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Computer NetworksComputer Networks

jamali@ iust.ac.irjamali@ iust.ac.ir Application Layer Application Layer 22--11June 23, 2010

Application Layer


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2.1 Principles of

app layer protocols clients and servers

app requirements

2.6 Socket

programming with TCP 2.7 Socket

programming with UDP

Chapter 2 OutlineChapter 2 Outline


. e an 2.3 FTP

2.4 Electronic Mail SMTP, POP3, IMAP

2.5 DNS

.server

2.9 Contentdistribution Network Web caching Content distribution

networks

P2P file sharing


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within same host, twoprocesses communicate

using inter-processcommunication (definedby OS).

implements userinterface &

application-levelprotocol Web: browser

- :

Network ApplicationsNetwork Applications


different hostscommunicate with anapplication-layer

protocol

streaming audio/video:media player

Process: program runningwithin a host.

user agent: interfaceswith user above andnetwork below.


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Application

processApplication Program Interface (API) Application Program Interface (API)

Communication Software & HardwareCommunication Software & Hardware

Application Process(Client Side)

Application Process(Server Side)

Client/Server ApplicationsClient/Server Applications


CommunicationCommunicationNetworkNetwork

Server Agents Examples: Internet Information

Sever + http, Appachi +http

SQL query engines + http

Communication SoftwareExamples: TCP, UDP; IP


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applicationtransportnetwork

data linkphysical

modem

modem

Application: communicating,distributed processes e.g., e-mail, Web, P2P file

sharing, instant messaging

running in end systems(hosts)

Applications and ApplicationApplications and Application--Layer ProtocolsLayer Protocols


applicationtransportnetworkdata linkphysical


implement applicationApplication-layer protocols

one piece of an app

define messages

exchanged by apps andactions taken

use communicationservices provided by lowerlayer protocols (TCP, UDP)


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Types of messagesexchanged: request &

response messages Syntax of messagetypes: what fields inmessages & how fields

Public-domain protocols:

defined in RFCs

allows forinteroperability

eg, HTTP, SMTP

ApplicationApplication--Layer ProtocolsLayer Protocols


re e ineate Semantics of the

fields: meaning ofinformation in fields

Rules for when andhow processes send &respond to messages

Proprietary protocols:

eg, Napster, KaZaA,


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modem

modem

Typical network app has twopieces: clientand server

Client: initiates contact with server

(speaks first)

typically requests service from

applicationtransportnetwork

data linkphysical

ClientClient--Server ParadigmServer Paradigm


,

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

replyServer: provides requested service to client e.g., Web server sends requested Web

page, mail server delivers e-mail


request


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Pure P2P architecturePure P2P architecture

no always on server

arbitrary end systems directly communicate peers are intermittently

modem

modem


connecte an c ange addresses example: Gnutella

Highly scalable But difficult to manage


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Hybrid of clientHybrid of client--server and P2Pserver and P2P

Napster File transfer P2P File search centralized:

Peers register content at central server Peers query same central server to locate content


Instant messaging Chatting between two users is P2P Presence detection/location centralized:

User registers its IP address with central server when itcomes online User contacts central server to find IP addresses of

buddies


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process sends/receivesmessages to/from itssocket

socket analogous to door sending process pushes

message out door

process

socket

host or

server

process

socket

host or

server

controlled by

app developer

Processes Communicating Across NetworkProcesses Communicating Across Network

Sockets: doors between process and host


sending process assumestransport infrastructureon other side of door whichbrings message to socket

at receiving process

TCP withbuffers,

variables

TCP withbuffers,

variablesInternet

controlled

by OS

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


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For a process toreceive messages, itmust have an identifier

Every host has a unique32-bit IP address

Q: does the IP address

Identifier includesboth the IP addressand port numbers

associated with theprocess on the host. Example port numbers:

HTTP ser er: 80

Process IdentifierProcess Identifier


the process runssuffice for identifyingthe process?

Answer: No, manyprocesses can berunning on same host

Mail server: 25 Example IP address:

113.45.12.201

Example Identifier: (80; 113.45.12.201)


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Data loss some apps (e.g., audio) can

tolerate some loss

other apps (e.g., filetransfer, telnet) require100% reliable data

Bandwidth

some apps (e.g.,multimedia) requireminimum amount ofbandwidth to beeffective

What transport service does an app need?What transport service does an app need?


Timing some apps (e.g.,

Internet telephony,

interactive games)require low delay to beeffective

other apps (elasticapps) make use ofwhatever bandwidththey get


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Application Data loss Bandwidth TimeSensitive

file transfer no loss elastic no

e-mail no loss elastic no

web documents no loss elastic (few kbps) no

real-time loss- audio: few kbps-1Mbps yes, 100s of

Requirements of Selected Network ApplicationsRequirements of Selected Network Applications


audio/video to erantvideo:10kbps-5Mbps

msec

storedaudio/video

loss-tolerant

same as above yes, few sec

interactivegames

loss-tolerant

few kbps-10kbps yes, 100s ofmsec

instantmessaging

no loss elastic yes and no


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TCP service: connection-oriented:setup

required between client and

server processes reliable transportbetween

sending and receiving process

low control:sender wont

UDP service: unreliable data transfer

between sending and

receiving process does not provide:

connection setup,reliability, flow control,

Internet Transport Protocols ServicesInternet Transport Protocols Services


overwhelm receiver congestion control:throttle

sender when networkoverloaded

no guarantee on:timing,minimum bandwidth

congestion control, timing,or bandwidth guarantee

Q: Why is there a UDP?


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Internet apps: application, transport protocolsInternet apps: application, transport protocols

applications application-layerprotocol

underlying transportprotocol

e-mail SMTP [RFC 2821] TCP

remote terminal access Telnet [RFC 854] TCP

web HTTP [RFC 2616] TCP


remote file server NFS [McKusik] UDP or TCP

streaming multimedia proprietary

(e.g., Dialpad)

UDP or TCP

internet telephony proprietary(e.g., Dialpad) Typically UDP


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2.1 Principles of app

layer protocols clients and servers

app requirements

2.6 Socket programming

with TCP 2.7 Socket programming

with UDP



. 2.3 FTP

2.4 Electronic Mail SMTP, POP3, IMAP

2.5 DNS

.server

2.9 Content distribution Network Web caching

Content distributionnetworks

P2P file sharing


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Web page consists of objects

Object can be HTML file, JPEG image, Javaapplet, audio file,

Web page consists of base HTML-file whichincludes several referenced objects

Each object is addressable by a URL

Web and HTTPWeb and HTTP


Example URL:

www.iust.ac.ir/computer/home.html

host name path name


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HTTP: hypertexttransfer protocol

Webs app layer protocol

client/server model client:browser that

requests, receives,

PC runningExplorer

Serverrunning

Apache Web

server

HTTP OverviewHTTP Overview


server:Web serversends objects inresponse to requests

HTTP 1.0: RFC 1945

HTTP 1.1: RFC 2068Mac runningNavigator

Storage

contents:

base files,

objects


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Uses TCP: client initiates TCP

connection (creates socket)

to server, port 80 server accepts TCP

connection from client

HTTP is stateless server maintains no

information about

past client requests

Protocols that maintainstate are com lex!

aside

HTTP Overview (cont.)HTTP Overview (cont.)


messages app ca on-layer protocol messages)exchanged between browser(HTTP client) and Webserver (HTTP server)

TCP connection closed

past history (state) mustbe maintained

if server/client crashes,their views of state may

be inconsistent, must bereconciled


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Nonpersistent HTTP

At most one object issent over a TCPconnection.

