Secure Routing algorithms in Mobile Ad Hoc Networks
Islamic Azad University of Tabriz
Faculty of Engineering,University of Isalmic Azad University,Tabriz
Abstract:- An ad hoc network is a collection of wireless mobile hosts forming a temporary network without the aid
of any established infrastructure or centralized administration. In this network, it may be necessary for one mobile
host to enlist the aid of other hosts in forwarding a packet to its destination, due to the limited range of each mobile
host’s wireless transmissions. This paper presents protocols for routing in ad hoc networks that uses dynamic source
routing. The protocol adapts quickly to routing changes when host movement is frequent, yet requires little or no
overhead during periods in which hosts move less frequently. Based on results from a packet-level simulation of
mobile hosts operating in an ad hoc network, the protocol performs well over a variety of environmental conditions
such as host density and movement rates. Most recent ad hoc network research has focused on providing routing
services without considering security. In this paper,we detail security threats against ad hoc routing
protocols,specifically examining AODV and DSR.
Key-words:-ad-hoc networks, secure routing , AODVR,DSR, SEAD,MANET
nodes can be reached with a single hop. The transmission
1 introduction medium works much the same way as in a wired
An ad-hoc network is a network that lacks all broadcast network. Complexity increases dramatically
infrastructure.The network is created in ad hoc fashion when distances increase and multi-hop routing is
by the participating nodes without any central required. Mobility means that the routes can change very
administration. There are no dedicated routers or network rapidly, which in combination with the limited resources
management nodes, but the participating nodes work in restrict the type of algorithms to use. A further challenge
peer-to-peer fashion and act as both servers and routers. for routing is that wireless communication can cause
The nodes are usually assumed to be independent and do links to be unidirectional, ie. work only in one direction.
not need to have any kind of affiliation from before, so Efficient routing of packets is a primary manet
both computational resources and link capacity might challenge. Conventional networks typically rely on
vary greatly from node to node. Nodes are not assumed distance-vector or link-state algorithms, which depend on
to be static, but are allowed to move freely inside a periodical broadcast advertisements of all routers to keep
network, as well as leave or enter the network at any routing tables up-to-date. In some cases, manets also use
time.Thus the network by definition needs to be wireless. these algorithms, which ensure that the route to every
 Ad hoc networks are primarily meant for use by host is always known. However, this approach presents
military forces or for emergency rescue situations. At the several problems:
state of war an army cannot rely on fixed infrastructure, • periodically updating the network toplogy
as it is an easy and attractive target for the enemy. ad hoc increases bandwidth overhead;
networks are optimal solution in such cases. For civil use • repeatedly awakening hosts to receive and send
ad hoc net-works are crucial if the fixed infrastructure information quickly exhausts batteries;
has been torn down by eg. some natural disaster, like a • the propogation of routing information, which
flood or an earth-quake.Then rescue operations could in depends on the number of existing hosts, causes
such a situation be managed through utilizing ad hoc overloading, thereby reducing scalability;
networks. • redundant routes accumulate needlessly; and
• communication systems often cannot respond
to dynamic changes in the network topology
2 Routing in ad hoc networks quickly enough.
Manets use multi-hop rather than single-hop routing to
deliver packets to their destination. a standard cellular
topology routes each packet in only one hop, from the
Routing in an ad hoc network is trivial, if all the nodes
base station to the mobile host. manets can route packets
are within transmission range of each other. All the
in multiple hops, enabling direct communication between overhear to update or maintain their cache. This
mobile hosts without the need for mobility support router increases power consumption as the node has to be in
mediation. active mode all the time, listening for all transmissions in
its broadcast range. It can however significantly reduce
the number of route requests in the network, which will
2.1 Proactive routing lead to less congestion and better bandwidth utilization.
Proactive routing is a scheme were the routes to other Well known reactive algorithms are AODV and DSR .
nodes are known in advance, before communication In general reactive algorithms are more efficient than
starts. The routes are updated with periodic updates and proactive ones[9, 3].
each node knows a route to every other node in the
network. Maintaining routes beforehand means that
starting transmission to a node is fast, as it requires no 3 ON-DEMAND ROUTING
initial route setup. Maintaining huge routing tables ALGORITHMS
requires storage space, network bandwidth and
Rather than relying on periodical broadcasts of available
processing time, all of which are scarce in ad hoc
routes, algorithms such as dynamic source routing (DSR)
and ad hoc on-demand distance vector routing AODVR)
A major problem with proactive routing is that if the
discover routes as needed. Because the route to every
topol-ogy of the network changes or when new nodes
mobile node is not known at any given time, these
enter or old ones leave the network. A node that moves
algorithms must build and maintain routes.
to a new location must make its presence known to all
the neighboring nodes. All the peers in the network need
to find a new optimal route to the node and vice versa.
