What Is an Application Layer?
An application layer is the topmost layer of the OSI model. It is responsible for displaying received information to the user or providing means to input data from the user to be sent down to lower layers.
It defines a communication partner, e.g., a process or software program, and how it communicates with other applications.
The application layer is responsible for providing interfaces and services to the user. It also supports services like message handling systems, shared database management systems, transaction processing systems, etc.
Examples of Application Layer Protocols
DHCP (Dynamic Host Configuration Protocol) – DHCP is a protocol used by network devices (servers, routers, modems, etc.) to provide IP addresses automatically for devices connected on a network using Transmission Control Protocol/Internet Protocol (TCP/IP).
DNS (Domain Name System) – DNS stands for Domain Name System. When you enter a URL into your browser’s address bar, the DNS records associated with that domain convert the domain name to an IP address, enabling your browser to connect to the website you want to visit.
HTTP (Hypertext Transfer Protocol) – A protocol used to transfer text, audio, video, and other multimedia files between the server and client. It is used online for sharing information through World Wide Web pages. HTTP is a TCP/IP-based application layer protocol that runs on top of Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) for the transmission of data packets.
SMTP (Simple Mail Transfer Protocol) – SMTP is an application layer TCP/IP protocol that sends e-mails from one machine to another over networks. It enables any machine connected to the internet to send e-mail messages to any other machine with an e-mail address.
FTP (File Transfer Protocol) – FTP is a protocol that enables file transfers between client and server applications over a TCP/IP-based network, such as the Internet. FTP is one of the oldest internet services and has been in use for decades.
The application is closest to the end-user, enabling direct interaction between the user and the software.
Functions of the Application Layer
User support and information services: The application layer consists of various user support and information services, such as remote login, file transfer, electronic mail, multimedia conferencing, and database access.
Resource sharing: The application layer also provides an interface between the network and the application program. This allows users to share resources such as files, printers, and databases over the network.
Standardization: Another important function of the application layer is to provide standardization among different types of networks. For example, when a user sends an electronic mail from one network to another using two different protocols, the application layer on both sides converts the data into a common format so that both parties can understand it.
Application layer protocols also require services from lower layers that address issues such as data integrity (whether data has been altered in transit), reliability (whether data was received), duplicate delivery (whether multiple copies were created), sequencing (whether segments arrive in order), and timing (whether segments arrive too early or too late).