CCM 4223 Lecture 10 Computer Networks: Wireless and Mobile Communication Systems Dr E. Ever

1

CCM 4223 Lecture 10Computer Networks: Wireless and Mobile Communication Systems

Dr E. Ever

School of Computing Science

ccm4300 2

RECAP: security holeMan (woman) in the middle attack: Trudy

poses as Alice (to Bob) and as Bob (to Alice)

I am Alice I am Alice

R

TK (R)

-

Send me your public key

TK

+A

K (R)-

Send me your public key

AK

+

TK (m)+

Tm = K (K (m))+

T-

Trudy gets

sends m to Alice encrypted

with Alice’s public key

AK (m)+

Am = K (K (m))+

A-

R

ccm4300 3

RECAP: Digital signature = signed message digest

large

message

m

H: Hash

functionH(m)

digital

signature

(encrypt)

Bob’s

private

key K B

-

+

Bob sends digitally signed message:

Alice verifies signature and integrity of digitally signed message:

KB(H(m))-

encrypted

msg digest

KB(H(m))-

encrypted

msg digest

large

message

mH: Hash

function

H(m)

digital

signature

(decrypt)

H(m)

Bob’s

public

key K B

+

equal

?

4

Session Content

||| Introduction to WWW as a digital library

||| HyperText Markup Language (HTML)

- overview

- format and representation

||| HyperText Transfer Protocol (HTTP)

- overview

- message format

- http example

||| Summary

5

Lesson objectives

At the completion of this lesson you should be able to

- understand and describe characteristics of the HTML

- understand and describe the concepts of the HTTP

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Introduction||| A conventional library is a more or less coherent and comprehensive collection of information: it so happens that the information is stored on paper.

||| The World Wide Web (WWW) can also be seen as a library: the fact that the information is stored electronically in a digital format leads us to refer to it as a digital library.

||| The web was first conceived in 1989 by Tim Berners-Lee (U.K.) at CERN. using traditional database was difficult to store informationthe first web client and server in 1990His specifications of URLs, HTTP and HTML were refined as Web technology spread. used hypertext network of information instead

- any document can contain a link to any other document Marc Andreessen (USA) led a team that wrote Mosaic, the first graphical web browser, by the end of 1993

- in 1994, Andreessen and some colleagues formed Netscape Communication Corporation, which is now owned by AOL.

MOSAIC WEB BROWSER (1993)

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Introduction - cont||| With a digital library, as with a conventional one, information must be stored and must then be made accessible.

||| On the Web, the information is stored in documents on Internet computers, which act as Web servers, and is made available from them on request by clients (i.e., uses a client-server model).

||| Documents are stored in mark-up language using HyperText Markup Language (HTML).

||| A document is accessed by using a browser to send a request for it and, when it is received, to display it in accordance with the way it is marked up.

||| The interchange takes place using the communication protocol known as HyperText Transfer Protocol (HTTP).

9

Network applications: some jargon

Process: program running within a host.

within same host, two processes communicate using interprocess communication (defined by OS).

processes running in different hosts communicate with an application-layer protocol

user agent: interfaces with user “above” and network “below”.

implements user interface & application-level protocol

Web: browser E-mail: mail reader streaming

audio/video: media player

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Applications and application-layer protocolsApplication: communicating,

distributed processes e.g., e-mail, Web, P2P file

sharing, instant messaging running in end systems

(hosts) exchange messages to

implement application

Application-layer protocols one “piece” of an app define messages

exchanged by apps and actions taken

use communication services provided by lower layer protocols (TCP, UDP)

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

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App-layer protocol defines Types of messages

exchanged, eg, request & response messages

Syntax of message types: what fields in messages & how fields are delineated

Semantics of the fields, ie, meaning of information in fields

Rules for when and how processes send & respond to messages

Public-domain protocols: defined in RFCs allows for

interoperability eg, HTTP, SMTP

Proprietary protocols: eg, KaZaA

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Client-server paradigmTypical network app has two

pieces: client and serverapplicatio

ntransportnetworkdata linkphysical

application

transportnetworkdata linkphysical

Client:initiates contact with server (“speaks first”)

typically requests service from server,

Web: client implemented in browser; e-mail: in mail reader

request

reply

Server:provides requested service to client

e.g., Web server sends requested Web page, mail server delivers e-mail

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Processes communicating across network

process sends/receives messages to/from its socket

socket analogous to door sending process shoves

message out door sending process asssumes

transport infrastructure on other side of door which brings message to socket at receiving process

process

TCP withbuffers,variables

socket

host orserver

process

TCP withbuffers,variables

socket

host orserver

Internet

controlledby OS

controlled byapp developer

API: (1) choice of transport protocol; (2) ability to fix a few parameters (lots more on this later)

