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ComputerNetworks

ArvindKrishnamurthy

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MaterialbasedoncoursesatStanford,Princeton,andMIT

Focusofthecourse

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???

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Focusofthecourse(2)• Three“networking”topics:

– Communications– Networking– Distributedsystems

• Ourfocusisonthe“middle”layer

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But,WhatisNetworking?

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APlethoraofProtocolAcronyms?

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BGPARP

HTTP

DNS

PPP

OSPF

DHCP

TCP

UDP

SMTP

FTP

SSH

MAC

IPRIP

NAT

CIDR

VLAN VTP

NNTP

POP

IMAP

REDECN

SACK

SNMP

TFTP

TLS

WAPSIP IPX

STUN

RTP

RTSP

RTCP

PIM

IGMPICMP

MPLS

LDP

HIP

LISP

LLDP

BFD

AHeapofHeaderFormats?

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TCP/IPHeaderFormatsinLego

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ABigBunchofBoxes?

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Router Switch

FirewallNAT

Loadbalancer

DHCPserver

DNSserver

Bridge

Hub

Repeater

Basestation

ProxyWANaccelerator

GatewayIntrusionDetectionSystem

Packetshaper

RouteReflector

LabelSwitchedRouter Scrubber

Packetsniffer

DeepPacket

Inspection

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ATonofTools?

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traceroutenslookup

ping

ipconfigrancid

whois

tcpdump

wiresharkNDT

iperf

dummynet

syslog

tratsnort

bro

arpwatch

mrtg

nmap

ntop

dig

wget

net-snmp

ATonofTools?

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traceroutenslookup

ping

ipconfigrancid

whois

tcpdump

wiresharkNDT

iperf

dummynet

syslog

tratsnort

bro

arpwatch

mrtg

nmap

ntop

dig

wget

net-snmp

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TheMainPoint1. TolearnhowtheInternetworks»

– Whatreallyhappenswhenyou“browsetheweb”?

– WhatareTCP/IP,DNS,HTTP,NAT,VPNs,802.11etc.anyway?

2. Tolearnthefundamentalsofcomputernetworks

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WhylearnabouttheInternet?

1. Curiosity»2. Impactonourworld»

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Fromthisexperimentalnetwork…

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ARPANET~1970

(a)Dec.1969. (b)July1970. (c)March1971.

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Tothis!

• Aneverydayinstitutionusedatwork,home,andon-the-go

• Visualizationcontainsmillionsoflinks

Attribution:ByTheOpte Project[CC-BY-2.5],viaWikimediaCommons

Internet~2005

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Question

• Whatdoyouthinkaretheissuesthatonehastotackletogrowfromasmallnetworktoanextremelylargenetwork?

Internet– SocietalImpact• Anenablerofsocietalchange

– Easyaccesstoknowledge– Electroniccommerce– Personalrelationships– Discussionwithoutcensorship

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Internet– Economicimpact• Anengineofeconomicgrowth

– Advertising-sponsoredsearch– “Longtail”onlinestores– Onlinemarketplaces– Crowdsourcing

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TheMainPoint(2)1. TolearnhowtheInternetworks2. Tolearnthefundamentalsof

computernetworks– Whathardproblemsmustthey

solve?– Whatdesignstrategieshave

provenvaluable?

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NotaCourseGoal

• TolearnITjobskills– Howtoconfigureequipment

• e.g.,Ciscocertifications

– Butcoursematerialisrelevant,andweusehands-ontools

CourseMechanics• Course Administration

– Everything you need to know will be on the course web page:

http://www.cs.washington.edu/csep561/

• Teaching Assistants:– Yuchen & Ming

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CourseLogistics

1. Readings2. Weeklyreports:25%3. Projects/Homeworks:75%

• Ingroupsoftwo• Ifyouwanttoworkindividually,thenyoucanjustdo

assignments1and2(withdifferentdeadlines)• Detailsonwebsite

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AssignmentFramework• Mininet – emulationsystemfromStanford• Allowsyoutocreatenodes,switches,linksallonasinglemachine

• Assignment1:TCPdynamics• Assignment2:Buildasimplerouter• Assignment3:BuildaNAT

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Topics

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Topics

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Topics

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Exercise

• Considerawebobjectfetch“index.html”from“nytimes.com”– Whatareallthestepsinvolvedinperformingthefetch?

