Review Of Networking Basics
Review of Networking BasicsYao WangPolytechnic University, Brooklyn, NY11201yao@vision.poly.edu
These slides are extracted from the slidesmade by authors of the book (J. F. Kuroseand K. Ross), available from the publisher sitefor instructors. We would like to thank theauthors for the excellent book and the slides.Slides based onComputer Networking:A Top Down ApproachFeaturing the Internet,Networking Basics2From [Kurose/Ross]2nd edition.Jim Kurose, Keith RossAddison-Wesley, July2002. Chapters 1-2
Overview of TelecommunicationNetworks and InternetBased on Chapter I -- ComputerNetworks and the InternetFrom [Kurose/Ross]Networking Basics3
Roadmap Whatis the Internet? Network edge Network core Internet structure and ISPs Protocol layers, service modelsFrom [Kurose/Ross]Networking Basics4
What’s the Internet: “nuts and bolts” view millions of connectedcomputing devices: hosts,routerserverend-systems PCs workstations, serversPDAs phones, toastersworkstationmobilelocal ISPrunning network apps communication links regional ISPfiber, copper, radio,satellitetransmission rate bandwidthrouters: forward packets(chunks of data)From [Kurose/Ross]companynetworkNetworking Basics5
What’s the Internet: “nuts and bolts” view protocols control sending,receiving of msgs e.g., TCP, IP, HTTP, FTP, PPPInternet: “network ofserverworkstationmobilelocal ISPnetworks” routerloosely hierarchicalpublic Internet versusprivate intranetregional ISPInternet standards RFC: Request for commentsIETF: Internet EngineeringTask ForceFrom [Kurose/Ross]companynetworkNetworking Basics6
A closer look at network structure: network edge:applications andhosts network core:routers network ofnetworks access networks,physical media:communication linksFrom [Kurose/Ross]Networking Basics7
Network edge: connection-oriented serviceGoal: data transferbetween end systems handshaking: setup(prepare for) datatransfer ahead of time Hello, hello back humanprotocolset up “state” in twocommunicating hostsTCP - TransmissionControl Protocol Internet’s connectionoriented serviceFrom [Kurose/Ross]TCP service [RFC 793] reliable, in-order bytestream data transfer flow control: loss: acknowledgementsand retransmissionssender won’t overwhelmreceivercongestion control: senders “slow down sendingrate” when networkcongestedNetworking Basics8
Network edge: connectionless serviceGoal: data transferApp’s using TCP:between end systems same as before!UDP - User DatagramProtocol [RFC 768]:Internet’sconnectionless service unreliable datatransfer no flow control no congestion controlFrom [Kurose/Ross]HTTP (Web), FTP (filetransfer), Telnet(remote login), SMTP(email)App’s using UDP: streaming media,teleconferencing, DNS,Internet telephonyNetworking Basics9
The Network Coremesh of interconnectedrouters the fundamentalquestion: how is datatransferred through net? circuit switching:dedicated circuit percall: telephone net packet-switching: datasent thru net indiscrete “chunks” From [Kurose/Ross]Networking Basics10
Network Core: Circuit SwitchingEnd-end resourcesreserved for “call”link bandwidth, switchcapacity dedicated resources:no sharing circuit-like(guaranteed)performance call setup required From [Kurose/Ross]Networking Basics11
Network Core: Circuit Switchingnetwork resources(e.g., bandwidth)divided into “pieces”pieces allocated to calls resource piece idle ifnot used by owning call dividing link bandwidthinto “pieces” frequency division time division(no sharing)From [Kurose/Ross]Networking Basics12
Circuit Switching: TDMA and TDMAExample:FDMA4 usersfrequencytimeTDMAfrequencyFrom [Kurose/Ross]timeNetworking Basics13
