Wenbing Zhao wenbing@ieee.org

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EEC-484/584 Computer Networks

Lecture 20

Wenbing Zhao wenbing@ieee.org

(Lecture nodes are based on materials supplied by Dr. Louise Moser at UCSB and Prentice-Hall)

2

Outline

Midterm #2 results

Review of lecture 19

World Wide Web

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20 November 2005 EEC484/584

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0.00 0

0 0

0 0 0

64.44 58

0 0

8 10 6

10 24

65.56 59

0 0

5 10 10 10 24

70.00 63

0 0

9 10 10 14 20

78.89 71

0 10 8

7 10 14 22

83.33 75

10 10

7 10 10 12 16

85.56 77

0 6

9 10 10 18 24

85.56 77

0 10 9

10 10 16 22

87.78 79

0 10 10

10 9

18 22

88.89 80

0 10 8

10 10 16 26

Total (normalized) Total

P7 P6

P5 P4

P3 P2 P1

EEC484 Midterm#2 Results

A

B+

A-

B B-

C F

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EEC584 Midterm#2 Results

75 0

4 9

10 10

18 24

75 0

5 8

10 10

16 26

76 10

0 7

10 7

16 26

77 10

0 5

10 10

20 22

78 10

0 6

10 10

16 26

80 5

6 9

10 10

16 24

80 10

0 6

10 10

16 28

81 10

0 9

10 10

18 24

84 10

6 6

10 10

14 28

86 10

5 5

10 10

16 30

87 10

3 6

10 10

20 28

Total P7

P6 P5

P4 P3

P2 P1

A-

A

(3)

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EEC584 Midterm#2 Results

62 0

0 6

10 10

14 22

64 0

0 8

10 10

12 24

65 0

0 9

10 10

14 22

65 0

0 9

10 10

14 22

67 0

0 7

10 10

16 24

67 0

0 9

10 10

12 26

68 3

5 8

10 10

10 22

69 10

0 9

10 2

14 24

69 0

0 7

10 10

14 28

71 10

0 4

7 10

18 22

71 10

5 8

0 10

14 24

72 0

5 9

10 10

18 20

72 3

5 9

10 9

14 22

73 10

0 6

10 9

14 24

73 9

6 8

0 10

14 26

74 3

5 10

10 10

12 24

74 10

0 9

10 9

12 24

B+

B

6

62.7 32.6

58 42

9.5 10

6.5

62.8 32.7

59 52

0 10

10

75.9 39.5

63 67

9.5 10

7

78.2 40.7

77 57.5

10 10

7

79.1 41.1

75 65

9 10

6

87.3 45.4

71 81

10 10

9

88.0 45.8

80 90

0 10

10

94.1 48.9

77 90

9 12

8.5

96.2 50.0

79 94

9.5 12.5

7

Total (normalized) Total

(52%) MT2

MT1 HW3

HW2 HW1

EEC484 Accumulated Credit

A A- B+

C

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EEC584 Accumulated Credit

78.8 41

81 64

10 10

10

79.2 41.2

67 81

10 10

9

80.8 42

72 78

10 10

10

81.2 42.2

86 65

10 10

10

81.5 42.4

76 79

8.5 10

10

83.8 43.6

72 91

8.5 10

9

83.8 43.6

73 87

9 10

10

84.0 43.7

69 89.5

10 10

10

85.0 44.2

77 85

9.5 10

10

85.8 44.6

84 79

10 10

10

87.3 45.4

75 94

9 10

10

87.7 45.6

75 95

9 10

10

87.7 45.6

80 90

10 10

9

88.7 46.1

87 84.5

9.5 10

10

Total (normalized) Total (52%)

MT2 MT1

HW3 HW2

HW1

A A-

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EEC584 Accumulated Credit

66.5 34.6

65 54

8 10

9

66.9 34.8

62 54

10 10

9

70.4 36.6

64 62

10 10

8.5

71.5 37.2

80 49

9 10

9.5

72.7 37.8

78 55

9 10

9

73.5 38.2

71 67

6.5 10

10

73.5 38.2

65 76

7 10

8

75.8 39.4

68 70

10 10

9.5

76.2 39.6

74 64

10 10

10

76.2 39.6

69 75

9 10

8

76.7 39.9

73 66.5

10 10

10

76.9 40

74 72

8 10

9

77.3 40.2

67 75

9.5 10

10

78.1 40.6

71 78

7 10

10

A- B+

B

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DNS – The Domain Name System

Hierarchical domain-based naming scheme and distributed database system for implementing it

Maps symbolic names (ASCII strings) to network addresses, i.e., maps hostnames and email

addresses to IP addresses

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The DNS Name Space

Each domain is named by the path upward from it to the unnamed root. E.g., eng.sun.com.

