computer networks - university of washington€¦ · computer networks 14 4 applicaon – programs...

ComputerNetworksShyamGollakota

ComputerNetworks 2

ProtocolsandLayers•  Protocolsandlayeringisthemainstructuringmethodusedtodivideupnetworkfunc>onality–  Eachinstanceofaprotocoltalksvirtuallytoitspeerusingtheprotocol

–  Eachinstanceofaprotocolusesonlytheservicesofthelowerlayer

ProtocolsandLayers(3)•  Protocolsarehorizontal,layersarever>cal

ComputerNetworks 3

X

YY

XInstanceofprotocolX

Peerinstance

Node1 Node2

Lowerlayerinstance(ofprotocolY)

ProtocolX

ServiceprovidedbyProtocolY

ProtocolsandLayers(4)•  Setofprotocolsinuseiscalledaprotocolstack

ComputerNetworks 4

ComputerNetworks 5

ProtocolsandLayers(6)•  Protocolsyou’veprobablyheardof:

–  TCP,IP,802.11,Ethernet,HTTP,SSL,DNS,…andmanymore

•  Anexampleprotocolstack–  UsedbyawebbrowseronahostthatiswirelesslyconnectedtotheInternet

HTTP

TCP

IP

802.11

Browser

ComputerNetworks 6

Encapsula>on•  Encapsula>onisthemechanismusedtoeffectprotocollayering–  Lowerlayerwrapshigherlayercontent,addingitsowninforma>ontomakeanewmessagefordelivery

–  Likesendingale[erinanenvelope;postalservicedoesn’tlookinside

Encapsula>on(3)•  Message“onthewire”beginstolooklikeanonion

–  Lowerlayersareoutermost

ComputerNetworks 7

HTTP

TCP

IP

802.11

HTTP

TCP HTTP

TCP HTTPIP

TCP HTTPIP802.11

Encapsula>on(4)

ComputerNetworks 8

HTTP

TCP

IP

802.11

HTTP

TCP HTTP

TCP HTTPIP

TCP HTTPIP802.11

HTTP

TCP

IP

802.11(wire)

HTTP

TCP HTTP

TCP HTTPIP

TCP HTTPIP802.11

TCP HTTPIP802.11

AdvantageofLayering•  Informa>onhidingandreuse

ComputerNetworks 9

HTTP

Browser

HTTP

Server

HTTP

Browser

HTTP

Server

or

AdvantageofLayering(2)•  Informa>onhidingandreuse

ComputerNetworks 10

HTTP

TCP

IP

802.11

Browser

HTTP

TCP

IP

802.11

Server

HTTP

TCP

IP

Ethernet

Browser

HTTP

TCP

IP

Ethernet

Server

or

AdvantageofLayering(3)•  Usinginforma>onhidingtoconnectdifferentsystems

ComputerNetworks 11

HTTP

TCP

IP

802.11

Browser

HTTP

TCP

IPEthernet

Server

AdvantageofLayering(4)•  Usinginforma>onhidingtoconnectdifferentsystems

ComputerNetworks 12

HTTP

TCP

IP

802.11

Browser

IP

802.11

IP

Ethernet

HTTP

TCP

IPEthernet

Server

IP TCP HTTP

802.11 IP TCP HTTP Ethernet IP TCP HTTP

ComputerNetworks 13

DisadvantageofLayering•  ??

InternetReferenceModel•  Afourlayermodelbasedonexperience;omitssomeOSIlayersandusesIPasthenetworklayer.

