Gateway Forwarding Schemes For Manet-Internet Connectivity

IJSRD - International Journal for Scientific Research & Development| Vol. 2, Issue 07, 2014 | ISSN (online): 2321-0613
All rights reserved by www.ijsrd.com 418
Gateway Forwarding Schemes For Manet-Internet Connectivity
Pramod Kumar Yadav1
Jay Prakash2
Yash Pal Singh3
1,2,3
Department Of Computer Science & Engineering
1,2,3
Madan Mohan Malaviya University of Technology, (U.P), Gorakhpur-273010 India
Abstract— In the real world one of the most important
challenge for the broad implementation of mobile ad hoc
network (MANET) technology is the finding way to capably
interconnect them with the Internet. Yet, such
interconnections are very difficult due to differences in
mobility, addressing and routing between MANETs and
reside IP networks. Imprecise address and routing
techniques are hard to integrate. In this paper we propose the
half tunnels as a powerful transition technique to integrate
various networks. In this paper, we will also discuss some
existing solutions like default routes host route etc to
interconnect MANETs with the Internet, but on analysis we
find them lacking in robustness and flexibility. For example,
many solutions do not consider the presence of multiple
gateways, and in such scenarios they either fail, or are less
efficient due to the lack of multi-homing capabilities.
Key words: MANETs, Forwarding Techniques, Default
Routes, Half tunnels, Indirection etc.
I. INTRODUCTION
It is very crucial to interconnect the MANET to the Internet
because MANETs are often inconceivable to have flat
addressing and flat routing. When mobile nodes are
connected to the Internet, it is possible that there will be
multiple gateways available for offering that service. This
environment is very challenging for ad hoc nodes that want
to make use of the services that the gateways offer. In this
paper, we argue ,that a solution for MANET Internet
connectivity mechanism must be robust enough to handle
with the challenges in many of the areas like MANETs,
sensors etc., and flexible enough to expand opportunities to
improve performance or reliability, e.g., by doing multi-
homing or load-balancing. We categorize mainly three
classes of gateway forwarding scheme along with a range
of proposals; host routes, i.e., one explicit routing entry for
each destination, default routes that aim to provide route
aggregation and tunneling that encapsulates packets for the
Internet with a gateway's address. But we focus on the two
latter strategies, because they are more widely proposed and
try to provide the most benefits. In this paper we basically
compare the default route and half tunneling. The default
route work best in a shared prefix non-transit network like a
single hop LAN with one default gateway, which is not a
simple case for an ad hoc network. As an alternative, we
imagine flat addressing in most scenarios, a combination of
one or more gateways with network addresses translation
(NAT) or Mobile IP connectivity [1].In a straight IP
network, addresses are securely assigned and nodes are
placed in a hierarchy according to their address prefixes.
The address prefix describes the location of the node and
forwarding can be performed by a longest prefix
complement. By using “flat addressing”, where mobile
nodes continue to use their home address or pick any one by
choosing, this does not work very longer. There is no
method to distinguish local addresses from remote
addresses. The lack of a routing hierarchy in ad hoc
networks has many restrained but serious implications. For
the example, for finding that when a packet should be
forwarded to a gateway, it is necessary to first search for the
destination inside the local ad hoc network. However, with a
reactive routing protocol each intermediate node has to
repeat this lookup course of action and it quickly becomes a
quite fruitless approach. Next problem is the existence of
multiple gateways and the use of network address
translation or Mobile IP [2]. A network address translation
gateway can be used to hide the network’s internal
“random” addresses on the way to the fixed network
therefore is a critical element for integrating the flat address
space into the Internet. Here we have need of ways to justly
the limit of the adaptive character of ad hoc routing
protocols and to allow mobile nodes attach to their
gateways. Mobile IP on the next side allows nodes to keep
their home network addresses while visiting a foreign ad hoc
network. The home agent will make sure that packets to the
ad hoc node are delivered to the foreign agent in the ad hoc
network using tunneling. In this paper we look at how
“default routes” can be adapted to handle multi-hop
forwarding in an ad hoc situation with several gateways
running either NAT or Mobile IP. Then we show how
forwarding to a gateway can be made more efficient by
using “half tunnels”. We propose an approach where several
gateways are supported and where the mobile nodes can
guide the forwarding path. In contrast to default routes, half
tunnels work healthy for both flat and shared prefix
networks and in networks running both proactive and
reactive routing protocols. We give the brief idea of various
routing protocols in section 2. In section 3 we introduce
gateway discovery approaches. Section 4 describes the
gateway forwarding schemes and section 5 includes all the
significance of half tunneling mechanism and section 6
contains the conclusion of this paper.
