CSE 302 ZRP Protocol

8/3/2019 CSE 302 ZRP Protocol

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The Zone Routing Protocol (ZRP)

Dr. R. B. Patel


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Brief Review of Reactive and Proactive protocols

A reactive routing protocol tries to find a routefrom S to D only on-demand, i.e., when the routeis required, for example, DSR and AODV aresuch protocols.

The main advantage of a reactive protocol is thelow overhead of control messages.

However, reactive protocols have higher latencyin discovering routes.


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Proactive Protocols

A proactive protocol maintains extensive routingtables for the entire network. As a result, a routeis found as soon as it is requested.

The main advantage of a proactive protocol is itslow latency in discovering new routes.

However, proactive protocols generate a highvolume of control messages required forupdating local routing tables.


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A Combined Protocol

It is possible to exploit the good features of bothreactive and proactive protcols and the Zonerouting protocol does that.

The proactive part of the protocol is restricted toa small neighbourhood of a node and thereactive part is used for routing across thenetwork.

This reduces latency in route discovery andreduces the number of control messages aswell.


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Routing Zones

Each node S in the network has a routing zone.This is the proactive zone for S as S collectsinformation about its routing zone in the mannerof the DSDV protocol.

If the radius of the routing zone is k, each nodein the zone can be reached within k hops from S.

The minimum distance of a peripheral nodefrom S is k (the radius).


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A Routing Zone

S

LK

G

H

I

J

AB

CD

E

All nodes except L are in the routing zone of S withradius 2.


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Nodes in a Routing Zone

The coverage of a nodes trasmitter is the set ofnodes in direct communication with the node.These are also called neighbours.

In other words, the neighbours of a node are thenodes which are one hop away.

For S, if the radius of the routing zone is k, thezone includes all the nodes which are k-hopsaway.


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Neighbour Discovery Protocol

Like other ad hoc routing protocols, each nodeexecutes ZRP to know its current neighbours.

Each node transmits a hello message at regularintervals to all nodes within its transmissionrange.

If a node P does not receive a hello message

from a previously known neighbour Q, Premoves Q from its list of neighbours.


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Basic Strategy in ZRP

The routing in ZRP is divided into two parts Intrazone routing : First, the packet is sent within the

routing zone of the source node to reach theperipheral nodes.

Interzone routing : Then the packet is sent from theperipheral nodes towards the destination node.

SD


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Intrazone Routing

Each node collects information about all thenodes in its routing zone proactively. Thisstrategy is similar to a proactive protocol likeDSDV.

Each node maintains a routing table for itsrouting zone, so that it can find a route to any

node in the routing zone from this table.


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Intrazone Routing

In the original ZRP proposal, intrazone routing isdone by maintaining a link state table at eachnode.

Each node periodically broadcasts a messagesimilar to a hello message kwon as a zonenotification message.

Suppose the zone radius is k for k>1


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Zone Notification Message

A hello message dies after one hop, i.e., afterreaching a nodes neighbours.

A zone notification mesage dies after k hops,i.e., after reaching the nodes neighbours at adistance of k hops.

Each node receiving this message decreasesthe hop count of the message by 1 and forwardsthe message to its neighbours.


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Keeping Track of Nodes in a Routing Zone

The message is not forwarded any more whenthe hop count is 0.

Each node P keeps track of its neighbour Q fromwhom it received the message through an entryin its link state table.

P can keep track of all the nodes in its routingzone through its link state table.


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ZRP: Example withZone Radius = K = 2

SCA

EF

B

D

S performs routediscovery for D

Denotes route request


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ZRP: Example with K = 2

SCA

EF

B

D


Denotes route reply

E knows route from E to D,so route request need not beforwarded to D from E


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ZRP: Example with K = 2

SCA

EF

B

D


Denotes route taken by Data


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Interzone Routing

The interzone routing discovers routes to thedestination reactively.

Consider a source (S) and a destination (D). If Dis within the routing zone of S, the routing iscompleted in the intrazone routing phase.

Otherwise, S sends the packet to the peripheralnodes of its zone through bordercasting.


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Bordercasting

The bordercasting to peripheral nodes can bedone mainly in two ways :

By maintaining a multicast tree for the peripheralnodes. S is the root of this tree.

Otherwise, S maintains complete routing table for itszone and routes the packet to the peripheral nodes byconsulting this routing table.


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Interzone Route Discovery

S sends a route request (RREQ) message to theperipheral nodes of its zone throughbordercasting.

Each peripheral node P executes the samealgorithm.

First, P checks whether the destination D is within its

routing zone and if so, sends the packet to D. Otherwise, P sends the packet to the peripheral

nodes of its routing zone through bordercasting.


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An Example of Interzone Routing

S

D

B

H

A

C


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Route Reply in Interzone Routing

If a node P finds that the destination D is withinits routing zone, P can initiate a route reply.

Each node appends its address to the RREQmessage during the route request phase. This issimilar to route request phase in DSR.

This accumulated address can be used to send

the route reply (RREP) back to the source nodeS.


