Web Essentials:
Clients, Servers, and Communication
PV219, spring 2024
Web Essentials
• Client: web browsers, used to surf the Web
• Server systems: used to supply information to these
browsers
• Computer networks: used to support the browserserver
communication
Client Server
Request “document A”
document A
Internet vs. Web
• The Internet: a inter-connected computer
networks, linked by wires, cables, wireless
connections, etc.
• Web: a collection of interconnected
documents and other resources.
• The world wide web (WWW) is accessible via
the Internet, as are many other services
including email, file sharing, etc.
How does the Internet Work?
• Through communication protocols
• A communication protocol is a specification of how
communication between two computers will be carried
out
– IP (Internet Protocol): defines the packets that carry blocks
of data from one node to another
– TCP (Transmission Control Protocol) and UDP (User
Datagram Protocol): the protocols by which one host sends
data to another.
– Other application protocols: DNS (Domain Name Service),
SMTP (Simple Mail Transmission Protocol), and FTP (File
Transmission Protocol)
The Internet Protocol (IP)
• A key element of IP is IP address, a 32/64-bit number
• The Internet authorities assign ranges of numbers to different
organizations
• IP is responsible for moving packet of data from node to node
• A packet contains information such as the data to be
transferred, the source and destination IP addresses, etc.
• Packets are sent through different local network through
gateways
• A checksum is created to ensure the correctness of the data;
corrupted packets are discarded
• IP-based communication is unreliable
Transmission Control Protocol (TCP)
• TCP is a higher-level protocol that extends IP to
provide additional functionality: reliable
communication
• TCP adds support to detect errors or lost data and
to trigger retransmission until the data is
correctly and completely received
• Connection
• Acknowledgment
TCP/IP Protocol Suites
HTTP, FTP, Telnet, DNS, SMTP
TCP, UDP
IP (IPv4, IPv6)
The World Wide Web
• WWW is a system of interlinked, hypertext documents
that runs over the Internet
• Two types of software:
– Client: a system that wishes to access the information
provided by servers must run client software (e.g., web
browser)
– Server: an internet-connected computer that wishes to
provide information to others must run server software
– Client and server applications communicate over the
Internet by following a protocol built on top of TCP/IP –
HyperText Transport Protocol (HTTP)
WWW History
• 1989 - Birth of WWW
– Tim Berners-Lee & his associates at CERN
• 1990 - First Web Browser
– Used within CERN
• 1991 - Public offering of WWW
• 1993 - Birth of Mosaic
– Graphical, multimedia browser from NCSA
• 1994 - First commercial browser
– By Netscape communications founded by
Jim Clark and Marc Andreessen
Basics of the WWW
• Hypertext: a format of information which
allows one to move from one part of a
document to another or from one document
to another through hyperlinks
• Uniform Resource Locator (URL): unique
identifiers used to locate a particular resource
on the network
• Markup language: defines the structure and
content of hypertext documents
Web Client: Browser
Makes HTTP requests on behalf of the user
• Reformat the URL entered as a valid HTTP request
• Use DNS to convert server’s host name to appropriate
IP address
• Establish a TCP connection using the IP address
• Send HTTP request over the connection and wait for
server’s response
• Display the document contained in the response
– If the document is not a plain-text document but instead is
written in HTML, this involves rendering the document
Web Servers
Main functionalities:
• Server waits for connect requests
• When a connection request is received, the server creates a new
process to handle this connection
• The new process establishes the TCP connection and waits for HTTP
requests (stateless!) – HTTP2+ is a solution
• The new process invokes software that maps the requested URL to
a resource on the server
• If the resource is a file, creates an HTTP response that contains the
file in the body of the response message
• If the resource is a program, runs the program, and returns the
output
Static Web: HTML/XHTML, CSS
• HTML stands for HyperText Markup Language
– It is a text file containing small markup tags (elements)
that tell the Web browser how to display the page
• XHTML stands for eXtensible HyperText Markup
Language
– It is identical to HTML 4.01
– It is a stricter and cleaner version of HTML
• CSS stands for Cascading Style Sheets
– It defines how to display HTML elements
Client-Side Programmability
• Scripting language: a lightweight programming
language
• Browser scripting: JavaScript (by Netscape in 1995)
– Designed to add interactivity to HTML pages
– Usually embedded into HTML pages
– What can a JavaScript do?
• Put dynamic text into an HTML page
• React to events
• Read and write HTML elements (generally, DOM manipulation)
• Validate data before it is submitted to a server
• Asynchronously communicate with server
• Create cookies
• …
Server-Side Programmability
• The requests cause the response to be generated
• Server scripting:
– CGI/Perl: Common Gate Way Interface (*.pl, *.cgi)
– PHP: Open source, strong database support (*.php)
– ASP: Microsoft product, uses .Net framework (*.asp)
– Java via JavaServer Pages (*.jsp)
– JavaScript via node.js (*.js)
– …