Chapppp yter 2: Application Layer - Department of Computer ... s... · Chapppp yter 2: Application...

1

Chapter 2: Application Layerp pp yOur goals: Conceptual aspects

learn about protocols by examining some

of network application protocols transport-layer

application-level protocols HTTP transport layer

service models client-server

HTTP SMTP DNS

P2Pparadigm peer-to-peer

paradigm

P2P Content distribution

networks (CDNs)paradigm

programming network applications sockets API

Application Layer (SSL) 1

sockets API10/3/2017

2

Chapter 2: outline

2 1 principles of 2.5 P2P applications2.1 principles of network applications

2.5 P2P applications2.6 video streaming

and content pp2.2 Web and HTTP2.3 electronic mail

distribution networks (CDNs)

• SMTP2.4 DNS

2.7 socket programming with UDP d TCPUDP and TCP

Application Layer 2-2

3

Creating a network appapplication

Write programs that run on (different) end

systems

applicationtransportnetworkdata linkphysical

systems communicate over network e.g., web server software

i t ith bcommunicates with browser software

No need to write software applicationt tfor network-core devices

This approach allows for rapid app development

applicationtransportnetworkdata link

transportnetworkdata linkphysical

p pp pmphysical

Application Layer (SSL) 310/3/2017

4

Application architectures Client-server

to scale –> racks of servers in a datacenter

to improve throughput and response time -> geographically distributed clusters of servers in CDNs

Peer-to-peer (P2P) difficult to manage difficult to manage

-> super nodes (with some server characteristics) needed in addition to regular peers


5

Client-server architectureserver:

always-on hostP dd permanent IP address

data centers for scalingscaling

clients: communicate with server

( k f )(speak first) may be intermittently

connectedclient/server

may have dynamic IP addresses

do not communicate


directly with each other10/3/2017

6

Pure P2P architecturel no always-on server

arbitrary end systems directly communicatedirectly communicate

peers are intermittently connected and may h IP ddchange IP addresses

difficult to manage find content or another

peer-peer find content or another

peer by broadcast (flooding)

NAT traversal problem NAT traversal problem

Example: BitTorrent has trackers (super nodes that manage the peers and chunks of a torrent)


peers and chunks of a torrent)

7

Processes communicatingProcess: program running

within a host.Client process: process

that initiates i ti within same host, two

processes communicate using inter-process

communicationServer process: process

that waits to beusing inter process communication (defined by OS).

that waits to be contacted

N t li t & processes in different

hosts communicate by

Note: client & server processes run in P2P as well as client-server hosts communicate by

exchanging messages apps

N A i h lid b h d i d


Note: A process in these slides may be a thread instead

8

Addressing processesd f l d h to receive messages,

process must have an identifier

identifier includes both IP address and port number associated with identifier

host device has unique 32-bit IP address

um wprocess on host.

Example port numbers:H P 0 Q: Is IP address of

host on which process runs enough for

HTTP server: 80 Mail server: 25

to send HTTP messageruns enough for identifying the process?

to send HTTP message to gaia.cs.umass.edu web server: IP dd ss: 128 119 245 12 A: No, many

processes can be running on same host

IP address: 128.119.245.12 Port number: 80

more shortly…


g m y

10/3/2017

9

App-layer protocol specs Types of messages

exchanged, Public-domain protocols:g e.g., request, response

Message syntax:h t fi ld i &

p defined in RFCs which

enable interoperability what fields in messages &

how fields are delineated Message semantics

e.g., HTTP, SMTP

Proprietary protocols: meaning of information in

fields Rules for when and how

Proprietary protocols: e.g., Skype

Rules for when and how processes send & respond to messages


10

What transport service does an app need?

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

tolerate some loss

Throughput some apps (e.g., audio,

video) require minimumtolerate some loss other apps (e.g., file

transfer, telnet, email) i 100% li bl d t

video) require minimum amount of throughput to be “effective”

require 100% reliable data transfer

Timing

other apps (“elastic apps”) make use of whatever throughput they getTiming

some apps (e.g., Internet telephony, i t ti )

throughput they get Security Encryption, data interactive games)

require low delay to be “effective”

yp ,integrity, … (in app layer, e.g. SSL/TLS)


11

Transport service requirements of common appsTransport service requirements of common apps

