Full HTML for

Basic foilset Introduction to Architecture of World Wide Web

Given by Nancy J. McCracken at CPS406/606 on Fall Semester 98. Foils prepared October 2 98
Outside Index Summary of Material

The World Wide Web (WWW) (the Web) is a hyperlinked collection of documents and programs that reside on computers all over the world, linked by the Internet.
This talk will show the underlying components and mechanisms that make the Web work.
  • Network protocols based on TCP/IP and a common Domain Name Service
  • Message-passing protocols based on MIME
  • Web Server architecture based on the HTTP protocol
This works on a world-wide basis is because these protocols are based on Open Standards which have been implemented by many vendors on a variety of machines. The Web software structure is strictly non- proprietary, while allowing proprietary pieces to fit in where needed.
The same architecture and software that makes the Web work is also suitable for implementing distributed applications between hetereogeneous machines and networks. This makes the architecture attractive for the corporate Intranet as well.

Table of Contents for full HTML of Introduction to Architecture of World Wide Web

 Denote Foils where Image Critical
Denote Foils where Image has important information
Denote Foils where HTML is sufficient
1 The Architecture of the World Wide Web
2 The Architecture of the World Wide Web
3 Top-level View of the World Wide Web
4 Top-level View of the Corporate Intranet
5 Networking Basics
6 Background on the Internet
7 Networking Basic Definitions
8 Networking Standards: OSI Layers
9 Simplified communication protocol model
10 The TCP/IP protocol suite
11 Typical message formats
12 Networking
13 Communications Issues
14 Networking Speeds
15 Internet 2
16 Open Standards
17 Internet Documents: Drafts, Memos and Standards
18 Internet Documents - Examples
19 Message-passing Protocols
20 Internet E-Mail (RFC-822)
21 Multi-purpose Internet Mail Extension (MIME)
22 MIME - "Content-Type" Header Field
23 MIME - Base Content Types
24 MIME - Base Content Types, continued
25 Web Services - HTTP Protocol
26 Applications based on information services typically use a Client/Server Architecture
27 The World Wide Web is a collection of clients and servers called browsers and Web sites
28 HTTP - Hypertext Transport Protocol
29 HTTPD - HTTP Daemon
30 URL - Uniform Resource Locator
31 Web Links can go to other Internet Services
32 HTTP - How does it work?
33 HTTP - GET Request Example
34 HTTP - Reply Example
35 HTTP - POST Request Example
36 Common Gateway Interface (CGI) - an introduction
37 Three-Tier Web Architecture

Outside Index Summary of Material

HTML version of Basic Foils prepared October 2 98

Foil 1 The Architecture of the World Wide Web

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
Nancy McCracken
NPAC
College of Engineering and Computer Science
Syracuse University
111 College Place
Syracuse NY 13244-4100
September 2, 1998
HTML version of Basic Foils prepared October 2 98

Foil 2 The Architecture of the World Wide Web

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
The World Wide Web (WWW) (the Web) is a hyperlinked collection of documents and programs that reside on computers all over the world, linked by the Internet.
This talk will show the underlying components and mechanisms that make the Web work.
  • Network protocols based on TCP/IP and a common Domain Name Service
  • Message-passing protocols based on MIME
  • Web Server architecture based on the HTTP protocol
This works on a world-wide basis is because these protocols are based on Open Standards which have been implemented by many vendors on a variety of machines. The Web software structure is strictly non- proprietary, while allowing proprietary pieces to fit in where needed.
The same architecture and software that makes the Web work is also suitable for implementing distributed applications between hetereogeneous machines and networks. This makes the architecture attractive for the corporate Intranet as well.
HTML version of Basic Foils prepared October 2 98

Foil 3 Top-level View of the World Wide Web

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
All over the world, users can use browsers to access information stored in multimedia document collections of web server machines. Programs are also accessible through the Common Gateway Interface (CGI).
Server returns
document
Browsers send URL
Internet
HTML version of Basic Foils prepared October 2 98