II. ROUTING PROTOCOLS
Usual distance-vector and link-state routing protocols are
proactive in nature, that they maintain routes to all nodes,
including all the nodes to which no packets are sent. For that
reason they require periodic control messages, which lead to
limited resources such as power and link bandwidth being
used more frequently for controlled traffic as mobility
increases. One example of a proactive routing protocol is
Optimized Link State Routing Protocol (OLSR) which
reduces the utilization of bandwidth significantly. Reactive
routing protocols, on the other hand, operate only when
there is a need of communication between two nodes. This
approach allows the nodes to focus either on routes that are
being used or on routes that are in process of being set up.
Examples of reactive routing protocols are Ad hoc On-
Demand Distance Vector (AODV), and Dynamic Source
Routing (DSR). Both proactive and reactive routing has
special advantages and disadvantages that make them
appropriate for definite types of scenarios. Proactive routing
protocols have their routing tables updated at all times, thus

(IJSRD/Vol. 2/Issue 07/2014/093)
the delay before sending a packet is minimal. However,
routing tables that are always updated required periodic
control messages that are flooded through the whole
network, an operation that consumes a lot of time,
bandwidth and energy. On the other hand, reactive routing
protocols find out routes between nodes only when they are
clearly needed to route packets. Whenever there is a need
for sending a packet, the mobile node must first send the
route if the route is not already known. This route discovery
process may result in substantial delay. Combining the
proactive and reactive approaches, results in a hybrid
routing protocol. A hybrid approach minimizes the
disadvantages, but also the advantages of the two combined
approaches. The Zone Routing Protocol is such a hybrid
routing protocol. Each mobile node proactively maintains
routes within a local region (referred to as the routing zone).
Mobile nodes residing outside the zone can be reached with
reactive routing.
III. GATEWAY DISCOVERY SCHEMES
How configuration phase with the gateway should be
initiated by the gateway (proactive method), by the mobile
node (reactive method) or by mixing these two approaches
(hybrid method) and Adaptive gateway discovery will be
discussed here. The classification of various gateway
discovery approaches is performed as per of their behavior
and working strategy in four major parts. Some of them lie
in Reactive, some of them in proactive and so on.
A. Proactive Gateway Discovery
The proactive gateway discovery [3] is initiated by the
gateway itself. The gateway periodically broadcasts a
gateway advertisement (GWADV) message with the period
determined by Advertisement Interval. The advertisement
period must be chosen with care so that the network is not
flooded unnecessarily. The mobile nodes that receive the
advertisement, create a (or update the) route entry for the
gateway and then rebroadcast the message. To assure that all
mobile nodes within the MANET receive the advertisement,
the number of retransmissions is determined by
NET_DIAMETER defined by AODV. However, this will
lead to extremely many unnecessary duplicated
advertisements. A feasible solution that prevents duplicated
advertisements is to introduce a “GWADV ID” field in the
advertisement message format similar to the “RREQ ID”
field in the RREQ message format. The advantage of this
approach is that there is a chance for the mobile node to
initiate a handover before it loses its Internet connection.
The disadvantage is that since a control message is flooded
through the whole MANET periodically, limited resources
in a MANET, such as power and bandwidth, will be used a
lot.
