Chapter 8. Exploring the Application Layer

Chapter Goals

You will learn about the following application layer protocols in this Chapter:

• Hypertext Transfer Protocol (HTTP)HTTP is the common web-based content transfer protocol on the Internet.

• Secure Sockets Layer (SSL)You can secure your web content using SSL.

• File Transfer Protocol (FTP)FTP is used to transfer files over the web.

The application layer is the topmost layer of the OSI model and provides you with high-level application development capabilities. As an application developer, you require little knowledge of how the content of your application traverses the network in terms of characteristics specific to the physical media and what network layer protocols the network uses. However, as a content networking administrator, you require knowledge of how the network functions in addition to how the application layer works.

Some TCP/IP-based application protocols implement the session, presentation, and application OSI reference layers of the OSI reference model. In TCP/IP, the application layer implements the OSI session layer to maintain state across TCP layer connections. For example, HTTP is stateless by design but can use cookies to maintain client state across TCP connections between clients and servers. Secure Sockets Layer (SSL) and Real-Time Streaming Protocol (RTSP), on the other hand, implement the OSI session layer in its specification directly, using a session identifier to associate the TCP connections to an application "session."

Application protocols often refer to well-known applications described in RFCs (for example, FTP, HTTP, SMTP, and Telnet). However, any process, either custom or well-known, that has a structured mechanism for communication between client and server devices is an application protocol. As such, each application layer protocol behaves according to the specifications that you set when developing it. Client-server application protocols are normally text-based, with a specified set of commands and with responses to these commands.

This Chapter discusses the following application layer protocols:

• HTTP

• SSL

• FTP

Although Cisco content networking products are aware of many other application layer protocols, these three are primarily used throughout the book in examples and scenarios.

Note

Because the application layer is Layer 5 of the TCP/IP stack, you may read Cisco.com documents or other books referring to the application layer as "Layer 5" instead of "Layers 57."