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Next Generation Mobile
Networks and Ubiquitous
Computing
Samuel Pierre
École Polytechnique de Montréal, Canada
Hershey • New York
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Next generation mobile networks and ubiquitous computing / Samuel Pierre, editor.
p. cm.
Includes bibliographical references and index. Summary: "This book provides a comprehensive and unied view of the latest
and most innovative research ndings on the many existing interactions between mobile networking, wireless communications,
and ubiquitous computing"- -Provided by publisher. ISBN 978-1-60566-250-3 (hardcover) -- ISBN 978-1-60566-251-0 (ebook)
1. Ad hoc networks (Computer networks) 2. Ubiquitous computing. I. Pierre, Samuel.
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2010010158
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All work contributed to this book is new, previously-unpublished material. The views expressed in this book are those of the
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105
Copyright © 2011, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited.
Chapter 11
Peer-to-Peer over Mobile
Ad-Hoc Networks
Nadia N Qadri
University of Essex, UK
Antonio Liotta
Eindhoven University of Technology, The Netherlands
The goal of mobile computing is to provide access
to information and resources at anytime, anywhere.
The traditional client-server architecture is insuf-
ficient to provide these needs, due to various issues
like scalability (as the number of nodes grows the
server can be overloaded) and link failure (which
may lead to server unreachable). To cope with
these issues there is a need for an architecture
which allows both decentralized and spontaneous
connectivity. Peer-to-Peer (P2P) systems offer the
means to realize decentralized networks, which
can be used to share resources over the internet.
On the other hand, a Mobile Ad-hoc Network
(MANETs) is a spontaneous network made of
mobile nodes connected wirelessly but without
relying on a specific infrastructure network. A
P2P network over an ad-hoc infrastructure is a
powerful combination that provides users with
means to access different kinds of information
anytime and anywhere. P2P systems and MANETs
have been developed by different communities in
order to address entirely different requirements.
Despite being fundamentally different, MANETs
and P2P networks share the common vision of
In this chapter we review various approaches for the convergence of Peer-to-Peer (P2P) and Mobile Ad
hoc Networks (MANETs), identifying strengths and weaknesses, and putting things in perspective. P2P
and MANETs are among the most active research topics in pervasive computing. The convergence of P2P
networks and MANETs would allow existing P2P applications such as P2P le sharing and P2P stream-
ing to benet from the ubiquitous connectivity of ad-hoc. A P2P network over an ad-hoc infrastructure is
a powerful combination that provides users with means to access different kinds of information anytime
and anywhere. Realizing such a system is, however, not straightforward.
DOI: 10.4018/978-1-60566-250-3.ch011
106
Peer-to-Peer over Mobile Ad-Hoc Networks
load distribution based on the user’s willingness
to share resources in a dynamic and decentral-
ized manner.
The P2P network concept has drawn increas-
ing attention, and has been widely deployed on
the Internet for various purposes, including dis-
tributed data storage, file sharing, collaborative
computing, and Internet telephony (Wu, 2006).
The P2P system is popular for being scalable,
fault-tolerant, and self-organized. Nowadays P2P
has turned out to be one the most active research
areas of distributed computing and networking.
P2P was originally used to describe the commu-
nication between two peers. Recently, it refers to
a system that enables two or more peers to share
resources (based on peer’s willingness) by using
some appropriate communication system, without
relying on central coordination. P2P applications
offer a way to take advantage of different computer
resources on the internet.
The term P2P became popular in 1999 with the
emergence of Napster (Napster, 1999). Napster
changed the way files were shared on the internet
because it let peers download digital materials
from each other, rather than from dedicated serv-
ers, increasing the system capacity and scalability.
Napster, though, was still a centralized P2P appli-
cation since it used a centralized indexing server.
By contrast, a second generation of P2P ap-
plications exemplified by Gnutella (Gnutella,
2000), Freenet (Clarke, 2000), KaZaA (KaZaA,
2002) and BitTorrent (BitTorrent, 2001) provided
a truly decentralized architecture.
On the other side, MANET technology also
draws great attention of worldwide researchers and
scientists. Since the first appearance of wireless ad-
hoc networks in the DARPA packet radio networks
in the 1970s (Jubin, 1987), it became an interesting
research object in the computer industry. In the
1990s, the concept of commercial ad-hoc networks
arrived with notebook computers and other viable
wireless communications equipment. At the same
time, the idea of a collection of mobile nodes
was proposed at several research conferences.
