World Wide Web & Email Protocols (PDF)

World Wide Web World Wide Web Brief History • 1990-91:  First web browser was created • End of 1992:  Several browsers were created. There were about 30 Web servers in the entire world. • 1993:  The first graphical web browser called Mosaic was created by a team of students at the University...

World Wide Web World Wide Web Brief History • 1990-91:  First web browser was created • End of 1992:  Several browsers were created. There were about 30 Web servers in the entire world. • 1993:  The first graphical web browser called Mosaic was created by a team of students at the University of Illinois. • End of 1993  There were about 200 Web servers in the world. • Today  There are more than 250 million separate websites, but many of these are hosted on the same servers by large hosting companies such as godaddy.com or Google sites. How the Web Works World Wide Web How the Web Works World Wide Web How the Web Works • The Web is a good example of a two-tier client–server architecture. • Client  Each client computer needs an application layer software package called a Web browser.  There are many different web browsers, such as Microsoft Internet Explorer. • Server  Each server on the network acts as a Web server  Server needs an application layer software package called a Web server.  There are many different Web servers, such as those produced by Microsoft and Apache. World Wide Web How the Web Works • To get a page from the Web, the user must type the Internet uniform resource locator (URL) for the page (e.g., www.yahoo.com) or click on a link that provides the URL. • The URL specifies:  1) the Internet address of the Web server  2) the directory and  3) the name of the specific page wanted. • If no directory and page are specified, the Web server will provide whatever page has been defined as the site’s home page. World Wide Web How the Web Works • Web browser and Web server must use the same standard protocol or language. • The standard protocol for communication between a Web browser and a Web server is Hypertext Transfer Protocol (HTTP). • To get a page from a Web server, the Web browser issues a special packet called an HTTP request that contains the URL and other information about the Web page requested. • Once the server receives the HTTP request, it processes it and sends back an HTTP response • HTTP response will be the requested page or an error message. World Wide Web How the Web Works • This request– response dialogue occurs for every file transferred between the client and the server. Inside an HTTP Request World Wide Web Inside an HTTP Request World Wide Web Inside an HTTP Request • An HTTP request from a Web browser to a Web server has three parts.  The request line (required)  The request header (required)  The request body (optional) World Wide Web Inside an HTTP Request • The request line  starts with a command (e.g., get), provides the Web page, and ends with the HTTP version number that the browser understands  the version number ensures that the Web server does not attempt to use a more advanced or newer version of the HTTP standard that the browser does not understand. • The request header  contains a variety of optional information such as the Web browser being used (e.g., Internet Explorer) and the date. • The request body  contains information sent to the server  E.g., information that the user has typed into a form. World Wide Web Inside an HTTP Request Inside an HTTP Response World Wide Web Inside an HTTP Response • The format of an HTTP response from the server to the browser is very similar to that of the HTTP request. • It has three parts too.  The response status (required)  The response header (optional)  The response body (optional) World Wide Web Inside an HTTP Response • The response status, which contains  the HTTP version number the server has used,  a status code (e.g., 200 means “OK”; 404 means “not found”), and  a reason phrase (a text description of the status code). • The response header, which contains a variety of optional information, such as  the Web server being used (e.g., Apache),  the date, and  the exact URL of the page in the response. • The response body, which is the Web page itself. Electronic Email Electronic Email Email Standards • Several standards have been developed to ensure compatibility between different email software packages. • Any software package that conforms to a certain standard can send messages that are formatted using its rules. • Any other package that understands that particular standard can then relay the message to its correct destination. • Many email packages send using one standard but can understand messages received in several different standards. • The most commonly used standard is SMTP (Simple Mail Transfer Protocol). How Email Works How Email Works Two-Tier Thick Client Architecture • Each client computer runs an application layer software package called a mail user agent  Commonly called an email client.  