Application Data loss Throughput Time SensitiveApplication

file transfere-mail

Data loss

no lossno loss

Throughput

elasticelastic

Time Sensitive

nono

Web documentsinteractive audio/video

no lossno lossloss-tolerant

elasticelastic (few kbps)audio: 5kbps-1Mbpsvideo:10kbps-5Mbps

noyes, 100’s of ms

streaming stored audio/video

interactive gamesi t t i

loss-tolerant

loss-tolerant

p psame as above

few kbps up

yes, few secs

yes, 100’s of msdinstant messaging no loss elastic yes and no


12

Internet transport protocols servicesTCP service: UDP service:TCP service: connection-oriented: setup

required between client and server processes; state info

UDP service: unreliable data

transfer between server processes; state info

reliable transport: between sending and receiving process

sending and receiving processes

does not provide: flow control: sender won’t

overwhelm receiver congestion control:

pconnection setup, reliability, flow control, congestion control, timing,

congestion control:throttle sender when network is overloaded

does not provide: timin

throughput guarantee, or security

does not provide: timing, minimum throughput guarantee, security

Q: why bother? Why is there a UDP?


13

Internet apps: application & transport protocolspp pp p p

ApplicationApplicationlayer protocol

Underlyingtransport protocolApplication

e-mailremote terminal access

layer protocol

SMTP [RFC 5321]

Telnet [RFC 854]

transport protocol

TCPTCPremote terminal access

Web file transfer

streaming multimedia

Telnet [RFC 854]

HTTP [RFC 2616]

FTP [RFC 959]

HTTP (e. g., Youtube)

TCPTCPTCPTCP g

Internet telephony

( g , )

SIP[RFC 3261],RTP[RFC 3550], proprietary(e.g., Skype) UDP or TCP


14

Chapter 2: outline





• SMTP2.4 DNS



15

Web and HTTPFirst some jargon Web page consists of base HTML-file which

includes several referenced objects

Obj t b HTML fil JPEG i J Object can be HTML file, JPEG image, Java applet, audio file,…

Each object is addressable by a URL, for example

www.someschool.edu/someDept/pic.gifwww.someschool.edu/someDept/pic.gif

host name path name


16

HTTP overviewHTTP: hypertext

transfer protocol Web’s application layer

protocol client/server model

PC runningExplorer

client: browser that requests, receives, displays Web objects

Explorer

p y j server: Web server

sends objects in response to requests

Server running

Apache Webserverp q

Mac runningChrome


17

HTTP overview (continued)HTTP overv ew (cont nued)

Uses TCP: HTTP is “stateless” client initiates TCP

connection (creates socket) to server, port 80

server maintains no information about past client requests, p

server accepts TCP connection from client

HTTP messages exchanged

p q

Protocols that maintain “state” are complex

aside HTTP messages exchanged

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

state are complex past history (state) must

be maintained if / li t h( )

TCP connection closed if server/client crashes,

their views of “state” may be inconsistent, must be reconciled


reconciled

10/3/2017

18

Nonpersistent HTTPSuppose user enters URL

www.someSchool.edu/someDepartment/home.index

(contains text, references to 10

jpeg images)

1a. HTTP client initiates TCP connection to HTTP server (process) at 1b. HTTP server at host

S h l d iti(process) atwww.someSchool.edu on port 80 www.someSchool.edu waiting

for TCP connection at port 80, “accepts” connection, notifying client

2. HTTP client sends HTTP request message (containing URL) into TCP connection socket Message indicates

cl ent

3. HTTP server receives request message forms responsesocket. Message indicates

that client wants object someDepartment/home.index

message, forms response message containing requested object, and sends message into its socket


time10/3/2017

19

Nonpersistent HTTP (cont.)

4. HTTP server closes TCP connection

5. HTTP client receives response message containing html file, displays html. Parsing html

connection.

p y gfile, finds 10 referenced jpeg objects

6. Steps 1-5 repeated for each time p pof 10 jpeg objects


20

Non-Persistent HTTP: Response timeDefinition of RTT: time for

a small packet to travel from client to serverfrom client to server and back.

Response time:initiate TCPconnectionp

one RTT to initiate TCP connection

RTT f HTTP time to

RTTrequestfile

one RTT for HTTP request and first few bytes of HTTP response

time to transmit file

RTT

filereceivedy p

to return file response time total

received

time time

2RTT+ file transmissi n time


= 2RTT+ file transmission time

21

Nonpersistent vs. Persistent HTTPNonpersistent HTTP delay requires 2 additional RTTs

f n bj t

Persistent HTTP delay server leaves connection

open after sending responsefor an object

most browsers open parallel P f h

open after sending response subsequent HTTP messages

between client/server are sent over same connectionTCP connections to fetch

referenced objects very little additional time

f ll f bj ts

sent over same connection client sends a new request

as soon as it encounters a referenced objectfor all reference objects

but OS overhead to allocate host resources for all TCP connections

referenced object (pipelining)

as little as one additional RTT for all referencedall TCP connections RTT for all referenced objects (plus their transmission time)