Network Core: Packet Switchingeach end-end data streamdivided into packets user A, B packets sharenetwork resources each packet uses full linkbandwidth resources used as neededBandwidth division into “pieces”Dedicated allocationResource reservationFrom [Kurose/Ross]resource contention: aggregate resourcedemand can exceedamount available congestion: packetsqueue, wait for link use store and forward:packets move one hopat a time transmit over link wait turn at nextlinkNetworking Basics14
Packet Switching: Statistical Multiplexing10 MbsEthernetABstatistical multiplexingC1.5 Mbsqueue of packetswaiting for outputlinkDESequence of A & B packets does not have fixedpattern · statistical multiplexing.In TDM each host gets same slot in revolving TDMframe.From [Kurose/Ross]Networking Basics15
Packet switching versus circuit switchingPacket switching allows more users to use network!1 Mbit link each user: circuit-switching: 100 kbps when “active”active 10% of timeN users1 Mbps link10 userspacket switching: with 35 users,probability 10 activeless than .0004From [Kurose/Ross]Networking Basics16
Packet switching versus circuit switchingIs packet switching a “slam dunk winner?”Great for bursty data resource sharing simpler, no call setup Excessive congestion: packet delay and loss protocols needed for reliable data transfer,congestion control Q: How to provide circuit-like behavior? bandwidth guarantees needed for audio/videoapps still an unsolved problem (chapter 6) From [Kurose/Ross]Networking Basics17
Packet-switched networks: forwarding Goal: move packets through routers from source todestination datagram network: we’ll study several path selection (i.e. routing)algorithms(chapter 4)destination address in packet determines next hoproutes may change during sessionanalogy: driving, asking directionsvirtual circuit network: each packet carries tag (virtual circuit ID), tagdetermines next hopfixed path determined at call setup time, remains fixedthru callrouters maintain per-call stateFrom [Kurose/Ross]Networking Basics18
Network tworksCircuit-switchednetworksFDMTDMNetworkswith VCsDatagramNetworks Datagram network is not either connection-orientedor connectionless. Internet provides both connection-oriented (TCP) andconnectionless services (UDP) to apps.From [Kurose/Ross]Networking BasicsInternet19
Internet structure: network of networksroughly hierarchical at center: “tier-1” ISPs (e.g., UUNet, BBN/Genuity,Sprint, AT&T), national/international coverage treat each other as equals Tier-1providersinterconnect(peer)privatelyFrom [Kurose/Ross]Tier 1 ISPTier 1 ISPNAPTier-1 providersalso interconnectat public networkaccess points(NAPs)Tier 1 ISPNetworking Basics20
Tier-1 ISP: e.g., SprintSprint US backbone networkFrom [Kurose/Ross]Networking Basics21
Internet structure: network of networks “Tier-2” ISPs: smaller (often regional) ISPs Connect to one or more tier-1 ISPs, possibly other tier-2 ISPsTier-2 ISP paystier-1 ISP forconnectivity torest of Internet tier-2 ISP iscustomer oftier-1 providerTier-2 ISPTier 1 ISPTier 1 ISPTier-2 ISPFrom [Kurose/Ross]Tier-2 ISPNAPTier 1 ISPTier-2 ISPsalso peerprivately witheach other,interconnectat NAPTier-2 ISPTier-2 ISPNetworking Basics22
Internet structure: network of networks “Tier-3” ISPs and local ISPs last hop (“access”) network (closest to end systems)localISPLocal and tier3 ISPs arecustomers ofhigher tierISPsconnectingthem to restof InternetTier 3ISPTier-2 ISPlocalISPlocalISPlocalISPTier-2 ISPTier 1 ISPTier 1 ISPTier-2 ISPlocallocalISPISPFrom [Kurose/Ross]NAPTier 1 ISPTier-2 ISPlocalISPNetworking BasicsTier-2 ISPlocalISP23
Internet structure: network of networks a packet passes through many networks!localISPTier 3ISPTier-2 ISPlocalISPlocalISPlocalISPTier-2 ISPTier 1 ISPTier 1 ISPTier-2 ISPlocallocalISPISPFrom [Kurose/Ross]NAPTier 1 ISPTier-2 ISPlocalISPNetworking BasicsTier-2 ISPlocalISP24
Internet protocol stack application: supporting networkapplications transport: host-host data transfer IP, routing protocolslink: data transfer betweenneighboring network elements TCP, UDPnetwork: routing of datagrams fromsource to destination FTP, SMTP, STTPapplicationtransportnetworklinkphysicalPPP, Ethernetphysical: bits “on the wire”From [Kurose/Ross]Networking Basics25
