Application - Layer Protocols

We have just learned that network processes chronicle with each other by sending messages into sockets. But how are these messages structured? What are the meanings of the varied fields in the messages? When do the processes send the messages? These questions bring us into the realm of application-layer protocols. An application-layer protocol defines how an application's processes, running on different end systems, pass messages to each other. In particular, an application-layer protocol defines:

o The types of messages exchanged, for example, ask messages and response messages
o The syntax of the varied message types, such as the fields in the message and how the fields are delineated
o The semantics of the fields, that is, the meaning of the facts in the fields
o Rules for determining when and how a process sends messages and responds to messages

Apache

Some application-layer protocols are specified in Rfcs and are therefore in the social domain. For example, the Web's application-layer protocol, Http (the Hyper Text replacement Protocol (Rfc 2616]), is available as an Rec. If a browser developer follows the rules of the Http Rfc, the browser will be able to retrieve Web pages from any Web server that has also followed the rules of the Http Rfc.

Many other application-layer protocols are rights and intentionally not available in the social domain. For example, many existing P2P file-sharing systems use rights application-layer protocols.

It is important to distinguish in the middle of network applications and application- layer protocols. An application-layer protocol is only one piece of a network application. Let's look at a integrate of examples. The Web is a client-server application that allows users to get documents from Web servers on demand. The Web application consists of many components, along with a proper for document formats (that is, Html), Web browsers (for example, Firefox and Microsoft Internet Explorer), Web servers (for example, Apache and Microsoft servers), and an application-layer protocol. The Web's application-layer protocol, H'I' defines the format and sequence of the messages that are passed in the middle of browser and Web server. Thus, Http is only one piece (albeit, an important piece) of the Web application. As another example, an Internet e-mail application also has many components, along with mail servers that house user mailboxes; mail readers that allow users to read and create messages; a proper for defining the buildings of an e-mail message; and application-layer protocols that define how messages are passed in the middle of servers, how messages are passed in the middle of servers and mail readers, and how the contents of confident parts of the mail message (for example, a mail message header) are to be interpreted. The needful application-layer protocol for electronic mail is Smtp (Simple Mail replacement Protocol) [Rfc 2821]. Thus, e-mail's needful application-layer protocol, Smtp, is only one piece (albeit, an important piece) of the e-mail application.

Application - Layer Protocols