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KeyInterfaces• Between(1)appsandnetwork,and(2)networkcomponents– Moreformaltreatmentlateron

host

app

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KeyInterfaces(2)1. Network-applicationinterfaces

definehowappsusethenetwork– Sockets arewidelyusedinpractice

host

app

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2. Network-networkinterfacesdefinehownodesworktogether– Traceroute canpeekinthenetwork

KeyInterfaces(3)

host

app

PeekinginsidetheNetworkwithTraceroute

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NetworkServiceAPIHidesDetails• Appstalktootherappswithnorealideaofwhatisinsidethenetwork– Thisisgood!Butyoumaybecurious…

host

appapp

host

???

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Traceroute• Widelyusedcommand-linetooltolethostspeekinsidethenetwork– OnallOSes (tracert onWindows)– DevelopedbyVanJacobson~1987– Usesanetwork-networkinterface(IP)inwayswewillexplainlater

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Traceroute (2)• Probessuccessivehopstofindnetworkpath

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...

LocalHost

RemoteHost

Traceroute (3)

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...

LocalHost Remote

Host

1hop 2hops3hops N-1hops

Nhops

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UsingTraceroute

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UsingTraceroute (2)• ISPnamesandplacesareeducatedguesses

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...

Mycomputer www.uw.edu(www1.cac.washington.edu)

tde3hops

Telefonica4hops

Level36 hops

pnw-gigapop1hop

UW3 hops

NYC SanJose Seattle

UW

Home1 hop

100ms180ms

>200ms

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Traceroutetoanothercommercialwebserver-bash-3.1$traceroute www.nyse.comtraceroute towww.nyse.com (209.124.184.150),30hopsmax,40bytepackets1acar-hsh-01-vlan75.cac.washington.edu(128.208.2.100)0.327ms 0.353ms 0.392ms2uwcr-hsh-01-vlan3904.cac.washington.edu(205.175.110.17)0.374ms 0.412ms 0.443ms3uwcr-hsh-01-vlan1901.cac.washington.edu(205.175.103.5)0.595ms 0.628ms 0.659ms4uwbr-ads-01-vlan1902.cac.washington.edu(205.175.103.10)0.445ms 0.472ms 0.501ms5ccar1-ads-ge-0-0-0-0.pnw-gigapop.net(209.124.176.32)0.679ms 0.747ms 0.775ms6a209.124.184.150.deploy.akamaitechnologies.com.184.124.209.in-addr.arpa(209.124.184.150)0.621ms 0.456ms 0.419ms

Whatisgoingon?

-bash-3.1$nslookup www.nyse.comName:a789.g.akamai.netAddress:209.124.184.137

TheSocketAPI(§1.3.4,6.1.2-6.1.4)

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Network-ApplicationInterface• Defineshowappsusethenetwork

– Letsappstalktoeachotherviahosts;hidesthedetailsofthenetwork

host

appapp

host

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MotivatingApplication• Simpleclient-serversetup

request

reply

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MotivatingApplication(2)• Simpleclient-serversetup

– Clientappsendsarequesttoserverapp– Serverappreturnsa(longer)reply

• Thisisthebasisformanyapps!– Filetransfer:sendname,getfile(§6.1.4)– Webbrowsing:sendURL,getpage– Echo:sendmessage,getitback

• Let’sseehowtowritethisapp…

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SocketAPI• Simpleabstractiontousethenetwork

– ThenetworkserviceAPIusedtowriteallInternetapplications

– PartofallmajorOSes andlanguages;originallyBerkeley(Unix)~1983

• Supportstwokindsofnetworkservices– Streams:reliablysendastreamofbytes»– Datagrams:unreliablysendseparatemessages.(Ignore

fornow.)– Question:whenwouldyouusestreamsvs.datagrams?