The IEEE 802.11 subcommittee (IETF-manet)
had adopted the term “ad-hoc networks” and
the research community had started to look into
the possibility of deploying ad-hoc networks in
other areas of application. During the last couple
of years tremendous improvements have been
made in the research of ad hoc networks. Due
to their ability to create and organize a network
without any central management, MANETs are
characterized as the art of networking without a
network (Jiang, 2004). A MANET is a special
kind of wireless ad-hoc network where there is
no fixed backbone infrastructure. Due to this, the
MANET can be flexible and rapidly deployed
(Goldsmith, 2002).
MANETs and P2P networks are decentralized,
autonomous and highly dynamic in a fairly similar
way. In both cases, network nodes contribute to
the overall system performance intermittently
and unpredictably. Both rely on mechanisms to
publish, share, index, cache, discover, and reor-
ganize resources dynamically. They both aim at
supporting efficient data distribution mechanisms
such as multicasting. The strong functional simi-
larities between MANETs and P2P networks, has
recently sparked a new research thread based on
a fundamental question: is it possible and benefi-
cial to envision a convergence between P2P and
MANETs? Such convergence would be appealing
as it would allow distributed applications to run
over spontaneous, infrastructure-less networks,
adding a new dimension to the current Internet.
In this chapter we review MANETs and P2P
systems from this new perspective. First we look at
each technology independently, considering their
commonalities and differences. Then we survey
existing literature on the topic of P2P over MA-
NETs. A critical discussion of the state-of-the-art
will lead us to the identification of the key areas
that require further research.
107
Peer-to-Peer over Mobile Ad-Hoc Networks
P2P can be broadly defined as a self organizing,
decentralized distributed system that consists
of potentially un-trusted, unreliable nodes with
symmetric roles (Borg, 2003; Wu, 2006). P2P
systems are designed for the sharing of resources
such as CPU cycles, storage, files and data directly
without any centralized control or hierarchical
organization. P2P architectures are characterized
by their ability to adapt to failures, handling and
accommodating large number of nodes while
maintaining acceptable connectivity and per-
formance (A-Theotokis, 2004). P2P networks
are mainly based on application-level overlays
(P2P overlays) built over the physical network,
enabling a range of P2P applications such as P2P
file sharing and P2P streaming. They have their
own routing protocols that permit computing de-
vices to share information and resources directly,
without dedicated servers (A-Theotokis, 2004). In
P2P networks, each peer can act as server or as a
client. Each peer can connect to one or more peer
at any time to provide resource sharing.
The P2P paradigm has largely adopted a layered
approach. A P2P overlay network built on top of
the Internet provides a general purpose substrate
that provides many common properties desired by
distributed applications, such as self-organization,
decentralization, diversity and redundancy.
Such an overlay shields distributed application
designers from the complexities of organizing
and maintaining secure overlays, tolerating node
failures, load balancing, and locating application
objects (Wu, 2006).
P2P network architectures can be classified in
three main categories: centralized, decentralized
and hybrid (Franciscani, 2003; Minar, 2001; Wu,
2006). Table 1 shows some categories, derived
from (Minar, 2001), whereas Table 2 shows the ad-
vantages and disadvantages of the P2P categories
considering different properties. In a centralized
P2P network, the topology is based on a central
indexing server (or servers) that maintains a
directory of the resources available on the peers
and coordinates the interaction between peers.
However, after receiving the information from
the central server, peers communicate directly.
The advantage of server based P2P systems is that
they can be more easily managed and therefore
fewer security issues are likely to be involved.
However they are less fault-tolerant as some of
the information is only held by the central server.
If anything goes wrong with the server, the whole
system goes down. The extensibility and scalabil-
ity of these systems depends on the capacity of the
server which is usually limited by its processing
power capabilities.
In decentralized P2P networks, all peers act
as both server and client equally. The communi-
cation is made through multiple unicasts, where
peers forward messages on behalf of others peer.
