WAN Technologies and Routing Realizing Physical Networks over Wider Distance

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WAN Technologies and Routing Realizing Physical
Networks over Wider Distance
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Why WANs?

• While you can extend LANs with bridging/repeaters,
  paradigm starts to break down for many endpoints
  and large traffic
• WANs must provide
• Open ended growth - can keep adding endpoints
• Reasonable performance for large size networks
• WANs built with packet switched networks glued
  together

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Packet Switches

• What is a packet switch? use a local star
  topology around a single computer connects
  locally and passes on to other PSs
• Move complete packets from one connection to
  another
• Does more than replicate bits
• Each switch really a special purpose computer
• Has CPU, memory, NICs
• Now we can connect Packet Switches (over
  distance) and create networks

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WANs

• Need not be symmetric
• Links can have different data rates
• Can have more than one link between switches for
  capacity or redundancy

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WAN Picture
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Store and Forward

• Packets buffered in memory when received
• Placed in switchs memory (stored)
• CPU notified, interrupted
• CPU examines and determines which interface to
  send packet to (forwarded)
• If output interface busy packet queued

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Addressing

• WANs use hierarchical routing to make forwarding
  more efficient
• Addresses broken into parts
• Only fraction of address used to determine
  outgoing interface
• Typical two parts, first part chooses interface
  ltPacket Switch number, Computer numbergt

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Addressing Picture
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Next Hop

• If destination local, send directly to given
  local computer
• If destination not local must forward to another
  switch need get the route
• Total route not kept in each switch
• Instead information on the next switch in line is
  kept
• Similar to airline flight lists

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Next Hop Picture
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Source Independence

• Next hop does not depend on source or path so far
• Only on destination
• Routing at airports similar
• People from multiple sources headed for multiple
  destinations only care about next flight

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Source Independence Continued

• Only one table required
• Only destination information need be extracted
• All forwarding handled uniformly
• For forwarding, get the packet switch number out
  of address, then index into table

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Routing

• Next hop information contained in routing table
• Process of moving packet to next hop called
  routing
• All packets with same first part of destination
  sent to same next hop
• Table indexed by destination address
• No searching, quicker just indexing

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Routing Continued

• Table contains list of destination packet
  switches, not computers
• Smaller amount of data to keep up

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Route Table
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Routes in a WAN

• WAN modeled as a graph
• Nodes are routers (the packet switches in
  network)
• Links (edges) are connections
• Edges can have weights

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WAN Graph
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Routing Table
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Default Route

• A lot of redundant information
• Router may only have single connection
• Or vast majority of traffic destined for single
  router
• This is a common situation
• Routing traverses the table in order so we can
  have a default at the bottom

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Default Route Continued

• Have default route for most traffic
• Can have only one default route
• Has lowest priority
• All non-default checked first
• If nothing matches use default

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Default Routes
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Table Computation

• Two approaches
• Static entered once and never changes
• Dynamic continually computed
• Most use dynamic
• Monitor network connections and respond

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Shortest Path

• If have graph with weights can use Dijkstras
  algorithm to compute shortest path between two
  nodes algorithm flexible for weight usage
• Can use result to compute next hop
• Shortest path can be defined various ways
  depending upon weights we use distance is sum
  of weights along a path
• Can use weights to represent various costs
  capacity, distance, etc.
• Can use this algorithm uniformly across whole
  network once and for all

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Distributed

• Each router can create table and send to
  neighboring routers
• One of best known is distance-vector think of
  the sum of the weights of all edges of a path
  (route) as the distance of that route
• Each router periodically sends route information
  to immediate neighbors
• Each message contains destination and distance
• If I find an immediate neighbor that has a
  shorter route to a destination than I have, I
  change my route table data to use that! (Actually
  a little more detailed the sum of my distance
  to that neighbor plus his distance to ultimate
  destination has to be minimal)

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Graph with Weighted Edges
A graph with weights assigned to edges. The
shortest path between nodes 4 and 5 is shown
darkened. The distance along the path is 19, the
sum of the weights on the edges.
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Computing Distance Vector Table and Abbreviated
Routing Table(For the network of the previous
slide from the perspective of Packet Switch 6)
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Distributed Continued

• Each neighbor updates own table if shorter route
  exists
• Alternative link-state
• AKA shortest path first
• Instead of destination and distance, status of
  link between switches sent
• Each switch then builds graph using Dijkstras
  algorithm

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Examples

• ARPANET
• 56Kbps leased lines
• Basis for Internet
• Started with distance/vector now uses link-state
• X.25
• ITU/CCITT character oriented
• Mostly in Europe
• In US was mostly terminal to host

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Example Continued

• Frame relay
• Block up to 8K octets
• Designed to bridge LANs
• 1.5 Mbps or 56Kbps
• SMDS
• Switched Multi-megabit Data Service
• Faster than FR, small header big payload (9188
  octets), very fast
• Need special interface hardware
• ATM
• Asynchronous Transfer Mode

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Exercise
From the perspective of Packet Switch C in the
graph below, compute and show the distance vector
table and the abbreviated routing table that C
must use. Assume the weights given on each edge
of the graph.
1
B
F
3
1
G
A
1
3
E
C
3
1
1
D