Application Layer Protocols, Networking

It is important to distinguish between network applications and application-layer protocols. An application-layer protocol is only one piece (albeit, a big piece) of a network application. Let's look at a couple of examples. The Web is a network application that allows users to obtain "documents" from Web servers on demand. The Web application consists of many components, including a standard for document formats (i.e., HTML), Web browsers (e.g., Netscape Navigator and Internet Explorer), Web servers (e.g., Apache, Microsoft and Netscape servers), and an application-layer protocol. The Web's application-layer protocol, HTTP (the HyperText Transfer Protocol [RFC 2068]), defines how messages are passed between browser and Web server. Thus, HTTP is only one piece (albeit, a big piece) of the

Web application. As another example, consider the Internet electronic mail application. Internet electronic mail also has many components, including mail servers that house user mailboxes, mail readers that allow users to read and create messages, a standard for defining the structure of an email message (i.e., MIME) and application-layer protocols that define how messages are passed between servers, how messages are passed between servers and mail readers, and how the contents of certain parts of the mail message (e.g., a

mail message header) are to be interpreted. The principal application-layer protocol for electronic mail is SMTP (Simple Mail Transfer Protocol [RFC 821]). Thus, SMTP is only one piece (albeit, a big piece) of the email application. 

As noted above, 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: 

•  the types of messages exchanged, e.g., request messages and response messages; l the syntax of the various message types, i.e., the fields in the message and how the fields are delineated;
•  the semantics of the fields, i.e., the meaning of the information in the fields;
•  rules for determining when and how a process sends messages and responds to messages.

Some application-layer protocols are specified in RFCs and are therefore in the public domain. For example, HTTP is available as an RFC. If a browser developer follows the rules of the HTTP RFC, the browser will be able to retrieve Web pages from any Web server (more precisely, any Web server that has also followed the rules of the HTTP RFC). Many other application-layer protocols are proprietary and intentionally not available in the public domain. For example, many of the existing Internet phone products use proprietary application-layer protocols.