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Wide Area Networks (WANs),

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Compares each item in list to local routes. Changes routes if better path exists. SEPT, 2003 ... Offered by phone companies for bridging LAN segments. ATM ... – PowerPoint PPT presentation

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Title: Wide Area Networks (WANs),


1
Part 1.3
  • Wide Area Networks (WANs),
  • Routing, and Shortest Paths

Robert L. Probert, SITE, University of Ottawa
2
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

3
Motivation
  • Connect multiple computers
  • Span large geographic distance
  • Cross public right-of-way
  • Streets
  • Buildings
  • Railroads

4
Building Blocks
  • Node-to-node links
  • Point-to-point long-distance connections
  • Packet switches
  • Routers
  • Protocols

5
Packet Switch
  • Hardware/Software device
  • Connects to
  • Other packet switches
  • Computers
  • Processes and Forwards packets
  • Uses unique (routable) addresses

6
Illustration of a Packet Switch
  • Special-purpose computer system
  • CPU
  • Memory
  • I/O interfaces
  • Firmware
  • Software

7
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

8
Illustration of a WAN
  • Interconnections depend on
  • Estimated traffic
  • Reliability needed

9
IP Network
10
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

11
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

12
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

13
Example of Dotted Decimal Notation
  • Four decimal values per 32-bit address
  • Each decimal number
  • Represents eight bits
  • Is between 0 and 255

14
Illustration of WAN Addressing
  • Two part address encoded as integer
  • Higher-order bits for switch number
  • Low-order bits for computer number

15
Address Classes Network Sizes
  • Maximum network size determined by class of
    address
  • Class A large
  • Class B medium
  • Class C small

16
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

17
Next-Hop Forwarding
  • Performed by packet switch
  • Uses table of routes
  • Table gives next hop

18
Forwarding Table Abbreviations
  • Many entries point to same next hop
  • Can be condensed (default)
  • Improves lookup efficiency

19
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

20
Relationship of Routing To Graph Theory
  • Graph
  • Node models switch
  • Edge models connection

21
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

22
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

23
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

24
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

25
Distance Vector Algorithm
26
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

27
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

28
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

29
Example of Link-State Information
  • Assume nodes 2 and 3
  • Test link between them
  • Broadcast information
  • Each node
  • Receives information
  • Recomputes routes as needed

30
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

31
Dijkstras Algorithm
32
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

33
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

34
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

35
(No Transcript)
36
Inside the Router-LSA
LSA Header
Router-LSAs
37
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
38
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
39
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
40
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

41
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

42
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

43
ATM Switching
VP - Virtual Path VC - Virtual Channel
44
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
45
Assessment of ATM
  • Failed to deliver on cost
  • Switches too expensive for LAN
  • QoS expensive to implement

46
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

47
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

48
Summary (continued)
  • Example WAN technologies
  • ARPANET
  • X.25
  • SMDS
  • Frame Relay
  • ATM
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