Preliminary Description of NACK-based Ad-hoc On-demand Distance

Vector Routing Protocol for MANETs

Alessandro Bianchi, Sebastiano Pizzutilo and Gennaro Vessio

Department of Informatics, University of Bari, Bari, Italy

Keywords:

MANET, AODV Variant, Abstract State Machines.

Abstract:

The present paper proposes a variant of the Ad-hoc On-demand Distance Vector (AODV) routing protocol

for Mobile Ad-hoc NETworks (MANETs) by means of an Abstract State Machine (ASM)-based model. The

variant introduces a new unicast message, which makes each host aware about the network topology more

quickly than in the original AODV.

1 INTRODUCTION

A Mobile Ad-hoc NETwork (MANET, for short)

is a network designed for wireless communications

among nomadic hosts (Agrawal and Zeng, 2003). It

does not need any ﬁxed infrastructure, and commu-

nication sessions between source and destination are

established and maintained by the cooperation of the

hosts in the network. Since each host can directly

communicate only within the area established by its

transmission range, communications with external ar-

eas need the contribution of intermediate hosts ac-

cording to a speciﬁc routing protocol. So, each host

can act both as message producer and consumer in a

communication session and as router supporting com-

munications among other hosts. Moreover, during

their lifetime, hosts can enter or leave the network at

will and continuously change their relative position.

So, the twofold role played by hosts in the network,

as well as the continuous change of the network topol-

ogy due to movement, requires the deﬁnition of spe-

ciﬁc routing protocols for properly managing the lack

of a ﬁxed infrastructure.

In this paper, we advance the proposal of a slight

variant of the the Ad-hoc On-demand Distance Vector

(AODV) routing protocol (Perkins et al., 2003): the

NACK-based AODV (N-AODV). N-AODV makes

each host aware about the network topology more

quickly than AODV. The new algorithm is formally

speciﬁed by means of the Abstract State Machine

(ASM) formalism (Gurevich, 2000), and its correct-

ness is proved.

The rest of this paper is structured as follows: Sec-

tion 2 is about related work; Section 3 provides a

background knowledge on both AODV and ASMs;

Section 4 deals with N-AODV and its ASM-based

model; ﬁnally, Section 5 concludes this paper and de-

picts the future development of our research.

2 RELATED WORK

AODV is one of the most popular routing protocol

for MANETs, and many variants have been proposed

aimed at its improvement. In most cases, modiﬁca-

tions deal with security and performance. For exam-

ple, an improvement for ensuring protection against

blackhole attacks is proposed in (Lakshmi et al.,

2010); and an optimization aimed at reducing cost,

delay and packet loss is presented in (Lanjewar and

Gupta, 2013). In both cases, the improvement is pro-

posed according to non-formal approaches.

The usefulness of the formal approach in this do-

main is emphasized in (Nakhaee et al., 2011), where

a method for route selection in AODV is provided

thanks to Coloured Petri nets; and in (H

ofner et al.,

2012), where an analysis of AODV and two variants is

conducted through AWN (Algebra for Wireless Net-

work), a process algebra speciﬁcally tailored for this

scope. These two variants concern the non-optimal

route selection and the failure in discovering routes.

More in general, formal validation and veriﬁca-

tion of MANETs’ behavior guarantees reliable re-

sults. For example, in (Singh et al., 2010) a process

calculus for reasoning about MANETs is proposed.

Similar approaches are followed in (Delzanno et al.,

2010), where processes are represented by ﬁnite state

machines; and in (Bianchi and Pizzutilo, 2010) and

500

Bianchi A., Pizzutilo S. and Vessio G..

Preliminary Description of NACK-based Ad-hoc On-demand Distance Vector Routing Protocol for MANETs.

DOI: 10.5220/0005105305000505

In Proceedings of the 9th International Conference on Software Engineering and Applications (ICSOFT-EA-2014), pages 500-505

ISBN: 978-989-758-036-9

 2014 SCITEPRESS (Science and Technology Publications, Lda.)

(Bianchi and Pizzutilo, 2013), where authors use Petri

nets-based models for investigating some properties.

Finally, to our best knowledge, the ASM-based

approach has been used in the MANET domain only

for specifying location services and position-based

routings among known locations (Bencz

ur et al.,

2003). However, we choose ASMs mainly because of

three different reasons. When the model expressivity

is considered, literature agrees that ASMs show versa-

tility in capturing both sequential and parallel compu-

tations, e.g. (N

emeth, 2002). Secondly, considering

methodological issues, the ASM formalism has been

successfully applied in both academia and industry

for the design and the analysis of complex systems in

several domains, and a speciﬁc development method

got prominence in the last years (B

orger and St

ark,

2003). Thirdly, considering the implementation point

of view, the capability of translating formal models

into executable code is provided by several tools, for

example CoreASM (Farahbod et al., 2007).