E.g., Outlook and Eudora. • The user creates the email message using these email clients. • Email client (E.g. Outlook) formats the message into an SMTP packet. • SMTP packet includes information such as the sender’s address and the destination address. How SMTP (Simple Mail Transfer Protocol) email works. IMAP = Internet Message Access Protocol; LAN = Local Area Network; POP = Post Office Protocol How Email Works Two-Tier Thick Client Architecture • The user agent then sends the SMTP packet to a mail server. • The mail server runs a special application layer software package called a mail transfer agent, • Mail transfer agent is commonly called as mail server software. How SMTP (Simple Mail Transfer Protocol) email works. IMAP = Internet Message Access Protocol; LAN = Local Area Network; POP = Post Office Protocol How Email Works Two-Tier Thick Client Architecture • Protocols  The SMTP standard covers message transmission:  Between mail servers (i.e., mail server to mail server) and  Between the originating email client and its mail server.  A different standard is used to communicate between the receiver’s email client and his or her mail server.  Two commonly used standards for communication between email client and mail server are:  Post Office Protocol (POP) and  Internet Message Access Protocol (IMAP). How SMTP (Simple Mail Transfer Protocol) email works. IMAP = Internet Message Access Protocol; LAN = Local Area Network; POP = Post Office Protocol How Email Works Two-Tier Thick Client Architecture • Protocols  The main difference between POP and IMAP is how the message is handled.  POP-3  The email message is copied to the client computer’s hard disk and deleted from the mail server.  IMAP  Email messages can remain stored on the mail server after they are read.  User can read their email from many different computers (e.g., home, office, computer labs) because the email is stored on the server as well until it is deleted. How SMTP (Simple Mail Transfer Protocol) email works. IMAP = Internet Message Access Protocol; LAN = Local Area Network; POP = Post Office Protocol How Email Works Three-Tier Thin Client Architecture HTTP = Hypertext Transfer Protocol; IMAP = Internet Message Access Protocol; LAN = Local Area Network; POP = Post Office Protocol; SMTP = Simple Mail Transfer Protocol How Email Works Comparison Two-Tier Thick Client Architecture Three-Tier Thin Client Architecture HTTP = Hypertext Transfer Protocol; IMAP = Internet Message Access Protocol; LAN = Local Area Network; POP = Post Office Protocol; SMTP = Simple Mail Transfer Protocol How Email Works Two- and Three-Tier Email Architecture Comparison • Three-tier approach using a web browser is more complicated than the two-tier approach. • So why do it? Because it is simpler to use to just have a Web browser to access the emails. • No need to install and maintain special email client on every client computer. • Users can check their email from a public computer anywhere on the Internet. • Sender and receiver do not have to use the same architecture for their email.  Sender can use two-tier architecture and the receiver, a three-tier architecture. • Because all communication is standardized using SMTP between the different mail servers, how the users interact with their mail servers is unimportant.  Each organization can use a different approach.  One single organization can use both architectures. Inside and SMTP Packet Inside an SMTP Packet Parts of an SMTP Packet • SMTP defines how message transfer agents operate and how they format messages sent to other message transfer agents. • An SMTP packet has two parts:  The header  lists source and destination email addresses  The body  Contains the actual data Inside an SMTP Packet Parts of an SMTP Packet Attachments in Emails Attachments in Emails Multipurpose Internet Mail Extension (MIME) • SMTP permits only the transfer of text messages. • How to transfer non-text files (attachments containing images, files, etc.,.)? • Several standards for nontext files have been developed that can operate together with SMTP  E.g., Multipurpose Internet Mail Extension (MIME), uuencode, and binhex. • Each of the standards is different, but all work in the same general way. Attachments in Emails Multipurpose Internet Mail Extension (MIME) • The MIME software exists as part of the email client • It takes the non-text file (E.g., PowerPoint file), and translates each byte in the file into a special code that looks like regular text. • This encoded section of “text” is then labeled with a series of special fields.  SMTP understands this encoded section as a MIME-encoded attachment. • It includes other information about the attachment (e.g., name of file, type of file). • When the receiver’s email client receives the SMTP message with the MIME attachment, it recognizes the MIME “text” and uses its MIME software (i.e., part of the email client) to translate the file from MIME “text” back into its original format. Thank You