Multipoint relaying for flooding broadcast messages in mobile wireless networks

TLDR: The mechanism of multipoint relays (MPRs) to efficiently flood broadcast messages in mobile wireless networks is discussed and a heuristic to select these MPRs in a mobile wireless environment is proposed, and it is proved that the computation of a multipoint relay set with minimal size is NP-complete.

Abstract: We discuss the mechanism of multipoint relays (MPRs) to efficiently flood broadcast messages in mobile wireless networks. Multipoint relaying is a technique to reduce the number of redundant re-transmissions while diffusing a broadcast message in the network. We discuss the principle and the functioning of MPRs, and propose a heuristic to select these MPRs in a mobile wireless environment. We also analyze the complexity of this heuristic and prove that the computation of a multipoint relay set with minimal size is NP-complete. Finally, we present some simulation results to show the efficiency of multipoint relays.

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Figure 6: Number of retransmitting nodes

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Multipoint relaying for ooding broadcast messages in
mobile wireless networks
Amir Qayyum, Laurent Viennot, Anis Laouiti
To cite this version:
Amir Qayyum, Laurent Viennot, Anis Laouiti. Multipoint relaying for ooding broadcast mes-
sages in mobile wireless networks. 35th Annual Hawaii International Conference on System Sciences
(HICSS’2002), Jan 2002, Maui, United States. pp.3866 - 3875. �inria-00471699�

Multipoint Relaying for Flooding Broadast Messages in
Mobile Wireless Networks
A m i r Q a y y u m , L a u r e n t V i e n n o t , A n i s L a o u i t i
Projet Hiperom, INRIA Roquenourt, B.P. 105, 78153 Le Chesnay Cedex, Frane
Email: Amir.Qayyuminria.fr, Laurent.Viennotinria.fr, Anis.Laouitiinria.fr
Abstrat
In this paper we disuss the mehanism of mul-
tipoint relays (MPRs) to eÆiently do the ood-
ing of broadast messages in the mobile wireless
networks. Multipoint relaying is a tehnique to
redue the number of redundant re-transmissions
while diusing a broadast message in the net-
work. We disuss the priniple and the funtion-
ing of MPRs, and propose a heuristi to selet
these MPRs in a mobile wireless environment. We
also analyze the omplexity of this heuristi and
prove that the omputation of a multipoint relay
set with minimal size is NP-omplete. Finally, we
present some simulation results to show the eÆ-
ieny of multipoint relays.
keywords:
multipoint relays, mobile wireless
networks, ooding of broadast messages
1 I n t r o d u  t i o n
The researh relating to mehanisms and proto-
ols used in the wired networks is beoming ma-
ture. As a result, these mehanisms and proto-
ols are now lassied aording to their relative
domains of appliation, based on performane re-
sults obtained in those spei areas.
For mobile wireless networks, the researh is
still in its earlier stage. There is less onsen-
sus about the appliability of dierent existing
tehniques and algorithms in these new type of
networks. To obtain a satisfatory performane
from these tehniques or algorithms, they must be
made adequate to this new and hallenging envi-
ronment.
Speially, ad-ho radio networks have an in-
herent apaity for broadasting, i.e., with one
emission, a node an reah all the nearby nodes.
Using this apability for optimizing broadast
messages in suh networks is a hallanging task.
A ompromise has to be made between a small
number of emissions and the reliability. Several
tehniques are desribed in the literature to limit
or optimize the ooding of broadast traÆ [1℄,
[2℄, [3℄, [4℄, [5℄, [6℄, [7℄. Some results of ompari-
son between these tehniques an be found in [8℄.
Our paper disusses in detail the mehanism of
\multipoint relaying" as one of the possible solu-
tion, rst presented in [2℄. Comparison with other
tehniques is reserved for future work.
1 . 1 R e q u i r e m e n t s o f a m o b i l e w i r e -
l e s s e n v i r o n m e n t
In \mobile" \wireless" networks, eah of these
two words put before us a list of requirements,
and the daunting task is to fulll them to their
best. The
mobility
implies the limited lifetime of
neighborhood or topology information reeived at
any time, beause of the movement of nodes. This
implies that the information be updated regularly,
otherwise it beomes invalid. More frequently
1