22

HTTP historyNonpersistent HTTP At most one object is

Persistent HTTP Version 1.1 (since 1999)

lti l bj t bsent over a TCP connection Version 0 9 (in 1991)

multiple objects can be sent over single TCP connection between Version 0.9 (in 1991)

• only one method (GET) • only one object type

(HTML)

client and server. pipelining of requests

(HTML)

Version 1.0 (in 1996)

Version 2 (since 2015)

allows server pushlti l i t Version 1.0 (in 1996)

multiplexing requests over a single TCP connection – without


HOL blocking10/3/2017

23

HTTP request message

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

ASCII (human-readable format)

GET /somedir/page.html HTTP/1.1\r\nHost: www someschool edu\r\n

request line(GET, POST,

HEAD commands) Host: www.someschool.edu\r\n User-agent: Mozilla/5.0\r\nConnection: close\r\n Accept-language:fr\r\n

HEAD commands)

headerlines

Accept language:fr\r\n\r\n Carriage return,

line feed indicateend of header lines


24

HTTP request message: general formatq g g


25

Uploading form inputUpload ng form nput

Post method: Web page often

includes form inputl

GET method: Uses GET method

Input is uploaded to server in entity body

Entity body is empty Input is uploaded in

URL field of requestURL field of request line:

www.somesite.com/animalsearch?monkeys&banana


26

Method typesMethod types

HTTP/1.0 HTTP/1.1 GET POST

GET, POST, HEAD PUT

HEAD asks server to leave

requested object out of

uploads file in entity body to path specified in URL fieldrequested object out of

response DELETE deletes file specified in

the URL fieldthe URL field

… (others)


27

HTTP response messagep m gstatus line(protocol

HTTP/1.1 200 OK\r\n Connection close\r\nDate: Tue, 09 Aug 2011 12:00:15 GMT\r\n

(protocolstatus code

status phrase)

Server: Apache/2.2.3 (CentOS)\r\n Last-Modified: Tue, 09 Aug 2011...\r\n Content-Length: 6821\r\n

/h l\ \

headerlines

Content-Type: text/html\r\n\r\n data data data data data ... data, e.g.,

requestedrequestedHTML file

Aside: several other methods to indicate


Aside: several other methods to indicate length of a response

28

Cookies: keeping state info p g

client serverusual http request msgusual http response +Set-cookie: 1678

Amazon servercreates ID

1678 for user

Cookie file

ebay: 8734 Set coo e: 6 8

usual http request msgcookie: 1678 cookie-

1678 for user

Cookie file

amazon: 1678

y

cookie: 1678

usual http response msgspecificaction

o : 678ebay: 8734

one week later:

usual http request msgcookie: 1678

usual http response msg

cookie-spectific

action

Cookie fileamazon: 1678ebay: 8734


usual http response msg actiony

10/3/2017

29

Cookies Cook es Application end points keep state at client

and server across multiple transactionsp http messages carry state info but http

protocol is stateless

What cookies can bring: authorization

Cookies and privacy: cookies permit sites

shopping carts recommendations i t t

(including third parties) to learn a lot about you

user session state (Web e-mail)

about you


30

Web cache (proxy server)

user sets browser: web accesses via

Goal: satisfy client request without involving origin servers

web accesses via cache

browser sends all Proxy

origin server

HTTP requests to cache if object is not in

client

Proxyserver

if object is not in cache, request object from origin server, then return object tothen return object to client

else cache returns object

clientorigin server


object server

10/3/2017

31

More about Web cachingMore about Web cach ng

cache acts as both Why Web caching?client and server

typically cache is i st ll d b ISP

y g reduce response time

for client requestinstalled by ISP (university, company, residential ISP)

reduce traffic on an institution’s access link.n .


