| Five layer network model |
| 5. Application layer |
| FTP • HTTP • SMTP • POP3 • IMAP4 • Oscar • ... |
| 4. Transport layer |
| TCP • UDP • ... |
| 3. Network layer |
| IP • ICMP • ... |
| 2. Data link layer |
| Token ring • ... |
| 1. Physical layer |
| Ethernet • IEEE 802.11 (Wireless) • ... |
The application layer is one of the abstract layers in the various layered network models, like the Five layer network model. The application layer is at the top of the model and uses the services provided by the layers beneath it. It's the layer that actually performs the functionality required by the application or program. The application layer generates contextually relevant messages, perhaps based on input from a user or another program, for instance, and uses the layers below to address and send the message. It likewise retrieves messages from the lower layers, and parses them based on context. The application layer is really the only layer that actually generates or understands message content, the others simply take what they're given, probably add a header and maybe do a checksum or some kind of encoding, and then pass it off to the next layer. In other words, the app layer is responsible for generating messages, the other layers are responsible for packaging and sending them.
Programs running on the application layer are generally an implementation of an application layer protocol, like HTTP for web browsing, SMTP for Email, or FTP for file transfer. There a literally hundreds of application layer protocols, these are just a few well known examples. The messages generated by these programs are based on the protocol they follow. Messages can be plain text commands, as is generally the case with the three protocols mentioned above, or they could be more complex binary values, it all depends on the protocol.
