計(jì)算機(jī)網(wǎng)絡(luò)自定向下設(shè)計(jì)課件第五章

Electronic

MailThree

major

components:r

user

agentsr

mail

serversr

simple

mail

transferprotocol:

smtpUser

Agentr

a.k.a.

“mail

reader”r

composing,

editing,

readingmail

messagesr

e.g.,

Eudora,

pine,

elm,NetscapeMessengerr

outgoing,

incoming

messagesstored

on

serveruser

mailboxoutgoingmessage

queuemailserveruseragentuseragentuseragentmailserveruseragentuseragentmailserveruseragentSMTP2:

Application

Layer1SMTPSMTPElectronic

Mail:

mail

serversMail

Serversr

mailbox

contains

incomingmessages

(yet

ot

be

read)for

userr

message

queue

of

outgoing(to

be

sent)

mail

messagesr

smtp

protocol

between

mailserver

to

send

emailmessagesm

client:

sending

mailserverm

“server”:

receivingmail

servermailserveruseragentuseragentuseragentmailserveruseragentuseragentmailserveruseragentSMTP2:

Application

Layer2SMTPSMTP2:

Application

Layer3Electronic

Mail:smtp

[RFC

821]r

uses

tcp

to

reliably

transfer

email

msg

from

client

toserver,

port

25r

direct

transfer:

sending

server

to

receiving

serverr

three

phases

of

transfer

m

handshaking(greeting)m

transferm

closurer

command/response

interactionm

commands:

ASCI

textm

response:

status

code

and

phrase2:

Application

Layer4Sample

smtp

interactionS:

220

C:

HELO

crepes.frS:

250

Hello

crepes.fr,

pleased

to

meet

youC:

MAIL

FROM:

<alice@crepes.fr>S:

250

alice@crepes.fr...

Sender

okC:

RCPT

TO:

<bob@>S:

250

bob@

...

Recipient

okC:

DATAS:

354

Enter

mail,

end

with

"."

on

a

line

by

itselfC:

Do

you

like

ketchup?C:

How

about

pickles?C:

.S:

250

Message

accepted

for

deliveryC:

QUITS:

221

closing

connection2:

Application

Layer5smtp:

final

wordstry

smtp

interaction

foryourself:r

telnet

servername

25r

see

220

reply

from

serverr

enter

HELO,

MAIL

FROM,RCPT

TO,

DATA,

QUITcommandsabove

lets

you

send

emailwithout

using

email

client(reader)Comparison

with

httpr

http:

pullr

email:

pushr

both

have

ASCIIcommand/responseinteraction,

status

codesr

http:

multiple

objects

infile

sent

in

separateconnectionsr

smtp:

multiple

messageparts

sent

in

oneconnectionMail

message

formatsmtp:

protocol

for

exchangingemail

msgsRFC

822:

standard

for

textmessage

format:r

header

lines,

e.g.,m

To:m

From:m

Subject:different

from

smtpcommands!r

bodym

the

“message”,

ASCIIcharacters

onlyr

line

containing

only

`.’headerbody.blankline2:

Application

Layer6Message

format:

multimedia

extensionsr

MIME:

multimedia

mail

extension,

RFC

2045,

2056r

additional

lines

in

msg

header

declare

MIME

contenttypeFrom:

alice@crepes.fr

To:

bob@Subject:

Picture

of

yummy

crepe.MIME-Version:

1.0Content-Transfer-Encoding:base64Content-Type:

image/jpegbase64

encoded

data

....................................base64

encoded

data.multimedia

datatype,

subtype,parameter

declarationmethod

usedto

encode

dataMIME

versionencoded

data2:

Application

Layer72:

Application

Layer8MIME

typesTextr

example

subtypes:

plain,htmlImager

example

subtypes:

jpeg,gifAudior

exampe

subtypes:

basic(8-bit

mu-law

encoded),32kadpcm

(32

kbpscoding)Videor

example

subtypes:

mpeg,quicktimeApplicationr

other

data

that

must

beprocessed

by

readerbefore

“viewable”r

example

subtypes:msword,

octet-streamMail

access

protocolsr

SMTP:

delivery/storage

to

receiver’s

serverr

Mail

access

protocol:

retrieval

from

serverm

POP:

