COMP/ELEC 429 Introduction to Computer Networks

T. S. Eugene Ng eugeneng at cs.rice.edu Rice University 1

COMP/ELEC 429/556Introduction to Computer Networks

Inter-domain routing

Some slides used with permissions from Edward W. Knightly, T. S. Eugene Ng, Ion Stoica, Hui Zhang

T. S. Eugene Ng eugeneng at cs.rice.edu Rice University 2

Autonomous Systems (AS)

• Internet is not a single network!

• The Internet is a collection of networks, each controlled by a different administration

• An autonomous system (AS) is a network under a single administrative control

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Example

AS-1

AS-2

AS-3

Interior router

Border router

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Implications

• ASs want to choose own local routing algorithm– AS takes care of getting packets to/from their own hosts– Intradomain routing: shortest path, load balance, etc.

• ASs want to choose own non-local routing policy– Interdomain routing must accommodate this to a degree

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Previous focus: Intra-Domain Routing

AS-1

AS-2

AS-3

Interior router

Border router

Intra-domain routing protocol aka Interior Gateway Protocol (IGP)

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Today’s focus: Inter-Domain Routing

AS-2

Interior router

Border router

AS-1

AS-3

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Inter-Domain Routing Considerations

• Global connectivity is at stake• Inevitably leads to one single protocol that everyone must speak

– Unlike many choices in intra-domain routing

• What are the requirements?– Scalability

– Flexibility in choosing routes

• If you were to choose, link state based or distance vector based?

• Border Gateway Protocol (BGP)– A hybrid between link state and distance vector

– “Path vector”

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Border Gateway Protocol Part I: E-BGP

border router interior router

E-BGPAS1

AS2

Two types of routers

Border router, Interior router

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BGP Operations (Simplified)

Establish session on TCP port 179

Exchange all active routes

Exchange incremental updates

AS1

AS2

While connection is ALIVE exchangeroute UPDATE messages

BGP session

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BGP Update Messages

• Update : Announcing new routes or withdrawing previously

announced routes.

Update=

Destination IP address prefix + attributes values(e.g. a routing path)

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Part I: E-BGP, Share connectivity information across ASs

E-BGP

A

AS1

AS2

you can reach netprefix A via addr and

the path is “AS2”

BGP table at R1:dest network prefix AS path next hop A AS2 addr

R1

addr

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E-BGP update

I-BGP updates

• I-BGP used to disseminate learned routes to all routers in AS

Part II: I-BGP, Carrying Info within an AS

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Part II: I-BGP, Carrying Info within an AS

AS1

B

AS3

A

AS2

you can reach netB via addr1 and the

path is “AS3”

E-BGP

you can reach netB via addr2 and

the path is “AS3 AS2”

E-BGP

addr2

addr1

I-BGP

you can reach netB via addr1 and the

path is “AS3”

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Attributes are Used to Select Best Routes

192.0.2.0/24pick me!

192.0.2.0/24pick me!

192.0.2.0/24pick me!

192.0.2.0/24pick me!

Given multipleroutes to the sameprefix, a BGP speakermust pick at mostone best route

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Example: Multiple AS Paths

AS701

AS73

AS7018

AS1239

AS9 128.2/16

128.2/169 701

128.2/169 7018 1239

Default choice: Pick shortest path

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Shorter Doesn’t Always Mean better

AS 4

AS 3

AS 2

AS 1

Is path 4 1 better than path 3 2 1?

AS can use custom policies other than shortest path

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Benefits of BGP Design

• Path Vector style routing– Distance vector algorithm with extra information– For each route, store the complete path (ASs)

• Advantages:– can make policy choices (choose among many possible

learned paths) based on set of ASs in path– can easily avoid loops

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Announcing and Choosing Routes

• BGP may learn many different paths for a destination network

• Learns only reachability information, no performance metrics– Not about optimizing anything– All about policy (business and politics)

• What a BGP speaker announces or not announces to a neighbor determines what routes may get used by that neighbor

• Router chooses among paths based on policy

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Nontransit vs. Transit ASes

ISP 1ISP 2

Nontransit ASmight be a corporateor campus network.

NET ATraffic NEVER flows from ISP 1through NET A to ISP 2(At least not intentionally!)

IP traffic

Internet Serviceproviders (often)are transit networks

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Selective Transit

NET BNET C

NET A provides transitbetween NET B and NET Cand between NET D and NET C NET A

NET D

NET A DOES NOTprovide transitBetween NET D and NET B

Most transit networks transit in a selective manner…

IP traffic

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Customers and Providers

Customer pays provider for access to the Internet

provider

customerIP traffic

provider customer

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The Peering Relationship

peer peer

customerprovider

Peers provide transit between their respective customers

Peers do not provide transit between peers

Peers (often) do not exchange $$$trafficallowed

traffic NOTallowed

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Peering Provides Shortcuts

Peering also allows connectivity betweenthe customers of “Tier 1” providers.

peer peer

customerprovider

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Import Routes

Frompeer

Frompeer

Fromprovider

Fromprovider

From customer

From customer

provider route customer routepeer route ISP route

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Export Routes

Topeer

Topeer

Tocustomer

Tocustomer

Toprovider

From provider

provider route customer routepeer route ISP route

filtersblock

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How can routes be marked as “provider”, “peer”, “customer”, “isp” ?

A community attribute is 32 bits

By convention, first 16 bits is ASN indicating who is giving itan interpretation

communitynumber

Very flexible BECAUSE it has no predefinedmeaning

Used for signalingwithin and betweenASs

Use “Community Attribute” in route announcement

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BGP Issues

• BGP designed for policy not performance

• Susceptible to router misconfiguration– Blackholes: announce a route you cannot reach

• Slow convergence time– Rate limiting and route flap dampening

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Combining IGP and BGP

Forwarding Table

OS kernel

IGP Process

IGP Routing tables

BGP Process

BGP Routing tables

Forwarding Table Manager

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Combine BGP and IGP Tables to Create Forwarding Table

AS 1 AS 2192.0.2.1

135.207.0.0/16

10.10.10.10

I-BGP: 135.207.0.0/16Next Hop = 192.0.2.1

192.0.2.0/30

Forwarding Table

135.207.0.0/16

destination next hop

10.10.10.10

192.0.2.0/30 10.10.10.10

BGP

192.0.2.1135.207.0.0/16

destination next hop

+

IGP

10.10.10.10192.0.2.0/30

destination next hop

E-BGP