Application 1 - Networking

8/12/2019 Application 1 - Networking

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2: Application Layer 1

CMPT 371

Data Communicationsand Networking

Chapter 2Application Layer


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Chapter 2: Application Layer

Our goals: conceptual,

implementationaspects of network

application protocols transport-layer

service models

client-server

paradigm peer-to-peer

paradigm

learn about protocolsby examining popularapplication-levelprotocols

HTTP FTP

SMTP / POP3 / IMAP

DNS

programming networkapplications socket API


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Chapter 2 outline

2.1 Principles of app layer protocols

2.2 Web and HTTP

2.3 FTP

2.4 Electronic Mail SMTP, POP3, IMAP

2.5 DNS

2.6 Content distribution

Network Web caching Content distribution networks

P2P file sharing


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Network applications

Applications -> Software

What exactly running in a computer ?


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Network applications: some jargon

Process:program runningwithin a host.

implements user

interface &application-levelprotocol Web: browser

E-mail: mail reader streaming audio/video:

media player

within same host, twoprocesses communicateusing interprocesscommunication(defined

by OS). processes running in

different hostscommunicate with an

application-layerprotocol


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Applications and application-layer protocols

Application: 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 messagesexchanged by apps andactions taken

use communication servicesprovided by lower layer

protocols (TCP, UDP)

applicationtransportnetworkdata linkphysical

applicationtransport

networkdata linkphysical

application

transportnetworkdata linkphysical


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App-layer protocol defines

Types of messagesexchanged, e.g., request& response messages

Syntax of message

types: what fields inmessages & how fieldsare delineated

Semantics of the fields,i.e., meaning ofinformation in fields

Rules for when and howprocesses send &respond to messages

Public-domain protocols:

defined in RFCs

allows for

interoperability eg, HTTP, SMTP

Proprietary protocols:

eg, Skype

BitTorrent (?)


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Client-server paradigm

Typical network app has twopieces: clientand serverapplicationtransportnetworkdata linkphysical

applicationtransport

networkdata linkphysical

Client: initiates contact with server

(speaks first) typically requests service from

server,

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

request

reply

Server: provides requested service to client

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


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Programming - Socket

process sends/receivesmessages to/from itssocket

socket analogous to door sending process shoves

message out door

sending process assumestransport infrastructureon other side of door whichbrings message to socketat receiving process

process

TCP/UDP

with

buffers,

variables

socket

host orserver

process

TCP/UDP

with

buffers,

variables

socket

host orserver

Internet

controlledby OS

controlled by

app developer

API: Application Programmers Interface

(1) choose transport protocol; (2) set parameters (You might havelearnt it; or you can study it in 471)


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Outside of the door Transport Layer Service

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

tolerate some loss

other apps (e.g., file transfer,

telnet) require 100% reliabledata transfer

Timing some apps (e.g.,

Internet telephony,interactive games)require low delay to beeffective

Bandwidth some apps (e.g.,

multimedia) requireminimum amount of

bandwidth to beeffective

other apps (elasticapps) make use of

whatever bandwidththey get


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Transport service requirements of common apps

Application

file transfer

e-mail

Web documentsreal-time audio/video

stored audio/video

interactive games

instant messaging

Data loss

no loss

no loss

no lossloss-tolerant

loss-tolerant

loss-tolerant

no loss

Bandwidth

elastic

elastic

elasticaudio: 5kbps-1Mbps

video:10kbps-5Mbps

same as above

few kbps up

elastic

Time Sensitive

no

no

noyes, 100s msec

yes, few secs

yes, 100s msec

yes and no


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Internet apps: application, transport protocols

Application

e-mail

remote terminal access

Webfile transfer

streaming multimedia

Internet telephony

Applicationlayer protocol

SMTP [RFC 2821]

Telnet [RFC 854]

HTTP [RFC 2616]FTP [RFC 959]

proprietary

(e.g. RealNetworks)

proprietary

(e.g., Dialpad)

Underlyingtransport protocol

TCP

TCP

TCPTCP

TCP or UDP

typically UDP


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Chapter 2 outline


2.2 Web and HTTP

2.3 FTP


2.5 DNS



P2P file sharing


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Web and HTTP

First some jargon

Web pageconsists of objects

Object can be HTML file, JPEG image, Java

applet, audio file, Most Web sites have a base HTML-filewhich

includes several referenced objects

Each object is addressable by a URL

Example URL:http://www.cs.sfu.ca/~jcliu/cmpt371/index.htm

host name path name


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HTTP overview

HTTP: hypertexttransfer protocol

Webs application layerprotocol

client/server modelclient:browser that

requests, receives,displays Web objects

server:Web server

sends objects inresponse to requests

HTTP 1.0: RFC 1945

HTTP 1.1: RFC 2068

PC runningExplorer

Serverrunning

Apache Web

server

Mac runningNavigator


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Is it difficult to write a Browser ?

