1. Web fundamentals: Clients, Servers, and Communication

2. SWE 444:: based on Jeffrey C. Jackson’s slides
The Internet
Internet: the network of networks connected via the public backbone and communicating using TCP/IP communication protocol
SWE 444:: based on Jeffrey C. Jackson’s slides

3. SWE 444:: based on Jeffrey C. Jackson’s slides
Internet Protocols
Communication protocol: how computers talk
how you answer and end call, what language you speak, etc.
Internet protocols developed as part of ARPANET research
ARPANET began using TCP/IP in 1982
Designed for use both within local area networks (LAN’s) and between networks
SWE 444:: based on Jeffrey C. Jackson’s slides

4. Internet Protocol (IP)
IP is the fundamental protocol defining the Internet (as the name implies!)
IP address:
32-bit number (in IPv4)
connected with at most one device at a time (although device may have more than one)
Written as four dot-separated bytes, e.g
SWE 444:: based on Jeffrey C. Jackson’s slides

5. SWE 444:: based on Jeffrey C. Jackson’s slidesIP
IP function: transfer data from source device to destination device
IP source software creates a packet representing the data
Header: source and destination IP addresses, length of data, etc.
Data itself
If destination is on another LAN, packet is sent to a gateway that connects to more than one network
SWE 444:: based on Jeffrey C. Jackson’s slides

6. SWE 444:: based on Jeffrey C. Jackson’s slidesIP
Source
Network 1
Gateway
Destination
Gateway
Network 3
Network 2
SWE 444:: based on Jeffrey C. Jackson’s slides

7. SWE 444:: based on Jeffrey C. Jackson’s slidesIP
Source
LAN 1
Gateway
Destination
Gateway
LAN 2
Internet Backbone
SWE 444:: based on Jeffrey C. Jackson’s slides

8. Transmission Control Protocol (TCP)
Limitations of IP:
No guarantee of packet delivery (packets can be dropped)
Communication is one-way (source to destination)
TCP adds concept of a connection on top of IP
Provides guarantee that packets delivered
Provide two-way (full duplex) communication
SWE 444:: based on Jeffrey C. Jackson’s slides

9. SWE 444:: based on Jeffrey C. Jackson’s slides
TCP {
Source
Destination
Can I talk to you?
Establish
connection.
OK. Can I talk to you?
OK. {
Here’s a packet.
Send packet
with
acknowledgment.
Got it. {
Here’s a packet.
Resend packet if
no (or delayed)
acknowledgment.
Here’s a resent packet.
Got it.
SWE 444:: based on Jeffrey C. Jackson’s slides

10. Domain Name Service (DNS)
DNS is the “phone book” for the Internet
Map between host names and IP addresses
Host names
Labels separated by dots, e.g.,
Final label is top-level domain
Generic: .com, .org, etc.
Country-code: .us, .il,.sa, etc.
SWE 444:: based on Jeffrey C. Jackson’s slides

11. SWE 444:: based on Jeffrey C. Jackson’s slides
World Wide Web
Originally, one of several systems for organizing Internet-based information
Characteristic of Web: support for hypertext (text containing links)
Communication via Hypertext Transport Protocol (HTTP)
Document representation using Hypertext Markup Language (HTML)
SWE 444:: based on Jeffrey C. Jackson’s slides

12. SWE 444:: based on Jeffrey C. Jackson’s slides
World Wide Web
The Web is the collection of machines (Web servers) on the Internet that provide information, above all HTML documents, via HTTP.
Machines that access information on the Web are known as Web clients.
A Web browser is software used by an end user to access the Web.
SWE 444:: based on Jeffrey C. Jackson’s slides

13. Hypertext Transport Protocol (HTTP)
HTTP is based on the request-response communication model:
Client sends a request
Server sends a response
HTTP is a stateless protocol:
The protocol does not require the server to remember anything about the client between requests.
SWE 444:: based on Jeffrey C. Jackson’s slides

14. SWE 444:: based on Jeffrey C. Jackson’s slides
HTTP
Normally implemented over a TCP connection
standard browser-server interaction:
User enters Web address in browser
Browser uses DNS to locate IP address
Browser opens TCP connection to server
Browser sends HTTP request over connection
Server sends HTTP response to browser over connection
Browser displays body of response in the client area of the browser window
SWE 444:: based on Jeffrey C. Jackson’s slides

