lect 8 applaction layer

7/23/2019 Lect 8 Applaction Layer

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Application Layer

Application Layer2-1


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Chapter 2: outline

2.1 principles of networkapplications

2.2 Web and HTTP

2.3 FTP

2.6 P2P applications

2.7 socket programmingwith UDP and TCP


. e ectron c ma SMTP, POP3, IMAP

2.5 DNS


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Chapter 2: application layer

our goals: conceptual,

implementation aspectsof network applicationprotocols

transport-layer

learn about protocols byexamining popularapplication-levelprotocols HTTP

FTP


service models

client-serverparadigm

peer-to-peerparadigm

SMTP / POP3 / IMAP

DNS

creating networkapplications

socket API


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Some network apps

e-mail

web

text messaging

remote login

P2P file sharin

voice over IP (e.g., Skype)

real-time videoconferencing

social networking

search


multi-user network games

streaming stored video(YouTube, Hulu, Netflix)


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Creating a network app

write programs that:

run on (different) end systems communicate over network

e.g., web server softwarecommunicates with browser

software

application

transport

network

data link

physical


no need to write software fornetwork-core devices

network-core devices do not

run user applications applications on end systems

allows for rapid appdevelopment, propagation

application

transport

network

data link

physical

application

transport

network

data link

physical


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Application architectures

possible structure of applications:

client-server

peer-to-peer (P2P)



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Client-server architecture

server: always-on host

permanent IP address

data centers for scaling


communicate with server

may be intermittentlyconnected

may have dynamic IPaddresses

do not communicate directlywith each other

client/server


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P2P architecture

no always-on server arbitrary end systems

directly communicate

peers request service fromother peers, provide service

in return to other peers

peer-peer


self scalability newpeers bring new servicecapacity, as well as newservice demands

peers are intermittentlyconnected and change IPaddresses

complex management


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Processes communicating

process: program runningwithin a host

within same host, twoprocesses communicate

using inter-process

client process: process thatinitiates communication

server process: process thatwaits to be contacted

clients, servers


OS)

processes in different hostscommunicate by exchanging

messages

aside: applications with P2P

architectures have clientprocesses & serverprocesses


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Sockets

process sends/receives messages to/from its socket

socket analogous to door

sending process showes message out door

sending process relies on transport infrastructure onother side of door to deliver message to socket at

receiving process


Internet

controlled

by OS

controlled byapp developer

transport

application

physical

link

network

process

transport

application

physical

link

network

processsocket


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Addressing processes

to receive messages,process must have identifier

host device has unique 32-bit IP address

Q: does IP address of host

on which process runs

identifierincludes both IPaddress and port numbersassociated with process onhost.

example port numbers:

HTTP server: 80


suffice for identifying theprocess?

mail server: 25

to send HTTP message togaia.cs.umass.edu webserver:

IP address: 128.119.245.12 port number: 80

more shortly

A: no, manyprocessescan be running on same

host


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App-layer protocol defines

types of messagesexchanged,

e.g., request, response

message syntax:

what fields in messages

open protocols: defined in RFCs

allows for interoperability

e.g., HTTP, SMTP

proprietary protocols:


delineated

message semantics

meaning of information

in fields rules for when and how

processes send & respondto messages

e.g., ype


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What transport service does an app need?

data integrity

some apps (e.g., file transfer,web transactions) require

100% reliable data transfer

other apps (e.g., audio) can

tolerate some loss

throughput

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


timing

some apps (e.g., Internettelephony, interactivegames) require low delayto be effective

make use of whateverthroughput they get

security encryption, data integrity,


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Transport service requirements: common apps

application

file transfer

e-mail

Web documents

data loss

no loss

no loss

no loss

throughput

elastic

elastic

elastic

time sensitive

no

no

no


rea - me au o v eo

stored audio/video

interactive games

text messaging

oss- o eran

loss-tolerant

loss-tolerant

no loss

au o: ps- ps

video:10kbps-5Mbps

same as above

few kbps up

elastic

yes, s

msec

yes, few secs

yes, 100s

msec

yes and no


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Internet transport protocols services

TCP service: reliable transport between

sending and receivingprocess

flow control: sender wont

overwhelm receiver

UDP service: unreliable data transfer

between sending andreceiving process

does not provide:


congestion control: throttle

sender when networkoverloaded

does not provide: timing,

minimum throughputguarantee, security connection-oriented: setup

required between client andserver processes

, ,congestion control,timing, throughputguarantee, security,orconnection setup,

Q: why bother? Why isthere a UDP?