Foil 4 Top-level View of the Corporate Intranet

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
All over the company, employees (and possibly affiliates and the public) can use browsers to access databases and use distributed applications stored on server machines, using web technology to interface to existing databases and applications.
Databases and
Applications
Employees access
data and programs
Corporate
Network
Firewall
Internet
HTML version of Basic Foils prepared October 2 98

Foil 5 Networking Basics

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
The first section of this talk covers basic networking terminology, the OSI networking layers, the TCP/IP protocol, and routing.
HTML version of Basic Foils prepared October 2 98

Foil 6 Background on the Internet

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
The Internet is a loose federation of networks.
Cooperative organization - no administration, no fees. Protocols and standards are evolved through the IETF, Internet Engineering Task Force.
Most national and international networks are members: NSFNET, ESNET, ARPANET, BITNET
All these networks are packet switched systems based on TCP/IP. Together these protocols allow for communication over a wide variety of technologies. Machines called gateways connect the networks.
Standard domain name system - names are looked up by name server to obtain routing information.
  • symbolic names: npac.syr.edu
  • internet addresses: 128.230.7.2
HTML version of Basic Foils prepared October 2 98

Foil 7 Networking Basic Definitions

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
A computer network is a communication system for connecting end-systems usually called hosts.
A local area network, LAN, connects computer systems within a few kilometers, usually within a single building. A common technology is Ethernet, which operates at 10Mbps (million bits per second). Computers or workstations connect to the LAN via an interface card.
A wide area network, WAN, connects computers in different cities or countries. A common technology is leased telephone lines operating between 9600 bps and 1.544 Mbps.
Computers in a network use a set of protocols to communicate.
HTML version of Basic Foils prepared October 2 98

Foil 8 Networking Standards: OSI Layers

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
Network communication protocols are usually described via a set of layering conventions from the International Standards Organization (ISO) known as the Open Systems Interconnection (OSI) Model.
Presentation
Session
Application
Network
Data Link
Transport
Physical
Message to Send
Presentation
Session
Application
Network
Data Link
Transport
Physical
Message Received
Web Request
Message
Packets
Cells
Message actually sent in Bits
over physical medium
HTML version of Basic Foils prepared October 2 98

Foil 9 Simplified communication protocol model

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
We simplify the model to the four lowest software layers - user applications use the process layer and the remaining three are usually included in the operating system, such as Unix, which has an OSI stack to process messages through the layers.
Process
Network
Data Link
Transport
User applications
Host-to-host layer: may establish virtual circuit for messages
Determines route to transmit packets
Transforms raw messages into one that appears free of transmission errors, transmits data cells and processes acknowledgement cells.
HTML version of Basic Foils prepared October 2 98

Foil 10 The TCP/IP protocol suite

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
TCP - Transmission Control Protocol. A connection-oriented protocol used by most Internet applications to provide a reliable, full-duplex, byte stream for a user process.
UDP - User Datagram Protocol. A connectionless protocol for user processes. Also not reliable.
ICMP - Internet Control Message Protocol. Handles error and control information between gateways and hosts.
IP - Internet Protocol. Provides the packet delivery service for the upper layers.
ARP - Address Resolution Protocol. Maps an Internet address into a hardware address.
RARP - Reverse Address Resolution Protocol.
User process
UDP
Hardware
interface
User process
TCP
IP
ICMP
ARP
RARP
HTML version of Basic Foils prepared October 2 98

Foil 11 Typical message formats

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
Each layer adds control information to the message - this process is called encapsulation.
data
data
Message
TCP message
IP packet
Ethernet frame
TCP
header
data
TCP
header
IP
header
data
TCP
header
IP
header
Ethernet
header
Ethernet
trailer
14 20 8 400 bytes 4
HTML version of Basic Foils prepared October 2 98

