WebEssentials_technologies Html5 css ppt

Web Essentials: Clients, Servers, and
Communication

2
The Internet
Technical origin: ARPANET (late 1960’s)
 One of earliest attempts to network heterogeneous,
geographically dispersed computers
 Email first available on ARPANET in 1972 (and
quickly very popular!)
ARPANET access was limited to select
DoD-funded organizations

3
The Internet
Open-access networks
 Regional university networks (e.g., SURAnet)
 CSNET for CS departments not on ARPANET
NSFNET (1985-1995)
 Primary purpose: connect supercomputer centers
 Secondary purpose: provide backbone to connect
regional networks

4
The Internet
The 6 supercomputer centers connected by the early NSFNET backbone

5
The Internet
Original NSFNET backbone speed: 56 kbit/s
Upgraded to 1.5 Mbit/s (T1) in 1988
Upgraded to 45 Mbit/s (T3) in 1991
In 1988, networks in Canada and France
connected to NSFNET
In 1990, ARPANET is decommissioned,
NSFNET the center of the internet

6
The Internet
Internet: the network of networks
connected via the public backbone and
communicating using TCP/IP communication
protocol
 Backbone initially supplied by NSFNET,
privately funded (ISP fees) beginning in 1995

7
Internet Protocols
Communication protocol: how computers talk
 Cf. telephone “protocol”: 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

8
Internet Protocol (IP)
IP is the fundamental protocol defining the
Internet (as the name implies!)
IP address:
 32-bit number (in IPv4)
 Associated with at most one device at a time
(although device may have more than one)
 Written as four dot-separated bytes, e.g.
192.0.34.166

9
IP
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

10
IP
Source
Gateway
Gateway
LAN 1
Internet Backbone
Destination
LAN 2

11
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

12
TCP
Source Destination
Can I talk to you?
OK. Can I talk to you?
OK.
Here’s a packet.
Got it.
Here’s a packet.
Here’s a resent packet.
Got it.
Establish
connection.
{
{
{
Send packet
with
acknowledgment.
Resend packet if
no (or delayed)
acknowledgment.

13
TCP
TCP also adds concept of a port
 TCP header contains port number representing an
application program on the destination computer
 Some port numbers have standard meanings
 Example: port 25 is normally used for email transmitted
using the Simple Mail Transfer Protocol (SMTP)
 Other port numbers are available first-come-first
served to any application

Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 14
TCP

15
User Datagram Protocol (UDP)
Like TCP in that:

Builds on IP

Provides port concept
Unlike TCP in that:

No connection concept

No transmission guarantee
Advantage of UDP vs. TCP:

Lightweight, so faster for one-time messages

16
Domain Name Service (DNS)
DNS is the “phone book” for the Internet
 Map between host names and IP addresses
 DNS often uses UDP for communication
Host names
 Labels separated by dots, e.g., www.example.org
 Final label is top-level domain
 Generic: .com, .org, etc.
 Country-code: .us, .il, etc.

17
DNS
Domains are divided into second-level domains,
which can be further divided into subdomains, etc.
 E.g., in www.example.com, example is a
second-level domain
A host name plus domain name information is
called the fully qualified domain name of the
computer
 Above, www is the host name, www.example.com
is the FQDN

18
DNS
nslookup program provides command-line
access to DNS (on most systems)
looking up a host name given an IP address is
known as a reverse lookup

Recall that single host may have multiple IP
addresses.

Address returned is the canonical IP address
specified in the DNS system.

19
DNS
ipconfig (on windows) can be used to
find the IP address (addresses) of your
machine
ipconfig /displaydns displays the
contents of the DNS Resolver Cache
(ipconfig /flushdns to flush it)

20
Analogy to Telephone Network
IP ~ the telephone network
TCP ~ calling someone who answers,
having a conversation, and hanging up
UDP ~ calling someone and leaving a
message
DNS ~ directory assistance

21
Higher-level Protocols
Many protocols build on TCP
 Telephone analogy: TCP specifies how we initiate
and terminate the phone call, but some other protocol
specifies how we carry on the actual conversation
Some examples:
 SMTP (email) (25)
 FTP (file transfer) (21)
 HTTP (transfer of Web documents) (80)

22
World Wide Web
Originally, one of several systems for
organizing Internet-based information
 Competitors: WAIS, Gopher, ARCHIE
Distinctive feature of Web: support for
hypertext (text containing links)
 Communication via Hypertext Transport Protocol
(HTTP)
 Document representation using Hypertext Markup
Language (HTML)

23
World Wide Web
The Web is the collection of machines (Web
servers) on the Internet that provide information,
particularly 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.

24
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.