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Internet apps: application, transport protocols

application

e-mail

remote terminal access

Web

applicationlayer protocol

SMTP [RFC 2821]

Telnet [RFC 854]

HTTP [RFC 2616]

underlyingtransport protocol

TCP

TCP

TCP


e rans er

streaming multimedia

Internet telephony

HTTP (e.g., YouTube),

RTP [RFC 1889]

SIP, RTP, proprietary

(e.g., Skype)

TCP or UDP

TCP or UDP


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Securing TCP

TCP & UDP

no encryption

cleartext passwds sentinto socket traverse

SSL is at app layer

Apps use SSL libraries,which talk to TCP

SSL socket API

SSL

provides encrypted

TCP connection data integrity

end-pointauthentication

c eartext passw s sentinto socket traverseInternet encrypted

See Chapter 7



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Chapter 2: outline

2.1 principles of networkapplications app architectures

app requirements

2.6 P2P applications

2.7 socket programmingwith UDP and TCP


.2.3 FTP

2.4 electronic mail SMTP, POP3, IMAP

2.5 DNS


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Web and HTTP

First, a review web page consists of objects

object can be HTML file, JPEG image, Java applet,audio file,

web a e consists of base HTML- ile which


includes several referenced objects

each object is addressable by a URL, e.g.,

www.someschool.edu/someDept/pic.gif

host name path name


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HTTP overview

HTTP: hypertexttransfer protocol Webs application layer

protocol client/server model

client: browser that

PC running

Firefox browser


requests, receives,(using HTTP protocol)and displays Webobjects

server: Web serversends (using HTTPprotocol) objects inresponse to requests

server

running

Apache Web

server

iphone running

Safari browser


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HTTP overview (continued)

uses TCP: client initiates TCP

connection (createssocket) to server, port 80

server accepts TCP

HTTP is stateless server maintains no

information aboutpast client requests


HTTP messages(application-layer protocolmessages) exchanged

between browser (HTTPclient) and Web server(HTTP server)

TCP connection closed

protocols that maintainstate are complex!

past history (state) must bemaintained

if server/client crashes, theirviews of state may beinconsistent, must bereconciled


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HTTP connections

non-persistent HTTP

at most one objectsent over TCPconnection

persistent HTTP

multiple objects canbe sent over singleTCP connection


closed

downloading multipleobjects required

multiple connections

,


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Non-persistent HTTPsuppose user enters URL:

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

80

1b. HTTP server at hostwww.someSchool.edu waiting

for TCP connection at ort 80.

(contains text,

references to 10jpeg images)www.someSchool.edu/someDepartment/home.index


2. HTTP client sends HTTP requestmessage (containing URL) intoTCP connection socket.

Message indicates that clientwants objectsomeDepartment/home.index

accepts connection, notifyingclient

3. HTTP server receives request

message, forms responsemessage containing requestedobject, and sends message intoits socket

time


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Non-persistent HTTP (cont.)

5. HTTP client receives responsemessage containing html file,displays html. Parsing html file,finds 10 referenced jpeg objects

4. HTTP server closes TCPconnection.


6. Steps 1-5 repeated for each of10 jpeg objects

time

N i t t HTTP ti


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Non-persistent HTTP: response time

RTT (definition): time for a

small packet to travel fromclient to server and back

HTTP response time:

one RTT to initiate TCP

connection

initiate TCPconnection

RTT


one or requestand first few bytes of HTTPresponse to return

file transmission time

non-persistent HTTPresponse time =

2RTT+ file transmissiontime

time totransmitfile

file

RTT

filereceived

time time


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Persistent HTTP

non-persistent HTTP issues: requires 2 RTTs per object

OS overhead for each TCPconnection

browsers often open

persistent HTTP: server leaves connection

open after sendingresponse

subsequent HTTP


to fetch referenced objects

client/server sent overopen connection

client sends requests assoon as it encounters areferenced object

as little as one RTT for allthe referenced objects


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HTTP request message

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

ASCII (human-readable format)

request line

carriage return character

line-feed character


(GET, POST,

HEAD commands)

header

lines

carriage return,

line feed at start

of line indicates

end of header lines

GET /index.html HTTP/1.1\r\nHost: www-net.cs.umass.edu\r\nUser-Agent: Firefox/3.6.10\r\nAccept: text/html,application/xhtml+xml\r\nAccept-Language: en-us,en;q=0.5\r\nAccept-Encoding: gzip,deflate\r\nAccept-Charset: ISO-8859-1,utf-8;q=0.7\r\nKeep-Alive: 115\r\nConnection: keep-alive\r\n\r\n


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HTTP request message: general format

requestline

headerlines

method sp sp cr lfversionURL

cr lfvalueheader field name

~~ ~~


body

cr lfvalueheader field name

cr lf

entity body~~ ~~


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Uploading form input

POST method: web page often includes

form input

input is uploaded toserver in entity body


URL method: uses GET method

input is uploaded in URLfield of request line:

www.somesite.com/animalsearch?monkeys&banana


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Method types

HTTP/1.0: GET

POST

HEAD

HTTP/1.1: GET, POST, HEAD

PUT

uploads file in entity

bod to ath s ecified


requested object outof response

in URL field

DELETE

deletes file specified inthe URL field

HTTP


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HTTP response message

status line(protocol

status code

status phrase)

HTTP/1.1 200 OK\r\nDate: Sun, 26 Sep 2010 20:09:20 GMT\r\nServer: Apache/2.0.52 (CentOS)\r\nLast-Modified: Tue, 30 Oct 2007 17:00:02

GMT\r\n" "


ea er

lines

data, e.g.,

requested

HTML file

- -Accept-Ranges: bytes\r\nContent-Length: 2652\r\nKeep-Alive: timeout=10, max=100\r\nConnection: Keep-Alive\r\nContent-Type: text/html; charset=ISO-8859-

1\r\n\r\ndata data data data data ...

