The Internet and the WWW
NET 3010
The Internet and the WWW
Internet
HTTP
Web Servers
Cookies
01/09/13
T01_NET3010_WWW
2
The Internet - History
ARPAnet
Advanced Research Projects Agency
1960's
Network of mainly universities
56 KB communication lines
Key in sharing information
Electronic-mail
Package switching technique
Designed to be operated without centralize control
Transmission Control Protocol
Different platforms ( hardware & software )
01/09/13
IP
T01_NET3010_WWW
3
Growing of the Internet
01/09/13
T01_NET3010_WWW
4
World Wide Web
WWW
Allows to locate and view multimedia-based documents
1990 – Tim Berners-Lee
Proposal for the World Wide Web – Nov. 1990
http://www.w3.org/Proposal
Hypertext – Nielsen, 1965
Xanadu-> word processor capable of sorting multiple version and
displaying the differences between the versions.
World Wide Web Consortium ( W3C )
Funded by Tim Berners-Lee
Develop non-proprietary and inter-operable technologies for WWW
Standards recommendations
www.w3c.org
01/09/13
T01_NET3010_WWW
5
Basic Ideas
Hypertext
Allows to move from one part of a document to another or
from one document to another.
Resource Identifiers
Unique id to locate a resource on the network
Client – Server
Model of computer in which a entity ( the client ) makes a
request to another entity ( the server ) that provides the
client with the resources / services
Markup language
Characters / code embedded in text which indicates
structure or advice in presentation
01/09/13
T01_NET3010_WWW
6
HyperText
01/09/13
T01_NET3010_WWW
7
URI – URN - URL
URI
Uniform Resource Name
Uniform Resource Locator
URN names a resource, while URL gives its address
URN vs URL analogous to DNS server and IP
address
Uniform Resource Locator Uniform Resource Name
URL
URN
Universal Resource Identifier ( URI )
01/09/13
T01_NET3010_WWW
8
URL
http://www.w3c.org/Addressing
Universal
General format
service://<user>:<passwd>@<host>:<port>/<dir_01>/<
page.html>
Several standards
Http, ftp, mailto, news, telnet, gopher, file ...
01/09/13
T01_NET3010_WWW
9
Web Architecture
01/09/13
T01_NET3010_WWW
10
Web Architecture ( 2 )
Universal readership principle
Browser clients
Present data
Do not know where the data is
Client server
Know how to extract data
Ignorant of how it will represented to the user
01/09/13
T01_NET3010_WWW
11
Client / Server Model 1
1
3
Browser request for a particular
HTML File
The browser displays the
file
The server locates and
sends it to the browser
01/09/13
T01_NET3010_WWW
2
12
Client / Server Model 2
1
Browser request for a particular HTML File
5 Browser displays the file
Server locates the CGI program and
passes request information
2
CGI program process the request
3
and send data to the server
Server sends data to the browser
01/09/13
T01_NET3010_WWW
4
13
Client / Server Model 3
1
Browser request for a particular HTML File
4 Browser displays the file
Server checks the file and executes
the embedded scripts
The final formatted document is
send to the browser
01/09/13
T01_NET3010_WWW
2
3
14
Client vs Server side
Server-side
CGI scripts
ASP, PHP, Server-side Javascript
Java Servlets & Java Server Pages
Client-side
Plug-ins & ActiveX controls
Java based
JavaScript, VBscript
01/09/13
T01_NET3010_WWW
15
Server Side Programming
Transactions
Multiple servers
Lack of standards
Servlet / applet pair
01/09/13
T01_NET3010_WWW
16
Client Side Programming
Client does the job
Server contacted only for data
All data send to the client
JavaScript, VB, Ajax
Plugins
Platform dependent
Java Applets
Platform independent
download
01/09/13
T01_NET3010_WWW
17
HTTP
01/09/13
T01_NET3010_WWW
18
HyperText Transfer Protocol
Provides a set of instructions for accurate
communication
Used for document delivery over the web
Client / server architecture
Initial design goals
Light protocol: use of not many resources
Fast protocol: Need to retrieve many widely distributed
protocols
Stateless: Servers don't retain info about past
requests
01/09/13
T01_NET3010_WWW
19
HTTP 1.0 – C / S interaction
Interaction between client / server has 4 stages:
Client connects to server
Client sends request to server
Server sends response to client
Server closes connection
01/09/13
T01_NET3010_WWW
20
HTTP 1.0 – Client Requests
Each client request has the format
method request – URI HTTP-version ( request-line )
header ( 0 or more lines )
< blank line > ( CRLF )
message-body ( if POST method )
Request methods:
GET: Request a representation of the request-URI
HEAD: Return only header information of request-URI
POST: Submit information to be processed by request-URI
Example:
GET /index.html HTTP/1.0
01/09/13
T01_NET3010_WWW
21
HTTP 1.0 – Server Response
Each server response has the format:
HTTP–version status-code reason-phrase (statusline)
headers ( 0 or more lines )
< blank line > ( CRLF )
message-body
01/09/13
T01_NET3010_WWW
22
HTTP/1.1 Status Codes
Server status codes are 3 digit numbers
First code gives code category
Code Range
Meaning
1XX
Informational
2XX
Successful
3XX
Redirection
4XX
Client error
5XX
Server error
01/09/13
T01_NET3010_WWW
23
Codes Meaning
Informational ( 1XX )
100 continue
101 switching protocols
Successful ( 2XX )
200
201
202
203
204
205
206
207
01/09/13
ok
created
accepted
non-authoritative information
no content
reset content
partial content
partial update ok
T01_NET3010_WWW
24
Codes Meaning ( 2 )
Redirection ( 3XX )
Client must take further action to complete the request
