Fixed Routing, Adaptive Routing and Flooding

8/13/2019 Fixed Routing, Adaptive Routing and Flooding

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CMPE 150- Introduction to Computer Networks1

CMPE 150

Fall 2005

Lectu re 21

Introduction to Computer

Networks


2/23


Announcements

Homework 4 up.

Due on 11.23.05.

Lab this week:

The Internet Behind the Web video.


3/23


Today

Finish DLL!


4/23


Last Class

Network Layer. Focus on packet switching networks.

Main functions.

Different network layer implementations. Datagrams versus virtual circuits.


5/23


Virtual-Circu i t versu s Datagram Subnets

5-4


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Rout ing


7/23CMPE 150- Introduction to Computer Networks7

Rout ing

One of the main functions of network layer.

Routing versus forwarding?

Datagram versus VC networks?



Rou t ing A lgor ithm

Computes routing tables.

Properties:

Correctness.

Robustness. Stability.

Optimality.

Try to optimize a certain metric.



Optimal i ty Princ iple

General statement about optimal routes(topology, routing algorithm independent).

If router J is on optimal path between I

and K, then the optimal path from J to Kalso falls along the same route.

Proof by contradiction.

Corollary: Set of optimal routes from all sources to

destination form a tree rooted at destination.

Sink tree.



Types o f Rou t ing A lgo r ithms

Non-adaptive versus adaptive.



Adapt ive and Non -adap t ive Rou t ing

Non-adaptive routing:

Fixed routing, static routing.

Do not take current state of the network (e.g.,

load, topology).

Routes are computed in advance, off-line, anddownloaded to routers when booted.

Adaptive routing:

Routes change dynamically as function of current

state of network.

Algorithms vary on how they get routing

information, metrics used, and when they change

routes.



Stat ic A lgor i thm s

(Non-Adaptive)

1.Shortest-path routing.

2.Flooding.



Shortest-Path Rou t ing

Problem: Given a graph, where nodesrepresent routers and edges, links, find

shortest pathbetween a given pair of nodes.

What is shortestin shortest path?

Depends on the routing metricin use.

Example: number of hops (static), geographic

distance (static), delay, bandwidth (raw versus

available), combination of a subset of these. Dijkstras shortest-path algorithm (19590.



Dijkstras Shortest-Path A lgor i thm

Initially, links are assigned costs.

As the algorithm executes, nodes are labeledwith its distance to source along best known

path.

Initially, no routes known, so all nodes arelabeled with infinity.

Labels change as the algorithm proceeds.

Labels can be temporary or permanent.

Initially all labels are tentative.A label becomes permanent if it represents the

shortest path from the source to the node.



Sho rtest Path Rout ingFind shortest-path from A to D:

Start

Label each adjacent node

with distance to

A.

B is made

permanent.



Flooding

Every incoming packet forwarded on every

outgoing link except the one it arrived on.

Problem: duplicates.

Constraining the flood: Hop count.

Keep track of packets that have been flooded.

Robust, shortest delay (picks shortest path asone of the paths).


17/23CMPE 150- Introduction to Computer Networks

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Flood ing: Example

Stallings Figure 12.4

(hop-count=3)



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Dynam ic Rout ing A lgor ithms

(Adaptive Routing)

Distance vector routing.

Link state routing.



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Dis tance Vecto r Rou t ing

Aka, Bellman-Ford (1957), Ford-Fulkerson(1962).

Original ARPANET routing; also used by

Internets RIP. Each router keeps routing table (or routing

vector) with best known distance to each

destination and corresponding outgoing

interface.

Routing tables are updated by exchanging

routing information with neighbors.



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Distance Vector (Contd)

Routing table at each router: One entry per participating router.

Each entry contains outgoing interface and distance to

corresponding destination.

Metric: number of hops, delay, queue length.

Each router knows distance to its neighbors.

Old ARPANET algorithm: DV where cost metric

is outgoing link queue length.



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Dis tance Vecto r Rou t ing

(a)A subnet. (b)Input from A, I, H, K, and thenew routing table for J.



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Rout ing Updates

Every T interval, routers exchangerouting updates.

Routing update from router X consists ofa vector with all dest inat ionsand the

corresponding distance from X to them. When router Y receives an update from

X, it can estimate its distance to router Z

through X as Dyz = Dyx+ Dxz. Router Y receives update from all its

ne ighborsand builds a new RT.


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Distance Vecto r: Example

1

4

6

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5

1Node Distance Next2

3

3

2

1

9

9

5

1

2 1 0 -

2 2 2

3 5 3

4 1 4

5 6 3

6 8 3

T=T0 T=T1

3 7 5

2 3 4

0 4 2

3 0 2

2 2 0

3 1 1

5 3 3

Node Distance Next

1 0 -

2 2 2

3 3 4

4 1 4

5 2 4

6 4 4

T=T2

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Fixed Routing, Adaptive Routing and Flooding