Ch 13 WAN Technologies and Routing

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Ch 13 WAN Technologies and Routing
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Motivation

• How to build a packet switching system that can
  span a large area
• Building blocks
• Point-to-point long-distance connections
• Packet switches
• Key issue that separate MAN/WAN from LAN is
  scalability
• Provide sufficient capacity to permit computers
  to communicate simultaneously

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Motivation
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Packet Switches

• A hardware device used to connects to
• Other packet switches
• Computers

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Illustration of a Packet Switch

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

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Illustration of a WAN
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Forming 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
• Interconnections depend on
• Estimated traffic (e.g., T1)
• Reliability needed

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Store and Forward

• Packets
• Sent from source computer
• Travels switch-to-switch to destination
• Switch
• Stores packet in buffer
• Examines packets destination address
• Forwards packet toward destination
• Output-port buffer

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Physical Addressing in a WAN

• Each WAN technology defines the exact frame
  format
• Each device connected to a WAN is assigned a
  unique physical address
• Many WANs use a hierarchical addressing scheme
  for efficient forwarding
• Packet switch number
• Port number

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Illustration of WAN Addressing

• An address is encoded as a single binary value
• Higher-order bits for switch number
• Low-order bits for computer number

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Next-Hop Forwarding

• The process of forwarding a packet to its next
  hop is known as routing
• Routing table for switch 2

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Forwarding Table Abbreviations

• Many entries point to same next hop can be
  condensed (default)
• Only examines the first part of the address
• Reduces the size of routing table (scalability)
• Improves lookup efficiency

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Source Independency

• Next-hop forwarding does not depends on
• Packets source address
• The path the packet has taken
• Next-hop forwarding does depends on
• Packets destination

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Relationship of Routing to Graph
edge
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Default Route

• Used to eliminate the case of duplication routing

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Source of Routing Table Information

• Static routing (manual)
• Table created by hand
• Useful in small networks (simplicity and low
  network overhead)
• Useful if routes never change
• Dynamic routing (automatic)
• Software creates / updates table
• Needed in large networks
• Changes routes when failures occur

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Dynamic Routing

• Distance Vector (DV)
• Switches exchange information in their routing
  tables
• Link-state
• Switches exchange link status information
• Can have a global view
• 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 Intuition

• 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

Dest. Next Dist.
V P 2
N
Cost2
A
Dest. Next Dist.
V X 6
Dest. Next Dist.
V N 4
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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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Link-State Routing
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Shortest Path Computation in a Graph

• Possible distance metric
• Geographic distance
• Economic cost
• Inverse of capacity
• Darkened path is minimum for node 4 to node 5

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Shortest Path Computation in a Graph

• Dijkstras Algorithms
• Routing table for switch 4 is constructed during
  the computation of the paths
• A switch
• Only communicates with directly attached
  neighbors
• Must learn route to each destination

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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

• Frame Relay
• Offered by phone companies
• Widely used commercial service
• SMDS (Switched Multi-megabit Data Service)
• Offered by phone companies
• Not as popular as Frame Relay
• ATM

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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

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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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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
Ru
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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

1 1 2 2 3 3 4 4 5 5 7 7
D R D R D R D R D R D R
S1,2,3,4,5,7 ? 0 8 2 2 3 ? 0 ? 0 5 7
S1,2,4,5,7 ? 0 5 3 13 3 ? 0 5 7
S1,4,5,7 ? 0 13 3 11 3 5 7
S1,4,5 ? 0 8 7 11 3
S1,5 ? 0 11 3
S1 20 3
S