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SocketAPI(2)• Sockets letappsattachtothe

localnetworkatdifferentports

Socket,Port#1

Socket,Port#2

SocketAPI(3)

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Primitive MeaningSOCKET CreateanewcommunicationendpointBIND AssociatealocaladdresswithasocketLISTEN Announcewillingnesstoacceptconnections;givequeuesizeACCEPT PassivelyestablishanincomingconnectionCONNECT ActivelyattempttoestablishaconnectionSEND SendsomedataovertheconnectionRECEIVE ReceivesomedatafromtheconnectionCLOSE Releasetheconnection

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UsingSocketsClient(host1) Server(host2)Time

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UsingSockets(2)Client(host1) Server(host2)Time

5:connect*

1:socket 2:bind1:socket

3:listen

9:send

6:recv*

4:accept*

7:send8:recv*

10:close 10:close

request

reply

disconnect

connect

*=callblocks

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ClientProgram(outline)socket() //makesocketgetaddrinfo() //serverandportname

//www.example.com:80connect() //connecttoserver[block]…send() //sendrequestrecv() //awaitreply[block]… //dosomethingwithdata!close() //done,disconnect

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ServerProgram(outline)socket() //makesocketgetaddrinfo() //forportonthishostbind() //associateportwithsocketlisten() //preparetoacceptconnectionsaccept() //waitforaconnection[block]…recv() //waitforrequest…send() //sendthereplyclose() //eventuallydisconnect

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ProtocolsandLayering(§1.3)

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NetworksNeedModularity• Thenetworkdoesmuchforapps:

– Makeandbreakconnections– Findapaththroughthenetwork– Transfersinformationreliably– Transfersarbitrarylengthinformation– Sendasfastasthenetworkallows– Sharesbandwidthamongusers– Securesinformationintransit– Letsmanynewhostsbeadded– …

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NetworksNeedModularity• Thenetworkdoesmuchforapps:

– Makeandbreakconnections– Findapaththroughthenetwork– Transfersinformationreliably– Transfersarbitrarylengthinformation– Sendasfastasthenetworkallows– Sharesbandwidthamongusers– Securesinformationintransit– Letsmanynewhostsbeadded– …

Weneedaformofmodularity,tohelpmanagecomplexityandsupportreuse

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ProtocolsandLayers• Protocols andlayering isthemainstructuringmethodusedtodivideupnetworkfunctionality– Eachinstanceofaprotocoltalksvirtuallytoitspeer usingtheprotocol

– Eachinstanceofaprotocolusesonlytheservicesofthelowerlayer

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ProtocolsandLayers(2)• Protocolsarehorizontal,layersarevertical

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X

YY

XInstanceofprotocolX

Peerinstance

Node1 Node2

Lowerlayerinstance(ofprotocolY)

ProtocolX

ServiceprovidedbyProtocolY

Question:whatisanexampleofaprotocolandwhatabstractiondoesitprovide/relyon?

ProtocolsandLayers(3)• Setofprotocolsinuseiscalledaprotocolstack

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ProtocolsandLayers(4)• Protocolsyou’veprobablyheardof:

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

• Anexampleprotocolstack– UsedbyawebbrowseronahostthatiswirelesslyconnectedtotheInternet

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Encapsulation• Encapsulation isthemechanismusedtoeffectprotocollayering– Lowerlayerwrapshigherlayercontent,addingitsowninformationtomakeanewmessagefordelivery

– Likesendingaletterinanenvelope;postalservicedoesn’tlookinside

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Encapsulation(2)

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HTTP

TCP

IP

802.11

Encapsulation(3)

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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

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Encapsulation(4)• Normallydrawmessagelikethis:

– Eachlayeraddsitsownheader

• Moreinvolvedinpractice– Trailersaswellasheaders,encrypt/compresscontents– Segmentation(dividelongmessage)andreassembly

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802.11 IP TCP HTTP

Firstbitsonthewire Lastbits

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Demultiplexing• Incomingmessagemustbepassedtotheprotocolsthatituses

??

