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Computer Networks

by: Jacey Olson
Jacey Olson

GPA 3.69


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Class Notes
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This 5 page Class Notes was uploaded by Jacey Olson on Thursday October 22, 2015. The Class Notes belongs to CSE 123 at University of California - San Diego taught by Staff in Fall. Since its upload, it has received 20 views. For similar materials see /class/226791/cse-123-university-of-california-san-diego in Computer Science and Engineering at University of California - San Diego.

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Date Created: 10/22/15
CSE 121 Spring 2003 Lecture 8 nter domain routing steran Savage Communications Software Time orimn Key issue The Internet is not just one network Interdomain versus intradomain routing Large cerperatlen You at home cximn mum lntzraamllnlautm Avnlm mm Solution Interdomain routing Separate routlng inside a domaln from routlng between oomalns lrlslde a uemaln use tradltlurlal lntener gateway preteeels osprl etc n dumalrls use Exterlur Gateway Prutuculs EGF s gtgt only excnange reachabllity lnrermatlen nu metncs De lde wnatte ue baseu on local pellcy Termlrlology Autonomous Systems ASs Unlt er abstractlen ln lrltErdEImalrl ruutlrlg anet Ruu nly netwerk vvlth eemm er were for dumaln en aumlnlstratlye urltrull a eenerent lnternal reutlng pullcy and presentlng a consistent external ylew er eenneetlylty Represented by a lBrblt number gtgt Example UUrlEi 7m Sprint 1239 UCSD 7377 ma mm cximn mum lntzraamllnlautlng Last two classes lntradomain routing protocols Also called lntenor Gateway Protocols lGP Create forwardlrlg tables at eacn routerso packets take best patn from any source to oestln l n Bestpatn shunes t patn m terms or metne hups eestl etc Dlstance yect Lucalexcharl eerglebaltepelegylnrermatlen Bellmanereru dlstrlbuted shunestpath algentnm Frublems durlrlgfallures eeunteteelnnmty Lll lkrs e Glubal Exchange erleeal tepelegy lnrermatlen Dllkstra s algentnm gt gt Complleateu te lmplement esp durlrlg rallure mm m cximn mum ntzraamllnlautmi Historic context Onglnal ARPArlet nao slngle routlng protoco el replaced a i o l wltn statlc metrlc LS algerltnm e NSFrletl CSrlEtl DDNl etc The total number or nudes was grewng Expurlerltlally Wltn tnelr ewn reutlng preteeels R F39l Hellul lSlS Arld tnelrewn rules e g F39 C Scalability Routll lg tables Wlth mllllol ls of entries Heterogeneity Network A uses nop count as a metrlcl Network B uses measured delay Network c uses llnk capacltyl wnat lr networks use olrrerent routlng protocols Policy Network A connects to Networks B arld c Network B ls only allowed to carry network C s traf c mm m cximn mum ntzraamllnlautmi InterDomain Routing Network comprised ofman Autonomous Systems ASes or domains To scale use hierarchy separate interdomain and intradomain routing 39 Also called interiorvs exterior gateway protocols lGPEGP lGP R P OSPF e EGP EGPl BGP mm m CXE inn mum m rum Mum InterDomain Routing Borderrouters summarlze arld adverllse lrlterrlal routes to external rlelghbors and woe yersa Borderrouters applypollc lrlternal routers Carl use rlotlorl oldefault routes Core l5 defaultyfree routers must have a route to all networks lrl the World mm mm chlzxn mum Wmmmung Problems with EGP In 1995 NSFnet got out ofthe backbone business Marly backbones MCl SprlrltAm Multlcorlrleoteo reglorlal networks Meshed topology loops A treebased structure didn t work anymore Need a new protocol mumps chlzxn mum Wamper Exterior Gateway Protocol First major interdomain routing protocol Spanning tree no loops BA re R R NEl glunal mm m chlzxn mum mmmmung What kind of protocol Llrlllt State gt TDD much state gtgt Currently ll EIEIEI ASS and gt lEIEIDEIEI rletwurks Relles on glubal metrll l pullcy Dlstarlce vector May rlut urlverge lumps Relles on glubal metrll and pullcy Solutlorl pathyector Reaepaplllyprpmepl no metrcs Route advenlsemerlts carry llst or s gt l arlreach UCSDthmughthlspath As7aAs7naAsll Autumatl loop detectlurl Why How mom will mum mmlequ Am 75 m3 Path Vectors Similar to distance vector except send entire paths e g 32l hears 71244 strorlger ayoloarloe of loops supports polloles later Modulo policy shorter paths 39 are chosen