The decentralized topology is further divided into
two categories i.e. Unstructured P2P overlays, is
Table 1. Classification of P2P Systems
Centralized Decentralized Hybrid
Unstructured Structured
Napster Freenet Chord KazaA
SETI@Home Gnutella CAN Morpheus
Bit Torrent Pastry Overnet/eDonkey2000
Tapastry
Kademlia
108
Peer-to-Peer over Mobile Ad-Hoc Networks
formed when the overlay links are established
arbitrarily by peers. Such networks can be easily
constructed as a new peer that wants to join the
network can copy existing links of another node
and then form its own links over time and struc-
tured P2P overlays, which is precisely controlled
and determined by some algorithm and were
developed to improve the search mechanism to
discover data. The advantage of decentralized
systems is that they can be more easily extended
and are more fault-tolerant than centralized P2P.
However, security is a big issue in these systems.
Decentralized Structured P2P system can be
managed more easily and are more scalable than
their unstructured counterpart. For instance, DHT
may be used to manage the P2P overlay (Dubnicki,
2004; Lua, 2005). In unstructured P2P systems
scalability is difficult to measure due to the lack
of deterministic overlay management mechanism.
Increasing number of nodes supports the resource
sharing system but also increases signaling
overheads as most of unstructured P2P protocols
periodically broadcast messages.
In hybrid (centralized + decentralized) P2P
networks, there are some super-nodes or super-
peers that play a more important role than others.
Peers forward their queries to super-peers which
communicate with each other in a decentralized
manner. Hybrid P2P systems have same advan-
tages as decentralized P2P systems but have
difficulty in managing the peers and in this case
the scalability depends on the actual number of
super peers and their capabilities.
A MANET can be defined as a self organizing and
autonomous system of mobile nodes that com-
municate over bandwidth-constrained wireless
links. Since the nodes are mobile, the network
topology may change rapidly and unpredictably
over time. The network is decentralized, where all
network activity including discovering the topol-
ogy, routing functionality and message delivering
is executed by the nodes themselves (Giordano,
2001; IETF-manet). MANETs represent an al-
ternative to the traditional infrastructure model
of mobile communication: they do not require a
fixed communication infrastructure, but instead
rely on wireless nodes to act as mobile routers.
As the network topology in such a network
may keep changing randomly, the routing proto-
cols used in traditional wired networks cannot be
directly applied in MANETs due to their highly
dynamic topology, absence of established infra-
structure for centralized administration (e.g base
station or access point), bandwidth constrained
wireless links, and resource (energy-constrained
nodes).
A variety of routing protocols for MANET
have been proposed in the literature (Royer, 1999).
These protocols can be classified into several
Table 2. Advantages and Disadvantages of P2P Systems based on different properties
Centralized Decentralized Hybrid
Unstructured Structured
Manageable Yes No Yes No
Coherent Yes No No partially
Extensible No Yes Yes Yes
Fault-Tolerant No Yes Yes Yes
Secure Yes No No No
Lawsuit-Proof No Yes Yes Yes
Scalable Depends on server May be Yes Depends on super peers
109
Peer-to-Peer over Mobile Ad-Hoc Networks
types based on different criteria. Ad hoc network
routing protocols can be classified into three ma-
jor categories based on the routing information
update mechanism.
1. Proactive routing protocols (table-driven)
2. Reactive routing protocols (on-demand)
3. Hybrid routing protocols (proactive +
reactive)
A detailed classification based on different
criteria can be found in (Murthy, 2004). A general
classification is shown in Figure 1.
Although conceptually and practically appealing,
deploying P2P over MANETs is not straightfor-
ward since many of the classic assumptions that
can be made for conventional P2P over wired
networks do not hold any longer. In fact, the ad-
hoc networks flexibility and convenience comes
with new issues (Goldsmith, 2002). MANETs
not only inherit the traditional problems of wire-
less networking such as bandwidth optimization,
power control, and transmission quality enhance-
ment (Giordano, 2001; IEEE-draft, 1999) but, due
to the lack of a fixed infrastructure suffer from
a number of complexities and design constraints
that are specific to ad-hoc networks (Chiasserini,
1999; Corson, 1999). Examples are: dynamically
changing network topology (Chlamtac, 1998),
energy constraints, signaling overheads, varia-
tion in link and node capabilities (Vaidya, 2004),
misbehaving nodes, and unreliable links.
A lot of research in the field of MANETs
addresses the crucial problem of packet routing
and service location issues - this is for example
reflected by the IETF MANET working group.