UDPTCP

ARPIP

Ethernet

SMTP HTTP DNS

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Demultiplexing (2)• Donewithdemultiplexing keys intheheaders

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Ethernet IP TCP HTTP

UDPTCP

ARPIP

Ethernet

SMTP HTTP

Ethertype value

IPprotocolfield

TCPportnumber

HostIncomingmessage

DNS

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Protocols• Whataretheimplicationsofprotocolsandlayering?– Arethereanydownsidestoprotocols/layering?– Arethereanyperformancecosts?

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AdvantageofLayering• Usinginformationhidingtoconnectdifferentsystems

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HTTP

TCPIP

802.11

Browser

HTTP

TCP

IPEthernet

Server

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Guidance• Whatfunctionalityshouldweimplementatwhichlayer?– Thisisakeydesignquestion– Referencemodels provideframeworksthatguideus»

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OSI“7layer”ReferenceModel• Aprincipled,internationalstandard,toconnectsystems

– Influential,butnotusedinpractice.(Woops)

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– Providesfunctionsneededbyusers– Convertsdifferentrepresentations– Managestaskdialogs– Providesend-to-enddelivery– Sendspacketsovermultiplelinks– Sendsframesofinformation– Sendsbitsassignals

InternetReferenceModel• Afourlayermodelbasedonexperience;omitssomeOSIlayersandusestheIPasthenetworklayer.

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4.Application – Programsthatusenetworkservice

3.Transport – Providesend-to-enddatadelivery

2.Internet – Sendpacketsovermultiplenetworks

1.Link – Sendframesoveralink

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InternetReferenceModel(2)• Whatareexamplesofcommonprotocolsineachlayer?

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4.Application3.Transport

2.Internet

1.Link

InternetReferenceModel(3)• IPisthe“narrowwaist”oftheInternet

– Supportsmanydifferentlinksbelowandappsabove

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4.Application3.Transport

2.Internet

1.Link Ethernet802.11

IP

TCP UDP

HTTPSMTP RTP DNS

3GDSLCable

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IPSuite:EndHostsvs.Routers

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HTTP

TCP

IP

Ethernetinterface

HTTP

TCP

IP

Ethernetinterface

IP IP

Ethernetinterface

Ethernetinterface

SONETinterface

SONETinterface

host host

router router

HTTP message

TCP segment

IP packet IP packetIP packet

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Layer-basedNames(2)• Fordevicesinthenetwork:

Network

LinkNetwork

Link

Link Link

Physical PhysicalRepeater

Switch(orbridge)

Router

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Layer-basedNames(3)• Fordevicesinthenetwork:

Proxyormiddleboxorgateway

Network

LinkNetwork

Link

AppTransport

AppTransport

Buttheyalllooklikethis!

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ANoteAboutLayers• Theyareguidelines,notstrict

– Mayhavemultipleprotocolsworkingtogetherinonelayer

– Maybedifficulttoassignaspecificprotocoltoalayer

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Best-EffortPacket-DeliveryService

Host-NetworkDivisionofLabor• Packetswitching

– Dividemessagesintoasequenceofpackets– Headerswithsourceanddestinationaddress

• Best-effortdelivery– Packetsmaybelost– Packetsmaybecorrupted– Packetsmaybedeliveredoutoforder

host host

network

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Host-NetworkInterface:WhyPackets?

• Datatrafficisbursty– Loggingintoremotemachines– Exchanginge-mailmessages

• Don’twanttowastebandwidth– Notrafficexchangedduringidleperiods

• Bettertoallowmultiplexing– Differenttransfersshareaccesstosamelinks

• Packetscanbedeliveredbymostanything– RFC1149:IPDatagramsoverAvianCarriers

Host-NetworkInterface:WhyBest-Effort?

• Neverhavingtosayyou’resorry…– Don’treservebandwidthandmemory– Don’tdoerrordetection&correction– Don’trememberfromonepackettonext

• Easiertosurvivefailures– Transientdisruptionsareokayduringfailover

• Canrunonnearlyanylinktechnology– Greaterinteroperabilityandevolution

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IntermediateTransportLayer• But,applications wantefficient,accuratetransferofdatainorder,ina

timelyfashion– Lettheendhostshandleallofthat– (Anexampleofthe“end-to-endargument”)