Persistent HTTP

Multiple objects canbe sent over singleTCP connection

HTTP ConnectionsHTTP Connections


.nonpersistent HTTP

server. HTTP/1.1 uses

persistent connections

in default mode


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Suppose user enters URLwww.iust.ac.ir/computer/home.html

1. HTTP client initiates TCPconnection to HTTP server(process) at www.iust.ac.iron port 80

2. HTTP server at hostwww.iust.ac.ir waiting forTCP connection at port 80.

(contains text,references to 10

jpeg images)

NonNon--persistent HTTPpersistent HTTP


3. HTTP client sends HTTPrequest message(containingURL) into TCP connection

socket. Message indicatesthat client wants objectComputer//home.html

accepts connection, notifyingclient

4. HTTP server receives request

message, forms responsemessagecontaining requestedobject, and sends messageinto its socket

time


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5. HTTP client receives responsemessage containing html file,displays html. Parsing html

file, finds 10 referenced jpegobjects

6. HTTP server closesTCP connection.

4.

NonNon--persistent HTTP (cont.)persistent HTTP (cont.)


7. Steps 1-6 repeated for eachof 10 jpeg objects

time


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Definition of RRT: time tosend a small packet totravel from client toserver and back.

Response time: one RTT to initiate TCP

connection

initiate TCP

connection

RTT

Response Time ModelingResponse Time Modeling


one RTT for HTTPrequest and first fewbytes of HTTP responseto return

file transmission timetotal = 2RTT+transmit time

time to

transmit

file

request

file

RTT

file

received

time time


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Nonpersistent HTTP issues: requires 2 RTTs per object

OS must work and allocatehost resources for each TCPconnection

but browsers often open

Persistent without pipelining:

client issues new request

only when previousresponse has been received

one RTT for eachreferenced object

Persistent HTTPPersistent HTTP


fetch referenced objectsPersistent HTTP server leaves connection

open after sending response

subsequent HTTP messagesbetween same client/serverare sent over connection

Persistent with pipelining: default in HTTP/1.1

client sends requests assoon as it encounters a

referenced object as little as one RTT for all

the referenced objects


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method SP URL SP version CR LF

header field name : value CR LF

--- - --- --- ---

request line

header lines

HTTPHTTP RequestRequestMessage: General FormatMessage: General Format


CR LF

Entity Body


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version SP status code SP phrase CR LF

header field name : value CR LF

--- - --- --- ---

status line

header lines

HTTPHTTP ResponseResponseMessage: General FormatMessage: General Format


CR LF

Entity Body


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HTTP request message: ASCII (human-readable format)

GET /somedir/page.html HTTP/1.1

request line(GET, POST,

HEAD commands)

HTTPHTTP RequestRequest MessageMessage


. .

User-agent: Mozilla/4.0Connection: close

Accept-language:fr

If-modified-since:Sat, 2 Nov 2002 13:45:12

(carriage return, line feed)

headerlines

Carriage return,line feedindicates end

of message


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HTTP/1.1 200 OK

Connection: close

Date: Thu, 06 Aug 1998 12:00:15 GMT

Server: Apache/1.3.0 (Unix)

-

status line(protocol

status codestatus phrase)

header

HTTPHTTP ResponseResponse MessageMessage


, ...

Content-Length: 6821Content-Type: text/html

data data data data data ...

ines

data, e.g.,requestedHTML file

MIME lines


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GETGETThe GET method means retrieve whatever information (in the formof an entity) is identified by the Request-URI.

The semantics of the GET method changes to a "conditional GET" ifthe request message includes an If-Modified-Since header field.

GET and HEAD CommandsGET and HEAD Commands


The HEAD method is identical to GET except that the server mustnot return any Entity-Body in the response.

There is no"conditional HEAD"request analogous to the conditional

GET.


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Goal: dont send object ifclient has up-to-date

cached version

client: specify date ofcached copy in HTTPrequest

client server

HTTP request msgIf-modified-since:

HTTP responseHTTP/1.0

objectnot

modified

Conditional GET: ClientConditional GET: Client--Side CachingSide Caching


-mo e -s nce:

server: response containsno object if cached copyis up-to-date:

HTTP/1.0 304 NotModified

HTTP request msgIf-modified-since:

HTTP responseHTTP/1.0 200 OK

object

modified

POST C dPOST C d


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POSTPOST

The POST method is used to request that the destination serveraccept the entity enclosed in the request as a new subordinate of

the resource identified by the Request-URL in the Request-Line.POST is designed to allow a uniform method to cover the followingfunctions:

Annotation of existin resources;

POST CommandPOST Command


Posting a message to a bulletin board, newsgroup, mailinglist,or similar group of articles;

Providing a block of data, such as the result of submitting aform, to a data-handling process;

Extending a database through an append operation.

U l di F I tU l di F I t


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Get method: Web page often

includes form input

Input is uploaded toserver in URL field ofrequest line:www.somesite.com/cgi.bin/form.cgi

URL encoding

A-Z A-Z

a-z a-z

0-9 0-9

*-_ *-_

Space +

Others %HEX

Uploading Form InputUploading Form Input


Post method: Web page often

includes form input Input is uploaded to

server in entity body

URL Encoded data

M th d TM th d T


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HTTP/1.0

GET

POST HEAD

asks server to leave

HTTP/1.1

GET, POST, HEAD

PUT uploads file in entity

body to path specifiedin URL field

Method TypesMethod Types


response

DELETE deletes file specified in

the URL field

Other MethodsOPTIONS, PATCH, COPY, MOVE, DELETE, LINK, UNLINK, TRACE,WRAPPED, extension method.