Foil 12 Networking

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
The Internet is a packet-switched network. Each message (or document) is broken up into a number of packets. Each packet has an address. A computer called a router sits on the local network and decides where to send it first on its way to its final address. Each computer along the network connection examines messages that come in and either keeps it or reroutes it along its way. The message is reassembled on the other end.
HTML version of Basic Foils prepared October 2 98

Foil 13 Communications Issues

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
Multiplexing - Different protocols can be used to send different messages through the same network.
Sequencing is the property that data is received by the receiver in the same order as transmitted by the sender, which is not true in a packet-switched network.
Error control guarantees that error-free data is received by the application programs. Data can either get corrupted by the transmission medium or get lost. Checksums are added to the data and received data is acknowledged. If there is any problem, retransmission occurs.
Flow control assures that the sender doesnąt overwhelm the receiver by sending data at a faster rate than it can process.
Error and flow control are handled on an end-to-end basis by TCP and on a hop-by-hop basis by IP. (A hop goes to only one intermediate machine on the network route.)
HTML version of Basic Foils prepared October 2 98

Foil 14 Networking Speeds

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
Performance of network delivery depends on the size of the message, the capacity of the various pieces of network that the message may travel along and the congestion of the network.
Modem 14,400bps 1.22sec 2.22min 2.78min 4.40min 22.2min
ISDN 56,000bps 0.31sec 34.3sec 42.9sec 1.13min 5.71min
T1 1.54Mbps .011sec 1.24sec 1.55sec 2.46sec 12.4sec
T3 45.0Mbps .0004sec 0.04sec 0.05sec 0.08sec 0.42sec
Network Speed
Email book picture audio video
(2.2KB) (240KB) (300KB) (475KB) (2.4MB - 1min)
HTML version of Basic Foils prepared October 2 98

Foil 15 Internet 2

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
The current demand for applications involving transfer of multimedia and real time events has resulted in additional protocols for the Internet currently under development as the Integrated Services model
RSVP - Reservation Protocol - a virtual circuit is established that can reserve a certain bandwidth for continuous transmission of packets.
  • Quality of service
  • Multicasting
RTP and RTSP - Real Time Protocol and, more specificately, Real Time Streaming Protocol - for continuous multimedia transmission
These protocols require network hosts which support them to save state regarding the virtual circuits.
HTML version of Basic Foils prepared October 2 98

Foil 16 Open Standards

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
All the network protocols just discussed are agreed on by various standards committees. The principal standards organization of the Internet is the Internet Engineering Task Force (IETF). The principal standards organization of the WWW is the World Wide Web Consortium (W3C).
HTML version of Basic Foils prepared October 2 98

Foil 17 Internet Documents: Drafts, Memos and Standards

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
Some material presented here comes from Internet documents. Here is a summary of various document formats you may find.
Internet Drafts
  • Working documents of the (IETF), its Area and Working Groups.
  • Other groups may also distribute Internet Drafts.
  • Some of these IDs are labelled by IETF-#.
  • IDs are valid for a maximum of 6 months and may be updated, replaced or made obsolete by other documents at any time.
Internet Memos
  • Referred to as RFC-# (Request for Comments)
  • More formal and complete than Internet Drafts, usually represent standard proposals/candidates.
  • Some RFCs become obsolete by subsequent RFCs, some others make it as standards
Internet Standards
  • Labelled by STD-# and often associated with the RFC-# specs (e.g. Internet E-Mail is referred to as FRC-822 or STD-11)
HTML version of Basic Foils prepared October 2 98

