Lecture 4 Basic Web Concepts

1 herbert van de sompel

CS 502 Computing Methods for Digital

Libraries

Cornell University – Computer ScienceHerbert Van de [email protected]

Lecture 4 Basic Web Concepts


HypertexT Transfer Protocol (HTTP)

web serverHTTP server

web browserHTTP client

renders response

HTTP request

HTTP response

IP address 1 IP address 2

TCP/IP network


Transmission Control Protocol/Internet Protocol (TCP/IP )

• is the protocol suite that drives the Internet

• handles network communications between network nodes (computers, printers, webcams, … connected to the Internet)

• protocol suite:

• TCP: communication of data between applications

• IP: communication of data between nodes

• UDP: communication between applications

• ICMP: error and stats


TCP/IP protocol architecture

Application layerClient sends HTTP request

Server receivesHTTP request

Transport layer TCP

Internet layer IP

Network Accesslayer

Ethernet, …


Transmission Control Protocol (TCP)

• breaks message up into chunks

• chunks get sequence number and IP address of addressee

• opens connection with addressee (handshake)

• hands chunks over to IP layer

• guarantees error-free delivery of chunks at addressee (through connection)


Internet Protocol (IP)

• handles the routing of chunks towards addressee (through routers)• IP Addressing:

• each node has an IP address: 157.193.101.6• each node can have readable name erlserv.rug.ac.be • DNS connects IP and readable name

• IP Data Transmission:• sender delivers chunk to router (via lower level protocol)• router delivers chunk to router or host• individual chunks can be delivered via different paths• routers decide on the path of least resistance• at addressee delivers chunk to TCP layer


Application layer

Transport layer

Internet layer

Network Accesslayer

IP, ICMP

TCP, UDP

HTTP, FTP, telnet

Ethernet, …

TCP/IP protocol architecture


HTTP request


HTTP request

no.good.com

GET / HTTP/1.1

Date: Wednesday, 02-Feb-99 23:04:12 GMTAccept-Language: en-usUser-Agent: Mozilla/4.0 (compatible; MSIE 5.01; Windows NT) Host: no.good.comConnection: Keep-Alive* a blank line *

method

header

entity-body



HTTP request

method URI HTTP-version GET - POST - HEAD – PUT - … GET / HTTP/1.1

header

entity-body

method

• general-header: optional, general informationDate: Wednesday, 02-Feb-99 23:04:12 GMTConnection: Keep-Alive

• request-header: about clientAccept-Language: en-usUser-Agent: Mozilla/4.0 (compatible; MSIE 5.01; Windows NT)

• entity-header: about entity-body

What is sent to the server


HTTP response

HTTP response

status

header

HTTP/1.1 200 OKDate: Wednesday, 02-Feb-99 23:04:25 GMTServer: Apache/1.3.6 (Unix)Last-Modified: Sun, 01 Feb 1999 13:54:26 GMTETag: “2f5cd-964-38js8”Content-length: 327Connection: closeContent-Type: text/html* a blank line * <title>Welcome to nogood</title><img src=“/images/nogood-logo.gif”>


no.good.com


entity-body


HTTP response

HTTP-version Status-code Reason-phrase HTTP/1.1 200 OK

header

entity-body

status

• general-header: optional, general informationDate: Wednesday, 02-Feb-99 23:04:25 GMT

• response-header: about serverServer: Apache/1.3.6 (Unix)

• entity-header: about entity-bodyContent-Type: text/htmlETag: “2f5cd-964-38js8”Content-length: 327What is sent to the clienttitle>Welcome to nogood</title><img src=“/images/nogood-logo.gif”>


HTTP request


HTTP request

no.good.com

GET /images/nogood-logo.gif HTTP/1.1




HTTP response

HTTP response

HTTP/1.1 200 OKDate: Wednesday, 02-Feb-99 23:04:29 GMTServer: Apache/1.3.6 (Unix)Last-Modified: Sun, 01 Feb 1999 08:20:00 GMTETag: “2f5cd-964-445e”Content-length: 220Connection: closeContent-Type: image/gif* a blank line * the GIF file


no.good.com



HypertexT Transfer Protocol (HTTP)



renders response

HTTP request

HTTP response

MIME type + file


Browser

file

Presentation software

Display

MIME type

• built into browser• plug-in• helper application


HTTP Proxies


no.good.com

web serverHTTP serverHTTP proxy

• Reduce network traffic: caching (Etag, Last-Modified)• IP-based authentication

server

client

cache


HTTP cookies

• HTTP protocol is stateless: once a server has given a response to a client, it forgets about it. No session information.• Fake state with cookies:

• server sends token to client• client sends token back to server• server understands the meaning of the token• for instance: server avoids to require input of username/password with every request by reading authorization from cookie


Dynamic content: Common Gateway Interface (CGI)


HTTP request

no.good.com


program

HTTP response

CGI

• Client interaction with non-web servers


CGI -- HTTP POST request


HTTP request

no.good.com

POST /cgi-bin/find HTTP/1.1

Date: Wednesday, 02-Feb-99 23:04:27 GMTAccept-Language: en-usUser-Agent: Mozilla/4.0 (compatible; MSIE 5.01; Windows NT) Host: no.good.comConnection: Keep-AliveContent-length: 26Content-type: application/x-www-form-urlencoded* a blank line * search=herbert&type=author


programfind

CGI


CGI -- HTTP GET request


HTTP request

no.good.com

GET /cgi-bin/find?search=herbert&type=author HTTP/1.1



programfind

CGI


CGI - the interface

programfind

CGI

no.good.com


find receives input from• STDIN• environment variables (about client, server, request …

search=herbert&type=authorSERVER-NAME server.good.comREMOTE-HOST 157.193.101.6…


CGI - the interface

programfind

CGI

no.good.com


find outputs to STDOUTContent-type: text/html

<title>Search results</title>…

web server adds header informationsends response to client


Dynamic content: Mobile code - JavaScript


no.good.com


HTTP response

• Executed by the browser• User interface, client-side validation, …

HTML

JavaScript


Dynamic content: Mobile code – Java applets


no.good.com


HTTP response

• Executed by virtual machine• Interaction with find not via HTTP

Java

programfind


Want to read a bit more?

• on Web Characterization http://www.w3.org/1999/05/WCA-terms/01

• on CGI http://www.ukans.edu/~acs/docs/other/forms-intro.shtml

• on Web, TCP/IP, CGI http://www.wdvl.com/Authoring/Tools/Tutorial/index4.html

• HTTP http://www.ietf.org/rfc/rfc1945.txt?number=1945 ; http://www.jmarshall.com/easy/http/

Lecture 4 Basic Web Concepts

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