ComputerNetworks 14

4Applica>on–Programsthatusenetworkservice3Transport–Providesend-to-enddatadelivery2Internet –Sendpacketsovermul>plenetworks

1Link –Sendframesoveralink

InternetReferenceModel(3)•  IPisthe“narrowwaist”oftheInternet

–  Supportsmanydifferentlinksbelowandappsabove

ComputerNetworks 15

4Applica>on3Transport

2Internet

1Link Ethernet802.11

IP

TCP UDP

HTTPSMTP RTP DNS

3GDSLCable

ComputerNetworks 16

Layer-basedNames(2)•  Fordevicesinthenetwork:

NetworkLink

NetworkLink

Link Link

Physical PhysicalRepeater(orhub)

Switch(orbridge)

Router

ComputerNetworks 17

Layer-basedNames(3)•  Fordevicesinthenetwork:

Proxyormiddleboxorgateway

NetworkLink

NetworkLink

AppTransport

AppTransport

Buttheyalllooklikethis!

18

ScopeofthePhysicalLayer•  Concernshowsignalsareusedtotransfermessagebitsoveralink– Wiresetc.carryanalogsignals– Wewanttosenddigitalbits

…1011010110…

Signal

SimpleLinkModel•  We’llendwithanabstrac>onofaphysicalchannel

–  Rate(orbandwidth,capacity,speed)inbits/second–  Delayinseconds,relatedtolength

•  Otherimportantproper>es:– Whetherthechannelisbroadcast,anditserrorrate

CSE461UniversityofWashington 19

DelayD,RateR

Message

MessageLatency•  Latencyisthedelaytosendamessageoveralink

–  Transmissiondelay:>metoputM-bitmessage“onthewire”

–  Propaga>ondelay:>meforbitstopropagateacrossthewire

–  Combiningthetwotermswehave:


MessageLatency(2)•  Latencyisthedelaytosendamessageoveralink

–  Transmissiondelay:>metoputM-bitmessage“onthewire”

T-delay=M(bits)/Rate(bits/sec)=M/Rseconds

–  Propaga>ondelay:>meforbitstopropagateacrossthewire

P-delay=Length/speedofsignals=Length/⅔c=Dseconds

–  Combiningthetwotermswehave:L=M/R+D



MetricUnits•  Themainprefixesweuse:

•  Usepowersof10forrates,2forstorage–  1Mbps=1,000,000bps,1KB=210bytes

•  “B”isforbytes,“b”isforbits

Prefix Exp. prefix exp. K(ilo) 103 m(illi) 10-3

M(ega) 106 µ(micro) 10-6

G(iga) 109 n(ano) 10-9


LatencyExamples(2)•  “Dialup”withatelephonemodem:

D=5ms,R=56kbps,M=1250bytes

L=5ms+(1250x8)/(56x103)sec=184ms!

•  Broadbandcross-countrylink:D=50ms,R=10Mbps,M=1250bytes

L=50ms+(1250x8)/(10x106)sec=51ms

•  Alonglinkoraslowratemeanshighlatency–  Open,onedelaycomponentdominates


Bandwidth-DelayProduct•  Messagestakespaceonthewire!

•  Theamountofdatainflightisthebandwidth-delay(BD)product

BD=RxD– Measureinbits,orinmessages–  SmallforLANs,bigfor“longfat”pipes


Bandwidth-DelayExample(2)•  Fiberathome,cross-country

R=40Mbps,D=50msBD=40x106x50x10-3bits

=2000Kbit=250KB

•  That’squitealotofdata“inthenetwork”!

110101000010111010101001011

weightsofharmonicfrequenciesSignalover>me

=

FrequencyRepresenta>on•  Asignalover>mecanberepresentedbyitsfrequencycomponents(calledFourieranalysis)

26am

plitu

de

Lost!

EffectofLessBandwidth•  Fewerfrequencies(=lessbandwidth)degradessignal

27

Lost!

27

Lost!Bandwidth

SignalsoveraWire(2)•  Example:

28

2:A[enua>on:

3:Bandwidth:

4:Noise:

Sentsignal


SignalsoverWireless•  Signalstransmi[edonacarrierfrequency,likefiber

•  Travelatspeedoflight,spreadoutanda[enuatefasterthan1/dist2

•  Mul>plesignalsonthesamefrequencyinterfereatareceiver

30

SignalsoverWireless(5)•  Variousothereffectstoo!