Foil 18 Internet Documents - Examples

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
Here are a few sample Internet documents relevant for Internet and WWW message-passing.
RFC-822: Crocker, D., "Standard for the Format of ARPA Internet Text Messages", SRD 11, RFC 822, UDEL, 1982.
RFC-1521: Borenstein, N. and Freed, N., "MIME (Multipurpose Internet Mail Extension) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies", RFC 1521, Bellcore, September 1993.
RFC-1524: Borenstein, N. "A User Agent Configuration Mechanism for Multimedia Mail Format Information", RFC 1524, Bellcore, September 1993.
Internet Draft: Tim Berners-Lee, "Basic HTTP", CERN, 1992/3.
RFP-1890: H. Schulzrinne, RTP Profile for Audio and Video Conferences with Minimal Control, Jan. 1996.
HTML version of Basic Foils prepared October 2 98

Foil 19 Message-passing Protocols

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
HTML version of Basic Foils prepared October 2 98

Foil 20 Internet E-Mail (RFC-822)

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
We all know and use it, but here is a formal specification.
Each message is a stream of 7-bit ASCII chars which contains a header and optional (newline separated) body.
Header consists of a set of entries with one entry per line given by a colon separated key:value pair.
Key contains no spaces or tabs and cannot exceed 63 chars.
Body is a fully unstructured sequence of ASCII chars.
There is a finite set of standard keys and an extension mechanism via the "X"-prefix. The standard set (as used by MH) is:
Date Bcc Resent-Date Resent-Fcc
From Fcc Resent-From resent-
Sender Message-ID Resent-To Message-Id
To Subject Resent-cc Forwarded
cc In-Reply-To Resent-Bcc Replied
HTML version of Basic Foils prepared October 2 98

Foil 21 Multi-purpose Internet Mail Extension (MIME)

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
Goals
  • Multimedia, multi-language, multi-component extension of RFC-822
  • Full backward compatibility with RFC-822
  • Open design to incorporate multiple well-known formats
  • Easy extension to new types and formats
Retain RFC-822 header+body format
Add new header fields
Allow for multipart multimedia bodies
Include media type and encoding information in new header fields such as: Content-Type, Content-Description, Content-Transfer-Encoding, Content-ID
Retain 7-bit ASCII for all valid encoding schemes
Implement multi-component bodies via a special 'magic type' Content-Type: multipart
HTML version of Basic Foils prepared October 2 98

Foil 22 MIME - "Content-Type" Header Field

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
Two level hierarchical typing scheme adopted of the form: basetype/subtype
Seven base media types are defined this minimal set is enforced, i.e. all extensions must pass the whole ID->RFC->STD process.
Allow for less restrictive subtyping the base types, for example:
  • Content-Type: text/plain
  • Content-Type: text/richtext
Some standard subtypes are specified and many more are expected. New subtypes must be registered with the IANA (Internet Assigned Numbers Authority).
Private experimental subtypes prefixed with "X-" may be used freely and without registration.
Seven base types are: text, image, audio, video, multipart, message, application.
HTML version of Basic Foils prepared October 2 98

Foil 23 MIME - Base Content Types

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
text
  • subtypes: plain (just ASCII) and richtext (a simple markup extension including <bold>, <italic> etc. tags)
  • character sets can be further specified in the header value field as follows:
    • Content-type: text/plain; charset=us-ascii
  • Other charsets can be used to support other languages such as iso-8859-1 (French) or iso-2022-JP (Japanese). These charsets need to be encoded in one of two encoding modes: base64 or quoted-printable. The latter retains ASCII subset and is more natural for non-ASCII extensions.
image
  • Standard subtypes: gif, jpeg. Others expected.
audio
  • Standard subtype: single-channel 8KHz u-law. Others expected.
video
  • Standard subtype: mpeg. Others plausible.
HTML version of Basic Foils prepared October 2 98

