COS461:ComputerNetworksMidtermReview

Spring2011

MikeFreedman

h@p://www.cs.princeton.edu/courses/archive/spr11/cos461/

1

2

Internetlayering:Message,Segment,Packet,andFrame

HTTP

TCP

IP

Ethernet interface

HTTP

TCP

IP

Ethernet interface

IP IP

Ethernet interface

Ethernet interface

SONET interface

SONET interface

host host

router router

HTTP message

TCP segment

IP packet IP packet IP packet

Ethernet frame Ethernet frame SONET frame 2

Topics•  Linklayer(Sl.4)

–  Sharingalink:TDMA,FDMA

–  EthernetandCSMA/CD

– WirelessandCSMA/CA–  Spanningtreeandswitching–  TranslaQngaddrs:DHCP/ARP

•  Networklayer(Sl.25)–  IPv4andaddressing–  IPforwarding– Middleboxes:NATs,firewalls,tunneling

•  Transportlayer(Sl.38)–  Socketinterface–  UDP–  TCP

•  Reliability•  CongesQonControl•  InteracQonsw/AcQveQueueManagement

•  ApplicaQonlayer(Sl.68)–  TranslaQngnames:DNS–  HTTPandCDNs–  Overlaynetworks

3

LinkLayer

4

Link‐LayerServices•  Encoding

– RepresenQngthe0sand1s

•  Framing– EncapsulaQngpacketintoframe,addingheaderandtrailer

– UsingMACaddresses,ratherthanIPaddresses

•  ErrordetecQon– Errorscausedbysignala@enuaQon,noise.– ReceiverdetecQngpresenceoferrors

5

MulQpleAccessProtocol•  Singlesharedbroadcastchannel

– AvoidhavingmulQplenodesspeakingatonce– Otherwise,collisionsleadtogarbleddata

•  MulQpleaccessprotocol– Distributedalgorithmforsharingthechannel– Algorithmdetermineswhichnodecantransmit

•  Classesoftechniques–  ChannelparQQoning:dividechannelintopieces

– Time‐divisionmulQplexing,frequencydivisionmulQplexing–  Takingturns:passingatokenforrighttotransmit–  Randomaccess:allowcollisions,andthenrecover

6

KeyIdeasofRandomAccess•  CarrierSense(CS)

–  Listenbeforespeaking,anddon’tinterrupt–  Checkingifsomeoneelseisalreadysendingdata– …andwaiQngQlltheothernodeisdone

•  CollisionDetecQon(CD)–  Ifsomeoneelsestartstalkingatthesame7me,stop–  Realizingwhentwonodesaretransmiangatonce– …bydetecQngthatthedataonthewireisgarbled

•  Randomness– Don’tstarttalkingagainrightaway– WaiQngforarandomQmebeforetryingagain

7

CSMA/CDCollisionDetecQon8

MediumAccessControlin802.11•  Collisionavoidance,notdetecQon

–  Firstexchangecontrolframesbeforetransmiangdata•  Senderissues“RequesttoSend”(RTS),includinglengthofdata

•  Receiverrespondswith“CleartoSend”(CTS)–  IfsenderseesCTS,transmitsdata(ofspecifiedlength)–  IfothernodeseesCTS,willidleforspecifiedperiod–  IfothernodeseesRTSbutnotCTS,freetosend

•  Link‐layeracknowledgmentandretransmission–  CRCtodetecterrors–  ReceivingstaQonsendsanacknowledgment

–  SendingstaQonretransmitsifnoACKisreceived

–  Givingupacerafewfailedtransmissions

9

ScalingtheLinkLayer•  EthernettradiQonallylimitedbyfadingsignalstrengthinlongwires–  IntroducQonofhubs/repeaterstorebroadcast

•  SQllamaximum“length”foraEthernetsegment–  Otherwise,twonodesmightbetoofarforcarriersensetodetectconcurrentbroadcasts

•  Further,toomanynodesinshorterEthernetcanyieldlowtransmissionsrates–  Constantlyconflictwithoneanother

