Routing Algorithms and Principles, Slides of Computer Networks

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Outline

1

• Routing Algorithm

• Shortest Path Routing

• Flow Based Routing

• Flooding

• Distance Vector

Routing

• Link State Routing

• Hierarchical Routing

Routing is the process of forwarding of a packet in a network so that it reaches its intended destination.

• Correctness : The routing should be done properly and correctly so that the packets may reach their proper destination.
• Simplicity : The routing should be done in a simple manner so that the overhead is as low as possible. With increasing complexity of the routing algorithms the overhead also increases.
• Robustness : Once a major network becomes operative, it may be expected to run continuously for years without any failures. The algorithms designed for routing should be robust enough to handle hardware and software failures and should be able to cope with changes in the topology and traffic without requiring all jobs in all hosts to be aborted and the network rebooted every time some router goes down. 2

Properties Of Routing

Algorithm

Types Of Routing

Algorithms

• (^) Nonadaptive

(Static)

Do not use measurements of current

conditions Static routes are downloaded at

• boot^ time

Adaptive Algorithms

• (^) Change routes

dynamically

• (^) Gather information at

runtime

locally

from adjacent

routers from all

• (^) Change other routers

routes

Every delta T

seconds

When load changes

When topology

5

d 3 > d 2 as d 1 + d 3 > d 1 + d 2 I   K J  Optimal path from I to K over J d 1 distance d 2 d 1 + d 2 is d^ minimal 3 Other path from J to K Set of all optimal routes

• (^) from all sources
• to a given destination is a tree: sink tree

Optimality

Principle

6

Dijkstra’s

Algorithm

A

B C

E F

D

G H

Each node is labeled (in parentheses) with its distance from the source node along the best known path.

(Cont’

d)

• (^) We want to find the shortest path from A to D.
• (^) Initially, no paths are known, so all nodes are labeled with infinity. A

B( C(

E( F(

D(

G( H(

(Cont’d)

A

B(2, A) C(

E( F(

D(

We make B with the smallest label permanent. B becomes the new working node. G(6, A) H(

(Cont’

d)

A

B(2, A) C(  

E(4, B F(

D(

We examine each of the nodes adjacent B, relabeling each one with the distance to B. G(6, A) H(

E(4, B)

(Cont’d)

A

B(2, A) C(9, B)

F(  

D(

We examine each of the nodes adjacent E, relabeling each one with the distance to E. G(5, E) H(

(Cont’d)

A

B(2, A) C(9, B)

E(4, B) F(6, E)

D(

We make G with the smallest label permanent. G becomes the new working node. G(5, E) H(

(Cont’d)

A

B(2, A) C(9, B)

E(4, B) F(6, E)

D(

G(5, E) H(9, G)

We make F with the smallest label permanent. F becomes the new working node.

E(4, B)

(Cont’d)

A

B(2, A) C(9, B)

D(

F(6,

E)

G(5, E) H(8, F)

We examine each of the nodes adjacent F, relabeling each one with the distance to F.

E(4, B)

(Cont’d)

A

B(2, A) C(9, B)

F(6, E)

D(10, F

We examine each of the nodes adjacent H, relabeling each one with the distance to H. G(5, E) H(8, F)

(Cont’d)

E(4, B)

A

B(2, A) C(9, B)

F(6, E)

D(10, F

We make C with the smallest label permanent. C becomes the new working node. G(5, E) H(8, F)