HTTP d


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HTTP response status codes

200 OK

request succeeded, requested object later in this msg

status code appears in 1st line in server-to-

client response message.

some sample codes:


ove ermanen y

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

400 Bad Request

request msg not understood by server404 Not Found

requested document not found on this server

505 HTTP Version Not Supported

T HTTP ( l d ) f lf


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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 cis.poly.edu.

anything typed in sent

to port 80 at cis.poly.edu

telnet cis.poly.edu 80


2. type in a GET HTTP request:

GET /~ross/ HTTP/1.1

Host: cis.poly.edu

by typing this in (hit carriage

return twice), you send

this minimal (but complete)GET request to HTTP server

3. look at response message sent by HTTP server!

(or use Wireshark to look at captured HTTP request/response)


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User-server state: cookies

many Web sites use cookiesfour components:

1) cookie header line ofHTTP response

message

example: Susan always access Internet

from PC

visits specific e-commercesite for first time


2) cookie header line innext HTTP requestmessage

3) cookie file kept onusers host, managedby users browser

4) back-end database atWeb site

arrives at site, site creates:

unique ID

entry in backenddatabase for ID

Cookies: keeping state (cont )


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Cookies: keeping state (cont.)

client server

cookie file

ebay 8734usual http request msg Amazon server

creates ID

1678 for user createentry

usual http responseset-cookie: 1678

ebay 8734

backend

database


usual http response msg

usual http response msg

one week later:

usual http request msgcookie: 1678 cookie-

specific

action

access

usual http request msgcookie: 1678 cookie-

specific

action

access

ebay 8734

amazon 1678

C ki ( ti d)


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Cookies (continued)

what cookies can be usedfor: authorization shopping carts recommendations

user session state Web

cookies and privacy: cookies permit sites to

learn a lot about you

you may supply name ande-mail to sites

aside


e-mail)

how to keep state: protocol endpoints: maintain state at

sender/receiver over multipletransactions

cookies: http messages carry state

W b h ( )


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Web caches (proxy server)

user sets browser: Webaccesses via cache

browser sends all HTTPrequests to cache

goal: satisfy client request without involving origin server

proxy

server


o ject in cac e: cac ereturns object

else cache requestsobject from origin

server, then returnsobject to client

client

client origin

server

origin

server

M b t W b hi


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More about Web caching

cache acts as bothclient and server server for original

requesting client

client to origin server

why Web caching? reduce response time

for client request

reduce traffic on an


typ ca y cac e sinstalled by ISP(university, company,residential ISP)

Internet dense with

caches: enables poorcontent providers to

effectively delivercontent (so too doesP2P file sharing)

Caching example:


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Caching example:

origin

serverspublic

Internet

assumptions: avg object size: 100K bits

avg request rate from browsers toorigin servers:15/sec

avg data rate to browsers: 1.50 Mbps

RTT from institutional router to any


institutional

network1 Gbps LAN

1.54 Mbps

access link

access link rate: 1.54 Mbps

consequences: LAN utilization: 15%

access link utilization = 99% total delay = Internet delay + access

delay + LAN delay

= 2 sec + minutes + usecs

problem!

Caching example: fatter access link


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Caching example: fatter access link

origin

serverspublic

Internet




access link utilization = 99%

total delay = Internet delay + accessdelay + LAN delay


1.54 Mbps

access link154 Mbps 154 Mbps

msecs

Cost: increased access link speed (not cheap!)

9.9%

institutional

network1 Gbps LAN

Caching example: install local cache


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origin

servers





public

Internet

institutional

network1 Gbps LAN


1.54 Mbps

access link

local webcache



access link utilization = 100%

total delay = Internet delay + accessdelay + LAN delay


??

How to compute linkutilization, delay?

Cost: web cache (cheap!)



