Internet And Web

1. REVIEW OF NETWORK CONCEPTS 1.1.

PACKETS, FRAMES AND ERROR DETECTION

Packets: - Logical grouping of information that includes a header containing control information and user data is called packet. Packets are most often used to refer to network layer units of data. Frame: - The basic package of information on a network channel is called frame. In other words, it is a logical grouping of information sent as a data link layer unit over a transmission medium. Error Detection: - Error control provides error detection and correction. There are two basic strategies for dealing with errors. These are: (a) To include only enough redundancy to allow the receiver to confirm that an error occurred, but not aware of which error and therefore request it for re-transmission. (b) Second method is to include enough unwanted data along with each block of data sent to enable the receiver to extract what the transmitted character must have been. 1.2. NETWORK TOPOLOGY, REPEATERS, BRIDGES, SWITCHES Network Topology: - Topology is the method in which networks are physically connected together. Topology determines the complexity of connecting computer, the strategy for physically expanding the network, in future. There are three type of topology is used: (i) Bus Topology: - Bus topology is a network geometric arrangement in which a single connecting line is shared by a number of nodes. In linear bus topology, all computers are connected by a single length of cable with a terminator at each end. Each node is connected to two others except the machines at either end of the cable, which are connected only to one other node. Examples of Bus Topology are Ethernet, Local Talk etc. Advantage of Bus Topology: (a) This topology is simple, reliable, and easy to use and understand in small sized LANs. (b) This topology requires least amount of cable to connect the computers together. (c) This type of topology is easy to extend. (d) This topology is less expensive than other cabling arrangements. Disadvantage of Bus Topology: (a) There is possibility of collision, thus in this case data packets may be lost. (b) Heavy network traffic can slow down a bus considerably. (c) It is difficult to troubleshoot a bus. (ii) Ring Topology: - Physical layout of Ring Topology based LAN is circular. Means that each workstation is connected with it neighbors. Transmission can be done in only one direction, either clockwise or anticlockwise. It is decided at the time of network design. Token Passing: - It is a mechanism i.e. used with Ring Topology based LAN to make transmission possible between two or more work stations. Token is nothing but a small program. When network is on, this program starts moving from one node to another in specific direction. If a work station has to access the service of network, then it has to wait for token. Once token becomes available to the node, it grabs the token make its transmission /work, after completion of transmission, it has to release the token. Advantage of Ring Topology: (a) Installation cost of Ring Topology may cheaper. (b) No one computer can monopolize the network. (c) The fair sharing of the network allows the network to degrade gracefully as more users can added. Disadvantage of Ring Topology: (a) Failure of a node may interrupt the system. (b) It is difficult to troubleshoot a ring network. (c) Adding or removing computers disrupts the network.

INTERNET AND WEB TECHNOLOGY (iii)

Star Topology: - Star Topology based LAN based on an electronic device namely Hub. All terminals including server are connected with central Hub. A Hub receives the signal from source and sends it to the destination. Example of Star Network is ATM (Asynchronous transmission Mode). There are two types of Hub: (a) Active Hub: - The active hub regenerates the electrical signal and sends it to all connected computers. (b) Passive Hub: - Passive hub doesn’t generate electrical signal but acts as a connecting point. Advantage of Star Topology: (a) It is easier to modify i.e. easy to add new terminal and easy to remove a particular terminal from a network. It doesn’t disturb/affect the network. (b) It is easy to troubleshoot. (c) Failure of a single terminal doesn’t interrupt the whole network. (d) To install this network several types of cables can be used. Disadvantage of Star Topology: (a) Failure of central hub interrupts the whole network. (b) It is considered as expensive network. (c) Additional resources are required in case of broadcast implementation. 1.3. WAN TECHNOLOGIES AND ROUTING WAN (Wide Area Network): - WANs operate at a much wider geographical distance than LANs. They combine the continuous error detection and correction techniques included in synchronous communications with robust network problem determination and data routing to form powerful backbones that ensure high-quality, reliable service for end users. These networks allow multiple users to access a variety of host computers simultaneously through the same physical medium, while separating each user’s session so that no user is aware of another on the network. Some of the functions that can be performed on WANs are given below:Remote Data and Job Entry: - It is possible to enter data of the sales and transactions at the point-of-sale terminals using WANs. It is also possible to centralize this data in a computer for processing or reporting purposes. Remote Job Entry at colleges and universities enable faculty and students to write and execute their own programs on a central computer. Centralizing information – It is often convenient for a Application Layer business to centralize regional/national information. WANs enable such dealers to query centralized databases. Facilitating Communications – Corporations in advanced Presentation Layer countries often use WANs to facilitate employee communications, to save on long-distance phone calls and letter writing, to cut costs on the preparation of written documents, and to overcome the time lags involved in Session Layer overseas communications. Video-conferencing, in which users communicate with each other through their computer systems, is another possible function of WANs. Transport Layer Routing: - A Router interconnects two or more physically and logically separate network segments and operates at the Network layer. Segments are joined together with a router, which maintains a separate logical identity. It constitutes an Network Layer Internet work-a group of networks linked by routers. The process of choosing the best path throughout the LAN. Routing lets PCs which are not directly Data Link Layer connected communicate by passing messages along to adjacent PCs. 1.4. OSI STACK AND TCP/IP MODEL