Domain names can be absolute (end with period), or relative

Domain names are case insentive. Component names <= 63 chars. Full path names <= 255 chars

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Resource Records

Every domain has set of resource records

Resolver returns set of resource record

¾ Domain-name, Time_to_live, Type, Class, Value

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Electronic Mail

Two subsystems

¾

User agents – allow users to read and send email.

− They are local programs that provide command-based, menu-based, or graphics for interacting with email system

− User agent builds message, passes it to message transfer agent, which uses header fields to construct envelop

¾

Message transfer agents – move messages from source to destination

− They are system daemons (software that runs in background) that move email through systems

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MIME –

Multipurpose Internet Mail Extensions

Problems with international languages:

¾ Languages with accents (French, German)

¾ Languages in non-Latin alphabets (Hebrew, Russian)

¾ Languages without alphabets (Chinese, Japanese)

¾ Messages not containing text at all (audio or images)

Solutions

¾ Add structure to message body

¾ Define encoding rules for non-ASCII messages

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MIME – Encoding Binary Messages

Base64 encoding or ASCII armor:

¾ Groups of 24 bits are broken up into four 6-bit units, with each unit being sent as a legal ASCII character

¾ The coding is ''A'' for 0, ''B'' for 1, and so on, followed by the 26 lower- case letters, the ten digits, and finally + and / for 62 and 63, respectively

¾ The == and = sequences indicate that the last group contained only 8 or 16 bits, respectively

¾ Carriage returns and line feeds are ignored,

Quoted-printable encoding: For messages that are almost entirely ASCII but with a few non-ASCII characters

This is just 7-bit ASCII, with all the characters above 127 encoded as an

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SMTP

Source machine (client) establishes TCP connection to port 25 of destination machine

¾ Email daemon listens to this port, accepts incoming connections, copies messages from them into appropriate mailboxes

¾ Error report returned to sender if message can’t be delivered

¾ Source machine waits for destination machine to reply

Destination machine (server) sends line of text, giving its identity and telling whether or not it can receive mail

If not, client releases connection and tries again later.

If so, client announces users at source and destination machines. If user exists at destination machine, server tells client to send message

Client sends, server acks

Connection is released

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Email Gateways

Required when two machines use different transport protocols or different message formats

¾

E.g., SMTP vs. X.400

Gateway does conversion

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Final Delivery

Sending and reading mail when the receiver has a permanent Internet connection and the user agent runs on the same machine as the message transfer agent

Reading e-mail when the receiver has a dial-up connection to an ISP

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POP3 – Post Office Protocol

Fetch email from

remote mailbox, store in user’s local machine to be read later

¾

Example: using POP3 to

fetch three messages

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IMAP –

Interactive Mail Access Protocol

For users who use multiple machines

Email server maintains central repository that can be accessed from any machine

Unlike POP3, IMAP does not copy email to user’s machines

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A Comparison of POP3 and IMAP

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Email Spoofing

Email spoofing: the forgery of an e-mail header so that the message appears to have originated from someone or somewhere other than the actual source

E-mail spoofing is possible because Simple Mail Transfer Protocol does not include an authentication mechanism

¾ To send spoofed e-mail, senders insert commands in headers that will alter message information.

¾ It is possible to send a message that appears to be from anyone, anywhere, saying whatever the sender wants it to say

Open Replay Problem

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An open relay is an SMTP email server that allows third-party relay of e-mail messages.