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Calculating access linkutilization, delay with cache:

suppose cache hit rate is 0.4 40% requests satisfied at cache,

60% requests satisfied at origin

origin

servers

access link utilization:

public

Internet


1.54 Mbps

access link

60% of requests use access link data rate to browsers over access link

= 0.6*1.50 Mbps = .9 Mbps utilization = 0.9/1.54 = .58

total delay = 0.6 * (delay from origin servers) +0.4

* (delay when satisfied at cache) = 0.6 (2.01) + 0.4 (~msecs) 0.4(0.01 seconds) + 0.6 (2.01 seconds) = ~ 1.2 secs less than with 154 Mbps link (and

cheaper too!)

institutional

network1 Gbps LAN

local webcache

Conditional GET


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Conditional GET

Goal: dont send object ifcache has up-to-datecached version no object transmission

delay

lower link utilization

HTTP request msgIf-modified-since:

HTTP response

HTTP/1.0

object

not

modified

before

client server


cac e: spec y a e ocached copy in HTTPrequestIf-modified-since:

server: response containsno object if cached copyis up-to-date:HTTP/1.0 304 NotModified

304 Not Modified

HTTP request msg

If-modified-since:

HTTP responseHTTP/1.0 200 OK

objectmodified

after

Ch t 2 tli


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Chapter 2: outline


app requirements

2.6 P2P applications2.7 socket programming

with UDP and TCP


.2.3 FTP


2.5 DNS

FTP: the file transfer protocol


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FTP: the file transfer protocol

file transfer

FTPserver

FTP

userinterface

FTPclient

local file

system

remote file

system

user

at host


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

FTP: separate control, data connections


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FTP: separate control, data connections

FTP client contacts FTP serverat port 21, using TCP

client authorized over controlconnection

client browses remote

FTPclient

FTPserver

TCP control connection,

server port 21

TCP data connection,server port 20


,over control connection

when server receives filetransfer command, serveropens 2ndTCP data

connection (for file) to client after transferring one file,

server closes data connection

server opens another TCPdata connection to transferanother file

control connection: out of

band

FTP server maintainsstate: current directory,earlier authentication

FTP commands responses


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FTP commands, responses

sample commands: sent as ASCII text over

control channel

USER username

PASS password

sample return codes status code and phrase (as

in HTTP)

331 Username OK,password required

125 data


current directory

RETR filenameretrieves (gets) file

STOR filename stores(puts) file onto remotehost

connectionalready open;transfer starting

425 Cant open

data connection 452 Error writingfile

Chapter 2: outline


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Chapter 2: outline


app requirements


with UDP and TCP


.2.3 FTP


2.5 DNS

Electronic mail outgoing


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Electronic mail

Three major components: user agents

mail servers

simple mail transferprotocol: SMTP

user mailbox

outgoing

message queue

mail

mail

server

SMTP

user

agent

user

agent

user


User Agent a.k.a. mail reader

composing, editing, reading

mail messages e.g., Outlook, Thunderbird,

iPhone mail client

outgoing, incomingmessages stored on server

mail

server

SMTP

SMTP

user

agent

user

agent

useragent

Electronic mail: mail servers


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Electronic mail: mail servers

mail servers: mailboxcontains incomingmessages for user

message queue of outgoing(to be sent) mail messages

mail

mail

server

SMTP

user

agent

user

agent

user


mail servers to send emailmessages

client: sending mailserver

server: receiving mailserver

mail

server

SMTP

SMTP

user

agent

user

agent

useragent

Electronic Mail: SMTP [RFC 2821]


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Electronic Mail: SMTP [RFC 2821]

uses TCP to reliably transfer email message fromclient to server, port 25

direct transfer: sending server to receivingserver

three phases of transfer


handshaking (greeting) transfer of messages

closure

command/response interaction (like HTTP, FTP) commands: ASCII text response: status code and phrase

messages must be in 7-bit ASCI

Scenario: Alice sends message to Bob


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g

1) Alice uses UA to compose

message [email protected]

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

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 messa e


useragent

TCP connection with Bobsmail server

mail

server

mailserver

1

2 3 4

5

6

Alices mail server Bobs mail server

useragent

Sample SMTP interaction


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p

S: 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 ok


C: 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

Try SMTP interaction for yourself:


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Try SMTP interaction for yourself:

telnet servername 25 see 220 reply from server

enter HELO, MAIL FROM, RCPT TO, DATA, QUITcommands


above lets you send email without using email client (reader)

SMTP: final words


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SMTP: final words

SMTP uses persistentconnections

SMTP requires message(header & body) to be in7-bit ASCII

comparison with HTTP: HTTP: pull

SMTP: push

both have ASCII


CRLF.CRLF todetermine end of message

comman responseinteraction, status codes

HTTP: each objectencapsulated in its own

response msg SMTP: multiple objects

sent in multipart msg

Mail message format


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g

SMTP: protocol forexchanging email msgs

RFC 822: standard for textmessage format:

header lines, e.g.,

headerblank

line


To:

From:

Subject:

different from SMTP MAILFROM, RCPT TO:commands!