the information is updated, more the mobility of
nodes an be handled orretly and eÆiently.
The
wireless
nature of the medium implies the
limited bandwidth apaity available in a fre-
queny band. It is further redued by the high bit
error rate in radio transmissions. This makes it a
sare and hene a preious resoure. Every ef-
fort is done to onsume it very prudently. Hene,
while designing a protool using wireless links,
the main task is to redue the unneessary use of
this bandwidth.
Therefore, the requirements of these two envi-
ronments are ompletely opposite to eah other.
Mobility requires more traÆ to be send in
the network to keep the nodes informed of the
hanges, and at the same time, wireless medium
does not allow to be used abundantly for unnees-
sary traÆ. The ompromise is to manage the mo-
bility of nodes while using minimum of the band-
width resoures.
1 . 2 F l o o d i n g o f b r o a d  a s t m e s s a g e s
i n t h e n e t w o r k
The type of ontrol traÆ that is generated to
manage the mobility of nodes in a network is
mostly the information that a node delares about
its relative movement, its new position, or its new
neighborhood, et. Some times, this information
is useful only in the neighborhood of the node
whih is delaring the information. Therefore,
the information is not required to be propagated
in whole of the network to reah every node. But
in many ases, not only the immediate neighbors
of the delaring node, but the other far away nodes
also need to know the topologial hanges our-
ring anywhere in the network. In these situations,
lot of message passing is required in the network
to keep the information onsistent and valid at
eah node, by regularly announing the hanges
due to mobility, or failure of links, et.
The announements about link hanges are des-
tined to eah node of the network. But often all
the nodes of network are not in the radio range of
eah other to ommuniate diretly. So there must
be a mehanism to reah the far away nodes in or-
der to keep them informed of the latest hanges.
The onept of
intermediate nodes
whih serve as
relays to pass the messages between the soure
and the destination is one of the solution.
If a message is for a spei destination, the
determination of intermediate nodes is simple: all
the nodes whih form the path (if it exists) from
the soure up to the destination are the
intermedi-
ate nodes
. These nodes agree upon a mehanism
to re-transmit the message, on their turn, so that
the message is suessfully transfered to the des-
tination. Dierent routing protools designate, in
dierent ways, these intermediate nodes for uni-
ast paket forwarding.
The problem arises when a paket is not des-
tined to a spei node, rather it is a broadast
message for all the nodes in the network. The
task of determining the intermediate nodes who
will forward the paket is not trivial in this ase.
The nodes should behave suh that the message is
reahed to every node in the network. A simple
solution is that eah node re-transmits the mes-
sage, when it reeives the rst opy of the mes-
sage. Fig 1 shows an example where a paket
originated by node
S
is diused up to 3-hops with
24 retransmissions. The paket is retransmitted
by all the intermediate nodes in order to diuse
it in the network. This tehnique is known as
\pure ooding". It is simple, easy to implement,
and gives a high probability that eah node, whih
is not isolated from the network, will reeive the
broadast message. The inonveniene of this
tehnique is that it onsumes a large amount of
bandwidth beause of so many redundant retrans-
missions.
In ertain onditions, and partiularly in the
\wireless" networks, the availability of limited re-
soures in terms of bandwidth apaity requires to
restrit the traÆ as muh as possible. If this on-
straint of wireless medium is not onsidered while
designing an algorithm, the network may suer
from performane degradations due to high over-
2

S
node
retransmitting
49
retransmissions
to diffuse
a message
upto
3-hops
Figure 1:
Diusion of a broadast message
using pure ooding
loads or ongestion, when the ooding of broad-
ast pakets is launhed in the network. On one
hand, broadast messages need some mehanism
of ooding, speially in mobile environment to
keep the mobile nodes remain in ontat by regu-
larly diusing the updates. But on the other hand,
it is not appreiatable either to aet the atual
working of the system due to this additional on-
trol traÆ.
Every protool uses some kind of ooding of
ontrol messages, for its funtioning [9℄, [10℄.
It beomes very advantageous to optimize the
resoure onsumption of the ooding proess.
Many tehniques are desribed in the literature
to limit the ooding of broadast traÆ and eah
tehnique has its own area of appliation and has
its own advantages and disadvantages. Here, we
will disuss the mehanism of \multipoint relay-
ing" as one of the possible solution.
2 M u l t i p o i n t r e l a y i n g
The onept of \multipoint relaying" is to re-
due the number of
dupliate re-transmissions
while forwarding a broadast paket. This teh-
nique restrits the number of re-transmitters to a
small set of neighbor nodes, instead of all neigh-
S
11
retransmissions
to diffuse
a message
upto
3-hops
retransmitting
nodes
Figure 2:
Diusion of a broadast message
using multipoint relays
bor, like in pure ooding. This set is kept small
as muh as possible by eÆiently seleting the
neighbors whih overs (in terms of one-hop ra-
dio range) the same network region as the om-
plete set of neighbors does. This small subset of
neighbors is alled
multipoint relays
of a given
network node. The tehnique of multipoint relays
(or MPRs) provides an adequate solution to re-
due ooding of broadast messages in the net-
work, while attaining the same goal of transfer-
ring the message to every node in the network
with a high probability. Fig 2 shows an exam-
ple where a broadast message of node
S
is dif-
fused in the network using the multipoint relays.
In this ase, it took only 11 retransmissions for a
message to reah up to 3-hops.
Multipoint relaying tehnique works in a dis-
tributed manner, designed in view of the mobile
and disperse nature of the network nodes. Eah
node alulates its own set of multipoint relays,
whih is ompletely independent of other nodes'
seletion of their MPRs. Eah node reats when
its neighborhood nodes hange and aordingly
modies its MPR set to ontinue overing its two-
hop neighbors.
An important aspet of the multipoint relays is
the manner in whih these multipoint relays are
3