Because of the broadcast property of route requests this 4 Dynamic source routing
causes a huge overhead and potential delay to the traffic DSR is a fairly simple algorithm based on the concept of
in the network.In small networks proactive routing can source routing, in which a sending node must provide the
be efficient, as normal communication does not involve sequence of all nodes through which a packet will travel.
the delay in the route setup. If only a few nodes Each node maintains its own route cache, essentially a
participate in the network the route updates can be kept routing table, of these addresses. Source nodes determine
small which reduces the overhead. Well known proactive routes dynamically and only as needed; there are no
algorithms are DSDV and WRP . periodical broadcasts from routers.
If there is a valid entry for the destination, the node
sends the packet using that route; if no valid route is
available in the route cache, the source node initiates the
2.2 Reactive routing
route discovery process by sending a special route
Reactive routing, like the name suggests, is a scheme
request (RREQ) packet to all neighboring nodes. The
were routes are discovered only after a node wants to
RREQ propagates through the network, collecting the
initiate communication with another node. To minimize
addresses of all nodes visited, until it reaches the
route requests every node has a cache were recently use
destination node or an intermediatenode with a valid
routes are kept. This scheme means that there is an initial
route to the destination node. This node in turn initiates
delay associated with route discovery for every
the route reply process by sending a special route reply
transmission.  The delay increases significantly when
(RREP) packet to the originating node announcing the
the network size increases. A problem with on-demand
newly discovered route. The destination node can
routing is keeping up with the nodes in the network.
accomplish this using inverse routing or by initiating the
Because of the reactive nature nodes do not have to
route discovery process backwards.
announce their arrival or departure from the network.
The DSR algorithm also includes a route main-tenance
This means that the intended recipient might al-ready
feature implemented via a hop-to-hop or end-to-end
have left the network when the sender wants to initiate
acknowledgement mechanism; the for-mer includes error
transmission. A route request still has to be transmitted
checking at each hop, while the latter checks for errors
throughout the whole network, consuming resources of
only on the sending and receiving sides. When the host
all the nodes. There also needs to exist some protocol by
encounters a broken link, it sends a route error (RERR)
which the nodes can announce the services they provide,
otherwise the peers will be unable to utilize them.
Dynamic source routing is easy to implement, can
Reactive protocols can be improved by using
work with asymmetric links, and involves no overhead
promiscuous route discovery. This means that nodes
when there are no changes in the network. The protocol
utilize route requests by other nodes that they can
can also easily be improved to support multiple routes to
the same destination. DSR’s main drawback is the large This algorithm also includes route maintenance
bandwidth overhead inherent in source routing. Because facilities. For every route in a routing table, a host
each route cache collects the addresses of all visited maintains a list of neighboring nodes using that
nodes,RREQ packets can become huge as they propagate route and informs them about potential link break-ages
through the network. Routing information can also with RERR messages. Each node also records
increase enough to exceed the accompanying message’s individual routing table entries and deletes those
usefulness. These problems limit the network’s not used recently.
acceptable diameter and therefore its scalability. AODVR offers several key advantages compared
to DSR :
The Dynamic Source Routing (DSR)  protocol is a • it supports multicast by constructing trees con-necting
distance-vector routing protocol for MANETs. When a all the multicast members along with
node generates a packet to a certain destination and it the required nodes;
does not have a known route to that destination, this node •smaller control and message packets result in
starts a route discovery procedure. Therefore, DSR is a less network bandwidth overhead; and
reactive protocol. One advantage of DSR is that no • the need for only two addresses when routing
periodic routing packets are required. DSR also has the destination and next hoprather than the entire sequence
capability to handle unidirectional links. Since DSR dis- ensures good scalability because packet size does not
covers routes on-demand, it may have poor performance depend on networkdiameter.
in terms of control overhead in networks with high However, AODVR only works with symmetric links
mobility and heavy traffic loads. Scalability is said to be and, because it does not allow for multipath routing, new
another disadvantage of DSR , because DSR relies on routes must be discovered when a link breaks down.
blind broadcasts to discover routes. There are two main The Ad hoc On-demand Distance Vector (AODV)
operations in DSR, route discovery and route routing protocol is a reactive MANET routing protocol.
maintenance. Similar to DSR, AODV broadcasts a route request to
discover a route in a reactive mode. The difference is that
in AODV, a field of the number of hops is used in the
5 Ad hoc on-demand distance vector route record, instead of a list of intermediate router
addresses. Each intermediate router sets up a temporary
routing reverse link in the process of a route discovery. This link
With AODVR a source node that wants to send a
points to the router that forwarded the request. Hence,
message to a destination for which it does not have a
the reply message can find its way back to the initiator
route broadcasts an RREQ packet across the network. All
when a route is discovered. When intermediate routers
nodes receiving this packet update their information for
receive the reply, they can also set up corresponding
the source node. Thus, unlike DSR, this approach does
forward routing entries. To prevent old routing
not use route caching. Instead, each node maintains only
information being used as a reply to the latest request, a
the next hop’s address in a routing table, and these
destination sequence number is used in the route
routing tables are updated all the way along the RREQ
discovery packet and the route reply packet. A higher
sequence number implies a more recent route request.
