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.
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:
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.
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."