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Chapter 13-14

Wide Area Networks (WANs),

Routing, and Shortest Paths

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MotivationMotivation

Connect multiple computersSpan large geographic distanceCross public right-of-way

StreetsBuildingsRailroads

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Building BlocksBuilding Blocks

Point-to-point long-distance connectionsPacket switches (nodes)

Forms a mesh network

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Packet SwitchPacket Switch

Hardware deviceConnects to

Other packet switchesComputers

Forwards packetsUses addresses

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Illustration of a Packet Switch

Illustration of a Packet Switch

Special-purpose computer systemCPUMemory I/O interfacesFirmware

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Building a WANBuilding a WAN

Place one or more packet switches at each site

Interconnect switchesLAN technology for local connectionsLeased digital circuits for long-distance

connections

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Illustration of a WANIllustration of a WAN

Interconnections depend onEstimated trafficReliability needed

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Store and ForwardStore and Forward

Basic paradigm used in packet switched networkPacket

Sent from source computerTravels switch-to-switchDelivered to destination

Switch“Stores” packet in memoryExamines packet’s destination address“Forwards” packet toward destination

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Addressing in a WANAddressing in a WAN

NeedUnique address for each computerEfficient forwarding

Two-part addressPacket switch number Computer on that switch

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Illustration of WAN Addressing

Illustration of WAN Addressing

Two part address encoded as integerHigher-order bits for switch numberLow-order bits for computer number

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Next-Hop ForwardingNext-Hop Forwarding

Performed by packet switchUses table of routesTable gives next hop

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Forwarding Table Abbreviations

Forwarding Table Abbreviations

Many entries point to same next hopCan be condensed (default)Improves lookup efficiency

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Source of Routing Table Information

Source of Routing Table Information

ManualTable created by handUseful in small networksUseful if routes never change

Automatic routingSoftware creates/updates tableNeeded in large networksChanges routes when failures occur

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Relationship of Routing To Graph Theory

Relationship of Routing To Graph Theory

GraphNode models switchEdge models connection

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Shortest Path ComputationShortest Path Computation

Algorithms from graph theoryNo central authority (distributed

computation)A switch

Must learn route to each destinationOnly communicates with directly attached

neighbors

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Illustration of Minimum Weight Path

Illustration of Minimum Weight Path

Label on edge represents “distance”Possible distance metric

Geographic distanceEconomic costInverse of capacity

Darkened path is minimum 4 to 5

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Algorithms for Computing Shortest Paths

Algorithms for Computing Shortest Paths

Distance Vector (DV)Switches exchange information in their routing

tablesLink-state

Switches exchange link status informationBoth used in practice

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Distance VectorDistance Vector

Periodic, two-way exchange between neighbors

During exchange, switch sendsList of pairsEach pair gives (destination, distance)

ReceiverCompares each item in list to local routesChanges routes if better path exists

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Distance Vector AlgorithmDistance Vector Algorithm

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Distance Vector IntuitionDistance Vector Intuition

LetN be neighbor that sent the routing messageV be destination in a pairD be distance in a pairC be D plus the cost to reach the sender

If no local route to V or local routed has cost greater than C, install a route with next hop N and cost C

Else ignore pair

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Example of Distance Vector Routing

Example of Distance Vector Routing

Consider transmission of one DV messageNode 2 send to 3, 5, and 6Node 6 installs cost 8 route to 2Later 3 sends update to 66 changes route to make 3 the next hop for

destination 2

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Link-State RoutingLink-State Routing

Overcomes instabilities in DVPair of switches periodically

Test link between themBroadcast link status message

SwitchReceives status messageComputes new routesUses Dijkstra’s algorithm

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Example of Link-State Information

Example of Link-State Information

Assume nodes 2 and 3Test link between themBroadcast information

Each node Receives informationRecomputes routes as needed

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Dijkstra’s Shortest Path Algorithm