However, little effort has been made to inves-
tigate other services required by higher-layer
protocols and applications. There is a magnitude
of potentially useful applications for mobile ad-
hoc networks, but application development for
mobile ad-hoc networks is not easy. This issue is
exacerbated in the case of P2P applications, which
are highly distributed and generate traffic surges
from and to unpredictable locations.
Recently, the synergy between MANETs and
P2P networks was recognized (Borg, 2003; Ding,
2004; Franciscani, 2003; Hu, 2003; Klemm,
2003; L. B. Oliveira, 2005; Schollmeier, 2002).
By analyzing the synergy of MANET and P2P
networks, we summarize the similarities between
them as follows.
• Dynamic network topology. Nodes in P2P
networks may randomly join and leave the
network without any signs, which causes
the network topology to change frequently
with time. It is the same with MANETs.
Furthermore, the mobility character of
mobile nodes in MANETs can make net-
Figure 1. Classification of MANET routing pro-
tocols
110
Peer-to-Peer over Mobile Ad-Hoc Networks
work topology changes to be even more
frequent.
• Multi-hop connection. Nodes in both types
of networks connect with each other via
multi-hop routing. Hop-to-hop connec-
tions in P2P network are typically via TCP
links with physically unlimited range,
while hop-to-hop connections in MANETs
are via wireless links, which are limited by
the radio transmission range.
• Decentralized control (infrastructure
less network). Both P2P networks and
MANETs have a decentralized structure.
And there is no central administration
point in the network
• Node’s multi-identity. Nodes in P2P net-
works and MANETs act both as client and
a server, as well as implementing routing
functionality. Each node can route and
transfer messages independently and their
nodes have equivalent functionalities and
capabilities. They can also provide re-
source downloading and communicate
with each other directly.
Besides, unstructured P2P networks such as
Gnutella share additional similarities with MA-
NET, including (i) flooding-based routing proto-
cols and (ii) limited scalability due to bandwidth
consuming traffic from flooding.
Despite these similarities there are some ma-
jor differences between both the networks that
are described in Table 3. A detailed discussion
on similarities and differences can be found in
(Schollmeier, 2002).
The similarities and differences between P2P and
MANETs discussed in previous sub section lead
to interesting although challenging research issues
on P2P systems over MANETs. Such as,
• Lack of Infrastructure. There are some P2P
protocols that rely on some infrastructure
components in their designs. For example,
CAN (Content Addressable Network)
assigns node identiers based on loca-
tions determined from static landmarks
(Ratnasamy, 2001). These protocols can-
not be adapted in ad hoc scenarios due to
lack of infrastructure.
• Frequent topology changes. In MANETs
due to node mobility and limited trans-
mission range, network topology changes
rapidly and unpredictably, which is not the
case in internet. Therefore P2P Protocols
integrated in MANETs need to update their
overlay topology frequently which results
in more overhead to maintain the overlay.
• Node Churn. The P2P protocol implement-
ed in MANETs could suffer from the fre-
quent node joining and leaving the network
due to mobility or limited transmission
range and may result in poor performance.
• Bandwidth limitation. The limited band-
width in MANETs may limit the usabil-
ity of P2P applications which are usually
bandwidth-intensive.
• Limited Battery Power. Many P2P appli-
cations on the internet are not designed to
use minimum message transmissions. In
MANETs it is necessary for nodes to re-
duce the number of message transmissions
while keeping the performance acceptable
to conserve battery power.
• Scalability. In structured P2P routing pro-
tocols, nodes can scale up to millions.
MANETs are likely to be much smaller in
scale and have limited capacity than P2P
overlays in the internet. Therefore it is dif-
cult to say that such a large number of
nodes can ever effectively be organized
into one infrastructure-less network.
111
Peer-to-Peer over Mobile Ad-Hoc Networks
• Addressing. No specic addressing archi-
tecture has been standardized for MANETs.
Due to node churn in MANETs, it is rea-
sonable to assume that nodes will have
changing IP addresses over the time. This
may be very challenging for structured P2P
protocols that store logical to physical ad-
dresses. In some cases node Identiers are
assigned as hashed IP addressed and there-
fore node identiers will be continuously
changing leading to inconsistency issues.
• Flooding. In MANETs many routing pro-
tocols are based on a ooding approach.
In the case of Unstructured P2P protocols,
which also rely on information ooding,
performance degradation will result from
signaling overheads.