H d Fi ldH d Fi ld


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1. General Header Fields

2. Request Header Fields

3. Res onse Header Fields

Header FieldsHeader Fields


4. Entity Body Hearer Fields

H d Fi ldH d Fi ld


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1. General Header Fields

Keep-Alive

Upgrade

Connection

MIME-Version

Data Pragma Type Subtypes

Multipurpose Internet Mail Extensions(rfcs822obs, 2045, 2046, 2077)



orwar e

Cache-Control TextImage

Audio

Multipart

Video

, ,

Msexel,/ Html, plain(ASCII),

/ Jpeg, gif, ief,

/

Basic, x-pn-realaudio,/ mixed

/ mpeg,quicktime



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2. Request Header Fields

Accept:

Accept-Language:

Authorization:

Range:

Refer:

Unless:



rom:

Host:

If-Modified-Since:

Proxy-Authorization:

ser- gen :

Accept-Encoding:

Cookie:



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3. Response Header Fields

Location:

Proxy-Authentication:

Public:



Retry-After:

Server:

WWW-Authenticate: Set-Cookie:

Header Fields (cont )Header Fields (cont )


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4. Entity Body Header Fields

Allow

Content-Encoding

Content-Language

Derived-From

Expires

Last-Modified

Header Fields (cont.)Header Fields (cont.)


on en - eng

Content-Range

Content-Type

Content-Version

Content-MD5

n

Title

Transfer-Encoding

URL-Header

extension-header

HTTP Response Status CodesHTTP Response Status Codes


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200 OK request succeeded, requested object later in this message

301 Moved Permanently

In first line in server -> client response message.

A few sample codes:

HTTP Response Status CodesHTTP Response Status Codes


requeste o ject move , new ocat on spec e ater n

this message (Location:)

400 Bad Request

request message not understood by server

404 Not Found requested document not found on this server

Status Codes CategoriesStatus Codes Categories


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Status Codes CategoriesStatus Codes Categories

Response Categories Informational :100

Successful :200

Redirection :300

Client Error :400 e . 404 Not found


Server Error :500

Status CodesStatus Codes


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Status-Codes "201" ; Created "202" ; Accepted "204" ; No Content "301" ; Moved Permanently "200" ; OK "302" ; Moved Temporarily "304" ; Not Modified

Status CodesStatus Codes


; a eques "401" ; Unauthorized "403" ; Forbidden "404" ; Not Found "500" ; Internal Server Error

"501" ; Not Implemented "502" ; Bad Gateway "503" ; Service Unavailable 505 ; HTTP Version Not Supported


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UserUser--Server Interaction: AuthorizationServer Interaction: Authorization


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Authorization : control access toserver content

authorization credentials:typically name, password

stateless: client must presentauthorization in eachrequest authorization: header line in

client server

usual http request msg

401: authorization req.WWW authenticate:

usual htt re uest ms

UserUser Server Interaction: AuthorizationServer Interaction: Authorization


if no authorization: header,server refuses access,sendsWWW authenticate:

header line in response

+ Authorization:

usual http response msg

usual http request msg

+ Authorization:

usual http response msgtime

Cookies: Keeping StateCookies: Keeping State--RFC2109RFC2109


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Cookies: Keeping StateCookies: Keeping State RFC2109RFC2109

Some Web sites store information in a small text file onyour computer. This file is called a cookie.

There are several types of cookies,

ou can choose whether to allow some none or all of


them to be saved on your computer.

If you do not allow cookies at all, you may not be able to

view some Web sites or

take advantage of customization features (such as localnews and weather, or stock quotes).

How cookies are usedHow cookies are used


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How cookies are usedHow cookies are used

Created by an Internet site to store information on yourcomputer. Example, you visit a Web site, it saves a record of the pages

you looked at, to help the site customize the view for you the

next time you visit.

Cookies can also store personally identifiable information,that can be used to identif or contact ou, such as


your name, e-mail address, or telephone number. A Web site only has access to the information you provide.

Once a cookie is saved on your computer, only the Web sitethat created the cookie can read it.

Persistent/Temporary CookiesPersistent/Temporary Cookies


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Persistent/Temporary CookiesPersistent/Temporary Cookies

A persistent cookie is one stored as a file on yourcomputer, and it remains there when you closeInternet browser.


your current browsing session, and is deleted fromyour computer when you close Internet browser.

FirstFirst--Party vs. ThirdParty vs. Third--Party cookiesParty cookies


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FirstFirst Party vs. ThirdParty vs. Third Party cookiesParty cookies

A first-party cookie either originates on or is sent to theWeb site you are currently viewing.

A third-party cookie either originates on or is sent to a Website different from the one you are currently viewing.


-

Web site you are viewing. For example, many sites useadvertising from third-party Web sites and those third-party Web sites may use cookies.

A common use for this type of cookie is to track your Web

page use for advertising or other marketing purposes.

Third-party cookies can either be persistent or temporary.

Unsatisfactory CookiesUnsatisfactory Cookies


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yy

Unsatisfactory cookies are cookies that might

allow access to personally identifiable information that


cou e use or a secon ary purpose w ou

your consent.

Cookies and PrivacyCookies and Privacy


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What cookies can bring:

authorization

shopping carts

recommendations

user session state

Cookies and privacy:

cookies permit sites tolearn a lot about you

you may supply nameand e-mail to sites

search en ines use

aside

yy


redirection & cookiesto learn yet more

advertising companiesobtain info acrosssites

HTTP and CookiesHTTP and Cookies


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There are four components:

1) cookie header line in theHTTP response message

2) cookie header line in HTTP

Example: Ali access Internet

always from same PC

He visits a specific e-

Many major Web sites use cookies


reques message

3) cookie file kept on usershost and managed by usersbrowser

4) back-end database at Web

site

commerce s e or rs

time When initial HTTP

requests arrives at site,site creates a unique ID

and creates an entry inbackend database forID

ExampleExample


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client serverusual http request msg

usual http response +

Set-cookie: 1678

usual http request msg cookie-

servercreates ID

1678 for user

Cookie file

amazon: 1678

Cookie file

ebay: 8734

pp



usual http request msgcookie: 1678


specificaction

cookie-

specificaction

ebay: 8734

Cookie file

amazon: 1678ebay: 8734

one week later:

Cookie File Samples

HTTP over TCPHTTP over TCP


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HTTPFiles


TCP

IP

Routers packetspacketspacketspacketspacketspacketsRouters



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2.1 Principles of applayer protocols clients and servers

app requirements 2.2 Web and HTTP

2.6 Socket programmingwith TCP


with UDP 2.8 Building a Web

server


2.4 Electronic Mail

SMTP, POP3, IMAP

2.5 DNS



P2P file sharing

FTP: the File Transfer ProtocolFTP: the File Transfer Protocol


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file transferFTP

server

FTPuser

interface

FTPclient

local filesystem

remote filesystem

userat host


transfer file to/from remote host

client/server model

client:side that initiates transfer (either to/fromremote)

server:remote host ftp: RFC 959

ftp server: port 21

File Transfer and Access Protocols

(FTP, TFTP, SFTP, NFS) RFCs1


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2640 Internationalization of the File Transfer

Protocol

2624 NFS Version 4 Design Considerations 2623 NFS Version 2 and Version 3 Security

Issues and the NFS Protocols Use of


RPCSEC_GSS and Kerberos V5 2577 FTP Security Considerations




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2428 FTP Extensions for IPv6 and NATs

2389 Feature negotiation mechanism for the File

Transfer Protocol 2349, 1784 - TFTP Timeout Interval and Transfer


2348, 1783 - TFTP Blocksize Option




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( , , , )

