A Review of Current Ad Hoc Routing Protocols:
 Assumptions
 Algorithm Description
 Advantages/Disadvantages
 References
By Olasubomi Awe
1:
Definition of Ad Hoc network
 An ad hoc network is a (possibly mobile) collection of
communications devices (nodes) that wish to communicate, but
have no fixed infrastructure available, and have no pre-determined
organization of available links [1].
 A mobile ad hoc network (MANET) is a self-organizing and selfconfiguring multihop wireless network, where the network
structure changes dynamically due to member mobility [2].
 Ad hoc networks consist of wireless hosts that communicate with
each other in the absence of a fixed infrastructure [3].
 An ad hoc network is a set of wireless mobile nodes (MNs) that
cooperatively form a network without specific user administration
or configuration [4].
 An ad hoc network is a group of wireless mobile devices (nodes)
that communicate with each other in a collaborative way, over
multi-hop wireless links, without any stationary infrastructure or
centralized management [5].
2:
Applications of Ad hoc networks
1
Military-use communication in a battlefield where a centralized
configuration is difficult.
2
Emergency communication in disaster areas.
3
Several people having meetings with computers that are equipped
with wireless interfaces can use ad hoc networks.
4
Ad hoc networks can be an interesting research for supporting
intelligent transport systems and sensor networks.
3:
Unicast Routing Protocols
Routing Protocols generally use two types of algorithms:
 Distance-Vector routing (DV), a vector containing the cost (e.g. hop
distance) and path (next hop) to all the destinations is kept and
exchanged at each node [2].
 Link-State routing (LS), which maintains global network topology
information at each router through periodical flooding of link
information about its neighbors [2].
DV protocols usually suffer from slow route convergence and a tendency
to create loops in mobile environments than LS protocols while LS
protocols advertisement scheme generates larger routing control
overhead than DV [2].
Routing Protocols for ad hoc networks can be into three broad categories
(Fig 1):
 Flat routing, which adopts a flat addressing scheme. Each node
participating in routing plays an equal role.
 Hierarchical routing, which assigns different roles to network nodes.
 Geographic position assisted routing, which requires each node to be
equipped with the Global Positioning System (GPS).
3.1:
Flat routing protocols can be further classified as:
1. Proactive or Table-driven routing protocols
2. Source-initiated or Reactive or On-Demand routing protocols
3.1.1:
Table-Driven Routing Protocols
These protocols require each node to maintain one or more tables containing
routing information such as the position of the other nodes in the network.
The objective is to maintain consistent, up-to-date routing information
amongst every node in the network [6]. The nodes respond to changes in the
network’s topology by propagating updates throughout the network thereby
maintaining a consistent network view. As a result, background routing
information is shared regardless of communication requests.
3.1.1.1: Fisheye State Routing (FSR) – This is a simple and efficient LS
type routing protocol that maintains a topology map at each node and
propagates link state updates [2]. A major difference between FSR and
other LS protocols is that FSR exchanges the entire link state information
only with neighbors instead of flooding it over the network.
Another difference is that the link state exchange is periodical instead
of event-triggered, which avoids frequent link state updates caused by link
breaks in an environment with unreliable wireless links and mobility.
Entries corresponding to faraway destination are propagated with lower
frequency than those corresponding to nearby destinations.
Advantages: The protocol achieves potential scalability by limiting the scope
of link state update dissemination in space and over time. FSR also
produces accurate distance and path information about the immediate
neighborhood of a node.
Disadvantages: The protocol provides imprecise knowledge of the best path
to a distant destination but this is compensated by the fact that the route on
which the packet travels becomes more accurate as the packet approaches
the destination.
3.1.1.2: Fuzzy Sighted Link State routing (FSLS) – FSLS includes an
optimal algorithm called Hazy Sighted Link State (HSLS), which sends a link
state update (LSU) every 2^k * T to a scope of 2^k, where k is hop distance
and T is the minimum LSU transmission period.
FSLS has the same advantages as FSR.
3.1.1.3: Destination-Sequenced Distance-Vector Routing (DSDV) – DSDV
is a table-driven algorithm based on the classical Bellman-Ford routing
mechanism. Every mobile node in the network consists of a routing table in
which all the possible destinations within the network and the number of
hops to each destination is recorded. Each entry is marked with a sequence
number assigned by the destination node [6]. These sequence numbers
allow stale routes to be differentiated from new ones by mobile nodes,
thereby eliminating the formation of loops. Routing updates care
periodically broadcasted throughout the network in order to maintain table
consistency. Route updates can either employ full dump or incremental
packets. The full dump packet carries all available information while the
incremental packet is used to relay only information that changed after the
last full dump. The mobile nodes maintain an additional table where data
sent in the incremental routing information is stored.
Broadcasts consist of:
 Address of the destination
 Number of hops to reach the destination
 Sequence number of the information received regarding the
destination
 New sequence number unique to broadcast
The route with the most recent sequence number is always used.
Mobile nodes also store settling time of routes, which is the weighted
average time that routes to a destination will fluctuate before receiving the
route with the best metric.
Advantages: Due to the fact that the broadcast of a routing update is delayed
by the length of the settling time, DSDV reduces network traffic and
optimizes routes by eliminating broadcasts that would occur if a better route
was discovered later in the future.
3.1.1.4: Wireless Routing Protocol (WRP) – The goal of WRP is to
maintain routing information among all nodes in the network. Each
maintains a total of four tables:
 Distance table
 Routing table
 Link-cost table
 Message Re-transmission list (MRL) table
The MRL consists of:
 Sequence number of the update message
 Retransmission counter
 Acknowledgement-required flag vector with one entry per neighbor
 List of updates (destination, distance of destination, and predecessor
of destination) sent in update message
Nodes inform each other of changes in the network using update messages.