API: Application Programming Interface

14

Processes communicating across network (2)

15

History of the Web World Wide Web, “Web”, “WWW” Tim Berners-Lee (3COM Founders , Director of the

World Wide Web Consortium (W3C), )at CERN in 1991 Demonstrated prototype at a conf. in ’91 Text-based

Marc Andreessen developed the first graphical Web browser in 1993: Mosaic

Andreessen founds Netscape Communications Browser war starts around 1995-96 America Online buys Netscape in 1998

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Some “Web” Terminology Web page may contain links to other pages

(sometimes also called Web Objects) Object can be HTML file, JPEG image, Java

applet, audio file,… Web pages are “Hypertexts”

One page points to another Proposed by Prof. Vannevar Bush in

1945! Each object is addressable by a URL:http://www.someschool.edu/someDept/pic.gif

path namehost nameprotocol

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Types of web pages

Static page/document: contents do not change unless the author changes it

Static Dynamic page/document: does not exist in a predefined form, it is created

whenever a browser requests it Page content and page layout are created separately. The

content is retrieved from a database and is placed on a webpage only when needed or asked.

http://www.cineworld.co.uk/

Active page/document: it is not fully specified, it consists of a programme that understands how to compute and display the value, the program can run on the client machine (http://www.calculator.net/scientific-calculator.html)

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Hypertext Mark-up Language (HTML) 0

||| Each Web pages are written in a language called HTML.

||| HTML allows users to produce a Web page that includes text, graphics, and pointers to other Web pages known as hyperlinks.

||| Technically, the web is a distributed hypermedia (i.e. hypertext mixed with other media e.g. audio tracks, video clips, or both) system that supports interactive access

Why is HTML called a mark-up language??Because it does not contain a detailed formatting instruction.

Instead of specifying a detailed document format, HTML allows a document to contain a general guidelines for display, and allows a browser to choose details. Consequently, two browsers may display an HTML document differently.

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||| Each HTML document is divided into two major parts: A head – contains details about the document A body – contains the majority of the information

||| Syntactically, each HTML document is represented as a text file that contains tags along with other information.

||| HTML tags provide structure for the document as well as formatting hints for the web browser.

||| To specify an immediate action or to start a formatting operation a tag name is bracketed by less-than and greater-than symbol.

<TAGNAME>||| To end operation - corresponding tag with two character sequence

less than and slash, and ends with a greater-than symbol.

</TAGNAME>

0

HTML format and representation

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0

HTML format and representation<html>...</html>

The Root element of an HTML document; all other elements are contained in this. The HTML element delimits the beginning and the end of an HTML document. Standardised in HTML 2.0; still current.

<head>...</head>Container for processing information and metadata for an HTML document. Standardised in HTML 2.0; still current.

<body>...</body>Container for the displayable content of an HTML document. Standardised in HTML 2.0; still current.

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||| The following simple example illustrates the form of an HTML document:<head><TITLE> Simple Example</TITLE></head><body><H1>Heading</H1><P>A short illustrative paragraph that includes a single hyperlink to the <A HREF=“http://www.w3.org/”>WWW Consortium home page</A>.</P></body></html>

||| The HTML code above will be interpreted and displayed by a browser in a form similar to that shown below:

HeadingA short illustrative paragraph that includes a single hyperlink to the WWW Consortium home page.

