Wide Area Networks (WANs),

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Part 1.3

• Wide Area Networks (WANs),
• Routing, and Shortest Paths

Robert L. Probert, SITE, University of Ottawa
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Ostap Monkewich

• Status at the University of Ottawa
• SITE NCIT Research Fellow
• Education
• Ph.D. Electrical Engineering, University of
  Ottawa
• M.A.Sc. Electrical Engineering, University of
  Ottawa
• B.Eng. Electrical Engineering, McGill University
• Work Experience
• Nortel Networks, 21 years, Hardware/Software
  design
• Department of Communications, 16 years,
  interoperability laboratory
• Standards
• ITU-T Vice-Chairman SG 17, Chairman WP on
  Languages and Telecommunications Software
• IETF/IRTF OSPF WG, Modelling and Simulation

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Motivation

• Connect multiple computers
• Span large geographic distance
• Cross public right-of-way
• Streets
• Buildings
• Railroads

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Building Blocks

• Node-to-node links
• Point-to-point long-distance connections
• Packet switches
• Routers
• Protocols

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Packet Switch

• Hardware/Software device
• Connects to
• Other packet switches
• Computers
• Processes and Forwards packets
• Uses unique (routable) addresses

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Illustration of a Packet Switch

• Special-purpose computer system
• CPU
• Memory
• I/O interfaces
• Firmware
• Software

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Building a WAN

• Place one or more packet switches at each site
• Interconnect switches
• LAN technology for local connections
• Leased digital circuits for long-distance
  connections

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Illustration of a WAN

• Interconnections depend on
• Estimated traffic
• Reliability needed

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IP Network
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Store and Forward

• Basic paradigm used in packet switched network
• Packet
• Sent from source computer
• Travels switch-to-switch
• Delivered to destination
• Switch
• Stores packet in memory
• Examines packets destination address
• Forwards packet toward destination

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Packet Switching

• Datagrams
• routing decision is based on the header only
• Datagrams are connectionless
• different packets making up the same message can
  take different path, can be lost or arrive out of
  sequence
• router is stateless, it does not store results of
  its decision
• Virtual Circuits
• routing decision is based on
• header information
• previous packets
• Virtual Circuits are connection-oriented (ATM,
  MPLS)
• subsequent packets use the same path (path may be
  shared)
• switches are stateful, they store virtual circuit
  state and identifiers to match incoming packets

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Addressing in a WAN

• Need
• Unique address for each computer
• Efficient forwarding
• Two-part address
• Packet switch number
• Computer on that switch
• IP addresses are represented as 32-bit binary
  numbers
• For human readability these are converted to the
  Dotted Decimal Notation

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Example of Dotted Decimal Notation

• Four decimal values per 32-bit address
• Each decimal number
• Represents eight bits
• Is between 0 and 255

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Illustration of WAN Addressing

• Two part address encoded as integer
• Higher-order bits for switch number
• Low-order bits for computer number

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Address Classes Network Sizes

• Maximum network size determined by class of
  address
• Class A large
• Class B medium
• Class C small

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One More Note on Addressing

• 129.52.18.6/20 means
• 20 leading bits are for network addresses
• the remaining 12 bits are for host addresses
• This means that some of the bits making up .18.
• belong to network addressing
• belong to host addressing
• Need to expand back to binary to resolve
• expand 10000001 00110100 00010010 00000110
• network 10000001 00110100 0001
• host 0010 00000110
• Need a subnet mask
• 11111111 11111111 11110000 00000000 or
  255.255.220.0

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Next-Hop Forwarding

• Performed by packet switch
• Uses table of routes
• Table gives next hop

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Forwarding Table Abbreviations

• Many entries point to same next hop
• Can be condensed (default)
• Improves lookup efficiency

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Source of Routing Table Information

• Manual
• Table created by hand
• Useful in small networks
• Useful if routes never change
• Automatic routing
• Software creates/updates table
• Needed in large networks
• Changes routes when failures occur

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Relationship of Routing To Graph Theory

• Graph
• Node models switch
• Edge models connection

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Best Path Computation

• Algorithms from graph theory
• No central authority (distributed computation)
• A switch
• Must learn route to each destination
• Only communicates with directly attached neighbors

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Link-state MetricMinimum Weight Path