MS Internet ExplorerSafari

Opera http://www.opera.com/ (15.3MB)

Mozilla http://www.mozilla.org/ (11.7MB)FireFox http://www.mozilla.com/en-US/firefox/

Chrome
http://www.opera.com/http://www.mozilla.org/http://www.mozilla.com/en-US/firefox/http://www.mozilla.com/en-US/firefox/http://www.mozilla.com/en-US/firefox/http://www.mozilla.com/en-US/firefox/http://www.mozilla.org/http://www.opera.com/


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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 www.cs.sfu.ca

Anything typed in sentto port 80

telnet www.cs.sfu.ca 80

2. Type in a GET HTTP request:GET

or

GET /~jcliu/index.htm

You have sent this minimal (but complete)GET request to HTTP server

And you have received HTML objects !


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Is it difficult to write a Browser ?

Implement HTTP - network

Implement a GUI - local

Anymore ?

Internet ExplorerChrome

FireFox

Efficiency, fault-tolerant, compatibility, security, Java-support, multi-language


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HTTP overview (continued)

Uses TCP: client initiates TCP

connection (creates socket)to server, port 80

server accepts TCPconnection from client

HTTP messages (application-layer protocol messages)exchanged between browser

(HTTP client) and Webserver (HTTP server)

TCP connection closed

HTTP is stateless server maintains no

information aboutpast client requests

Protocols that maintainstate are complex!

past history (state) mustbe maintained

if server/client crashes,their views of state maybe inconsistent, must bereconciled

aside


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HTTP request message

two types of HTTP messages: request, response

HTTP request message: ASCII (human-readable format)

GET /somedir/page.html HTTP/1.1

Host: www.someschool.edu

User-agent: Mozilla/4.0

Connection: 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


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HTTP request message: general format


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Uploading form input

Post method:

Web page oftenincludes form input

Input is uploaded toserver in entity body

URL method:

Uses GET method Input is uploaded in

URL field of requestline:

www.somesite.com/animalsearch?monkeys&banana


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Method types

HTTP/1.0

GET

POST

HEAD asks server to leave

requested object out ofresponse

debugging

HTTP/1.1

GET, POST, HEAD

PUT

uploads file in entitybody to path specifiedin URL field

DELETE deletes file specified in

the URL field


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HTTP response message

HTTP/1.1 200 OK

Connection close

Date: 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)

headerlines

data, e.g.,

requestedHTML file


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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 server404 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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HTTP connections

Nonpersistent HTTP

At most one object issent over a TCPconnection.

HTTP/1.0 usesnonpersistent HTTP

Persistent HTTP

Multiple objects canbe sent over singleTCP connectionbetween client andserver.

HTTP/1.1 usespersistent connections

in default mode


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Nonpersistent HTTPSuppose user enters URLwww.someSchool.edu/someDepartment/home.index

1a.HTTP client initiates TCPconnection to HTTP server(process) atwww.someSchool.edu on port

80

2.HTTPclient sends HTTPrequest message(containingURL) into TCP connection

socket. Message indicatesthat client wants objectsomeDepartment/home.index

1b.HTTPserver at hostwww.someSchool.edu waiting

for TCP connection at port 80.accepts connection, notifyingclient

3.HTTPserver receives request

message, forms responsemessagecontaining requestedobject, and sends messageinto its socket

time

(contains text,

references to 10

jpeg images)


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Nonpersistent HTTP (cont.)

5.HTTP client receives responsemessage containing html file,displays html. Parsing htmlfile, finds 10 referenced jpegobjects

6.Steps 1-5 repeated for eachof 10 jpeg objects

4.HTTPserver closes TCPconnection.

time


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Response time modeling

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

time to

transmit

file

initiate TCP

connection

RTT

request

file

RTT

file

received

time time

Response time: one RTT to initiate TCP

connection

one RTT for HTTP

request and first fewbytes of HTTP responseto return

file transmission time

total = 2RTT+transmit time


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Nonpersistent HTTP issues

requires at least 2 RTTs per object so browsers often open parallel TCP connections

to fetch referenced objects

but OS must work and allocate host resources

for each TCP connection


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Persistent HTTP

server leaves connection open after sending

response subsequent HTTP messages between same

client/server are sent over connection

Persistent without pipelining:

client issues new request only when previous responsehas 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


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User-server interaction: authorization

