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Handout # 9: Internet Topology and RoutingProfessor Yashar GanjaliDepartment of Computer ScienceUniversity of Toronto

yganjali@cs.toronto.eduhttp://www.cs.toronto.edu/~yganjaliCSC 458/2209 Computer Networks1AnnouncementsDont forget the programming assignmentExtended deadline: Friday Oct. 30th at 5pm (sharp)Submit electronically on MarkUs.

This weeks tutorialProgramming assignment 1 Q&A

Reading for this week: Chapter 5CSC 458/CSC 2209 Computer Networks2University of Toronto Fall 20152Announcements ContdMidterm examSection L0101: Tue. Oct. 27th, 1-3 PM Section L5101: Tue. Oct. 27th, 6-8 PMSection L0201: Thu. Oct. 29th, 1-3 PMSame room and time as the lectureFor undergraduate and graduate studentsCSC 458/CSC 2209 Computer Networks3University of Toronto Fall 2015CSC 458/CSC 2209 Computer Networks4University of Toronto Fall 2015OutlineInternets TopologyInternets two-tiered topologyAS-level topologyRouter-level topology

Routing in the InternetHierarchy and Autonomous SystemsInterior Routing Protocols: RIP, OSPFExterior Routing Protocol: BGP4Internet Routing ArchitectureDivided into Autonomous SystemsDistinct regions of administrative controlRouters/links managed by a single institutionService provider, company, university, Hierarchy of Autonomous SystemsLarge, tier-1 provider with a nationwide backboneMedium-sized regional provider with smaller backboneSmall network run by a single company or universityInteraction between Autonomous SystemsInternal topology is not shared between ASs but, neighboring ASs interact to coordinate routingCSC 458/CSC 2209 Computer Networks5University of Toronto Fall 2015

5AS TopologyNode: Autonomous SystemEdge: Two ASs that connect to each otherCSC 458/CSC 2209 Computer Networks6University of Toronto Fall 2015

12345676What is an Edge, Really?Edge in the AS graphAt least one connection between two ASsSome destinations reached from one AS via the other

CSC 458/CSC 2209 Computer Networks7University of Toronto Fall 2015

AS 1AS 2

Exchange PointAS 1AS 2AS 37Identifying Autonomous SystemsCSC 458/CSC 2209 Computer Networks8University of Toronto Fall 2015AS Numbers are 16 bit values.Level 3: 1 MIT: 3Harvard: 11Yale: 29U of T: 239AT&T: 7018, 6341, 5074, UUNET: 701, 702, 284, 12199, Sprint: 1239, 1240, 6211, 6242, Currently just over 52,000 in use.8CSC 458/CSC 2209 Computer Networks9University of Toronto Fall 2015Interdomain Paths

1234567ClientWeb serverPath: 6, 5, 4, 3, 2, 19Business RelationshipsNeighboring ASs have business contractsHow much traffic to carryWhich destinations to reachHow much money to payCommon business relationshipsCustomer-providerE.g., Princeton is a customer of AT&TE.g., MIT is a customer of Level 3Peer-peerE.g., Princeton is a peer of Patriot MediaE.g., AT&T is a peer of SprintCSC 458/CSC 2209 Computer Networks10University of Toronto Fall 201510Customer-Provider RelationshipCustomer needs to be reachable from everyoneProvider tells all neighbors how to reach the customerCustomer does not want to provide transit serviceCustomer does not let its providers route through itCSC 458/CSC 2209 Computer Networks11University of Toronto Fall 2015ddprovidercustomercustomerproviderTraffic to the customerTraffic from the customeradvertisementstraffic11Peer-Peer RelationshipPeers exchange traffic between customers AS exports only customer routes to a peerAS exports a peers routes only to its customersOften the relationship is settlement-free (i.e., no $$$)CSC 458/CSC 2209 Computer Networks12University of Toronto Fall 2015peerpeerTraffic to/from the peer and its customersdadvertisementstraffic12Princeton ExampleInternet: customer of AT&T and USLECResearch universities/labs: customer of Internet2Local residences: peer with Patriot Media Local non-profits: provider for several non-profitsCSC 458/CSC 2209 Computer Networks13University of Toronto Fall 2015

