Each application or service uses protocols that define the standards and data formats to be used. Without protocols, the data network would not have a common way to format and direct data. To understand the function of various network services, familiarize yourself with the underlying protocols that govern their operation.
Many different types of applications communicate across data networks. Therefore, application layer services must implement multiple protocols to provide the desired range of communication experiences. Each protocol has a specific purpose and contains the characteristics required to meet that purpose. The right protocol details in each layer must be followed so that the functions at one layer interface properly with the services in the lower layer. Click the Play button in the figure to see an animation about applications and services.
Applications and services may also use multiple protocols in the course of a single conversation. One protocol may specify how to establish the network connection and another describe the process for the data transfer when the message is passed to the next lower layer.
While applications provide people with a way to create messages and application layer services establish an interface to the network, protocols provide the rules and formats that govern how data is treated. All three components may be used by a single executable program and can even use the same name. For example, when discussing Telnet, we could be referring to the application, the service, or the protocol.