Authorization :control access toserver content

authorization credentials:typically name, password

stateless:client must present

authorization in eachrequest authorization:header line in

each request

if no authorization:header,server refuses access,

sendsWWW authenticate:

header line in response

client serverusual http request msg

401: authorization req.WWW authenticate:

usual http request msg+ Authorization:

usual http response msg

usual http request msg+ Authorization:

usual http response msg time


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Cookies: keeping state

Many Web sites usecookies

Example: You access Internet

always from same PC

You visit a specific e-

commerce site for firsttime

When initial HTTPrequests arrives at site,site creates a unique ID

and creates an entry inbackend database forID


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Cookies: keeping state

Many major Web sitesuse cookies

Four components:1) cookie header line in

the HTTP responsemessage

2) cookie header line inHTTP request message

3) cookie file kept onusers host and managedby users browser

4) back-end database atWeb site


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Cookies: keeping state (cont.)

client serverusual http request msg

usual http response +Set-cookie: 1678

usual http request msgcookie: 1678


usual http request msgcookie: 1678


cookie-specificaction

cookie-spectific

action

servercreates ID

1678 for user

Cookie file

amazon: 1678

ebay: 8734

Cookie file

ebay: 8734

Cookie file

amazon: 1678

ebay: 8734

one week later:


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Cookies (continued)

What cookies can bring: authorization

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 & cookiesto learn yet more

advertising companiesobtain info acrosssites

aside


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Cookies: Enable/Disable


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Cookies: Delelte


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Chapter 2 outline


2.2 Web and HTTP

2.3 FTP


2.5 DNS



P2P file sharing


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FTP: the file transfer protocol

transfer file to/from remote host

client/server model

client:side that initiates transfer (either to/from

remote)server:remote host

ftp: RFC 959

ftp server: port 21

file transferFTP

server

FTPuser

interface

FTPclient

local file

system

remote file

system

userat host


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FTP commands, responses

Sample commands: sent as ASCII text over

control channel

USER username

PASS password LISTreturn list of file in

current directory

RETR filenameretrieves

(gets) file STOR filenamestores

(puts) file onto remotehost

Sample return codes status code and phrase (as

in HTTP) 331 Username OK,

password required 125 data connection

already open;

transfer starting

425 Cant open data

connection 452 Error writing

file


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FTP Client Software


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Port 21 ?


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Secure FTP (http://www.ssh.com/)

ssh never trusts the net; somebodyhostile who has taken over the networkcan only force ssh to disconnect, butcannot decrypted or play back thetraffic, or hijack the connection.


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Key differences between HTTP & FTP

Stateless vs. stateful

Inband control vs. outband control

Why ?


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Chapter 2 outline


2.2 Web and HTTP

2.3 FTP


2.5 DNS



P2P file sharing


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Electronic Mail

Three major components: user agents

mail servers

mail transfer protocol

User Agent

a.k.a. mail reader

composing, editing, readingmail messages

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

outgoing, incoming messagesstored on server

user mailbox

outgoingmessage queue

mail

server

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent


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Electronic Mail: mail servers

Mail Servers mailboxcontains incoming

messages for user

messagequeueof outgoing

(to be sent) mail messages SMTP protocolbetween mail

servers to send emailmessages

client: sending mail

server server: receiving mail

server

mailserver

useragent

useragent

useragent

mail

server

useragent

useragent

mailserver

user

agent

SMTP

SMTP

SMTP

?


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Electronic Mail: SMTP [RFC 2821]

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

S i Ali d t B b


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Scenario: Alice sends message to Bob

1) Alice uses UA to compose

message and [email protected]

2) Alices UA sends messageto her mail server; messageplaced in message queue

3) Client side of SMTP opensTCP connection with Bobsmail server

4) SMTP client sends Alices

message over the TCPconnection

5) Bobs mail server places themessage in Bobs mailbox

6) Bob invokes his user agent

to read message

useragent

mailserver

mailserver user

agent

1

2 3 4 56


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Sample SMTP interactionS: 220 hamburger.edu

C: HELO crepes.frS: 250 Hello crepes.fr, pleased to meet you

C: MAIL FROM:

S: 250 [email protected]... Sender ok

C: RCPT TO:

S: 250 [email protected] ... Recipient okC: DATA

S: 354 Enter mail, end with "." on a line by itself

C: Do you like ketchup?

C: How about pickles?

