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1
Chapter 13-14
Wide Area Networks (WANs),
Routing, and Shortest Paths
2
MotivationMotivation
Connect multiple computersSpan large geographic distanceCross public right-of-way
StreetsBuildingsRailroads
3
Building BlocksBuilding Blocks
Point-to-point long-distance connectionsPacket switches (nodes)
Forms a mesh network
4
Packet SwitchPacket Switch
Hardware deviceConnects to
Other packet switchesComputers
Forwards packetsUses addresses
5
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
10
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 route 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
SMDS (Switched Multimegabit Data Service)Offered 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
37
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)
40
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
41
Assessment of ATMAssessment of ATM
Failed to deliver on promiseSwitches too expensive for LANQoS impossible to implement
42
SummarySummary
Wide Area Networks (WANs)Span long distancesConnect many computersBuilt from packet switchesUse store-and-forward
WAN addressingTwo-part addressSwitch/computer
43
Summary (continued)Summary (continued)
RoutingEach switch contains routing tableTable gives next-hop for destination
Routing tables createdManuallyAutomatically
Two basic routing algorithmsDistance vectorLink state
44
Summary (continued)Summary (continued)
Example WAN technologiesARPANETX.25SMDSFrame RelayATM