2347, 1782 - TFTP Option Extension

2228 FTP Security Extensions

2224 NFS URL Scheme 2204 ODETTE File Transfer Protocol


2090 TFTP Mu ticast Option

2055 WebNFS Server Specification

2054 WebNFS Client Specification




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( , , , )

1986 Experiments with a Simple File Transfer

Protocol for Radio Links using

Enhanced Trivial File Transfer Protocol (ETFTP)

1813 NFS Version 3 Protocol Specification

1785 TFTP Option Negotiation Analysis


1639, 1545 - FTP Operation Over Big AddressRecords (FOOBAR)

1635 How to Use Anonymous FTP

1579 Firewall-Friendly FTP




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( , , , )

1440 SIFT/UFT: Sender-Initiated/UnsolicitedFile Transfer

1415 FTP-FTAM Gateway Specification

1350, 783 - The TFTP Protocol (Revision 2) 1282, 1258 - BSD Rlogin


1094 NFS: Network File System Protocolspecification

1068 Background File Transfer Program (BFTP)

1037 NFILE - a file access protocol




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( )

959, 765, 542, 354, 265, 172, 114 - File TransferProtocol.

949 FTP unique-named store command.

913 Simple File Transfer Protocol. 906 Bootstrap loading using TFTP.


.

743 FTP extension: XRSQ/XRCP. 737 FTP extension: XSEN.




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697 CWD command of FTP 691 One more try on the FTP

686 Leaving well enough alone

683 FTPSRV - Tenex extension for paged files 662 Performance improvement in ARPANET file


640 Revised FTP reply codes 630 FTP error code usage for more reliable mail

service




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624 Comments on the File Transfer Protocol

614 Response to RFC 607: "Comments on the FileTransfer Protocol"

607 Comments on the File Transfer Protocol 571 Tenex FTP problem


532 UCSD-CC Server-FTP facility 520 Memo to FTP group: Proposal for File Access

Protocol




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506 FTP command naming problem

505 Two solutions to a file transfer accessproblem

501 Un-muddling "free file transfer" 487 Freefile transfer


480 Host-dependent FTP parameters 479 Use of FTP by the NIC Journal

478 FTP server-server interaction - II




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468 FTP data compression

463 FTP comments and response to RFC 430

448 Print files in FTP

438 FTP server-server interaction

430 Comments on File Transfer Protocol


418 Server file transfer under TSS/360 atNASA Ames

414 File Transfer Protocol (FTP) status andfurther comments




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412 User FTP Documentation 385 Comments on the File Transfer Protocol

310 Another Look at Data and File Transfer

Protocols 294 The Use of "Set Data Type" Transaction in


281 Suggested addition to File Transfer Protocol 269 Some Experience with File Transfer

264, 171 - The Data Transfer Protocol




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250 Some thoughts on file transfer

242 Data Descriptive Language for Shared Data

238 Comments on DTP and FTP proposals

163 Data transfer protocols

141 Comments on RFC 114: A File Transfer


ro oco

133 File Transfer and Recovery

FTP: Separate Control, Data Connections1FTP: Separate Control, Data Connections1


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FTP client FTP server

TCP control connectionport 21 (persistent)

TCP data connectionport 20(nonpersistent)


c en con ac erver a por , pec y ng

TCP as transport protocol. Client obtains authorization over control connection.

Client browses remote directoryremote directory by sending

commands over control connection. When server receives a command for a file transfer,

server opens a TCP data conn. to client at port 20.

FTP MODEL1FTP MODEL1


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USERUSERUSERINTERFACE

USERPI

SERVERPI FTP REPLIESFTP REPLIES

FTP COMMANDSFTP COMMANDS

PI: Protocol InterpreterDTP: Data Transfer Process


FILESYSTEM

USERDTP

SERVERDTP

FILESYSTEM

DATACONNECTION

Control Connection InitiationControl Connection Initiation


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In the model, the user-protocol interpreter (PI)initiates the control connection.

The control connection follows the Telnet

protocol.

FTP commands are enerated b the user-PI and


transmitted to the server process via the controlconnection.

Standard replies are sent from the server-PI to

the user-PI over the control connection inresponse to the commands.

Data Connection InitiationData Connection Initiation


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The FTP commands specify the parameters forthe data connection (data port, transfer mode, representation type, and

structure) and

the nature of file system operation (store, retrieve,append, delete, etc.).

- " "


port, and the server initiate the data connectionand data transfer in accordance with the specifiedparameters.

Note that the data connection may be used for

simultaneous sending and receiving.

Relationship between FTP and TelnetRelationship between FTP and Telnet


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The FTP uses the Telnet protocol on thecontrol connection.

This can be achieved in two ways: First, the user-PI or the server-PI may

implement the rules of the Telnet Protocol


rec y n e r own proce ures; or,

Second, the user-PI or the server-PI may makeuse of the existing Telnet module in thesystem.

FTP: Separate Control, Data ConnectionsFTP: Separate Control, Data Connections


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After transferring a file, server closes port20.

Server opens a second TCP data connection to

transfer the next file. Control connection: out of band


.

stateless, in-band and a push/pull protocol.FTP is also a push/pull one)

FTP server maintains state: currentdirectory, earlier authentication

FTP Commands, ResponsesFTP Commands, Responses


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Sample commands: sent as ASCII text over

control channel(4 uppercase ASCII

characters) USERusername

PASS password

Sample return codes status code and phrase (as

in HTTP) 331 Username OK,

password required

125 data connection

already open;


LIST re urn s o e n

current directory RETR filename retrieves

(gets) file

STOR filename stores

(puts) file onto remotehost

transfer starting

425 Cant open dataconnection

452 Error writing

file

ftp://ftpserver.iust.ac.ir/dir1/filename.extension



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2.3 FTP




server


2.4 Electronic Mail SMTP, POP3, IMAP,

HTTP

2.5 DNS



P2P file sharing

outgoing

Electronic MailElectronic Mail


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Three major components: user agents mail servers simple mail transfer

protocol: SMTPUser Agent

:

user mailbox

g g

message queue

mailserver

user

agent

mail

server

useragent

P


mail reader

composing, editing, readingmail messages

e.g., Eudora, Outlook, elm,Netscape Messenger

outgoing, incoming messagesstored on server

agent

useragent

mailserver

useragent

useragent

SMT

Electronic Mail: Mail ServersElectronic Mail: Mail Servers


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Mail Servers mailbox contains incoming

messages for user

message queue of outgoing

(to be sent) mail messages SMTP protocol between mail

servers to send email

mailserver

useragent

usera ent

mailserver

useragent

TP


messages

client: sending mailserver (sending agent)

server: receiving mailserver (receiving agent)

useragent

mailserver

user

agentuseragent

S

Electronic Mail: SMTP [RFC 2821]Electronic Mail: SMTP [RFC 2821]


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uses TCP to reliably transfer email message from clientto server, port 25

direct transfer: sending server to receiving server

three phases of transfer

handshaking (greeting)

transfer of messages


closure

command/response interaction commands: ASCII text

response: status code and phrase

messages must be in 7-bit ASCII

1) User1 (sender) uses UA to 4) SMTP client sends User1s

Scenario: User1 sends message to User2Scenario: User1 sends message to User2


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) User (sender) uses UA tocompose message [email protected] .