32

Caching example origin

Assumptions average object size = 1 Mbits avg request rate from institution’s

originservers

publicI t t avg. request rate from institution s

browsers to origin servers = 15/sec Internet delay (ISP router to any

origin server and back) = 2 sec

Internet

origin server and back) = 2 secConsequences utilization on LAN = 15% institutional

15 Mbps access link

utilization on access link = 100% total delay = Internet delay + access

link delay + LAN delay

network 100 Mbps LAN

= 2 seconds + minutes + 0.01 second

many lost packets in reponses

institutionalcache


Assumption: request is very short and takes no time to transmit

33

Caching example (2)g mp ( )possible solution in s b nd idth f ss

originservers

increase bandwidth of access link to, say, 100 Mbps public

Internet

consequence utilization on LAN = 15% utilization on access link = 15%

100 Mbps access link

Total delay = Internet delay + access link delay + LAN delay

= 2 + 0.01 + 0.01 seconds

institutionalnetwork 100 Mbps LAN

access l nk

often a costly upgradeinstitutional

cache


cache

10/3/2017

34

Caching example (3)possible solution: install cache suppose hit rate is 0.4

originservers

consequence 40% requests will be satisfied

almost immediately

publicInternet

almost immediately 60% requests satisfied by

origin servers utilization of access link

d d 60% l i i

15 Mbps access linkreduced to 60%, resulting in

negligible delay (say 70 ms) ave. total delay = Internet

delay + access link delay + LAN

institutionalnetwork 100 Mbps LAN

access l nk

y ydelay= 0.6*(2 + 0.07 + 0.01) +

0.4* (0 + 0 + 0.01) seconds1 25

institutionalcache


= 1.25 sec cache

10/3/2017

35

Conditional GET Goal: don’t send object if

cache has up-to-date cached version

cache serverversion

cache: indicates date of cached copy in HTTP request to server

HTTP request msgIf-modified-since:

<date>object

notto serverIf-modified-since:

<date>

server: response contains no

HTTP responseHTTP/1.1

304 Not Modified

not modified

server: response contains no object if cached copy is up-to-date: HTTP/1.1 304 Not

HTTP request msgIf-modified-since: /

Modified <date>

HTTP responseHTTP/1 1 200 OK

object modified


HTTP/1.1 200 OK<data>

10/3/2017

36

Chapter 2: outline





• SMTP2.4 DNS



37

Electronic MailMajor components:

outgoing message queueMajor components:

user agents mail servers

user mailbox

message queue

useragent

simple mail transfer protocol: SMTP

Us A nt

useragent

mailserver

agent

SMTPUser Agent a.k.a. “mail reader” composing, editing, reading mail

mailserver user

agent

SMTP

SMTPmessagese.g., Outlook, alpine, Mozilla Thunderbird, iPhone mail client

useragent

mailserver

SMTP

useragent

user


agent

10/3/2017

38

Electronic Mail: mail serversMail Servers mailbox contains incoming

messages for useruseragentmessages for user

message queue of outgoing (to be sent) mail messages

useragent

mailserver

agent

SMTPSMTP protocol

between mail servers to

mailserver user

agent

SMTP

SMTPbetween mail servers to send email messages “client”: sender “server”: receiver

useragent

mailserver

SMTP

server : receiver server

useragent

user

Looks like it is the very first (hybrid) P2P

!Application Layer (SSL) 38

useragent

10/3/2017

system!

39

Mail access protocolsMa l access protocols

useruserSMTP SMTP Mail access

t l agent

sender’s mail

useragent protocol

receiver’s mail

SMTP: for delivery to receiver’s server Mail access protocol: for retrieval from server

server server

p POP: Post Office Protocol [RFC 1939]

• authorization (agent <-->server) and download IMAP: Internet Mail Access Protocol [RFC 3501] IMAP: Internet Mail Access Protocol [RFC 3501]

• more features (keeps user state across sessions)• manipulation of stored msgs in folders on server

W b l HTTP f b h d d


Web email: use HTTP for both send and receive10/3/2017

40

Electronic Mail: SMTP uses TCP to reliably transfer email message from

client to server, port 25 direct transfer: sending server to receiving server three phases of transfer

h ndsh kin ( tin ) handshaking (greeting) transfer of messages (persistent connection) closure closure

command/response interaction commands: ASCII text response: status code and phrase

messages must be in 7-bit ASCII


41

Sample smtp interaction(after TCP connection established)S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <[email protected]> S: 250 [email protected]... Sender ok C: RCPT TO: <[email protected]> S: 250 [email protected] ... Recipient ok C: DATA S 354 E t il d ith " " li b it lfS: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C:C: . S: 250 Message accepted for delivery C: QUIT S: 221 hamburger.edu closing connection


g g

10/3/2017

42

Try SMTP interaction for yourself:Try SMTP interaction for yourself

telnet servername 25

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

commandsabove lets you send email without using email client

(reader)(reader)

Log on a UTCS linux machine. Then doLog on a UTCS linux machine. Then dotelnet mailbox2.cs.utexas.edu 25 (opens TCP conn.)