Foil 24 MIME - Base Content Types, continued

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
multipart
  • Specifies a MIME message composed of several parts with possible different Content-Type fields.
  • Parts are separated by a boundary string, specified in the multipart header entry
  • Subtypes: mixed (serial combination of media), parallel (for parallel presentation if possible), alternative (multiple representations of the same data) and digest (all parts are messages)
message
  • Subtypes: rfc822 (standard ARPA e-mail format), partial (a single chunk of a larger message, chopped into pieces for transmission and then reassembled), external-body (pointer to a remote data - similar to typerlink/URL but different representation)
application
  • Current subtypes: postscript, ODA
  • Placeholder for "anything else" - several interactive/custom/creative extensions expected here
  • Already registered: Andrew-inset,t ATOMICMAIL (Bellcore)
HTML version of Basic Foils prepared October 2 98

Foil 25 Web Services - HTTP Protocol

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
HTML version of Basic Foils prepared October 2 98

Foil 26 Applications based on information services typically use a Client/Server Architecture

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
Server: A program in charge of a resource or information.
  • Operation is defined by list of services.
  • Normal mode is to listen for requests, stopping to fulfill a request when it arrives.
Client: Any program that makes a request for service from the server.
Server with
Resource
Clients
Send request
Return data or acknowledgement
HTML version of Basic Foils prepared October 2 98

Foil 27 The World Wide Web is a collection of clients and servers called browsers and Web sites

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
Web servers provide access to a collection of files containing hyperlinked information
  • primary service is to send text files, images, digitized video
  • can also provide customized services through the form/CGI script interface
Browsers provide an easy graphical interface for users to request information. The client machine also provides viewers for a standard set of image and video formats.
The interface is kept very simple to run on all networks and most machines.
May also use CGI interface to access other services
Browser sends URL
Viewer displays file based on format
Server returns file with text, image or video
(may also send requests for CGI service)
HTML version of Basic Foils prepared October 2 98

Foil 28 HTTP - Hypertext Transport Protocol

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
HTTP provides an upper level to the Internet, that is, it is built on top of a back-bone network with all the packets flowing from client to server and vice versa using the standard TCP/IP protocol.
It uses MIME formats and concepts, but does not fully conform to MIME as the WWW is not a mail system.
HTTP protocol is compatible with other network services such as FTP (File Transfer Protocol), NNTP (Network News Transport Protocol).
  • On a UNIX-based machine, the basic services are enumerated in the file /etc/services. Each service cooresponds to a standard port. For example, telnet is mapped to port 43, and FTP is mapped to port 21. All ports below 1024 are privileged - only the system administrator can determine port use.
The HTTP service is standardly assigned to port 80 - it provides a much shorter service connection than the other services.
HTML version of Basic Foils prepared October 2 98

Foil 29 HTTPD - HTTP Daemon

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
The HTTP daemon is the server which responds to the Internet service requests on standard port 80 (or on another custom port). The server program is available from NCSA and is easily installed by editing a set of configuration files which give directory locations for documents, cgi scripts, error messages and icons, and which allows for options regarding path names, domain access, and so on.
Ports:
21
43
80
FTP
Telnet
HTTP
Network messages specify host name and port
Operating system
HTML version of Basic Foils prepared October 2 98

Foil 30 URL - Uniform Resource Locator

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
A URL has the standard form
  • service://machine:port/file.file-extension
HTML hyperlinks typically use the service http for linking to other documents and media files. Some other internet services can also be used such as
  • ftp://machine/file.file-extension.
In this way, a Web server can provide other Internet services through the browser interface.
The machine is an Internet address and can either be a symbolic name provided by the Domain Name Service (DNS) or the IP numbers.
If the port is not specified, it defaults to 80.
The file.file-extension is given by any Unix path name starting from the directory known to the server as "document root". Which path names are valid is one of the options of the server - whether "public_html" is automatically put into the path name and whether paths starting with "~username" are allowed.
In the http service, the file-extension is used to tell the browser what helper application to use to view the file. Typical file extensions are html, gif, jpeg, mpeg, au, ram, etc.
HTML version of Basic Foils prepared October 2 98

Foil 31 Web Links can go to other Internet Services

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
For other services, the Web server transfers the connection to the appropriate server.
Protocol URL identifier Example
E-mail mailto: mailto:njm@npac.syr.edu
FTP ftp:// ftp://ftp.npac.syr.edu
Telnet telnet:// telnet://gamera.syr.edu
Usenet News (NNTP) news: news:comp.infosystems.www
WWW(HTTP) http:// http://www.npac.syr.edu
HTML version of Basic Foils prepared October 2 98

Foil 32 HTTP - How does it work?

