© 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, Ph.D. chaiporn.j@ku.ac.th

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

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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 Provider NAP: 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

WAN DTE DCE DCE DTE

DCE – Data Circuit-terminating Equipment DTE – 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

Germany New York

Alaska

Bangkok

Next hop keep changing

Bangkok Alaska Bangkok 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)