Chapter 1Introduction

Computer Networking: A Top Down Approach Featuring the Internet,

3rd edition. Jim Kurose, Keith RossAddison-Wesley, July

2004.

A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!) If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.

Thanks and enjoy! JFK/KWR

All material copyright 1996-2005J.F Kurose and K.W. Ross, All Rights Reserved

What’s the Internet: “nuts and bolts” view

• millions of connected computing devices: hosts = end systems

• running network apps• communication links

– fiber, copper, radio, satellite

– transmission rate = bandwidth

• routers: forward packets (chunks of data)

local ISP

companynetwork

regional ISP

router workstation

servermobile

What’s the Internet: “nuts and bolts” view

• protocols control sending, receiving of msgs– e.g., TCP, IP, HTTP, FTP, PPP

• Internet: “network of networks”– loosely hierarchical

– public Internet versus private intranet

• Internet standards– RFC: Request for comments

– IETF: Internet Engineering Task Force

local ISP

companynetwork

regional ISP

router workstation

servermobile

What’s a protocol?human protocols:• “what’s the time?”• “I have a question”• introductions

… specific msgs sent… specific actions

taken when msgs received, or other events

network protocols:• machines rather than

humans• all communication

activity in Internet governed by protocols

protocols define format, order of msgs sent and received among network entities,

and actions taken on msg transmission, receipt

What’s a protocol?a human protocol and a computer network protocol:

Q: Other human protocols?

Hi

Hi

Got thetime?

2:00

TCP connection request

TCP connectionresponseGet http://www.awl.com/kurose-ross

<file>time

Some network apps

• E-mail• Web• Instant messaging• Remote login• P2P file sharing• Multi-user network

games• Streaming stored

video clips

• Internet telephone• Real-time video

conference• Massive parallel

computing

“Cool” internet appliances

World’s smallest web serverhttp://www-ccs.cs.umass.edu/~shri/iPic.html

IP picture framehttp://www.ceiva.com/

Web-enabled toaster +weather forecaster

Internet phones

Internet protocol stack• application: (L7 & L6 of OSI) supporting

network applications– FTP, SMTP, HTTP

• transport: (L5 & L4 of OSI) host-host data transfer– TCP, UDP

• network: routing of datagrams from source to destination– IP, routing protocols

• link: data transfer between neighboring network elements– PPP, Ethernet

• physical: bits “on the wire”

application

transport

network

link

physical

messagesegment

datagram

frame

sourceapplicatio

ntransportnetwork

linkphysical

HtHnHl M

HtHn M

Ht M

M

destination

application

transportnetwork

linkphysical

HtHnHl M

HtHn M

Ht M

M

networklink

physical

linkphysical

HtHnHl M

HtHn M

HtHnHl M

HtHn M

HtHnHl M HtHnHl M

router

switch

Encapsulation

Background: Addressing• For a process to receive

messages, it must have an identifier

• A host has a unique32-bit IP address

• Q: does the IP address of the host on which the process runs suffice for identifying the process?

• Answer: No, many processes can be running on same host

• Identifier includes both the IP address and port numbers associated with the process on the host.

• Example port numbers:– HTTP server: 80

– Mail server: 25

• More on this later

Web and HTTPFirst some jargon• Web page consists of objects• Object can be HTML file, JPEG image, Java

applet, audio file,…• Each object is addressable by a URL• Web page consists of base HTML-file which

includes several referenced objects• Example URL:

www.someschool.edu/someDept/pic.gif

host name path name

HTTP overview• HTTP: hypertext transfer protocol• Web’s application layer protocol• client/server model

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

– server: Web server sends objects in response to requests

• HTTP 1.0: RFC 1945– http://www.rfc-editor.org/rfc/rfc1945.txt

• HTTP 1.1: RFC 2068– http://www.rfc-editor.org/rfc/rfc2068.txt

• HTTP state management (cookies): RFC 2109

– http://www.rfc-editor.org/rfc/rfc2109.txt

PC runningExplorer

Server running

Apache Webserver

Mac runningNavigator

HTTP request

HTTP request

HTTP response

HTTP response

HTTP overview (continued)Uses TCP:• client initiates bi-directional

TCP connection (via 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)– Messages encoded in text

• 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

URL(Uniform Resource Locator)

Way of identifying and accessing a web page:

Examplehttp://www.lclark.edu/~jmache/index.html

“how”Type of transaction

(protocol)

“where”Address or name of server

“what”Resource requested

URI(Uniform Resource Identifier)

Identifies a resource and includes URLs, but broader in context.

See http://www.w3.org/Addressing/ for more details

Mark-up Languages

• A way of describing information in a document.• Standard Generalized Mark-Up Language

(SGML) - a specification for a mark-up language ratified in 1986.

