3  Application layer

• The application layer consists of many different protocols because it needs to provide an interface between the many different application programs that may be installed on a device.
• The protocols in the application layer ensure that data is exchanged between devices using an agreed format and in an agreed manner.
• Some common application protocols and the programs they support include:
  • Simple Mail Transfer Protocol (SMTP): used by email programs such as Outlook and Thunderbird when they need to send emails.
  • Post Office Protocol 3 (POP3): used by email programs when they need to receive emails.
  • Hyper Text Transfer Protocol (HTTP): used by web browsers such as Edge and Firefox to request and transfer webpages from the WWW.
  • File Transfer Protocol (FTP): often used by file sharing programs in order to send and receive files between users across the Internet.

Figure 2