10

Bridges/Switches:TrafficIsolaQon

•  SwitchbreakssubnetintoLANsegments

•  Switchfilterspackets–  Frameonlyforwardedtothenecessarysegments–  Segmentscansupportseparatetransmissions

hub hub hub

switch/bridge

segment segment

segment

11

ComparingHubs,Switches,Routers

Hub/

Repeater

Bridge/

Switch

Router

Traffic isolation no yes yes

Plug and Play yes yes no

Efficient routing no no yes

Cut through yes yes no

12

SelfLearning:BuildingtheTable•  Whenaframearrives

–  InspectthesourceMACaddress

–  Associatetheaddresswiththeincominginterface

–  Storethemappingintheswitchtable

–  UseaQme‐to‐livefieldtoeventuallyforgetthemapping

A

B

C

D

Switch learns how to reach A

13

SoluQon:SpanningTrees•  Ensurethetopologyhasnoloops

–  Avoidusingsomeofthelinkswhenflooding–  …toavoidformingaloop

•  Spanningtree–  Sub‐graphthatcoversallverQcesbutcontainsnocycles–  Linksnotinthespanningtreedonotforwardframes

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EvoluQonTowardVirtualLANs

Red VLAN and Orange VLAN Switches forward traffic as needed

R O

RO

R

R

R

O O O R O R R R

O

O

O

15

Group users based on organizational structure, rather than the physical

layout of the building.

Wireless

16

CSMA:CarrierSense,MulQpleAccess

•  MulQpleaccess:channelissharedmedium–  StaQon:wirelesshostoraccesspoint– MulQplestaQonsmaywanttotransmitatsameQme

•  Carriersense:sensechannelbeforesending–  StaQondoesn’tsendwhenchannelisbusy–  Topreventcollisionswithongoingtransfers–  But,detecQngongoingtransfersisn’talwayspossible

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A B

C A B C

A’s signal strength

space

C’s signal strength

CA:CollisionAvoidance,NotDetecQon•  CollisiondetecQoninwiredEthernet

–  StaQonlistenswhiletransmiang– Detectscollisionwithothertransmission– Abortstransmissionandtriessendingagain

•  Problem#1:cannotdetectallcollisions– Hiddenterminalproblem–  Fading

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CA:CollisionAvoidance,NotDetecQon•  CollisiondetecQoninwiredEthernet

–  StaQonlistenswhiletransmiang– Detectscollisionwithothertransmission– Abortstransmissionandtriessendingagain

•  Problem#1:cannotdetectallcollisions– Hiddenterminalproblem–  Fading

•  Problem#2:listeningwhilesending–  Strengthofreceivedsignalismuchsmaller–  Expensivetobuildhardwarethatdetectscollisions

•  So,802.11doescollisionavoidance,notdetecQon

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HiddenTerminalProblem

•  AandCcan’tseeeachother,bothsendtoB

•  Occursb/c802.11reliesonphysicalcarriersensing,whichissuscepQbletohiddenterminalproblem

20

CBA

Virtualcarriersensing•  Firstexchangecontrolframesbeforetransmiangdata–  Senderissues“RequesttoSend”(RTS),incl.lengthofdata

–  Receiverrespondswith“CleartoSend”(CTS)

•  IfsenderseesCTS,transmitsdata(ofspecifiedlength)

•  IfothernodeseesCTS,willidleforspecifiedperiod

•  IfothernodeseesRTSbutnotCTS,freetosend

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HiddenTerminalProblem

•  AandCcan’tseeeachother,bothsendtoB

•  RTS/CTScanhelp–  BothAandCwouldsendRTSthatBwouldseefirst–  BonlyrespondswithoneCTS(say,echo’ingA’sRTS)–  CdetectsthatCTSdoesn’tmatchandwon’tsend

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CBA

ExposedTerminalProblem

•  BsendingtoA,CwantstosendtoD•  AsCreceivesB’spackets,carriersensewouldpreventitfromsendingtoD,eventhoughwouldn’tinterfere

•  RTS/CTScanhelp–  ChearsRTSfromB,butnotCTSfromA–  Cknowsit’stransmissionwillnotinterferewithA