B. Reactive Gateway Discovery
The reactive gateway discovery [4] is initiated by a mobile
node that is to create or update a route to a gateway. The
mobile node broadcasts a RREQ with an ’I’ flag (RREQ_I)
to the ALL_MANET_GW_MULTICAS address, i.e. the IP
address for the group of all gateways in a MANET. Thus,
only the gateways are addressed by this message and only
they process it. Intermediate mobile nodes that receive a
RREQ_I are not allowed to answer it, so they just
rebroadcast it. When a gateway receives a RREQ_I, it
unicast back a RREP_I which, among other things, contains
the IP address of the gateway. The advantage of this
approach is that control messages are generated only when a
mobile node needs information about reachable gateways.
The disadvantage of reactive gateway discovery is that a
handover cannot be initiated before a mobile node loses its
Internet connection.
C. Hybrid Gateway Discovery
Proactive approach requires much traffic overhead on the
MANET, while the reactive approach causes higher delay.
To minimize the disadvantages of the proactive and reactive
strategies, they can be combined into a hybrid
proactive/reactive [3] method for gateway discovery. For
mobile nodes in a certain range around a gateway, proactive
gateway discovery is used while mobile nodes residing
outside this range use reactive gateway discovery to obtain
information about the gateway. The gateway periodically
broadcasts a GWADV message. Upon receipt of the
message, the mobile nodes update their routing table and
then rebroadcast the message. The maximum number of
hops a GWADV can move through the MANET is
determined by ADVERTISEMENT_ZONE. This value
defines the range within which proactive gateway discovery
is used. When a mobile node residing outside this range
needs gateway information, it broadcasts a RREQ_I to the
ALL_MANET_GW_MULTICAST address. Mobile nodes
receiving the RREQ_I just rebroadcast it. When a gateway
receives a RREQ_I, it sends a RREP_I towards the source.
D. Adaptive Gateway Discovery
Information is easily delivered by the gateway only if it is
routing those datagram that it would collect anyway.
Adaptive gateway discovery [3] algorithm is based on this
idea. In this approach number of hops of its active source
location is maintained. In adaptive gateway discovery
schemes one can consider the optimal set of configuration
parameters e.g. time to live and threshold. In other words a
modified Hybrid Gateway Discovery mechanism which
dynamically adjusts value of TTL and periodicity of
GW_ADV messages depending on the MANET
characteristics in order to achieve a good trade-off between
performance and network overhead is called an Adaptive
Gateway Discovery Mechanism.
IV. FORWARDING SCHEMES
The gateway forwarding schemes in the MANET plays a
vital role for the flexibility of Internet connectivity. Here we
discuss the common approaches.
A. Direct forwarding strategies
Direct forwarding strategies [5] try to always use the
shortest route available, and there is no way to identify the
indirection points. There are two direct forwarding
approaches: host route forwarding and the default route
forwarding.
1) Host route forwarding
A host route scheme consists of forwarding state dispersed
over the nodes that include the route, leading from the
source to the destination. In a hop-by-hop scheme, such state
is a number of consecutive mappings between a destination

and a next hop. There is one set of mappings for each
destination and hence there is no aggregation. Since the state
is dispersed, it is possible to do local changes without
involving any other nodes. For example, the route can be
optimized by eliminating nodes, or it can be repaired by
adding new nodes. Routes within a MANET are normally
host routes, and the approach works well as long as the
destination is local to the MANET. However, when the
destination is located in the Internet, the gateway can be
added or excluded as any other intermediate node along the
path leading from the source in the MANET to the
destination in the Internet.
2) Default route forwarding
A default route [5] is basically a routing table entry, pointing
to a gateway connected to the Internet. It is the entry for
using the packets that do not match any other routing table
entry. In compare to a host route, a default route provides
route aggregation. However, there are two main problems
with default routes in MANETs: (1) Like host routes, the
default route only maps to a next hop, and hence it suffers
from the same lack of indirection support. (2) The
aggregation is in conflict with reactive routing approaches,
because they only maintain least routing state. Thus, it is not
possible to map a destination to the default route just
because there is no other routing state. As an alternative, the
destination must first be searched in the MANET using a
route request. Only if there is no corresponding reply, the
destination must be mapped to the default route. More
disturbing is that this process is repeated on each
intermediate hop, which leads to cascading route requests.
a) Representing Multi-hop Default
Routes
In the single hop LAN environment, the default route entry
points to the default gateway that a host uses to forward
packets to the Internet. When extending this principle to the
multi-hop environment, there are two ways to view this
setup. In Figure 1 we have :(a) The default route indicates
the next hop to the default gateway. (b) It indicates which
gateway is currently selected to be the default. In the single
hop case the two views are the same, but in the multi-hop
case there are various differences. In (a) the default route
maps to the next hop (63.3.5.23) and there is no need for an
explicit entry for the gateway host in (b) the default route
maps to the gateway address (192.168.1.1) and this gateway
entry is used to find out the corresponding next hop.