• Transportlayercanoptionally…– Detectandretransmitlostpackets– Putout-of-orderpacketsbackinorder– Detectandhandlecorruptedpackets– Avoidoverloadingthereceiver– <insertyourrequirementhere>

DirectoriesandRouting

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RelationshipBetweenLayers

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linksession

path

name

address

Directories:MappingNametoAddress

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linksession

path

name

address

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TypesofDirectories• Simplisticdesigns

– Askeveryone(e.g.,floodinginARP)– Telleveryone(e.g.,pushing/etc/hosts)– Centraldirectory

• Scalabledistributeddesigns– Hierarchicalnamespace(e.g.,DNS)– Flatnamespace(e.g.,DistributedHashTable)

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Routing:MappingLinktoPath

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linksession

path

name

address

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PathComputation• Spanningtree(e.g.,Ethernet)

– Onetreethatconnectseverypairofnodes• Shortestpaths(e.g.,OSPF,IS-IS,RIP)

– Shortest-pathtreerootedateachnode

• Locallyoptimalpaths(e.g.,BGP)– Eachnodeselectsthebestamongitsneighbors

• End-to-endpaths(e.g.,sourcerouting)– Eachnodepicksthebestend-to-endpath

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NetworkDiscoveryandBootstrapping

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RelationshipBetweenLayers

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linksession

path

name

address

ThreeKindsofIdentifiersHost Name IP Address MAC Address

Example www.cs.princeton.edu 128.112.7.156 00-15-C5-49-04-A9

Size Hierarchical, human readable, variable length

Hierarchical, machine readable, 32 bits (in IPv4)

Flat, machine readable, 48 bits

Read by Humans, hosts IP routers Switches in LAN

Allocation, top-level

Domain, assigned by registrar (e.g., for .edu)

Variable-length prefixes, assigned by ICANN, RIR, or ISP

Fixed-sized blocks, assigned by IEEE to vendors (e.g., Dell)

Allocation, low-level

Host name, local administrator

Interface, by DHCPor an administrator

Interface, by vendor

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LearningaHost’sAddress

• WhoamI?– Hard-wired:MACaddress– Staticconfiguration:IPinterfaceconfiguration– Dynamicallylearned:IPaddressconfiguredbyDHCP

• Whoareyou?– Hard-wired:IPaddressinaURL,orinthecode– Dynamicallylookedup:ARPorDNS

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me youadapter adapter

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MappingBetweenIdentifiers• DynamicHostConfigurationProtocol(DHCP)

– GivenaMACaddress,assignauniqueIPaddress– …andtellhostotherstuffabouttheLocalAreaNetwork– Toautomatetheboot-strappingprocess

• AddressResolutionProtocol(ARP)– GivenanIPaddress,providetheMACaddress– ToenablecommunicationwithintheLocalAreaNetwork

• DomainNameSystem(DNS)– Givenahostname,providetheIPaddress– GivenanIPaddress,providethehostname

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DynamicHostConfigurationProtocol

arrivingclient DHCP server

Host learnsIP address,Subnet mask, Gateway address, DNS server(s), and a lease time.

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AddressResolutionProtocol(ARP)• EveryhostmaintainsanARPtable

– (IPaddress,MACaddress)pair• Consultthetablewhensendingapacket

– MapdestinationIPaddresstodestinationMACaddress– Encapsulateandtransmitthedatapacket

• But,whatiftheIPaddressisnotinthetable?– Senderbroadcasts:“WhohasIPaddress1.2.3.156?”– Receiverresponds:“MACaddress58-23-D7-FA-20-B0”– SendercachestheresultinitsARPtable

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requesting hostcis.poly.edu

gaia.cs.umass.edu

root DNS server

local DNS serverdns.poly.edu

1

23

4

5

6

authoritative DNS serverdns.cs.umass.edu

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TLD DNS server

DomainNameSystemHostatcis.poly.eduwants

IPaddressforgaia.cs.umass.edu

Recursive query: #1Iterative queries: #2, 4, 6

Questions• Shouldaddressescorrespondtotheinterface(pointofattachment)ortothehost?

• Whydowehaveallthreeidentifiers?Doweneedallthree?

• Whatshouldbedonetopreventspoofingofaddresses?