–  CissafetotransmittoD

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CBA D

ImpactonHigher‐LayerProtocols•  WirelessandmobilitychangepathproperQes

– Wireless:higherpacketloss,notfromcongesQon– Mobility:transientdisrupQons,andchangesinRTT

•  Logically,impactshouldbeminimal…–  Best‐effortservicemodelremainsunchanged–  TCPandUDPcan(anddo)runoverwireless,mobile

•  But,performancedefinitelyisaffected–  TCPtreatspacketlossasasignofcongesQon–  TCPtriestoesQmatetheRTTtodriveretransmissions–  TCPdoesnotperformwellunderout‐of‐orderpackets

•  Internetnotdesignedwiththeseissuesinmind

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NetworkLayer

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IPPacketStructure

4-bit Version

4-bit Header Length

8-bit Type of Service

(TOS) 16-bit Total Length (Bytes)

16-bit Identification 3-bit Flags 13-bit Fragment Offset

8-bit Time to Live (TTL) 8-bit Protocol 16-bit Header Checksum

32-bit Source IP Address

32-bit Destination IP Address

Options (if any)

Payload

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SourceAddress:WhatifSourceLies?•  Sourceaddressshouldbethesendinghost

–  But,who’schecking,anyway?–  Youcouldsendpacketswithanysourceyouwant

•  Whywouldsomeonewanttodothis?–  Launchadenial‐of‐servicea@ack

•  SendexcessivepacketstothedesQnaQon•  …tooverloadthenode,orthelinksleadingtonode

–  EvadedetecQonby“spoofing”•  But,thevicQmcouldidenQfyyoubythesourceaddress•  So,youcanputsomeoneelse’ssourceaddressinpackets

–  Also,ana@ackagainstthespoofedhost•  Spoofedhostiswronglyblamed•  Spoofedhostmayreceivereturntrafficfromreceiver

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HierarchicalAddressing:IPPrefixes•  IPaddressescanbedividedintotwoporQons

– Network(lec)andhost(right)•  12.34.158.0/24isa24‐bitprefix

– Whichcovers28addresses(e.g.,upto255hosts)

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00001100 00100010 10011110 00000101

Network (24 bits) Host (8 bits)

12 34 158 5

ClassfulAddressing•  Intheoldendays,onlyfixedallocaQonsizes

– ClassA:0*•  Verylarge/8blocks(e.g.,MIThas18.0.0.0/8)

– ClassB:10*•  Large/16blocks(e.g,.Princetonhas128.112.0.0/16)

– ClassC:110*•  Small/24blocks(e.g.,AT&TLabshas192.20.225.0/24)

– ClassD:1110*•  MulQcastgroups

– ClassE:11110*•  Reservedforfutureuse

•  Thisiswhyfolksusedo@ed‐quadnotaQon!

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CIDR:HierarchalAddressAllocaQon30

12.0.0.0/8

12.0.0.0/16

12.254.0.0/16

12.1.0.0/16 12.2.0.0/16 12.3.0.0/16

: : :

12.3.0.0/24 12.3.1.0/24

: :

12.3.254.0/24

12.253.0.0/19 12.253.32.0/19 12.253.64.0/19 12.253.96.0/19 12.253.128.0/19 12.253.160.0/19

: : :

• Prefixes are key to Internet scalability – Address allocated in contiguous chunks (prefixes) – Routing protocols and packet forwarding based on prefixes – Today, routing tables contain ~200,000 prefixes (vs. 4B)

Twotypesofaddresses•  Providerindependent(fromIANA)

•  Providerallocated(fromupstreamISP)

•  ProviderallocatedaddressesseemtooffermorepotenQalforaggregaQon(andreducingrouQngtablesize),butnotalwaysso…

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Scalability:AddressAggregaQon32

Provider is given 201.10.0.0/21

201.10.0.0/22 201.10.4.0/24 201.10.5.0/24 201.10.6.0/23

Provider

RoutersinrestofInternetjustneedtoknowhowtoreach201.10.0.0/21.ProvidercandirectIPpackets

toappropriatecustomer.