– Wirelesspropaga>oniscomplex,dependsonenvironment

•  Somekeyeffectsarehighlyfrequencydependent,–  E.g.,mul>pathatmicrowavefrequencies

WirelessMul>path•  Signalsbounceoffobjectsandtakemul>plepaths

–  Somefrequenciesa[enuatedatreceiver,varieswithloca>on– Messesupsignal;handledwithsophis>catedmethods(§2.5.3)

31

32

Wireless•  Senderradiatessignaloveraregion

–  Inmanydirec>ons,unlikeawire,topoten>allymanyreceivers

–  Nearbysignals(samefreq.)interfereatareceiver;needtocoordinateuse

33

WiFi

WiFi

Wireless(2)•  Microwave,e.g.,3G,andunlicensed(ISM)frequencies,e.g.,WiFi,arewidelyusedforcomputernetworking

34

802.11b/g/n

802.11a/g/n

35

Topic•  We’vetalkedaboutsignalsrepresen>ngbits.How,exactly?–  Thisisthetopicofmodula>on

…1011010110…

Signal

ASimpleModula>on•  Letahighvoltage(+V)representa1,andlowvoltage(-V)representa0– ThisiscalledNRZ(Non-ReturntoZero)

36

Bits

NRZ

0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0

+V

-V

ASimpleModula>on(2)•  Letahighvoltage(+V)representa1,andlowvoltage(-V)representa0– ThisiscalledNRZ(Non-ReturntoZero)

37

Bits

NRZ

0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0

+V

-V

Modula>on

38

NRZsignalofbits

Amplitudeshipkeying

Frequencyshipkeying

Phaseshipkeying

39

Topic•  Howrapidlycanwesendinforma>onoveralink?–  Nyquistlimit(~1924)»–  Shannoncapacity(1948)»

•  Prac>calsystemsaredevisedtoapproachtheselimits

40

KeyChannelProper>es•  Thebandwidth(B),signalstrength(S),andnoisestrength(N)–  Blimitstherateoftransi>ons–  SandNlimithowmanysignallevelswecandis>nguish

BandwidthB SignalS,NoiseN

41

NyquistLimit•  Themaximumsymbolrateis2B

•  ThusifthereareVsignallevels,ignoringnoise,themaximumbitrateis: R=2Blog2Vbits/sec

1010101010101010101

42

ClaudeShannon(1916-2001)•  Fatherofinforma>ontheory

–  “AMathema>calTheoryofCommunica>on”,1948

•  Fundamentalcontribu>onstodigitalcomputers,security,andcommunica>ons

Credit:CourtesyMITMuseum

Electromechanicalmousethat“solves”mazes!

ShannonCapacity•  Howmanylevelswecandis>nguishdependsonS/N

–  OrSNR,theSignal-to-NoiseRa>o–  Notenoiseisrandom,hencesomeerrors

•  SNRgivenonalog-scaleindeciBels:–  SNRdB=10log10(S/N)

43

0

1

2

3

N

S+N

44

ShannonCapacity(2)•  Shannonlimitisforcapacity(C),themaximuminforma>oncarryingrateofthechannel:

C=Blog2(1+S/(BN))bits/sec

Wired/WirelessPerspec>ve•  Wires,andFiber

– EngineerlinktohaverequisiteSNRandB→ Canfixdatarate

•  Wireless– GivenB,butSNRvariesgreatly,e.g.,upto60dB!→ Can’tdesignforworstcase,mustadaptdatarate

45

Wired/WirelessPerspec>ve(2)•  Wires,andFiber

– EngineerlinktohaverequisiteSNRandB→ Canfixdatarate

•  Wireless– GivenB,butSNRvariesgreatly,e.g.,upto60dB!→ Can’tdesignforworstcase,mustadaptdatarate