Body: the message ASCII characters only

body

Mail access protocols


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p

senders mailserver

SMTP SMTPmail access

protocol

receivers mailserver

(e.g., POP,IMAP)

user

agent

user

agent


mail access protocol: retrieval from server

POP: Post Office Protocol [RFC 1939]: authorization,download

IMAP: Internet Mail Access Protocol [RFC 1730]: more

features, including manipulation of stored msgs onserver

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

POP3 protocol3 d


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authorization phase client commands: user: declare username

pass: password

server responses

+OK

C: list

S: 1 498

S: 2 912

S: +OK POP3 server ready

C: user bob

S: +OK

C: pass hungryS: +OKuser 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

POP3 (more) and IMAP


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( )

more about POP3 previous example uses

POP3 download anddelete mode

Bob cannot re-read e-

mail if he chan es

IMAP keeps all messages in one

place: at server

allows user to organizemessages in folders


client

POP3 download-and-keep: copies of messageson different clients

POP3 is stateless acrosssessions

sessions:

names of folders andmappings betweenmessage IDs and folder

name

Chapter 2: outline


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C apte : out e


app requirements


with UDP and TCP


.2.3 FTP


2.5 DNS

DNS: domain name system


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y

people: many identifiers: SSN, name, passport #

Internet hosts, routers:

IP address (32 bit) -used for addressing

Domain Name System: distributed database

implemented in hierarchy ofmany name servers

application-layer protocol: hosts,


name, e.g.,www.yahoo.com -used by humans

Q: how to map between IPaddress and name, andvice versa ?

resolve names (address/nametranslation)

note: core Internet function,implemented as application-

layer protocol complexity at networks

edge

DNS: services, structure


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why not centralize DNS? single point of failure traffic volume

distant centralized database

maintenance

DNS services hostname to IP address

translation

host aliasing canonical, alias names

mail server aliasin


load distribution

replicated Webservers: many IPaddresses correspond

to one name

A: doesnt scale!

DNS: a distributed, hierarchical database


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Root DNS Servers

com DNS servers org DNS servers edu DNS servers

poly.edu

DNS servers

umass.edu

DNS serversyahoo.com

amazon.com

pbs.org

DNS servers


client wants IP for www.amazon.com; 1st approx:

client queries root server to find com DNS server

client queries .com DNS server to get amazon.com DNS server

client queries amazon.com DNS server to get IP address forwww.amazon.com

DNS: root name servers


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contacted by local name server that can not resolve name

root name server: contacts authoritative name server if name mapping not known

gets mapping

returns mapping to local name server


13 root nameserversworldwide

a. Verisign, Los Angeles CA

(5 other sites)

b. USC-ISI Marina del Rey, CA

l. ICANN Los Angeles, CA

(41 other sites)

e. NASA Mt View, CA

f. Internet Software C.

Palo Alto, CA (and 48 other

sites)

i. Netnod, Stockholm (37 other sites)

k. RIPE London (17 other sites)

m. WIDE Tokyo

(5 other sites)

c. Cogent, Herndon, VA (5 other sites)

d. U Maryland College Park, MD

h. ARL Aberdeen, MD

j. Verisign, Dulles VA (69 other sites )

g. US DoD Columbus,

OH (5 other sites)

TLD, authoritative servers


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top-level domain (TLD) servers: responsible for com, org, net, edu, aero, jobs, museums,

and all top-level country domains, e.g.: uk, fr, ca, jp

Network Solutions maintains servers for .com TLD

Educause for .edu TLD


authoritative DNS servers:

organizations own DNS server(s), providingauthoritative hostname to IP mappings for organizationsnamed hosts

can be maintained by organization or service provider

Local DNS name server


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does not strictly belong to hierarchy each ISP (residential ISP, company, university) has

one also called default name server


,local DNS server has local cache of recent name-to-address translation

pairs (but may be out of date!)

acts as proxy, forwards query into hierarchy

root DNS serverDNS nameresolution example


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local DNS server

2

3

4

5

TLD DNS server

resolution example

host at cis.poly.eduwants IP address forgaia.cs.umass.edu


requesting hostcis.poly.edu

gaia.cs.umass.edu

ns.po y.e u

16

authoritative DNS server

dns.cs.umass.edu

78

contacted server

replies with name ofserver to contact

the remaining queries

are iterative. I dont know this

name, but ask thisserver

root DNS serverDNS nameresolution example


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6

3

recursive query: puts burden of name

resolution oncontacted name local DNS server

2

7

resolution example

TLD DNSserver


5server The query from the

requesting host tothe local DNS server

is recursive, and theremaining queriesare iterative.

heavy load at upper

levels of hierarchy?


gaia.cs.umass.edu

ns.po y.e u

1


dns.cs.umass.edu

8

DNS caching


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DNS caching in order to improve the delay performance and to reduce

the number of DNS messages In a query chain, when a DNS server receives a DNS reply (containing,

for example, a mapping from a hostname to an IP address), it can cache

the mapping in its local memory.

For example,in Figure 2.21, each time the local DNS server dns.poly.edu


,contained in the reply.