Both P2P and MANETs have recently become
popular research areas due to the wide deploy-
ment of P2P applications over the Internet and
rapid progress of wireless communication, but
little research has been done on the convergence
of these two overlay network technologies. Many
decentralized applications have been built upon
P2P overlays and these same ideas may be use-
ful in developing higher level services in an ad
hoc setting.
A number of publications have been devoted
to bringing P2P protocols into ad hoc networks.
Many of those publications are mainly concep-
tual, presenting architectural proposals but not
evaluating them (Conti, 2004; Datta, 2003; Ding,
2004; Eberspacher, 2004; Hu, 2003; Yan, 2004).
A first approach for implementing P2P file shar-
Table 3. Differences between P2P and MANETs
P2P MANET
Layer level Application layer overlay Network layer
Reason of creating a network To provides service over logical infrastructure To provides Connectivity over physical in-
frastructure
Purpose of communication b/w
nodes
Search data do not necessarily communicate Communicate with other users
Communication pattern Multiple unicast with virtual broadcasting Physical broadcasting
Nodes behavior Static Mobile
Communication link Direct Indirect
Relay nodes Not required Required intermediate nodes
Connection medium Fixed wired Wireless
Connection establishment Hop by Hop via TCP links, Whereas the single hop
path length is not physically limited
Hop by Hop via radio links, which are thus
limited by the radio transmission
Connection maintenance Comparatively Easy Difficult
Node location Anywhere on internet Limited area
Network Structure Logical structure apart from physical structure Logical structure corresponds to physical
Structure
Physical position of the node Difficult to find Can be roughly estimated
Routing Reactive only (reliable algorithms not implemented
yet) stops when TTL field is 0
Proactive, Reactive and Hybrid (reliable algo-
rithms exist), Stops when destination is found
Network Topology Changes less frequently Changes more frequently due to mobility
Network Size Large networks (millions of nodes) Usually small networks (few nodes)
Network connectivity Not effected by nodes joining or leaving Effected by nodes joining and leaving
Bandwidth Large bandwidth Small bandwidth
Available Resources Practical unlimited limited
112
Peer-to-Peer over Mobile Ad-Hoc Networks
ing for MANETs constitutes the protocol 7DS
(Papadopouli, 2001), which focus on single-hop
settings. 7DS uses local broadcast transmissions
for sharing Web documents among peers in order
to enable online web browsing without connecting
to the Internet.
PROEM (Kortuem, 2001) aspires to provide
a platform to develop P2P applications for MA-
NETs. Its messages are based on TCP, UDP or
HTTP and employ XML for the representation of
the messages. However it also does not provide
any mechanism to adapt its virtual to the physical
topology. Thus its feasibility in larger MANET
scenarios is doubtful.
Lawrence proposes an approach to use JADE-
LEAP in an ad hoc environment (Lawrence, 2002).
The author proposes the removal of some man-
datory components and adds a Discovery Agent
to support ad hoc networks. Unfortunately this
does not address the question of routing, which
is one of the most critical questions in a MANET
scenario. Later on, the concept of Passive Dis-
tributed Indexing (PDI) (Lindemann, 2002) and
Optimized Routing Independent Overlay Network
(ORION) were introduced (Klemm, 2003). Both
of them focused on building a file sharing sys-
tem on MANET, based on flooding mechanisms
to discover files on demand. A protocol named
MPP emphasized on communication between
application layer and network layer to deploy
P2P systems effectively in mobile environments
(Schollmeier, 2003). This protocol establishes an
intermediate layer which helps passing queries
from application layer to network layer. Although
this protocol is an important step towards an ef-
ficient P2P application on MANET, it still relies
on flooding the network with query lookup. The
use of flooding quickly becomes the performance
bottleneck as the size of MANET grows.
To deploy a P2P system on MANET, main-
taining the overlay connectivity is the most chal-
lenging issue due to node mobility. If we apply
existing Internet P2P solutions to MANET, the
static overlay structure will cause extremely high
connection overheads and inaccurate search for a
file because of node mobility. In addition, flat static
addressing mechanisms are assumed in MANET.
As a consequence, routing is done on demand by
flooding or, otherwise, routing tables have to be
maintained dynamically. Intuitively, routing or
searching a file by flooding yields extremely high
cost. Therefore, the flooding approach is only ap-
plicable to small-scale MANET. Scalability is a
critical challenge for deploying a P2P system to
large MANET with thousands of nodes(Cheng,
2005). In next section we will be discussing the
various design approaches for the convergence
of P2P and MANET.