An update message, which contains the list of updates as well as a list of
responses indicating which mobiles should acknowledge (ACK) the update,
is sent only between neighboring nodes. If a node is not sending messages,
it must send a hello message within the specified time period to ensure its
connectivity. Otherwise the lack of messages from the node may cause a
false alarm.
Once a mobile node receives a hello message from a new node, the
new node is added to the mobile node’s routing table and the mobile node
sends the new node a copy of its routing table information.
Advantages: WRP achieves loop freedom by avoiding the “count-toinfinity” problem. It forces each node to perform consistency checks of
predecessor information reported by all its neighbors. This also leads to a
faster-route convergence when a link failure event occurs [6].
3.1.1.5: Optimized Link State Routing Protocol (OLSR) – OLSR, a LS
protocol periodically exchanges topology information with other nodes in
the network. The protocol’s use of multipoint relays (MPRS) to reduce the
number of excessive broadcast retransmissions and also the size of the LS
update packets, leads to an efficient flooding of control messages in the
network. A node periodically broadcasts hello messages to immediate
neighbors in order to exchange neighborhood information and to compute
the MPR set. From the neighbor lists, a node can figure out nodes that are
two hops away and compute the minimum set of one-hop relay points
required to reach the two-hop neighbors [6].
Each node informs its neighbors about its MPR set in the hello
message. Upon receiving a hello message each nodes records the nodes that
select it as one of their MPRs.
OLSR differs from other LS protocols in that only the MPR nodes of
a particular node need to forward the link state updates issued by that node
and the link state update of a node is reduced in size because it includes only
neighbors that select that node as one of their MPR nodes. As a result, a
specific node can be reached only from its MPR selectors.
Advantages: OLSR is well suited for dense networks. OLSR reduces the
size of control packets. It also minimizes flooding of this control traffic by
using only the selected nodes, which are the only nodes responsible for
retransmitting broadcast messages.
3.1.1.6: Topology Broadcast Based on Reverse Path Forwarding (TBRPF)
– TBRPF is a LS protocol. It consists of two separate modules: the neighbor
discovery module (TND) and the routing module. TND is performed
through periodical hello messages that report only changes of neighbors.
The routing module operates based on partial topology information obtained
from both periodic and differential topology updates [2]. TBRPF works as
follows: Assume node S is the source of update messages. Every node j in
the network chooses its next hop (say p) on the minimum-hop path toward S
as its parent with respect to node S. Node j only accepts topology updates
originated at node S from parent p, and then forwards them to the children
pertaining to S. Also, only the links that will result in changes to j’s source
tree are includes in updates. This allows for the propagation of a smaller
subset of source tree.
Advantages: TBRPF adapts to topology change faster, generates less routing
overhead and uses a smaller topology update packet [2].
3.1.2:
On-Demand Routing Protocols
With on-demand routing protocols, each node tries to reduce overhead by
only sending routing packets when a communication is waiting. A route
discovery process within the network is only initiated when a node requires
a route to a destination. Once a route has been established, it is maintained
by a route maintenance procedure until either the destination becomes
inaccessible along every path from the source or until the route is no longer
desired.
FSR
OLSR
TBRPF
Routing metric
Shortest path
Shortest path
Shortest path
Frequency of updates
Periodically
Periodically
Use sequence
Yes
Yes
Periodically, as needed
(link changes)
Yes (HELLO)
Loop-free
Yes
Yes
Yes
Worst case exists
No
Yes (pure LS)
No
Multiple paths
Yes
No
No
numbers
Table 1. Characteristics of Table-driven protocols.
3.1.2.1: Ad Hoc On-Demand Vector Routing (AODV) – The AODV
protocol is a an improvement on the DSDV algorithm described above
because it tends to minimize the number of required broadcasts by creating
routes on a demand basis instead of maintaining a complete list of routes as
observed in the DSDV algorithm. A path discovery process is initiated
when a source node wishes to send a message to a destination node and does
not have a valid route to that destination. The source node broadcasts a
route request (RREQ) packet to its neighbors, which in turn forward the
request to their neighbors. The forwarding continues until either the
destination or an intermediate node with a “fresh enough” route to the
destination is located.
AODV makes use of destination sequence numbers to ensure all routes are
loop-free and contain the most recent route information [6].
Each node maintains its own sequence number and a broadcast ID.
The broadcast ID is incremented for every RREQ the node initiates, and
together with the node’s IP address, uniquely identifies and RREQ. The
source node, along with its sequence number and broadcast ID includes the
most recent sequence number it has for the destination in the RREQ.
Intermediate nodes reply to the RREQ only if they have a route to the
destination whose corresponding destination sequence number is equal or
greater than that contained in the RREQ.
When forwarding the RREQ, intermediate nodes record in their
routing tables the address of the neighbor from which the first copy of the
broadcast packet is received, thereby establishing a reverse path. If
additional copies of the same RREQ are later received, these packets are
discarded. Associated with each route entry is a route timer which will
cause the deletion of the entry if it is not used within the specified lifetime
[6]. As a result, AODV only supports the use of symmetric links.
Route maintenance is as follows: If a source node moves, it is able to
reinitiate the route discovery protocol to find a new route to the destination.
If a node along the route moves, its upstream neighbor notices the move and
propagates a link failure notification message to each of its active upstream
neighbors to inform them of the erasure of the part of the route [6]. These
nodes then propagate the link failure notification to their upstream
neighbors, and so on until the source node is reached.
3.1.2.2: Dynamic Source Routing (DSR) – With DSR, the source
determines the complete route from itself to the destination, and includes the
route in the packet. Assumptions made by DSR are that the network
diameter is relatively small and that the mobile nodes can enable a
promiscuous receive mode, whereby every received packet is delivered to
the network driver software without filtering by destination address. Mobile
nodes are required to maintain route caches that contain the source routes of
which the mobile node is aware. Entries in the route cache are continually
updated as new routes are learned.