43

Mail message formatSMTP (rfc 5321): protocol for

exchanging email msgsf 5322 t d d f t trfc 5322: standard for text

message format:header

blankline

header lines, e.g., To: From:

body

Subject:different from SMTP

commands and responsescommands and responses body

the “message”, ASCII characters only


characters only

10/3/2017

44

Message format: multimedia extensions

Multipurpose Internet Mail Extensions (MIME) additional lines in msg header declare media additional lines in msg header declare media

content types (also multipart content types)

From: [email protected] To: [email protected] Subject: Picture of yummy crepe. MIME-Version: 1.0method used

MIME version

MIME Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg

b 64 d d d t

multimedia datatype subtype

to encode data

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

type, subtype, parameter declaration

encoded data


45

Chapter 2: outline





• SMTP2.4 DNS



46

DNS: Domain Name SystemD D m m y mPeople: many identifiers:

SSN, name, passport #Domain Name System: distributed database, , p p

Internet hosts, routers: IP address (32 bit)

d f dd i

distributed databaseimplemented as a hierarchy of many name servers

- used for addressing datagrams

“name”, e.g.,

application-layer protocolused to resolve names (address/name translation)

www.yahoo.com - used by humans

Required: map between

(address/name translation) note: core Internet function,

implemented as application-layer protocolequ red map between

name and IP addresseslayer protocol

complexity at network’s “edge”


47

DNS Why not centralized DNS? single point of failure

DNS services Hostname to IP single point of failure

traffic volume distant centralized

address translation Host alias

C i l d li database maintenance/update

Canonical and alias names

Mail server alias

doesn’t scale!

Mail server alias Load distribution

Replicated Web pserver: a set of IP addresses for one canonical host name


canonical host name10/3/2017

48

Distributed, Hierarchical DatabaseRoot DNS Servers

Top-level domains

.com DNS servers .org DNS servers .edu DNS servers

poly edu umass edub poly.eduDNS servers

umass.eduDNS serversyahoo.com

DNS serversamazon.comDNS servers

pbs.orgDNS servers

Client wants IP address for www amazon com: 1st approx:Client wants IP address for www.amazon.com: 1 approx: Client queries a root server to find .com DNS server Client queries .com DNS server to get amazon.com q g

DNS server Client queries amazon.com DNS server to get IP

dd f


address for www.amazon.com10/3/2017

49

DNS: root name servers 13 root name13 root name “servers”worldwide

c. Cogent, Herndon, VA (5 other sites)d. U Maryland College

e. NASA Mt View, CAf Internet Software C

i. Netnod, Stockholm (37 other sites)

k. RIPE London (17 other sites)

m WIDE Tokyo

y gPark, MDh. ARL Aberdeen, MDj. Verisign, Dulles VA (69 other sites )

a. Verisign, Los Angeles (5 other sites)

f. Internet Software C.Palo Alto, CA (and 48 other sites)

m. WIDE Tokyo(5 other sites)

(5 other sites)b. USC-ISI Marina del Rey, CAl. ICANN Los Angeles, CA

(41 other sites)

g. US DoD Columbus, OH (5 other sites)


50

TLD and Authoritative Servers Top-level domain (TLD) servers: responsible

for .com, .org, .net, .edu, …, and all top-level t d i k f jcountry domains .uk, .fr, .ca, .jp

Verisign maintains servers for .com TLD Educause for edu TLD Educause for .edu TLD

Authoritative DNS servers: organization’s DNS servers, providing authoritative p ghostname-to-IP mappings for organization’s named hosts (e.g., Web and mail). Can be locally maintained by organization or a Can be locally maintained by organization or a

service provider


51

Local Name ServerLocal Name Server

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

university) has one.y) also called “default name server”

when host makes DNS query, query is sent q y, q yto its local DNS server which acts as proxy, forwards query into

hi hhierarchy


52

root DNS serverDNS name s l ti l

23

TLD DNS

resolution example Host at cis.poly.edu

t IP dd f4

5

TLD DNS serverwants IP address for gaia.cs.umass.edu

It sends a recursivelocal DNS server

dns.poly.edu

1678

It sends a recursive query to local DNS server

1

authoritative DNS serverdns.cs.umass.edu

8iterated query: contacted server

replies with name of requesting host

cis.poly.edu

gaia.cs.umass.edu

dns.cs.umass.edupserver to contact“I don’t know this name, but ask this


gaia.cs.umass.edu,other server”

10/3/2017

53

DNS: caching when (any) name server learns mapping, it caches

mapping cached entries timeout (disappear) after

some time (e.g., 2 days) TLD servers typically cached in local name TLD servers typically cached in local name

servers -Thus root name servers not often visited

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

if h st h d IP dd ss this t b if host changed IP address, this may not be known Internet-wide until all TTLs expire


