Which protocols belong to the application layer?

Application layer protocols are the ones used directly by applications to work over the network: browsers, email clients, messengers, APIs, and so on.

In simpler terms: if a protocol is needed by a program rather than by the network's internal plumbing, it is application layer.

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#Main application layer protocols

#Web and APIs

• HTTP - transferring web pages and API requests
• HTTPS - the same, but encrypted
• WebSocket - a persistent bidirectional connection (chats, online games)

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#Email

• SMTP - sending mail
• POP3 - receiving mail (downloads it, then deletes it from the server)
• IMAP - receiving mail while keeping messages stored on the server

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#File transfer

• FTP - file transfer (unencrypted)
• FTPS - FTP with encryption
• SFTP - file transfer over SSH
• TFTP - simplified file transfer (often used for network equipment)

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#Utility network services

• DNS - converts a domain name into an IP address
• DHCP - automatically hands out IP addresses to devices
• NTP - time synchronization

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#Remote access and management

• SSH - secure remote access to a server
• Telnet - remote access without encryption (obsolete)
• SNMP - monitoring and managing network devices

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#An important point for interviews

An application layer protocol is not the same thing as a program

Examples:

• a browser uses HTTP / HTTPS
• an email client uses SMTP, IMAP, POP3
• ping is a utility, and ICMP is not an application layer protocol