IP on Mobile Ad Hoc Networks

IP on Mobile Ad Hoc Networks
A Mobile Ad Hoc Network (MANET) [518] consists of autonomous mobile users and their communications
devices (PDAs, for example), which all act as wireless network nodes. When users activate their
devices, the network self-organizes and the nodes find one another automatically. Once the network  topology is discovered, the nodes collaborate to establish a stream of communication. In that stream,
each node can act as a source, relay point, or destination. The communication flow starts with the
source node and, in the case of out-of-range nodes, may hop across a number of intermediary nodes
before reaching the destination node. These multiple hops use less power, cause less interference and
utilize available frequencies better than direct links, and may enable more traffic to be carried on the
MANET.
In addition, there is no single point of failure on a MANET, as there could be on a WLAN
(access points) or cellular network (base stations). If a MANET node joins or leaves the network, the
MANET can reconfigure itself appropriately [520].
IP-based technologies can be advantageously applied to MANETs. The protocols employed by
such MANETs are standards-based and enjoy routing flexibility, efficiency, and robustness. Their
interoperability with the Internet is greatly enhanced, and many QoS questions are taken care of by
IP standardization [524].
When the MANET nodes utilize IP, they are assigned unique IP addresses. It is not necessary for
all nodes to be in range of all the others – two nodes that are communicating and in the range of each
other at one point in time might find themselves still communicating (via intermediary nodes), but
out of range (due to their mobility) at a later time. One concern regarding MANETs is whether the
nodes should keep track of routes to all possible destinations on the network, or only keep track of
destinations that are of immediate use. There are trade-offs to consider with either approach. Keeping
track of all possible routes means that initial latency is minimized, but additional control traffic needs
to be constantly exchanged, lowering network efficiency and raising battery use. If routes are only
discovered as needed, initial communication delays will be high, but power consumption and control
traffic are kept low [523].
Some of the challenges faced by the developers of MANET technology and protocols stem
directly from Internet connectivity. How many of the nodes in an ad hoc network should be allowed
to directly connect to the Internet? Mobile IP protocol assigns a mobile node a care-of address along
with a HA, effectively adding a new IP address to the mobile node. Decisions about which nodes in
a MANET can function as Internet gateways and what to do when one of them leaves the network
are still being deliberated. MANET routing becomes complicated when packets are routed across the
MANET’s boundary, and routing protocols for MANETs are still evolving [521].
One of the problems associated with MANETs stems from the lack of any centralized authority
in an ad hoc network and the need for all the nodes to collaborate in order to perform infrastructural
tasks like routing and forwarding: nodes need to cooperate in a “disinterested” manner to keep the
network up and running. The fear is that, in the absence of an authority figure, some nodes may begin
to function in a self-interested way, refusing to expend its resources for the good of the network. This
may occur because of a particular device’s internal set of battery conservation rules, or because a
device may be programmed to “hoard” available bandwidth rather than relay packets for other nodes,
for instance. Worse, a device may fail to abide by the network’s back-off protocol or contention
resolution rules. Current protocol proposals require that all nodes cooperate to correct route failures
when a node leaves the network. This, in turn, requires that nodes transmit route failure messages
to a sender “disinterestedly.” If they fail to do so, the sender will erroneously interpret the lack of
acknowledgements as a congestion situation and take inappropriate action. Research is under way to
modify ad hoc network protocols to account for these possibilities [522].
Research is also under way to ensure the security of MANETs and put intrusion detection systems
in place, especially for MANETs that arise when first responders (police, fire, and health officials)
arrive on the scene of a public safety incident. The first responders’ PDAs and laptops could quickly
establish a network to work together, and researchers are developing secure routing protocols that
do not rely on preexisting trust associations between nodes or the availability of an online service
to establish trust associations. Intrusion detection is of obvious importance in such situations, first to
maintain the privacy of affected individuals and second to prevent malicious nodes from entering and
disrupting the network [519].Because of their dynamic topology and variable link capacity, MANETs require special attention
to QoS issues. The current model in existence relies on “best effort” routing and queuing mechanisms,
but better methods are under research. This will become increasingly important as services such as
streaming video are implemented in MANET devices [517].