46

EngineerSNRfordatarate

AdaptdataratetoSNR

Puzngitalltogether–DSL•  DSL(DigitalSubscriberLine)iswidelyusedforbroadband;manyvariantsoffer10sofMbps–  Reusestwistedpairtelephonelinetothehome;ithasupto~2MHzofbandwidthbutusesonlythelowest~4kHz

47

DSL(2)•  DSLusespassbandmodula>on(calledOFDM)

–  Separatebandsforupstreamanddownstream(larger)– Modula>onvariesbothamplitudeandphase(calledQAM)–  HighSNR,upto15bits/symbol,lowSNRonly1bit/symbol

48

Upstream Downstream

26–138kHz

0-4kHz 143kHzto1.1MHz

Telephone

Freq.

Voice Upto1Mbps Upto12Mbps

ADSL2:


Topic•  Somebitswillbereceivedinerrordue

tonoise.Whatcanwedo?–  Detecterrorswithcodes»–  Correcterrorswithcodes»–  Retransmitlostframes

•  Reliabilityisaconcernthatcutsacrossthelayers–we’llseeitagain

Later

Problem–Noisemayflipreceivedbits


Signal0 0 0 0

11 10

0 0 0 011 1

0

0 0 0 011 1

0

SlightlyNoisy

Verynoisy


Approach–AddRedundancy•  Errordetec>oncodes

–  Addcheckbitstothemessagebitstoletsomeerrorsbedetected

•  Errorcorrec>oncodes–  Addmorecheckbitstoletsomeerrorsbecorrected

•  Keyissueisnowtostructurethecodetodetectmanyerrorswithfewcheckbitsandmodestcomputa>on


Mo>va>ngExample•  Asimplecodetohandleerrors:

–  Sendtwocopies!Errorifdifferent.

•  Howgoodisthiscode?–  Howmanyerrorscanitdetect/correct?–  Howmanyerrorswillmakeitfail?


Mo>va>ngExample(2)•  Wewanttohandlemoreerrorswithlessoverhead– Willlookatbe[ercodes;theyareappliedmathema>cs

–  But,theycan’thandleallerrors–  Andtheyfocusonaccidentalerrors(willlookatsecurehasheslater)


UsingErrorCodes•  CodewordconsistsofDdataplusR

checkbits(=systema>cblockcode)

•  Sender:–  ComputeRcheckbitsbasedontheDdatabits;sendthecodewordofD+Rbits

D R=fn(D)Databits Checkbits


UsingErrorCodes(2)•  Receiver:

–  ReceiveD+Rbitswithunknownerrors–  RecomputeRcheckbitsbasedontheDdatabits;errorifRdoesn’tmatchR’

D R’Databits Checkbits

R=fn(D)=?


Intui>onforErrorCodes•  ForDdatabits,Rcheckbits:

•  Randomlychosencodewordisunlikelytobecorrect;overheadislow

AllcodewordsCorrect

codewords


R.W.Hamming(1915-1998)•  Muchearlyworkoncodes:

–  “ErrorDetec>ngandErrorCorrec>ngCodes”,BSTJ,1950

•  Seealso:–  “YouandYourResearch”,1986

Source:IEEEGHN,©2009IEEE


HammingDistance•  DistanceisthenumberofbitflipsneededtochangeD1toD2

•  Hammingdistanceofacodeistheminimumdistancebetweenanypairofcodewords


HammingDistance(2)•  Errordetec>on:

–  Foracodeofdistanced+1,uptoderrorswillalwaysbedetected


HammingDistance(3)•  Errorcorrec>on:

–  Foracodeofdistance2d+1,uptoderrorscanalwaysbecorrectedbymappingtotheclosestcodeword


Topic•  Somebitsmaybereceivedinerror

duetonoise.Howdowedetectthis?–  Parity»–  Checksums»–  CRCs»

•  Detec>onwillletusfixtheerror,forexample,byretransmission(later).