AT&TUSLEC

Internet2Patriotpeer13AS Structure: Tier-1 ProvidersTier-1 providerHas no upstream provider of its ownTypically has a national or international backboneUUNET, Sprint, AT&T, Level 3, Top of the Internet hierarchy of 12-20 ASsFull peer-peer connections between tier-1 providersCSC 458/CSC 2209 Computer Networks14University of Toronto Fall 2015

14AS Structure: Other ASsTier-2 providersProvide transit service to downstream customers but, need at least one provider of their ownTypically have national or regional scopeE.g., Minnesota Regional NetworkIncludes a few thousand of the ASsStub ASsDo not provide transit service to othersConnect to one or more upstream providersIncludes vast majority (e.g., 85-90%) of the ASsCSC 458/CSC 2209 Computer Networks15University of Toronto Fall 201515Characteristics of the AS GraphAS graph structureHigh variability in node degree (power law)A few very highly-connected ASsMany ASs have only a few connectionsCSC 458/CSC 2209 Computer Networks16University of Toronto Fall 20151101001000CCDF10.10.010.001AS degreeAll ASs have 1 or more neighborsVery few have degree >= 10016Characteristics of AS PathsAS path may be longer than shortest AS pathRouter path may be longer than shortest pathCSC 458/CSC 2209 Computer Networks17University of Toronto Fall 2015

sd3 AS hops, 7 router hops2 AS hops, 8 router hops17Backbone networksMultiple Points-of-Presence (PoPs)Lots of communication between PoPsAccommodate traffic demands and limit delayBackbone NetworksCSC 458/CSC 2209 Computer Networks18University of Toronto Fall 201518CSC 458/CSC 2209 Computer Networks19University of Toronto Fall 2015Example: Abilene Internet2 Backbone

19Points-of-Presence (PoPs)Inter-PoP linksLong distancesHigh bandwidthIntra-PoP linksShort cables between racks or floorsAggregated bandwidthLinks to other networksWide range of media and bandwidthCSC 458/CSC 2209 Computer Networks20University of Toronto Fall 2015Intra-PoPOther networksInter-PoP20Where to Locate Nodes and LinksPlacing Points-of-Presence (PoPs)Large population of potential customersOther providers or exchange pointsCost and availability of real-estateMostly in major metropolitan areasPlacing links between PoPsAlready fiber in the groundNeeded to limit propagation delayNeeded to handle the traffic loadCSC 458/CSC 2209 Computer Networks21University of Toronto Fall 201521Customer Connecting to a Provider

CSC 458/CSC 2209 Computer Networks22University of Toronto Fall 2015Provider

Provider1 access link2 access links

Provider2 access routers

Provider2 access PoPs22Multi-Homing: Two or More ProvidersMotivations for multi-homingExtra reliability, survive single ISP failureFinancial leverage through competitionGaming the 95th-percentile billing modelBetter performance by selecting better path

CSC 458/CSC 2209 Computer Networks23University of Toronto Fall 2015

Provider 1Provider 223Inferring the AS-Level TopologyCollect AS paths from many vantage pointsLearn a large number of AS pathsExtract the nodes and the edges from the pathExample: AS path 1 7018 88 impliesNodes: 1, 7018, and 88Edges: (1, 7018) and (7018, 88)Ways to collect AS paths from many placesMapping traceroute data to the AS levelMap using whoisExample: try whois h whois.arin.net MCI WorldcomMeasurements of the interdomain routing protocolCSC 458/CSC 2209 Computer Networks24University of Toronto Fall 201524Map Traceroute Hops to ASsCSC 458/CSC 2209 Computer Networks25University of Toronto Fall 2015 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 9 209.247.9.17010 66.185.138.3311 * 12 66.185.136.1713 64.236.16.52Traceroute output: (hop number, IP)AS25AS25AS25AS25AS11423AS3356AS3356AS3356AS3356AS1668AS1668AS1668AS5662BerkeleyCNNCalrenLevel3AOL25Define what data path is: forwarding path for the trafficTo identify routing anomalies, understand how traffic is affected by routing dynamicsW: Good to recap data path and BGP path. I still dont know what is a BGP path.