C: .S: 250 Message accepted for delivery

C: QUIT

S: 221 hamburger.edu closing connection


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Try SMTP interaction for yourself:

telnet servername 25


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Try SMTP interaction for yourself:

telnet smtp.sfu.ca25

(mailgate.sfu.ca 465 SSLv2http://www.sfu.ca/itservices/sfuconnect/getstarted/accessmethods/ssl_background_

info.html)

see 220 reply from server enter HELO, MAIL FROM, RCPT TO, DATA, QUIT

commandsabove lets you send email without using email client

(reader)

-- note: SFU IT service has upgraded their systemand now supports only SSL connections


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Message format: multimedia extensions

ASCII: http://www.asciitable.com/ MIME: multimedia mail extension, RFC 2045, 2056

additional lines in msg header declare MIME contenttype

From: [email protected]

To: [email protected]: Picture of yummy crepe.

MIME-Version: 1.0

Content-Transfer-Encoding: base64

Content-Type: image/jpeg

base64 encoded data ..............................

......base64 encoded data

multimedia data

type, subtype,parameter declaration

method usedto encode data

MIME version

encoded data

MIME


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MIME typesContent-Type: type/subtype; parameters

Text example subtypes:plain,

html

Image example subtypes: jpeg,gif

Audio

example subtypes:basic(8-bit mu-law encoded),32kadpcm (32 kbpscoding)

Video example subtypes:mpeg,

quicktime

Application other data that must be

processed by readerbefore viewable

example subtypes:msword, octet-stream


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Multipart Type

From: [email protected]

To: [email protected]: Picture of yummy crepe.

MIME-Version: 1.0

Content-Type: multipart/mixed; boundary=StartOfNextPart

--StartOfNextPart

Dear Bob, Please find a picture of a crepe.

--StartOfNextPartContent-Transfer-Encoding: base64

Content-Type: image/jpeg

base64 encoded data .....

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

......base64 encoded data

--StartOfNextPartDo you want the recipe?

MIME t


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MIME typesContent-Type: type/subtype; parameters

An Example: MailEncodingExample.txt------=_NextPart_000_0010_8CEC7DC6.92E25D8CContent-Type: application/octet-stream;

name="readme.scr"

Content-Transfer-Encoding: base64

Content-Disposition: attachment;

filename="readme.scr"

TVqQAAMAAAAEAAAA//8AALgAAAAAAAAAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAqAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAUEUA

AEwBAwAAAAAAAAAAAAAAAADgAA8BCwEHAABQAAAAEAAAAGAAAGC+AAAAcAAAAMAAAAAASgAAEAAA

AAIAAAQAAAAAAAAABAAAAAAAAAAA0AAAABAAAAAAAAACAAAAAAAQAAAQAAAAABAAABAAAAAAAAAQ

AAAAAAAAAAAAAADowQAAMAEAAADAAADoAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAAAAAAAAAAAAAABVUFgwAAAAAABgAAAAEAAAAAAAAAAEAAAAAAAAAAAAAAAAAACAAADg

VVBYMQAAAAAAUAAAAHAAAABQAAAABAAAAAAAAAAAAAAAAAAAQAAA4C5yc3JjAAAAABAAAADAAAAA

BAAAAFQAAAAAAAAAAAAAAAAAAEAAAMAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA





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Coding for MIME

Key issue: Binary data (bit string) to plain text (SMTPmessage)

Simple solution: 8 bit -> one plain text symbol (?)


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ASCII Table

Beyond 7 bit? Extended ASCII

Never well defined


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Base 64 coding more info (RFC1521)

A 65-character subset of US-ASCII is used, enabling 6bits to be represented per printable character. (Theextra 65th character, "=", is used to signify a specialprocessing function.)

NOTE: This subset has the important property that it is representedidentically in all versions of ISO 646, including US ASCII, and allcharacters in the subset are also represented identically in all versionsof EBCDIC. Other popular encodings, such as the encoding used by theuuencode utility and the base85 encoding specified as part of Level 2PostScript, do not share these properties, and thus do not fulfill theportability requirements a binary transport encoding for mail mustmeet.


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Base 64 coding more info (RFC1521)

The 64 characters

A-Z a-z 0-9 + / =


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Mail access protocols

useragent

senders mailserver

useragent

? SMTP ?

receivers mailserver


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Mail access protocols

SMTP: delivery/storage to receivers server 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 msgs on server

HTTP: Hotmail , Yahoo! Mail, etc.

useragent

senders mailserver

useragent

SMTP SMTP accessprotocol

receivers mailserver


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WebMail Hotmail/Gmail SFU Webmail:

webmail.sfu.ca (old)/connect.sfu.ca


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POP3 protocol

authorization phase client commands:

user:declare username

pass:password

server responses +OK

-ERR

transaction phase, client: list:list message numbers

retr:retrieve message bynumber

dele:delete

quit

C: list

S: 1 498

S: 2 912

S: .C: retr 1

S:

S: .

