what is a distributed system? - postechdpnm.postech.ac.kr/cs600/lecture/e600-module1.pdf · a...

Module 1: Introduction to Networking & Internetworking

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What is a Distributed System?


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Working DefinitionA distributed system is several computers doing something together

Three primary features of a distributed systemMultiple computersCommunications“Virtual” Computer

A well designed distributed system should be perceived as a single, integrated computing facility

World Wide WebAutomatic Teller MachinesCell Phones


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Typical Layering in DSs

Computer and network hardware

PlatformOperating system

Middleware

Applications, services


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DS LayersPlatform

Windows NT / Pentium processorSolaris / SPARC processor

Middleware:

Achieve transparency of heterogeneity at platform levelAchieve communication and resource sharing, e. g., remote method invocationExamples:

CORBA (OMG)DCOM (Microsoft)ODP (ITU- T/ ISO)Java Remote Method Invocation (Sun)


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Module 1Networking &

Internetworking


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Goal/OverviewGoal:

get context, overview, “feel” of networkingApproach:

descriptiveuse Internet as example

Overview:what’s the Internetwhat’s a protocol?network edgenetwork coreaccess net, physical mediaperformance: loss, delayprotocol layers, service modelsbackbones, NAPs, ISPs


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What’s the Internet: “nuts and bolts” view

millions of connected computing devices: hosts, end-systems

pc’s workstations, serversPDA’s phones, toasters

running network appscommunication links

fiber, copper, radio, satellite

routers: forward packets (chunks) of data thru network

local ISP

companynetwork

regional ISP

router workstationserver

mobile


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What’s the Internet: “nuts and bolts” view

protocols: control sending, receiving of msgs

e.g., TCP, IP, HTTP, FTP, PPPInternet: “network of networks”

loosely hierarchicalpublic Internet versus private intranet

Internet standardsRFC: Request for commentsIETF: Internet Engineering Task Force

local ISP

companynetwork

regional ISP

router workstationserver

mobile


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What’s the Internet: a service view

communication infrastructure enables distributed applications:

WWW, email, games, e-commerce, database., voting, more?

communication services provided:

connectionlessconnection-oriented

cyberspace [Gibson]:“a consensual hallucination

experienced daily by billions of operators, in every nation, ...."


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Some good hyperlinksInternet Engineering Task Force (IETF)http://www.ietf.org

World Wide Web Consortium (W3C)http://www.w3c.org/Consortium

Association for Computing Machinery (ACM)http://www.acm.org/sigcom”Special interest group in Data Communications”

Institute of Electrical and Electronics Engineers (IEEE)http://www.comsoc.org ”Communications Society”http://www.computer.org/ ”Computer Society”


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Some other good hyperlinksConnected: An Internet Encyclopediahttp://www.FreeSoft.org/CIE/index.htm

Data Communications (tutorials)http://www.data.com/Tutorials

Media history projectshttp://www.mediahistory.com


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What’s a protocol?human protocols:

“what’s the time?”“I have a question”introductions

… specific msgs sent… specific actions taken

when msgs received, or other events

network protocols:machines rather than humansall communication activity in Internet governed by protocols

protocols define format, order of msgs sent and received among network

entities, and actions taken on msg

transmission, receipt


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What’s a protocol?a human protocol and a computer network protocol:

Q: Other human protocol?

Hi

HiGot thetime?2:00

TCP connectionreq.

TCP connectionreply.

Get http://www.cs.postech.kr/DistSys_course/index.htm<file>

time


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A closer look at network structure:

network edge:applications and hostsnetwork core:

routersnetwork of networks

access networks, physical media:communication links


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The network edge:end systems (hosts):

run application programse.g., WWW, emailat “edge of network”

client/server modelclient host requests, receives service from servere.g., WWW client (browser)/ server; email client/server

peer-peer model:host interaction symmetric

e.g.: teleconferencing


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Network edge: connection-oriented service

Goal: data transfer between end sys.handshaking: setup (prepare for) data transfer ahead of time

Hello, hello back human protocolset up “state” in two communicating hosts

TCP - Transmission Control Protocol

Internet’s connection-oriented service

TCP service [RFC 793]reliable, in-order byte-stream data transfer

loss: acknowledgements and retransmissions

flow control:sender won’t overwhelm receiver

congestion control:senders “slow down sending rate” when network congested


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Network edge: connectionless service

Goal: data transfer between end systems

same as before!UDP - User Datagram Protocol [RFC 768]: Internet’s connectionless service

unreliable data transferno flow controlno congestion control

App’s using TCP:HTTP (WWW), FTP (file transfer), Telnet (remote login), SMTP (email)

App’s using UDP:streaming media, teleconferencing, Internet telephony


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The Network Core

mesh of interconnected routersthe fundamental question: how is data transferred through net?

circuit switching:dedicated circuit per call: telephone netpacket-switching: data sent thru net in discrete “chunks”

Restaurant Analogy !