Destination Next Hop Hop Count
63.3.5.23 63.3.5.23 1
66.35.250.151 Default -
default 63.3.5.23 3
(a)
Destination Next Hop Hop Count
192.168.1.1 63.3.5.23 3
63.3.5.23 63.3.5.23 1
63.23.250.151 Default -
default 192.168.1.1 3
(b)
Fig. 1: Two different routing table configurations to the
same end. The address 66.35.250.151 is a destination on the
internet.
If the gateway can be reached over multiple hops,
solution (a) will only indicate the next hop and a node has
no state forceful so that to which gateway this default route
actually leads to. Solution 1(b) is the solution suggested in
the Globalv6 proposal. It is different because the default
route maps to the address of the presently selected gateway.
Note that table 1(a) requires two routing table accesses and
table 1(b) in worst case requires three routing tables. Route
look-ups may be necessary at each intermediate node on the
path to the gateway and will be costly. One look-up is saved
if the nodes in the ad hoc network have a shared prefix or if
they run a proactive routing protocol. In the reactive case,
nodes have to configure host route entry to avoid subsequent
route discoveries. Once the default route concept has been
augmented for multi-hop scenarios, like in table 1 (b) the
default route entry is extra because each destination will
have its own entry in the route table.
b) Avoiding Inconsistent Routes
There are some reasons why approach table 1(b) is preferred
in a multi-hop environment. There is a persistent mapping
between a default route and the gateway address. It is very
significant when a node receives a control message with
new information about the default route. Otherwise the node
would not be able to tell if the control message will update
the default route to point to a new gateway, and the
inconsistencies in the path to the gateway can occur if the
message is forwarded. Control messages that conflicts with
the currently configured default route can be lost. Other
compensation are that one can configure multiple default
routes which are distinguished by their gateway address. At
last, if the routing protocol requires destination sequence
numbers, it is not compulsory to keep a sequence number
for the default route since it is just a mapping to a host entry.
In (a), the default route is a “destination” and therefore
needs a sequence number of its individual.
B. Indirect forwarding strategies
An indirect forwarding strategy [5] is a way to get packets
from point A to point B, via a specified indirection point C.
Such forwarding hence allows non shortest route
forwarding, which is useful if a source node wants flows to
navigate a specific point between the source and the
destination. This is an important characteristic as a gateway
can also act as a Mobile IP home agent, or as a proxy. Such
indirection points hold status essential to the flows
traversing them on their path towards the Internet.
Therefore, these state holders are necessary indirection
points that cannot be changed without also moving the states
to the new points. Indirection can be achieved through
source routing or flow based routing. Another way is to use
encapsulation or say tunneling. With a tunneling approach to
indirection, packets destined for an Internet destination are
encapsulated in an extra IP header, which specifies the
gateway as an indirection point. The tunnel ends at the
gateway, which decapsulates the packets before they are
sent further. Fig. 2 shows tunneling between a source node
and a gateway in a MANET. For example, intermediate
nodes could be made gateway conscious by marking
gateways with an extra flag in the routing tables. The benefit
of this is that intermediate nodes do not have to perform
gateway discovery themselves when they also want to
communicate with the Internet. If nodes furthermore have
configured a common IP prefix, they can directly determine
whether packets should be tunneled to the Internet on
already available gateway routes.

V. HALF TUNNELING
For getting an increased level of indirection, we suggest to
use half tunnel that creates a one hop- illusion between
MANET node and gateway. This set up provides a
lightweight indirection point through the gateway onto the
path to the internet destination.. In this section we try
tunneling – proposed by Jonsson et al in [1] as an alternative
approach to default routes.