The protocol consists of two main phases: route discovery and route
maintenance. When a mobile node has a packet to send to some destination,
it first consults its route cache to determine whether it already has a route to
the destination. If it has an unexpired route to the destination, it will use this
route to send the packet. Otherwise, if the node does not have such a route,
it initiates route discovery by broadcasting a route request packet. This
route request contains the address of the destination, along with the source
node’s address and a unique identification number. Each node receiving the
packet checks whether it knows of a route to the destination. If it does not, it
adds its own address to the route record of the packet and then forwards the
packet along its ongoing links [6].
A route reply is generated when the route request reaches either the
destination itself, or an intermediate node, which contains in its route cache
an unexpired route to the destination. If the node generating the route reply
is the destination, it places the route record contained in the route request
into the route reply. If the responding node is an intermediate node, it will
append its cached route to the route record and then generate the route reply.
Route maintenance is accomplished through the use of route error
packets and acknowledgements. Route error packets are generated at a node
when the data link layer encounters a fatal transmission problem. When a
route error packet is received, the hop in error is removed from the node’s
route cache and all routes containing the hop are truncated at that point. In
addition to route error messages, acknowledgements are used to verify the
correct operation of the route links.
Advantages: DSR does not make use of periodic routing advertisements,
thereby saving bandwidth and reducing power consumption. Therefore, the
protocol does not incur any overhead when there is no change to the network
topology. DSR also enables nodes to keep multiple routes to a destination in
their cache. Hence, route recovery is faster.
DSR can quickly adapt to topological change caused by node movement.
Also, DSR is able to compute correct routes in the presence of asymmetric
(uni-directional) links.
Disadvantages: DSR is not scalable to large networks due to the small
diameter assumption and the source routing requirement.
The need to place the entire route in both route replies and data packets
creates a greater control overhead than in AODV.
3.1.2.3: Temporally Ordered Routing Algorithm (TORA) – TORA is a
highly adaptive loop-free distributed routing algorithm based on the concept
of link reversal. TORA is proposed to operate in a highly dynamic mobile
networking environment. In addition to being source-initiated, it provides
multiple routes for any desired source/destination pair. The main aspect of
TORA is the localization of control messages to a small set of nodes near the
occurrence of a topological change. As a result, nodes need to maintain
routing information about adjacent (one-hop) nodes. TORA performs three
basic functions:
 Route creation
 Route maintenance
 Route erasure
During the route creation and maintenance phases, nodes use a “height”
metric to establish a directed acyclic graph (DAG) rooted at the destination.
Thereafter, links are assigned a direction (upstream or downstream) based on
the relative height metric of neighboring nodes. In times of node mobility
the DAG route is broken, and route maintenance is necessary to reestablish a
DAG rooted at the same destination.
Time is an important factor for TORA because the “height” metric is
dependent on the logical time of a link failure. TORA assumes that all
nodes have synchronized clocks (accomplished via an external time source
such as the Global Positioning System). TORA’s metric comprises of five
elements:
 Logical time of a link failure
 The unique ID of the node that defined the new reference level
 A reflection indicator bit
 A propagation ordering parameter
 The unique ID of the node
The first three elements collectively represent the reference level. TORA’s
route erasure phase essentially involves flooding a broadcast clear packet
(CLR) throughout the network to erase invalid routes. In TORA, there is a
potential for oscillations to occur. Because it uses inter-nodal coordination,
its instability problem is similar to the “count-to-infinity” problem in
distance-vector routing protocol except that such oscillations are temporary
and route convergence will ultimately occur.
Advantages: TORA supports multiple routes. It retains multiple route
possibilities for a single source/destination pair. Bandwidth is conserved
because of the fewer route rebuilding. TORA also supports multicasts.
Disadvantages: TORA’s reliance on synchronized clocks limits its
applicability. If the external time source fails, the algorithm ceases to
operate. Also, route rebuilding may not occur as quickly due to oscillations
during this period. This can lead to lengthy delays while waiting for the new
routes to be determined.
3.1.2.4: Associated-Based Routing (ABR) – ABR is free from loops,
deadlock, and packet duplicates, and defines a new routing metric for ad hoc
mobile networks. This metric is known as the degree of association
stability. Each node periodically generates a beacon to signify its existence.
For each beacon received, the associativity tick of the current node with
respect to the beaconing node is incremented. Association stability is
defined by connection stability of one node with respect to another node
over time and space.
A high degree of association stability may indicate a low state of node
mobility, while a low degree may indicate a high state of node mobility.
Associativity ticks are reset when the neighbors of a node or the node itself
moves out of proximity. The fundamental objective of ABR is to derive
long-lived routes for ad hoc mobile networks.
The three phases of ABR are:
 Route discovery
 Route reconstruction (RRC)
 Route deletion
The route discovery phase is accomplished by a broadcast query and awaitreply (BQ-REPLY) cycle. A node desiring a route broadcasts a BQ message
in search of mobiles that have a route to the destination. All nodes (except
the destination) receiving the query append their addresses and their
associativity ticks with their neighbors along with QoS (Quality of Service)
information to the query packet.
A successor node erases its upstream node neighbor’s associativity
tick entries and retains only the entry concerned with itself and its upstream
node. Therefore, each resultant packet arriving at the destination will
contain the associativity ticks of the nodes along the route to the destination.
The destination is then able to select the best route by examining the
associativity ticks along each of the paths. When multiple paths have the
same overall degree of association stability, the route with the minimum
number of hops is selected. The destination then sends a REPLY packet
back to the source along with this path.
RRC may consist of partial route discovery, invalid route erasure,
valid route updates, and new route discovery, depending on which node(s)
along the route move. Movement by the source results in a new BQ-REPLY
process. When a discovered route is no longer desired, the source node
initiates a route delete (RD) broadcast so that all nodes along the route
update their routing tables. The RD message is propagated by a full
broadcast, as opposed to a directed broadcast, because the source node may
not be aware of any route node changes during RRCs.
Advantages: The resulting path tends to be longer-lived than other routes.
