Networking Fundamentals - ecs-network.serv.pacific.edu · ì Computer Systems and Networks ECPE 170 –Jeff Shafer –University of the Pacific Networking Fundamentals

ìComputer Systems and NetworksECPE170– JeffShafer– UniversityofthePacific

NetworkingFundamentals

Lab Schedule

Activitiesì ThisWeek

ì Networkprogrammingì Endiannessì Lab8– Network

Programming

AssignmentsDueì Lab8

ì DuebyMar27th 5:00am

ì Lab9ì DuebyApr3rd 5:00am

Spring2017ComputerSystemsandNetworks

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Persons of the Day: Vint Cerf / Bob Kahn

ì Co-designersofTCP/IPprotocolsuiteì Enablesreliable

communicationacrossunreliablenetwork

ì FoundationofInternet

ì 2004ACMTuringAwardwinners(shared)

ì 2005PresidentialMedalofFreedomwinners (shared)


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Person of the Day: Tim Berners-Lee

ì Inventorof“WorldWideWeb”ì Firstimplementationof

HTTP(HyperText TransferProtocol)tocommunicatebetweenclientandserver

ì KnightedbyQueenElizabethIIin2004


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ìComputer Networks


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Disclaimer

ì ThesetopicstakeanentiresemesterofCOMP177(ComputerNetworking)toexplore!

ì Afewdays(mostofwhichislabtime) isonlysufficientforthebriefestofoverviews…


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Network Model

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Application Layer(Myriad examples: Web browser, web server, etc…)

Transport Layer(Reliability – e.g. TCP)

Network Layer(Global Network – e.g. IP)

Link Layer(Local Area Network – e.g. Ethernet)

Physical Layer (“Bit on a Wire”)


Application Layer

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Application Layer

Transport LayerNetwork Layer

Link LayerPhysical Layer

HTTP DNS IMAP

Sockets

…andmanymore!

Skype BitTorrent RDP

SSH NTP NFS


Application Layer

ì Theapplicationlayer programmercanmakemany(fantastic)assumptionsaboutthenetworkì Thenetworkisreliable

ì Messagesarenotlostì Messagesarereceivedintheordertheyaresent

ì Thenetworkcantransferdataofinfinitelength(youcansendasmuchdataasdesired)

ì Youcandelivermessagesdirectlytoaspecificapplicationonaspecificcomputeranywhereontheplanet

ì Thelowerlayers(transport,network,link,…)doalltheheavy-liftingtomaketheseassumptionstrue


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Client-Server Architecture

Serverì Always-onhost

ì AlwayshasaknownIPaddress

ì Lotsofbandwidth

ì Serverprocess:processthatwaitstobecontacted

Clientì Communicatewithserver

ì Maybeintermittentlyconnected

ì MayhavedynamicIPaddresses

ì Donotcommunicatedirectlywitheachother

ì Clientprocess:processthatinitiatescommunication


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Why Do We Have Sockets?

ì Challenge– Inter-processcommunication

ì Aprocessisanindependentprogramrunningonahostì Separatememoryspace

ì Howdoprocessescommunicatewithotherprocessesì Onthesamehost?ì Ondifferenthosts?

ì Sendmessages betweeneachother

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What is a Socket?

ì Aninterfacebetween process(application)andnetworkì Theapplicationcreatesasocketì Thesockettypedictatesthestyleofcommunication

ì Reliablevs.besteffortì Connection-orientedvs.connectionless

ì Onceconfiguredtheapplicationcanì Passdatatothesocketfornetworktransmissionì Receivedatafromthesocket(transmittedthrough

thenetworkbysomeotherhost)


What is a Socket?ì Processsends/receivesmessages

to/fromitssocket

ì Socketanalogoustodoorì Sendingprocessshovesmessage

outdoorì Transportinfrastructureonother

sideofdoorcarriesmessagetosocketatreceivingprocess

ì Imagineyouarejustwritingtoafile…

ì APIallowcustomizationofsocketì Choosetransportprotocolì Chooseparametersofprotocol

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process

TCPwithbuffers,variables

socket

hostorserver

process

TCPwithbuffers,variables

socket

hostorserver

Internet

controlledbyOS

controlledbyappdeveloper


Addressing Processes

ì Toreceivemessages,eachprocessonahostmusthaveanidentifierì IPaddressesareuniqueì Isthissufficient?

ì No,therecanthousandsofprocessesrunningonasinglemachine(withoneIPaddress)

ì Identifiermustincludeì IPaddressì and portnumber(example:80forweb)

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ì Eachhosthas65,536ports

ì Someportsarereservedforspecificappsì FTP(20,21),Telnet(23),HTTP(80),etc…

ì Outgoingports(onclients)canbedynamicallyassignedbyOSinupperregion(above49,152)– calledephemeralports

ì Seehttp://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers

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Ports

Port0

Port1

Port65535


Socket Usage: Client Program

ì Basicsocketfunctionsforconnection-oriented(TCP)clients

1. socket() createthesocketdescriptor

2. connect() connecttotheremoteserver

3. send(),recv() communicatewiththe server

4. close() endcommunicationbyclosingsocketdescriptor

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Application-Layer Protocol

ì Socketsjustallowustosendrawmessagesbetweenprocessesondifferenthostsì Transportservicetakescareofmovingthedata

ì What exactlyissentisuptotheapplicationì Anapplication-layer protocolì HTTP,NTP,IMAP,SFTP,Skype,etc…

