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Transcript of © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. © The McGraw-Hill...
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.© The McGraw-Hill Companies, Inc.
Internetworking, WANs, and Dynamic Routing
Asst. Prof. Chaiporn Jaikaeo, [email protected]
http://www.cpe.ku.ac.th/~cpjComputer Engineering Department
Kasetsart University, Bangkok, Thailand
Adapted from the notes by Lami Kaya and lecture slides from Anan Phonphoem
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Internetworks
Two or more networks connected become an internetwork, or internet
KU Network
CU Network
TU Network
Internetwork = Network of networks
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Internetworks Internetworking two LANs with a MAN or a
WAN
Obvious example The Internet
Bangkhen Kampangsaen
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The Internet (Conceptual View)
ISP: Internet Service ProviderNAP: Network access point (switching station)
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Wide Area Network (WAN)
Enterprise Network: WAN owned by a company
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Traditional WAN Architecture
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Traditional WAN Architecture
LAN WAN
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WAN Connection DCE generates clock for DTE
WANWANDTE DCE DCE DTE
DCE – Data Circuit-terminating EquipmentDTE – Data Terminal Equipment
generates clock
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WAN Devices
V.35 serial cable
CSU/DSU or Modem(DCE)
Router (DTE)
To WAN
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Example of WAN Topology These packet switches form a packet
switching network
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Store and Forward Paradigm
A packet switch stores packets in memory The forward operation occurs once a packet has
arrived and is waiting in memory. The processor examines the packet determines its destination and sends the packet over the I / O interface that leads to
the destination
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Addressing in a WAN
WANs addresses follow a key concept that is used in the Internet: hierarchical addressing Hierarchical addressing divides each
address into two parts:(site, computer at the site)
In practice, instead of a identifying a site, each packet switch is assigned a unique number
first part of an address identifies a packet switch
second part identifies a specific computer
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Addressing in WAN
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Next-Hop Forwarding
GermanyNew York
Alaska
Bangkok
Next hop keep changing
Bangkok AlaskaBangkok Germany New York Alaska
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Source Independence Next hop depends on
destination of the packet Not the source !
Source Independence
Bangkok Germany New York Alaska
Forwarding packet uses the destination address in the packet
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Next-Hop Forwarding
Source E [2,1] Destination C [3,2]
Forwarding Table in Switch 2
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Routing Tables The next-hop table is called
Routing Table Process of forwarding packet
Routing Large network
Routing table can be very large
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Dynamic Routing in a WAN
We use the term routing software to describe software that automatically reconfigures forwarding tables
Route computation in a WAN is to think of a graph that models the network Each node corresponds to a packet switch
(individual computers are not part of the graph)
An edge (link) denotes a direct connection between a pair of packet switches
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WAN Routing
WAN A Graph representation
node
Edge
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Routing Table
Edge = (u,v)
node
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Default Routes
> 1 destination with same next-hop
• One default• Lowest priority
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Routing Table Construction Static Routing
Manual configure Simple and low overhead Inflexible
Dynamic Routing Automatic changing Change according to network problems Mostly use
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Distributed Route Computation
In practice, networks need to perform distributed route computation All packet switches must participate in
distributed route computation No central entity to do computation
There are two general forms: Link-State Routing (LSR) Distance-Vector Routing (DVR)
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Link-State Routing (LSR) Also known as Shortest Path First (SPF) routing Dijkstra algorithm used it to characterize the way it
works To use LSR, packet switches periodically send
messages across the network that carry the status of a link
Every switch collects incoming status messages
and uses them to build a graph of the network
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Dijkstra's Algorithm Uses a greedy
approach to select the next node into the shortest path tree
Assumes non-negative weight edges
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Dijkstra’s Algorithm Animation
http://www-b2.is.tokushima-u.ac.jp/~ikeda/suuri/dijkstra/Dijkstra.shtml
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Distance Vector Routing (DVR)
Uses Distributed Bellman-Ford Algorithm Like LSR, DVR arranges for packet switches to
exchange messages periodically In DVR, a switch sends a complete list of
destinations and the current cost of reaching each
When it sends a DVR message a switch is sending a series of individual
statements, of the form: “I can reach destination X, and its current
distance from me is Y”
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DVR Concept
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Hop Count
1 hop
2 hops
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Routing table distribution
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Updating routing table For router A
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Final routing tables
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Updating the routing table Example
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Routing Problems In theory, either LSR or DVR will compute shortest
paths Furthermore, each approach will eventually converge
meaning that the forwarding tables in all packet switches agree
However, problems do occur For example, if LSR messages are lost, two packet switches
can disagree about the shortest path DVR problems can be more severe
because a link failure can cause two or more packet switches to create a routing loop
in which each packet switch thinks the next packet switch in the set is the shortest path to a particular destination
As a result, a packet can circulate among the switches indefinitely
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WAN Technologies
ARPANET X.25 Frame Relay Switched Multi-Megabit Data Service
(SMDS) Asynchronous Transfer Mode (ATM) Multi-Protocol Label Switching (MPLS) Integrated Services Digital Network
(ISDN)