25
HTTP
Normally implemented over a TCP connection (80 is
standard port number for HTTP)
Typical 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

26
HTTP
The information transmitted using HTTP is
often entirely text
Can use the Internet’s Telnet protocol to
simulate browser request and view server
response

27
HTTP
$ telnet www.example.org 80
Trying 192.0.34.166...
Connected to www.example.com
(192.0.34.166).
Escape character is ’^]’.
GET / HTTP/1.1
Host: www.example.org
HTTP/1.1 200 OK
Date: Thu, 09 Oct 2003 20:30:49 GMT
…
{
Send
Request
{
Receive
Response
Connect {

28
HTTP Request
Structure of the request:
 start line
 header field(s)
 blank line
 optional body

29
HTTP Request
 start line
 header field(s)
 blank line
 optional body

30
HTTP Request
Start line
 Example: GET / HTTP/1.1
Three space-separated parts:
 HTTP request method
 Request-URI (Uniform Resource Identifier)
 HTTP version

31
HTTP Request
Start line

Example: GET / HTTP/1.1

HTTP request method

Request-URI

HTTP version
 We will cover 1.1, in which version part of start line
must be exactly as shown

32
HTTP Request
Start line
 Request-URI
 HTTP version

33
HTTP Request
Uniform Resource Identifier (URI)
 Syntax: scheme : scheme-depend-part
 Ex: In http://www.example.com/
the scheme is http
 Request-URI is the portion of the requested URI
that follows the host name (which is supplied by the
required Host header field)
 Ex: / is Request-URI portion of
http://www.example.com/

34
URI
URI’s are of two types:
 Uniform Resource Name (URN)
 Can be used to identify resources with unique names, such
as books (which have unique ISBN’s)
 Scheme is urn
 Uniform Resource Locator (URL)
 Specifies location at which a resource can be found
 In addition to http, some other URL schemes are
https, ftp, mailto, and file

35
HTTP Request
Start line
 Request-URI
 HTTP version

36
HTTP Request
Common request methods:
 GET
 Used if link is clicked or address typed in browser
 No body in request with GET method
 POST
 Used when submit button is clicked on a form
 Form information contained in body of request
 HEAD
 Requests that only header fields (no body) be returned in the
response

37
HTTP Request
 start line
 header field(s)
 blank line
 optional body

38
HTTP Request
Header field structure:
 field name : field value
Syntax
 Field name is not case sensitive
 Field value may continue on multiple lines by
starting continuation lines with white space
 Field values may contain MIME types, quality
values, and wildcard characters (*’s)

39
Multipurpose Internet Mail
Extensions (MIME)
Convention for specifying content type of a
message
 In HTTP, typically used to specify content type
of the body of the response
MIME content type syntax:
 top-level type / subtype
Examples: text/html, image/jpeg

40
HTTP Quality Values and
Wildcards
Example header field with quality values:
accept:
text/xml,text/html;q=0.9,
text/plain;q=0.8, image/jpeg,
image/gif;q=0.2,*/*;q=0.1
Quality value applies to all preceding items
Higher the value, higher the preference
Note use of wildcards to specify quality 0.1 for
any MIME type not specified earlier

41
HTTP Request
Common header fields:
 Host: host name from URL (required)
 User-Agent: type of browser sending request
 Accept: MIME types of acceptable documents
 Connection: value close tells server to close connection after
single request/response
 Content-Type: MIME type of (POST) body, normally
application/x-www-form-urlencoded
 Content-Length: bytes in body
 Referer: URL of document containing link that supplied URI for
this HTTP request

42
HTTP Response
Structure of the response:
 status line
 header field(s)
 blank line
 optional body

43
HTTP Response
 status line
 header field(s)
 blank line
 optional body

44
HTTP Response
Status line
 Example: HTTP/1.1 200 OK
 HTTP version
 status code
 reason phrase (intended for human use)

45
HTTP Response
Status code
 Three-digit number
 First digit is class of the status code:
 1=Informational
 2=Success
 3=Redirection (alternate URL is supplied)
 4=Client Error
 5=Server Error
 Other two digits provide additional information

See http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html

46
HTTP Response
 status line
 header field(s)
 blank line
 optional body

47
HTTP Response
Common header fields:
 Connection, Content-Type, Content-Length
 Date: date and time at which response was generated (required)
 Location: alternate URI if status is redirection
 Last-Modified: date and time the requested resource was last
modified on the server
 Expires: date and time after which the client’s copy of the
resource will be out-of-date
 ETag: a unique identifier for this version of the requested
resource (changes if resource changes)

48
Client Caching
A cache is a local copy of information
obtained from some other source
Most web browsers use cache to store
requested resources so that subsequent
requests to the same resource will not
necessarily require an HTTP request/response
 Ex: icon appearing multiple times in a Web page

49
Client Caching
Browser Web
Server
1. HTTP request for image
2. HTTP response containing image
Client Server
Cache
3. Store image

50
Client Caching
Browser Web
Server
Client Server
Cache
I need that
image
again…

51
Client Caching
Browser Web
Server
Client Server
Cache
I need that
image
again…
HTTP request for image
HTTP response containing image
This…

52
Client Caching
Browser Web
Server
Client Server
Cache
I need that
image
again…
Get
image
… or this

53
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)

54
Client Caching
Validating cached resource:
 Send HTTP HEAD request and check Last-
Modified or ETag header in response
 Compare current date/time with Expires header
sent in response containing resource
 If no Expires header was sent, use heuristic
algorithm to estimate value for Expires
 Ex: Expires = 0.01 * (Date – Last-Modified) + Date

