COS461:ComputerNetworksSpring2011

MikeFreedman

h>p://www.cs.princeton.edu/courses/archive/spring11/cos461/

IPPacketSwitchingReading:Sect4.1.1–4.1.4,4.3.5

GoalsofToday’sLecture•  ConnecQvity

–  Circuitswitching–  Packetswitching

•  IPservicemodel–  Best‐effortpacketdelivery–  IPastheInternet’s“narrowwaist”– DesignphilosophyofIP

•  IPpacketstructure–  FieldsintheIPheader–  TracerouteusingTTLfield–  Source‐addressspoofing

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RecalltheInternetlayeringmodel3

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

Review:CircuitSwitching‐MulQplexingaLink

•  Time‐division– EachcircuitallocatedcertainQmeslots

•  Frequency‐division– Eachcircuitallocatedcertainfrequencies

4

time frequency

time

CircuitSwitching(e.g.,PhoneNetwork)

1.  SourceestablishesconnecQontodesQnaQon– NodealongthepathstoreconnecQoninfo– NodesmayreserveresourcesfortheconnecQon

2.  SourcesendsdataovertheconnecQon– NodesQnaQonaddress,sincenodesknowpath

3.  SourcetearsdownconnecQonwhendone

5

CircuitSwitchingWithHumanOperator6

Telephone switch

“Operator, please connect me to

555-1212”

AdvantagesofCircuitSwitching•  Guaranteedbandwidth

–  Predictableperformance:not“besteffort”

•  SimpleabstracQon–  ReliablecommunicaQonchannelbetweenhosts– Noworriesaboutlostorout‐of‐orderpackets

•  Simpleforwarding–  ForwardingbasedonQmeslotorfrequency– Noneedtoinspectapacketheader

•  Lowper‐packetoverhead–  ForwardingbasedonQmeslotorfrequency– NoIP(andTCP/UDP)headeroneachpacket

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DisadvantagesofCircuitSwitching•  Wastedbandwidth

–  Burstytrafficleadstoidleconnduringsilentperiod

•  BlockedconnecQons–  ConnecQonrefusedwhenresourcesarenotsufficient

•  ConnecQonset‐updelay– Unabletoavoidextralatencyforsmalldatatransfers

•  Networkstate– Networknodesmuststoreper‐connecQoninformaQon

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PacketSwitching:StaQsQcal(TimeDivision)MulQplexing

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Packets

•  IntuiQon:Trafficbycomputerend‐pointsisbursty!–  Versus:Telephonetrafficnotbursty(e.g.,constant56kbps)

•  Nodesdifferinnetworkdemand–  Peakdatarate(e.g.,Mbps)–  Dutycycle(howmuchQmespetnsending/receiving)

•  Packetswitching:Packetsqueue,handledinFIFOorder–  Eachsendergets#Qmeslots~demand

PacketSwitching(e.g.,Internet)1.  Datatrafficdividedintopackets

– Eachpacketcontainsheader(withsrcanddstaddr)2.  Packetstravelseparatelythroughnetwork

– Packetforwardingbasedontheheader– Networknodesmaystorepacketstemporarily– Besteffort:Packetsmaybeloss,corrupted,reordered

3.  DesQnaQonreconstructsthemessage

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IPServiceModel:WhyPackets?•  Datatrafficisbursty

– Websurfing,email,etc.

•  Don’twanttowastebandwidth–  Notrafficexchangedduringidleperiods

•  Be>ertoallowmulQplexing–  Differenttransfersshareaccesstosamelinks

•  Don’twantcomplex,statefulrouters–  Don’tneedtoreservebandwidth/memory,–  Don’tneedtorememberfromonepkttonext

•  Packetscanbedeliveredbymostanything–  RFC1149:IPDatagramsoverAvianCarriers

•  SQll,canbeinefficient:headerbitsineverypackets

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IPService:Best‐EffortisEnough•  NoerrordetecQonorcorrecQon

–  Higher‐levelprotocolcanprovideerrorchecking•  Successivepacketsmaynotfollowthesamepath

