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Chapter 27Chapter 27

Internetwork Routing(Static and automatic routing;

route propagation; BGP, RIP, OSPF; multicast routing)

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TerminologyTerminology

ForwardingRefers to datagram transferPerformed by host or routerUses routing table

RoutingRefers to propagation of routing informationPerformed by routersInserts / changes values in routing table

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Two Forms of Internet Routing

Two Forms of Internet Routing

Static routingTable initialized when system bootsNo further changes

Automatic routingTable initialized when system bootsRouting software learns routes and updates tableContinuous changes possible

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Static RoutingStatic Routing

Used by most Internet hostsTypical routing table has two entries:

Local network → direct deliveryDefault → nearest router

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Example of Static RoutingExample of Static Routing

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Automatic RoutingAutomatic Routing

Used by IP routersRequires special softwareEach router communicates with neighborsPass routing informationUse route propagation protocol

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Example of Route Propagation

Example of Route Propagation

Each router advertises destinations that lie beyond it

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The Point of Routing Exchange

The Point of Routing Exchange

Each router runs routing software that learns about destinations other routers can reach, and informs other routers about destinations that it can reach. The routing software uses incoming information to update the local routing table continuously.

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Autonomous System Concept

Autonomous System Concept

Set of networks and routers under one administrative authority

Flexible, soft definitionIntuition: a single corporationNeeded because no routing protocol can

scale to entire InternetEach AS chooses a routing protocol

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Classifications of Internet Routing Protocols

Classifications of Internet Routing Protocols

Two broad classesInterior Gateway Protocols (IGPs)

Used among routers within autonomous systemDestinations lie within IGP

Exterior Gateway Protocols (EGPs)Used among autonomous systemsDestinations lie throughout Internet

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Illustration of IGP / EGP Use

Illustration of IGP / EGP Use

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The Concept of Route and Data Flow

The Concept of Route and Data Flow

Each ISP is an autonomous system that uses an Exterior Gateway Protocol to advertise its customers’ networks to other ISPs. After an ISP advertises destination D, datagrams destined for D can begin to arrive

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Specific Internet Routing Protocols

Specific Internet Routing Protocols

Border Gateway Protocol (BGP)Routing Information Protocol (RIP)Open Shortest Path First Protocol (OSPF)

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Border Gateway Protocol (BGP)

Border Gateway Protocol (BGP)

Provides routing among autonomous systems (EGP)Policies to control routes advertisedUses reliable transport (TCP)Gives path of autonomous systems for each

destinationCurrently the EGP of choice in the InternetCurrent version is four (BGP-4)As of January, 2009, the backbone Internet routing

table has approximately 300,000 entries

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The Routing Information Protocol (RIP)

The Routing Information Protocol (RIP)

Routing within an autonomous system (IGP)Hop count metricUnreliable transport (uses UDP)Broadcast or multicast deliveryDistance vector algorithmCan propagate a default routeImplemented by Unix program routedPassive version for hosts

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Illustration of RIP Packet Format

Illustration of RIP Packet Format

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The Open Shortest Path First Protocol (OSPF)

The Open Shortest Path First Protocol (OSPF)

Routing within an autonomous system (IGP)Full CIDR and subnet supportAuthenticated message exchangeAllows routes to be imported from outside

the autonomous systemUses link-status (SPF) algorithmSupport for multi-access networks (e.g.,

Ethernet)

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OSPF Areas and EfficiencyOSPF Areas and Efficiency

Allows subdivision of AS into areasLink-status information propagated within

areaRoutes summarized before being propagated

to another areaReduces overhead (less broadcast traffic)

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Link-Status in the InternetLink-Status in the Internet

Router corresponds to node in graphNetwork corresponds to edgeAdjacent pair of routers periodically

Test connectivityBroadcast link-status information to area

Each router uses link-status messages to compute shortest paths

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Illustration of OSPF GraphIllustration of OSPF Graph

(a) an interconnect of routers and networks, and (b) an equivalent OSPF graphRouter corresponds to a node in the graph

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OSPF and ScaleOSPF and Scale

Because it allows a manager to partition the routers and networks in an autonomous system into multiple areas, OSPF can scale to handle a much larger number of routers than other IGPs

Some networks are to big even for OSPF. For these networks BGP can be used as IGP.

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Internet Multicast RoutingInternet Multicast Routing

Difficult because Internet multicast allowsArbitrary computer to join multicast group at any timeArbitrary member to leave multicast group at any timeArbitrary computer to send message to a group (even if

not a member)Internet Group Multicast Protocol (IGMP)

Used between computer and local routerSpecifies multicast group membership

Multicast architectureMulticast architecture

IGMP is used both by the client computer and the adjacent network switches to connect the client to a local multicast router. Protocol Independent Multicast (PIM) is then used between the local and remote multicast routers, to direct multicast traffic from the video server to many multicast clients

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Multicast Routing Protocols

Multicast Routing Protocols

Several protocols existDistance Vector Multicast Routing Protocol (DVMRP)Core Based Trees (CBT)Protocol Independent Multicast – Sparse Mode (PIM-SM)Protocol Independent Multicast – Dense Mode (PIM-DM)Multicast extensions to the Open Shortest Path First

(MOSPF)None best in all circumstances

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SummarySummary

Static routing used by hostsRouters require automatic routingInternet divided into autonomous systemsTwo broad classes of routing protocols

Interior Gateway Protocols (IGPs) provide routing within an autonomous system

Exterior Gateway Protocols (EGPs) provide routing among autonomous systems

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Summary (continued)Summary (continued)

Border Gateway Protocol (BGP) is current EGP used in Internet

Interior Gateway Protocols include:Routing Information Protocol (RIP)Open Shortest Path First protocol (OSPF)

Internet multicast routing difficultProtocols proposed include: DVMRP, PIM-SM,

PIM-DM, MOSPF