A. Half Tunneling Mechanism
A half tunnel is basically a transparent two-step for-warding
mechanism for ad hoc network. It can be viewed as a
translation function for intercontext forwarding or loose
source routing with one indirection point. Fig 2 provides an
overview of the mechanism of half tunnel forwarding.
Generally the name half tunnel refers to the fact that
tunneling actually is only needed in the forward direction. A
half tunnel takes care of the forwarding of packets to an
Internet gateway; it does not pact with how a mobile node is
attached to the Internet. The half tunnel mechanism provide
a smooth way of the transition of the packets to the gateway
in the MANET-Internet connection and it has many
advantages over the default route scheme for the packet
transition in various areas like sensors and MANET to the
connection with the Internet.
Fig. 2: Half tunnel forwarding showing the routing table state at each node and how encapsulation (Enc.) and decapsulation
(Dec.) changes the destination address of packets for Internet destinations (I flag). Because of the encapsulation, intermediate
nodes only have to maintain the gateway route (G flag).The gateway acts as a NAT or mobile IP home foreign agent.
By using a half tunnel the one-hop false intuition is
created between a mobile source node and the internet
gateway. Fig 2 depicts a detailed view of the mechanism
behind this process. In the first step, when a packet is found
to be for an Internet host, it is routed through a half tunnel to
the gateway by encapsulation. An encapsulated packet is
sent to the gateway using the gateway’s overt IP address and
the IP forwarding mechanism as configured by the ad hoc
routing protocol and at the gateway, the packet will exit the
tunnel and is decapsulated. In the second step, the initial
packet is routed towards the final destination in the
comprehensive Internet. It is very important to note that a
packet like this is not using a default route, but it uses the
standard host route entries. Address to the ad hoc network
address of the gateway and “override” a packet’s destination
by tunneling over numerous hops. In the invalidate direction
no tunnel is needed, since packets entering the ad hoc-
network from the fixed Internet will be translated to a local
address by the address translation or will already have a
valid “return address” if the gateway is the source node’s
Mobile IP home agent. If the gateway is a foreign agent, it
will be the end point of a Mobile IP reverse tunnel and will
thus decapsulates the incoming packet to a nearby routable
one where the destination is the source node’s home address
B. Benefits
Half tunnels have following striking properties:
 Route aggregation: Tunnelling achieves route
aggregation at intermediate nodes because it
requires individual host entries pointing to the few
gateways instead of an entry for several global IP
address.
 Stability and reduced overhead: Once a source
node has configured a tunnel to a gateway, that
tunnel will not be abstracted to another gateway
unless connectivity with the gateway is completely
lost. In that case the source node will be notified
and can take proper actions.
 Multiple gateways: Intermediate nodes can
preserve routes to multiple gateways so that end
nodes may tunnel Internet traffic bound for
different gateways over a general intermediate hop,
which is not the case for default route forwarding.

End nodes could even keep up flows through
different gateways at the same time by configuring
multiple tunnel entry points. In this case extra
tunnels can be used as back up routes if the
connectivity to one gateway is lost. This will avoid
route request flooding for all its current Internet
destinations. Additionally, tunnels to multiple
gateways are helpful when a node want to do a give
up between gateways or wants load balancing.
 Efficient forwarding: In terms of routing table
lookups, half tunnels are more efficient than the
default route counterpart. A source node needs to
Look-ups in the routing table .On intermediate
nodes, only one look-up is needed, which is a clear
advantage over the default route approach that most
likely needs three table accesses, both at the source
node and at intermediate nodes.
 Implementation simplicity: Half tunnels needs only
minor changes to the routing protocol. The tunnel
state needs to be maintained at the source node
only and intermediate nodes can forward tunnelled
packets without extra sense or state. As opposite to
loose source routing, tunnels do not require
extraordinary processing when forwarding a
packet.
 Security: Half tunnels are not as security perceptive
as default routes; only the source node adds
redirection state on the source node itself. Default
routes on the next side can be redirected at any
intermediate hop and all Internet traffic distracted
by a malicious node. It is true that any node can act
as if to be a gateway.