54

DNS recordsDNS: distributed db storing resource records (RR)

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

Type=A name is hostname

Type=CNAMEl f

T NS

name is hostname value is IP address

name is alias name for some “canonical” (the real) namee.g., www.ibm.com is really

Type=NS name is domain (e.g.

foo.com)

servereast.backup2.ibm.com

value is canonical name

value is hostname of authoritative name server for this domain

Type=MX value is host name of mail

server associated with name


55

DNS protocol messagesDNS protocol : query and reply messages, both with

same message format

msg header identification: 16 bit #

f lfor query, reply to query uses same #

flags: query or reply recursion desired recursion availabler cur n a a a reply is authoritative


56

DNS protocol messages (cont.)D p m g ( .)

N t fi ldName, type fieldsfor a query

RR iRRs in responseto query

records forrecords forauthoritative servers

additional “helpful”additional helpfulinfo that may be used


57

Inserting records into DNS Example: just created startup “Network Utopia” Register name networkutopia.com at a registrar (e.g.,

Network Solutions)Network Solutions) Need to provide registrar with names and IP addresses of

your authoritative name servers (primary and secondary)

Registrar inserts NS and A resource records for each authoritative server into .com TLD servers:(networkutopia.com, dns1.networkutopia.com, NS)(dns1 networkutopia com 212 212 212 1 A)(dns1.networkutopia.com, 212.212.212.1, A)

Put into authoritative servers, CNAME record for www networkutopia com MX record forwww.networkutopia.com, MX record for networkutopia.com, and …

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


How do people get the IP address of your Web site?10/3/2017

58

Chapter 2: outline





• SMTP2.4 DNS



59

P2P architecture no always-on server arbitrary end systems arbitrary end systems

directly communicate peers are intermittently

d d h IPconnected and change IP addresses peer-peer

Hybrid deployment examples: File distribution, e.g.,

BitTorrent (has trackers)BitTorrent (has trackers) Internet telephony, e.g.,

original Skype (had super peers)


peers),

10/3/2017

60

File Distribution: Client-Server vs P2PQuestion : How much time to distribute a file

from one server to N peers?p

S

us server upload capacity (bps)

uu2d1 d2

u1

Server ui peer i upload capacity (bps)di peer i downloadus

d2

dN Network (with

File, size Fdi peer i download capacity (bps)

uN

Network (with abundant bandwidth)


61

Time to distribute file to N users : Client-Server

u

Server

F

server sequentially sends N copies:

us

u2d1 d2u1

dNNetwork (with

F NF/us time

client i takes F/di ti t d l d

uN

dN abundant bandwidth)time to download

Tcs ≥ max { NF/us , F/min(di) }Time to distribute Fto N clients is

Wh N is l mi (d ) > /N

cs { s , ( i) }i(lower bound)


When N is large, min(di) > us/N10/3/2017

i

62

Time to distribute file to N users: P2P

u

Server

F

server must send one copy: F/us time

us

u2d1 d2u1

dNNetwork (with

Fpy s

client i takes F/di time to download

l d duN

dN abundant bandwidth) NF bits must be uploaded (aggregate)

fastest possible upload rate: u + Σ ufastest possible upload rate: us + Σ1≤i≤N-1 ui

TP2P ≥ max { F/us , F/min(di) , NF/(us + Σui) } Largest TP2P ≥ max { F/us , F/min(di) , NF/(us Σui) }i

us > (us + Σui)/N and, for large N, min(di) > (us + Σui)/N

gvalue as N ↑


s ( s i) , g , ( i) ( s i)i

63

Client-server vs. P2P: exampleCli t l d t f ll i F/ 1 i 10 d

3.5P2P

Client upload rate = u for all i, F/u = 1 min., us = 10u, dmin ≥ u

2.5

3

tion

Tim

e

Client-Server

/ / (10 )sNF u NF u=

1.5

2

m D

istri

but ( )/ 10 min.