But,AggregaQonNotAlwaysPossible33

201.10.0.0/21

201.10.0.0/22 201.10.4.0/24 201.10.5.0/24 201.10.6.0/23

Provider 1 Provider 2

Mul7‐homedcustomer(201.10.6.0/23)hastwoproviders.OtherpartsoftheInternetneedtoknow

howtoreachthesedesQnaQonsthroughbothproviders.

CIDRMakesPacketForwardingHarder•  Forwardingtablemayhavemanymatches

–  E.g.,entriesfor201.10.0.0/21and201.10.6.0/23–  TheIPaddress201.10.6.17wouldmatchboth!

–  UseLongestPrefixMatching

•  CanleadtorouQngtableexpansion–  TosaQfyLPM,needtoannounce/23fromboth1and2

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201.10.0.0/21

201.10.0.0/22 201.10.4.0/24 201.10.5.0/24 201.10.6.0/23

Provider 1 Provider 2

Internet‐wideInternetRouQng•  AS‐leveltopology

– DesQnaQonsareIPprefixes(e.g.,12.0.0.0/8)– NodesareAutonomousSystems(ASes)– EdgesarelinksandbusinessrelaQonships

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1

2

3 4

5

6 7

Client Web server

Middleboxes•  Middleboxesareintermediaries

–  Interposedin‐betweenthecommunicaQnghosts

–  OcenwithoutknowledgeofoneorbothparQes

•  Myriaduses–  Networkaddresstranslators–  Firewalls–  Tunnelendpoints–  Trafficshapers

–  IntrusiondetecQonsystems

–  TransparentWebproxycaches

–  ApplicaQonaccelerators

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“AnabominaQon!”–  ViolaQonoflayering–  Hardtoreasonabout–  Responsibleforsubtlebugs

“ApracQcalnecessity!”–  Solvereal/pressingproblems

–  Needsnotlikelytogoaway

Port‐TranslaQngNAT

•  Mapoutgoingpackets–  ReplacesourceaddresswithNATaddress–  Replacesourceportnumberwithanewportnumber

–  Remotehostsrespondusing(NATaddress,newport#)

•  MaintainatranslaQontable–  Storemapof(srcaddr,port#)to(NATaddr,newport#)

•  Mapincomingpackets–  ConsultthetranslaQontable– MapthedesQnaQonaddressandportnumber

–  Localhostreceivestheincomingpacket

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TransportLayer

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TwoBasicTransportFeatures•  Demul0plexing:portnumbers

•  Errordetec0on:checksums

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Web server (port 80)

Client host

Server host 128.2.194.242

Echo server (port 7)

Service request for 128.2.194.242:80

(i.e., the Web server) OS Client

IP payload

detect corruption

UserDatagramProtocol(UDP)•  Datagrammessagingservice

–  DemulQplexingofmessages:portnumbers–  DetecQngcorruptedmessages:checksum

•  LightweightcommunicaQonbetweenprocesses–  Sendmessagestoandreceivethemfromasocket

–  Avoidoverheadanddelaysofordered,reliabledelivery

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SRC port DST port

checksum length

DATA

TransmissionControlProtocol(TCP)•  Stream‐of‐bytesservice

–  Sendsandreceivesastreamofbytes,notmessages

•  Reliable,in‐orderdelivery–  Checksumstodetectcorrupteddata–  Sequencenumberstodetectlossesandreorderdata–  Acknowledgments&retransmissionsforreliabledelivery

•  ConnecQonoriented–  Explicitset‐upandtear‐downofTCPsession

•  Flowcontrol–  Preventoverflowofthereceiver’sbufferspace

•  CongesQoncontrol–  AdapttonetworkcongesQonforthegreatergood

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EstablishingaTCPConnecQon

•  Three‐wayhandshaketoestablishconnecQon– HostAsendsaSYNchronize(open)tothehostB– HostBreturnsaSYNACKnowledgment(SYNACK)– HostAsendsanACKtoacknowledgetheSYNACK

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SYN

SYN ACK

ACK Data

A B

Data

Each host tells its ISN to the other host.

