What is the application layer?
The application layer is at Layer 7, the top of the Open Systems Interconnection (OSI) communications model. It ensures that an application can communicate with other applications on different computer systems and networks.
The application layer isn't an application. Instead, it's a component within an application that controls the communication method to other devices. It's an abstraction layer service that masks the rest of the application from the transmission process.
The application layer relies on all the layers below it to complete its process. At this stage, the data or the application is presented in a visual form that the user can understand.
How does the application layer work?
The application layer lets users and software interact with network services. It operates on a client-server model, where the client, such as a user's device, sends requests to a server, which responds with the requested data.
The application layer doesn't handle data transport but ensures data -- such as websites, streamed videos and downloaded files -- is presented correctly. It is the gateway for applications, such as web browsers and email clients, to communicate with an underlying network. This ensures data is sent or received in a usable format.
For example, when a domain name is typed into a browser, the application layer translates it into a request that the network can process.
Functions of the application layer
The application layer handles the following functions:
- Ensures the receiving device is identified, reachable and ready to accept data.
- When appropriate, enables authentication between devices for an extra layer of network security.
- Confirms that necessary communication interfaces exist, such as an Ethernet or Wi-Fi interface in the sender's computer.
- Gets both ends to agree on error recovery procedures, data integrity and privacy.
- Determines protocol and data syntax rules at the application level.
- Presents the data to the user application on the receiving end.
Two types of software provide access to the network within the application layer:
- Network-aware applications, such as email.
- Application-level services, such as file transfer or print spooling.
How the OSI model works
Before the creation of the OSI model, applications ran on separate systems. They used different network technologies that couldn't communicate or exchange information.
To solve that issue, the International Organization for Standardization designed a layered approach to accommodate technological changes. It breaks down inconsistencies to foster greater information exchange.
Devices like routers and switches have OSI protocols embedded in their firmware to facilitate computer networking among different systems and network infrastructures. Applications contain protocols to facilitate the data transfer process with lower OSI layers.
Each layer of the OSI model handles different network functions. They transfer specific information between upper and lower layers during data processing and information exchange.
OSI layers and their functions
The seven OSI layers are the following:
- Physical layer. In the physical layer, devices such as a local area network or router physically link to a network.
- Data link layer. The data link layer defines how data is transferred over a connecting link.
- Network layer. The network layer determines how messages move across a network, such as end-to-end movement of data packets using logical addresses.
- Transport layer. The transport layer facilitates error recovery and reorders data packets that are in the wrong sequence.
- Session layer. The session layer manages all phases of a session, including setup, in-process control functions and message takedown, and it supports concurrent transmission of multiple bidirectional messages.
- Presentation layer. The presentation layer supports message formatting and encryption.
- Application layer. This layer facilitates communication with applications running on other computers.
Application layer protocols
The application layer was developed with the following two sublayers:
- Common application service element. CASE provides services for the application layer and requests services from the session layer. Its activities include remote operation control and data transfer. CASE services apply to a variety of application requirements.
- Specific application service element. SASE provides application-specific services, such as file transfer, remote database access, application management information and transaction processing.
Early application layer protocols the Internet Engineering Task Force specified in Request for Comments 1233 included the following four protocols that are still in use:
- File Transfer Protocol. FTP handles file transfer activities.
- Simple Mail Transfer Protocol. SMTP handles the movement of electronic mail.
- Domain Name System. DNS maps the name used for a website to its IP address.
- Simple Network Management Protocol. SNMP provides remote host management.
The following are examples of well-known application layer protocols in use today:
- Bitcoin for digital currency.
- Hypertext Transfer Protocol for message communications.
- H.323 for packet-based communications, such as voice over IP;
- Lightweight Directory Access Protocol for queries of user information.
- Network File System for data storage and retrieval from various media.
- Remote Procedure Call to execute a procedure or subroutine on a different system or network; and
- X.400 Message Handling Service Protocol that facilitates the transfer of email among compatible systems.
Many communications protocols and interface methods have been developed based on the OSI model. Communication with remote hosts is common and uses a variety of web browsers, the application layer and its six complementary layers.
Who uses the application layer?
Several IT professionals use the application layer as part of their jobs. The most common include the following:
- Software engineers. Their programs must ensure data flows correctly between clients and servers via the application layer.
- Systems architects. They plan the implementation of network services, ensuring the application layer integrates with lower-level infrastructure.
- Web developers. Their web designs rely on the application layer.
- Cybersecurity analysts. They analyze vulnerabilities in the application layer, such as vulnerabilities tied to domain misuse and protocol exploits.
- Network administrators. They monitor and secure the layer against targeted network attacks.
OSI vs. Transmission Control Protocol/Internet Protocol
The OSI model and TCP/IP models are often compared. In the OSI model, the application layer is a distinct, topmost layer. It sits above the presentation and session layers, providing network services to end-user applications. IT demarcates tasks. For instance, in the OSI model, the application layer handles protocols such as HTTP and SMTP, while the lower layers manage data transport.
The TCP/IP model combines the application, presentation and session layers into one application layer. This version still allows browsing, file transfers and other client-server interactions, but it doesn't differentiate the layers. The TCP/IP model prioritizes practical implementation over conceptual clarity, making it a popular option for modern internet infrastructure.
Find out about the common networking protocols you should know and understand.