2) User1s UA sends messageto his mail server; message

placed in message queue.3) Client side of SMTP opens

TCP connection with

)message over the TCPconnection.

5) User2s mail server placesthe message in User2s

mailbox.6) User2 invokes his/her user

agent to read message.

U

A:UserAgent


User2s mail server.

useragent

mail

server

mailserver user

agent

1

23 4 5

6User1

User2

[email protected]

[email protected]

webmail.iust.ac.ir

yahoo.com

Try SMTP Interaction for Yourself:Try SMTP Interaction for Yourself:


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telnet servername 25

see 220 reply from server

enter HELO, MAIL FROM, RCPT TO, DATA, QUITcommands above lets you send email without using


ema c en rea er

SMTP: Final WordsSMTP: Final Words


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SMTP uses persistentconnections.

SMTP requiresmessage (header &

-

Comparison with HTTP:

HTTP: pull

SMTP: push

both have ASCII


ASCII SMTP server usesCRLF.CRLF todetermine end of

message

comman response

interaction, status codes. HTTP: each object

encapsulated in its ownresponse message.

SMTP: multiple objectssent in multipart message.

Mail Message FormatMail Message Format


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SMTP: protocol forexchanging email msgs

RFC 822: standard for textmessage format:

header lines, e.g.,

headerblank

line


o:

From: Subject:

differentfrom SMTPcommands!

body the message, ASCII

characters only

bodybody

..

SMTP Message Transfer Between Mail ServersSMTP Message Transfer Between Mail Servers

sms = sending mail server


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sms rms: TCP connection establishment on port 25

sms rms: SMTP handshake (HELO, MAIL, FROM, RCPT TO, DATA,commands, and 220, 250, 354, 221 status codes)

sms header

g

rms = receiving mail server


Mail

Massage

body.

sms QUIT

rms 221 yahoo.com closing connection

ExampleExample


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mailserver

mailserver user

agent13 4 6

User1

User2webmail.iust.ac.ir


[email protected]

[email protected]

yahoo.com

sms: TCP connection request on port 25

Example: SMTP InteractionExample: SMTP Interaction


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rms: 220 yahoo.com accepts connectionsms: HELO webmail.iust.ac.ir is readyrms: 250 Hello webmail.iust.ac.irsms: MAIL FROM:

rms: 250 [email protected]... Sender oksms: RCPT TO: rms: 250 user2@ ahoo.com ... Reci ient ok


Mail

Massage

from

user1user1 to

user2user2

sms: DATA

rms: 354 Enter mail, end with "." on a line by itselfsms: From: [email protected]: To: [email protected].

sms: .rms: 250 Message accepted for deliverysms: QUITrms: 221 yahoo.com closing connection

Message Format: Multimedia ExtensionsMessage Format: Multimedia Extensions


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MIME: multimedia mail extension, RFC 2045, 2056 additional lines in message header declare MIME content

type

From: [email protected]

To: [email protected]: Picture of iust

cc: [email protected]

-

MIME version


Content-Transfer-Encoding: base64

Content-Type: image/jpeg

base64 encoded data .....

.........................

......base64 encoded data

.

to encode data

multimedia datatype, subtype,

parameter declaration

encoded data

MIMEMIME (RFCs(RFCs20452045,, 20462046,, 20772077))TypesTypesContentContent--Type: type/subtype; parametersType: type/subtype; parameters


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Text example subtypes:plain,

html

Image example subtypes: jpeg,

Video example subtypes:mpeg,

quicktime

Application


Audio example subtypes:basic

(8-bit mu-law encoded),32kadpcm(32 kbpscoding)

processed by readerbefore viewable

example subtypes:msword, octet-stream

From: [email protected]

T 2@ h

Multipart TypeMultipart Type


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To: [email protected]: photo of iust.

MIME-Version: 1.0

Content-Type: multipart/mixed;

boundary=StartOfNextPart

--StartOfNextPart

Dear User1, Hear is a photo of iust in 1383 winter.


-- ar ex ar

Content-Transfer-Encoding: base64

Content-Type: image/jpeg

base64 encoded data .....

.........................

......base64 encoded data

--StartOfNextPart

Do you like to receive more pictures?

.

SMTP SMTP

Mail Access ProtocolsMail Access Protocols


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useragent

senders mailserver

useragent

SMTP SMTP accessprotocol

receivers mailserver

Sender Receiver


Mail access protocol: retrieval from server

POP: Post Office Protocol [RFC 1939] authorization (agent server) and download

IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex) manipulation of stored messages on server

HTTP: Hotmail , Yahoo! Mail, IUST, etc.

POP3POP3--Post Office ProtocolPost Office Protocol -- Version 3Version 3


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RFC 1939 / Std 53

To retrieve emails from server (POP3 server) Server port number: 110


Three states Authorization State Transaction State

Update State


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S: +OK POP3 server ready

C: user USER1

POP3 ProtocolPOP3 Protocol


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authorization phase client commands:

user: declare usernamepass: password

server responses +OK

C: listS: 1 498

S: 2 912

C: user USER1S: +OK

C: pass zxcdvf

S: +OK user successfully logged on


-ERR

transaction phase, client: list: list message numbers retr: retrieve message by

number dele: delete quit

.

C: retr 1

S: S: .

C: dele 1

C: retr 2

S:

S: .C: dele 2

C: quit

S: +OK POP3 server signing off

More about POP3 IMAP

POP3 (more) and IMAPPOP3 (more) and IMAP


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More about POP3 Previous example uses

download and delete mode. Download brings the mail

into client computer. Delete removes the mail

from the mail server. User1cannot re-read e-mail if

IMAP Keep all messages in

one place: the server

Allows user toorganize messages infolders


computer.

Download-and-keep: leavesthe message on the mailserver.

POP3 is stateless acrosssessions.

IMAP keeps user stateacross sessions: names of folders and

mappings between

message IDs and foldername

2 1 Principles of app 2 6 Socket programming



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.

2.4 Electronic Mail SMTP, POP3, IMAP,

HTTP

2.5 DNS



P2P file sharing

DNS RFCs1DNS RFCs1

2673 Binary Labels in the Domain Name System


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2673 Binary Labels in the Domain Name System. 2672 Non-Terminal DNS Name Redirection.

2671 Extension Mechanisms for DNS (EDNS0).

2606 Reserved Top Level DNS Names.

2541 DNS Securit O erational Considerations.


2540 Detached Domain Name System (DNS)Information.

2539 Storage of Diffie-Hellman Keys in the

Domain Name System (DNS).

DNS RFCs2DNS RFCs2

2535 Domain Name System Security Extensions


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2535 Domain Name System Security Extensions. 2517 Building Directories from DNS: Experiences from

WWWSeeker.

