CSCI-1680 Network Layer: - PowerPoint Presentation

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CSCI-1680Network Layer:Inter-domain Routing

Based partly on lecture notes by Rob Sherwood, David Mazières, Phil Levis, John Jannotti

Rodrigo Fonseca

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AdministriviaMidterm moved up from 3/17 to 3/15IP due on Friday

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TodayLast time: Intra-Domain Routing (IGP)RIP distance vector

OSPF link stateInter-Domain Routing (EGP)Border Gateway ProtocolPath-vector routing protocol

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Why Inter vs. IntraWhy not just use OSPF everywhere?E.g., hierarchies of OSPF areas? Hint: scaling is not the only limitation

BGP is a policy control and information hiding protocolintra == trusted, inter == untrustedDifferent policies by different ASsDifferent costs by different ASs

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Types of ASsLocal Traffic – source or destination in local ASTransit Traffic – passes through an AS

Stub ASConnects to only a single other ASMultihomed ASConnects to multiple ASsCarries no transit trafficTransit AS

Connects to multiple

ASs

and carries transit traffic

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AS RelationshipsHow to prevent X from forwarding transit between B and C?How to avoid transit between CBA ?

B: BAZ -> XB: BAZ -> C ? (=> Y: CBAZ and Y:CAZ)

B

A

C

X

Y

Z

Example from Kurose and Ross, 5

th

Ed

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Choice of Routing AlgorithmConstraintsScaling

Autonomy (policy and privacy)Link-state?Requires sharing of complete informationInformation exchange does not scaleCan’t express policyDistance Vector?Scales and retains privacy

Can’t implement policy

Can’t avoid loops if shortest path not taken

Count-to-infinity

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Path Vector ProtocolDistance vector algorithm with extra informationFor each route, store the complete path (

ASs)No extra computation, just extra storage (and traffic)AdvantagesCan make policy choices based on set of ASs in pathCan easily avoid loops

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BGP - High LevelSingle EGP protocol in use todayAbstract each AS to a single node

Destinations are CIDR prefixesExchange prefix reachability with all neighborsE.g., “I can reach prefix 128.148.0.0/16 through

ASes

44444 3356 14325 11078

”

Select a single path by routing

policy

Critical: learn many paths, propagate one

Add your ASN to advertised path

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Why study BGP?Critical protocol: makes the Internet runOnly widely deployed EGPActive area of problems!

EfficiencyCogent vs. Level3: Internet PartitionSpammers use prefix hijackingPakistan accidentally took down YouTubeEgypt disconnected for 5 days

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BGP Example

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BGP Example

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BGP Example

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BGP Example

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BGP Example

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BGP Protocol DetailsSeparate roles of speakers and

gatewaysSpeakers talk BGP with other ASsGateways are routes that border other AssCan have more gateways than speakersSpeakers know how to reach gatewaysSpeakers connect over TCP on port 179

Bidirectional exchange over long-lived connection

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BGP ImplicationsExplicit AS Path == Loop freeExcept under churn, IGP

/EGP mismatchReachability not guaranteedDecentralized combination of policiesNot all ASs know all pathsAS abstraction -> loss of efficiencyScaling

37K

ASs

350K+ prefixes

ASs

with one prefix: 15664

Most prefixes by one AS: 3686 (AS6389, BellSouth)

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BGP Table Growth

Source: bgp.potaroo.net

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Integrating EGP and IGPStub ASs

Border router clear choice for default routeInject into IGP: “any unknown route to border router”Inject specific prefixes in IGPE.g., Provider injects routes to customer prefixBackbone networksToo many prefixes for IGPRun internal version of BGP,

iBGP

All routers learn mappings: Prefix -> Border Router

Use IGP to learn: Border Router -> Next Hop

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iBGP

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iBGP

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BGP MessagesBase protocol has four message types

OPEN – Initialize connection. Identifies peers and must be first message in each direction UPDATE – Announce routing changes (most important message) NOTIFICATION – Announce error when closing connection

KEEPALIVE

– Make sure peer is alive

Extensions can define more message types

E.g., ROUTE-REFRESH [RFC 2918]

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Anatomy of an UPDATEWithdrawn routes: list of withdrawn IP prefixesNetwork Layer