–  NotaproblemaslongaspacketsreachthedesQnaQon

•  Packetscanbedeliveredout‐of‐order–  Receivercanputpacketsbackinorder(ifnecessary)

•  Packetsmaybelostorarbitrarilydelayed–  Sendercansendthepacketsagain(ifdesired)

•  NonetworkcongesQoncontrol(beyond“drop”)–  Sendercanslowdowninresponsetolossordelay

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TheInternetProtocolSuite13

UDP TCP

Data Link

Physical

Applications

The Hourglass Model

Waist

The waist facilitates interoperability

FTP HTTP TFTP DNS

TCP UDP

IP

Ethernet SONET 802.11

History:WhyIPPackets?•  IPproposedintheearly1970s

– DefenseAdvancedResearchProjectAgency(DARPA)

•  Goal:connectexisQngnetworks– MulQplexeduQlizaQonofexisQngnetworks–  E.g.,connectpacketradionetworkstotheARPAnet

•  MoQvaQngapplicaQons–  Remotelogintoservermachines–  Inherentlyburstytrafficwithlongsilentperiods

•  PriorARPAnetexperiencewithpacketswitching–  Previouslyshowedstore‐and‐forwardpacketswitching

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OtherMainDrivingGoals(InOrder)

•  CommunicaQonshouldconQnuedespitefailures–  Surviveequipmentfailureorphysicala>ack–  TrafficbetweentwohostsconQnueonanotherpath

•  SupportmulQpletypesofcommunicaQonservices– Differingrequirementsforspeed,latency,&reliability

–  BidirecQonalreliabledeliveryvs.messageservice

•  Accommodateavarietyofnetworks–  BothmilitaryandcommercialfaciliQes

– MinimizeassumpQonsabouttheunderlyingnetwork

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OtherDrivingGoals,SomewhatMet•  Permitdistributedmanagementofresources

– NodesmanagedbydifferentinsQtuQons– …thoughthisissQllratherchallenging

•  Cost‐effecQveness–  StaQsQcalmulQplexingthroughpacketswitching– …thoughpacketheadersandretransmissionswasteful

•  Easeofa>achingnewhosts–  StandardimplementaQonsofend‐hostprotocols– …thoughsQllneedafairamountofend‐hostsooware

•  Accountabilityforuseofresources– MonitoringfuncQonsinthenodes– …thoughthisissQllfairlylimitedandimmature

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IPPacketStructure4-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

IPHeader:Version,Length,ToS•  IPVersionnumber(4bits)

–  Necessarytoknowwhatotherfieldstoexpect:howtoparse?

–  “4”(forIPv4),“6”(forIPv6)

•  Headerlength(4bits)–  #of32‐bitwordsinheader–  Typically“5”for20‐byteIPv4header,moreif“IPopQons”

•  Type‐of‐Service(8bits)–  Allowpacketstobetreateddifferentlybasedonneeds–  E.g.,lowdelayforaudio,highb/wforbulktransfer–  (We’lldiscussmoreduring“QualityofService”lecture)

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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

IPHeader:Length,Fragments,TTL•  Totallength(16bits)

–  #ofbytesinthepacket– Maxsizeis63,535bytes(216‐1)–  Linksmayhaveharderlimits:Ethernet“MaxTransmissionUnit”(MTU)commonly1500bytes

•  FragmentaQoninformaQon(32bits)–  PacketidenQfier,flags,andfragmentoffset–  SplitlargeIPpacketintofragmentsiflinkcannothandlesize–  …sowhytypicallysendmaxMTUpackets?

