Lecture 7 Application layer protocols_v1_Part2.pdf

Outline • Domain Name System (DNS) • Hypertext Transfer Protocol (HTTP) • Simple Mail Transfer Protocol (SMTP) • File Transfer Protocol (FTP) • Telnet and Secure Socket Shell (SSH) • Dynamic Host Configuration Protocol (DHCP) 21 WWW • The most popular service on the Internet • Immense collection...

Outline • Domain Name System (DNS) • Hypertext Transfer Protocol (HTTP) • Simple Mail Transfer Protocol (SMTP) • File Transfer Protocol (FTP) • Telnet and Secure Socket Shell (SSH) • Dynamic Host Configuration Protocol (DHCP) 21 WWW • The most popular service on the Internet • Immense collection of Web pages and other resources that can be downloaded across the Internet and displayed on a web browser • The main concept is hypertext • WWW is the set of linked hypertext documents that can be viewed on web browsers 22 Web Client • To communicate with a web server, using Hypertext Transfer Protocol (HTTP), – Firefox, IE, Chrome, etc. • To submit an HTTP request to the server 23 Web Server • To provide response message to the client • To provide web content that can be accessed through the Internet – Proprietary: Microsoft’s Internet Information Services (IIS) – Open source: Apache developed by Apache Software Foundation 24 WWW’s Major functional components • HyperText Markup Language (HTML) • Hypertext Transfer Protocol (HTTP) • Uniform Resource Locator (URI) 25 HyperText Markup Language (HTML) • Is the language used to create webpages • Describes the visual appearance of a document to be displayed by an Internet browser • Defines the structure and layout of a Web document by using a variety of tags and attributes – Title, lists, paragraph – Fonts, colors, positioning using CSS (Cascading Style Sheets) – Linking to another document or to a file 26 HTML (cont.-) 27 HTTP: Protocol of the World Wide Web • Originally, its main purpose was to transfer static web pages written in HTML • Now, it is also used for general file transfer and downloading/displaying multimedia files • The standard (and default) port for HTTP (Web) servers to listen on is 80, though they can use any port. • HTTP is an application-layer protocol based on TCP as its Transport-layer protocol • Functions as a request-response protocol in the clientserver computing model • Identifies and locates network resources by a Uniform Resource Locator (URL) 28 HTTP Encapsulation • Example of how the layers work together: – You start your Web browser with a Web site address – The web browser formats a request for your home page by using the Application layer protocol HTTP – The request looks something like: 29 HTTP Encapsulation (cont.-) • Example continued: – The Application-layer protocol HTTP passes the request down to the TCP – TCP adds a header to the request – The unit of information the Transport layer works with is called a segment – TCP passes the segment to the Internetwork layer protocol (IP) 30 HTTP Encapsulation (cont.-) • Example continued: – IP places its header on the segment: IP header TCP header Get the Website’s home page – The unit of information is now called a packet – The packet is passed down to the Network access layer, where the NIC operates – A frame header and trailer are added Frame header IP header TCP header Get the Website’s home page Frame Trailer – The frame is delivered to the network medium as bits • on its way to the Web server – The Web server processes it and returns a Web page 31 Locating a Resource Object on the Internet • Every object on the Internet has a unique Uniform Resource Locator (URL) • All URLs consist of four parts: – Service type – Host or domain name – Directory or subdirectory information – Filename http://www.qut.edu.au/study/example.htm ftp://opensaurce.com/public/utilities/installer.exe 32 Web Client/Server Communication The Client initiates a contact with the Web server to request for a service. The user types a URL into a Web browser: 1. The browser first contacts to a DNS for resolving the target’s IP address 2. The DNS replies with the corresponding IP address for the web server. 3. The web browser connects to the web server sending an HTTP request for the target website with a TCP 3-way handshake. 4. The web server receives the request and checks for the request message. If the requested page exists, then the web server replies it; else it sends an HTTP 404 error message. DNS server Client Web server 1 2 3 4 5 6 7 8 33 Source: https://web.stanford.edu/class/msande91si/www-spr04/readings/week1/InternetWhitepaper.htm Web Client/Server Communication (cont.) 5. The web browser receives the requested page and then the connection is closed. 6. The browser then parses through the web page information and looks for other page elements it needs to complete the web page. 7. For each element needed, the browser makes additional connections and HTTP requests to the server for each element. 8. When the browser has finished loading all images, info, etc. the page will be completely loaded in the browser window. Source: https://web.stanford.edu/class/msande91si/www-spr04/readings/week1/InternetWhitepaper.htm DNS server Client Web server 1 2 3 4 5 6 7 8 34 HTTP Summary • Roles for HTTP servers and clients • WWW’s major functional components – HTTP, HTML, URL • Web client/server communication process 35 Outline • Domain Name System (DNS) • Hypertext Transfer Protocol (HTTP) • Simple Mail Transfer Protocol (SMTP) • File Transfer Protocol (FTP) • Telnet and Secure Socket Shell (SSH) • Dynamic Host Configuration Protocol (DHCP) 36 Three E-mail protocols • Post Office Protocol version 3 (POP3) • Internet Message Access Protocol version 4 (IMAP4) • Simple Mail Transfer Protocol (SMTP) 37 Post Office Protocol version 3 (POP3) • Email clients use the POP3 to download incoming messages from an e-mail server to their local desktops • POP3 clients download e-mail from the mail server running at the user’s ISP, and these messages are then deleted from the server • POP3 uses TCP port 110 38 Internet Message Access Protocol version 4 (IMAP4) • IMAP4 has advanced message controls: – The capability to manage messages locally yet store them on a server – IMAP4 downloads only e-mail headers initially and then downloads the message body and attachments when the message is selected – IMAP4 uses TCP port 143 39 Simple Mail Transfer Protocol (SMTP) • It is the standard protocol for sending email over the Internet. • POP3 is used to retrieve e-mail and SMTP is used to send it. • SMTP uses TCP port 25 40