• Key aspect - using pairs of tags that surround information - a begin tag <tag_name> and a matching end tag </tag_name> .

Example<title> CS 393 home page </title>

HyperText Markup Language (HTML)

A mark-up language used in web pages.

“Hypertext” refers to the text’s ability to link to other documents.

“Markup” refers to providing information to tell browser how to display page and other things.

HTML page format

<HTML><HEAD>

</HEAD><BODY>

</BODY></HTML>

Signifies an HTML document

Head section includes information about document - “metadata”

Body section contains text and references to images to be displayed

End of document

HTML Tags

• Tags specify details such as type of text.

Example

<B> to start bold text

</B> to end bold text

<I> to start italic text

</I> to end italic text

HTML page<HTML><HEAD>

</HEAD><BODY>

Hello world<I> My name is <B>Jens</B> </I>

</BODY></HTML>

QuestionWhat does the previous HTML page display?

Answer

Hello World My name is Jens

HTML page<HTML><HEAD>

</HEAD><BODY><BR>Hello world <P><I> My name is <B>Jens</B> </I>

</BODY></HTML>

Line break tag - some tags in HTML are not in pairs

Attributes

Many tags can have attributes which specify something about the body between tag pair.

Example

<FONT COLOR=red SIZE=3 FACE=Times>This text is displayed in red in Times font, about 12 pt.</FONT>

Attributes

HTTP request message

• two types of HTTP messages: request, response• HTTP request message:

– ASCII (human-readable format)GET /somedir/page.html HTTP/1.1Host: www.someschool.edu User-agent: Mozilla/4.0Connection: close Accept-language:fr

(extra carriage return, line feed)

request line(GET, POST,

HEAD commands)

header lines

Carriage return, line feed

indicates end of message

HTTP request message: general format

HTTP request line (methods)

HTTP/1.0• GET

– Return object specified by URI

• POST– Send data to server (forms)

• HEAD– asks server to leave

requested object out of response

– Return headers only of GET 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

• OPTIONS, TRACE, CONNECT

HTTP request line (cont.)

• URI– Object to retrieve

• E.g. http://www.cs.pdx.edu/index.html with a proxy• E.g. /index.html if no proxy

• HTTP version– Version being used– HTTP 1.1

• Host: header required• Connection: header supported

Common HTTP request headers• Accept

– Acceptable document types, encodings, languages, character sets

• If-Modified-Since– For use with caching

• Referer – URL which caused this page to be requested

• User-Agent• Host

– For multiple web sites hosted on same server

• Connection– Keep connection alive for subsequent request or close connection

Other HTTP request headers• Authorization

– Authentication info for HTTP authentication

• From– User email (when privacy is disabled)

Rest of HTTP request

Blank-line Separate request headers from POST information End of request

Body If POST, send POST information

Handling user input (forms)

POST method:• Input is uploaded to

server in entity body

GET method:• Input is uploaded in

URL field of request line

GET search?name=george&animal=monkey HTTP/1.1Host: www.somesite.com

POST search HTTP/1.1Host: www.somesite.comContent-type: application/x-www-form-urlencoded

name=george&animal=monkey

HTTP response message

HTTP/1.1 200 OK Connection closeDate: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... 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 response message: general format

HTTP response format

• Status-line– HTTP version– 3 digit response code

• 1XX – informational• 2XX – success• 3XX – redirection• 4XX – client error• 5XX – server error

– Reason phrase

HTTP response status codes

200 OK– request succeeded, requested object later in this message

301 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

A few sample codes:

Common HTTP response headers

• Server– server software

• Content-Encoding – x-gzip

• Content-Length• Content-Type• Expires• Last-Modified• ETag

Other HTTP response headers

• Location – redirection

• WWW-Authenticate – request for authentication

• Allow– list of methods supported (GET, HEAD, etc)

Rest of HTTP response Blank-line

Separate headers from data Body

Data being returned to client

HTTP headers by function

• Authentication– Client

• Authorization, Proxy-Authorization

– Server• WWW-authenticate,

Proxy-Authenticate

• User, server tracking– Client

• Cookie, Referer, From, User-agent

– Server• Set-cookie, Server

• Caching– General

• Cache-control, Pragma

– Client• If-Modified-Since, If-

Unmodified-Since, If-Match

– Server• Last-Modified, Expires,

ETag, Age

Trying out HTTP (client side) for yourself

1. Telnet to your favorite Web server:

Opens TCP connection to port 80(default HTTP server port) at cis.poly.edu.Anything typed in sent to port 80 at cis.poly.edu

telnet cis.poly.edu 80

2. Type in a GET HTTP request:

GET /~ross/ HTTP/1.1Host: cis.poly.edu

By typing this in (hit carriagereturn twice), you sendthis minimal (but complete) GET request to HTTP server