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
On each hyperlink click, the browser (client) initiates a connection with the server at the "machine" (e.g. using UNIX BSD connect call on the default port 80, or a custom user-defined port)
A request is sent to the server, formatted as a MIME-like message.
The server replies with another MIME-like message which is received by the browser and either formatted in the browser window or viewed with a helper application.
The connection is closed on both sides. (The exception to this is the "server push" connection.)
HTML version of Basic Foils prepared October 2 98

Foil 33 HTTP - GET Request Example

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
GET /document.html HTTP/1.0 Accept: www/source Accept: text/html Accept: image/gif User-Agent: Lynx/2.2 libww/2.14 From: mnotulli@ukonaix.cc.ukans.edu -- blank-line-terminating-the-request --
First line syntax is always: METHOD URL ProtocolVersion
The following lines form a header of an (extended) MIME message
"User-Agent" specifies the browser type
"Accept" specifies MIME types recognized by the browser
The server is expected to provide the requested data in one of these acceptable formats.
HTML version of Basic Foils prepared October 2 98

Foil 34 HTTP - Reply Example

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
HTTP/1.0 200 OK Date: Wednesday, 02-Feb-95 23:04:12 GMT Server: NCSA/1.1 MIME-version: 1.0 Last-modified: Monday, 15-Nov-94 23:33:16 GMT Content-type: text/html Content-length: 2345 ---- blank-line-separating-header-and-body--<HTML><HEAD> <TITLE> Document Title </TITLE>. . .
This message contains both header and body
Some replies contain only header (e.g. error reports, such as HTTP/1.0 404 Not Found)
GET request also contained header only, whereas POST request (see next example) contains both header and body
HTML version of Basic Foils prepared October 2 98

Foil 35 HTTP - POST Request Example

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
POST /cgi-bin/post-query HTTP/1.0 Accept: www/sourceAccept: text/html Accept: video/mpeg Accept: image/x-rgb Accept: application/postscript User-Agent: Lynx/2.2 libwww/2.14 From: grobe@unanaix.cc.ukans.edu Content-type: application/x-www-form-urlencoded Content-length: 150 --blank-line-separating-header-and-body---org=Academic%20Computing%20Services &users=10000 &browser=lynx &contact=Michael%20Grobe%20grobe@kuhbuh.cc.ukans.edu
Both header and body present in POST requests - the body is typically used to pass a form contents to the server.
HTML version of Basic Foils prepared October 2 98

Foil 36 Common Gateway Interface (CGI) - an introduction

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
CGI is an interface for running programs on the server at the request of the client.
When the user clicks on a CGI link, the server calls the corresponding process and returns its output, not the data/file/code associated with the process.
Typical Applications
  • Support for dynamic generation of HTML documents, such as on-the-fly conversions from other formats.
  • Interface to and integration with Forms/GUI area of HTML - submitted forms are handled by suitable CGI processes.
  • Interfacing with other (non-HTTP) remote services such as databases, video-on-demand, simulations, etc.
    • This is current area of major development of the web.
Look at a simple example of an HTML form with its CGI Perl program.
HTML version of Basic Foils prepared October 2 98

Foil 37 Three-Tier Web Architecture

From Introduction to Architecture of World Wide Web CPS406/606 -- Fall Semester 98. *
Full HTML Index
For most major web applications, the web server layer is just the "middleware" to support access to applications.
Databases
Parallel Compute Server
Application
Server
Server with CGI interface translates and directs requests for services
Browser
(client)
May require secure connection
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