3 BACKGROUND

3.1 AODV Routing Protocol

AODV is a reactive protocol that discovers and main-

tains routes on-demand, i.e. routes are built only as

desired by source nodes using a route request/route

reply cycle, which allows updating routing tables

stored in each node (Perkins et al., 2003). When an

initiator host needs to start a communication session

to a destination node, and it does not know a proper

route, it broadcasts a route request (RREQ) packet to

all its neighbors. An RREQ packet includes, among

the others: initiator address and broadcast id (this

pair uniquely identiﬁes the packet); destination ad-

dress; destination sequence number, which expresses

the freshness of the information about destination;

and hop count, which expresses the distance. Because

of broadcast transmissions, each intermediate node

can receive several instances of the same RREQ: for

avoiding the packet duplication, yet received RREQs

with the same identifying pair are discarded.

Knowledge of routes is stored into routing tables,

recorded into a cache memory of each node. More

precisely, a routing table in a node lists all other nodes

in the network, and the best (known) route to reach

each of them. To this end, each entry of the routing

table includes the address of the node, its sequence

number, the hop count to reach it, and the next hop

ﬁeld identifying the next node in the route to reach it.

When a node n receives an RREQ, it updates in-

formation about initiator and about the host that di-

rectly sent that RREQ to n. Then, n checks if one of

the following holds: it is the destination, or the des-

tination is one of its neighbors, or it knows a route

to the destination with corresponding sequence num-

ber greater than or equal to the one contained in the

RREQ (this means that its knowledge about the route

is recent). If so, n unicasts a route reply (RREP)

packet back to initiator; otherwise, it updates the hop

count ﬁeld, and rebroadcasts the RREQ.

The process is so reiterated until a route to des-

tination is found, or until the route discovery pro-

cess times out. An RREP packet contains: initiator

and destination address; destination sequence num-

ber; and hop count. While RREP travels towards ini-

tiator, routes are set up inside the routing tables of

the traversed hosts. Once initiator receives the RREP,

communication simply starts.

The protocol also includes a mechanism for

recording the up-to-date information about the broken

physical links, but this issue is outside our purposes.

3.2 Abstract State Machines

Informally speaking, ASMs are ﬁnite sets of so-called

rules of the form if condition then updates (possi-

bly with the else clause) which transform abstract

states (B

orger and St

ark, 2003). The concept of ab-

stract state extends the usual notion of state occur-

ring in ﬁnite state machines: it is an arbitrary com-

plex structure, i.e. a domain of objects with functions

and relations deﬁned on them. On the other hand,

the concept of rule reﬂects the notion of transition

occurring in traditional transition systems: condition

is a ﬁrst-order formula whose interpretation can be

true or false; while updates is a ﬁnite set of assign-

ments of the form f (t

,...t

) := t, whose execution

consists in changing in parallel the value of the spec-

iﬁed functions to the indicated value. In each state,

all conditions are checked, so that all updates in rules

whose conditions evaluate to true are simultaneously

executed, and the result is the transition of the ma-

chine from one state to another.

The framework also includes constructs aimed

at directly supporting reﬁnements to parallel or dis-

tributed implementations; among them, of particular

interest for us, is the forall construct, which allows

executing all rules satisfying a given condition. Also,

there are constructs aimed at supporting the mecha-

nism of procedure calls; this is achieved by the deﬁ-

nition of ASM submachines, i.e. parameterized rules,

which allow the declaration of local functions, so that

each call of a submachine works with its own instan-

tiation of its local functions.

A generalization of basic ASMs is represented

PreliminaryDescriptionofNACK-basedAd-hocOn-demandDistanceVectorRoutingProtocolforMANETs

501

by Distributed ASMs (DASMs) (B

orger and St

ark,

2003), capable to capture the formalization of multi-

agent systems. Essentially, a DASM is intended as

an arbitrary but ﬁnite number of independent agents,

each executing its own underlying ASM. In a DASM,

the keyword self is used for supporting the relation

between local and global states and for denoting the

speciﬁc agent which is executing a rule.