2352, 2240 - A Convention For Using Legal Names as Domain

Names.

2317 Classless IN-ADDR.ARPA delegation.


2308 Negative Caching of DNS Queries (DNS. NCACHE)

2230 Key Exchange Delegation Record for the DNS. 2219 Use of DNS Aliases for Network Services.

2182 Selection and Operation of Secondary DNS Servers.

DNS RFCs3DNS RFCs3

2181 Clarifications to the DNS Specification


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2181 Clarifications to the DNS Specification. 2137 Secure Domain Name System Dynamic Update.

2136 Dynamic Updates in the Domain Name System (DNS

UPDATE).

2065 Domain Name System Security Extensions.

2052 A DNS RR for specifying the location of services


(DNS SRV).

2010 Operational Criteria for Root Name Servers.

1996 A Mechanism for Prompt Notification of Zone

Changes (DNS NOTIFY).

1995 Incremental Zone Transfer in DNS.

DNS RFCs4DNS RFCs4

1982 Serial Number Arithmetic


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1982 Serial Number Arithmetic.

1912, 1537 - Common DNS Operational andConfiguration Errors.

1876 A Means for Expressing LocationInformation in the Domain Name System.


uppor or oa a anc ng.

1713 Tools for DNS debugging. 1712 DNS Encoding of Geographical Location.

1706, 1637, 1348 - DNS NSAP Resource Records.

1591 Domain Name System Structure andDelegation.

DNS RFCs5DNS RFCs5

1536 Common DNS Implementation Errors andS d Fi


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1536 Common DNS Implementation Errors andSuggested Fixes. 1535 A Security Problem and Proposed Correction

With Widely Deployed DNS Software 1480, 1386 - The US Domain. 1464 Using the Domain Name System To Store


r rary r ng r u e .

1394 Relationship of Telex Answerback Codes toInternet Domains. 1183 New DNS RR Definitions. 1101 DNS encoding of network names and other

types. 1035 Domain names - implementation and

specification.

DNS RFCs6DNS RFCs6


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1034 Domain names - concepts and facilities

1033 Domain administrators operations guide

1032 Domain administrators guide 1031 MILNET name domain transition


973 Domain system changes and observations

953, 811 - Hostname Server 921, 897 - Domain name system implementation

schedule - revised

920 Domain requirements

DNS RFCs7DNS RFCs7

883 Domain names: Implementation specification


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883 Domain names Implementation specification 882 Domain names: Concepts and facilities

881 Domain names plan and schedule

830 Distributed system for Internet nameservice


applications

799 Internet name domains

756 NIC name server - a datagram-basedinformation utility

752 Universal host table

People: many identifiers: Domain Name System:

DNS: Domain Name SystemDNS: Domain Name SystemRFC1034RFC1034


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People many dent f ers SSN, name, passport #

Internet hosts, routers:

IP address (32 bit) -used for addressingdatagrams

Doma n Name System distributed database

implemented in hierarchy ofmany name servers

application-layer protocolhost, routers, name servers to


name, e.g.,

www.iust.ac.ir - used byhumans

Q: map between IPaddresses and name ?

(address/name translation)

note: core Internetfunction, implemented asapplication-layer protocol

complexity at networks

edge

Why DNSWhy DNS

Although DNS is most commonly associated withth I t t i t t k l DNS


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g ythe Internet, private networks also use DNSbecause of the following benefits:

Convenience: User-friendly names are easier for peopleto remember than numerical IP addresses.


Consistency: IP a resses may c ange, ut t e server

names can remain constant. Simplicity: Users need to learn only one naming

convention to find resources on either the Internet or onan intranet.

DNS: Domain Name SystemDNS: Domain Name System

The Domain Name System is a distributed


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ydatabase system that can serve as the foundationfor name resolution in a TCP/IP network.

DNS is used by most internetworking software,


programs, to locate servers and to resolve, or map,a user-friendly name of a computer to its IPaddress.

DNS ComponentsDNS Components

To understand how DNS maps names to IPaddresses you will need to understand the


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paddresses, you will need to understand thefollowing five components:

1. Domain Name Space Name Structure,

2. Zone


Zone data base file,

3. Name Server (DNS Server) DNS Root Servers,

4. DNS Resolver,

5. Protocol Forward Lookup Query

Caching.

1. Domain Name Space1. Domain Name Space

The domain name space has a hierarchicalstructure


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pstructure. The root domain is at the top, and it is

represented by a period. Below the root domain, the top-level, or 1st-level

domains can be an organizational type, such as com


., , .IRAN.

Second-level domains are registered to individualsor organizations, such as microsoft.com, theMicrosoft Corporation domain.

Second-level domains can have many sub domains.and any domain can have hosts.

1. Domain Name Space1. Domain Name Space

.

Root Domain


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Top Level or First Level Domain

org. edu. com. ir.

Second Level Domain (Registered to Individuals or Organizations)ac.ir.

me,orF

QDN


iust.ac.ir.

ce.iust.ac.ir.

Any domain can have hosts.

A host is a specific computer within a domain.

Iran University of Science & Technology s Domain: iust.ac.ir.

host

subdomain 2nd-leveldomain

top-leveldomain

webmail . iust . ac . ir .

root

fully

qualifieddom

ainn

1. Domain Name Space: Name Structure1. Domain Name Space: Name Structure

d

Max: 63 Characters Root


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Host name . . Subdomain-name . 2nd-level name. 1st-level-name .

Max: 255 Characters

ICANN: Internet Corporation For Assigned Names


.

Levelm-name authority appoints levelm-1-names.

Example: ir. is controlled by ) ( iust.ir. is appointed by ) ( and controlled by Iran

University of science and Technology.

Names are ValuableNames are Valuable


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new


1. Domain Name Space: Top Levels1. Domain Name Space: Top Levels

com: commercial edu: educational institute

ir: Iran nl: Netherlands


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u ucat ona nst tut ac: Academic, research institute int: international organization gov: US federal goverment org: US armed forces

nl: Netherlands

jp: Japan

uk: England


net: network providers

biz: business name: peoples name pro: professions

tv: TV stations and networks

For the most upFor the most up--toto--datedate

information about new top levelinformation about new top leveldomains, consult:domains, consult:http://www.icann.org/tldshttp://www.icann.org/tlds

For administrative purposes,domains can be organized

2. Zones2. Zonesir.

Name

NameServer

Zone Database File


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domains can be organized

into zones.

As you can see in this example,

a zone is a discrete and

contiguous area of the

ac.ir.NameServer

Name

ZoneDatabase File

ZoneDatabase File


oma n name space.

One reason to divide a name space into zones is todelegate authority for different portions of it.

One very large domain could be difficult to administer.

The name-to-IP address data for computers located in a

zone, is stored in a zone database file on a DNS nameserver.

A zone database storing resource records (RR) of the zonefor which a server is authoritative.