Layering: logical communicationEach layer: distributed “entities”implementlayer functionsat each node entitiesperformactions,exchangemessages withpeersFrom g cal26
Layering: logical communicationE.g.: transport take data from app add addressing,reliability checkinfo to form“datagram” send datagram topeer wait for peer toack receipt analogy: postofficeFrom icalackdataNetworking worklinkphysical27
Layering: physical ysicalapplicationtransportnetworklinkphysicalFrom rtnetworklinkphysicalNetworking 8
Protocol layering and dataEach layer takes data from above adds header information to create new data unit passes new data unit to layer belowsourceMHt MHn Ht MHl Hn Ht MapplicationtransportnetworklinkphysicalFrom [Kurose/Ross]destinationapplicationHttransportHn HtnetworklinkHl Hn HtphysicalNetworking BasicsMmessageMsegmentMdatagramMframe29
Application Layer Functions andProtocols with focus on ContentDistribution/Retrieval on theWebBased on Chapter II --Application LayerFrom [Kurose/Ross]Networking Basics30
Roadmap Principles of app layer protocols Client-server paradigm Transport service requirements of common app. DNS Web and HTTP Content distribution Network Web caching Content distribution networksFrom [Kurose/Ross]Networking Basics31
Applications and application-layer protocolsApplication: communicating,distributed processes e.g., e-mail, Web, P2P filesharing, instant messagingrunning in end systems(hosts)exchange messages toimplement applicationapplicationtransportnetworkdata linkphysicalApplication-layer protocols one “piece” of an appdefine messagesexchanged by apps andactions takenuse communication servicesprovided by lower layerprotocols (TCP, UDP)From [Kurose/Ross]applicationtransportnetworkdata linkphysicalNetworking Basicsapplicationtransportnetworkdata linkphysical32
App-layer protocol defines Types of messagesexchanged, e.g., request& response messages Syntax of messagetypes: what fields inmessages & how fieldsare delineated Semantics of the fields,i.e., meaning ofinformation in fields Rules for when and howprocesses send &respond to messagesFrom [Kurose/Ross]Public-domain protocols: defined in RFCs allows forinteroperability eg, HTTP, SMTPProprietary protocols: eg, KaZaANetworking Basics33
Client-server paradigmTypical network app has twopieces: client and serverClient:applicationtransportnetworkdata linkphysicalrequest initiates contact with server(“speaks first”) typically requests service fromserver, Web: client implemented inbrowser; e-mail: in mail readerServer: provides requested service to clientreplyapplicationtransportnetworkdata linkphysical e.g., Web server sends requested Webpage, mail server delivers e-mailFrom [Kurose/Ross]Networking Basics34
What transport service does an app need?Data loss some apps (e.g., audio) cantolerate some loss other apps (e.g., filetransfer, telnet) require100% reliable datatransferTiming some apps (e.g.,Internet telephony,interactive games)require low delay to be“effective”From [Kurose/Ross]Bandwidth some apps (e.g.,multimedia) requireminimum amount ofbandwidth to be“effective” other apps (“elasticapps”) make use ofwhatever bandwidththey getNetworking Basics35
Transport service requirements of common appsData lossBandwidthTime Sensitivefile transfere-mailWeb documentsreal-time audio/videono lossno lossno lossloss-tolerantnononoyes, 100’s msecstored audio/videointeractive gamesinstant messagingloss-tolerantloss-tolerantno losselasticelasticelasticaudio: 5kbps-1Mbpsvideo:10kbps-5Mbpssame as abovefew kbps upelasticApplicationFrom [Kurose/Ross]Networking Basicsyes, few secsyes, 100’s msecyes and no36