If a hostname/IP address pair is cached in a DNS server and another

query arrives to the DNS server for the same hostname, the DNS server

can provide the desired IP address, even if it is not authoritative for the

hostname. Because hosts and mappings between hostnames and IP addresses are

by no means permanent, DNS servers discard cached information after a

period of time (often set to two days).

root DNS serverDNS caching


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local DNS server

2

3

4

5

TLD DNS server



gaia.cs.umass.edu

ns.po y.e u

16


dns.cs.umass.edu

78

DNS: caching, updating records once (any) name server learns mapping it caches


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once (any) name server learns mapping, it cachesmapping cache entries timeout (disappear) after some time (TTL)

TLD servers typically cached in local name servers

thus root name servers not often visited

cached entries may be out-of-date (best effort


name-to-a ress trans ation if name host changes IP address, may not be known

Internet-wide until all TTLs expire

update/notify mechanisms proposed IETF standard

RFC 2136

DNS records


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DNS: distributed db storing resource records (RR)that provide hostname-to-IP address mappings. Each DNS reply message carries one or more

resource records.

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

type=A name is hostname

type=CNAME


type=NS name is domain (e.g.,

foo.com)

value is hostname ofauthoritative nameserver for this domain

(foo.com, dns.foo.com,NS)is a Type NS record.

value is IP address canonical (the real) name

www.ibm.comis really

servereast.backup2.ibm.com

value is canonical name

type=MX value is name of mailserver

associated with name

DNS protocol, messages


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queryand replymessages, both with same message

format

msg header

identification:16 bit # forquery, reply to query uses

identification flags

# questions

# answer RRs

2 bytes 2 bytes


same #

flags:

query or reply

recursion desired

recursion available reply is authoritative

questions (variable # of questions)

answers (variable # of RRs)

authority (variable # of RRs)

additional info (variable # of RRs)

DNS protocol, messages


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identification flags

# questions

# answer RRs

2 bytes 2 bytes


name, type fieldsfor a query

RRs in responseto query

records forauthoritative servers

additional helpfulinfo that may be used

questions (variable # of questions)

answers (variable # of RRs)

authority (variable # of RRs)

additional info (variable # of RRs)

Inserting records into DNS


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example: new startup Network Utopia

register name networkuptopia.com at DNS registrar(e.g., Network Solutions) provide names, IP addresses of authoritative name server

(primary and secondary)


reg s rar nser s wo s n o .com server:(networkutopia.com, dns1.networkutopia.com, NS)

(dns1.networkutopia.com, 212.212.212.1, A)

create authoritative server type A record forwww.networkuptopia.com; type MX record fornetworkutopia.com

Attacking DNS


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DDoS attacks Bombard root servers

with traffic Not successful to date

Redirect attacks Man-in-middle

Intercept queries

DNS poisoning

Local DNS servers

cache IPs of TLDservers, allowing rootserver bypass

Bombard TLD servers Potentially more

dangerous

en ogus re es oDNS server, whichcaches

Exploit DNS for DDoS

Send queries withspoofed sourceaddress: target IP

Requires amplificationApplication Layer2-76

Chapter 2: outline


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app requirements


with UDP and TCP


.2.3 FTP


2.5 DNS

Pure P2P architecture


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no always-on server

arbitrary end systemsdirectly communicate

peers are intermittentlyconnected and change IPaddresses


examples: file distribution

(BitTorrent)

Streaming (KanKan)

VoIP (Skype)

File distribution: client-server vs P2P

Q i h h i di ib fil ( i F) f


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Question: how much time to distribute file (size F) from

one server to N peers? peer upload/download capacity is limited resource

us: server uploadcapacity


us

uN

dN

server

network (with abundant

bandwidth)

file, size F

ui: peer i uploadcapacity

di: peer i downloadcapacityu2 d2

u1 d1

di

ui

File distribution time: client-server


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File distribution time: client-server

i i


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server transmission: mustsequentially send (upload) Nfile copies: time to send one copy: F/us time to send N copies: NF/us

client: client must download

us

network

di

ui

F


increases linearly in N

time to distribute F

to N clients usingclient-server approach

Dc-s > max{NF/us,,F/dmin}

file copy dmin = min client download rate

min client download time: F/dmin

The distribution time is the time it takes to

get a copy of the file to all N peers.

File distribution time: P2P

t i i t


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server transmission: mustupload at least one copy

time to send one copy: F/usus

network

di

ui

F

client: each client mustdownload file copy min client download time: F/dmin


time to distribute F

to N clients using

P2P approach

DP2P

> max{F/us,,F/d

min,,NF/(u

s

+ Sui

)}

c ients: as aggregate must up oa its max upload rate (limting max download rate) is us + Sui

but so does this, as each peer brings service capacity

increases linearly in N

File distribution time: P2Pwe first make the following observations


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Client-server vs. P2P: example

li t l d t F/ 1 h 10 d


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2.5

3

3.5

tionTime P2P

Client-Server

client upload rate = u, F/u = 1 hour, us = 10u, dmin us


0

0.5

1

1.5

2

0 5 10 15 20 25 30 35

N

MinimumD

istribu

P2P file distribution: BitTorrent

file divided into 256Kb chunks


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tracker: tracks peersparticipating in torrent

torrent: group of peersexchanging chunks of a file

file divided into 256Kb chunks

peers in torrent send/receive file chunks


Alice arrives

obtains listof peers from tracker and begins exchanging

file chunks with peers in torrent

P2P file distribution: BitTorrent


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peer joining torrent:

has no chunks, but willaccumulate them over timefrom other peers

registers with tracker to get


subset of peers(neighbors)

while downloading, peer uploads chunks to other peers

peer may change peers with whom it exchanges chunks

churn: peers may come and go once peer has entire file, it may (selfishly) leave or

