The application layer has hundreds of protocols. Some common protocols were developed and standardized very early in the 1980s. Some are deprecated and we rarely see them, such as Telnet and CharGEN. New protocols are developed as needed to keep up with today's demands, such as Bitcoin and MQTT. The following is a shortlist of application layer protocols:
| Protocol | Purpose |
| Simple Mail Transfer Protocol (SMTP) | Transports email |
| Hypertext Transfer Protocol (HTTP) | Ensures delivery (transfer) of web pages the proper format. |
| File Transfer Protocol (FTP) | Transfers files between a client and server across the network |
| Message Queuing Telemetry Transport (MQTT) |
Used with IoT devices, sensors, and mobile devices as a lightweight messaging protocol |
All protocols have a specific purpose on the network. Today, Wireshark has dissectors for most, but not all protocols. However, new dissectors are added all the time.
The PDU for the application layer is data. In many cases, the protocols in this layer are involved in a series of client requests and server responses.
The header structure will vary as each is application specific. In addition, the header for the client is generally different than the header for the server.
Most protocols will have an associated port, which will be found in the transport layer header. The following is a summary of the application layer:
| Layer | Purpose | Protocols | PDU |
| Application | Initiates contact with the network | HTTP, DNS, FTP | Data |
After the data leaves the application layer, it is then passed to the presentation layer, in order to properly format the data. Let's explore this next.