Longer-lived routes require fewer route constructions and therefore yields
higher throughput. ABR is guaranteed to be free of packet duplicates: only
the best route is marked valid while, all other possible routes remain passive.
Disadvantages: ABR assumes that each node is beaconing periodically. The
beaconing interval must be short enough to accurately reflect the spatial,
temporal, and connectivity state of the mobile hosts. This requirement may
result in additional power consumption although experimental results show
that the periodic beaconing has a minute influence on the overall battery
consumption.
3.1.2.5: Signal Stability Routing (SSR) - Unlike other algorithms, SSR
selects routes based on the signal strength between nodes and a node’s
location stability. This route selection criterion has the effect of choosing
routes that have “stronger” connectivity. SSR can be divided into two
cooperative protocols: the Dynamic Routing Protocol (DRP) and the Static
Routing Protocol (SRP).
The DRP is responsible for the maintenance of the Signal Stability
Table (SST) and Routing Table (RT). The SST records the signal strength
of neighboring nodes, which is obtained by periodic beacons from the link
layer of each neighboring node. All transmissions are received by, and
processed in, the DRP. After updating all appropriate table entries, the DRP
passes a received packet to the SRP.
The SRP processes packets by passing the packet up the stack if it is
the intended receiver or looking up the destination in the RT and then
forwarding the packet if it is not. If no entry is found in the RT of the
destination, a route-search process is initiated to find a route. Route-search
packets arriving at the destination have necessarily chosen the path of
strongest signal stability, since the packets are dropped at a node if they have
arrived over a weak channel. If there is no route-reply message received at
the source within a specific timeout period, the source changes the PREF
field in the header to indicate that weak channels are acceptable, since these
may be the only links over which the packet can propagate.
When a failed link is detected within the network, the intermediate
nodes send an error message to the source indicating which channel has
failed. The source then initiates another route-search process to find a new
path to the destination. The source also sends an erase message to notify all
nodes of the broken link.
Advantages: The resulting paths tend to be more stable and longer-lived,
resulting in fewer route reconstructions.
Disadvantages: Unlike in AODV and DSR, intermediate nodes cannot reply
to route requests sent toward a destination; this results in potentially long
delays before a route can be discovered.
3.1.2.6: Load-Balanced Ad-hoc Routing (LBAR) – LBAR is an on-demand
protocol intended for delay-sensitive applications where users are most
concerned with packet transmission delay. LBAR is concerned with finding
a path that would reflect least traffic load so that the data packets are routed
with the least delay. It assumes that packets are of the same size and traffic
is at a constant rate. The LBAR consists of four components:
 Route discovery – This is initiated when a source node needs to
communicate with another node for which it does not have a known
route. This process starts when the source broadcasts setup messages
to its neighbors. A node receiving the setup message forwards it to its
neighbors. The destination node collects arriving setup messages
within a route-select waiting period. An ACK message is forwarded
backward towards the source node from the destination node along the
selected path, which is called the active path. When the source
receives an ACK message, it starts transmission.
 Path maintenance – In the instant when the source node moves away
from the active path, the source node would have to reinstate a new
route discovery procedure to establish a route to the destination. If an
intermediate or destination node moves away from the active path, the
node upstream of the broken hop propagates an error message to the
destination node. After receiving a notification of the broken link, the
destination node picks up an alternative best-cost partial route passing
through the node propagating the error message and then sends an
ACK message to the initiator of the error message.
 Local Connectivity Management – Nodes can learn of neighboring
nodes in two ways. When a node receives a broadcast from a
neighbor, it updates its local connectivity information in its
Neighborhood table to ensure the inclusion of its neighbor. If a node
has not sent out a packet to any active neighboring node within a
certain time, it broadcasts a hello message to its neighbors, containing
its identity and activity. Neighbors receiving the hello message
update their local connectivity information in their Neighborhood
tables.
 Cost Computation – The cost function is used to find the path with the
least traffic so that data packets can be transmitted to the destination
as far as possible while achieving the goal of balancing load over the
network.
Advantages: LBAR provides quick response to link failure by patching up
the broken routes in use, thus guaranteeing reliability of data transmission.
The average time delay is lower than with AODV and DSR.
3.2:
Hierarchical routing is one way to solve the problem of dealing with
an increase in the wireless network size. It provides a scalable and efficient
solution. Wireless hierarchical routing is based on the idea of organizing
nodes in groups and then assigning nodes different functionalities inside and
outside a group. Both routing table size and update packet size are reduced
by including in them only part of the network; thus, control overhead is
reduced. Hierarchical routing protocols have smaller routing tables
compared to proactive routing protocols. The smaller message size greatly
reduces routing overhead. However, Hierarchical routing induces additional
overhead in order to maintain its hierarchical structure. Hierarchical routing
also requires complex management for hierarchy ID (HID) registration and
translation as discussed below.
Performance
Parameters
Loop free
Multicast
capabilities
Beaconing
requirements
Multiple route
possibilities
Routes maintained
in
Utilizes route
cache/table
expiration timer
Route
reconfiguration
methodology
Routing metric
AODV
DSR
TORA
ABR
SSR
Yes
Yes
Yes
No
Yes
No
Yes
No
Yes
No
No
No
No
Yes
Yes
No
Yes
Yes
No
No
Route table
Route
cache
Route table
Route table
Route table
Yes
No
No
No
No
Erase route,
notify
source
Erase
route,
notify
source
Link
reversal,
route repair
Localized
broadcast
query
Erase route,
notify source
Shortest
path
Associativity Associativity
and shortest and stability
path
Freshest and Shortest
shortest
path
path
Table 2: Comparisons of the characteristics of on-demand ad hoc routing protocols.
The most popular method of building hierarchy is to group nodes
geographically close to each other into explicit clusters. Each cluster has a
leading node (clusterhead) to communicate to other nodes on behalf of the
cluster. An implicit way of forming a hierarchy is to allocate each node to a
local scope.