s

N=1

as

/ ( ) 1 min.N

s i

N

NF u u = F / u−

→ ∞

+ =

2 servers

0.5

1

Min

imum 1

/ ( ) .s ii

N u u / u=

00 5 10 15 20 25 30 35

N


What if more than 1 server? I.e., a data center

64

Chapter 2: outline





• SMTP2.4 DNS



65

Sockets process sends/receives

messages to/from its socket

host orserver

host orserversocket

socket analogous to “door” sending process shoves

server se ve

controlled byapp developer

message out of door it relies on transport

infrastructure on other

process

TCP with

socket

process

TCP with

socket

infrastructure on other side of door which delivers message to

k t t i i

C w tbuffers,variables

buffers,variables

Internet

socket at receiving process

controlledby OS


66

Socket programmingGoal: learn how to write client and server programs

which communicate by sending data into sockets, di d t t f k t

Socket API introduced in BSD4 1 UNIX 1981

reading data out of sockets

introduced in BSD4.1 UNIX, 1981 sockets are explicitly created, used, then

released by applications y pp client/server paradigm API: choice of a transport protocol and ability

t if f tto specify a few parameters reliable byte stream unreliable datagram


unreliable datagram 10/3/2017

67

Socket programming with TCPBefore client contact: server process must first

be running

When contacted by client, server TCP creates a new connection socket for server be runn ng

server must have created socket (door) that welcomes client’s contact

process to communicate with client allows server to talk with

l l lw m

Client contacts server by: creating client-local TCP

s k t

multiple clients source IP address and port

number used to distinguish l ( h )socket

specifying IP address, port number of server process

h l k

clients (more in Chap 3)

CP d l bl dapplication viewpoint

When client makes connect call: client TCP establishes connection to server TCP

TCP provides reliable, in-ordertransfer of byte stream

between client and server


68

Socket API for TCP (BSD Unix) S id Client side

socket(), returns client socket id

Server side socket(), returns server

socket idsocket id connect(), need to

specify server IP address and port sends

bind(), binds server socket to server IP address and portaddress and port, sends

conn req send(), sends to client

socket

listen(), willing to accept conn req on server socket

accept(), accepts new conn socket recv(), receives from

client socket close() closes

p preq and returns its connection socket id

recv(), receives from close(), closes

connection

note: OS supplies local IP address

connection socket send(), sends to connection

socket


note: OS supplies local IP address and port for client close(), closes connection

10/3/2017

69

Client/server socket interaction: TCPserver (running on hostid) client

create socket: port=xserverSocket = socket()

S k t bi d()

server (running on hostid) client

wait for incoming connection request

serverSocket.bind()serverSocket.listen()

create socket:clientSocket = socket()connection request

connectionSocket =serverSocket.accept()

connect to hostid, port=xclientSocket socket()

TCP connection setup

send request usingclientSocketrecv request from

connectionSocket

send reply toconnectionSocket

close

recv reply fromclientSocket

close

Application Layer

connectionSocket closeclientSocket

10/3/2017

69Note: syntax is neither complete nor precise

70

Socket programming with UDPUDP: no “connection”

between client and serverh d h k

application viewpoint

UDP provides unreliable transfer no handshaking sender explicitly attaches IP

address and port of destination k

pof groups of bytes (“datagrams”)

between client and server

to every packet server must extract IP

address, port of sender from i d kevery received packet

UDP: transmitted data may be received out of orderbe received out of order, or lost


71

Socket API for UDP (BSD Unix) Client side

socket(), returns client socket id

Server side socket(), returns server

socket idsocket id sendto(), sends to client

socket, need to specify destination’s IP address and

bind(), binds server socket to server IP address and portdestination s IP address and

port recvfrom(), receives from

client socket: data and

p recvfrom(), receives

from server socket: data and sender’s IP client socket data and

sender’s IP address and port

bind() optional

address and port sendto(), sends to

server socket, need to bind(), optional note: needs timeout

management;OS supplies local IP address and port for

,specify destination’s IP address and port


OS supplies local IP address and port for client if bind() not used10/3/2017

72

Client/server socket interaction: UDP

create socket:cli ntS ck t

create socket: port= xserverSocket =

server (running on serverIP) client

clientSocket =socket(AF_INET, SOCK_DGRAM)

create datagram with server IP

socket(AF_INET, SOCK_DGRAM)serverSocket.bind()

create datagram with server IP address and port=x; send datagramto clientSocket

recv datagram with client’s address fromserverSocket

recv datagram with server’s address froml k

serverSocket

send reply withclient’s address to

clientSocket

closeclientSocket

serverSocket

clientSocket

10/3/201772Application Layer

Note: syntax is neither complete nor precise

73

Chapter 2: outline





• SMTP, POP3, IMAP2.4 DNS



74

Video Streaming and CDNs: context

• Netflix, YouTube: 37%, 16% of downstream

video traffic: major consumer of Internet bandwidth

Netfl x, You ube 37%, 6% of downstreamresidential ISP traffic in 2015

• ~1B YouTube users, ~75M Netflix users challenge: scale ~1B users?