TCP“StreamofBytes”Service43

Byte 0

Byte 1

Byte 2

Byte 3

Byte 0

Byte 1

Byte 2

Byte 3

Host A

Host B

Byte 80

Byte 80

…EmulatedUsingTCP“Segments”44

Byte 0

Byte 1

Byte 2

Byte 3

Byte 0

Byte 1

Byte 2

Byte 3

Host A

Host B

Byte 80

TCP Data

TCP Data

Byte 80

Segmentsentwhen:1.  Segmentfull(MaxSegmentSize),2.  Notfull,butQmesout,or3.  “Pushed”byapplicaQon.

Reliability:TCPAcknowledgments45

Host A

Host B

TCP Data

TCP Data

TCP HDR

TCP HDR

ISN (initial sequence number)

Sequence number = 1st byte

ACK sequence number = next expected byte

DetecQnglosses46

Packet

Tim

eout

Packet

ACK

Tim

eout

Packet

Tim

eout

Packet

ACK

Tim

eout

Packet

ACK

Tim

eout

Packet

ACK

Tim

eout

ACK lost DUPLICATE

PACKET

Packet lost Early timeout DUPLICATE PACKETS

Flowcontrol:Slidingwindow•  Allowalargeramountofdata“inflight”

– Allowsendertogetaheadofthereceiver– …thoughnottoofarahead

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Sending process Receiving process

Last byte ACKed

Last byte sent

TCP TCP

Next byte expected

Last byte written Last byte read

Last byte received

WhereCongesQonHappens:Links•  SimpleresourceallocaQon:FIFOqueue&drop‐tail

•  Accesstothebandwidth:first‐infirst‐outqueue– Packetstransmi@edintheordertheyarrive

•  Accesstothebufferspace:drop‐tailqueuing–  Ifthequeueisfull,droptheincomingpacket

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TCPCongesQonWindow•  EachTCPsendermaintainsacongesQonwindow

– Maximumnumberofbytestohaveintransit–  I.e.,numberofbytessQllawaiQngacknowledgments

•  AdapQngthecongesQonwindow– Decreaseuponlosingapacket:backingoff–  Increaseuponsuccess:opQmisQcallyexploring– Alwaysstrugglingtofindtherighttransferrate

•  Bothgoodandbad–  Pro:avoidshavingexplicitfeedbackfromnetwork–  Con:under‐shooQngandover‐shooQngtherate

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LeadstotheTCP“Sawtooth”50

t

Window

halved

Loss

But, could take a long time to get started!

SlowStartandtheTCPSawtooth51

t

Window

halved

Loss

Exponential “slow start”

Duplicate ACK

RepeaQngSlowStartAcerTimeout52

t

Window

halved

Loss

Slow start in operation until it reaches half of

previous cwnd.

Timeout

Extensions

•  TaildropinroutersleadtoburstylossandsynchronizaQonofsenders– LedtoRandomEarlyDetecQon(RED)

•  Packetsdroppedandretransmissionwhenunnecessary– LedtoExplicitCongesQonNoQficaQon(ECN)

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Problemswithtaildrop•  UnderstablecondiQons,queuealmostalwaysfull–  Leadstohighlatencyforalltraffic

•  Possiblyunfairforflowswithsmallwindows–  Largerflowsmayfastretransmit(detecQnglossthroughTripDupACKs),smallflowsmayhavetowaitforQmeout

•  WindowsynchronizaQon– Moreonthislater…

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✗

FairQueuing(FQ)•  Maintainsseparatequeueperflow

•  Ensuresnoflowconsumesmorethanits1/nshare–  VariaQon:weightedfairqueuing(WFQ)

•  Ifallpacketsweresamelength,wouldbeeasy

•  Ifnon‐work‐conserving(resourcescangoidle),alsowouldbeeasy,yetloweruQlizaQon

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RoundRobinService

Egress Link

Flow 1

Flow 2

Flow 3

Flow 4

FairQueuingBasics

•  TrackhowmuchQmeeachflowhasusedlink

– ComputeQmeusedifittransmitsnextpacket

•  Sendpacketfromflowthatwillhavelowestuseifittransmits– Whynotflowwithsmallestusesofar?

– Becausenextpacketmaybehuge!

