T. S. Eugene Ngeugeneng at cs.rice.edu Rice University1 COMP/ELEC 429/556 Introduction 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 Ngeugeneng at cs.rice.edu Rice University2 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

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University3 Example AS-1 AS-2 AS-3 Interior router Border router

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University4 Implications ASs want to choose own local routing algorithm –AS takes care of getting packets to/from their own hosts –Intradomain routing: RIP, OSPF, etc ASs want to choose own non-local routing policy –Interdomain routing must accommodate this

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University5 Previous focus: Intra-Domain Routing AS-1 AS-2 AS-3 Interior router Border router Intra-domain routing protocol aka Interior Gateway Protocol (IGP), e.g. OSPF, RIP

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University6 Today’s focus: Inter-Domain Routing AS-2 Interior router Border router AS-1 AS-3

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University7 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”

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University8 Border Gateway Protocol Part I: E-BGP border routerinterior router E-BGP AS1 AS2  Two types of routers  Border router, Interior router

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University9 BGP Operations (Simplified) Establish session on TCP port 179 Exchange all active routes Exchange incremental updates AS1 AS2 While connection is ALIVE exchange route UPDATE messages BGP session

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University10 BGP Update Messages Update : Announcing new routes or withdrawing previously announced routes. Update = prefix + attributes values

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University11 Part I: E-BGP, Share connectivity information across ASs E-BGP A AS1 AS2 you can reach net prefix A via addr and the path is “AS2” BGP table at R1: dest network prefix AS path next hop A AS2 addr R1 addr

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University12 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

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University13 Part II: I-BGP, Carrying Info within an AS AS1 B AS3 A AS2 you can reach net B via addr1 and the path is “AS3” E-BGP you can reach net B via addr2 and the path is “AS3 AS2” E-BGP addr2 addr1 I-BGP you can reach net B via addr1 and the path is “AS3”

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University14 Attributes are Used to Select Best Routes /24 pick me! /24 pick me! /24 pick me! /24 pick me! Given multiple routes to the same prefix, a BGP speaker must pick at most one best route

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University15 Example: Multiple AS Paths AS701 AS73 AS7018AS1239AS / /

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University16 Shorter Doesn’t Always Mean Shorter AS 4 AS 3 AS 2 AS 1 Path 4 1 is “better” than path 3 2 1

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University17 Multiple Routing Processes on a Single Router Forwarding Table OS kernel IGP Process IGP Routing tables BGP Process BGP Routing tables Forwarding Table Manager

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University18 Combine BGP and IGP to Create Forwarding Table AS 1AS / I-BGP: /16 Next Hop = /30 Forwarding Table /16 destinationnext hop / BGP /16 destinationnext hop + IGP /30 destinationnext hop E-BGP

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University19 Benefits of BGP Design Path Vector style routing –Distance vector algorithm with extra information –For each route, store the complete path (ASs) –No extra computation, just extra storage Advantages: –can make policy choices (choose among many possible learned paths) based on set of ASs in path –can easily avoid loops

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University20 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

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University21 Nontransit vs. Transit ASes ISP 1 ISP 2 Nontransit AS might be a corporate or campus network. NET A Traffic NEVER flows from ISP 1 through NET A to ISP 2 (At least not intentionally!) IP traffic Internet Service providers (often) have transit networks

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University22 Selective Transit NET B NET C NET A provides transit between NET B and NET C and between NET D and NET C NET A NET D NET A DOES NOT provide transit Between NET D and NET B Most transit networks transit in a selective manner… IP traffic

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University23 Customers and Providers Customer pays provider for access to the Internet provider customer IP traffic provider customer

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University24 The Peering Relationship peer customerprovider Peers provide transit between their respective customers Peers do not provide transit between peers Peers (often) do not exchange $$$ traffic allowed traffic NOT allowed

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University25 Peering Provides Shortcuts Peering also allows connectivity between the customers of “Tier 1” providers. peer customerprovider

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University26 Import Routes From peer From peer From provider From provider From customer From customer provider routecustomer routepeer routeISP route

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University27 Export Routes To peer To peer To customer To customer To provider From provider provider routecustomer routepeer routeISP route filters block

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University28 How Can Routes be “Colored”? “Community” Value A community value is 32 bits By convention, first 16 bits is ASN indicating who is giving it an interpretation community number Very powerful BECAUSE it has no predefined meaning Used for signaling within and between ASs

T. S. Eugene Ngeugeneng at cs.rice.edu Rice University29 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