Reachability Information (NLRI)List of prefixes to which path attributes applyPath attributesORIGIN, AS_PATH, NEXT_HOP, MULTI-EXIT-DISC, LOCAL_PREF, ATOMIC_AGGREGATE, AGGREGATOR, …Each attribute has 1-byte type, 1-byte flags, length, contentCan introduce new types of path attribute – e.g., AS4_PATH for 32-bit AS numbers

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ExampleNLRI: 128.148.0.0/16AS Path: ASN

44444 3356 14325 11078Next Hop IP: same as in RIPv2Knobs for traffic engineering:Metric, weight, LocalPath, MED, CommunitiesLots of voodoo

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BGP StateBGP speaker conceptually maintains 3 sets of stateAdj

-RIB-In“Adjacent Routing Information Base, Incoming”Unprocessed routes learned from other BGP speakersLoc-RIBContains routes from Adj-RIB-In selected by policyFirst hop of route must be reachable by IGP or static route

Adj

-RIB-Out

Subset of Loc-RIB to be advertised to peer speakers

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DemoRoute views project: http://www.routeviews.orgtelnet route-

views.linx.routeviews.orgshow ip bgp 128.148.0.0/16 longer-prefixesAll path are learned internally (iBGP)

Not a production device

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Route SelectionMore specific prefixNext-hop reachable?

Prefer highest weightComputed using some AS-specific local policyPrefer highest local-prefPrefer locally originated routesPrefer routes with shortest AS path lengthPrefer eBGP

over

iBGP

Prefer routes with lowest cost to egress point

Hot-potato routing

Tie-breaking rules

E.g., oldest route, lowest router-id

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Customer/Provider AS relationshipsCustomer pays for connectivityE.g. Brown contracts with OSHEANCustomer is stub, provider is a transit

Many customers are multi-homedE.g., OSHEAN connects to Level3, CogentTypical policy: prefer routes from customers

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Peer RelationshipsASs agree to exchange traffic for freePenalties/Renegotiate if imbalance

Tier 1 ISPs have no default route: all peer with each otherYou are Tier i + 1 if you have a default route to a Tier i

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Peering DramaCogent vs. Level3 were peersIn 2003, Level3 decided to start charging Cogent

Cogent said no Internet partition: Cogent’s customers couldn’t get to Level3’s customers and vice-versaOther ISPs were affected as wellTook 3 weeks to reach an undisclosed agreement

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“Shutting off” the InternetStarting from Jan 27

th, 2011, Egypt was disconnected from the Internet2769/2903 networks withdrawn from BGP (95%!

Source:

RIPEStat

- http://

stat.ripe.net/egypt

/

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Egypt Incident

Source: BGPMon (http://bgpmon.net/blog/?p=480)

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Some BGP ChallengesConvergenceScaling (route reflectors)Traffic engineeringHow to assure certain routes are selected

Security

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ConvergenceGiven a change, how long until the network re-stabilizes?Depends on change: sometimes never

Open research problem: “tweak and pray”Distributed setting is challengingEasier: is there a stable configuration?Distributed: open research problemCentralized: NP-Complete problem!Multiple stable solutions given policies (e.g.

“

Wedgies

”

, RFC 4264)

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Scaling iBGP: route reflectors

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Scaling iBGP: route reflectors

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Route EngineeringRoute filteringSetting weightsMore specific routes: longest prefixAS

prepending: “477 477 477 477”More of an art than science

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BGP SecurityAnyone can source a prefix announcement!To say BGP is insecure is an understatement

Pakistan Youtube incidentYoutube’s has prefix 208.65.152.0/22

Pakistan’s government order

Youtube

blocked

Pakistan Telecom (AS 17557) announces

208.65.153.0/

24 in the wrong direction (outwards!)

Longest prefix match caused worldwide outage

http://www.youtube.com/watch?v=

IzLPKuAOe50

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Many other incidentsSpammers steal unused IP space to hideAnnounce very short prefixes

For a short amount of timeChina incident, April 8th 2010China Telecom’s AS23724 generally announces 40 prefixesOn April 8th, announced ~37,000 prefixes

About 10% leaked outside of China

Suddenly, going to

www.dell.com

might have you routing through AS23724!

Secure BGP is in the works

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BGP RecapKey protocol that holds Internet routing togetherPath Vector Protocol among Autonomous SystemsPolicy, feasibility first; non-optimal routes

Important security problems

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Next LectureNetwork layer wrap-upIPv6Multicast

MPLSNext Chapter: Transport Layer (UDP, TCP,…)