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Application-Layer Protocol

ì Boththeclientandserverspeakingtheprotocolmustagreeonì Typesofmessagesexchanged

ì e.g.,request,responseì Messagesyntax

ì Whatfieldsareinmessagesì Howfieldsaredelineated

ì Messagesemanticsì Meaningofinformationinfields

ì Rulesforwhen andhow processessendandrespondtomessages

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Hypertext Transfer Protocol Overview

ì HTTP istheapplicationlayerprotocol fortheweb

ì Itishowtheclientandservercommunicate

ì Client/servermodelì Client:browserthat

requests,receives,“displays”Webobjects

ì Server:Webserversendsobjectsinresponsetorequests

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PCrunningChrome

Serverrunning

ApacheWebserver

MacrunningSafari


Web and HTTP

ì Webpage consistsofbaseHTMLfileand(potentially)manyreferencedobjectsì HTMLfile,PNGimage,Flashvideo,…

ì EachobjectisaddressablebyaURL

ì ExampleURL:


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www.somecompany.com/someDept/image.png

hostname pathname

HTTP Request Message (Client->Server)


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GET /about/ HTTP/1.1Host: www.google.comUser-agent: Mozilla/13.0Connection: close Accept-language:en<line with only \r\n>

requestline(GET,POST,

HEADcommands)

headerlines

Carriagereturn,linefeed

indicatesendofmessage

HTTPisatext-basedprotocol.TheclientsendsASCII bytesintherequest,andtheserverrespondswithASCIIbytesinthereply.

HTTP Response Message (Server -> Client)


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HTTP/1.1 200 OKVary: Accept-EncodingContent-Type: text/htmlLast-Modified: Tue, 10 Apr 2012 09:33:47Date: Tue, 10 Apr 2012 17:50:51 GMTExpires: Tue, 10 Apr 2012 17:50:51 GMTCache-Control: private, max-age=0X-Content-Type-Options: nosniffServer: sffeX-XSS-Protection: 1; mode=blockTransfer-Encoding: chunked<line with only \r\n><Data begins here...>

statusline(protocol

statuscode,statusphrase)

headerlines

data,e.g.,requestedHTMLfile

HTTP Response Status Codes

200 OKì Requestsucceeded,requestedobjectlaterinthismessage

301 Moved Permanentlyì Requestedobjectmoved,newlocationspecifiedlaterinthis

message(Location:)

400 Bad Requestì Requestmessagenotunderstoodbyserver

404 Not Foundì Requesteddocumentnotfoundonthisserver

505 HTTP Version Not Supported


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Afewexamplesoutofmany!

ìOther Layers


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Link Layer

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Application LayerTransport LayerNetwork Layer

Link Layer

Physical Layer

Framing

Ethernet!

MAC addressesHubs & Switches

Transfer between

neighbors


Network Layer

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Application LayerTransport Layer

Network Layer

Link LayerPhysical Layer

IP – Internet Protocol!

IP Addresses Routers Routing Protocols

End-to-End packet

transfer


IP Properties

ì Datagramì Eachpacketisindividually

routedì Packetsmaybe

fragmented orduplicatedbyunderlyingnetworks

ì Connectionlessì Noguaranteeofdeliveryin

sequence

ì Unreliableì Noguaranteeofdeliveryì Noguaranteeofintegrity

ofdata

ì Besteffortì Onlydroppacketswhen

necessaryì Notimeguaranteefor

delivery


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Ethernetnetworksprovidethesame“guarantees”

Transport Layer

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Application LayerTransport Layer

Network LayerLink Layer

Physical Layer

TCP UDP

End-to-End message transfer

Sockets

Flow Control Congestion Control


“Magic” of the Internet

ì IP:Un-reliable,ordernotguaranteed,deliveryofindividualmessages

ì TCP:Reliable,in-orderdeliveryofdatastream

ì Magicì TCPisbuiltontopofIP!

ì GreatclownanalogybyJoelSpolskyhttp://www.joelonsoftware.com/articles/LeakyAbstractions.html

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Clown Delivery

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Broadway,NYC

NeedtomoveclownsfromBroadwaytoHollywoodforanewjob


Clown Delivery – Problems?

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Carcrash/lost

DifferentroutesShavedhead/toouglytowork!

Manycars,manyclownsBadthingsareguaranteedtohappentoatleastsome ofthem


Clown Delivery – Problems?

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PeopleinHollywoodgetfrustrated–It’shardtomakemovieswithclownsinthiscondition!


Clown Delivery - Solution

ì Newcompanyì HollywoodExpress

ì Guaranteesthatallclownsì (1)Arriveì (2)InOrderì (3)InPerfectCondition

ì Mishap?Callandrequestclown’stwinbrotherbesentimmediately

ì UFOcrashinNevadablockshighway?

ì Clownsre-routedviaArizonaì Directorneverevenhears

abouttheUFOcrashì Clownsarrivealittlemore

slowly

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Networking Abstraction

ì TCPprovidesasimilarreliabledeliveryserviceforIP

ì Abstractionhasitslimitsì Ethernetcablechewed

throughbycat?ì Nousefulerrormessage

forthatproblem!ì Theabstractionis

“leaky”– itcouldn’tsavetheuserfromlearningaboutthechewedcable

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ìDemos


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Demos

1. ImpersonatewebbrowserviaTelnet

2. Walkthroughofclient.py andserver.pydemoprograms

3. Rundisplay.py withexampleimage

4. Monitordisplay.py withWireshark andexaminepackettrace


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Documents

Networking Fundamentals - ecs-network.serv.pacific.edu · ì Computer Systems and Networks ECPE 170 –Jeff Shafer –University of the Pacific Networking Fundamentals