HTML format and representation - cont

HEADBODY

HTML EXAMPLE

<html>

<head>

<title>Hello</title>

</head>

<body>

<p>Hello, world!</p>

<p>This is a picture: <img src="some-image.gif"></p>

<p>This is a <a href="another-page.html">link</a> to another page.</p>

</body>

</html>

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||| When a Web browser interacts with a Web server, the two programs follow the HTTP.

||| HTTP is an layer protocol

The HTTP protocol ….

||| HTTP - client/server model

client: browser that requests, receives, “displays” Web objects

server: Web server sends objects in response to requests

||| Before 1998 - http1.0: RFC 1945

||| http1.1: RFC 2068

PC runningExplorer

Server running

apache Webserver

Mac runningNavigator

http request

http re

quest

http response

http re

sponse

application

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||| The generic client-server interaction proceeds through four stages:

1. Send request.

e.g. ‘http://www.mdx.ac.uk/cs/lecture6.html’

2. Receive request.

The named computer (a Web server) receives the request and locates the file in its file store.

3. Send response.

The Web server sends the requested file to the requesting computer.

4. Receive response.

The client machine receives the file and its browser displays it in accordance with the way it is marked up.

The HTTP protocol - cont

25

The HTTP protocol - cont

http: TCP transport service:

||| client initiates TCP connection (creates socket) to server, port 80

||| server accepts TCP connection from client

||| http messages (application-layer protocol messages) exchanged between browser (http client) and Web server (http server)

||| TCP connection closed

http is “stateless”

||| server maintains no information about past client requests

Protocols that maintain “state” are complex!

||| past history (state) must be maintained

||| if server/client crashes, their views of “state” may be inconsistent, must be reconciled

aside

26

Concepts

||| A web page (also referred to as document) consists of objects.

||| An object is simply a file such as an HTML file, JPEG file, a Java applet, an audio clip, etc.

||| Most web pages consist of a base HTML file and several referenced objects

||| For example, if a Web page contains HTML text and five JPEG images, then the Web page has six objects.

The HTTP protocol - contHTTP : Uniform Resource Locator (URL)

||| Each object is addressable by a single URL

||| Each URL has two components: - the host name of the server that houses the object and - the object’s path name

||| For example:

www.mdx.ac.uk/cs/alasebae/pec.jpg

- www.mdx.ac.uk for a host name

- /someDepartment/logo.gif for a path name

27

application

transportNetworkNetwork access

1. SYN 1. SYN

2. SYN + ACK2. SYN + ACK3. ACK 3. ACKapplicatio

ntransportNetworkNetwork access

TCP three-way handshake

4. Data flow begins 4. Receive data

Three-wayThree-wayhandshakehandshake

CloseClose

timetime

SYNSYN

ACKACK

SYN +SYN +ACKACKOpenOpen

FINFIN

ACKACK

ACKACK

FINFIN

Initiates a connection

Accepts and acknowledges

Acknowledges and begins tx

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HTTP example (1.0)Suppose user enters URL www.someSchool.edu/someDepartment/home.index

1a. http client initiates TCP connection to http server (process) at www.cs.mdx.ac.uk Port 80 is default for http server.

2. http client sends http request message (containing URL) into TCP connection socket

1b. http server at host www.cs.mdx.ac.uk waiting for TCP connection at port 80 (HTTP uses port 80 ). “accepts” connection, notifying client

3. http server receives request message, forms response message containing requested object (someDepartment/home.index), sends message into sockettime

(e.g. contains text, references to 5

jpeg images)

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HTTP example - cont

5. http client receives response message containing html file, displays html. Parsing html file, finds 5 referenced jpeg objects

6. Steps 1-5 repeated for each of 5 jpeg objects

4. http server closes TCP connection.

time

30

||| Steps above above uses non-persistent connection because each TCP connection is closed after the server sends the object

the connection does not persist for other objects

||| Thus, in this example, when a user request the Web page, 6 TCP connections are generated:

for each connection, TCP buffers must be allocated

- serious burden on web server each object suffers two round-trip time (RTT)

- one RTT to establish the TCP connection and one RTT to request and receive an object

each object suffers from TCP slow start due to congestion control mechanism employed by TCP

- slow start initially before ramp up to relative high rate

HTTP example shortcomings

31

Slow Start

32

Non-persistent, persistent connectionsNon-persistent||| http/1.0: server parses

request, responds, closes TCP connection

||| 2 round-trip times (RTTs) to fetch object

TCP connection object request/transfer

||| each transfer suffers from TCP’s initially slow sending rate

||| many browsers open multiple parallel connections

Persistent

||| default for http/1.1 Pipelined Non-pipelined

||| on same TCP connection: server, parses request, responds, parses new request,..

||| client sends requests for all referenced objects as soon as it receives base HTML.

||| fewer RTTs, less slow start.

