Standard application layer protocols

Overview

Recall questions on FTP, HTTP, HTTPS, POP3, SMTP and SSH, which of them expose data to an eavesdropper, and how anonymous FTP access differs.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is FTP used for?

   Model answer

   File Transfer Protocol is used to transfer files between a client and a server across a network. It supports uploading and downloading, listing the contents of directories, and creating, renaming and deleting files and folders on the server. It is designed for moving whole files, in bulk if necessary, rather than for viewing documents.

2. 2

   What is HTTP used for?

   Model answer

   Hypertext Transfer Protocol is the protocol used between a web browser and a web server to request and deliver web pages and the resources that go with them, such as images, stylesheets and scripts. The client sends a request naming a method and a resource; the server sends back a response containing a status code and, usually, the resource itself. The data travels in plain text.

3. 3

   What is HTTPS, and how does it differ from HTTP?

   Model answer

   Hypertext Transfer Protocol Secure does the same job as HTTP — requesting and delivering web pages — but the connection is encrypted. Before any request is sent, the client and server carry out a handshake in which the server presents a digital certificate proving its identity and the two agree a shared symmetric key, and all subsequent traffic is encrypted with it. This means an eavesdropper cannot read or alter the pages and data in transit, and the client can be confident it is talking to the genuine server rather than an impostor.

4. 4

   What is POP3 used for?

   Model answer

   Post Office Protocol version 3 is used by an email client to retrieve messages from a mail server. In its usual mode the client connects, downloads the waiting messages to the user's own device, and the messages are then deleted from the server, so the mail is stored and read locally.

5. 5

   What is SMTP used for?

   Model answer

   Simple Mail Transfer Protocol is used to send email: from a mail client to its outgoing mail server, and from one mail server on to the next as the message is relayed towards the recipient's mail server. It handles sending and forwarding only, not retrieval.

6. 6

   What is SSH used for?

   Model answer

   Secure Shell provides an encrypted connection to a remote computer over a network. It is used to log in securely to that computer and execute commands on it as though sitting at it, to transfer files securely, and to tunnel other protocols inside its encrypted connection. Everything sent, including the password used to log in, is encrypted.

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