How do loss and delay occur?packets queue in router bufferspacket arrival rate to link exceeds output link capacity packets queue, wait for turn packet being transmitted (delay)ABFrom [Kurose/Ross]packets queueing (delay)free (available) buffers: arriving packetsdropped (loss) if no free buffersNetworking Basics37
Packet loss queue (aka buffer) preceding link in bufferhas finite capacity when packet arrives to full queue, packet isdropped (aka lost) lost packet may be retransmitted byprevious node, by source end system, ornot retransmitted at allFrom [Kurose/Ross]Networking Basics38
Internet transport protocols servicesTCP service:UDP service: unreliable data transfer connection-oriented: setuprequired between client andserver processesreliable transport betweensending and receiving processflow control: sender won’toverwhelm receivercongestion control: throttlesender when networkoverloadeddoes not providing: timing,minimum bandwidthguaranteesFrom [Kurose/Ross]between sending andreceiving process does not provide:connection setup,reliability, flow control,congestion control, timing,or bandwidth guaranteeQ: why bother? Why isthere a UDP?Networking Basics39
Internet apps: application, transport protocolsApplicatione-mailremote terminal accessWebfile transferstreaming multimediaInternet telephonyFrom [Kurose/Ross]Applicationlayer protocolUnderlyingtransport protocolSMTP [RFC 2821]Telnet [RFC 854]HTTP [RFC 2616]FTP [RFC 959]proprietary(e.g. RealNetworks)proprietary(e.g., Dialpad)TCPTCPTCPTCPTCP or UDPNetworking Basicstypically UDP40
DNS: Domain Name SystemDomain Name System:People: many identifiers: SSN, name, passport # distributed database application-layer protocolInternet hosts, routers: IP address (32 bit) used for addressingdatagrams“name”, e.g.,gaia.cs.umass.edu - usedby humansQ: map between IPaddresses and name ?From [Kurose/Ross]implemented in hierarchy ofmany name servershost, routers, name servers tocommunicate to resolve names(address/name translation) note: core Internetfunction, implemented asapplication-layer protocol complexity at network’s“edge”Networking Basics41
DNS name serversWhy not centralize DNS? single point of failure traffic volume distant centralizeddatabase maintenancedoesn’t scale!From [Kurose/Ross]no server has all nameto-IP address mappingslocal name servers: each ISP, company haslocal (default) name serverhost DNS query first goesto local name serverauthoritative name server: for a host: stores thathost’s IP address, namecan perform name/addresstranslation for that host’snameNetworking Basics42
DNS: Root name servers contacted by local name server that can not resolve name root name server: contacts authoritative name server if name mapping not knowngets mappingreturns mapping to local name servera NSI Herndon, VAc PSInet Herndon, VAd U Maryland College Park, MDg DISA Vienna, VAh ARL Aberdeen, MDj NSI (TBD) Herndon, VAk RIPE Londoni NORDUnet Stockholmm WIDE Tokyoe NASA Mt View, CAf Internet Software C. Palo Alto,CA13 root nameservers worldwideb USC-ISI Marina del Rey, CAl ICANN Marina del Rey, CAFrom [Kurose/Ross]Networking Basics43
Simple DNS examplehost surf.eurecom.frwants IP address ofgaia.cs.umass.eduroot name server251. contacts its local DNSserver, dns.eurecom.fr2. dns.eurecom.fr contacts local name serverdns.eurecom.frroot name server, ifnecessary163. root name server contactsauthoritative name server,dns.umass.edu, ifrequesting hostnecessarysurf.eurecom.frFrom [Kurose/Ross]Networking Basics34authorititive name serverdns.umass.edugaia.cs.umass.edu44
DNS exampleroot name serverRoot name server: may not know7authoritative nameserver may knowintermediate nameserver: who tocontact to findauthoritative nameserver62local name serverdns.eurecom.fr18requesting host3intermediate name serverdns.umass.edu45authoritative name .eduFrom [Kurose/Ross]Networking Basics45