33

Persistent HTTP

Nonpersistent HTTP issues: requires 2 RTTs per object OS overhead for each TCP

connection browsers often open parallel

TCP connections to fetch referenced objects

Persistent HTTP server leaves connection

open after sending response subsequent HTTP messages

between same client/server sent over open connection

Persistent without pipelining: client issues new request

only when previous response has been received

one RTT for each referenced object

Persistent with pipelining: default in HTTP/1.1 client sends requests as

soon as it encounters a referenced object

as little as one RTT for all the referenced objects

34

Performance Comparison

HTML page (of size 5Kbp)

10 images (each of size 5Kb)

RTT 100 m seconds

5 parallel connections

David J. Yates

HT

TP

Res

pons

e tim

e( s

econ

ds)

35

||| two types of http messages: request, response

||| http request message: ASCII (human-readable format) Get to request object, Post when user fills in form,

Head respond without object for debug purpose 1st line request(GET, POST,

HEAD commands)

Subsequent linesCalled header lines

Carriage return, line feed

indicates end of message

GET /somedir/page.html HTTP/1.0 Host www.someschool.eduConnection: closeUser-agent: Mozilla/4.0Accept-language:fr

(extra carriage return, line feed)

HTTP message format: request

Indicates non-persistent connection

36

Response time modelingDefinition of RTT: time to send

a small packet to travel from client to server and back.

Response time: one RTT to initiate TCP

connection one RTT for HTTP request

and first few bytes of HTTP response to return

file transmission time

total = 2RTT+transmit time

time to transmit file

initiate TCPconnection

RTT

requestfile

RTT

filereceived

time time

37

HTTP message format: request - cont

38

Uploading form input

Post method: Web page often

includes form input Input is uploaded to

server in entity body

URL method: Uses GET method Input is uploaded in

URL field of request line:

www.yoursite.com/yoursearch?cw&exams

Submits data to be processed (e.g., from an HTML form) to the identified resource. The data is included in the body of the request.

Requests a representation of the specified resource

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Method types

HTTP/1.0 GET POST HEAD

asks server to leave requested object out of response

HTTP/1.1 GET, POST, HEAD PUT

uploads file in entity body to path specified in URL field

DELETE deletes file specified

in the URL field

40

Classical HTTP/1.0http://www.somewhere.com/index.html

index.html references: page1.jpg,page2.jpg, page3.jpg.

time to transmit index.hml

initiate TCPconnection

RTT

GET index.html

RTT

filereceived

GET page1.jpg

time to transmit page1.jpg

GET: Retrieves a read-only copy of a file

POST: sending forms that interactively completed by user browser

HEAD: it returns information about data

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HTTP/1.0 200 OK Connection: closeDate: Thu, 06 Aug 2006 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 2006 …... Content-Length: 6821 Content-Type: text/html data data data data data ...

status line(protocol

status codestatus phrase)

header lines

data, e.g., requestedhtml file

HTTP message format: response

42

HTTP response status codes

200 OK request succeeded, requested object later in this

message301 Moved Permanently

requested object moved, new location specified later in this message (Location:)

400 Bad Request request message not understood by server

404 Not Found requested document not found on this server

505 HTTP Version Not Supported

In first line in server->client response message.

A few sample codes:

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User-server interaction: authentication

Authentication: control access to server content

||| authorisation credentials: typically name, password

||| stateless: client must present authorisation in each request

- authorisation: header line in each request

- if no authorisation: header, server refuses access, sends

WWW authenticate: header line in response

client server

usual http request msg

401: authorisation req.WWW authenticate:

usual http request msg+ Authorisation: <cred>

usual http response msg

usual http request msg+ Authorisation: <cred>

usual http response msg time

Despite HTTP server is “stateless” – HTTP provides two mechanism:

44

Cookies: keeping “state”

||| server-generated # , server-remembered #, later used for:- authentication- remembering user

preferences, previous choices

||| server sends “cookie” to client in response msgSet-cookie: 1678453

||| client presents cookie in later requestscookie: 1678453

client server

usual http request msg

usual http response +Set-cookie: #

usual http request msgcookie: #

usual http response msg

usual http request msgcookie: #

usual http response msg

cookie-specificaction

cookie-specificaction

45

Cookies: keeping “state” (cont.)