Physical Layer

OSI Stack: -

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OSI STACK 14

INTERNET AND WEB TECHNOLOGY Main objectives of OSI Models are:  Allow to interconnected two systems through standard interface.  Each layer performs a well-defined function.  The function of each layer should be chosen according to international standard protocols.  Each lower level protocol provides its services to higher-level protocol.  The seven layers of OSI reference Model are: (i) Physical Layer: - All electrical and mechanical devices, these are used to connect two terminals, two nodes, medium etc. considered as the devices of physical layer. In other words mostly hardware are categorized into physical layer. These are involved in interconnecting two points of a network as well as carry bit stream. (ii) Data Link Layer: - The Data Link Layer is the second layer of OSI Model. The Data Link Layer together with physical layer and the interconnectivity medium provide a data link connection between source station and destination station for reliable transfer of database. Services provide by Data Link Layer: (a) Flow Control – Flow Control deals with how to keep the fast sender from over flowing a slow receiver by buffer at the receiver sides and acknowledgement. (b) Framing – Some control bits are added to the data packets these are received from network layer. The bits are associated with start and end frames. (c) Error Detection and Correction Course – Various methods are use for such purpose. Some popular methods are parity bit, checksum etc. (iii) Network Layer: - The Network Layer provides services to the transport layer. It can be based on ether virtual circuits or data grams. In both cases its main job is routing packets from source to destination. In case of virtual circuit subnet, a routing decision is made when virtual circuit is setup. In case of data gram subnet, routing decision is made on every incoming package.

(iv) Transport Layer: - This Layer is responsible provide reliable cost effective data transport from source machine to destination machine. Transport Layer also provides some additional services associated with data transportation. Two protocols are used with transport layer to provide such services. These are: TCP (Transmission Control Protocol) UDP (User Data Protocol)

(a) (b)

(v)

Session Layer: - Main functionality of this layer is to establish and release the session. The session layer allows users on different machines to establish session between them. Session can allow going both directions. One of the services of the session layer is to manage dialog control.

(vi) Presentation Layer: - Presentation Layer mainly manages the formats of information. (vii) Application Layer: - Several protocols are available under this layer. Large number

of

terminals are connected with the system and requires verify of services. TCP/IP: - TCP reference model is implemented with or suitable form those computer network architecture that allow communication across multiple divers network. TCP/IP network architecture consists of four Application Layer layers: -

(i) Application

Layer: - The Application Layer provides services that can be used by other applications for example protocols have been developed for remote login, for e-mail, for file transfer etc. the application layer program run directly over the transport layer.

(ii)

Transport Layer: - Two basic services are provided by this layer: -

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Transport Layer Internet Layer Network Interface Layer 14

TCP/IP Suite

INTERNET AND WEB TECHNOLOGY The first service consists of reliable connection Oriented transfer of byte stream, which is provided by TCP. The second service consists of connectionless transfer of individual package i.e. provided by UDP. UDP is used for applications that require quick but reliable delivers. The TCP/IP Model doesn’t require strict layering. In other words the application layer has the option of by passing intermediate layer.