(altruistically) remain in torrent

BitTorrent: requesting, sending file chunks


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requesting chunks: at any given time, different

peers have different subsetsof file chunks

periodically, Alice asks each

sending chunks: tit-for-tat Alice sends chunks to those

four peers currently sending herchunks at highest rate other peers are choked by Alice

do not receive chunks from her


they have

Alice requests missingchunks from peers, rarestfirst

re-evaluate top 4 every10 secs

every 30 secs: randomly selectanother peer, starts sendingchunks optimistically unchoke this peer

newly chosen peer may join top 4

Distributed Hash Table (DHT)


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Hash table

DHT paradigm

rcu ar an over ay networ s

Peer churn

Simple database with(key value) pairs:

Simple Database


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Key Value

John Washington 132-54-3570

Simple database with(key, value) pairs:

key: human name; value: social security #

ana ou se ones - -Xiaoming Liu 385-41-0902

Rakesh Gopal 441-89-1956

Linda Cohen 217-66-5609

.

Lisa Kobayashi 177-23-0199

key: movie title; value: IP address

More convenient to store and search on

Hash Table


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Original Key Key Value

John Washin ton 8 62458 132-54-3570

More convenient to store and search on

numerical representation of key key = hash(original key)

Diana Louise Jones 7800356 761-55-3791

Xiaoming Liu 1567109 385-41-0902

Rakesh Gopal 2360012 441-89-1956

Linda Cohen 5430938 217-66-5609

. Lisa Kobayashi 9290124 177-23-0199

Distribute (key, value) pairs over millions of peers

Distributed Hash Table (DHT)


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st bute ( ey, va ue) pa s ove o s o pee s

pairs are evenly distributed over peers Any peer can query database with a key

database returns value for the key

To resolve query, small number of messages exchanged among

Each peer only knows about a small number of otherpeers

Robust to peers coming and going (churn)

Assign key-value pairs to peers

rule: assign key-value pair to the peer that has the


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rule: assign key value pair to the peer that has the

closest ID. convention: closest is the immediate successor of

the key.

e.g., ID space {0,1,2,3,,63}

suppose 8 peers: 1,12,13,25,32,40,48,60 If key = 51, then assigned to peer 60

If key = 60, then assigned to peer 60

If key = 61, then assigned to peer 1

Circular DHT

each peer only aware of


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1

each peer onlyaware of

immediate successor andpredecessor.

12

13

25

3240

48

60

overlay network

Resolving a query


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1

12

60

What is the valueassociated with key 53

value

13

25

3240

48

O(N) messageson avgerage to resolve

query, when there

are N peers

Circular DHT with shortcuts1 What is the value for

key 53

value


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12

13

60

key 53

each peer keeps track of IP addresses of predecessor,

successor, short cuts. reduced from 6 to 3 messages. possible to design shortcuts with O(log N) neighbors, O(log N)

messages in query

25

3240

Peer churn1

handling peer churn:peers may come and go (churn)

each peer knows address of its


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3

4

12

15


two successorseach peer periodically pings itstwo successors to check aliveness

if immediate successor leaves,

example: peer 5 abruptly leaves8

10immediate successor

Peer churn1

handling peer churn:peers may come and go (churn)



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3

4

12

15


two successorseach peer periodically pings itstwo successors to check aliveness

if immediate successor leaves,

example: peer 5 abruptly leaves

peer 4 detects peer 5s departure; makes 8 its immediatesuccessor

4 asks 8 who its immediate successor is; makes 8simmediate successor its second successor.

810

immediate successor

Chapter 2: outline


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app requirements

2.6 P2P applications2.7 socket programmingwith UDP and TCP


.

2.3 FTP


2.5 DNS

Socket programming

goal: learn how to build client/server applications that


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goal: learn how to build client/server applications that

communicate using socketssocket: door between application process and end-

end-transport protocol


Internet

controlled

by OS

controlled byapp developer

transport

application

physical

link

network

process

transport

application

physical

link

network

processsocket

Socket programming

Two socket types for two transport services:


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Two socket types for two transport services:

UDP: unreliable datagram TCP: reliable, byte stream-oriented

Application Example:


1. Client reads a line of characters (data) from itskeyboard and sends the data to the server.

2. The server receives the data and convertscharacters to uppercase.

3. The server sends the modified data to the client.4. The client receives the modified data and displays

the line on its screen.

