On-demand routing, also known as reactive routing, is a routing technique used in computer networks, particularly in mobile ad-hoc networks (MANETs) and wireless sensor networks (WSNs). Unlike proactive (table-driven) routing protocols, where routes are established and maintained regardless of whether they are needed, on-demand routing protocols create routes on an as-needed basis, which helps reduce overhead and conserves network resources. Here are the key characteristics and principles of on-demand routing:
- Route Discovery: In on-demand routing, routes are established only when a source node needs to send data to a destination. When a node wants to transmit data to another node and does not have a pre-established route, it initiates a route discovery process.
- Route Maintenance: Routes are typically maintained for as long as data is being transmitted between the source and destination nodes. Once data transmission ceases, routes are no longer needed and are allowed to expire.
- Route Request (RREQ) and Route Reply (RREP): When a source node needs to find a route to a destination, it broadcasts a Route Request (RREQ) packet to discover a path. Nodes that receive the RREQ packet can respond with a Route Reply (RREP) if they have a valid route to the destination or forward the request to other neighboring nodes.
- Dynamic Route Establishment: On-demand routing protocols are well-suited for scenarios where network topology changes frequently or where nodes are highly mobile. Routes are created based on the current network conditions.
- Reduced Control Traffic: On-demand routing can lead to reduced control traffic in the network, as routes are only established when necessary. This is particularly advantageous in resource-constrained networks like wireless sensor networks.
- Examples of On-Demand Protocols: Prominent on-demand routing protocols include AODV (Ad-hoc On-Demand Distance Vector), DSR (Dynamic Source Routing), and TORA (Temporally-Ordered Routing Algorithm), among others.
- Route Caching: Some on-demand routing protocols incorporate route caching, allowing nodes to store recently discovered routes for future use. This can speed up route discovery by avoiding the need for a new RREQ.
- Loop Avoidance: On-demand routing protocols typically employ mechanisms to avoid routing loops, such as sequence numbers to ensure freshness of route information.
- Scalability: On-demand routing protocols can be more scalable than proactive protocols because they do not require every node in the network to maintain routing information for all possible destinations.
- Energy Efficiency: In scenarios with energy-constrained nodes, on-demand routing can be more energy-efficient as it minimizes routing-related overhead.
On-demand routing is suitable for networks where the network topology changes frequently, such as mobile ad-hoc networks or sensor networks, or in situations where network resources should be conserved. It offers flexibility and adaptability to changing network conditions, but it can introduce some latency in the route discovery process, especially if no suitable routes are readily available in the network.