Parameters
On-demand
Table-driven
Availability of routing
information
Periodic route updates
Available when needed
Always available regardless
of need
Not required
Required
Coping with mobility
Use localized route
discovery as in ABR and
SSR
Grows with increasing
mobility of active routes as
in ABR
Few can support QoS,
although most support
shortest path
Inform other nodes to
achieve a consistent routing
table
Greater than that of ondemand routing
Signaling traffic
generated
Quality of service
support
Mainly shortest path as the
QoS metric
Table 3. Comparisons of on-demand versus table-driven routing protocols
3.2.1: Clusterhead-Gateway Switch Routing (CGSR) – CGSR, an
on-demand protocol uses a stable clustering algorithm, Least Clusterhead
Change (LCC) to partition the whole network into clusters. A clusterhead
is elected in each cluster. A mobile node that belongs to two or more
clusters is a gateway connecting the clusters.
CGSR is a distance vector routing algorithm. Each mobile node
maintains two tables:
 A cluster member table which records the clusterhead for each node
 A distance vector (DV) routing table, which maintains one entry for
each cluster recording the path to its clusterhead.
The diameter of a cluster is only two hops and clusters overlap. The cluster
head is just the node whose IP address is the smallest among its neighbors.
To route a data packet, the current node first looks up the clusterhead of the
destination node from the cluster member table. Then it consults its routing
table to find the next hop to that destination cluster and routes the packet
toward the destination clusterhead. The destination clusterhead will finally
route the packet to the destination node, which is a member of it and can be
directly reached [2].
Advantages: CGSR can greatly reduce the routing table size compared to
DV protocols. Only one entry is needed for all nodes in the same cluster.
Thus, the broadcast packet size of routing table is reduced.
Disadvantages: The drawback of CGSR is the difficulty of maintaining the
cluster structure in a mobile environment. The LCC clustering algorithm
introduces additional overhead and complexity in the formation and
maintenance of clusters.
3.2.2: Hierarchical State Routing (HSR) – HSR is a multilevel clusteringbased LS routing protocol. It maintains a logical hierarchical topology by
using the clustering scheme recursively. Nodes at the same logical level are
grouped as clusters. Elected clusterheads at the lower level become
members of the next higher level. These new members in turn organize
themselves in clusters and so on. The goal of clustering is to reduce routing
overhead (routing table storage, processing, and transmission) at each level.
A clusterhead acts as a local coordinator for transmissions within the cluster.
At the first level of clustering, each node monitors the state of the link
to each neighbor and broadcasts it within the cluster. The clusterhead
summarizes link state information within its cluster and propagates it to the
neighbor cluster heads (via gateways). In HSR, the hierarchical ID (HID)
of a node is defined as the sequence of MAC addresses of the nodes on the
path from the top hierarchy to the node itself.
Advantages: Each node can dynamically and locally update its own HID on
receiving the routing updates from the nodes higher up in the hierarchy. The
hierarchical address is sufficient to deliver a packet to its destination from
anywhere in the network using HSR tables [2]. Gateway nodes can
communicate with multiple clusterheads and thus can be reached from the
top hierarchy via multiple paths.
Disadvantages: Longer hierarchical addresses, frequent updates of the
cluster hierarchy and the hierarchical addresses as nodes move. A
continuously changing hierarchical address makes it difficult to locate and
keep track of nodes.
3.2.3: Zone Routing Protocol (ZRP) – ZRP combines both proactive and
on-demand routing strategies. Each zone has a predefined zone centered at
itself in terms of number of hops. For nodes within the zone, it uses
proactive routing protocols to maintain routing information. For nodes
outside its zone, it does not maintain routing information in a permanent
base. On-demand routing strategy is adopted when interzone connections
are required.
ZRP is made up of three components:
 Proactive Intrazone Routing Protocol (IARP): This is used to
maintain routing information. IARP provides a route to nodes within
a node’s zone.
 Reactive Interzone Routing Protocol (IERP): IERP uses the route
query (RREQ)/ route reply (RREP) packets to discover a route in a
way similar to typical on-demand routing protocols.
 Bordercast Resolution Protocol (BRP): When the intended
destination is not known at a node, that node must be outside its zone.
Thus a RREQ packet is broadcast via the nodes on the border of the
zone. Route queries are only broadcast from one node’s border nodes
to other border nodes until one node knows the exact path to the
destination node.
ZRP limits the proactive overhead to only the size of the zone, and the
reactive search overhead to only selected border nodes. However, potential
inefficiency may occur when flooding of the RREQ packets goes through
the entire network.
3.2.4: Landmark Ad Hoc Routing Protocol (LANMAR) – LANMAR, a
proactive routing, is used for networks that exhibit group mobility. One can
identify logical subsets in which the members have a commonality of
interests and are likely to move as a group. LANMAR uses an IP-like
address consisting of a group ID (or subset ID) and a host ID: < GroupID,
HostID>. Each logical group has one dynamically elected node serving as a
landmark. A global distance vector mechanism propagates the routing
information about all the landmarks in the entire network.
LANMAR works in a symbiosis with a local scope routing scheme
such as FSR. When a node needs to relay a packet to a destination within its
scope, it uses the FSR routing tables directly. Otherwise, the packet will be
routed toward the landmark corresponding to the destination’s logical
subnet, which is read from the logical address carried in the packet header.
When the packet arrives within the scope of the destination, it is routed
using local tables (that contain the destination), possibly without going
through the landmark.
Advantages: LANMAR reduces both routing table size and control overhead
effectively through the truncated local routing table and “summarized”
routing information for remote groups of nodes. LANMAR provides a
flexible routing framework for scalable routing while still preserving the
benefits introduced by the associated local scope routing scheme.
Disadvantages: The main limitation of LANMAR is the assumption of
group mobility.