Post

Office

Protocol

[RFC

1939]authorization

(agent

<-->server)

and

downloadm

IMAP:

Internet

Mail

Access

Protocol

[RFC

1730]more

features

(more

complex)manipulation

of

stored

msgs

on

serveruseragentsender’s

mailserveruseragentSMTP

SMTPPOP3

orIMAPreceiver’s

mailserver2:

Application

Layer9POP3

protocolauthorization

phaser

client

commands:m

user:

declare

usernamem

pass:

passwordr

server

responsesm

+OKm

-ERRtransaction

phase,

client:r

list:

list

message

numbers

r

retr:

retrieve

message

bynumberr

dele:

deleter

quitS:

+OK

POP3

server

readyC:

user

aliceS:

+OKC:

pass

hungryS:

+OK

user

successfully

logged

onC:

listS:

1

498S:

2

912S:

.C:

retr

1S:

<message

1

contents>S:

.C:

dele

1C:

retr

2S:

<message

1

contents>S:

.C:

dele

2C:

quitS:

+OK

POP3

server

signing

off2:

Application

Layer102:

Application

Layer11DNS:

Domain

Name

SystemPeople:

many

identifiers:m

SSN,

name,

Passport

#Internet

hosts,

routers:m

IP

address

(32

bit)

-used

for

addressingdatagramsm

“name”,

e.g.,

-used

by

humansQ:

map

between

IPaddresses

and

name

?Domain

Name

System:r

distributed

databaseimplemented

in

hierarchy

ofmany

name

serversr

application-layer

protocolhost,

routers,

name

servers

tocommunicateto

resolve

names(address/name

translation)m

note:

core

Internetfunction

implemented

asapplication-layer

protocolm

complexity

at

network’s“edge”2:

Application

Layer12DNS

name

serversr

no

server

has

all

name-to-IP

address

mappingslocal

name

servers:m

each

ISP,

company

haslocal

(default)

name

serverm

host

DNS

query

first

goesto

local

name

serverauthoritative

name

server:m

for

a

host:

stores

thathost’s

IP

address,

namem

can

perform

name/addresstranslation

for

thathost’s

nameWhy

not

centralize

DNS?r

single

point

of

failurer

traffic

volumer

distant

centralizeddatabaser

maintenancedoesn’t

scale!DNS:

Root

name

serversr

contacted

by

localname

server

that

cannot

resolve

namer

root

name

server:m

contactsauthoritative

nameserver

if

namemapping

not

knownm

gets

mappingm

returns

mapping

tolocal

name

serverr

~

dozen

root

nameservers

worldwide2:

Application

Layer13Simple

DNS

examplehost

surf.eurecom.frwants

IPaddress

ofContacts

its

local

DNS server,

dns.eurecom.frdns.eurecom.fr

contacts root

name

server,

if necessaryroot

name

server

contacts authoritative

name

server, ,

if necessaryrequesting

hostsurf.eurecom.frroot

name

serverauthorititive

name

serverlocal

name

serverdns.eurecom.fr1234562:

Application

Layer14DNS

exampleRoot

name

server:r

may

not

knowauthoratiative

nameserverr

may

knowintermediate

nameserver:

who

tocontact

to

findauthoritative

nameserverroot

name

serverlocal

name

serverdns.eurecom.fr123456authoritative

name

serverrequesting

hostsurf.eurecom.frintermediate

name

server2:

Application

Layer1578DNS:

iterated

queriesrecursive

query:r

puts

burden

of

nameresolution

oncontacted

nameserverr

heavy

load?iterated

query:r

contacted

serverreplies

with

name

ofserver

to

contactr

“I

don’t

know

thisname,

but

ask

thisserver”requesting

hostsurf.eurecom.frroot

name

serverlocal

name

serverdns.eurecom.fr1234authoritative

name

serverintermediate

name

server5

678iterated

query2:

Application

Layer162:

Application

Layer17DNS:

caching

and

updating

recordsr

once

(any)

name

server

learns

mapping,

it

cachesmappingm

cache

entries

timeout

(disappear)

after

sometimer

update/notify

mechanisms

under

design

by

IETFm

RFC

2136m

/html.charters/dnsind-charter.htmlDNS

recordsDNS:

distributed

db

storing

resource

records

(RR)RR

format:

(name,

value,

type,ttl)Ty