Overview of Application Layer in OSI Model
- 1. Domain Name System(DNS) • An application layer protocol defines how the application processes running on different systems, pass the messages to each other. • 1. DNS is a service that translates the domain name into IP addresses. This allows the users of networks to utilize user-friendly names when looking for other hosts instead of remembering the IP addresses. • 2. DNS is required for the functioning of the internet. • 3. Each node in a tree has a domain name, and a full domain name is a sequence of symbols specified by dots. • 4. For example, suppose the Http site at EduSoft had an IP address of 132.147.165.50, most people would reach this site by specifying www.EduSoft.com.
- 2. The DNS Name Space A portion of the Internet domain name space.
- 3. DNS • The top-level domains come in two flavors: generic and countries. • The original generic domains were com (commercial), edu (educational institutions), gov (the U.S. Federal Government), int (certain international organizations), mil (the U.S. armed forces), net (network providers), and org (nonprofit organizations). • The country domains include one entry for every country, as defined in ISO 3166.
- 4. DNS • Working of DNS • DNS Resource Records is an entry in DNS Server database . Resource Records are usually a name to IP Address (IPv4or IPv6) mapping (or vice versa). • To map a name onto an IP address, an application program calls a library procedure called the Resolver, passing it the name as a parameter. • The resolver sends a UDP packet to a local DNS server, which then looks up the name and returns the IP address to the resolver, which then returns it to the caller. • Armed with the IP address, the program can then establish a TCP connection with the destination.
- 5. Simple Network Management Protocol (SNMP) • SNMP is an application layer protocol used to manage and monitor the network devices(PCs,Routers,Switches). • SNMP components – • There are 3 components of SNMP: 1. SNMP Manager – It is a centralized system used to monitor network. It is also known as Network Management Station (NMS) • 2. SNMP agent – It is a software management software module installed on a managed device. Managed devices can be network devices like PC, router, switches, servers etc. • 3. Management Information Base – MIB consists of information of resources that are to be managed. These information is organized hierarchically.
- 7. SNMP SNMP messages – Different variables are: • 1. GetRequest –SNMP manager sends this message to request data from SNMP agent. • 2. GetNextRequest –This message can be sent to discover what data is available on a SNMP agent. • 3. GetBulkRequest –This message is used to retrieve large data at once by the SNMP manager from SNMP agent. • 4. SetRequest – It is used by SNMP manager to set the value of an object instance on the SNMP agent. • 5. Response –It is a message send from agent upon a request from manager. • 6. Trap –These are the message send by the agent without being requested by the manager. It is sent when a fault has occurred. • 7. InformRequest –An asynchronous alert similar to a TRAP but requires confirmation of receipt by the SNMP manager.
- 8. Electronic mail • Electronic mail (e-mail) is a process by which digital information can be sent, received, forwarded, and stored using telecommunications networks. • Using the Internet, e-mail can be transmitted outside of corporate networks • E-mail systems consist of two subsystems. • 1. User Agents, which allow people to read and send e-mail • 2. Message Transfer Agents, which move messages from source to destination.
- 9. Electronic mail- Architecture • The e-mail is sent through the internet. It has two user agents (UAs) and a pair of message transfer agents (MTAs). • When Alice sends the mail to Bob, then Alice runs the user agent (UA) and message transfer agents (MTAs) program to prepare the e-mail through the internet. After that, this e-mail is stored in the mailbox of Bob.
- 10. Electronic mail • E-mail systems support 5 basic functions:- • a) Composition: It refers to the process of creating messages and answers • (b) Reporting: It has to do with telling the originator what happened to the message that is, whether it was delivered, rejected (or) lost. • (c ) Transfer: It refers to moving messages from originator to the recipient. • (d) Displaying: Incoming messages are to be displayed so that people can read their email. • (e ) Disposition: It concerns what the recipient does with the message after receiving it. Possibilities include throwing it away before reading (or) after reading, saving it and so on.
- 11. Electronic mail • The common protocols for email delivery are • Post Office Protocol (POP), Internet Message Access Protocol (IMAP), and Simple Mail Transfer Protocol (SMTP). • .
- 12. Simple Mail TransferProtocol • The Simple Mail Transfer Protocol (SMTP) is used to deliver e-mail messages over the Internet. • This protocol is used by e-mail clients to deliver messages to the server, and is also used by servers to forward messages to their final destination. • Working of SMTP • The client announces the server whom the e-mail is coming from and whom it is going to. • If such a recipient exists at the destination, the server gives the client the go-ahead to send the message. • Then the client sends the message and the server acknowledges it. . When all the e-mail has been exchanged in both directions, the connection is released.
- 14. SMTP Let us take a look at the characteristics of the SMTP: • SMTP makes use of Port 25. • It makes use of persistent TCP connections and thus can send multiple emails all at once. • It is a connection-oriented protocol. • It makes use of TCP at the transport layer. • Sending message can include text, voice, video • or graphics. • SMTP offers reliability in terms of the outgoing email messages.
- 15. World Wide Web • The World Wide Web abbreviated as WWW and commonly known as the web. • The web consists of a vast, worldwide connection of documents or web pages. • The pages can be retrieved and viewed by using browsers of which internet explorer, Netscape Navigator, Google, Chrome, etc are the popular ones. • The browser fetches the page requested displays the page, properly formatted, on the screen.
- 16. WWW Components of Web: There are 3 components of web: • 1. Uniform Resource Locator (URL): Serves as system for resources on web. • 2. HyperText Transfer Protocol (HTTP): Specifies communication of browser and server. • 3. Hyper Text Markup Language (HTML): Defines structure, organisation and content of webpage.
- 17. WWW • Hypertext Markup Language (HTML): • HTML is a standard markup language which is used for creating web pages. • It describes the structure of web pages through HTML elements or tags. • These tags are used to organize the pieces of content such as 'heading,' 'paragraph,' 'table,' 'Image,' and more.
- 18. WWW • Hypertext Transfer Protocol (HTTP): • Hyper Text Transfer Protocol (HTTP) is an application layer protocol which enables WWW to work smoothly and effectively. • It is based on a client-server model. The client is a web browser which communicates with the web server which hosts the website. • When you enter a URL in the browser, an HTTP command is sent to the Web server, and it transmits the requested Web Page.
- 19. WWW • HTTP • When we open a website using a browser, a connection to the web server is opened, and the browser communicates with the server through HTTP and sends a request. • HTTP is carried over TCP/IP to communicate with the server. • The server processes the browser's request and sends a response, and then the connection is closed. • Thus, the browser retrieves content from the server for the user.