Challenges of Inter-AS MappingMapping traceroute hops to ASs is hardNeed an accurate registry of IP address ownershipWhois data are notoriously out of dateCollecting diverse interdomain data is hardPublic repositories like RouteViews and RIPE-RISCovers hundreds to thousands of vantage pointsEspecially hard to see peer-peer edgesCSC 458/CSC 2209 Computer Networks26University of Toronto Fall 2015

AT&TSprintHarvardHarvardB-schoold1d2???26Inferring AS RelationshipsKey ideaThe business relationships determine the routing policiesThe routing policies determine the paths that are chosenSo, look at the chosen paths and infer the policiesExample: AS path 1 7018 88 impliesAS 7018 allows AS 1 to reach AS 88AT&T allows Level 3 to reach PrincetonEach triple tells something about transit serviceCollect and analyze AS path dataIdentify which ASs can transit through the other and which other ASs they are able to reach this wayCSC 458/CSC 2209 Computer Networks27University of Toronto Fall 201527Paths You Should Never See (Invalid)CSC 458/CSC 2209 Computer Networks28University of Toronto Fall 2015Customer-providerPeer-peertwo peer edgestransit through a customer28Challenges of Relationship InferenceIncomplete measurement dataHard to get a complete view of the AS graphEspecially hard to see peer-peer edges low in hierarchyReal relationships are sometime more complexPeer in one part of the world, customer in anotherOther kinds of relationships (e.g., backup)Special relationships for certain destination prefixes

Still, inference work has proven very usefulQualitative view of Internet topology and relationshipsCSC 458/CSC 2209 Computer Networks29University of Toronto Fall 201529CSC 458/CSC 2209 Computer Networks30University of Toronto Fall 2015OutlineInternets TopologyInternets two-tiered topologyAS-level topologyRouter-level topology

Routing in the InternetHierarchy and Autonomous SystemsInterior Routing Protocols: RIP, OSPFExterior Routing Protocol: BGP30Routing Story So Far TechniquesFloodingDistributed Bellman Ford AlgorithmDijkstras Shortest Path First Algorithm

Question 1. Can we apply these to the Internet as a whole?Question 2. If not, what can we do?CSC 458/CSC 2209 Computer Networks31University of Toronto Fall 201531Routing in the InternetThe Internet uses hierarchical routing.Within an AS, the administrator chooses an Interior Gateway Protocol (IGP)Examples of IGPs: RIP (rfc 1058), OSPF (rfc 1247, ISIS (rfc 1142).Between ASs, the Internet uses an Exterior Gateway ProtocolASs today use the Border Gateway Protocol, BGP-4 (rfc 1771)CSC 458/CSC 2209 Computer Networks32University of Toronto Fall 201532Routing in the InternetCSC 458/CSC 2209 Computer Networks33University of Toronto Fall 2015

Stub ASTransit ASe.g. backbone service providerStub ASAS AAS BAS CInterior GatewayProtocolInterior GatewayProtocolInterior GatewayProtocolBGPBGP33Interior Routing ProtocolsRIPUses distance vector (distributed Bellman-Ford algorithm).Updates sent every 30 seconds.No authentication.Originally in BSD UNIX.Widely used for many years; not used much anymore.

OSPFLink-state updates sent (using flooding) as and when required.Every router runs Dijkstras algorithm.Authenticated updates.Autonomous system may be partitioned into areas.Widely used.CSC 458/CSC 2209 Computer Networks34University of Toronto Fall 201534Interdomain RoutingAS-level topologyDestinations are IP prefixes (e.g., 12.0.0.0/8)Nodes are Autonomous Systems (ASs)Links are connections & business relationshipsCSC 458/CSC 2209 Computer Networks35University of Toronto Fall 2015

1234567ClientWeb server35Challenges for Interdomain RoutingScalePrefixes: 150,000-500,000, and growingASs: 52,000 visible ones, and growingAS paths and routers: at least in the millionsPrivacyASs dont want to divulge internal topologies or their business relationships with neighborsPolicyNo Internet-wide notion of a link cost metricNeed control over where you send traffic and who can send traffic through youCSC 458/CSC 2209 Computer Networks36University of Toronto Fall 201536Link-State Routing is ProblematicTopology information is flooded High bandwidth and storage overheadForces nodes to divulge sensitive informationEntire path computed locally per nodeHigh processing overhead in a large networkMinimizes some notion of total distanceWorks only if policy is shared and uniformTypically used only inside an ASE.g., OSPF and IS-ISCSC 458/CSC 2209 Computer Networks37University of Toronto Fall 201537Distance Vector is on the Right TrackAdvantagesHides details of the network topologyNodes determine only next hop toward the destDisadvantagesMinimizes some notion of total distance, which is difficult in an interdomain settingSlow convergence due to the counting-to-infinity problem (bad news travels slowly)Idea: extend the notion of a distance vectorCSC 458/CSC 2209 Computer Networks38University of Toronto Fall 201538Path-Vector RoutingExtension of distance-vector routingSupport flexible routing policiesAvoid count-to-infinity problemKey idea: advertise the entire pathDistance vector: send distance metric per dest dPath vector: send the entire path for each dest dCSC 458/CSC 2209 Computer Networks39University of Toronto Fall 2015