301 moved permanently
302 moved temporatily
303 see other
304 not modified
305 use proxy
307 temporary redirect
Client error ( 4XX )
The request cannot be processed because the client has
made an error
400 bad request
401 unauthorized
402 payment required
01/09/13
T01_NET3010_WWW
25
Codes Meaning ( 3 )
Client error ( 4XX )
403
404
405
406
407
408
409
410
411
412
413
414
415
01/09/13
forbidden
not found
method is not allowed
not acceptable
proxy authentication required
request time-out
conflict
gone
length required
precondition failed
request entity too large
request-uri too large
unsupported media type
T01_NET3010_WWW
26
Codes Meaning ( 4 )
Client error ( 4XX )
416
417
418
419
requested range not satisfiable
expectation failed
reauthentication required
proxy reauthentication required
Server error ( 5XX )
The server is incapable of performing the request
500 internal server error
501 not implemented
502 bad gateway
503 server unavailable
504 gateway time-out
505 http version not supported
506 partial uptade not implemented
01/09/13
T01_NET3010_WWW
27
HTTP/1.1
More methods and headers
Negotiation to select representation of resources
Persistent connections
More secure authentication
More sophisticated caching model
01/09/13
T01_NET3010_WWW
28
HTTP/1.1 C/S model
Client
Program which makes the connection to send requests
Server
Program accepting connections to service requests
Origin server – server where resources is or need to be
created
Proxy
Intermediary program acting both as client and server
Requests to proxy can be serviced internally ( cache )
Client's request must be explicitly address to proxy
Gateway
Program acting as intermediary for a server ( firewall )
Client does not know it is not communicating with the
origin server
01/09/13
T01_NET3010_WWW
29
HTTP /1.1 Improved method
GET
Can send input to the server program
Conditional GET for certain header
Partial GET
HEAD
Last-Modified date useful for cache
Content-Length useful for page layout, arrival time, etc
Content-Type useful to see if client can handle
01/09/13
T01_NET3010_WWW
30
HTTP/1.1 New Methods
POST
Submits data in message-body to be processed by the
request-URI
PUT
Requests that message-body to be stored at request-URI
DELETE
Request removal of data at request-URI
Others
TRACE: Echoes back
CONNECT: Use with proxy that can be change to be a SSL
tunnel
OPTIONS: Returns HTTP methods that the server supports
01/09/13
T01_NET3010_WWW
31
HTTP/1.1 Headers
General
Date:
date/time when message originated
Via:
used by gateways/proxies to indicate path
Entity
Content-Base: base URI to resolve relative URIs
Content-Length: size of message body in bytes
Request
Host: Allow to identify multiple host with same IP
Response
Server: name and version of server
01/09/13
T01_NET3010_WWW
32
HTTP/1.1 Persistent
Connections
HTTP/1.0 used separate connections for each
request
HTTP/1.1 send multiple request and response
interactions over single connection
01/09/13
T01_NET3010_WWW
33
Advantages
Save CPU time and memory
Requests / responses can be pipelined
Less packages send => less network load
No need to close connection if errors
01/09/13
T01_NET3010_WWW
34
Pipelining
Allows clients to make multiple requests without
waiting for each response
Server process request concurrently
Server must send response in same order as
requests
01/09/13
T01_NET3010_WWW
35
HTTP/1.1 Caching
Cache
Storage for temporary responses
Purpose to reduce:
response time
network time
cost of Internet access
Can be located at
Client
Server
Intermediary ( proxy )
01/09/13
T01_NET3010_WWW
36
Proxy caching algorithm
1. client configured to use a proxy p as a cache
2. When client requests URI http:/s/doc.html
request is sent to p ( not s )
3. if p has doc.html and it has not expired p returns
to the client
4. else if doc.html has expired, validate copy if
original s . return either:
1. Code 304 -> p's copy is up to date or
2. New version of doc.html
5. Else p doesn't have doc.html; issues GET to s
01/09/13
T01_NET3010_WWW
37
Proxy caching
Cache hit
Items 3 and 4.1
Cache miss
Items 4.2 and 5
Cache consistency
Expiry model ( 3 & 4 )
Validation model ( 4.1 )
01/09/13
T01_NET3010_WWW
38
Expiry Model
Responses have a lifetime
If fresh => before lifetime has expired
If stale => after lifetime has expired
Expiry set by origin server
Expires header or max-age directive of Cache-Control
header
Can force revalidation of stale response by mustrevalidate directive of Cache-Control
Can disallow caching using no-cache directive
Heuristic expiry
01/09/13
T01_NET3010_WWW
39
Validation Model
Stale may still valid
No need to resend resource
Cache checks to see if resource is usable
If usable server sends 304 else full resource
Validator types
Last-Modified
Etag
01/09/13
T01_NET3010_WWW
40
HTTP Authentication
Basic authentication
Secure HTTP
HTTPS
Extension of HTTP
Use of Secure Socket Layers ( SSL )
Supported by all browser today
01/09/13
T01_NET3010_WWW
41
Cookies
Piece of information generated by a Web server
and stored in the user's computer, ready for future
access.
Embedded in the HTML information flowing back
and forth between client and server
Allow user-side customization of Web information
Two stage process
Cookie is stored in the user's computer, it may happened
without user consent
When the user access the server, the browser sends the
cookie and it information to the server
01/09/13
T01_NET3010_WWW
42