Dijkstra’s Shortest Path Algorithm

InputGraph with weighted edgesNode, n

OutputSet of shortest paths from n to each nodeCost of each path

Called Shortest Path First (SPF) algorithm

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Dijkstra’s AlgorithmDijkstra’s Algorithm

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Algorithm IntuitionAlgorithm Intuition

Start with self as source nodeMove outwardAt each step

Find node u such that itHas not been consideredIs “closest” to source

Compute Distance from u to each neighbor vIf distance shorter, make path from u go through v

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Result of Dijkstra’s Algorithm

Result of Dijkstra’s Algorithm

Example routes from node 6To 3, next hop = 3, cost = 2To 2, next hop = 3, cost = 5To 7, next hop = 7, cost = 5To 4, next hop = 7, cost = 8To 5, next hop = 3, cost = 11To 1, next hop = 3, cost = 20

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Early WAN TechnologiesEarly WAN Technologies

ARPANETHistorically important in packet switchingFast when invented, slow by current standards

X.25Early commercial serviceStill UsedMore popular in Europe

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Recent WAN TechnologiesRecent WAN Technologies

SMDSOffered by phone companiesNot as popular as Frame Relay

Frame RelayWidely used commercial serviceOffered by phone companies

ATM

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Asynchronous Transfer Mode ( ATM )

Asynchronous Transfer Mode ( ATM )

Designed by phone companiesSingle technology meant to handle

VoiceVideoData

Intended as LAN or WANGoal: replacement for Internet

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ATM CharacteristicsATM Characteristics

End-to-end (application to application)Connection-oriented interface:

Establish “connection”Send dataClose connection

Performance guarantees (statistical)Uses cell switching

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ATM CellATM Cell

Fixed size packet (for highest speed electronics)

Size chosen as compromise between voice (small) and data (large)

5 octet header48 octet payload

Note: size not optimal for any application

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ATM Cell HeaderATM Cell Header

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ATM SwitchATM Switch

Building block of ATM networkConnections to

ComputersOther ATM switchesAccepts and forwards cells

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Cell ForwardingCell Forwarding

Performed directly by hardwareIncoming cell sent to an outgoing interfaceUses label in cellMotivation: highest speed

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Label SwitchingLabel Switching

ATM connection identified with 24-bit binary value

Known as Virtual Path Identifier / Virtual Channel Identifier (VPI / VCI)

Generically called labelVPI / VCI rewritten at each switch

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Example of VPI/VCI Rewriting

Example of VPI/VCI Rewriting

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ATM Quality of ServiceATM Quality of Service

Fine-grained (per connection)Specified when connection establishedEndpoint specifies

Type of data transferThroughput desiredMaximum packet burst sizeMaximum delay tolerated

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Type of Data TransferType of Data Transfer

Constant Bit Rate (CBR)Example: audio

Variable Bit Rate (VBR)Example: video with adaptive encoding

Available Bit Rate (ABR)Example: data

Unspecified Bit Rate (UBR)Each type has detailed parameters (e.g., mean, max,

burst duration)

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Sending Data Over ATMSending Data Over ATM

Uses ATM Adaptation Layer (AAL5)Accepts and delivers large, variable-size

packetsAAL5 divides into cells for transmission

Called segmentation and reassembly

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Assessment of ATMAssessment of ATM

Failed to deliver on promiseSwitches too expensive for LANQoS impossible to implement

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SummarySummary

Wide Area Networks (WANs)Span long distancesConnect many computersBuilt from packet switchesUse store-and-forward

WAN addressingTwo-part addressSwitch/computer

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Summary (continued)Summary (continued)

RoutingEach switch contains routing tableTable gives next-hop for destination

Routing tables createdManuallyAutomatically

Two basic routing algorithmsDistance vectorLink state

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Summary (continued)Summary (continued)

Example WAN technologiesARPANETX.25SMDSFrame RelayATM