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1dd: path (2,1)d: path (1)data trafficdata traffic39Faster Loop DetectionNode can easily detect a loopLook for its own node identifier in the pathE.g., node 1 sees itself in the path 3, 2, 1Node can simply discard paths with loopsE.g., node 1 simply discards the advertisementCSC 458/CSC 2209 Computer Networks40University of Toronto Fall 2015

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1d: path (2,1)d: path (1)d: path (3,2,1)40Border Gateway Protocol (BGP-4)BGP is a path-vector routing protocol.BGP advertises complete paths (a list of ASs).Also called AS_PATH (this is the path vector)Example of path advertisement: The network 171.64/16 can be reached via the path {AS1, AS5, AS13}.Paths with loops are detected locally and ignored.Local policies pick the preferred path among options.When a link/router fails, the path is withdrawn.CSC 458/CSC 2209 Computer Networks41University of Toronto Fall 201541BGP OperationsCSC 458/CSC 2209 Computer Networks42University of Toronto Fall 2015Establish session on TCP port 179 Exchange all active routes Exchange incremental updates

AS1AS2While connection is ALIVE exchangeroute UPDATE messagesBGP session42Incremental ProtocolA node learns multiple paths to destinationStores all of the routes in a routing tableApplies policy to select a single active route and may advertise the route to its neighborsIncremental updatesAnnouncement Upon selecting a new active route, add node id to path and (optionally) advertise to each neighborWithdrawalIf the active route is no longer available send a withdrawal message to the neighborsCSC 458/CSC 2209 Computer Networks43University of Toronto Fall 201543BGP MessagesOpen : Establish a BGP session. Keep Alive : Handshake at regular intervals. Notification : Shuts down a peering session. Update : Announcing new routes or withdrawing previously announced routes.

Attributes include: Next hop, AS Path, local preference, Multi-exit discriminator, Used to select among multiple options for pathsCSC 458/CSC 2209 Computer Networks44University of Toronto Fall 2015BGP announcement = prefix + path attributes44BGP RouteDestination prefix (e.g,. 128.112.0.0/16)Route attributes, includingAS path (e.g., 7018 88)Next-hop IP address (e.g., 12.127.0.121)CSC 458/CSC 2209 Computer Networks45University of Toronto Fall 2015AS 88Princeton128.112.0.0/16AS path = 88Next Hop = 192.0.2.1AS 7018AT&T AS 12654RIPE NCCRIS project

192.0.2.1128.112.0.0/16AS path = 7018 88Next Hop = 12.127.0.12112.127.0.12145BGP Path SelectionSimplest caseShortest AS pathArbitrary tie breakExampleThree-hop AS path preferred over a four-hop AS pathAS 12654 prefers path through Global CrossingBut, BGP is not limited to shortest-path routingPolicy-based routingCSC 458/CSC 2209 Computer Networks46University of Toronto Fall 2015AS 3549Global Crossing AS 12654RIPE NCCRIS project AS 1129Global Access135.207.0.0/16AS Path = 1129 1755 1239 7018 6341135.207.0.0/16AS Path = 3549 7018 634146AS_PATH AttributeCSC 458/CSC 2209 Computer Networks47University of Toronto Fall 2015AS 7018135.207.0.0/16AS Path = 6341AS 1239SprintAS 1755EboneAT&TAS 3549Global Crossing 135.207.0.0/16AS Path = 7018 6341135.207.0.0/16AS Path = 3549 7018 6341AS 6341135.207.0.0/16AT&T ResearchPrefix OriginatedAS 12654RIPE NCCRIS project AS 1129Global Access135.207.0.0/16AS Path = 7018 6341135.207.0.0/16AS Path = 1239 7018 6341135.207.0.0/16AS Path = 1755 1239 7018 6341135.207.0.0/16AS Path = 1129 1755 1239 7018 6341Pick shorter AS path47Flexible PoliciesEach node can apply local policiesPath selection: Which path to use?Path export: Which paths to advertise?ExamplesNode 2 may prefer the path 2, 3, 1 over 2, 1Node 1 may not let node 3 hear the path 1, 2CSC 458/CSC 2209 Computer Networks48University of Toronto Fall 2015