55
Character Sets
Every document is represented by a string of integer
values (code points)
The mapping from code points to characters is defined
by a character set
Some header fields have character set values:
 Accept-Charset: request header listing character sets that the
client can recognize
 Ex: accept-charset: ISO-8859-1,utf-8;q=0.7,*;q=0.5
 Content-Type: can include character set used to represent the
body of the HTTP message
 Ex: Content-Type: text/html; charset=UTF-8

56
Character Sets
Technically, many “character sets” are
actually character encodings

An encoding represents code points using variable-
length byte strings

Most common examples are Unicode-based
encodings UTF-8 and UTF-16
IANA maintains complete list of Internet-
recognized character sets/encodings

57
Character Sets
Typical US PC produces ASCII documents
US-ASCII character set can be used for such
documents, but is not recommended
UTF-8 and ISO-8859-1 are supersets of US-ASCII and
provide international compatibility
 UTF-8 can represent all ASCII characters using a single byte
each and arbitrary Unicode characters using up to 4 bytes each
 ISO-8859-1 is 1-byte code that has many characters common in
Western European languages, such as é

58
Web Clients
Many possible web clients:
 Text-only “browser” (lynx)
 Mobile phones
 Robots (software-only clients, e.g., search engine
“crawlers”)
 etc.
We will focus on traditional web browsers

59
Web Browsers
First graphical browser running on general-
purpose platforms: Mosaic (1993)

61
Web Browsers
Primary tasks:
 Convert web addresses (URL’s) to HTTP
requests
 Communicate with web servers via HTTP
 Render (appropriately display) documents
returned by a server

62
HTTP URL’s
Browser uses authority to connect via TCP
Request-URI included in start line (/ used for path
if none supplied)
Fragment identifier not sent to server (used to
scroll browser client area)
http://www.example.org:56789/a/b/c.txt?t=win&s=chess#para5
host (FQDN) port
authority
path query fragment
Request-URI

63
Web Browsers
Standard features

Save web page to disk

Find string in page

Fill forms automatically (passwords, CC numbers, …)

Set preferences (language, character set, cache and HTTP
parameters)

Modify display style (e.g., increase font sizes)

Display raw HTML and HTTP header info (e.g., Last-Modified)

Choose browser themes (skins)

View history of web addresses visited

Bookmark favorite pages for easy return

64
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

65
Web Servers
Basic functionality:
 Receive HTTP request via TCP
 Map Host header to specific virtual host (one of many host
names sharing an IP address)
 Map Request-URI to specific resource associated with the
virtual host
 File: Return file in HTTP response
 Program: Run program and return output in HTTP response
 Map type of resource to appropriate MIME type and use to
set Content-Type header in HTTP response
 Log information about the request and response

66
Web Servers
httpd: UIUC, primary Web server c. 1995
Apache: “A patchy” version of httpd, now the most
popular server (esp. on Linux platforms)
IIS: Microsoft Internet Information Server
Tomcat:
 Java-based
 Provides container (Catalina) for running Java servlets
(HTML-generating programs) as back-end to Apache or IIS
 Can run stand-alone using Coyote HTTP front-end

67
Web Servers
Some Coyote communication parameters:
 Allowed/blocked IP addresses
 Max. simultaneous active TCP connections
 Max. queued TCP connection requests
 “Keep-alive” time for inactive TCP connections
Modify parameters to tune server
performance

68
Web Servers
Some Catalina container parameters:
 Virtual host names and associated ports
 Logging preferences
 Mapping from Request-URI’s to server
resources
 Password protection of resources
 Use of server-side caching

69
Tomcat Web Server
HTML-based server administration
Browse to
http://localhost:8080
and click on Server Administration link
 localhost is a special host name that means
“this machine”

Tomcat Web Server
Some Connector fields:
 Port Number: port “owned” by this connector
 Max Threads: max connections processed
simultaneously
 Connection Timeout: keep-alive time

Tomcat Web Server

Secure Servers
Since HTTP messages typically travel over a
public network, private information (such as credit
card numbers) should be encrypted to prevent
eavesdropping
https URL scheme tells browser to use encryption
Common encryption standards:
 Secure Socket Layer (SSL)
 Transport Layer Security (TLS)

Secure Servers
Browser
Web
Server
I’d like to talk securely to you (over port 443)
Here’s my certificate and encryption data
Here’s an encrypted HTTP request
Here’s an encrypted HTTP response
Here’s an encrypted HTTP request
Here’s an encrypted HTTP response
TLS/
SSL
TLS/
SSL
HTTP
Requests
HTTP
Responses
HTTP
Requests
HTTP
Responses

Secure Servers
Man-in-the-Middle Attack
Browser
Fake
DNS
Server
What’s IP
address for
www.example.org?
100.1.1.1
Fake
www.example.org
100.1.1.1
Real
www.example.org
My credit card number is…

Secure Servers
Preventing Man-in-the-Middle
Browser
Fake
DNS
Server
What’s IP
address for
www.example.org?
100.1.1.1
Fake
www.example.org
100.1.1.1
Real
www.example.org
Send me a certificate of identity

WebEssentials_technologies Html5 css ppt

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