client server

usual http request msgusual http response

+Set-cookie: 1678

usual http request msg

cookie: 1678usual http response

msg

usual http request msg

cookie: 1678usual http response msg

cookie-specificaction

cookie-specificaction

servercreates ID

1678 for user

entry in backend

database

access

acce

ss

Cookie file

amazon: 1678ebay: 8734

Cookie file

ebay: 8734

Cookie file

amazon: 1678ebay: 8734

one week later:

46

Cookies (continued)What cookies can bring:

authorisation

shopping carts

recommendations

user session state (Web e-mail)

Cookies and privacy: cookies permit sites to

learn a lot about you you may supply name

and e-mail to sites search engines use

redirection & cookies to learn yet more

advertising companies obtain info across sites

aside

47

Conditional GET: client-side caching

||| Goal: don’t send object if client has up-to-date cached version

||| client: specify date of cached copy in http request

If-modified-since: <date>

||| server: response contains no object if cached copy is up-to-date:

HTTP/1.0 304 Not Modified

client server

http request msgIf-modified-since:

<date>

http responseHTTP/1.0

304 Not Modified

object not

modified

http request msgIf-modified-since:

<date>

http responseHTTP/1.1 200 OK

<data>

object modified

48

Web Caches (proxy server)

||| user sets browser: Web accesses via web cache

||| client sends all http requests to web cache

- object in web cache: web cache returns object

- else web cache requests object from origin server, then returns object to client

Goal: satisfy client request without involving origin server

client

Proxyserver

client

http request

http re

quest

http response

http re

sponse

http request

http response

origin server

origin server

49

Web Caching Hierarchy

client

local proxy cache(e.g., local ISP,

University)

regional proxy cache

national/international proxy cache

50

Why Web Caching?

Assume: cache is “close” to client (e.g., in same network)

||| smaller response time: cache “closer” to client

||| decrease traffic to distant servers

- link out of institutional/local ISP network often bottleneck

originservers

public Internet

institutionalnetwork 10 Mbps LAN

1.5 Mbps access link

institutionalcache

51

Issues of Web Caching...||| Read/Write Operation

- read operation vs. write operation

||| Consistency :accessing from cache vs. accessing from server

-Strong consistency

-Weak consistency

||| Main memory buffer

- Cache size vs. performance

||| All objects cannot be cached

-e.g, dynamic objects

52

Weak Consistency Each cached copy has a TTL beyond which it

must be validated with the origin server TTL = freshness life time – age

freshness life time: often heuristically calculated; sometimes based on MAX_AGE or EXPIRES headers

age = current time (at client) – timestamp on object (time at which server generated response)

Age Penalty?

53

Content Distribution Networks (CDNs)

The content providers are the CDN customers.

Content replication

CDN company installs hundreds of CDN servers throughout Internet in lower-tier ISPs,

close to usersCDN replicates its customers’

content in CDN servers. When provider updates content, CDN updates servers

origin server in North America

CDN distribution node

CDN serverin S. America CDN server

in Europe

CDN serverin Asia

54

HTTP Performance Modelling (roughly)

Assume Web page consists of:

1 base HTML page (of size X bits)

M images (each of size X bits)

Non-persistent HTTP:

M+1 TCP connections in series

Response time = (M+1)Y/R + (M+1)2RTT + sum of idle times

55

HTTP Performance Modelling (2)

Persistent connections:

2 RTT to request and receive base HTML file

1 RTT to request and receive M images

Response time = (M+1)X/R + 3RTT + sum of idle times

Non-persistent HTTP with Y parallel connections

Suppose M/Y integer.

1 TCP connection for base file

M/Y sets of parallel connections for images.

Response time = (M+1)X/R + (M/Y + 1)2RTT + sum of idle times

56

Summary!||| The WWW is a distributed hypermedia repository of information (i.e. digital library) that is accessed with an interactive browser

||| A browser displays a Web page of information.

||| Web pages are written in the HTML

||| Web browser interacts with a Web server via the HTTP