(iii)

Internet Layer: - The Internet layer handles the transfer of information access multiple networks through the use of gateways or routers. It has similar function as network layer of OSI Model. A key aspect of the Internet layer is the definition of global unique address for the machines that are attached with the Internet. The Internet layer provides a single service namely best effort connectionless packet transfer. For this packets are also called data grams. The connectionless approach makes the system robust i.e. if failures occur in the network the packets are routed around the point of failure. There is no need to setup the connection.

(iv)

Network Interface Layer: - Network Interface Layer is concern with the network specific aspects of the transfer of packets. It must deal with port of network layer and data link layer. The network interface layer is particularly concern with the protocols that access the intermediate networks.

2. IP ADDRESSING 02.01. SCHEME An IP address is a software address, not a hardware address, which is a hard code in the machine or network interface card. This address is made up of 32 bits of information. These bits are divided into four sections containing one byte (8 bit each). These sections are referred to as Octets. There are three methods for depicting an IP address: • Dotted-decimal, as in 130.57.30.56 • Binary, as in 10000010.00111001.00011110.00111000 • Hexadecimal, as in 82 39 1E 38 All the three examples given above represent the same IP address. The 32 bit IP addressing is a two level hierarchical addressing scheme. The first part is designated as network address and the other part of the address is designated as a node address. The network address uniquely identifies each network. Every machine on the same network shares that network address as part of its IP address. The node address is assigned to, and uniquely identifies, each machine on the network. This part of the address must be unique because it identifies a particular machine. The number can also be referred to as a host address. 02.02. HIERARCHY CLASS The designers of the Internet decided to create classes of networks based on network size. For a small number of networks possessing a very large number of nodes, they created the rank Class A Network. At the other stream is the Class C Network, reserved for the numerous networks with a small number of nodes. The class distinction for networks in between very large and very small is called a Class B Network. (a) Class A Network – In Class A Network, the first byte is assigned to the network address, and the three remaining bytes are used for the node addresses. The Class A format is: Network.Node.Node.Node For example, in the IP address 49.22.102.70, 49 is the network address and 22.102.70 is the node address. (b) Class B Network – In Class B Network, the first two bytes are assigned to the network address, and the remaining two bytes are used for the node addresses. The Class B format is:

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INTERNET AND WEB TECHNOLOGY Network.Network.Node.Node For example, in the IP address 130.57.30.56, 130.57 is the network address and 30.56 is the node address. (c) Class C Network – In Class C Network, the first three bytes are assigned to the network address, and the remaining one byte is used for the node address. The Class C format is: Network.Network.Network.Node For example, in the IP address 198.21.74.102, 198.21.74 is the network address and 102 is the node address. (d) Additional Classes of Networks – Another class of network is Class D. this range of addresses is used for multicast pakets. The range of numbers is from 224.0.0.0 to 239.255.255.255. a multicast transmission is used when a host wants to broadcast to multiple destinations. Hosts do this when attempting to learn of all the routers on its network. There is also a Class E range of numbers starting at 240.0.0.0 and running to 255.255.255.255. These numbers are reserved for future use. One should not assign Class D and E addresses to nodes on any networks.

3. IP DATAGRAM 03.01. HEADER During Transportation TCP/IP adds some additional information to the data packet i.e. associated with transportation service, called TCP Header. Sections of TCP Header: (i) Source and destination port number – values of this field identifies the local port number. (ii) Sequence Number – sequence number identifies the current sequence number of data segment. (iii) Data offset – values of this field identifies the start of data. (iv) Flags – (a) URG (Urgent Flag) – value of this field identifies the urgent pointer. It is considered as urgent flag. (b) ACK (Acknowledgement Flag) – value of this field associated with acknowledgement of the packet. (c) PSH (Push Flag) – This flag performs push function. (d) RST (Reset Flag) – it performs reverse of push function. (e) SYN (Synchronization Flag) – value of this field associated with sequence synchronization. (f) FIN (Final) – End of transmission flag. (v) Window Size – This field contains the values associated with strength of destination station, how many byte (data block) the receiving host can accept at a time? (vi) Checksum – Checksum for data and header. (vii) Urg Ptr(Urgent Point) – The value of this field identifies specific type of data area. 03.02. VIRTUAL PACKET

