Network Partitioning Detection

Table 2-3: The features of conflict resolution approaches in MANETs

Approaches Features

Route table-

based Address table-based Casual chang-ing changing Uniform

Average signaling cost &

reassignment latency MEDIUM MEDIUM / HIGH Very HIGH LOW Possible address reas-

signment by the node de-

tecting the conflicts NO YES NO YES

Coupling with routing

protocol YES NO NO NO

Algorithm Complexity HIGH MEDIUM LOW LOW

2.5 Network Partitioning Detection

In MANETs, network partitioning takes place and if all partitions keep working independent- ly there will be no critical problem (i.e. no resulting address conflicts). However, if later those partitions merge together again, the problem of conflicted addresses may arise. This problem can be clarified in two cases as follows:

• The first case arises if two nodes from two independent partitions allocate identical

free addresses to new nodes. This may happen when every node has permission to al- locate a free address from a table containing all available addresses in the network, as in the MANETconf protocol. Here, if two nodes from different partitions allocate identical addresses at the same time there will be no rejection from the other nodes in either partition.

• The second case is when, before the merger with the other partitions, a partition reas-

signs addresses which have been regarded as abrupt departure nodes because they be- long to the other unreachable partitions.

The detection of such conflicts may be difficult because each partition has identical network configuration (network ID). Therefore, partitioning detection is required so that each partition can be assigned with unique identifier. This will enable them to be regarded as different net- works, distinguished by their unique identifiers. In this case a merger can be detected and the merger conflicts can then be handled by using one of the methods described in the above section.

Basically, there would be no assignment of identical free addresses in different partitions in protocols following local first or local only approaches because every node in those protocols owns a disjoint block of free addresses. However, if there is a need to reuse the addresses of ab-

2.5 Network Partitioning Detection

ruptly departing nodes (nodes not found, i.e. missing node) due to the lack of available address- es, the detection of the network partitioning is important.

Because the detection of the partitioning may be a complex and bandwidth consuming process some authors depend on the tables of routing protocols to detect the departure of nodes, as de- scribed in [54]. In MANETs, the dependency on the routing table information is inappropriate because there is such a wide variety of routing protocols. Aside from the routing service, the par- titioning detection can be achieved in a proactive or reactive means, depending on the impact of the partitioning on the assignment process of each protocol.

• Proactive detection: protocols which utilize synchronization among nodes such as

centralized protocols (one manager per network class or one manager per cluster class) can proactively detect any partitioning in the network. After the detection, each partition has to change its partition identifier and select the manager in the partition (if not already found). In this way, each partition can resume the allocation of free ad- dresses or reallocate the addresses which are of nodes detected as missing. Of course, the detection of a partitioning increases the signaling cost in the network. Moreover, if the partitions merge again later, another change needs to be done. Therefore, the pro- tocols using proactive detection will perform poorly in networks with highly frequent partitioning and merging.

• Reactive detection: in this mechanism, applied by most protocols following the dis-

tributed approach, partitioning in a network will be detected when there is an event such as the assignment of free address in the network. In reactive detection there are two basic methods, reflecting the relation between the assignment process and the kind of assigned address (an address not assigned before or an address of a missing node): o During the assignment of any free address: the protocols following this meth-

od require an acknowledgment in every assignment process from all config- ured nodes, similarly to protocols from the global assignment decision class. Basically, the abrupt departure nodes (signifying partition) can be detected by any node assigning a free address to a requesting node. In MANETconf, for example, if there is a partitioning in the network, the nodes in every partition with no response during the assignment process are considered as abruptly de- parting nodes and their addresses will be cleaned out of the allocation table to permit the reuse in the future. To prevent possible collisions after the merger, the absence of the lowest IP address node in a partition is the key for changing the partition identifier. This means that only the partition with no such node will change its identifier. This, however, may cause address duplication in a scenario when only the partition with the lowest IP address node makes the ad- dress assignment process. In this case, this partition will reuse the nodes from another partition (in which the node with the lowest IP address is missing) without the need for a change of the partition identifier. If later the both parti- tions merge, there is no way to detect the possible address conflicts. To solve this problem, the authors of MANETconf protocol introduced an additional method (proactive) in which this node (with lowest IP address) periodically