The Application Layer

The Application Layer is concerned with network processes and applications.  The application layer provides services such as email, file transfer and terminal emulator services.

The following application layer protocols are discussed:

• FTP (File Transfer Protocol)
• HTTP (HyperText Transfer Protocol)
• HTTPS (HyperText Transfer Protocol Secure)
• POP3 (Post Office Protocol V3)
• SMTP (Simple Mail Transfer Protocol)
• SSH (Secure Shell)

HTTP

• HTTP is implemented in two programs: a client program and a server program
• Web browsers implement the client program
• Web servers implement the server program eg Apache, IIS
• HTTP defines both the structure of the message and how they exchange messages

HTTPS

• A secure version of HTTP
• All HTTP requests are encrypted using SSL (Secure Socket Layer) or TLS (Transport Layer Security).
• These secure protocols start by sending a certificate containing a public key to encrypt the transmissions.

FTP

• Used to transfer files from one host to another over the internet.
• Use an FTP client to manage the process.

Emailing

• POP3 (Post Office Protocol v3) and SMTP (Simple Mail Transfer Protocol) are used for emailing.

SSH

• Encrypted communication between two hosts on port 22.
• Remote command-line communication