Figure 2. Taxonomy of P2P over MANET design Approaches
113
Peer-to-Peer over Mobile Ad-Hoc Networks
P2P can be mapped on the top of MANET in
different ways. Various design approaches can be
adopted, based on the classification of decentral-
ized P2P overlays, as shown in Figure 2.
As discussed previously, P2P overlays are
classified into unstructured P2P and structured
P2P overlays. In addition, the nodes in MANETs
have to act as end hosts and routers and are need
to involve in supporting the P2P overlay. The P2P
overlay can be implemented either at the network
layer or at application layer. If a P2P protocol is
layered on top of existing MANETs routing pro-
tocol the corresponding design approach is termed
layered approach. Figure 3 shows a general sce-
nario of P2P application layered on the top of a
MANETs.
If a protocol is embedded with a MANETs
routing protocol at the network layer then this
approach is called integrated or cross-layer ap-
proach, which are used interchangeably. Figure
4 shows a simple example of P2P application
integrated with network layer of MANET.
In (L.B. Oliveira, 2003), the performance of ad
hoc routing protocol under an unstructured P2P
application in MANETs is observed. The authors
studied the performance of three existing MANETs
routing protocols, i.e. AODV (Perkins, 1999),
DSR (Johnson, 1996) and DSDV (Perkins, 1994),
when Gnutella-like (Gnutella, 2000) application
was layered on top. Their results show that each
protocol analyzed, performed well under some
conditions and according to some metrics but had
drawbacks in others; particularly the delivery rate
was quite good but with more number of hops
and latency. Furthermore, it was observed that
the performance of these protocols was different
from those observed in previous studies (Broch,
1998; Das, 2000) for ad hoc network. Later on,
different unstructured P2P protocols (Franciscani,
Figure 3. A P2P application Layered over
MANET(L. B. Oliveira, 2005)
Figure 4. A P2P application integrated with
MANET(L. B. Oliveira, 2005)
114
Peer-to-Peer over Mobile Ad-Hoc Networks
2005; Heer. T, 2006; Lau. G, 2005) were introduced
but their performance over MANET has not yet
been evaluated.
Optimized Routing Independent Overlay Net-
work (ORION) (Klemm et al., 2003) is also based
on the integrated approach. ORION proposes a
P2P file sharing system for MANET. The basis of
ORION is AODV (Perkins, 1999) and the Simple
Multicast and Broadcast protocol for MANET.
ORION combines application-layer query pro-
cessing with the network layer process of the
route discovery from AODV. The results show
that ORION significantly increases the search ac-
curacy as the number of nodes grows and reduces
overheads from searching. In layered approach
the search accuracy was always lower than that
of integrated approach due to more overheads at
large network size.
In (Conti, 2005) a cross-layer Gnutella-like
P2P application is proposed for MANET and its
performance over MANET is investigated. The
comparison with a similar layered design (L.B.
Oliveira, 2003) is made. Results show that a
straightforward implementation of the protocol is
not satisfactory under the point of view of signal-
ing overheads and average overlay connectivity.
A similar work is done in (Rajagopalan, 2006)
who propose a cross-layer decentralized BitTorrent
for MANET. A comparison is made with a modi-
fied version of BitTorrent (BitTorrent, 2001) over
MANET. The results observed were analogous to
those obtained in the cross-layered Gnutella study
mentioned above, which shows that the integrated
approach is more resistant to node mobility and
network partitions and performs better and more
consistently than the layered one.
In summary, the key factor responsible for the
worst performance of the layered design is not the
inefficiency of the search mechanism adapted by
unstructured P2P protocol but due to the overheads
generated in maintaining the overlay topology
whose neighbor connections are not adapted at
the dynamic physical topology.
The layered design not only the limited
scalability of unstructured systems but also the
limited scalability of flooding based ad hoc rout-
ing protocols as both system strongly rely on
information flooding. Additionally, this approach
does not take cross-layer optimization techniques
into consideration so both layers flood the net-
work without being aware of each other (Olaf,
2006). The results of the study indicate that the
integrated unstructured design has significantly
lower overhead compared to the layered design
while achieving better performance according to
application-specific metrics.