3. Look at response message sent by HTTP server!

User-server state: cookies

Many major Web sites use cookies

Four components:1) cookie header line of HTTP

response messageSet-cookie:

2) cookie header line in HTTP request messageCookie:

3) cookie file kept on user’s host, managed by user’s browser

4) back-end database at Web site

Example:– Susan access Internet

always from same PC

– She visits a specific e-commerce site for first time

– When initial HTTP requests arrives at site, site creates a unique ID and creates an entry in backend database for ID

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-spectificaction

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:

Cookies (continued)What cookies can bring:• authorization• shopping carts• Site preferences• 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

Web caches (proxy server)

Why Web caching?• Reduce response time for client

request.

• Reduce traffic on an institution’s access link.

• Reduce load on servers.

• Enables “poor” content providers to effectively deliver content (but so does P2P file sharing)

Goal: satisfy client request without involving origin server (i.e. do not send content that has not changed)

Info on web caching http://www.ircache.net/ http://www.squid.org ICP

http://www.rfc-editor.org/rfc/rfc2186.txt

http://www.rfc-editor.org/rfc/rfc2187.txt

More about Web caching• Browser sends all HTTP requests

to cache– object in cache: cache returns object – else cache requests object from origin

server, then returns object to client

• Done directly at client– Via browser web cache

• Along path from client to origin server– Via proxy web cache– Proxy acts as both client and server– Typically cache is installed by ISP

(university, company, residential ISP)

client

Proxyserver

client

HTTP request

HTTP request

HTTP response

HTTP response

HTTP request

HTTP response

origin server

origin server

Caching example Assumptions• average object size = 100,000

bits• avg. request rate from

institution’s browsers to origin servers = 15/sec

• delay from institutional router to any origin server and back to router = 2 sec

Consequences• utilization on LAN = 15%• utilization on access link = 100%• total delay = Internet delay + access

delay + LAN delay = 2 sec + minutes + milliseconds

originservers

public Internet

institutionalnetwork 10 Mbps LAN

1.5 Mbps access link

institutionalcache

Caching example (cont)Possible solution

• increase bandwidth of access link to, say, 10 Mbps

Consequences• utilization on LAN = 15%

• utilization on access link = 15%

• Total delay = Internet delay + access delay + LAN delay

= 2 sec + msecs + msecs

• often a costly upgrade

originservers

public Internet

institutionalnetwork 10 Mbps LAN

10 Mbps access link

institutionalcache

Caching example (cont)

Install cache• suppose hit rate is .4

Consequence• 40% requests will be satisfied

almost immediately• 60% requests satisfied by origin

server• utilization of access link reduced

to 60%, resulting in negligible delays (say 10 msec)

• total avg delay = Internet delay + access delay + LAN delay = .6*(2.01) secs + .4*milliseconds < 1.4 secs

originservers

public Internet

institutionalnetwork 10 Mbps LAN

1.5 Mbps access link

institutionalcache

Conditional GET

• Goal: don’t send object if cache has up-to-date cached version

• cache: specify date of cached copy in HTTP requestIf-modified-since: <date>

• server: response contains no object if cached copy is up-to-date: HTTP/1.0 304 Not Modified

cache 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/1.0 200 OK

<data>

object modified

HTTP caching

• Additional caching methods– ETag and If-Match

• HTTP 1.1 has file signature as well

• When/how often should the original be checked for changes?– Check every time?

– Check each session? Day? Etc?

– Use Expires header• If no Expires, often use Last-Modified as estimate

Example Cache Check Request

GET / HTTP/1.1Accept: */*Accept-Language: en-usAccept-Encoding: gzip, deflateIf-Modified-Since: Mon, 29 Jan 2001 17:54:18 GMTIf-None-Match: "7a11f-10ed-3a75ae4a"User-Agent: Mozilla/4.0 (compatible; MSIE 5.5;

Windows NT 5.0)Host: www.cs.pdx.eduConnection: Keep-Alive

Example Cache Check Response

HTTP/1.1 304 Not ModifiedDate: Tue, 27 Mar 2001 03:50:51 GMTServer: Apache/1.3.14 (Unix) (Red-Hat/Linux)

mod_ssl/2.7.1 OpenSSL/0.9.5a DAV/1.0.2 PHP/4.0.1pl2 mod_perl/1.24

Connection: Keep-AliveKeep-Alive: timeout=15, max=100ETag: "7a11f-10ed-3a75ae4a"