By processing mail that is neither for nor from a local user, an open relay makes it possible for an

unscrupulous sender to route large volumes of spam

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Relayed Mail Scenario

Step 1 -- Source Host to Relay Host

¾ R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready

¾ S: HELO MIT-AI.ARPA

¾ R: 250 USC-ISIE.ARPA

¾ S: MAIL FROM:<JQP@MIT-AI.ARPA>

¾ R: 250 OK

¾ S: RCPT TO:<@USC-ISIE.ARPA:Jones@BBN-VAX.ARPA>

¾ R: 250 OK

¾ S: DATA

¾ R: 354 Start mail input; end with <CRLF>.<CRLF>

¾ S: Date: 2 Nov 81 22:33:44

¾ S: From: John Q. Public <JQP@MIT-AI.ARPA>

¾ S: Subject: The Next Meeting of the Board

¾ S: To: Jones@BBN-Vax.ARPA

¾ S:

¾ S: Bill:

¾ S: The next meeting of the board of directors will be on Tuesday. John.

¾ S: .

¾ R: 250 OK

¾ S: QUIT

¾ R: 221 USC-ISIE.ARPA Service closing transmission channel

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Relayed Mail Scenario

Step 2 -- Relay Host to Destination Host

¾ R: 220 BBN-VAX.ARPA Simple Mail Transfer Service Ready

¾ S: HELO USC-ISIE.ARPA

¾ R: 250 BBN-VAX.ARPA

¾ S: MAIL FROM:<@USC-ISIE.ARPA:JQP@MIT-AI.ARPA>

¾ R: 250 OK

¾ S: RCPT TO:<Jones@BBN-VAX.ARPA>

¾ R: 250 OK

¾ S: DATA

¾ R: 354 Start mail input; end with <CRLF>.<CRLF>

¾ S: Received: from MIT-AI.ARPA by USC-ISIE.ARPA ; 2 Nov 81 22:40:10 UT

¾ S: Date: 2 Nov 81 22:33:44

¾ S: From: John Q. Public <JQP@MIT-AI.ARPA>

¾ S: Subject: The Next Meeting of the Board

¾ S: To: Jones@BBN-Vax.ARPA

¾ S:

¾ S: Bill:

¾ S: The next meeting of the board of directors will be on Tuesday. John.

¾ S: .

¾ R: 250 OK

¾ S: QUIT

¾ R: 221 USC-ISIE.ARPA Service closing transmission channel

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The World Wide Web

Creation of Tim Berners-Lee, in 1989 CERN nuclear physics research

Mosaic – first graphical interface, creation of Marc Andersson (and others), precursor to Netscape

How does WWW work inside?

¾ To host a web browser, a machine must be directly on Internet, or have a connection to router or machine on Internet

¾ To fetch a Web page, browser establishes TCP connection to the machine where the page is and sends a message over the connection asking for the page

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The World Wide Web

Architectural Overview

Static Web Documents

Dynamic Web Documents

HTTP – The HyperText Transfer Protocol

Performance Enhancements

The Wireless Web (not required)

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Architectural Overview

WWW consists of a vast collection of documents or Web pages

Each page may contain links to other pages. Users can follow a link by clicking on it, which then takes them to the page pointed to

The idea of having one page point to another is called hypertext

¾ Invented by Vannevar Bush, a MIT EE professor, in 1945

Browser – program used to view Web pages

Hyperlinks – strings of text that are links to other pages

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Architectural Overview

A Web page

The page reached by clicking on Department of Animal Psychology

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Architectural Overview

Every Web site has server process listening to TCP port 80 for incoming connections from clients (browsers)

After connection is established, client sends request and server sends reply

Then connection is released

The Client Side

Plug-in: a code module that the browser fetches from a special directory on the disk and installs as an

extension to itself

Helper application: a complete program, running as

a separate process. It accepts the name of a scratch

file where the content file has been stored, opens the

file, and displays the contents

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The Server Side

Steps the server performs in its main loop (to a first approximation):

¾

Accept a TCP connection from a client (a browser)

¾

Get the name of the file requested

¾

Get the file (from disk)

¾

Return the file to the client

¾

Release the TCP connection

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The Server Side

Steps the server performs in its main loop (more realistic)

¾ Resolve the name of the Web page requested

¾ Authenticate the client

¾ Perform access control on the client

¾ Check the cache

¾ Fetch the requested page from disk

¾ Determine the MIME type to include in the response

¾ Take care of miscellaneous odds and ends (e.g., building user profile or gathering certain statistics)

¾ Return the reply to the client

¾ Make an entry in the server log

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The Server Side

A multithreaded Web server with a front end and processing modules

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The Server Side

A server farm

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The Server Side

Normal request-reply message sequence

Sequence when TCP handoff is used

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URLs – Uniform Resource Locaters