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INTERNET AND WEB TECHNOLOGY Connectionless internet service allows a sender to transmit individual packets of data across an internet. Each packet travels independently and contains information that identifies the internet recipient. These types of packets are called virtual packet. A router is used to send and receive these packets from one network to another. As the term virtual implies, protocol software creates and handles internet packets, the underlying hardware does not understand or recognize the internet packet format. 03.03. ROUTING TABLES The Routing table contains a set of entries that each specifies a destination and the next hop used to reach that destination. Each destination listed in a routing table is a network, not an individual host. The distinction is important because an internet can contain over 1000 times as many hosts as networks. For example in given below figure, router R 2 connects directly to the networks labeled Net 2 and Net 3. Therefore R2 deliver a datagram to any destination attached to those networks. When a datagram is destined for network 4, R2 sends the datagram to router R3.

R1

Destination Net 1 Net 2 Net 3 Net 4 03.04. DATAGRAM FORWARDING

R2

R3

Next Hop R1 Deliver direct Deliver direct R3

The process of using a routing table to select a next hop for a given datagram is called routing or forwarding. Now we assume that the datagram contains a destination IP address D. the routing software must find an entry in the routing table that specifies a next hop for D. to do so, the software examines each entry in the table by using the mask in the entry to extract a prefix of address D and comparing the result to the Destination field of the entry. If the two are equal, the datagram will be forwarded to the Next hop in the entry. 03.05. ENCAPSULATION, FRAGMENTATION, REASSEMBELY Encapsulation – Encapsulation is a technique to transmit datagram across a physical layer. When an IP datagram is encapsulated in a frame, the entire datagram is placed in the data area of a frame. The network hardware treats a frame that contains a datagram exactly like any other frame.

Fragmentation – The technique IP uses to divide a large datagram into smaller datagrams called fragments. The ultimate destination reassembles the fragments. The fragmentation scheme has been planned carefully to make it possible to further fragment a fragment. Another router along the path divides the fragment into smaller fragments. In a poorly designed internet where networks are arranged in a sequence of decreasing MTUs, each router along the path must further fragment each fragment. Reassembly – The process of creating a copy of the original datagram from fragments is called reassembly. Each fragment begins with a copy of the original datagram header, all fragments have the same destination address as the original datagram from which they were derived. The fragment that carries the final piece of data has an additional bit set in the

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INTERNET AND WEB TECHNOLOGY header. Thus a receiver performing reassembly can tell whether all fragments have arrived successfully.

4. FDD 04.01. DATAGRAM FORMAT The general form of an IPv6 datagram given below. An IPv6 datagram begins with a base header, which is followed by zero or more extension headers, followed by data. In particular, some extension headers are larger than the base header, while other can be smaller.

04.02. FRAGMENTATION IPv6 fragmentation resembles IPv4 fragmentation, the details differ. Like IPv4, a prefix of the original datagram is copied into each fragment, and the payload length is modified to be the length of the fragment. Unlike Ipv4, however, IPv6 does not include fields for fragmentation information in the base header. Instead, IPv6 places them in a separate fragment extension header; the presence of the header identifies the datagram as a fragment. Figure illustrates IPv6 fragmentation.

04.03. REASSEMBLY The process of creating a copy of the original datagram from fragments is called reassembly. Each fragment begins with a copy of the original datagram header, all fragments have the same destination address as the original datagram from which they were derived. The fragment that carries the final piece of data has an additional bit set in the header. Thus a receiver performing reassembly can tell whether all fragments have arrived successfully. 04.04. PATH MTU, ADDRESSING AND HEXADECIMAL NOTATION Path MTU - The minimum MTU along a path from a source to destination is known as the path MTU and the process of learning the path MTU is known as path MTU discovery. In general, path MTU discovery is an iterative procedure. A host sends a sequence of various-size datagrams to the destination to see if they arrive without error. Once a datagram is small enough to pass through without fragmentation, the host chooses a datagram size equal to the path MTU.