3.3
Geographic Position Information Assisted Routing uses GPS to
provide location information with a precision within a few meters and a
universal timing. The location information is used for directional routing in
distributed ad hoc systems while the universal clock provides global
synchronizing among GPS equipped nodes. Research shows that
geographical location information can improve routing performance in ad
hoc networks. All protocols assume that the nodes know their position.
3.3.1: Geographical Addressing and Routing (GeoCast) – GeoCast allows
messages to be sent to all nodes in specific geographical area using
geographical information instead of logical node addresses. A geographic
destination address is expressed in three ways:
 Point (latitude and longitude)
 Circle (center point and radius)
 Polygon (a list of points)
When the destination of a message is a polygon or circle, every node within
the geographic region of the polygon/circle receives the message. Data
communications starts from a computer host capable of receiving and
sending geographic messages (GeoHost). Data Packets are then sent to the
local GeoNode, which is responsible for forwarding the packets to the local
GeoRouter.
CGSR
HSR
ZRP
LANMAR
Hierarchy
Explicit two
levels
Implicit two
levels
Implicit two levels
Routing
philosophy
Loop-free
Proactive,
distance vector
Explicit
multiple
levels
Proactive, link
state
Hybrid, DV and
LS
Proactive, DV and LS
Yes
Yes
Yes
Yes
Routing metric
Via critical
nodes
Yes
(clusterhead)
Via critical
nodes
Yes
(clusterhead)
Local shortest
path
No
Local shortest path
O(M*H)
O(L) +O(e)
O(L)+O(G)
O(M*H)
O(N)
O(N)
Critical nodes
O(N/M)
Storage
Complexity
Communication O(N)
complexity
Yes (landmark)
Abbreviations: N (number of nodes in the network), M (the average number of nodes in a cluster)
H (number of hierarchical levels of HSR), L (average number of nodes in a node’s local scope)
G (number of logical groups in LANMAR),
Table 4. Characteristics of hierarchical routing protocols.
A GeoRouter checks whether its service area intersects the destination
polygon. As long as a part of the destination area is not covered, the
GeoRouter sends a copy of the packet to its parent router for further routing
beyond its own service area. Then it checks the service area of its child
routers for possible intersections. When a router’s service area falls within
the target area, the router picks up the packet and forwards it to the
GeoNodes attached to it.
Advantages: GeoCast provides effective group communication to a
geographic area. The hierarchy structure reduces the size of the routing
tables
3.3.2: Location-Aided Routing (LAR) – This on-demand protocol uses
location information to limit the area for discovering a new route to a
smaller request zone. As a result, the number of route request messages is
reduced. LAR performs route discovery through limited flooding (floods
requests to a request zone). Only nodes in the request zone forward route
requests. LAR uses two schemes to determine the request zone.
Scheme 1: The source estimates a circular area in which the destination is
expected to be found at the current time. The position and size of the circle
is calculated based on the knowledge of the previous destination location,
the time instant associated with the previous location record, and the average
moving speed of the destination. The smallest rectangular zone that includes
the expected zone and the source is the request zone. The coordinates of the
four corners of the zone are attached to a route request by the source. Only
nodes inside the request zone forward the request message.
Scheme 2: The source calculates the distance to the destination based on the
destination known to it. This distance as well as the destination distance is
included in the route request message and sent to neighbors. When a node
receives the request, it calculates its distance to the destination. A node will
relay a request message only if its distance to the destination is less than or
equal to the distance included in the request message.
Advantages: LAR reduces DSR overhead by restricting the propagation of
route request packets.
Disadvantages: Control overhead increases when the network grows
3.3.3: Distance Routing Effect Algorithm for Mobility (DREAM) –
DREAM, a proactive routing protocol provides distributed, loop-free,
multipart routing and is able to adapt to mobility. It minimizes the routing
overhead by using two new principles for the routing update frequency and
message lifetime. These principles are distance effect and mobility rate.
With the distance effect, the greater the distance separating two nodes, the
slower they appear to be moving relative to each other. With the mobility
rate, the faster a node moves, the more frequently it needs to advertise its
new location.
In DREAM, each node maintains allocation table (LT). Each node
periodically broadcasts control packets to inform all other nodes of its
location. The distance effect is realized by sending more frequently to nodes
that are more closely positioned. Also, the frequency of sending a control
packet is adjusted based on its moving speed. Data packets are partially
flooded to nodes in direction of the destination. The source calculates the
direction toward the destination and then selects a set of one-hop neighbors
that are located in that direction. If the set is empty, the data is flooded to
the entire network. Otherwise, the set is enclosed in the data header and
transmitted within the data. Only nodes specified in the header are qualified
to receive and process the data packet.
When the destination receives the data, it propagates an ACK to the
source in the same way. The destination does not generate an ACK when it
receives the data through flooding. If the source does not receive an ACK, it
retransmits by pure flooding.
Advantages: The occurrence of multiple deliveries increases the probability
of reception and protects DREAM from mobility.
Disadvantages: Similar to LAR, control overhead increases when the
network size increases.
3.3.4: Greedy Perimeter Stateless Routing (GPSR) – GPSR uses only
neighbor location information in forwarding data packets. It requires only a
small amount of per-node routing state, has low routing message
complexity, and works best for dense wireless networks. Beacon messages
are periodically broadcast at each node to inform its neighbors of its
position, which results in minimized one-hop-only topology information at
each node.
GPSR assumes that sources can determine through separate means the
location of destinations and include such location in the data packet header.
The protocol uses two data forwarding schemes: greedy forwarding and
perimeter forwarding. The former is the primary forwarding strategy, while
the letter is used in regions where the primary fails. In greedy forwarding,
when a node receives a packet with the destination’s location it chooses
from its neighbors the node that is geographically closest to the destination
and then forwards the data packet to it. Before performing the perimeter
forwarding, the forwarding node needs to calculate a relative neighborhood
graph (RNG). Perimeter forwarding traverses the RNG using the right hand
rule hop by hop along the perimeter of the region.