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FQAlgorithm•  ImagineclockQckperbit,thentxQme~length

FinishQmeFi=max(Fi‐1,ArriveQmeAi)+LengthPi

•  CalculateesQmatedFiforallqueuedpackets

•  TransmitpacketwithlowestFinext

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FQAlgorithm(2)•  Problem:Can’tpreemptcurrenttxpacket

•  Result:InacQveflows(Ai>Fi‐1)arepenalized– Standardalgorithmconsidersnohistory

– Eachflowgetsfairshareonlywhenpacketsqueued

58

FQAlgorithm(3)

•  Approach:givemorepromptnesstoflowsuQlizinglessbandwidthhistorically

•  BidBi=max(Fi‐1,Ai–δ)+Pi–  IntuiQon:withlargerδ,schedulingdecisionscalculatedbylasttxQmeFi‐1morefrequently,thuspreferringslowerflows

•  FQachievesmax‐minfairness–  Firstpriority:maximizetheminimumrateofanyacQveflows

–  Secondpriority:maximizethesecondminrate,etc.

59

Usesof(W)FQ•  Scalability

– #queuesmustbeequalto#flows

– But,canbeusedonedgerouters,lowspeedlinks,orsharedendhosts

•  (W)FQcanbeforclassesoftraffic,notjustflows– UseIPTOSbitstomark“importance”

– Partof“DifferenQatedServices”architecturefor“Quality‐of‐Service”(QoS)

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BurstyLossFromDrop‐TailQueuing

•  TCPdependsonpacketloss–  PacketlossisindicaQonofcongesQon– AndTCPdrivesnetworkintolossbyaddiQverateincrease

•  Drop‐tailqueuingleadstoburstyloss–  Iflinkiscongested,manypacketsencounterfullqueue–  Thus,losssynchronizaQon:

• Manyflowsloseoneormorepackets•  Inresponse,manyflowsdividesendingrateinhalf

61

SlowFeedbackfromDropTail•  Feedbackcomeswhenbufferiscompletelyfull

– …eventhoughthebufferhasbeenfillingforawhile

•  Plus,thefillingbufferisincreasingRTT– …makingdetecQonevenslower

•  Mightbebe@ertogiveearlyfeedback– Andget1‐2connecQonstoslowdownbeforeit’stoolate

62

RandomEarlyDetecQon(RED)•  BasicideaofRED

–  RouternoQcesthatqueueisgeangbacklogged– …andrandomlydropspacketstosignalcongesQon

•  Packetdropprobability– Dropprobabilityincreasesasqueuelengthincreases–  Else,setdropprobabilityasfuncQonofavgqueuelengthandQmesincelastdrop

63

Average Queue Length

Dro

p Pr

obab

ility

0

1

ProperQesofRED•  Dropspacketsbeforequeueisfull

–  Inthehopeofreducingtheratesofsomeflows

•  DropspacketinproporQontoeachflow’srate– High‐rateflowshavemorepackets– …and,hence,ahigherchanceofbeingselected

•  DropsarespacedoutinQme– WhichshouldhelpdesynchronizetheTCPsenders

•  TolerantofbursQnessinthetraffic– Bybasingthedecisionsonaveragequeuelength

64

ProblemsWithRED•  Hardtogettunableparametersjustright

– Howearlytostartdroppingpackets?– Whatslopeforincreaseindropprobability?– WhatQmescaleforaveragingqueuelength?

•  REDhasmixedadopQoninpracQce–  Ifparametersaren’tsetright,REDdoesn’thelp– Hardtoknowhowtosettheparameters

•  ManyothervariaQonsinresearchcommunity– Nameslike“Blue”(self‐tuning),“FRED”…

65

Feedback:FromlosstonoQficaQon

•  Earlydroppingofpackets– Good:givesearlyfeedback– Bad:hastodropthepackettogivethefeedback

•  ExplicitCongesQonNoQficaQon– RoutermarksthepacketwithanECNbit– SendinghostinterpretsasasignofcongesQon

66

ExplicitCongesQonNoQficaQon•  Mustbesupportedbyrouter,sender,ANDreceiver

–  End‐hostsdetermineifECN‐capableduringTCPhandshake

•  ECNinvolvesallthreeparQes(and4headerbits)1. Sendermarks“ECN‐capable”whensending