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INTERNET AND WEB TECHNOLOGY Addressing – Like IPv4, IPv6 assigns a unique address for each connection between a computer and a physical network. Thus, if a computer (e.g. a router) connects to three physical networks, the computer is assigned three addresses. Also like IPv4, IPv6 separates each such address into a prefix that identifies the network and a suffix that identifies a particular computer on the network. Hexadecimal Notation – An address that occupies 128 bits can accommodate Internet growth, writing such numbers can be unwidely. For example, consider a 128-bit number written in dotted decimal notation: 105.220.136.100.255.255.255.255.0.0.18.128.140.10.255.255 The designers of Ipv6 propose using a more compact syntactic form known as colon hexadecimal notation in which each group of 16 bits is written in hexadecimal with a colon separating groups. For example, when the above number is written in colon hex, it becomes: 69DC:8864:FFFF:FFFF:0:1280:8C0A:FFFF

5. TCP 05.01. PROTOCOL Protocol means the set of rules or standards designed to enable computers to be connected with one another and to exchange information among them with very little error. Protocol can describe low level details of machine-to-machine interfaces or high-level exchange between allocation programs. Protocol specifies the computer language used to transfer information. Specifically, a protocol tells the browser where the information is located. It also tells the browse what to expect from the document retrieval process. Transport protocols provide reliability, which is fundamental for many applications. The Transmission Control Protocol (TCP) is the transport level protocol that provides reliability in the TCP/IP protocol suite. 05.02. SERVICE TO APPLICATION TCP provides a completely reliable, connection-oriented, full-duplex stream transport service hat allows two application programs to form a connection, send data in either direction, and then terminate the connection. Each TCP connection is started reliably and terminated gracefully, with all data being delivered before the termination occurs. From an application program’s point of view, the service offered by TCP has seven major features:  Connection Oriented – TCP provides connection oriented service in which an application must first request a connection to a destination, and then use the connection to transfer data.  Point-to-Point Communication – Each TCP connection has exactly two endpoints.  Complete Reliability – TCP guarantees that the data sent across a connection will be delivered exactly as sent, with no data missing out of order.  Full Duplex Communication – TCP connection allows data to flow in either direction, and allows either application program to send data at any time.  Stream Interface – TCP provides a stream interface in which an application sends a continuous sequence of octets across a connection.  Reliable Connection Startup – TCP requires that when two applications create a connection, both must agree to the new connection.  Graceful Connection Shutdown – An application program can open a connection, send arbitrary amounts of data, and than request that the connection be shut down. 05.03. RELIABILITY, RETRANSMISSION, FLOW CONTROL

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INTERNET AND WEB TECHNOLOGY Reliability – TCP must be carefully designed to achieve reliability. The major problems are: unreliable delivery by the underlying communication system and computer reboot. To understand the scope of the problem, consider a situation in which two application programs from a TCP connection, communicate, close the connection and then form a new connection. Because any message can be lost, duplicated, delayed, or delivered out of order, messages from the first connection can be duplicated and a copy delayed long enough for the second connection to be established. Messages must be unambiguous, or the protocol will accept duplicate messages from the old connection and allow them to interface with the new connection. Retransmission – TCP uses a variety of techniques to handle parts of the problem. One of the most important techniques is retransmission. When TCP sends data, the sender compensates for packet loss by implementing a retransmission scheme. Both sides of a communication participate. When TCP receives data, it sends an acknowledgement back to the sender. Whenever it sends data, TCP starts a timer. If the timer expires before an acknowledgement arrives, the sender retransmits the data. Figure illustrates retransmission. Flow Control – TCP uses a window mechanism to control the flow of data. When a connection is established, each end of the connection allocates a buffer to hold incoming data, and sends the size of the buffer to the other end. As data arrives, the receiver sends acknowledgements, which also specify the remaining buffer size. The amount of buffer space available at any time is called the window, and a notification that specifies the size is called a window advertisement. A receiver sends a window advertisement with each acknowledgement. 05.04. THREE WAY HANDSHAKE To guarantee that connections are established or terminated reliably, TCP uses a 3-way handshake in which three messages are exchanged. TCP uses term synchronization segment (SYN segment) to describe messages in a 3-way handshake used to create a connection, and the term FIN segment to describe messages in a 3-way handshake used to close a connection. Part of 3-way handshake used to create a connection requires each end to generate a random 32-bit sequence number. If an application attempts to establish a new TCP connection after a computer reboot, TCP chooses a new random number. 05.05. CONGESION CONTROL, SEGMENT FORMAT Congestion Control – If congestion triggers excessive retransmission, the entire system can reach a state of congestion collapse, analogous to a traffic jam on a highway. To avoid the problem, TCP always uses packet loss as a measure of congestion, and responds to congestion by reducing the rate at which it retransmits data.