•  Time‐To‐Live(8bits)–  HelpsidenQfypacketsstuckinforwardingloops–  …andeventuallydiscardfromnetwork

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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

IPHeader:MoreonTime‐to‐Live(TTL)

•  PotenQalrobustnessproblem–  Forwardingloopscancausepacketstocycleforever–  Confusingifthepacketarrivesmuchlater

•  Time‐to‐livefieldinpacketheader–  TTLfielddecrementedbyeachrouteronpath–  PacketisdiscardedwhenTTLfieldreaches0…– …and“Qmeexceeded”message(ICMP)senttosource

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Aside:Tracerouteasnetworktool

•  Commonusesoftraceroute– DiscoverthetopologyoftheInternet– Debugperformanceandreachabilityproblems

•  OnUNIXmachine– “traceroutecnn.com”or“traceroute12.1.1.1”

•  OnWindowsmachine– “tracertcnn.com”or“tracert12.1.1.1”

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ExampleTraceroute:BerkeleytoCNN22

1 169.229.62.1

2 169.229.59.225

3 128.32.255.169

4 128.32.0.249

5 128.32.0.66

6 209.247.159.109

7 *

8 64.159.1.46

9 209.247.9.170

10 66.185.138.33

11 *

12 66.185.136.17

13 64.236.16.52

Hop number, IP address, DNS name

inr-daedalus-0.CS.Berkeley.EDU

soda-cr-1-1-soda-br-6-2

vlan242.inr-202-doecev.Berkeley.EDU

gigE6-0-0.inr-666-doecev.Berkeley.EDU

qsv-juniper--ucb-gw.calren2.net

POS1-0.hsipaccess1.SanJose1.Level3.net

?

?

pos8-0.hsa2.Atlanta2.Level3.net

pop2-atm-P0-2.atdn.net

?

pop1-atl-P4-0.atdn.net

www4.cnn.com

No response from router

No name resolution

IPHeader:UseofTTLinTraceroute•  Time‐To‐LivefieldinIPpacketheader

– SourcesendsapacketwithaTTLofn– EachrouteralongthepathdecrementstheTTL– “TTLexceeded”sentwhenTTLreaches0

•  TraceroutetoolexploitsthisTTLbehavior

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source destination

TTL=1 Time

exceeded

TTL=2

SendpacketswithTTL=1,2,…andrecordsourceof“;meexceeded”message

IPHeaderFields:TransportProtocol•  Protocol(8bits)

–  IdenQfiesthehigher‐levelprotocol•  E.g.,“6”forTCP,“17”forUDP

–  ImportantfordemulQplexingatreceivinghost•  Indicateswhatkindofheadertoexpectnext

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protocol=6 protocol=17

IP header

UDP header

Ethernet hdr IP header

TCP header

Ethernet hdr

IPHeader:ChecksumonHeader

•  Checksum(16bits)– Sumofall16‐bitwordsinIPheader–  Ifanybitsofheaderarecorruptedintransit,checksumwon’tmatchatreceivinghost

– Receivinghostdiscardscorruptedpackets•  Sendinghostwillretransmitthepacket,ifneeded

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134 + 212

= 346

134 + 216

= 350 Mismatch!

IPHeader:ToandFromAddresses•  TwoIPaddresses

– SourceanddesQnaQon(32bitseach)

•  DesQnaQonaddress– UniqueidenQfierforreceivinghost– Allowseachnodetomakeforwardingdecisions

•  Sourceaddress– UniqueidenQfierforsendinghost– Enablesrecipienttosendareplybacktosource

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SourceAddress:WhatifSourceLies?•  Sourceaddressshouldbethesendinghost

–  But,who’schecking?Youcan“spoof”anyaddress!

•  Whywouldsomeonewanttodothis?–  Launchadenial‐of‐servicea>ack

•  SendexcessivepacketstodesQnaQon•  …tooverloadnode,orlinksleadingtoit

–  EvadedetecQonby“spoofing”•  But,vicQmcouldidenQfyyoubysourceaddr,solie!

– Also,ana>ackagainstthespoofedhost•  Spoofedhostiswronglyblamed•  Spoofedhostmayreceivereturntrafficfromreceiver

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Summary:PacketSwitchingReview•  Efficient

–  Cansendfromanyinputthatisready

•  General– MulQpletypesofapplicaQons

•  Accommodatesburstytraffic– AddiQonofqueues

•  Storeandforward–  Packetsareselfcontainedunits–  Canusealternatepaths–reordering

•  ContenQon(i.e.,noisolaQon)–  CongesQon– Delay

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