The RREQ contains the source node’s address,
Route maintenance in AODV is similar to that in
broadcast ID, and current sequence number as well as the
DSR . One advantage of AODV is that AODV is loop-
destination node’s most recent sequence number. Nodes
free due to the destination sequence numbers associated
use these sequence numbers to detect active routes. A
with routes. The algorithm avoids the Bellman-Ford
node that receives an RREQ can send an RREP if it is
“count to infinity” problem. Therefore, it offers quick
either the destination or has a route to the destination
convergence when the ad hoc network topology changes
with a corresponding sequence number greater than or
which, typically, occurs when a node moves in the
equal to the sequence number the RREQ contains. In the
latter case, the node returns an RREP to the source with
Similar to DSR, poor scalability is a disadvantage of
an updated sequence number for that destination;
otherwise, it rebroadcasts the RREQ. Nodes keep track
of the RREQ source address and broadcast ID,
discarding any RREQ they have already processed. As
the RREP propagates back to the source, nodes set up 6 LINK-STATE ROUTING
entries to the destination in their routing tables. The ALGORITHMS
route is established once the source node receives the Link-state routing algorithms exploit the periodic
RREP. exchange of control messages between routers,ensuring
that the route to every host is alwaysknown and routing algorithms work under the assumption that nodes
immediately providing required routesas needed. are benign and cooperate to make the network work as
However, this proactivity comes at thecost of high efficiently as possible.
bandwidth overhead. Ad hoc linkstaterouting algorithms A black hole attack uses this assumption and makes all
attempt to conserve bandwidthby reducing the size and traffic travel through the malicious node by claiming to
number of control messages. have the shortest route to any node outside direct
transmission range. The node then drops the traffic,
instead of forwarding it. Packets are transmitted to the
7 SEAD, Secure Efficient Distance Vector node but none come out, thus the name.
routing A more subtle variant of the black hole attack is the
The research in ad hoc networks usually recognizes the gray hole attack. This works the same way, but the ill-
limitations of the nodes in the network. The problem is willed node selectively transmits some packets and drops
how to achieve secure routing protocols without using others. The node could e.g. transmit all the routing
inefficient public-key cryptography. In the authors packets, but drop all data packets. This kind of attack is
present a new proactive secure routing protocol called much harder to notice for the other nodes, because a
SEAD. SEAD, or Secure Efficient Ad hoc Distance recipient has no way to know that somebody sent it a
vector routing protocol that does not rely on asymmetric packet. Everything seems to work, as the nodes still have
encryption primitives, but in-stead it relies on oneway valid routes to the other nodes.Another serious attack
hash chains for security. The SEAD protocol is based on towards an ad hoc network is partitioning. The attacker
on the DSDV-SQ-protocol which is a distance vector can analyze the network traffic and check for any nodes
protocol. Distance vector (DV) protocols are efficient in that connect different independent parts of the network.
respect to the required storage space, because the routing By taking out such a node, the attacker could split the
table only stores the next hop to any remote node in the network in two. With a single wellplaced node the
network, not the whole route. DSDV-SQ extends this attacker can control all traffic between the network
scheme by adding a sequence number to each segments.
destination. This sequence number is incremented each Replay attacks are attacks where the enemy replays
time the route is updated and thus it protects against earlier sent packets to the network. If the packets contain
replay attacks and routing loops that many DV protocols data this should cause no trouble, the recipient just
normally suffer from. This condition is usually referred discards erroneous packets. This behavior as such is
as the ’Count to infinity’ prrouting updates by keeping a nothing special as the network might duplicate and
weighted delay timer that it oblem. DSDV-SQ tries to reorder packets on the route. If the enemy however
minimize redundant uses to decide when to send route replays route requests old locations and routing
updates to its neighbors. information might cause nodes to get unreachable.
A wormhole attack is one type of a replay attack. The
malicious user tunnels packets from one part of the
network to another, creating a wormhole through the
8 Routing disruption network.
In routing disruption the attacker uses the weaknesses in The current proposed routing protocols for ad hoc
routing protocols to prevent the network from working in wireless networks allow for many different types of
an optimal way. Routing loops can with some routing attacks. Analogous exploits exist in wired networks ,
algorithms be inserted with carefully selected routing but are more easily defended against by infrastructure
packets.a routing loop is a path that travels through the present in a wired network. In this section, we classify
same node more than once. Routing loops cause packets modification, impersonation, and fabrication exploits
to be sent by the same nodes over and over again until
against ad hoc routing protocols .Table 1 shows the
the TTL-field is exhausted.
comparision of protocols against the attacks.
Routing loops can be used to create DoS, because
sending packets consumes resources from all the nodes
in the loop. The destination node or nodes can also be
isolated from the rest of the network, if none of the
packets sent to them reach their destination. 
Attack AODV DSR ARAN
Routing loops can be created also by accident, because
the nodes are allowed to move relative to each other.
This is why most routing algorithms contain some kind Remote redirection
modif. of seq. numbers yes no no
of protection against routing loops. More straightforward
attacks on routing exist. Like mentioned earlier most modif. of hop counts yes no no
modif. of source routes no yes no aodv-13.txt.
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fabr. of error messages yes non-
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