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INTERNET AND WEB TECHNOLOGY TCP congestion control takes over when a message is lost. Instead of retransmitting enough data to fill the receiver’s buffer, TCP begins by sending a single message containing data. If the acknowledgement arrives without additional loss, TCP doubles the amount of data begin sent, and sends two additional messages. If acknowledgements arrive for those two, TCP sends four more, and so on. The exponential increase continues until TCP is sending half of the receiver’s advertised window, at which time TCP slows down the rate of increase. TCP then increases the window size linearly. TCP Segment Format – TCP uses a single format for all messages, including messages that carry data, those that carry acknowledgements, and messages that are part of the 3-way handshake used to create or terminate a connection. TCP uses the term segment to refer to a message. SOURCE PORT DESTINATION PORT SEQUENCE NUMBER ACKNOWLEDGEMENT NUMBER HLEN NOT CODE WINDOW USED BITS CHECKSUM URGENT POINTER OPTIONS (if any) BEGINNING OF DATA • Source Port – It identifies the application program that sent the data. • Destination Port – It identifies which application program on the receiving computer should receive the data. • Sequence Number – It gives the sequence number of the first octet of data being carried in the segment. The receiver uses the sequence number to reorder segments that arrive out of order and to compute an acknowledgement number. • Acknowledgement Number – It specifies the sequence number of the data that is expected next. • Window – It specifies how much additional buffer space is available for data starting at the position given by the acknowledgement. • Checksum – This field contains a checksum that covers the TCP segment header and the data.

6. DOMAIN NAME SYSTEM 06.01. STRUCTURE Structure of Computer Names – The naming scheme used in the internet is called the Domain Name System (DNS). Syntactically, each computer name consists of a sequence of alpha-numeric segments separated by periods. For example, a computer in the Computer Science Department at Purdue University has the domain name: Mordred.cs.purdue.edu The domain name system does specify values for the most significant segment, which is called a top-level domain (TLD). The lists of top-level domains of the DNS are given below: Domain Name Assigned To Domain Name Assigned To Aero Air transport Int International industry treaty organizations Arpa Infrastructure Mil United states domain military Biz Businesses Museum Museums Com Commercial Name Individuals organization Coop Cooperative Net Major network associations support center

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INTERNET AND WEB TECHNOLOGY Edu