• Label on edge represents a metric between nodes
• Possible path metric
• Geographic distance
• Economic cost
• Inverse of capacity
• Darkened path is minimum metric from 4 to 5

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Algorithms for Computing Shortest Paths

• Distance Vector (DV)
• Switches exchange information in their routing
  tables
• Fewest number of intermediate hops
• Link-state
• Switches exchange link status information in
  their routing tables
• Sum of path metrics with smallest value
• Both used in practice

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Distance Vector

• Periodic, two-way exchange between neighbors
• During exchange, switch sends
• List of pairs
• Each pair gives (destination, distance)
• Receiver
• Compares each item in list to local routes
• Changes routes if better path exists

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Distance Vector Algorithm
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Distance Vector Intuition

• Let
• N be neighbor that sent the routing message
• V be destination in a pair
• D be distance in a pair
• C 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

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

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Link-State Routing

• Overcomes instabilities in DV
• Pair of switches periodically
• Test link between them
• Broadcast link status message
• Switch
• Receives status message
• Computes new routes
• Uses Dijkstras algorithm

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Example of Link-State Information

• Assume nodes 2 and 3
• Test link between them
• Broadcast information
• Each node
• Receives information
• Recomputes routes as needed

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Dijkstras Shortest Path Algorithm

• Input
• Graph with weighted edges
• Node, n
• Output
• Set of shortest paths from n to each node
• Cost of each path
• Called Shortest Path First (SPF) algorithm

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Dijkstras Algorithm
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Algorithm Intuition

• Start with self as source node
• Move outward
• At each step
• Find node u such that it
• Has not been considered
• Is closest to source
• Compute
• Distance from u to each neighbor v
• If distance shorter, make path from u go through v

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Result of Dijkstras Algorithm

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

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Link State Update Traffic

• To apply the routing algorithm, the current state
  of each link is required
• Every half hour routers exchange packets to
  update the state of their links for route
  calculations
• The traffic can be significant and peaks every
  half hour
• Older routers can be brought down as they are
  unable to handle the traffic peaks

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Inside the Router-LSA
LSA Header
Router-LSAs
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Number of LSAs in the 11- Router Network
Model Over 1.5-Hour Time Interval Simultaneous
Router Starts Random Number lt 100 of LSAs per
Router
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Number of LSAs in the 11- Router Network
Model Over 1-Hour Time Interval Equally Spaced
Router Starts Random Number lt 100 of LSAs per
Router
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Number of LSAs in the 11- Router Network
Model Over 1-Hour Time Interval Random Router
Starts Random Number lt 100 of LSAs per Router
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Early WAN Technologies

• ARPANET
• Historically important in packet switching
• Fast when invented, slow by current standards
• X.25
• Early commercial service
• Still Used
• More popular in Europe

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Recent WAN Technologies

• Swithced Multimegabit Digital Service
• Offered by phone companies for data
• Not as popular as Frame Relay
• Frame Relay
• Widely used commercial service
• Offered by phone companies for bridging LAN
  segments
• ATM
• Designed for fiberoptic links of 155 Mbps and
  higher
• Switches run at Gigabit speeds
• Voice, data and video
• Bandwidth assignment on request by application

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IP Network Protocol

• Works across multiple links
• Links may be of different technology
• telephone lines
• Ethernet
• packet radio
• ATM
• IP makes them all look the same - a homogeneous
  network

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ATM Switching
VP - Virtual Path VC - Virtual Channel
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Asynchronous Transfer Mode - ATM

• The layers

• ATM Adaptation Layer (AAL) - segment/reassemble
  frames
• ATM Layer (ATM) - analyze/switch VPIs/VCIs
• Physical Layer (PHY) - bit timing on physical
  medium

VCC - concatenation of VCs VPC -
concatenation of VP
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Assessment of ATM

• Failed to deliver on cost
• Switches too expensive for LAN
• QoS expensive to implement

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Summary

• Wide Area Networks (WANs)
• Span long distances
• Connect many computers
• Built from packet switches
• Use store-and-forward
• WAN addressing
• Two-part address
• Switch/computer

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Summary (continued)

• Routing
• Each switch contains routing table
• Table gives next-hop for destination
• Routing tables created
• Manually
• Automatically
• Two basic routing algorithms
• Distance vector
• Link state

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Summary (continued)

• Example WAN technologies
• ARPANET
• X.25
• SMDS
• Frame Relay
• ATM