15. SWE 444:: based on Jeffrey C. Jackson’s slides
HTTP
The information transmitted using HTTP is often totally text
Can use the Internet’s Telnet protocol to simulate browser request and view server response
SWE 444:: based on Jeffrey C. Jackson’s slides

16. SWE 444:: based on Jeffrey C. Jackson’s slides
Client Caching
A cache is a local copy of information gets from some other source
SWE 444:: based on Jeffrey C. Jackson’s slides

17. SWE 444:: based on Jeffrey C. Jackson’s slides
Client Caching
Client
Server
Browser
Web
Server
1. HTTP request for image
2. HTTP response containing image
3. Store image
Cache
SWE 444:: based on Jeffrey C. Jackson’s slides

18. SWE 444:: based on Jeffrey C. Jackson’s slides
Client Caching
Client
Server
Browser
Web
Server
This…
HTTP request for image
HTTP response containing image
I need that
image
again…
Cache
SWE 444:: based on Jeffrey C. Jackson’s slides

19. SWE 444:: based on Jeffrey C. Jackson’s slides
Client Caching
Client
Server
Browser
Web
Server
I need that
image
again…
Get image
… or this
Cache
SWE 444:: based on Jeffrey C. Jackson’s slides

20. SWE 444:: based on Jeffrey C. Jackson’s slides
Client Caching
Cache advantages
(Much) faster than HTTP request/response
Less network traffic
Less load on server
Cache disadvantage
Cached copy of resource may be invalid (inconsistent with remote version)
SWE 444:: based on Jeffrey C. Jackson’s slides

21. SWE 444:: based on Jeffrey C. Jackson’s slides
Web Clients
Many possible web clients:
Text-only “browser”
Mobile phones
Robots (software-only clients)
etc.
We will focus on traditional web browsers
SWE 444:: based on Jeffrey C. Jackson’s slides

22. SWE 444:: based on Jeffrey C. Jackson’s slides
Web Browsers
Primary tasks:
Convert web addresses (URL’s) to HTTP requests
Communicate with web servers via HTTP
make (properly display) documents returned by a server
SWE 444:: based on Jeffrey C. Jackson’s slides

23. SWE 444:: based on Jeffrey C. Jackson’s slides
Web Browsers
Standard features
Save web page to disk
Fill forms automatically (passwords, CC numbers, …)
Set preferences (language, character set, cache and HTTP parameters)
Modify display style (e.g., increase font sizes)
Choose browser themes (skins)
View history of web addresses visited
Bookmark favorite pages for easy return
SWE 444:: based on Jeffrey C. Jackson’s slides

24. SWE 444:: based on Jeffrey C. Jackson’s slides
Web Browsers
Additional functionality:
Execution of scripts (e.g., drop-down menus)
Event handling (e.g., mouse clicks)
GUI for controls (e.g., buttons)
Secure communication with servers
Display of non-HTML documents (e.g., PDF) via plug-ins
SWE 444:: based on Jeffrey C. Jackson’s slides

25. URL Uniform Resource Locator
Represents the address of a resource on the Internet. 25
Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides

26. Web Standards and the W3C Consortium
W3C – World Wide Web Consortium
takes a proactive role in developing recommendations and prototype technologies related to the Web
produces specifications, called Recommendations, in an effort to standardize web technologies
WAI – Web Accessibility Initiative

27. Web Accessibility WAI – Web Accessibility Initiative
Develops recommendations for
web content developers,
web authoring tool developers,
developers of web browsers, and
developers of other user agents to facilitate use of the web by those with special needs.
Section 508 of the Rehabilitation Act
requires that government agencies must give individuals with disabilities access to information technology that is comparable to the access available to others

28. Markup Languages SGML HTML Standard Generalized Markup Language
A standard for specifying a markup language or tag set
HTML
Hypertext Markup Language
The set of markup symbols or codes placed in a file intended for display on a web browser.

29. Markup Languages (2) XML XHTML eXtensible Markup Langauge
A text-based language designed to describe, deliver, and exchange structured information.
It is not intended to replace HTML
extends the power of HTML by separating data from presentation.
XHTML
eXtensible Hypertext Markup Language
Developed by the W3C as the reformulation of HTML 4.0 as an application of XML.
It combines the formatting strengths of HTML 4.0 and the data structure and extensibility strengths of XML.

30. Markup Languages (3) The relationship between XHTML, HTML, and XML
XML Syntax