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Route Reply in Interzone Routing

An alternative strategy is to keep forward andbackward links at every nodes routing tablesimilar to the AODV protocol. This helps inkeeping the packet size constant.

A RREQ usually results in more than one RREPand ZRP keeps track of more than one path

between S and D. An alternative path is chosenin case one path is broken.


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Route Maintenance

When there is a broken link along an active pathbetween S and D, a local path repair procedureis initiated.

A broken link is always within the routing zone ofsome node.

AB


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Route Maintenance

Hence, repairing a broken link requiresestablishing a new path between two nodeswithin a routing zone.

The repair is done by the starting node of thelink (node A in the previous diagram) by sendinga route repair message to node B within its

routing zone. This is like a RREQ message from A with B as

the destination.


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How to Prevent Flooding of the Network

Interzone routing may generate many copies ofthe same RREQ message if not directedcorrectly.

The RREQ should be steered towards thedestination or towards previously unexploredregions of the network.

Otherwise, the same RREQ message may reachthe same nodes many times, causing theflooding of the network.


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Routing Zones Overlap Heavily

Since each node has its own routing zone, therouting zones of neighbouring nodes overlapheavily.

Since each peripheral node of a zone forwardsthe RREQ message, the message can reach thesame node multiple times without proper control.

Each node may forward the same RREQmultiple times.


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Guiding the Search in InterZone Routing

The search explores new regions of the network.


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Query Forwarding and Termination Strategy

When a node P receives a RREQ message, Precords the message in its list of RREQmessages that it has received.

If P receives the same RREQ more than once, itdoes not forward the RREQ the second timeonwards.

Also P can keep track of passing RREQmessages in several different ways.


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Termination Strategies

In the promiscuous mode of operation accordingto IEEE 802.11 standards, a node can overhearpassing traffic.

Also, a node may act as a routing node duringbordercasting in the intrazone routing phase.

Whenever P receives a RREQ message throughany of these means, it remembers which routingzone the message is meant for.


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Termination Strategies

Suppose P has a list of nodes A, B,C,...,N suchthat the RREQ message has already arrived inthe routing zones of the nodes A, B, C, ...,N.

Now P receives a request to forward a RREQmessage from another node Q.

This may happen when P is a peripheral nodefor the routing zone of Q.


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Early Termination of Unnecessary RREQs

P receives a RREQ from Q since P is a peripheral nodefor the routing zone of Q.

P

QA

B

C

NX

P does not bordercast the RREQ to A,B,...,N but only toX whichis not in its list.


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Evaluation of ZRP

When the radius of the routing zone is 1, thebehaviour of ZRP is like a pure reactive protocol,for example, like DSR.

When the radius of the routing zone is infinity (orthe diameter of the network), ZRP behaves likea pure proactive protocol, for example, like

DSDV. The optimal zone radius depends on node

mobility and route query rates.


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Control Traffic

Control traffic generated by a protocol is thenumber of overhead packets generated due toroute discovery requests.

In ZRP, control traffic is generated due tointerzone and intrazone routing.

Hello messages transmitted for neighbourdiscovery are not considered as control trafficsince mobility has no effect on it.


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Control Traffic for Intrazone Routing

In the intrazone routing, each node needs toconstruct the bordercast tree for its zone.

With a zone radius of r, this requires completeexchange of information over a distance of 2r-1hops.

For unbounded networks with a uniformdistribution of nodes, this results in O( )intrazone control traffic.

2

r


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Control Traffic for Intrazone Routing

However, for a bounded network, thedependence is lower than .

There is no intrazone control traffic when r=1.

The intrazone control traffic grows fast inpractice with increase in zone radius. So, it isimportant to keep the zone radius small.

2

r


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Control Traffic for Interzone Routing

When the zone radius is 1, the control traffic ismaximum since ZRP degenerates into floodsearch.

In other words, every RREQ message potentiallyfloods the entire network. This is due to the factthat all the neighbours of a node n are its

peripheral nodes. However, control traffic drops considerably even

if the zone radius is just 2.


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The control traffic can be reduced drasticallywith early query termination, when a RREQmessage is prevented from going to the sameregion of the network multiple times.

However, the amount of control traffic dependsboth on node mobility and query rate.

The performance of ZRP is measured bycompairing control traffic with call-to-mobility(CMR) ratio.


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The call-to-mobility ratio (CMR) is the ratio ofroute query rate to node speed.

As CMR increases, the number of controlmessages is reduced by increasing the radius ofthe routing zones.

This is because, it is easier to maintain largerrouting zones if mobility is low. Hence, routediscovery traffic also reduces.


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On the other hand, CMR is low if mobility is high.

In such a case, the routing zone maintenance

becomes very costly and smaller routing zonesare better for reducing control traffic.

An optimally configured ZRP for a CMR of 500[query/km] produces 70% less traffic than floodsearching.


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Route Query Response Time

For a fixed CMR, the route query response timedecreases initially with increased zone radius.

However, after a certain radius, the responsetime increases with zone radius.

This is due to the fact that the network takeslonger time to settle even with small changes inlarge routing zones.

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CSE 302 ZRP Protocol