• single mega-video server won’t workch ll n : h t n it challenge: heterogeneity different users have different capabilities

and bandwidths (e.g., wired versus mobile)( g , m ) solution: distributed, application-level

infrastructure

Application Layer

2-74

75

Multimedia: videospatial coding example: instead of sending N values of same color (all purple), send only

CBR: (constant bit rate): video encoding rate fixed ……………………..

co or (a purp ), s n on ytwo values: color value (purple) and number of repeated values (N)

……………….…….

VBR: (variable bit rate): video encoding rate changes as g gamount of spatial, temporal coding changes examples: frame imp• MPEG-1 (CD-ROM) 1.5

Mbpstemporal

• MPEG-2 (DVD) 3-6 Mbps• MPEG-4 (often used in

Internet < 1 Mbps)frame i+1

mpcoding example: instead of sending complete frame at i+1, send only Internet < 1 Mbps) differences from frame i

Application Layer

2-75

76

Streaming stored video: g

simple scenario:p

video server client

Internet

(stored video)client

Video can be compressed to essentially any bit rate by trading image qualityrate by trading image qualityE.g., Internet video from 100Kbps for low quality

to over 3 Mbps for high-definition movies

Application Layer

2-76

77

Streaming multimedia: DASH Dynamic, Adaptive Streaming over HTTP server:

di id id fil i t lti l h k divides video file into multiple chunks chunks stored, encoded at different rates manifest file: provides URLs for different chunks manifest file: provides URLs for different chunks

client: periodically measures server-to-client bandwidthp y consulting manifest, requests one chunk at a time

• chooses maximum coding rate sustainable at current bandwidth

• can choose different coding rates at different points in timetime

Application Layer

2-77

78

Streaming multimedia: DASH (cont.) “intelligence” at client:

client determinesclient determines when to request chunk - so that buffer

starvation, or overflow does not occur what encoding rate to request - higher quality

when more bandwidth available

where to request chunk – may possibly request from another server that has higher available f m gbandwidth (less congested path) [actually client will need help from CDN]

Application Layer

2-78

79

Content distribution networkschallenge: how to stream content (selected

from millions of videos) to hundreds of thousands of simultaneous users?

option 1: single, large “mega-server”single point of failurepoint of network congestionlong path to distant clients

lti l i f id t t imultiple copies of same video sent over outgoing links

this solution doesn’t scale….this solution doesn t scale

Application Layer

2-79

80

Content distribution networks option 2: third-party CDN multiple copies

of videos at geographically distributed , t / itstore/serve sites: enter deep: place CDN servers deep into many

access networksaccess networks • close to users• e.g., used by Akamai, 1700 locations

bring home: smaller number (10’s) of larger bring home: smaller number (10 s) of larger clusters in IXPs/POPs near (but not within) access networks

• e.g., used by Limelight

option 3: private CDN, e.g., Google and NetFlix– use both enter-deep and bring-home servers


81

CDN content access: DNS redirectBob (client) requests video http://video.netcinema.com/6Y7B23V video stored in KingCDN.com

11. Bob clicks on URL for video http://video netcinema com/6Y7B23Vhttp://video.netcinema.com/6Y7B23Von netcinema.com web page

22. Client resolves video.netcinema.comvia local DNS

4&5. Local DNS resolves 56. request video from Bob’s

netcinema.com 3. netcinema’s DNS returns hostname 1105 KingCDN com 4

1105.KingCDN.comfrom KingCDN’s authoritative DNS, which returns the IP address of a KingCDN server

6. request video fromKINGCDN server,streamed via HTTP

local DNSserver

netcinema.com

3

hostname 1105.KingCDN.com 4 g

Bob’s client then establishes a direct TCP connection with the KingCDN server and issues an HTTP GET request for video

KingCDN.comnetcinema’s

authoratative DNSHTTP GET request for video

KingCDNauthoritative DNS Application

Layer2-81

82

Cluster selection strategy (proprietary)f d D

subscriber requests content from CDN

copies of content stored at CDN servers

subscriber requests content from CDN• directed to nearby copy, retrieves content• may choose different copy if network path is m y ff py f p

congested

if t filwhere’s Madmen?

manifest file

Application Layer

2-82

83

Case study: Netflix

Amazon upload copies of lti l i f

Netflix

Netflix registration,

Amazon cloud CDN

server

multiple versions of video to CDN servers

gaccounting servers

22. Bob browsesNetflix video

3. Manifest filereturned for requested video

CDNser er

1

1. Bob manages

2Netflix video 3 requested video server

1. Bob manages Netflix account

4. DASH

CDNserver

4. DASH streaming

Application Layer

2-83

84

End of Chapter 2End of Chapter 2


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