HTTP overviewHTTP: hypertexttransfer protocol Web’s application layerprotocol client/server model client: browser thatrequests, receives,“displays” Web objects server: Web serversends objects inresponse to requests HTTP 1.0: RFC 1945 HTTP 1.1: RFC 2068From [Kurose/Ross]HTTPrequestPC running HTTPresExplorerponsesteuqrese ServerPnTorunningHTesprPTApache WebHTserverMac runningNavigatorNetworking Basics46
HTTP overview (continued)HTTP is “stateless”Uses TCP: client initiates TCPconnection (creates socket)to server, port 80 server accepts TCPconnection from client HTTP messages (applicationlayer protocol messages)exchanged between browser(HTTP client) and Webserver (HTTP server) TCP connection closedFrom [Kurose/Ross] server maintains noinformation aboutpast client requestsasideProtocols that maintain“state” are complex! past history (state) mustbe maintained if server/client crashes,their views of “state” maybe inconsistent, must bereconciledNetworking Basics47
HTTP connectionsNonpersistent HTTP At most one object issent over a TCPconnection. HTTP/1.0 usesnonpersistent HTTPFrom [Kurose/Ross]Persistent HTTP Multiple objects canbe sent over singleTCP connectionbetween client andserver. HTTP/1.1 usespersistent connectionsin default modeNetworking Basics48
HTTP request messagetwo types of HTTP messages: request, response HTTP request message: ASCII (human-readable format)request line(GET, POST,HEAD commands)GET /somedir/page.html HTTP/1.1Host: www.someschool.eduUser-agent: Mozilla/4.0header Connection: closelines Accept-language:frCarriage return,line feedindicates endof messageFrom [Kurose/Ross](extra carriage return, line feed)Networking Basics49
HTTP request message: general formatFrom [Kurose/Ross]Networking Basics50
HTTP response messagestatus line(protocolstatus codestatus phrase)headerlinesdata, e.g.,requestedHTML fileFrom [Kurose/Ross]HTTP/1.1 200 OKConnection closeDate: Thu, 06 Aug 1998 12:00:15 GMTServer: Apache/1.3.0 (Unix)Last-Modified: Mon, 22 Jun 1998 .Content-Length: 6821Content-Type: text/htmldata data data data data .Networking Basics51
HTTP response status codesIn first line in server- client response message.A few sample codes:200 OK request succeeded, requested object later in this message301 Moved Permanently requested object moved, new location specified later inthis message (Location:)400 Bad Request request message not understood by server404 Not Found requested document not found on this server505 HTTP Version Not SupportedFrom [Kurose/Ross]Networking Basics52
Cookies: keeping “state”Many major Web sitesuse cookiesFour components:Example: 1) cookie header line inthe HTTP responsemessage2) cookie header line inHTTP request message3) cookie file kept onuser’s host and managedby user’s browser4) back-end database atWeb siteFrom [Kurose/Ross] Susan access Internetalways from same PCShe visits a specific ecommerce site for firsttimeWhen initial HTTPrequests arrives at site,site creates a unique IDand creates an entry inbackend database forIDNetworking Basics53
Cookies: keeping “state” (cont.)clientebay: 8734Cookie fileamazon: 1678ebay: 8734usual http request msgusual http response Set-cookie: 1678usual http request msgcookie: 1678usual http response msgCookie fileamazon: 1678ebay: 8734From [Kurose/Ross]cookiespecificactionac cessacceone week later:enserverda try itab n bcreates IDas acekend1678 for userssCookie fileserverusual http request msgcookie: 1678usual http response msgNetworking Basicscookiespectificaction54
Cookies (continued)What cookies can bring: authorization shopping carts recommendatio
From [Kurose/Ross] Networking Basics 2 Slides based on Computer Networking: A Top Down Approach Featuring the Internet, 2nd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2002. Chapters 1-2 These slides are extracted from the slides made by authors of the book (J. F. Kurose and K.
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