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148So Many ChoicesCSC 458/CSC 2209 Computer Networks49University of Toronto Fall 2015Which route shouldFrank pick to 13.13.0.0./16? AS 1AS 2AS 4AS 313.13.0.0/16Franks Internet Barnpeerpeercustomerprovider49Franks ChoicesCSC 458/CSC 2209 Computer Networks50University of Toronto Fall 2015AS 1AS 2AS 4AS 313.13.0.0/16local pref = 80local pref = 100local pref = 90Set appropriate local prefto reflect preferences:Higher Local preference valuesare preferredpeerpeercustomerproviderRoute learned from customer preferred over route learned from peer, preferred over route learned from provider50BGP Route Selection SummaryCSC 458/CSC 2209 Computer Networks51University of Toronto Fall 2015Highest Local PreferenceShortest ASPATHLowest MEDi-BGP < e-BGPLowest IGP cost to BGP egressLowest router IDtraffic engineering Enforce relationshipsE.g. prefer customer routes over peer routesThrow up hands andbreak ties51BGP Policy: Applying Policy to RoutesImport policyFilter unwanted routes from neighborE.g. prefix that your customer doesnt ownManipulate attributes to influence path selectionE.g., assign local preference to favored routesExport policyFilter routes you dont want to tell your neighborE.g., dont tell a peer a route learned from other peerManipulate attributes to control what they seeE.g., make a path look artificially longer than it isCSC 458/CSC 2209 Computer Networks52University of Toronto Fall 201552BGP Policy: Influencing DecisionsCSC 458/CSC 2209 Computer Networks53University of Toronto Fall 2015Best Route Selection Apply Import PoliciesBest Route TableApply Export PoliciesInstall forwardingEntries for bestRoutes. ReceiveBGPUpdatesBestRoutesTransmitBGP UpdatesApply Policy =filter routes & tweak attributesBased onAttributeValuesIP Forwarding TableApply Policy =filter routes & tweak attributes Open ended programming.Constrained only by vendor configuration language53Import Policy: Local PreferenceFavor one path over anotherOverride the influence of AS path lengthApply local policies to prefer a pathExample: prefer customer over peerCSC 458/CSC 2209 Computer Networks54University of Toronto Fall 2015

AT&TSprintYaleTier-2Tier-3Local-pref = 100Local-pref = 9054Import Policy: FilteringDiscard some route announcementsDetect configuration mistakes and attacksExamples on session to a customerDiscard route if prefix not owned by the customerDiscard route that contains other large ISP in AS pathCSC 458/CSC 2209 Computer Networks55University of Toronto Fall 2015

AT&TPrinceton

USLEC128.112.0.0/1655Export Policy: FilteringDiscard some route announcementsLimit propagation of routing informationExamplesDont announce routes from one peer to anotherDont announce routes for network-management hosts

CSC 458/CSC 2209 Computer Networks56University of Toronto Fall 2015

AT&TPrinceton

Sprint128.112.0.0/16

UUNET

network operator56Export Policy: Attribute ManipulationModify attributes of the active routeTo influence the way other ASs behaveExample: AS prependingArtificially inflate the AS path length seen by othersTo convince some ASs to send traffic another wayCSC 458/CSC 2209 Computer Networks57University of Toronto Fall 2015

AT&TPrinceton

USLEC128.112.0.0/16

Sprint88 888857BGP Policy ConfigurationRouting policy languages are vendor-specificNot part of the BGP protocol specificationDifferent languages for Cisco, Juniper, etc.Still, all languages have some key featuresPolicy as a list of clausesEach clause matches on route attributes and either discards or modifies the matching routesConfiguration done by human operatorsImplementing the policies of their ASBusiness relationships, traffic engineering, security, http://www.cs.princeton.edu/~jrex/papers/policies.pdfCSC 458/CSC 2209 Computer Networks58University of Toronto Fall 201558AS is Not a Single NodeMultiple routers in an ASNeed to distribute BGP information within the ASInternal BGP (iBGP) sessions between routersCSC 458/CSC 2209 Computer Networks59University of Toronto Fall 2015

