Lecture 10 - Routing Part I

8/2/2019 Lecture 10 - Routing Part I

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Routing & forwarding Routing & forwarding Routing & forwarding Routing & forwarding

Not the same thing!Routing- routing refers to deciding the route of the packet from one

system (a computer) to another system (a computer). It involves

many intermediate routers and routing table knowledge.

Forwarding – forwarding is a rather simple process of passing a

packet from one port/interface to another port /interface. It

generally involves one of the routers and routers forwarding table

information

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How to find the best path from A to F?

How does R1 chooses the best route to R4?

What is routing?

“A part of the network software responsible for deciding which output line an

incoming packet should be transmitted on.” –Tanenbaum

In Internetworking multiple paths exist between two networks. Routing helps

to identifying the path.

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A

B

C

D

E

F

R1R2

R3

R4

1. Next-hop routing

To reduce the contents of a routing table

Routing table holds only the information that leads to the next hop instead of holding information about the complete route

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2. Network-specific routing

To reduce the routing table and simplify the searching process

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3. Host-specific routing

Inverse of network-specific routing

Destination host address is given in the routing table

Not efficient, but administrator has greater control over routing

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4. Default routing

Another technique to simplify routing

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Route method Vs NextRoute method Vs NextRoute method Vs NextRoute method Vs Next----hop methodhop methodhop methodhop method



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Routing Routing Routing Routing ---- propertiespropertiespropertiesproperties

1. correctness

2. simplicity

3. Robustness

should cope with changes in the topology and traffic that happenover days…

updating possibility to meet the above requirement

4. Stability

must converge to equilibrium

5. fairness

6. optimality

min mean packet delay

max total network throughput

Note: 5 & 6 often contradictory

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Routing Algorithm Analogy

A person traveling from Madurai to Sivakasi.

If they were using a non-adaptive algorithm, they would make the decision to pass

through Thirumangalam and Virudhunagar before they left Madurai.

If they were using an adaptive algorithm, they would possibly travel to

Thirumangalam first and depending on how busy the routes are, they may travel

to Srivilliputtur instead of Virudhunagar and then onto Sivakasi.

In non-adaptive, the hops or jumps taken is decided before hand.

In adaptive, the next hop is decided at each stage.

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Critaria used for selection of route

Minimum hop

Least cost

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Routing algorithmsRouting algorithmsRouting algorithmsRouting algorithms

Grouped into

Non-adaptive algorithm or Static routing

Non-adaptive routing algorithms make routing decisions based

on the network topology.

The routes are chosen in advance, off line and downloaded to

the routers when the network is booted.Adaptive algorithm

Adaptive algorithms change their routing decisions in line with

changes in network traffic or network topology.

This is sometimes known as dynamic routing.

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Lecture 10 - Routing Part I