Q14Mobile Computing

Dynamic Source Routing (DSR) is a reactive (on-demand) routing protocol designed specifically for use in multi-hop wireless ad hoc networks. Unlike proactive protocols, DSR does not maintain routing tables of up-to-date routes for every possible destination; instead, routes are discovered only when needed, which conserves bandwidth and battery power in nodes that are not actively communicating. A distinguishing feature of DSR is source routing: the source node determines the complete, ordered sequence of nodes through which a packet must travel and places this list in the packet's header, so intermediate nodes simply forward according to the embedded route rather than making independent routing decisions.

When a source node needs a route to a destination for which it has no cached route, it broadcasts a Route Request (RREQ) packet containing the source and destination addresses and a unique request ID. Each intermediate node that receives the RREQ appends its own address to a route record in the packet and rebroadcasts it, unless it has already seen that request or is the destination. When the RREQ reaches the destination, or a node with a cached route to the destination, a Route Reply (RREP) is generated containing the accumulated route and sent back to the source, either by reversing the recorded route or using a route the replying node already has to the source.

Since nodes are mobile, links along an active route can break. Route Maintenance detects such failures using link-layer acknowledgments or passive observation. When a node cannot forward a packet because the next hop is unreachable, it generates a Route Error (RERR) packet, sent back to the source, identifying the broken link. The source then removes the affected route from its cache and, if a cached alternative route exists, uses it, or initiates a fresh Route Discovery. Because intermediate nodes overhear source routes in transit, they can also cache useful route fragments, improving efficiency for future discoveries.