Education Org Non-commercial institution organizations Gov United states pro Credentialed government professionals Info information Country code A country Top-level domains are controlled by the Internet Corporation for Assigned Names and Numbers (ICANN). ICANN designates one or more domain registrars to administer a given top-level domain and approve specific names. To obtain a domain, an organization must register with an approved registrar. A unique domain suffix is assigned to each organization. Geographic Structure – In addition to the familiar organizational structure, the DNS allows organizations to use a geographic registration. For example the Corporation For National Research Initiatives registered the domain: cnri.reston.va.us because the corporation is located in the town of Reston, Virginia in the United States. Thus, names of computer at the corporation end in .us instead of .com. 06.02. DNS CLIENT, SERVER MODEL One of the main features of the Domain Naming System is autonomy. The naming hierarchy helps achieve autonomy by allowing an organization to control all names with a particular suffix. In addition to hierarchy names, the DNS uses client-server interaction to aid autonomy. In essence, the entire naming system operates as a large, distributed database. Most organizations that have an Internet connection run a domain name server. Each server contains information that links the server to the other domain name servers; the resulting set of servers functions as a large, coordinated database of names. Whenever an application needs to translate a name to an IP address, the application becomes a client of the naming system. The client places the name to be translated in a DNS request message, and sends the request to a DNS server. The server extracts the name from the request, translates the name to an equivalent IP address, and returns the resulting address to the application in a reply message. 06.03. HIERARCHY MULTIPLE SEREVER DNS servers are arranged in a hierarchy that matches the naming hierarchy, with each being the authority for part of the naming hierarchy. A root server occupies the top of the hierarchy, and is an authority for the top-level domains. Although it does not contain all possible domain names, a root server contains information about how to reach other servers. Although the hierarchy of DNS servers follows the naming hierarchy, the structure is not identical. A corporation can choose to place all its domain names in a single server, or can choose to run servers. For example, figure 1 illustrates two ways Foobar Corporation might choose to allocate its naming hierarchy to two servers. In figure 1, the server for foobar.com also includes the soap division, while in figure 2 the server for foobar.com includes everything except the walnut subdivision.

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INTERNET AND WEB TECHNOLOGY com Server for candy.foobar.com

Server for Foobar.com

foobar

candy

peanut

Root server

soap

almond

walnut

Figure-1

com

Server for foobar.com

foobar

soap

candy

peanut

almond

Root server

walnut

Figure-2

06.04. RESOLVING A NAME The translation of a domain name into an equivalent IP address is called name resolution, and the name is said to be resolved to an address. Software to perform the translation is known as name resolver software. Today, the Domain Name System (DNS) is the standard for resolving names to addresses. However, the hosts file still plays a role in name resolution during the booting of a system and as a means to provide LAN resolution when DNS is down. In a nutshell, DNS is a distributed database whose structure looks like the UNIS file system. DNS is a client/server system in which the resolvers query name servers to find an address record for a domain name. The query process begins with the root name servers. If the root name server does not know the answer, it returns the addresses of a name server that knows more details about the domain name. The resolver then queries the new name server. This iterative process continues until a name server responds with the address for the domain name.

7. E-MAIL AND FILE TRANSFER 07.01. SMTP SMTP stands for Simple Mail Transfer Protocol. It is a TCP/IP protocol that specifies how computers exchange electronic mail. It works with post office protocol (POP).

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INTERNET AND WEB TECHNOLOGY Working of SMTP – SMTP is a simple ASCII protocol. After establishing the TCP connection, the sending machine, operating as the client, waits for the receiving machine, operating as the server, to talk first. The server starts by sending a line of text giving its identity and telling whether or not it is prepared to receive mail. If it is not, the client releases the connection and tries again later. If the server is willing to accept e-mail, the client announces whom the email is coming from and whom it is going to. If such a recipient exists at the destination, the server gives the client the o-ahead to send the message. Then the client sends the message and the server acknowledges it. 07.02. MAIL TRANSFER A user interacts with an e-mail interface program when composing or reading messages. The underlying e-mail system contains a mail transfer program that handles the details of sending a copy of a message to a remote computer. When a user finishes composing an outgoing message, the e-mail interface places the message in a queue that the mail transfer program handles. The mail transfer program waits for a message to be placed on its queue, and then transfers a copy of the message to each recipient. Sending a copy of a message to a recipient on the local computer is trivial because the transfer program can append the message to the user’s mailbox. Sending a copy to a remote user is more complex. The mail transfer program becomes a client that contacts a server on the remote machine. The client sends the message to the server, which places a copy of the message in the recipient’s mailbox. The given below figure illustrates the interaction.