SimpleErrorDetec>on–ParityBit•  TakeDdatabits,add1checkbitthatisthesumoftheDbits–  Sumismodulo2orXOR


ParityBit(2)•  Howwelldoesparitywork?

– Whatisthedistanceofthecode?–  Howmanyerrorswillitdetect/correct?

•  Whataboutlargererrors?


Checksums•  Idea:sumupdatainN-bitwords

– Widelyusedin,e.g.,TCP/IP/UDP

•  Strongerprotec>onthanparity

1500bytes 16bits


InternetChecksum•  Sumisdefinedin1scomplementarithme>c(mustaddbackcarries)–  Andit’sthenega>vesum

•  “Thechecksumfieldisthe16bitone'scomplementoftheone'scomplementsumofall16bitwords…”–RFC791


InternetChecksum(2)Sending:1. Arrangedatain16-bitwords

2. Putzeroinchecksumposi>on,add

3. Addanycarryoverbacktoget16bits

4. Negate(complement)togetsum

0001 f203 f4f5 f6f7

+(0000) ------ 2ddf0

ddf0

+ 2 ------ ddf2

220d


InternetChecksum(3)Sending:1. Arrangedatain16-bitwords2. Putzeroinchecksumposi>on,add


4. Negate(complement)togetsum

0001 f203 f4f5 f6f7

+(0000) ------ 2ddf0

ddf0

+ 2 ------ ddf2

220d


InternetChecksum(4)Receiving:1. Arrangedatain16-bitwords2. Checksumwillbenon-zero,add


4. Negatetheresultandcheckitis0

0001 f203 f4f5 f6f7

+ 220d ------ 2fffd

fffd

+ 2 ------ ffff

0000


InternetChecksum(5)Receiving:1. Arrangedatain16-bitwords2. Checksumwillbenon-zero,add


4. Negatetheresultandcheckitis0

0001 f203 f4f5 f6f7

+ 220d ------ 2fffd

fffd

+ 2 ------ ffff

0000


InternetChecksum(6)•  Howwelldoesthechecksumwork?

– Whatisthedistanceofthecode?–  Howmanyerrorswillitdetect/correct?

•  Whataboutlargererrors?


CyclicRedundancyCheck(CRC)•  Evenstrongerprotec>on

–  Givenndatabits,generatekcheckbitssuchthatthen+kbitsareevenlydivisiblebyageneratorC

•  Examplewithnumbers:–  n=302,k=onedigit,C=3


CRCs(2)•  Thecatch:

–  It’sbasedonmathema>csoffinitefields,inwhich“numbers”representpolynomials

–  e.g,10011010isx7+x4+x3+x1

•  Whatthismeans:– Weworkwithbinaryvaluesandoperateusingmodulo2arithme>c


CRCs(3)•  SendProcedure:1.  Extendthendatabitswithkzeros2.  DividebythegeneratorvalueC3.  Keepremainder,ignorequo>ent4.  Adjustkcheckbitsbyremainder

•  ReceiveProcedure:1.  Divideandcheckforzeroremainder

CRCs(4)


Databits:1101011111

Checkbits:C(x)=x4+x1+1C=10011

k=4

100111101011111

CRCs(5)



CRCs(6)•  Protec>ondependongenerator

–  StandardCRC-32is100000100110000010001110110110111

•  Proper>es:–  HD=4,detectsuptotriplebiterrors–  Alsooddnumberoferrors–  Andburstsofuptokbitsinerror–  Notvulnerabletosystema>cerrorslikechecksums


ErrorDetec>oninPrac>ce•  CRCsarewidelyusedonlinks

– Ethernet,802.11,ADSL,Cable…•  ChecksumusedinInternet

–  IP,TCP,UDP…butitisweak•  Parity

–  Isli[leused


Topic•  Twostrategiestohandleerrors:1.  Detecterrorsandretransmitframe

(Automa>cRepeatreQuest,ARQ)

2.  Correcterrorswithanerrorcorrec>ngcode

Donethis


ContextonReliability•  Whereinthestackshouldweplacereliabilityfunc>ons?