2. Zone Database File2. Zone Database File


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Type=A name is hostname

Type=CNAME name is alias name for some

RR format: (Name, TTL, Class, Type, Value)


Type=NS name is domain (e.g. iust.ac.ir) value is IP address of authoritative

name server for this domain

value is IP address

canonical (the real) name

www.ibm.com is reallyservereast.backup2.ibm.com

value is canonical name

Type=MX value is name of mail-server

associated with name

2. Zone Database File: Type Description2. Zone Database File: Type Description

Example ResourceRecord FormatDescriptionTypeNumber

1D N 10 1 1 1N k dd1


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www 1D IN A 10.1.1.1Network addressA1

@ 1D IN NS na.av.comAuthoritative nameserver

NS2

Mail destination; now replaced by MXMD3

Mail forwarder; now replaced by MXMF4


anon ca a as name

Start of zone authoritySOA6Mailbox domain nameMB7

Mailbox memberMG8

Mail rename domainMR9

Null resource recordNULL10Well-Known serviceWKS11


2. Zone Database File: Type Description2. Zone Database File: Type Description

Example Record FormatDescriptionTypeNumberPointer to a domain namePTR12


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www 1D IN HINFO SolarisHost informationHINFO13

Mailbox informationMINFO14

Mail exchangeMX15

System 1 IN TXT This is a

Text stringsTXT16


new sys em

Responsible personRP17

AFS-type servicesAFSDB18

X.25 addressX.2519

ISDN addressISDN20

Route throughRT21



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A DNS server is aWhy not centralize DNS? single point of failure

3. DNS Name Servers3. DNS Name Servers


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A DNS server is acomputer that runs aDNS server program,such as BerkeleyInternet NAME Domain

single point of failure traffic volume

distant centralizeddatabase


.

DNS Servers containDNS database files. They resolve name

resolution queries issued

by DNS clients.

doesnt scale!

no server has all name-to-IP address mappings.

does scale!

Distributed, Hierarchical DatabaseDistributed, Hierarchical Database

Root DNS Servers


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org DNS Servers edu DNS Servers com DNS Servers ir DNS Servers

ac.ir DNS


Servers

iust.ac.ir

DNS Servers

3. DNS Name Servers3. DNS Name Servers

One name server holds the

primary zone database file

ir.NameServer

NameServer

Primary ZoneDatabase File


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primary zone database file

for a zone.

That name server is said to"have authority" for making

chan es to the zone file.

ac.ir.Secondary ZoneDatabase File


A copy of the zone file may

be stored on other servers to balance network load. These copies are referred to as secondary zone database

files.

The secondary zone database files are updated regularly

from the primary zone database file by a process called zonetransfer.

Zone Owners ResponsibilitiesZone Owners Responsibilities

Authoritatively maintain the zones data


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Authoritatively maintain the zone s data.

Arrange for replicated name servers for the zone

Typically, zone data is maintained in a primary file andloaded into a primary server

Re licated servers use TCP-based zone trans ers


specified in DNS protocol to refresh their data.

Therefore, either servers can answer name resolutionqueries for that zone.

A name server authoritative for a zone does nothave to be in that zone.

A name server can handle any number of zones,which dont have to be contiguous.

contacted by local name server that can not resolve name root name server:

Either, contacts authoritative name server gets mapping

3. DNS Root Name Servers3. DNS Root Name Servers


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(i)(i) NORDUnet Stockholm, Sweden

(k)(k) RIPE London, UK

(a)(a) NSI Herndon, VA(c)(c) PSInet Herndon, VA

(d)(d) U Maryland College Park, MD

g pp g returns mapping to local name server

Or, introduces authoritative name server to local name server.


(b)(b) USC-ISI Marina del Rey, CA(l)(l) ICANN Marina del Rey, CA

(e)(e) NASA Mt View, CA(f)(f) Internet Software C. Palo Alto, CA

(m)(m) WIDE Tokyo, Japan

gg enna,(h)(h) ARL Aberdeen, MD

(j)(j) NSI (TBD) Herndon, VA

13 root name serversworldwide

13 Root Name Servers13 Root Name Servers

F gets 270,000,000+ hits per day.


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p y Other root servers are comparable.

Top Level Domain servers answer 5,000,000,000 per day.



Root ServersServer Operator Locations IP Addr HomeASN


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A VeriSign Global RegistryServices Dulles VA 198.41.0.4 19836

B

Information Sciences

InstituteMarina Del Rey CA

IPv4: 192.228.79.201IPv6:2001:478:65::53

tba

C Cogent Communications Herndon VA; Los Angeles;New York City; Chicago 192.33.4.12 2149


n vers y o ary an o ege ar . . .

E NASA Ames Research Center Mountain View CA 192.203.230.10 297

F Internet Systems Consortium,Inc.

Ottawa; Palo Alto; San Jose CA;New York City; San Francisco;Madrid; Hong Kong; Los Angeles;Rome; Auckland; Sao Paulo;Beijing; Seoul; Moscow; Taipei;Dubai; Paris; Singapore; Brisbane;Toronto; Monterrey; Lisbon;Johannesburg;Tel Aviv;Jakarta;

Munich;

IPv4: 192.5.5.241IPv6:2001:500::1035

3557

G U.S. DOD NetworkInformation Center Vienna VA 192.112.36.4 568


Root ServersServer Operator Locations IP Addr

HomeASN


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H U.S. Army Research Lab Aberdeen MD IPv4: 128.63.2.53IPv6:2001:500:1::803f:235 13

I Autonomica/NORDUnet

Stockholm; Helsinki; Milan;London; Geneva; Amsterdam;Oslo; Bangkok; Hong Kong;Brussels; Frankfurt;Bucharest; Ankara;Chicago; Washington DC;Tok o Kuala Lum ur

192.36.148.17 29216


J VeriSign Global RegistryServices

Dulles VA (2 locations); MountainView CA;Seattle WA; Amsterdam;Atlanta GA; Los Angeles CA;Miami; Stockholm; London;Tokyo; Seoul; Singapore;Sterling VA (2 locations,standby)

192.58.128.30 26415

KReseaux IP Europeens -

Network CoordinationCentre

London (UK); Amsterdam (NL);Frankfurt (DE); Athens (GR);Doha (QA); Milan (IT)

IPv4: 193.0.14.129IPv6: 2001:7fd::1

25152

LInternet Corporation for

Assigned Names andNumbers

Los Angeles 198.32.64.12 20144

M WIDE Project Tokyo; Seoul (KR); Paris (FR) 202.12.27.33IPv6: 2001:dc3::35 7500


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4. Resolver4. Resolver

A DNS resolver is a service that uses the DNS


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protocol to query for information from DNS

servers.


DNS client service.

DNS service uses UDP in transport layer.