User interface on sender’s computer Mail transfer program on sender’s computer

Recipient’s mailbox Server on recipient’s computer

Internet 07.03. DIAL UP AND POP Dial up – This is the simplest and cheapest type of connection to internet. All we need is a computer with a modem and any of the popular telecommunications packages. Windows 9.X and other OS have bulletin facility. The individual user dials-in to an internet service provider. Each user has an account on the provider’s internet services host computer to the internet. Pop – It is a simple protocol used for opening the remote e-mail boxes. This protocol is defined in RFC 1225. it has the commands for the user to • Log in the remote mailbox • Log out the remote mailbox • Fetch messages from the remote mailbox • Delete messages from the remote mailbox This protocol also defines the method for transfer of the e-mail from the remote mailbox to the user’s local machine for reading mail later. 07.04. FTP GENERAL MODEL AND USER INTERFACE FTP is designed to be run from a program (browser) or for interactive use. When a program invokes FTP, the program must handle all details and then inform the user whether the operation succeeded or failed; the user never sees the FTP interface. When a user invokes FTP interactively, the user communicates with a command-driven interface. FTP issues a prompt to which the user responds by entering a command. FTP executes the command, and then issues another prompt.

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INTERNET AND WEB TECHNOLOGY 07.05. FILE NAME TRANSLATION AND NETWORK FILE SYSTEM File Name Translation – File name differences can be especially important when using an abbreviation. In such commands, a user can specify an abbreviation that FTP expands into a list of names. Unfortunately, a file name that is valid on one computer may be illegal on another. To handle incompatibilities among computer systems, the BSD interface to FTP permits a user to define rules that specify how to translate a file name when moving to a new computer system. Network File System – The file access mechanism used with TCP/IP is known as Network File System (NFS). NFS allows an application to open a remote file, move to a specified position in the file, and read or write data starting at that position. In addition to reducing the load on the network, the file access scheme used by NFS allows shared file access. A file that resides at an NFS server can be accessed by multiple clients. To prevent other from interfering with file updates, NFS allows a client to lock a file. The interface to NFS is unlike the interface to FTP. NFS provides the conventional file operations such as open, read, and write. To configure NFS, a special directory is created in the computer’s file system and associated with a remote computer.

8. WORLD WIDE WEB(WWW) 08.01. INTERFACE A browser is an interactive program used to access the web. A browser implements a point and click interface-the browser displays information on the computer’s screen and permits a user to navigate using the mouse. A browser can display both text and graphics. In other words we can say that a browser is an interactive program that permits a user to view information from the World Wide Web. 08.02. HYPERTEXT To ensure that web browsers correctly interpret the contents of each page, the web uses a standard representation known as the Hyper Text Markup Language (HTML). HTML allows a document to contain general guidelines for display, and allows a browser to choose details; consequently, two browsers may display an HTML document differently. 08.03. HYPERMEDIA The term hypermedia is used to distinguish the web from a traditional hypertext system which restricts documents to textual information-the web allows documents to contain digitized photographic images or graphics. A hypermedia document available on the web is called a page. 08.04. HTML FORMAT AND REPRESENTATION Each HTML document is divided into a head that contains information about the document, and a body that contains the information to be displayed. An HTML document consists of a text file that contains markup tags embedded in the document. A tag is bracketed by less-than and greater-than symbols: and its corresponding tag has the form: . An HTML document starts with the tag . The pair of tags and bracket the head, while the pair of tags and bracket the body. In the head, the tags <TITLE> and bracket the text that forms the document title. Example shown below:

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INTERNET AND WEB TECHNOLOGY <TITLE> Text that forms the document title Body of the document appears here 08.05. EMBEDDING GRAPHICS AND IMAGES In general, nontextual information such as a graphics image or digitized photo is not inserted directly in an HTML document. Instead, the data resides in a separate location, and the document contains a reference to the data. When a browser encounters such a reference, the browser goes to the specified location, obtains a copy of the image, and inserts the image in the displayed document. HTML uses the IMG tag to encode a reference to an external image. For example, the tag: specifies that the file fred_phoo.gif contains an image that the browser should insert in the document. 08.06. HTTP HTTP protocol is used to manage the links between one hypertext document and another. HTTP is the mechanism that opens the related documents when we select a hypertext link. HTTP allows a browser to request a specific item, which the server then returns. HTTP request are sent as text encoded in ASCII. HTTP supports four basic operations that a browser can specify when making a request: • GET requests a specified item from the server. • HEAD requests status information about an item. • POST sends data to server. The server appends the data to a specified item. • PUT sends data to the server. The server uses the data to replace a specified item. ***** *** *

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