in preference to longer ones Reachability only no metrics chlzxn mum Wmmmung Pol c es Choice ofroutes may depend on owner cost AUP BuSlrleSS oorlsloeratlorls more Ol l HlS later Local policy dictates what route will be chosen and what routes will be advertised e g x doesrl t proylde trarlslt forB orA prerers not to use x mom will mum mmlequ How BGP operates roughly 5 L V As1 Proscons ofuslng TOP BGP session A While connection is ALIVE exchange route UPDATE messages Two types of BGP neighbor relationships iBGP keeps eBGP consistent ampEGP quotPMe iBGP is needed to avoid routing loops within an AS Injecting external routes into GP does not scale and causes BGP policy information to be lost ilGP neiunuors uo not nnounoe routes reoeiveu vi ilGPto otherilGP neighbors kw 75 m3 1 Important BGP attributes Local pref Statically con gured ranking of routes within AS AS path ASs the announcement traversed Origin Route came 39om IGP or EGP Multi Exit Discriminator preference for where to exit Commun39 opaque data used for interISP policy Nexthop where the route was heard 39om enema cximn innquot monumequ n BGP Decision process Default decision for route selection Highest iocai prerr shortestAs path iowest MED prefer eBGP over rBePr iowest iGP cost router rd Many policies built on default decision process but 0 5 Any criteria EGP attributes suurce aeerese perm is prrrner Can have separate puhcy rer inbuund ruutes instaiied mute and uutbuund ruutes Limited oniy by power or vendorespecrrrc routing ianguage enema cximn rnnm nornnnmnn w Example local pref Local preference 0 y used In iBGP s 4 preference values 7 A571 1 1 are m ore preferred Awe m u Example AS Path shorter As Paths are 159 128216 9 CM HIMnit more preferred 15701 K e no AS7018 39 V AHJ A e x 7 128216 A573 9 70181239 unwnrwash Shortest AS path doesn t mean best path Mr asp says that path is better than path 3 Maxim chmb mam nhraamlinlwung Example Using IGP cost for Hot potato routing egress 1 IGP distances This Ronterhas two BGP routes to 1924418024 Hot potato get traffic off of your network as soon as possible Go for egress 1 Problems with hot potato a High bandwidth 2355 content Providerb ckbo e s39 Web rann SFF Low bandwidth On customer backbone 15 san Diego customers want 0 Man their proVIdert t e bits Ongoing Problems wBGP instabiiity Ruute apping ong Asapth oeeision eriteria derauits to Dvriike behaviur booneing tguaranteedtu eonverge NF ahard to teii ir it dues Scaiabiiity stiii a probiem 1EEEIEIEI netwurk prefixes in oeraoitrree taoie tuday Tensiurv Want to manage traffic to very speeirie netvvurks eg muitihumed cuntervt pruviders but aisu Want El aggregate inrorrnation Performance Nunruptimai duesn t baianee iuad aeross oaths Security Miami chmb mam inhraamlinlwung 2 Routing policy So far we ve discussed mechanism How and why are basic routing policies decided maxim chmb mam nhraomlinlouuni History First policies for political reasons netAUP eyen today lntemet2 Emergence of commercial policies 19944995 NSFnettran ltlon NSF ceases to run lnternet backbone Commerclal camer M cl Sprlnt ANS start selling lF39 backbone serylce lnterconnecteu Wltn each other and reglunal networks at seyeral bubllc NAPs Eyerybne talks to eyerybne Tnen nye years wen all 7 mm emu mum hummhmun On the Internet No regulation One lSP doesn t haye to talllt to another Founded on shared goodwillquot Pay forconnecllvlty not perpacket Not clear who should pay anyway No standard settlement en 7 mm emu mum hummhmun Example peering Routing I01 Tables knlmmm u Background Settlement Thetelephone word LECslocal exchange carners lXCsUntenexcnangec LECs MUST provide lXCs access to customers regulation When a call goes 39om one phone companyto another Call bllleo to the caller Tn e money l5 Spill up among the phone Systems rtnls l5 called Settlement same um emu mum hrhmn Peering vs Transit Peering Two lSPs proyloe connectlylty to each others customers traoltlonally rorrree Nonctl ansltlve relatlonshlp Transit One lSP oroyloes connectlylty to eyery place lt knows about usually tor money emu um emu mum hrhmn Example transit By EastNet purchasing transit Eastnet is announced by USNet to USNet peering and transit interconnections alike emu um emu mum hrhmn


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