected to each other via a physical link to communicate. Consider a desktop computer. Fixed and Wireless: In a Fixed and Wireless arrangement, the gadgets are fixed in place and connected to each other via a wireless link to communicate with one another. Communication towers, for example, and Wi-Fi routers are examples. Mobile and Wired: Some devices are wired, while others are mobile under the Mobile and Wired setup. They enable communication with the other devices when used together. Laptops, for example. Mobile and Wireless: The devices in a Mobile and Wireless configuration can communicate with one another regardless of their location. Devices can also connect towards any network without a cable connection. Consider the Wi-Fi Dongle. Mobile Hardware The term \"mobile hardware\" refers to mobile devices or identified having that can be utilised to receive or access mobility services. Smartphones, laptops, portable PCs, tablet PCs, and Personal Digital Assistants are all examples of mobile hardware. Figure 7.2 Smart Devices 149 CU IDOL SELF LEARNING MATERIAL (SLM)

A receptor media that can send and receive messages is integrated into these gadgets. Full- duplex operation is possible with these devices. They may send and receive signals, in other words. They wouldn’t have to wait for one device to finish communicating before the other starts communicating. Mobile Software A programme that operates on mobile hardware is known as mobile software. This is built to handle the unique characteristics and needs of mobile applications. This really is the operating system that mobile devices run on. To put it another way, it’s the brains of mobile systems. This is a necessary component for the mobile device to function. Figure 7.3 Mobile OS This allows mobile devices to be more portable, allowing for wireless communication. Mobile Computing Applications The list consists of some of the most important fields where mobile computing is commonly used: • Access to the web or the internet. • GPS stands for Global Positioning System (GPS). • Emergency services are available. • Services relating to entertainment. • Educational services are available. 7.2 MOBILE COMPUTING TECHNOLOGIES: Computing technologies are really the tools that are used to store, handle, and communicate data. Wireless simply refers to interacting with the other devices without the use of a wire, i.e., without the use of a physical connection. The term \"wireless computing\" refers to the transfer of data and information among computers or devices which are not physically attached to one another and have a \"wireless network connection.\" Mobile devices, Wi-Fi, wireless copiers, and so on. Although mobile phones are not physically connected, we can nevertheless send and receive data. A mobile device is one that does not require a network connection or any other type of connectivity to send information and data between devices. Laptops, tablets, smartphones, 150 CU IDOL SELF LEARNING MATERIAL (SLM)