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| Overview

The application layer is the top layer in the TCP/IP reference model. When the ISO developed the OSI Reference Model, the application layer functions were divided into three separate, more detailed layers. Although the OSI version is more detailed, it is more common to refer to the application layer of TCP/IP since it is more encompassing.

The application layer is the interface that separates application software from the transport layer, and deals with high-level protocols rather than segments, bytes, packets, or bits.

It provides network services for users and their programs and is the layer in which user-access network processes reside. These processes include all of those that users interact with directly, as well as other processes of which the users are not aware.

This layer includes all application layer protocols that use the host-to-host transport protocols to deliver data. Other functions that process user data such as data encryption, decryption, compression and decompression, can also reside at the application layer.

Most of the application layer protocols provide user services. Application layer protocols are typically used for network management, file transfer, distributed file services, terminal emulation, and electronic mail. However, new user services are often added (for example, VPNs, VoIP, and so on).

The most widely known and implemented TCP/IP application layer protocols are listed below:

- Telnet enables users to establish terminal session connections with remote hosts.
- HyperText Transfer Protocol (HTTP) supports the exchanging of text, graphic images, sound, video, and other multimedia files on the World Wide Web.
- File Transfer Protocol (FTP) performs interactive file transfers between hosts.
- Trivial File Transfer Protocol (TFTP) performs basic interactive file transfers typically between hosts and networking devices (for example, routers, switches, and so on).
- Simple Mail Transfer Protocol (SMTP) supports basic message delivery services.
- Post Office Protocol (POP) is used to connect to mail servers and download e-mail.
- Simple Network Management Protocol (SNMP) is used to collect management information from network devices.
- Domain Name Service (DNS) maps IP addresses to the names assigned to network devices. Commonly called name service.
- Network File System (NFS) enables computers to mount drives on remote hosts and operate them as if they were local drives. Originally developed by Sun Microsystems, it combines with two other application layer protocols, external data representation (XDR), and remote-procedure call (RPC), to allow transparent access to remote network resources.

Other application layer protocols are listed below.

- Finger — User Information Protocol
- IMAP4 — Internet Message Access Protocol
- IPDC — IP Device Control
- ISAKMP — Internet Message Access Protocol
- LDAP — Lightweight Directory Access Protocol
- NTP — Network Time Protocol
- POP3 — Post Office Protocol version 3
- RLOGIN — Remote Login
- RTSP — Real-time Streaming Protocol
- SCTP — Stream Control Transmission Protocol
- S-HTTP — Secure Hypertext Transfer Protocol
- SLP — Service Location Protocol
- TFTP — Trivial File Transfer Protocol
- WCCP — Web Cache Coordination Protocol
- X-Window

These and other network applications use the services of TCP/IP and other lower-layer Internet protocols to provide users with basic network services.