In (Cramer, 2006) a Structured P2P protocol,
chord (I.Stoica, 2001) is layered on the top of
three existing MANET routing protocols, AODV,
DSR, and OLSR (Clausen, 2003). In the majority
of all tested scenarios, which varied in network
size, node mobility and application load, it was
found that Chord’s ability to consistently resolve
lookups was significantly impaired. The results
show that chord is unable to perform well under
high mobility.
Similar results were observed in (Hu, 2003;
Pucha, 2004) when slightly modified version of
pastry (A.Rowstron, 2001) was layered on the
DSR ad-hoc routing Protocol. The results show
that it is difficult to take advantage of many
optimization techniques from the interactions
between DHT and ad hoc routing protocol with
layered approach. For example, it is difficult for
the routing structures of the DHT and the route
cache of DSR to coordinate with each other to
optimally discover and maintain source routes
(Pucha, 2004).
In (Delmastro, 2005) the performance of a
structured P2P system (Freepastry), running
on a real ad hoc network, is evaluated. The re-
sults observed were almost same as in (Cramer,
2006; Pucha, 2004). From the implementation
of FreePastry (Freepastry) it was observed that
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Peer-to-Peer over Mobile Ad-Hoc Networks
carrying out the overlay management through a
high number of remote connections, introduces
a heavy overhead on the ad hoc networks, reduc-
ing the overall system performances. In order to
improve the system, each Pastry node should be-
come aware of the network topology, maintaining
locally a correspondence between physical and
logical address spaces.
Hu and others (Hu, 2003) proposed DPSR,
a network layer unicast routing protocol, which
uses a cross-layer approach for implementing the
structured P2P overlay Pastry as a network-layer
routing protocol, and integrates it with the MANET
routing protocol DSR. No performance evalua-
tion of DPSR is available though. Ekta, which
is based on DPSR, is a DHT for MANETs. Ekta
and DPSR share the essence of tightly integrating
Pastry and DSR at the network layer. However,
unlike DPSR, Ekta does not use DHT for unicast
routing. By the optimizing the cross-layer tech-
niques, integrating route cache of DSR and DHT
protocol into one structure can help to prevent
overheads and increase the overall efficiency of
the system in Mobile ad hoc environment. IP ad-
dresses are directly mapped to pastry node IDs,
enabling efficient routing to particular nodes.
Ekta uses eavesdropping to learn about paths to
close nodes. This knowledge is used to find close
nodes within certain ID spaces for proximity rout-
ing and thus optimize the performance of Pastry.
However this work does not address the issues
like network splits, mergers, and the existence of
multiple DHTs and mostly focuses on the methods
to improve the efficiency of the routing techniques.
The simulation scenario which was chosen makes
network splits impossible, and therefore does not
allow making statements about the performance
of Ekta in these extreme but common situations
(Heer, 2006).
CrossROAD (Delmastro, 2005) is also based
on Pastry. In CrossROAD, the performance of rout-
ing protocols and middleware platforms have been
evaluated in terms of overhead and reconfiguration
delays, in case of mobility scenarios and network
topology changes. In addition, a real prototype
of an optimized P2P system (CrossROAD) that
exploits the cross-layer interactions with a proac-
tive routing protocol (OLSR) has been presented.
However, the evaluation is made on a network of
only eight nodes, so it is still doubtful that this
will work on large MANETs.
In (Cramer, 2005b) the performance of Prox-
imity Neighbor Selection (PNS) for DHT (Dis-
tributed Hash Table) over MANET have been
evaluated. The authors used Chord as their DHT for
simulations. It was observed that the performance
of PNS is strongly dependent on network density,
degrading with increasing density. A different
strategy than PNS for adapting a virtual network
to its underlying network is proximity identifier
selection (PIS) (Gummadi, 2003) where a node’s
overlay address is chosen according to the node’s
current location (Du, 2004; Zahn, 2004).
Iterative Successor Pointer Rewiring Protocol
(ISPRP) (Cramer, 2005a, 2005c) is a P2P routing
protocol for MANET based on Chord. ISPRP is a
self-stabilizing protocol that creates the basic ring
structure of a Chord overlay for overlay-based
routing at the network layer of an ad-hoc network.