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Network Core: Circuit Switching

End-end resources reserved for “call”link bandwidth, switch capacitydedicated resources: no sharingcircuit-like (guaranteed) performancecall setup required


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Network Core: Circuit Switchingnetwork resources

(e.g., bandwidth) divided into “pieces”pieces allocated to callsresource piece idle if not used by owning call (no sharing)dividing link bandwidth into “pieces”

frequency divisiontime division


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Circuit Switching: A numerical exampleAssumptions:

A sent file to B: 640Kbits All links use TDM with:

24 slotsbit rate of 1.536 Mbps

Establishing end-end circuit: 500 msec

How long does it take to send the file?

http://www.att.com/technology/forstudents/brainspin/networks/bandwidth/game.html


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Network Core: Packet Switchingeach end-end data stream

divided into packetsuser A, B packets sharenetwork resourceseach packet uses full link bandwidth resources used as needed,

resource contention:aggregate resource demand can exceed amount availablecongestion: packets queue, wait for link usestore and forward: packets move one hop at a time

transmit over linkwait turn at next link

Bandwidth division into “pieces”Dedicated allocationResource reservation


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Network Core: Packet Switching

Packet-switching versus circuit switching: human restaurant analogy

A

B

C10 MbsEthernet

1.5 Mbs

45 Mbs

D E

statistical multiplexing

queue of packetswaiting for output

link


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Network Core: Message Switching

Time (sec.)

15

10

5

0Message

Message

Message

Message

Message switching = Packet-switching without segmentation Packet switching: Store and Forward

Message remains in tact as it traverses the network

Example: time to move message from source to destination?msg size = 7.5 Mbits; link rate = 1.5Mbps; no congestion


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Network Core: Packet SwitchingPacket-switching:

store and forward behavior


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Packet switching versus circuit switching

1 Mbit linkeach user:

100Kbps when “active”active 10% of time

circuit-switching: 10 users

packet switching: with 35 users, probability > 10 active less that .004

Packet switching allows more users to use network!

N users1 Mbps link


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Packet switching versus circuit switching

Great for bursty dataresource sharingno call setup

Excessive congestion: packet delay and lossprotocols needed for reliable data transfer, congestion control

Q: How to provide circuit-like behavior?bandwidth guarantees needed for audio/video apps

still an unsolved problem in the Internet

Is packet switching a “slam dunk winner?”


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Packet-switched networks: routing

Goal: move packets among routers from source to destination

There are several routing algorithms (RIP, OSPF, BGP, …)datagram network:

destination address determines next hoproutes may change during sessionanalogy: driving, asking directions

virtual circuit network:each packet carries tag (virtual circuit ID), tag determines next hopfixed path determined at call setup time, remains fixed thru callrouters maintain per-call state


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Network Taxonomy

telecommunication networks

Packet-switchednetworks

Circuit-switchednetworks

FDM TDM Networkswith VCs

DatagramNetworks

connection-oriented

connection-oriented & connectionless


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Access networks and physical media

Q: How to connect end systems to edge router?residential access netsinstitutional access networks (school, company)mobile access networks

Keep in mind: bandwidth (bits per second) of access network?shared or dedicated?


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Residential access: point to point access

Dialup via modemup to 56Kbps direct access to router (conceptually)

ISDN: integrated services digital network: 128Kbps all-digital connect to routerADSL: asymmetric digital subscriber line

up to 1 Mbps home-to-routerup to 8 Mbps router-to-homedeployment: available via telephone companies, e.g., KT


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Residential access: cable modems

HFC: hybrid fiber coaxasymmetric: up to 10Mbps upstream, 1 Mbps downstream

network of cable and fiber attaches homes to ISP router

shared access to router among homeissues: congestion, dimensioning

deployment: available via cable companies, e.g., …


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Institutional access: local area networks

company/univ local area network (LAN) connects end system to edge routerEthernet:

shared or dedicated cable connects end system and router10 Mbs, 100Mbps, Gigabit Ethernet

deployment: institutions, home LANs soonLANs


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Wireless access networks

shared wireless access network connects end system to routerwireless LANs:

radio spectrum replaces wiree.g., IEEE 802.11a/b

wider-area wireless access

CDPD: wireless access to ISP router via cellular networkIEEE 802.20

basestation

mobilehosts

router


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Physical Media

physical link:transmitted data bit propagates across linkguided media:

signals propagate in solid media: copper, fiber

unguided media:signals propagate freely, e.g., radio

Twisted Pair (TP)two insulated copper wires

Category 3: traditional phone wires, 10 Mbps ethernetCategory 5 TP: 100Mbps ethernet


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Physical Media: coax, fiber

Coaxial cable:wire (signal carrier) within a wire (shield)

baseband: single channel on cablebroadband: multiple channel on cable

bidirectionalcommon use in 10Mbs Ethernet

Fiber optic cable:glass fiber carrying light pulseshigh-speed operation:

100Mbps Ethernethigh-speed point-to-point transmission (e.g., 5 Gps)

low error rate


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Physical media: radio

signal carried in electromagnetic spectrumno physical “wire”bidirectionalpropagation environment effects:

reflection obstruction by objectsinterference

Radio link types:microwave

e.g. up to 45 Mbps channelsLAN

2Mbps, 11Mbpswide-area (e.g., cellular)

e.g. CDPD, 10’s Kbpssatellite

Bandwidth in the Gbps range250 msec end-end delay -two waysgeostationary vs low altitude