4 NACK-BASED AODV

4.1 Description

In the original formulation of AODV, when an inter-

mediate node n receives an instance of an RREQ and

does not know a proper route to the speciﬁc destina-

tion, it simply rebroadcasts the RREQ. A bit more

reﬁned mechanism is the NACK-based AODV (N-

AODV): in addition to rebroadcasting the RREQ, n

unicasts a NACK (Not ACKnowledgement) packet

back to initiator. The NACK is so used to inform

all nodes beetwen n and initiator that, roughly speak-

ing, n “does not know anything” about the destination.

Each NACK packet includes the IP addresses and the

sequence numbers of n and of initiator.

In N-AODV each host updates its own routing

table when it receives an RREQ or an RREP, ac-

cording to AODV, and when it receives a NACK. In

this case, N-AODV updates the entries concering all

nodes sending the NACK. So, the usage of NACKs

allows improving the network topology awareness of

each host. Moreover, if in a ﬁnite amount of time ini-

tiator receives only NACKs, then it knows that desti-

nation is currently outside the MANET space; if ini-

tiator does not receive any packet (neither NACKs or

RREPs), then it can realize it is isolated.

4.2 ASM-based Model

A MANET that adopts N-AODV can be modeled by a

DASM including a set of agents = {a

,...,a

}, where

each agent models the behavior of each node. In

general, a host is characterized by several features,

e.g. the amplitude of its transmission range, the di-

rection and the speed of its movement, and so on.

However, for our purposes, these features can be ab-

stracted away, and we only consider the IP address,

which univocally identiﬁes each a

in the MANET.

Since all agents implement the same protocol,

each agent behaves according to the same ASM, so

only one ASM is discussed in the following. Each

ASM can be in one of the following states:

idle: the agent is inactive. Its conﬁgura-

tion, ∀ dest ∈ agents, is given by: wish-

ToInitiate(self, dest) = false; receivedRREQ(self,

dest) = false; isEmpty(replies(self)) = true; and

isEmpty(nacks(self)) = true;

initiator: the agent has to start a new communication

session. It is characterized by wishToInitiate(self,

dest) = true;

router: the agent has received an RREQ. It is char-

acterized by receivedRREQ(self, dest) = true;

forwarding: the agent is forwarding an RREP or

a NACK to another destination. It is char-

acterized by isEmpty(replies(self)) = false or

isEmpty(nacks(self)) = false.

It is worth noting that a destination state is not

necessary: a host knows to be the destination when,

in router state, it receives an RREQ directed to it.

The functions dealing with the initiator and router

states are:

wishToInitiate: agents × agents → boolean, indicat-

ing whether a new communication session to dest

is required by the environment;

receivedRREQ: agents × agents → boolean, indicat-

ing whether an RREQ packet has been received.

Moreover, for the sake of simplicity, each agent

is associated with three types of queues of messages:

requests, replies, and nacks, which include RREQ,

RREP, and NACK packets, respectively. This allows

us working with abstract messages, simply inserting

or deleting them into the corresponding queue. Each

RREQ, RREP, or record (i.e. an entry of the routing

table) is built by concatenation of the information de-

scribed in Section 3.1 (in the pseudo-code the dot no-

tation helps in identifying the value of a speciﬁc ﬁeld

of a packet). The functions dealing with the queues

are deﬁned as follows:

requests: agents → {requestFrom(n) | n ∈ neighb},

represents the queue storing the RREQs received

from the other nodes;

replies: agents → {replyFrom(n) | n ∈ agents},

represents the queue storing the RREPs received

from the other nodes;

nacks: agents → {nackFrom(n) | n ∈ agents}, repre-

sents the queue storing the NACKs received from

the other nodes;

isEmpty: {requests, replies, nacks} → boolean,

evaluates to true if the corresponding queue is

empty; false otherwise;

top: {requests, replies, nacks} → {requestFrom(n),

replyFrom(n), nackFrom(n)}, is used for referring

the ﬁrst element of the corresponding queue.

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Note that when the MANET starts operating, each

agent is idle, i.e. for each agent both wishToIniti-

ate(self, dest) and receivedRREQ(self dest) evaluate

to false for each dest; and both isEmpty(replies(self))

and isEmpty(nacks(self)) evaluate to true.

In addition, each ASM includes the following:

neighb: agents → PowerSet(agents), speciﬁes the

nodes in the neighborhood of each agent;

routingTable: agents → PowerSet(records), repre-

sents the information about the nodes recorded

into the agent’s routing table. Whenever a node

checks its routing table, it considers only the most

recent information.

The values of the neighb and routingTable func-

tions, as well as the set agents, depend on the partic-

ular scenario: they are dynamically set according to

the MANET evolution, with respect to both the host

mobility and the computational history.