Socket programming with UDP

UDP: no connection between client & server


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no handshaking before sending data sender explicitly attaches IP destination address and

port # to each packet

rcvr extracts sender IP address and port# from


UDP: transmitted data may be lost or receivedout-of-order

Application viewpoint:

UDP provides unreliable transfer of groups of bytes(datagrams) between client and server

Client/server socket interaction: UDP

server (running onserverIP) client


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create socket:

clientSocket =

socket(AF_INET,SOCK_DGRAM)

Create datagram with server IP and

create socket, port= x:

serverSocket =

socket(AF_INET,SOCK_DGRAM)

( g )

close

clientSocket

read datagram from

clientSocket

port=x; send datagram viaclientSocketread datagram from

serverSocket

write reply to

serverSocket

specifyingclient address,

port number

Application 2-102

Example app: UDP client

Python UDPClientinclude Pythons socket


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from socket import *

serverName = hostname

serverPort = 12000

clientSocket = socket(socket.AF_INET,

include Python s socket

library

create UDP socket for

server


socket.SOCK_DGRAM)message = raw_input(Input lowercase sentence:)

clientSocket.sendto(message,(serverName, serverPort))

modifiedMessage, serverAddress =

clientSocket.recvfrom(2048)print modifiedMessage

clientSocket.close()

get user keyboard

input

Attach server name, port to

message; send into socket

print out received string

and close socket

read reply characters from

socket into string

Example app: UDP server

Python UDPServer


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serverPort = 12000

serverSocket = socket(AF_INET, SOCK_DGRAM)

serverSocket.bind(('', serverPort))

create UDP socket

bind socket to local port

number 12000


print The server is ready to receivewhile 1:

message, clientAddress = serverSocket.recvfrom(2048)

modifiedMessage = message.upper()

serverSocket.sendto(modifiedMessage, clientAddress)

loop forever

Read from UDP socket intomessage, getting clientsaddress (client IP and port)

send upper case string

back to this client

Socket programming with TCP

client must contact server when contacted by client,TCP k


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server process must first berunning

server must have createdsocket (door) thatwelcomes clients contact

server TCP creates new socketfor server process tocommunicate with thatparticular client

allows server to talk withmulti le clients


client contacts server by: Creating TCP socket,

specifying IP address, portnumber of server process

when client creates socket:

client TCP establishesconnection to server TCP

source port numbers usedto distinguish clients(more in Chap 3)

TCP provides reliable, in-orderbyte-stream transfer (pipe)between client and server

application viewpoint:

Client/server socket interaction: TCP

server (running on hostid) client


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wait for incoming

connection re uest

create socket,port=x, for incoming

request:serverSocket = socket()

create socket,connect to hostid ort=x

TCP


connectionSocket =serverSocket.accept()

clientSocket = socket()

send request using

clientSocketread request from

connectionSocket

write reply to

connectionSocket

connec on se up

close

connectionSocket

read reply fromclientSocket

close

clientSocket

Example app: TCP client

f k t i t *

Python TCPClient


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serverName = servername

serverPort = 12000

clientSocket = socket(AF_INET, SOCK_STREAM)

create TCP socket for

server, remote port 12000


clientSocket.connect((serverName,serverPort))sentence = raw_input(Input lowercase sentence:)

clientSocket.send(sentence)

modifiedSentence = clientSocket.recv(1024)

print From Server:, modifiedSentenceclientSocket.close()

No need to attach server

name, port

Example app: TCP server


Python TCPServer


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from socket import

serverPort = 12000

serverSocket = socket(AF_INET,SOCK_STREAM)

serverSocket.bind((,serverPort))

serverSocket.listen 1

create TCP welcoming

socket

server begins listening for


print The server is ready to receive

while 1:

connectionSocket, addr = serverSocket.accept()

sentence = connectionSocket.recv(1024)capitalizedSentence = sentence.upper()

connectionSocket.send(capitalizedSentence)

connectionSocket.close()

loop forever

server waits on accept()

for incoming requests, newsocket created on return

read bytes from socket (butnot address as in UDP)

close connection to this

client (but notwelcoming

socket)

Chapter 2: summary

our study of network apps now complete!


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application architectures client-server

P2P

application service

specific protocols: HTTP

FTP


requ rements: reliability, bandwidth, delay

Internet transport servicemodel

connection-oriented,

reliable: TCP unreliable, datagrams: UDP

DNS

P2P: BitTorrent, DHT

socket programming: TCP,UDP sockets

Chapter 2: summary

most importantly: learned about protocols!


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typical request/replymessage exchange:

client requests info orservice

important themes:

control vs. data msgs

in-band, out-of-band


server responds withdata, status code

message formats:

headers: fields givinginfo about data

data: info beingcommunicated

centralized vs. decentralized stateless vs. stateful

reliable vs. unreliable msgtransfer

complexity at networkedge


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Chapter 1Additional Slides

Introduction1-111

application

(www browser,packet


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Transport (TCP/UDP)

Network IP

email client)application

OS

packet

analyzer

co of all

Link (Ethernet)

Physical

cap ure

(pcap)

Ethernet

frames

sent/receive

d

lect 8 applaction layer

Documents