Root name server: t k

root name server

23

6

7

5. DNS Protocol: Forward Lookup Query5. DNS Protocol: Forward Lookup Query

a:answer

r:referral

q:query

qa


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may not knowauthoritative name

server may know

3

7q:query q

q a

a

a

a

q


server:who to

contact to findauthoritative nameserver

requesting host

www.icann.org

oca name serverdns.iust.ac.ir

1 4 5

authoritative name serverdns.icann.org

n erme a e name server(Top level) dns.org

8

recursive query: puts burden of name

root name server

2iterated query


a:answerr:referral q


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puts burden of nameresolution on

contacted nameserver

hea load

23

4

7

q yq:query

qa

r


iterated query: contacted serverreplies with name ofserver to contact

I dont know thisname, but ask thisserver

requesting host

www.icann.org

local name server

dns.iust.ac.ir

15 6

authoritative name server

dns.icann.org

intermediate name server(Top Level) dns.org

8


root name server

2 iterated query

a:answer

r:referral

q:query

q


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3

4

5

intermediate name server

7

q:query

a

rq

qr


requesting host

www.icann.org

local name serverdns.iust.ac.ir

1 6

authoritative name serverdns.icann.org

op eve dns.org

8


.

RootName ServerAuthorized nameserver for org. zone

IP Address of www.icann.org is

142.12.01.23


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org. edu. com. ir.


The query contains the FQDN of the requested computer. A forward lookup query is a request

to map a name to an IP address.

ac.ir.

iust.ac.ir.LocalName Server

Client

www.icann.org.

Forward Lookup Query:

What is IP Address of www.icann.org

DNS Resolver and Local DNS ServerDNS Resolver and Local DNS Server

Root server

3


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Application DNS cache

4

Top-level

5


DNS resolver

Local DNS

server

1 10

2

DNS response 9

domain server6

Second-level

domain server

7

8

Caching based on a time-to-live (TTL) assigned by the DNS serverresponsible for the host name to reduce latency in DNS translation.

5. DNS Protocol: Caching and Updating Records5. DNS Protocol: Caching and Updating Records


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once (any) name server learns mapping, it cachesmapping

cache entries timeout (disappear) after some time Top Level Domain servers typically cached in local name


erver . u roo name erver no o en v e .

update/notify mechanisms under design by IETF

RFC 2136

http://www.ietf.org/html.charters/dnsind-charter.html

DNS protocol : queryand replymessages, both with samemessage format

5. DNS Protocol Messages5. DNS Protocol Messages

identification flags


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msg header identification: 16 bit #for uer , re l to uer

g

number of questions number of answer RRs

number of authority RRs number of additional RRs

questions

12byte

s


uses same #

flags: query or reply

recursion desired

recursion available

reply is authoritative

var a e num er o ques ons

answers

(variable number of resource records)

authority


additional information


identification flags

number of questions number of answer RRsName, Type fields

5. DNS Protocol Messages5. DNS Protocol Messages

12byt


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number of authority RRs number of additional RRs

questions(variable number of questions)

, ypfor a query

RRs in responseto uer

tes



authority(variable number of resource records)

additional information


records forauthoritative servers

additional helpfulinfo that may be used

Inserting records into DNSInserting records into DNS

Example: just created startup Network Utopia

Register name networkuptopia com at a registrar


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Register name networkuptopia.com at a registrar

(e.g., Network Solutions) Need to provide registrar with names and IP addresses of your

authoritative name server (primary and secondary)


eg s rar nser s wo s n o e com server:

(networkutopia.com, dns1.networkutopia.com, NS)

(dns1.networkutopia.com, 212.212.212.1, A)

Put in authoritative server Type A record forwww.networkuptopia.com and Type MX record fornetworkutopia.com

How do people get the IP address of your Web site?

Example: Set Debug CommandExample: Set Debug Command

> set debug

> www.goole.comServer: jupitr.iust.ac.irAddress: 194.225.228.4-------------------------------

Got answer:

AUTHORITY RECORDS:

-> 1.google.comnameserver = b.1.google.comttl = 37098


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Got answer:HEADER:

opcode = QUERY, id = 11, rcode =NOERROR

header flags: response, want recursion,recursion avail.questions = 1, answers = 4, authorityrecords = 6, additional = 0

ttl = 37098 -> 1.google.com

nameserver = c.1.google.com

ttl = 37098 -> 1.google.com

nameserver = d.1.google.comttl = 37098


QUESTIONS:www.google.com, type = A, class = IN

ANSEWRS:

-> www.google.comcannonical name = www.1.google.comttl = 900

-> www.1.google.cominternet address = 66.102.9.147ttl = 176

-> www.1.google.com

internet address = 66.102.9.99ttl = 176 -> www.1.google.com

internet address = 66.102.9.104ttl = 176

-> .goog e.comnameserver = e.1.google.comttl = 37098

-> 1.google.comnameserver = g.1.google.comttl = 37098

-> 1.google.comnameserver = a.1.google.comttl = 37098

--------------------Non-authoritative answer:

Name: www.1.google.comAddresses: 66.102.9.147, 66.102.9.99, 66.102.9.104Aliases: www.google.com


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Using Ports to Identify ServicesUsing Ports to Identify Services

Web server(port 80)

Client hostServer host 128.2.194.242Service request for

128.2.194.242:80(i th W b s )


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(port 80)

Echo server(port 7)

(i.e., the Web server)OSClient


Web server(port 80)

Echo server(port 7)

Service request for128.2.194.242:7

(i.e., the echo server)OS

Client

Client host

Network ApplicationNetwork Application--CommunicationCommunication

Network Applications communicates through a network. API is a programming means, either a library or part ofOS.

API provides the communication functionality for the


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API provides the communication functionality for theNetwork Applications (Processes).


Application Program Interface (API)

Communication Software & HardwarePlatform (OS + Hardware)

pp ca on o ware(Network Application)

OSOS

API comes as a library

API TechnologiesAPI Technologies--SocketsSockets

Sockets (earliest technology) Low level functions for the communication

S k t


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Socket,

ServerSocket, ConnectionSocket,


,

DatagramSocket,

Send, Read,

Write,

Close, Accept


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API TechnologiesAPI Technologies--ComponentsComponents

Components: Components Look and act likebj t


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objects, CORBA (common object request Broker

Architecture)


COM,

COM+ , DCOM (Distributed Component Object Model)

k P

Goal:Goal: learn how to build client/server application thatlearn how to build client/server application thatcommunicate using socketscommunicate using sockets

Socket ProgrammingSocket Programming


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Socket API introduced in BSD4.1UNIX, 1981 ex licitl created used

Socket a host-local, application-

created, OS-controlled


released by apps

client/server paradigm two types of transport

service via socket API: unreliable datagram reliable, byte stream-

oriented

n er ace

a door into which

application process can both send and

receive

messages to/from another

application process


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Client must contact server server process must first

be runningh d

When contacted by client,server TCP creates newsocket for server process to

Socket ProgrammingSocket Programming with TCPwith TCP


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server must have created

socket (door) thatwelcomes clients contact

communicate with client

allows server to talk withmultiple clients

jamali@ iust.ac.irjamali@ iust.ac.ir App

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