## 4 Lifetime Improvement Mechanisms in Routing
### Node-Disjoint Multipath  
_Micro Sensor Multi-Path Routing Protocol (MSMRP)_[42] is an energy-aware and node-disjoint multi-path algorithm that can extend the network lifetime by distributing traffic load all over the field. It can move around the unavailable areas at the time of route construction. These areas caused by node unavailability due to energy depletion, non-uniform coverage, disasters, or extreme congestion. The protocol consists of the following procedures: (1) _Route Discovery Procedure:_ as shown in Figure 6, the source node starts to broadcast a RREQ message when it has some data for transmission. The RREQ will be discarded directly, if it is forwarded to an unavailable node. (2) _Route Reply Procedure:_ the sink node may receive several RREQ messages, but it chooses only two optimal paths among them and sends RREP messages back to the source node. In some circumstances, an unavailable node may appear along the inverse path. In this state, the previous hop tries to select another neighbor node specified from the cached RREP messages one by one. If it succeeds in finding one, it sends an ACK to the sink and then forwards the RREP to the source node along a new inverse path. In failure state when there is no route to the source, it sends a No message to the sink and then from the remaining paths, the sink will select another best path to send out a RREP again. (3) Route Maintenance and Error Handling Process: MSMRP uses a HELLO mechanism in the RERR process to specify a route failure. In this method, when a node does not receive HELLO message from one or several neighbors during a period, it realizes that the route through these neighbors is expired. Therefore, it prepares a RERR message and sends to their precursor node separately. It also deletes unreachable entries in its routing table. Recursively, the precursor nodes do the same process until all of the relay nodes on the route will be aware about the route expiration. To apply energy-efficiency to the proposed scheme, the protocol refuses to send RERR when it detects another unavailable node on the route [42]. Sometimes a node receives more than one RREP message from one of the precursor nodes. To ensure node disjointness among multiple paths, it sends out its neighboring table to that precursor node. After that, the precursor node searches for a common neighbor node that exists in the received table and its own table. When the precursor node chooses one common neighbor node, it sends a RREP message to it. If there is a route to the destination node of the RREP message, the common neighbor node sends an ACK message back to the precursor node. Otherwise, the precursor node selects another common neighbor node. At the worst case when there is no common neighbor, the joint node should forward the RREP by itself and the approach converts to link-disjoint multi-path method [42]. The test-bed experiments [42] show that the MSMRP algorithm could successfully build the routes crossing around the unavailable areas. However, there is no comparison with other similar protocols to  
Figure 6: RREQ dissemination procedure in MSMRP [42]. justify the proposed method. Another drawback of the MSMRP protocol is that in comparison with its predecessor MSRP, it has more routing control packets such as RREQ, RREP, RERR, advertisement message of neighbor node table, HELLO message and delete message of neighbor node. These control packets impose a high overhead to the sensor nodes that cause unnecessary energy consumption across the network. _Energy-Balance Multipath Rumor Routing (EBMRR)_[43] is another energy-efficient approach that uses a probabilistic manner to find multiple paths between the source and the sink. In order to prolong network lifetime, it uses the available paths alternatively to distribute the energy consumption all over the network. In contrast with the original Rumor Routing [31], EBMRR considers residual energy and power usage level at the neighboring nodes.