Download Internetwork - Department of Computer Engineering

Internetworking, WANs, and
Dynamic Routing
Asst. Prof. Chaiporn Jaikaeo, Ph.D.
[email protected]
http://www.cpe.ku.ac.th/~cpj
Computer Engineering Department
Kasetsart University, Bangkok, Thailand
Adapted from the notes by Lami Kaya and lecture slides from Anan Phonphoem
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
© The McGraw-Hill Companies, Inc.
Internetworks
Internetwork = Network of networks

Two or more networks connected become an
internetwork, or internet
TU
Network
KU Network
CU Network
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Internetworks

Internetworking two LANs with a MAN or a WAN
Bangkhen

Kampangsaen
Obvious example The Internet
3
The Internet (Conceptual View)
ISP: Internet Service Provider
NAP: Network access point
(switching station)
4
Wide Area Network (WAN)
Enterprise Network: WAN owned by a company
5
Traditional WAN Architecture
6
Traditional WAN Architecture
LAN
WAN
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WAN Connection

DCE generates clock for DTE
generates clock
DTE
DCE
WAN
DCE
DTE
DCE – Data Circuit-terminating Equipment
DTE – Data Terminal Equipment
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WAN Devices
CSU/DSU or Modem
(DCE)
Router (DTE)
To WAN
V.35 serial cable
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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
Alaska
New York
Germany
Bangkok
Bangkok  Germany
Bangkok 
 New
Alaska
York  Alaska
Next hop keep changing
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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
Forwarding Table in Switch 2
Source E [2,1] Destination C [3,2]
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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
node
Edge
WAN
A Graph representation
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Routing Table
node
Edge = (u,v)
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Default Routes
• One default
• Lowest priority
> 1 destination with same next-hop
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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 nonnegative 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
2 hops
1 hop
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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


However, problems do occur


meaning that the forwarding tables in all packet switches agree
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)
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