VI. EVALUATION
On the basis of our simulation we have compare the two
gateway forwarding scheme default route forwarding and
the half tunneling. We have compared these two techniques
for throughput, control traffic per data and for packet
delivery ratio.
On comparison we found that half tunnel solution
shows better result in compare to the default route
forwarding, due to the cascading effect, multiple changing
and gateway problem and state replication problem which is
found in default route forwarding.
Throughput with respect to no. of nodes
Control traffic data with respect to no. of nodes.
Packet delivery ratio with respect to no. of nodes.
VII. CONCLUSION
On making the study for the solutions to Internet
connectivity, we found an important inspiration that many of
them do not support indirection and are hence not robust
enough, not even in less difficult scenarios. Furthermore,
many of the solutions are only proposals that have not been
implemented and evaluated, neither in simulation nor in
reality. In contrast, our proposed scheme is very interesting,
and is further a complete Internet connectivity solution for
AODV routing. We expect that other routing protocols can
be supported with little effort, reactive ones as well as
proactive ones. In our study, we have compared two Internet
connectivity designs in a multiple gateway scenario – one
design using default routes and one using tunneling for
indirection. Normally default route scheme’s lack of
indirection can have serious impact on the performance due
to incorrect routing. In comparison, the indirection approach
does not suffer from the default route’s inherent problems,
and is also more flexible in terms of multi-homing
capabilities. We have presented half tunnels as a way to
improve efficiency in forwarding of packets to an Internet
gateway in ad hoc networks. We argued that half tunnels are
simple to implement. Half tunnels also have elements of
self-configuration, since tunnels are configured
automatically by the route reply. Half tunnels can be
implemented proactively or reactively, whichever suits the
target routing protocol. Our conclusion is that tunneling
schemes, or other approaches that allow indirection [6], are
essential for building robust and flexible Internet
connectivity for Mobile ad hoc networks. The main
advantage of a tunneling solution is that the source node of a
flow is always in full control and alone carries all the state
necessary to forward a packet to a gateway. No state is
replicated at intermediate nodes, except the state for the
route to the gateway. This makes tunneling transparent to
existing routing protocols and route aggregation is achieved

at intermediate nodes. A robust forwarding strategy is
necessary to build Internet connectivity solutions for
MANETs. We have compared the efficiency of using
default routes to that of using tunneling. Our conclusion is
that tunneling packets to a gateway in ad hoc networks with
flat addressing and reactive routing is more efficient and
flexible in compared to default routes.
REFERENCES
[1] U. Jonsson, F. Alriksson, T. Larsson, P. Johansson,
and G. Q. Maguire Jr. MIPMANET - Mobile IP for
Mobile Ad hoc Networks. In 1st ACM international
symposium on Mobile ad hoc networking and
computing (Mobihoc’00), 2000.
[2] C. Perkins. IP mobility support for IPv4, January
2002. IETF Internet RFC 3220.
[3] Koushik Majumder, Dr. Sudhabindu Ray, Prof.
Subir Kar Sarkar, “Implementation and
Performance Evaluation of the Gateway Discovery
Approaches in the Integrated MANET Internet
Scenerio”, International Journal on Computer
Science and Engineering, Vol.3 No.3, March
2011, PP: 1213-1226.
[4] Kumar Rakesh, Anil Kumar Surje and Manoj
Mishra “Review Strategies And Analysis of
Mobile Ad hoc Networks-Internet Integration
Solutions” IJCSI International Journal of Computer
Science Issues, Vol. 7, Issue 4, No 6, July 2010.
[5] Eric Nordstrom et.al “Robust and flexible Internet
connectivity for mobile ad hoc networks”, © 2010
Elsevier B.V. All right reserved.
[6] R. Wakikawa, J. Malinen, C. Perkins, A. Nilsson, A.
Tuominen, Global connectivity for IPv6 mobile ad
hoc networks, March 2006, IETF Internet Draft,
draft-wakikawa-manet-globalv6-05.txt.

Gateway Forwarding Schemes For Manet-Internet Connectivity

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