During perimeter forwarding, if the packet reaches a location that is
closer to the destination than the position where the pervious greedy
forwarding of the packet failed, the greedy process is resumed. In the worst
case, GPSR will possibly generate a very long path before a loop is detected.
Advantage: GPSR achieves its scalability by being insensitive to the number
of nodes in the network.
GeoCast
LAR
DREAM
GRSR
Yes
Yes
Yes
No
Yes
No
Yes
Yes
Proactive
Ondemand
Proactive
Proactive
No
Yes
No
No
Shortest
path
Yes
Shortest
path
Yes
Closest distance
Loop-free
Shortest
path
Yes
Worst-case exists
No
No
Multiple receivers
Yes
Yes (full
flooding)
No
Yes (loops and
longer paths)
no
Support location
propagation
Data forwarding
by location
Routing
philosophy
Sensitive to
mobility
Routing metric
No
No
Table 4: Characteristics of GPS assisted routing protocols.
1.1.4 A comparison between AODV, DSR and CGSR shows that [16]:
 DSR and CGSR have very high throughput.
 AODV exhibits a very short eng-to-end delay of data packets.
 CBRP has a higher routing overhead than DSR.
 DSR has much smaller routing overhead than AODV and CGSR.
 AODV has the largest overhead.
Table 6
4: Multicasting Routing Protocols
A multicasting protocol is a set of standards and parameters that two ends
of communication agree upon to route packets [13]. Multicasting involves
efficiently sending copies of the same information to any subset of nodes in
a network. Network utilization can be improved by sending one message to
a relay node, which would send a copy to each of the downstream relay
nodes eventually delivering the message to each of the destinations.
Multicasting protocols can be classified into two categories based on how
multicast trees are constructed:
 Source-based protocol, which tries to maintain a per-source multi-cast
tree from each source host to every member in the multicast group.
 Core-based or group-shared, uses only one multicast tree rooted at a
core host. The tree then spans from the core host to every member of
the multicast group.
Multicast protocols are more applicable for ad hoc networks than unicast
protocols because, in a typical ad hoc environment, it is more likely that
mobile nodes work as a group and are involved in collaborative computing
[7].
4.1:
On-demand Multicast Routing Protocol (ODMRP) –
ORMRP is a mesh based multicast protocol in which a mesh of nodes
for forwarding packets is created between the senders and receivers. The
mesh is created using the forwarding group concept.
The source host’s periodical JOIN packets establish the multiple trees.
The source node desiring to send data packets to multicast members floods a
JOIN_DATA to the whole network. When a host receives a JOIN_DATA
for the first time, it will rebroadcast the packet and establish a reverse path to
the previous host. Then each host that is a multicast receiver and has
received the JOIN_DATA will reply a JOIN_TABLE packet to its upstream
host on the reverse path. Each host that receives the JOIN_TABLE for the
first time will repeat the process until the source host is reached [7].
Disadvantages: ODMRP has excessive overhead that is incurred while
keeping the forwarding group current and in the global flooding of the JOIN
packets.
4.2:
Multicast AODV - This protocol is extended from the unicast AODV
protocol. When a host joins a multicast group, it has to be added to the
corresponding multicast tree. A route request (RREQ) packet can be
broadcast for this purpose. If a host receives a RREG for a multicast group
of which it is not a member or to which it does not have a route, it will
rebroadcast the RREQ to its neighbors.
When a multicast group member receives the RREQ, it will unicast
Route Reply (RREP) packet to the sending host. As hosts along the path to
the sending host receive the RREP, they will add entries to their multicast
routing tables for the hosts from which they received the RREP. Eventually,
when one or more RREP reaches the source, the source can pick the host to
which the RREP is returned with the minimum hop count as its next hop
leading to the multicast tree. Then, the source unicasts a multicast activation
(MACT) packet to its next hop [7]. The next hop, on receiving the MACT
packet, will enable for the source host the route entry with the minimum hop
count leading to the multicast tree and send the MACT.
4.3:
Multicast Routing Protocol Based on Zone Routing (MZR) –
MZR is a source-initiated on-demand protocol in which a multicast
delivery tree is created using the mechanism of zone routing. MZR uses the
zone routing mechanism in ZRP for creating and maintaining multicast trees.
It assumes bi-directional links between mobile nodes. It is a source tree
based protocol. For each multicast session, identified by a <source_id,
group_id> pair, a multicast delivery tree rooted at the source is created.
MZR consists of two parts:
 A proactive protocol, which runs inside each zone, maintaining an upto-date zone routing table at each table.
 A reactive multicast tree creation, which is initiated when a source
needs to send multicast data to its group members.
For each multicast session in the ad hoc network, a tree rooted at the
source and identified by a <source, group> pair is created. Every node
maintains a multicast routing table, which contains route entries
corresponding to each active multicast session in the ad hoc network. A
multicast route entry is identified by the multicast session id, a <source,
group> pair, and contains the IP address of the upstream node and a list of
downstream nodes on the corresponding multicast tree.
A multicast data delivery table is initiated by a multicast source. This is
achieved in two stages:
 The source initially forms the tree inside its zone and then tries to
extend the tree to the entire network. The source sends a TREECREATE to each zone node, through unicast routes obtained from
the zone routing table. When a zone node, interested in the
multicast group session, receives the TREE-CREATE packet, it
creates a multicast route entry and replies to the source with a
TREE-CREATE-ACK packet. This packet travels back to the
source through the reverse route created by the TREE-CREATE.
 Once the source is done with its zone, it tries to extend the
multicast tree to the entire network. The source identifies all the
border nodes in its zone and sends a TREE-PROPAGATE packet
message to each of them. The packet is set to the zone radius and
the packet is unicasted to these border nodes. The TREEPROPAGATE packet tells each node to extend the multicast tree
inside its zone. When a border node receives a TREEPROPAGATE packet, it creates a multicast route entry for the
session, and then sends a TREE-CREATE packet to all its zone
nodes [7]. A node in the border node’s zone then replies with a
TREE-CREATE-ACK packet if it is interested.