It does not employ flooding for discovering the
routes to the successor nodes. However, it does
not handle node mobility at present. Recently,
Zupeng Li and others in (Zupeng Li, 2006) have
proposed a new routing protocol named Peer Com-
puting based Dynamic Source Routing (PDSR)
for MANET which integrates P2P computing
with MANETs routing algorithms. PDSR adopts
a node naming mechanism similar to Chord. The
performance of PDSR over MANET is not yet
evaluated.
In summary, virtually in every scenario the
integrated architecture appear to perform better
than the layered one. The integration of the P2P
structured layer and ad hoc routing layers is crucial
to achieve efficiency, as it significantly reduces
the duplication of routing information and main-
tenance efforts. On the other hand, unstructured
P2P does not have precise control over the overlay
116
Peer-to-Peer over Mobile Ad-Hoc Networks
topology. As a result of lacking structure, the query
protocols in unstructured P2P networks in general
revert to flooding that limits scalability. A sum-
mary of the advantages and disadvantages of the
approaches discussed above is given in Table 4.
This chapter focuses on the interesting develop-
ments surrounding P2P protocols over MANETs.
It is observed that most work involves Gnutella
as an unstructured P2P overlay and CHORD and
Pastry as a structured P2P overlay, in both layered
and integrated approaches over MANETs. The per-
formance of these protocols were observed along
with few new P2P protocols for MANETs but it
would be interesting to see how other structured
P2Pand unstructured P2P protocols will perform
over MANETs. On the other hand if we look at
the literature we observe that only few of the
available MANET routing protocols have been
investigated in relation to P2P, including mainly
AODV, DSR, DSDV and OLSR. The feasibility
of other MANET routing protocols under P2P
applications is still not known. Along with that,
the protocols mentioned in previous sections are
only applicable in some specific scenarios or are
feasible under some metrics. It is unclear which
of the P2P overlay, structured or unstructured,
would be more efficient for supporting distributed
application over MANET. Which design approach
would support scalable unicast in MANETs?
Would it will be possible to apply to the case of
MANETs all the incentive-based and security
techniques which are used when P2P systems
run over internet?
In this chapter we reviewed the convergence
of P2P over MANETs. First we discussed each
technology separately, providing background as
to where and when they emerged. We also dis-
cussed the classification of P2P overlays based
different topologies along with examples. Then
we made the comparison between those topolo-
gies in respect of different properties, giving an
insight on which topology works well under
which scenarios. Furthermore we discussed the
similarities and dissimilarities between P2P and
MANET technologies from various perspectives.
It was interesting to see that although both work
under almost the same scenarios, but have many
differences as well. From this point it will be
easier to understand that in order to implement
P2P protocols over MANET we need to mitigate
those differences to get an optimal system for
MANET. Our review identifies areas that require
further investigation, indicating that a promising
way to tackle the problem is to build P2P-aware
MANETs, rather than attempting a layered ap-
proach.
Table 4. Summarized advantages and disadvantages of different architectures
Architectures
Unstructured Structured
Layered Integrated Layered Integrated
Manageable No No partial Yes
Scalable No Medium Limited Yes
Handle mobility No Yes At low mobility Yes
Overheads High Low Medium Low
Implementation Easy Partially difficult Partially difficult Difficult
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Peer-to-Peer over Mobile Ad-Hoc Networks
Hybrid Routing: This type of protocols com-
bines the advantages of proactive and of reactive
routing. The routing is initially established with
some proactively prospected routes and then
serves the demand from additionally activated
nodes through reactive flooding.
Mobile Ad Hoc Networks: is a kind of wire-
less ad-hoc network, and is a self-configuring
network of mobile routers (and associated hosts)
connected by wireless links.
Overlay Network: is a computer network
which is built virtually or logically on top of
another network.
Peer-to-Peer (P2P): can be broadly defined as
self organizing; decentralized distributed systems
that consist of potentially untrusted, unreliable
nodes with symmetric roles.
Proactive Routing: This type of protocols
maintains fresh lists of destinations and their
routes by periodically distributing routing tables
throughout the network.
Reactive Routing: This type of protocols finds
a route on demand by flooding the network with
Route Request packets.
Structured P2P Overlays: Structured P2P
network employ a globally consistent protocol to
ensure that any node can efficiently route a search
to some peer that has the desired file
Unstructured P2P Overlays: is formed when
the overlay links are established arbitrarily. Such
networks can be easily constructed as a new peer
that wants to join the network can copy existing
links of another node and then form its own links
over time.