2. Ifroutersees“ECN‐capable”andexperiencingcongesQon,routermarkspacketas“ECNcongesQonexperienced”

3. Ifreceiversees“congesQonexperienced”,marks“ECNecho”flaginresponsesunQlcongesQonACK’d

4. Ifsendersees“ECNecho”,reducescwndandmarks“congesQonwindowreduced”flaginnextTCPpacket

•  WhyextraECNflag?CongesQoncouldhappenineitherdirecQon,wantsendertoreacttoforwarddirecQon

•  WhyCRWACK?ECN‐echocouldbelost,butweideallyonlyrespondtocongesQoninforwarddirecQon

67

ApplicaQonlayer

DNSHTTPandCDNs

P2PandDHTs

68

ThreeHierarchicalAssignmentProcesses

•  Hostname:www.cs.princeton.edu–  Domain:registrarforeachtop‐leveldomain(e.g.,.edu)–  Hostname:localadministratorassignstoeachhost

•  IPaddresses:128.112.7.156–  Prefixes:ICANN,regionalInternetregistries,andISPs–  Hosts:staQcconfiguraQon,ordynamicusingDHCP

•  MACaddresses:00‐15‐C5‐49‐04‐A9–  Blocks:assignedtovendorsbytheIEEE–  Adapters:assignedbythevendorfromitsblock

69

MappingBetweenIdenQfiers•  DomainNameSystem(DNS)

–  Givenahostname,providetheIPaddress–  GivenanIPaddress,providethehostname

•  DynamicHostConfiguraQonProtocol(DHCP)–  GivenaMACaddress,assignauniqueIPaddress–  …andtellhostotherstuffabouttheLocalAreaNetwork–  Toautomatetheboot‐strappingprocess

•  AddressResoluQonProtocol(ARP)–  GivenanIPaddress,providetheMACaddress–  ToenablecommunicaQonwithintheLocalAreaNetwork

DHCPandARPuseL2broadcast….DNSisapp‐layerprotocol

70

Recursivevs.IteraQveQueries•  Recursivequery

– Askservertogetanswerforyou

– E.g.,request1andresponse8

•  IteraQvequery– Askserverwhotoasknext

– E.g.,allotherrequest‐responsepairs

71

requesting host cis.poly.edu

root DNS server

local DNS server dns.poly.edu

1

2 3

4 5

6

authoritative DNS server dns.cs.umass.edu

7 8

TLD DNS server

Onepage,lotsofobjects

•  DynamicHTML: 19.6KB

•  StaQccontent: 6.2MB•  1flashmovie•  18images

•  5stylesheets•  3scripts

TCPInteracQon:ShortTransfers•  MulQpleconnecQonsetups

–  Three‐wayhandshakeeachQme

•  Round‐tripQmeesQmaQon– MaybelargeatthestartofaconnecQon(e.g.,3seconds)–  LeadstolatencyindetecQnglostpackets

•  CongesQonwindow–  SmallvalueatbeginningofconnecQon(e.g.,1MSS)– Maynotreachahighvaluebeforetransferisdone

•  DetecQngpacketloss–  Timeout:slow–  DuplicateACK

•  Requiresmanypacketsinflight•  Whichdoesn’thappenforveryshorttransfers 73

PersistentHTTPNon‐persistentHTTPissues:•  Requires2RTTsperobject•  OSmustallocateresources

foreachTCPconnecQon

•  ButbrowsersocenopenparallelTCPconnecQonstofetchreferencedobjects

PersistentHTTP:•  ServerleavesconnecQon

openacersendingresponse

•  SubsequentHTTPmessagesbetweensameclient/serveraresentoverconnecQon

Persistentwithoutpipelining:•  Clientissuesnewrequestonly

whenpreviousresponsehasbeenreceived

•  OneRTTforeachobject

Persistentwithpipelining:•  DefaultinHTTP/1.1

•  Clientsendsrequestsassoonasitencountersreferencedobject

•  Asli@leasoneRTTforallthereferencedobjects

74