URL – Uniform Resource Locator

URL encodes three types of information

¾

What is the page called – local name uniquely indicating the specific page

¾

Where is the page located – DNS name of the server on which the page is located

¾

How can the page be accessed – protocol, e.g.,

http, ftp

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Some Common URLs

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Statelessness and Cookies

HTTP is basically stateless

Sometimes, it is necessary to have stateful Web client/server communications

Cookies: name derives from ancient programmer slang in which a program calls a procedure and gets something back that it may need to present later to get some work done

¾ Some examples of cookies

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HTML – HyperText Markup Language

(b)

The HTML for a sample Web page The formatted page

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Common HTML Tags

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Tables

An HTML table

A possible rendition of this table

Forms

(a) The HTML for an order form

(b) The formatted page

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Forms

A possible response from the browser to the server with information filled in by the user:

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XML and XSL

Drawback of HTML

¾

Mixes content with formatting

¾

Does not provide structure to Web pages

XML: eXtensible Markup Language

¾

Describes Web content in a structured way

XSL: eXtensible Style Language

¾

Describes the formatting independently of the

content

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A Simple Web Page in XML

46

A Style Sheet in XSL

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Dynamic Web Documents

Server-side dynamic Web page generation

¾

CGI – Common Gateway Interface

− Standard interface to allow Web servers to talk to back- end programs and scripts that can accept input and generate HTML pages in response

¾

PHP – PHP: Hypertext Preprocessor

− PHP scripts embeded inside HTML pages and have them be executed by the server itself to generate response page

¾

JSP - Java Server Pages

¾

ASP – Active Server Pages

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Server-Side Dynamic Web Page Generation

Steps in processing the information from an HTML form

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Server-Side Dynamic Web Page Generation

A Web page containing a form

A PHP script for handling the output of the form

Output from the PHP script when

the inputs are "Barbara" and 24 respectively

Client-Side Dynamic Web Page Generation

50

Use of

JavaScript for

processing a

form.

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Client-Side Dynamic Web Page Generation

Server-side scripting with PHP Client-side scripting with JavaScript

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Client-Side Dynamic Web Page Generation

The various ways to generate and display content

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HyperText Transfer Protocol

HTTP – HyperText Transfer Protocol

¾ It specifies what messages clients may send to servers and what responses they get back in return

¾ Each interaction consists of one ASCII request, followed by one RFC 822 MIME-like response

¾ Defined in RFC 2616

HTTP

¾ Connection

− In HTTP 1.0: make connection, sends a request, gets a response, tears down connection

− In HTTP 2.0: connection can be reused

¾ Methods: had some provision for object-oriented programming

¾ Message header

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HTTP Methods

The built-in HTTP request methods

¾

Method names are case sensitive!

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HTTP Methods

The status code response groups

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HTTP Message Headers

Redirection

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Example HTTP Usage

Content of the log file

>telnet www.ietf.org 80 >log

>GET /rfc.html HTTP/1.1

>Host: www.ietf.org

>

>close

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Performance Enhancement

Caching

¾ Save pages that have been requested in case they are used again

¾ Client-side technique

Server replication

¾ Replicate server’s contents at multiple locations

¾ Sometimes called mirroring

Content delivery networks

¾ Deliver contents for their providers to end users efficiently for a fee

¾ Whole process starts with URL replacement so all contents point to a CDN server

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Caching

Hierarchical caching with three proxies

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Content Delivery Networks

Original Web page

Same page after transformation

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Content Delivery Networks

Steps in looking up a URL when a CDN is used

WAP –

62

The Wireless Application Protocol

The WAP protocol stack

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WAP

The WAP architecture

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I-Mode

Structure of the i-mode data network showing

the transport protocols

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I-Mode

Structure of the i-mode software

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I-Mode

An example of cHTML file.

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Second-Generation Wireless Web

Comparison of 1st-generation WAP and i-mode

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Second-Generation Wireless Web

New features of WAP 2.0

Push model as well as pull model

Support for integrating telephony into apps

Multimedia messaging

Inclusion of 264 pictograms

Interface to a storage device

Support for plug-ins in the browser

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Second-Generation Wireless Web

WAP 2.0 supports two protocol stacks

70