PhysicalLink

NetworkTransportApplica>on


ContextonReliability(2)•  Everywhere!Itisakeyissue

–  Differentlayerscontributedifferently

PhysicalLink

NetworkTransportApplica>on

Recoverac>ons(correctness)

Maskerrors(performanceop>miza>on)


ARQ•  ARQopenusedwhenerrorsarecommonormustbecorrected–  E.g.,WiFi,andTCP(later)

•  Rulesatsenderandreceiver:–  Receiverautoma>callyacknowledgescorrectframeswithanACK

–  Senderautoma>callyresendsapera>meout,un>lanACKisreceived


ARQ(2)•  Normalopera>on(noloss)

Frame

ACKTimeout Time

Sender Receiver


ARQ(3)•  Lossandretransmission

Frame

Timeout Time

Sender Receiver

Frame

ACK

X


SoWhat’sTrickyAboutARQ?•  Twonon-trivialissues:

–  Howlongtosetthe>meout?»–  Howtoavoidaccep>ngduplicateframesasnewframes»

•  Wantperformanceinthecommoncaseandcorrectnessalways


Timeouts•  Timeoutshouldbe:

–  Nottoobig(linkgoesidle)–  Nottoosmall(spuriousresend)

•  FairlyeasyonaLAN–  Clearworstcase,li[levaria>on

•  FairlydifficultovertheInternet– Muchvaria>on,noobviousbound– We’llrevisitthiswithTCP(later)


Duplicates•  WhathappensifanACKislost?

X

Frame

ACKTimeout

Sender Receiver

NewFrame?


Duplicates(2)•  WhathappensifanACKislost?

Frame

ACK

X

Frame

ACKTimeout

Sender Receiver

NewFrame?

NewFrame??


Duplicates(3)•  Orthe>meoutisearly?

ACK

Frame

Timeout

Sender Receiver

NewFrame?


Duplicates(4)•  Orthe>meoutisearly?

Frame

ACK

Frame

ACK

Timeout

Sender Receiver

NewFrame?

NewFrame??


SequenceNumbers•  FramesandACKsmustbothcarrysequencenumbersforcorrectness

•  Todis>nguishthecurrentframefromthenextone,asinglebit(twonumbers)issufficient–  CalledStop-and-Wait


Stop-and-Wait•  Inthenormalcase:

Time

Sender Receiver


Stop-and-Wait(2)•  Inthenormalcase:

Frame0

ACK0Timeout Time

Sender Receiver

Frame1

ACK1


Stop-and-Wait(3)•  WithACKloss:

X

Frame0

ACK0Timeout

Sender Receiver

NewFrame?


Stop-and-Wait(4)•  WithACKloss:

Frame0

ACK0

X

Frame0

ACK0Timeout

Sender Receiver

NewFrame?

It’saResend!


Stop-and-Wait(5)•  Withearly>meout:

ACK0

Frame0

Timeout

Sender Receiver

NewFrame?


Stop-and-Wait(6)•  Withearly>meout:

Frame0

ACK0

Frame0

ACK0

Timeout

Sender Receiver

NewFrame?

It’saResend

OK…


Limita>onofStop-and-Wait•  Itallowsonlyasingleframetobeoutstandingfromthesender:–  GoodforLAN,notefficientforhighBD

•  Ex:R=1Mbps,D=50ms–  Howmanyframes/sec?IfR=10Mbps?


SlidingWindow•  Generaliza>onofstop-and-wait

–  AllowsWframestobeoutstanding–  CansendWframesperRTT(=2D)

–  Variousop>onsfornumberingframes/ACKsandhandlingloss

•  WilllookatalongwithTCP(later)

computer networks - university of washington€¦ · computer networks 14 4 applicaon – programs...

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