To ensure maintenance of the multicast tree, each route entry has a timer
associated with it. When the timer expires, the corresponding multicast
route entry is removed.
Advantages: MZR scales well for different group sizes. In terms of
throughput, MZR delivers near 100 percent of the data traffic for low
mobility speeds.
Disadvantages: At high speeds, the tree links break down quite often,
leading to constant branch reconstructions and therefore larger packet losses.
4.4: Robust Multicast Routing Protocol (RoMR) – The main aspect of this
algorithm is to build multiple reliable multicast trees that adapt to topology
changes in a dynamic fashion. Its main characteristics are robustness and
mobility awareness in efficiently supporting multicast communication.
RoMR is designed to be both proactive and reactive. It is proactive because
it creates trees with all the information currently available in anticipation of
future events [14]. It is reactive in the sense that it reacts to changes in
network topology and group membership.
4.5: Multicast Core-Extraction Distributed Ad hoc Routing (MCEDAR) –
MCEDAR addresses both the issue of robustness and efficiency in the
network. In MCEDAR, the infrastructure of a multicast group resides
entirely within a core and a core broadcast mechanism is used to perform
data forwarding on the infrastructure. MCEDAR has the following salient
characteristics:
 It uses a mesh-based routing infrastructure that attempts to provide
increasing robustness with an increase in the network mobility.
 It uses a tree based forwarding protocol that approximates the optimal
forwarding overhead in terms of the time taken to deliver each data
packet [9].
 It relies on the core broadcast mechanism for both control packer
forwarding and data forwarding, and hence does not use the highly
ineffective local broadcasts in its key mechanisms.
 It decouples the control infrastructure from the data forwarding
infrastructure thus minimizing control overhead and maximizing data
forwarding efficiency.
4.6: Core-Assisted Mesh Protocol (CAMP) – CAMP is designed to support
multicast routing in very dynamic ad hoc network with broadcast links. A
mapping service is assumed to exist that provides routers with the addresses
of groups identified by their names [10]. CAMP assumes the availability of
routing information from a unicast routing protocol. CAMP differs from
other multicast protocols in that it builds and maintains a multicast mesh for
information distribution within each multicast group. A multicast mesh is a
subset of the network topology that provides at least one oath from each
source to the each receiver in a multicast group.
Packets are forwarded through the mesh along the paths that first
reach the routers from the sources, i.e., the shortest paths from sources to
receivers that can be defined within the mesh [10]. A router keeps a cache
of the identifiers of those packets it has forwarded recently and forwards a
multicast packet received from a neighbor if the packet identifier is not
found in its cache.
Advantages: CAMP reduces control traffic associated with the establishment
and maintenance of multicast meshes. CAMP is more robust than shared
tree structures and is more scalable than the flood-based mesh approach used
in ODMRP. CAMP always loop less, forwards packets around failed links
of a mesh, and is resilient to any core failure and network partitions.
4.7: Protocol Independent Multicast Routing (PIM) – PIM has the
following characteristics:
 It maintains the traditional IP multicast service model of receiverinitiated membership [11].
 It supports both shared and source-specific distribution trees [11].
 It is not dependent on a specific unicast routing protocol [11].
The robustness, flexibility, and scaling properties of the protocol make it
well suited for large heterogeneous networks. PIM differs from other
multicast protocols in the following ways:
 Routers with local (or downstream) members join a PIM sparse mode
distribution tree by sending explicit join messages. In other multicast
protocols, membership is assumed.
 Dense mode IP multicast tree construction is all data-driven.
Two versions of PIM exist: PIM-SM (sparse mode) and PIM-DM (dense
mode).
4.8: Core Based Trees (CBT) – The main objective of CBT is keeping the
routing state to a minimum. CBT is based on the idea of using only one
routing tree per multicast group instead of having a routing tree for each
source transmitting to a multicast group.
In CBT, each multicast group has its own core router. A routing tree
is formed from the core router to the members of the multicast group [12].
Sources send the multicast data packets by unicast routing to the core router.
When a router on the routing tree receives one of the data packets and
determines that it is addressed to the core system for this routing tree, it
checks the multicast address carried in the packet. If the address is for the
group serviced by this tree, the router’s child interfaces forward the packets
to group members and are sent through the parent interface toward the core.
Advantages: The algorithm is well suited for sparsely populated networks.
Disadvantages: CBT places a burden on sources and routers during the
forwarding process. The use of only one routing tree per multicast group
causes some scalability problems. CBT is not very compatible with other
multicast routing algorithms.
4.9: Distance Vector Multicast Routing Protocol (DVMRP) – A modified
version of the DVRMP is the Group Membership near First-DVMRP
(GMNF-DVMRP). The major difference between the two protocols is that
if there is a member on the neighboring network, GMNF-DVMRP considers
the router attached to this neighboring subnetwork first to become the
designated multicast router of its subnetwork. By so doing, GMNFDVMRP decreases the network costs of the multicast tree formed by
DVRMP.
Each multicast router of a subnetwork sends the message of group
membership to its neighboring multicast routers periodically [13]. After
receiving the message, the receiving multicast router records these multicast
group addresses in a table. Whenever multicast packets from a particular
source arrive, this multicast router compares the multicast address in the
packet header with those in the small table. If the multicast address of the
incoming packets exists in the small table, the source address of the
multicast router is set to be true.
Protocol Tree
Group
Protocol
State
Scalability
Type
Distribution Independence
Mechanism
DVMRP
Sourcebased
dense
NO
Hard
bad
CBT
Sharedbased
Sparse
NO
Hard
Good
PIM-SM
Mixed
Sparse
YES
Soft
Good
PIM-DM
Source-
Dense
YES
Soft
Bad
based
Table 5: A comparison of multicast routing protocols
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