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Delay in packet-switched networkspackets experience delay

on end-to-end pathfour sources of delay at each hop

nodal processing: check bit errorsdetermine output link

queueingtime waiting at output link for transmission depends on congestion level of router

A

B

propagation

transmission

nodalprocessing queueing


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Delay in packet-switched networksTransmission delay:

R=link bandwidth (bps)L=packet length (bits)time to send bits into link = L/R

Propagation delay:d = length of physical links = propagation speed in medium (~2x108 m/sec)propagation delay = d/s

A

B

propagation

transmission

nodalprocessing queueing

Note: s and R are very different quantitites!


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Queueing delay (revisited)

R=link bandwidth (bps)L=packet length (bits)a=average packet arrival rate

traffic intensity = La/R

La/R ~ 0: average queueing delay smallLa/R -> 1: delays become largeLa/R > 1: more “work” arriving than can be serviced, average delay infinite!


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Protocol “Layers”Networks are complex!

many “pieces”:hostsrouterslinks of various mediaapplicationsprotocolshardware, software

Question:Is there any hope of organizing structure of

network?

Or at least our discussion of networks?


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Organization of air travel

a series of steps

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing


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Organization of air travel: a different view

Layers: each layer implements a servicevia its own internal-layer actionsrelying on services provided by layer below

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing


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Layered air travel: services

Counter-to-counter delivery of person+bags

baggage-check-to-baggage-claim delivery

people transfer: loading gate to arrival gate

runway-to-runway delivery of plane

airplane routing from source to destination


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Distributed implementation of layer functionality

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing

Dep

arti

ng a

irpo

rt

arri

ving

air

port

intermediate air traffic sitesairplane routing airplane routing


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Example of a 4 layers Protocol Stack

source destination

M

M1

M1

M1

H3

H3H2H3H2H1

Originalmessage

M2

M2

M2

H3

H3H2H3H2H1

M

M1

M1

M1

H3

H3H2H3H2H1

M2

M2

M2

H3

H3H2H3H2H1

3-PDU2-PDU1-PDU


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Interoperation between layersInteroperation between layers achieved through standard interfacesEach layer may perform one or more generic tasks:

Error Control, to make logical channel between 2 layers reliableFlow Control, to avoid overwhelming a slower peer with PDUsSegmentation, to divide large data chunks into smaller pieces at transmitting sideReassembly, to reassemble the smaller pieces into original large chunk at receiving sideMultiplexing, to allow several higher-level sessions to share a single lower-level connectionConnection setup, to provide handshaking with a peer


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Why layering?Dealing with complex systems:

explicit structure allows identification, relationship of complex system’s pieces

layered reference model for discussionmodularization eases maintenance, updating of system

change of implementation of layer’s service transparent to rest of systeme.g., change in gate procedure doesn’t affect rest of system

layering considered harmful?


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Internet protocol stackapplication: supporting network applications

ftp, smtp, httptransport: host-host data transfer

tcp, udpnetwork: routing of datagrams from source to destination

ip, routing protocolslink: data transfer between neighboring network elements

ppp, ethernetphysical: bits “on the wire”

application

transport

network

link

physical


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Layering: logical communication

applicationtransportnetwork

linkphysical


linkphysical application

transportnetwork

linkphysical


linkphysical

networklink

physical

Each layer:distributed“entities”implement layer functions at each nodeentities perform actions, exchange messages with peers


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Layering: logical communication


linkphysical


linkphysical application

transportnetwork

linkphysical


linkphysical

networklink

physical

data

dataE.g.: transport

take data from appadd addressing, reliability check info to form “datagram”send datagram to peerwait for peer to ack receiptanalogy: post office

data

transport

transport

ack


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Layering: physical communication


linkphysical


linkphysical


linkphysical


linkphysical

networklink

physical

data

data


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Protocol layering and data

Each layer takes data from aboveadds header information to create new data unitpasses new data unit to layer below


linkphysical


linkphysical

source destinationMMMM

Ht

HtHnHtHnHl

MMMM

Ht

HtHnHtHnHl

messagesegmentdatagramframe


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Internet structure: network of networks

roughly hierarchicalnational/international backbone providers (NBPs)

e.g. BBN/GTE, Sprint, AT&T, IBM, UUNetinterconnect (peer) with each other privately, or at public Network Access Point (NAPs)

regional ISPsconnect into NBPs

local ISP, companyconnect into regional ISPs

NBP A

NBP B

NAP NAP

regional ISP

regional ISP

localISP

localISP


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US National Backbone Providere.g. BBN/GTE US backbone network


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Summary so far!Covered a “ton” of

material!Internet overviewwhat’s a protocol?network edge, core, access networkperformance: loss, delaylayering and service modelsbackbones, NAPs, ISPs

You now hopefully have:context, overview, “feel” of networking

what is a distributed system? - postechdpnm.postech.ac.kr/cs600/lecture/e600-module1.pdf · a...

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