AS1AS2eBGPiBGP

59Joining BGP and IGP InformationBorder Gateway Protocol (BGP)Announces reachability to external destinationsMaps a destination prefix to an egress point128.112.0.0/16 reached via 192.0.2.1Interior Gateway Protocol (IGP)Used to compute paths within the ASMaps an egress point to an outgoing link192.0.2.1 reached via 10.10.10.10CSC 458/CSC 2209 Computer Networks60University of Toronto Fall 2015

192.0.2.110.10.10.1060Joining BGP with IGP InformationCSC 458/CSC 2209 Computer Networks61University of Toronto Fall 2015Forwarding TableForwarding TableAS 7018AS 88

192.0.2.1128.112.0.0/1610.10.10.10

BGP192.0.2.1128.112.0.0/16destinationnext hop10.10.10.10192.0.2.0/30destinationnext hop128.112.0.0/16Next Hop = 192.0.2.1128.112.0.0/16destinationnext hop10.10.10.10+192.0.2.0/3010.10.10.1061Causes of BGP Routing ChangesTopology changesEquipment going up or downDeployment of new routers or sessionsBGP session failuresDue to equipment failures, maintenance, etc.Or, due to congestion on the physical pathChanges in routing policyReconfiguration of preferencesReconfiguration of route filtersPersistent protocol oscillationConflicts between policies in different ASsCSC 458/CSC 2209 Computer Networks62University of Toronto Fall 201562Routing Change: Before and AfterCSC 458/CSC 2209 Computer Networks63University of Toronto Fall 2015

0123

0123(1,0)(2,0)(3,1,0)(2,0)(1,2,0)(3,2,0)63AS 1Delete the route (1,0)Switch to next route (1,2,0)Send route (1,2,0) to AS 3AS 3Sees (1,2,0) replace (1,0)Compares to route (2,0)Switches to using AS 2

Routing Change: Path ExplorationCSC 458/CSC 2209 Computer Networks64University of Toronto Fall 2015

0123(2,0)(1,2,0)(3,2,0)64Initial situationDestination 0 is aliveAll ASs use direct pathWhen destination diesAll ASs lose direct pathAll switch to longer pathsEventually withdrawnE.g., AS 2(2,0) (2,1,0) (2,1,0) (2,3,0) (2,3,0) (2,1,3,0)(2,1,3,0) nullRouting Change: Path ExplorationCSC 458/CSC 2209 Computer Networks65University of Toronto Fall 2015

1230(1,0)(1,2,0)(1,3,0)(2,0)(2,1,0)(2,3,0)(2,1,3,0)(3,0)(3,1,0)(3,2,0)65BGP runs over TCPBGP only sends updates when changes occurTCP doesnt detect lost connectivity on its ownDetecting a failureKeep-alive: 60 secondsHold timer: 180 secondsReacting to a failureDiscard all routes learned from the neighborSend new updates for any routes that changeBGP Session Failure CSC 458/CSC 2209 Computer Networks66University of Toronto Fall 2015

AS1AS266BGP Converges Slowly, if at AllPath vector avoids count-to-infinityBut, ASs still must explore many alternate paths to find the highest-ranked path that is still availableFortunately, in practiceMost popular destinations have very stable BGP routesAnd most instability lies in a few unpopular destinationsStill, lower BGP convergence delay is a goalCan be tens of seconds to tens of minutesHigh for important interactive applications or even conventional application, like Web browsingCSC 458/CSC 2209 Computer Networks67University of Toronto Fall 201567ConclusionsBGP is solving a hard problemRouting protocol operating at a global scaleWith tens of thousands of independent networksThat each have their own policy goalsAnd all want fast convergenceKey features of BGPPrefix-based path-vector protocolIncremental updates (announcements and withdrawals)Policies applied at import and export of routesInternal BGP to distribute information within an ASInteraction with the IGP to compute forwarding tablesCSC 458/CSC 2209 Computer Networks68University of Toronto Fall 201568