The ASM pseudo-code of the i-th agent is:

AGENTPROGRAM(a

) =

if wishToInitiate(self, dest) = true then

Initiator(dest)

if ¬isEmpty(requests(self)) then {

let dest = top(requests(self)).dest

let nextHop = sender of top(requests(self))

update routingTable(self)

receivedRREQ(self, dest) := true

Router(dest, nextHop)

}

if ¬isEmpty(replies(self)) {

let dest = top(replies(self)).dest

let nextHop = select c.nextHop

from routingTable(self)

with dest = c.dest

update routingTable(self)

UnicastRREP(nextHop)

dequeue top(replies(self))

}

if ¬isEmpty(nacks(self)) {

let dest = top(nacks(self)).dest

let nextHop = select c.nextHop

from routingTable(self)

with dest = c.dest

update routingTable(self)

UnicastNACK(nextHop)

dequeue top(nacks(self))

}

UnicastRREP(n) =

enqueue replyFrom(self) into replies(n)

UnicastNACK(n) =

enqueue nackFrom(self) into nacks(n)

In the pseudo-code above, the instruction “update

routingTable(self)” is not speciﬁed: it simply indi-

cates that the agent’s routing table is updated accord-

ing to the received packet, RREQ or RREP or NACK.

Informally speaking, each agent is inactive until

a new communication session is required by the en-

vironment; or until its computation is solicited by the

receipt of an RREQ; or until it has to forward a unicast

packet (either RREP or NACK) to the next hop in the

route to reach the receiver of that packet. Activation

of an agent unfolds different computational branches:

two of them lead to the execution of a new instance of

the Initiator or Router submachine, respectively; in

the other two cases, forwarding activities of RREPs

and NACKs, respectively, are executed. It is worth

noting that all these activities evolve concurrently.

4.2.1 Initiator

The ASM pseudo-code of the Initiator submachine is:

Initiator(dest) =

if dest /∈ neighb(self) ∧ dest /∈ routingTable(self)

then {

BroadcastRREQ

sentRREQ(self) := true

}

else {

StartCommunicationSession(dest)

stopInitiator

}

if sentRREQ(self) ∧ ¬isEmpty(replies(self))

then {

select r from replies(self)

with maximum destSeqNum

StartCommunicationSession(dest)

empty replies(self)

stopInitiator

}

if sentRREQ(self) ∧ isEmpty(replies(self)) ∧

¬(timeout(self) = 0) then

timeout(self) := timeout(self)−1

if sentRREQ(self) ∧ ¬isEmpty(nacks(self))

then {

update routingTable(self)

empty nacks(self)

}

if sentRREQ(self) ∧ timeout(self) = 0 then {

stopInitiator

}

BroadcastRREQ =

forall n ∈ neighb(self) do {

forall r ∈ requests(n) do {

if requestFrom(self).dest = r.dest ∧

PreliminaryDescriptionofNACK-basedAd-hocOn-demandDistanceVectorRoutingProtocolforMANETs

503

requestFrom(self).id = r.id then

discard requestFrom(self)

}

enqueue requestFrom(self) into requests(n)

}

stopInitiator =

wishToInitiate(self, dest) := false

sentRREQ(self) := false

In the pseudo-code above StartCommunicationS-

ession(dest) is not described because it is not strictly

part of the protocol.

The Initiator submachine is characterized by four

local functions: sentRREQ: agents → boolean, act-

ing as a ﬂag indicating whether an RREQ has been

sent; timeout: agents → integer, which models the

maximum waiting time for RREPs; and the afore-

mentioned replies and nacks queues. A new queue of

RREPs and NACKs is instantiated for each speciﬁc

communication session. This submachine includes

two additional states:

waiting: it indicates that the agent is waiting for re-

sponses concerning that dest from the other agents

in the DASM. Its conﬁguration is given by: wish-

ToInitiate(self, dest) = true; sentRREQ(self) =

true; timeout(self) > 0;

endInitiating: it indicates that the computational ac-

tivities executed by initiator, concerning the route

discovery for that dest, are completed. Its conﬁg-

uration is: wishToInitiate(self, dest) = false; sen-

tRREQ(self) = false.

If a route to dest is known, then the communi-

cation session simply starts; otherwise, BroadcastR-

REQ is executed. Its result consists in inserting a

new RREQ into the requests queue of all the agent’s

neighbors and in evolving the current state to waiting.

