The Electronic Mail and WWW Protocols

The Electronic Mail and WWW Protocols

1. The Electronic Mail System

An Overview of Electronic Mail

Electronic mail (email) is one of the most widely used communication services on the Internet, and it supports many formats, from plain text to images and attached files.

The System Structure

The user agent (UA):

• Mail client: Outlook, Thunderbird, Foxmail
• Web mail: Gmail, QQ Mail, 163 Mail
• Mobile mail: the mail application on a phone

The mail server:

• Sending server: an SMTP server
• Receiving server: a POP3 or IMAP server
• Relay server: a mail forwarding server

The mailbox:

• Inbox: the mail that has arrived
• Outbox: the mail waiting to be sent
• Drafts: the mail that is not finished
• Trash: the mail that has been deleted

The Mail Format and MIME

The structure of a message:

• Header: the sender, the recipients, the subject, the date, and so on
• Body: the content of the message
• Attachments: the attached files

MIME (Multipurpose Internet Mail Extensions):

• Text types: plain text, HTML text
• Image types: JPEG, PNG, GIF
• Audio types: MP3, WAV, MIDI
• Video types: MP4, AVI, MOV
• Application types: PDF, DOC, ZIP

The SMTP Protocol

Function: sending mail

How it works:

1. The connection is established: the client connects to the SMTP server
2. Authentication: the client presents a user name and password
3. The mail is transferred: the client sends the content of the message
4. The connection is closed: the connection is closed once the transfer is complete

Common commands:

• HELO: the client identifies itself
• AUTH: authentication
• MAIL FROM: the sender’s address
• RCPT TO: the recipient’s address
• DATA: the mail data
• QUIT: close the connection

Port number: 25 (plaintext), 587 (encrypted)

The POP3 Protocol

Function: receiving mail

How it works:

1. The connection is established: the client connects to the POP3 server
2. Authentication: the client presents a user name and password
3. The mail is downloaded: the client downloads the messages
4. The connection is closed: the connection is closed once the download is complete

Characteristics:

• Simple: the protocol is simple and easy to implement
• Offline: the mail is downloaded to the local machine
• Deleted: the server deletes the mail after the download
• Single device: suits use from a single device

Port number: 110 (plaintext), 995 (encrypted)

The IMAP Protocol

Function: receiving mail

How it works:

1. The connection is established: the client connects to the IMAP server
2. Authentication: the client presents a user name and password
3. The mail is synchronised: the client and the server synchronise the mailbox
4. The connection is kept: the connection state is maintained

Characteristics:

• Online: the mail is stored on the server
• Several devices: it supports synchronisation across devices
• Folders: it supports managing folders of mail
• Search: it supports searching the mailbox

Port number: 143 (plaintext), 993 (encrypted)

2. The WWW and HTTP

An Overview of the WWW

The WWW (World Wide Web) is a distributed hypermedia system built on the HTTP protocol.

The Structure of the WWW

Hypertext:

• HTML: the Hypertext Markup Language
• Links: the hyperlinks between pages
• Multimedia: text, images, audio, and video

The browser:

• Function: parses HTML and displays the page
• Types: Chrome, Firefox, Safari, Edge
• Extensions: support for various additional functions

The server:

• Web server: Apache, Nginx, IIS
• Application server: Tomcat, WebLogic, WebSphere
• Database server: MySQL, Oracle, SQL Server

The HTTP Protocol

Basic concepts:

• Connectionless: every request is handled independently
• Stateless: no state is kept between requests
• Request-response: the client requests, the server responds

HTTP methods:

• GET: retrieve a resource
• POST: submit data
• HEAD: retrieve the header information of a resource
• PUT: update a resource
• DELETE: delete a resource
• OPTIONS: retrieve the methods that are supported
• TRACE: echo the request back

HTTP status codes:

• 1xx: informational
• 2xx: success (200 OK)
• 3xx: redirection (301, 302)
• 4xx: client error (404, 403)
• 5xx: server error (500, 502)

The HTTP Request Format

GET /index.html HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0
Accept: text/html,application/xhtml+xml
Accept-Language: zh-CN,zh;q=0.9
Connection: keep-alive

The HTTP Response Format

HTTP/1.1 200 OK
Date: Mon, 23 May 2005 22:38:34 GMT
Server: Apache/1.3.3.7
Content-Type: text/html; charset=UTF-8
Content-Length: 138

<html>
<head>
<title>Example</title>
</head>
<body>
Hello World
</body>
</html>

The HTTPS Protocol

Security properties:

• Encrypted transport: the traffic is encrypted with SSL/TLS
• Authentication: the identity of the server is verified
• Data integrity: tampering with the data is prevented

How it works:

1. The handshake phase: the encryption parameters are negotiated
2. The key exchange: a session key is generated
3. The data transfer: the data is transferred in encrypted form

Port number: 443

3. Application Scenarios

Electronic Mail Applications

Personal communication:

• Everyday correspondence
• Handling work mail
• Receiving notification mail

Enterprise use:

• Internal corporate communication
• Customer support
• Sending marketing mail

Secure mail:

• Encrypted mail transport
• Digital signature verification
• Mail archiving and management

WWW Applications

Information browsing:

• Browsing news sites
• Using search engines
• Visiting social media

E-commerce:

• Online shopping
• Online banking
• Online payment

Web applications:

• Online office work
• Cloud storage services
• Online education

Exercises

Exercise 1

What are the respective roles of SMTP and POP3 in an electronic mail system?

Exercise 2

What are the main characteristics of the HTTP protocol? Which HTTP methods are common?

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Glossary