1 chapter 13-14 wide area networks (wans), routing, and shortest paths
TRANSCRIPT
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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 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
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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