When an RREP is received (i.e. isEmpty(replies(self))

evaluates to false), then the computation continues;

otherwise, when the timeout expires, the route discov-

ery process ends. If a NACK is received, the routing

table is updated. The stopInitiator rule simply resets

the conﬁguration to the endInitiating state.

4.2.2 Router

The ASM pseudo-code of the Router submachine is:

Router(dest, nextHop) =

if dest = self ∨ dest ∈ neighb(self) ∨

dest ∈ routingTable(self) then {

UnicastRREP(nextHop)

dequeue top(requests(self))

receivedRREQ(self, dest) := false

}

else {

BroadcastRREQ

UnicastNACK(nextHop)

dequeue top(requests(self))

receivedRREQ(self, dest) := false

}

Note that BroadcastRREQ behaves as well as in

the Initiator submachine; instead, UnicastRREP and

UnicastNACK behave as well as in the main program.

The Router submachine includes the state en-

dRouting, which speciﬁes that the execution of the

routing activities due to the route discovery process

is completed. This state is only characterized by the

value false for the receivedRREQ(self, dest) function.

If router is the destination of the RREQ (dest eval-

uates to self) or if it knows a route to dest (dest ∈

neighb(self) ∨ dest ∈ routingTable(self)), then it uni-

casts an RREP packet back to initiator. Otherwise, it

rebroadcasts the RREQ to all its neighbors and uni-

casts a NACK packet back to initiator. In both cases,

the computation evolves to the endRouting state for

that value of dest.

4.3 Correctness

The proof of the correctness of N-AODV is quite sim-

ple, so, for the sake of brevity, it is only sketched.

First of all, it is worth noting that the execution of

each ASM does not starve: both the program of each

agent a

, and Initiator and Router submachines allow

their execution always evolve: even when they must

wait for external events, the waiting time is ﬁnite.

For what concerns the overall machine, both branches

dealing with the unicast forwarding of packets restore

the state, so that the computation can continue after

their activities. The Router submachine surely returns

back an RREP or a NACK packet, depending on the

values of the guard conditions, and then evolves to the

endRouting state. The Initiator submachine starts the

communication session or enables neighbors to exe-

cute the protocol. If the latter happens, then, if initia-

tor receives back an RREP, it appropriately continues

the execution by starting the communication session;

otherwise, it stops waiting for the timeout expiration.

Depending on the receiving of NACKs, initiator pos-

sibly manages them.

Secondly, the pseudo-code shows that the correct

packet is received back by initiator. In fact:

a. initiator receives back one or more RREPs if and

only if a route to destination exists;

b. initiator receives back only NACKs but no RREPs

if and only if no route to destination exists;

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504

c. initiator does not receive back any packet if and

only if it is isolated.

In order to prove issue (a) above, note that receiv-

ing RREPs means that the replies queue in the Initia-

tor submachine is not empty, and this occurs if and

only if the UnicastRREP rule has been executed, be-

cause only this rule enqueues an RREP into replies.

In turn, the UnicastRREP rule is executed if and only

if that router or one of its neighbors is the destination,

or a route to destination is recorded into its routing

table. Analogous proofs for issues (b) and (c).

Finally, it is easy to show that the execution of

the protocol goes only through the described states:

the execution of the rules transform the values of the

functions only to the desired states. So, no unexpected

behavior can occur.

5 CONCLUSION

In this paper, we have formally presented N-AODV: a

variant of the AODV routing protocol for MANETs.

The main advantage of the variant is that the hosts in

the MANET can indirectly obtain information about

the network topology more quickly than in AODV

thanks to NACKs. In fact, in AODV the routing tables

are updated on RREQ or RREP receiving, and more

precisely, when the node n

receives a packet sent by

that traversed the chain (n

, n

, . . . , n

i−1

, n

), n

updates its routing table only in entries concerning n

and n

i−1

. Instead, N-AODV updates all entries n

, n

. . . , n

− 1.

The research will continue with the aim to investi-

gate how properly managing the knowledge carried

on by NACKs and recording it into routing tables.

Secondly, we are interested into formally analyze

some computationally interesting properties, such as

starvation-freedom, reversibility of the execution, and

so on. Finally, if protocol performance issues are con-

sidered, it is worth noting that the use of NACKs in-

jects overhead in the computation activities carried

out by hosts, so, this overhead will be investigated

through simulations, for evaluating if it is adequately

balanced by the information gain obtained.

ACKNOWLEDGEMENT

This work has been partially funded by the Ital-

ian Ministry of Education, University and Research

(MIUR), within the Piano Operativo Nazionale –

PON02 00563 3489339.

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