seleted by eah node. The goal is to ahieve
the maximum performane by seleting an opti-
mal set of MPRs by eah node. But this task is
not a trivial one. If the mehanism of seleting the
MPRs is too simple, it may not selet eÆiently
the MPRs in a dynami and omplex situation,
and the expeted performane gain would not be
ahieved. If the algorithm of MPR seletion is
very omplex and sophistiated to provide a
near
to optimal
MPR set, it may beome diÆult to im-
plement it. A highly sophistiated algorithm may
generate its own ontrol traÆ, to gather informa-
tion for its funtioning, whih beomes ompara-
ble to the saving in ooding of messages. Thus,
there must be a ompromise in designing suh an
algorithm for the seletion of multipoint relays: it
should be easy to implement, and it should give
near to optimal MPR set in \majority" of ases.
The information required to alulate the mul-
tipoint relays is the set of one-hop neighbors and
the two-hop neighbors, i.e. the neighbors of
the one-hop neighbors. To obtain the informa-
tion about one-hop neighbors, most protools use
some form of HELLO messages, that are sent lo-
ally by eah node to delare its presene. In a
mobile environment, these messages are sent pe-
riodially as a
keep alive
signals to refresh the in-
formation. To obtain the information of two-hop
neighbors, one solution may be that eah node at-
tahes the list of its own neighbors, while sending
its HELLO messages. With this information, eah
node an independently alulate its one-hop and
two-hop neighbor set. One a node has its one-
and two-hop neighbor sets, it an selet a mini-
mum number of one-hop neighbors whih
overs
all its two-hop neighbors.
2 . 1 H e u r i s t i  f o r t h e s e l e  t i o n o f m u l -
t i p o i n t r e l a y s
We propose here one heuristi for the seletion
of multipoint relays. To selet the multipoint re-
lays for the node
x
, let us all the the set of one-
hop neighbors of node
x
as
N
(
x
)
, and the set of
its two-hop neighbors as
N
2
(
x
)
. Let the seleted
multipoint relay set of node
x
be
MPR
(
x
)
. The
heuristi an be stated as:
1. Start with an empty multipoint relay set
MPR
(
x
)
2. First selet those one-hop neighbor nodes in
N
(
x
)
as multipoint relays whih are the only
neighbor of some node in
N
2
(
x
)
, and add
these one-hop neighbor nodes to the multi-
point relay set
MPR
(
x
)
3. While there still exist some node in
N
2
(
x
)
whih is not overed by the multipoint relay
set
MPR
(
x
)
:
(a) For eah node in
N
(
x
)
whih is not
in
MPR
(
x
)
, ompute the number of
nodes that it overs among the unov-
ered nodes in the set
N
2
(
x
)
(b) Add that node of
N
(
x
)
in
MPR
(
x
)
for
whih this number is maximum.
To analyze the above heuristi, rst notie that
the seond step permits to selet some one-hop
neighbor nodes as MPRs whih must be in the
MPR
(
x
)
set. Otherwise the
MPR
(
x
)
will not over
all the two-hop neighbors. These nodes will be
seleted as MPRs in the proess, sooner or later.
Therefore, if the seond step is omitted, the mul-
tipoint relay set an still be alulated with su-
ess, i.e. it will over all the two-hop neighbors.
The presene of step 2 is for optimizing the MPR
set. Those nodes whih are neessary to over
the two-hop set
N
2
(
x
)
are all seleted in the be-
ginning, whih helps to redue the number of un-
overed nodes of
N
2
(
x
)
at the start of the normal
reursive proedure of step 3.
4