C: dele 1

C: retr 2

S:

S: .

C: dele 2

C: quit

S: +OK POP3 server signing off

S: +OK POP3 server ready

C: user bob

S: +OK

C: pass hungry

S: +OKuser successfully logged on


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POP3 (more) and IMAP

More about POP3 Previous example uses

download and deletemode.

Bob cannot re-read e-mail if he changesclient

Download-and-keep:copies of messages ondifferent clients

POP3 is statelessacross sessions

newpop.sfu.ca 995SSLv2

IMAP IMAP keeps user state

across sessions: names of folders and

mappings betweenmessage IDs and foldername

Allows user toorganize messages in

folders Partial retrieving -

header


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Chapter 2 outline

2.1 Principles of app layer protocols 2.2 Web and HTTP

2.3 FTP


2.5 DNS

2.6 Content distribution Network Web caching

Content distribution networks

P2P file sharing


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Name vs. ID

People:many identifiers: name, passport #, driving license ID, SIN

Example:

I want to talk to J.C. Liu. -- from Friend

Please tell me you driving license ID. -- fromVisa card service


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DNS: Domain Name System

People:many identifiers: SIN, name, passport #

Internet hosts, routers: IP address (32 bit) -

used for addressingdatagrams

name, e.g.,gaia.cs.umass.edu - usedby humans

Q:map between IPaddresses and name ?

Domain Name System: distributed database

implemented in hierarchy ofmany name servers

application-layer protocolhost, routers, name servers tocommunicate to resolvenames(address/name translation)

note: core Internetfunction, implemented asapplication-layer protocol

complexity at networksedge


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DNS name servers

no server has all name-to-IP address mappings

local name servers: each ISP, company has

local (default) name server host DNS query first goes

to local name server

authoritative name server: for a host: stores that

hosts IP address, name can perform name/address

translation for that hostsname

Centralize DNS? No single point of failure

traffic volume

distant centralized

database maintenance

doesnt scale!

DNS Cli t S tti


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DNS: Client Settings

DN


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DNS name servers

local name servers: each ISP, company has local (default) name server

host DNS query first goes to local name server

authoritative name server: for a host: stores that hosts IP address, name can perform name/address translation for that

hosts name

no server has all name-to-IP address mappings


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Si l DNS l


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Simple DNS example

host surf.eurecom.frwants IP address ofgaia.cs.umass.edu

1.contacts its local DNS

server, dns.eurecom.fr2.dns.eurecom.frcontacts

root name server, ifnecessary

3.root name server contacts

authoritative name server,dns.umass.edu,ifnecessary requesting host

surf.eurecom.frgaia.cs.umass.edu

root name server

authorititive name serverdns.umass.edu

local name serverdns.eurecom.fr

1

23

4

5

6

DNS l


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DNS example

Root name server: may not know

authoritative nameserver

may know

intermediate nameserver:who tocontact to findauthoritative nameserver

requesting hostsurf.eurecom.fr

gaia.cs.umass.edu

root name server


1

23

4 5

6

authoritative name serverdns.cs.umass.edu

intermediate name serverdns.umass.edu

7

8

DNS it t d i


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DNS: iterated queries

recursive query: puts burden of name

resolution oncontacted nameserver

heavy load?

iterated query: contacted server

replies with name of

server to contact I dont know this

name, but ask thisserver

requesting hostsurf.eurecom.fr

gaia.cs.umass.edu

root name server


1

23

4

5 6

authoritative name serverdns.cs.umass.edu

intermediate name serverdns.umass.edu

7

8

iterated query

DNS hi d d ti d


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DNS: caching and updating records

once (any) name server learns mapping, it cachesmapping

cache entries timeout (disappear) after sometime

update/notify mechanisms under design by IETF RFC 2136 http://www.ietf.org/html.charters/dnsind-charter.html

DNS d


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DNS records

DNS:distributed db storing resource records (RR)

Type=NS nameis domain (e.g.

foo.com) valueis IP address of

authoritative nameserver for this domain

RR format: (name, value, type,ttl)

Type=A

nameis hostname valueis IP address

Type=CNAME

nameis alias name for somecanonical (the real) name

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

valueis canonical name

Type=MX valueis name of mailserver

associated with name

DNS t l


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DNS protocol, messages

DNS protocol :queryand replymessages, both withsame message format

msg header identification:16 bit #

for query, reply to queryuses same #

flags:

query or reply

recursion desired

recursion available reply is authoritative

DNS t l


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DNS protocol, messages

Name, type fieldsfor a query

RRs in response

to query

records forauthoritative servers

additional helpfulinfo that may be used

Documents

Application 1 - Networking