Ad Hoc Routing with Distributed Ordered Sequences

Abstract

Ad hoc routing with distributed ordered sequences Marc Mosko ∗ Palo Alto Research Center 3333 Coyote Hill Road Palo Alto, CA 94304 Email: mmosko@parc.com Abstract— We propose a new hop-by-hop routing protocol for ad hoc wireless networks that uses a novel sequence number scheme to ensure loop-freedom at all times. We use a single large per-destination label space to order nodes in a topological sort (directed acyclic graph). Nodes manipulate the label set in- network without needing destination-controlled resets, so path repair is localized. The label size is large enough that it should never be exhausted in the lifetime of any given network. Route request flooding is performed through a new method that exploits the inherent partial order of the network, so nodes can share RREQ floods. Whereas most previous route request pruning techniques create a request tree, the new technique creates a directed acyclic request graph. Simulation results compared to AODV, DSR and OLSR show that the new protocol has in most cases equivalent or better packet delivery ratio and latency, but with a fraction of the network load. I. I NTRODUCTION Wireless ad hoc computer networks are communications networks in which each node may be mobile and has at least one radio interface. There is no central infrastructure, such as cell towers, base stations, for access points. Exam- ples of these networks include tactical military applications, commercial vehicle-to-vehicle systems such as DSRC [1], or emergency rescue impromptu networks. The Internet Engi- neering Task Force (IETF) studies such networks under the mobile ad hoc networks (MANET) working group. Three MANET routing protocols have request for comments (RFC) status and two have internet draft (ID) status. The three MANET RFC protocols are the Adhoc On-demand Distance Vector (AODV) routing protocol [2], the Optimized Link State Routing (OLSR) protocol [3], and the Topology Dissemination Based on Reverse-Path Forwarding (TBRPF) [4]. The two Internet Drafts are the Dynamic Source Routing Protocol (DSR) [5], and the Dynamic MANET On-demand (DYMO) Routing [6]. TBRPF and OLSR are examples of link-state protocols, where nodes exchange topology information and execute a shortest path algorithm (e.g. Dijkstra’s) using the topology information they maintain to find routing paths. Unfortunately, neither of these protocol are loop-free, which means that the routing tables at nodes may point in a directed cycle at times. AODV, DSR, DYMO operate as on-demand protocols, which means that they do not maintain routes for all destinations, only those for which there is traffic. Nodes discover paths in on-demand routing protocols through route request (RREQ) floods in the network and unicast route reply J.J. Garcia-Luna-Aceves ∗† Computer Engineering Department University of California at Santa Cruz Santa Cruz, CA 95064 Email: jj@soe.ucsc.edu (RREP) advertisements. The IETF on-demand protocols attempt to maintain loop- free operation through different techniques. DSR is a source- routing protocol, so each source node must maintain complete path information to each in-use destination. If there are path changes, then the protocol must either drop the traffic or use a recovery technique, which has been shown to be prone to looping. AODV uses distance labels (hop count) to order nodes along shortest paths. If a node needs to repair a path, it increments a destination sequence number and broadcasts a RREQ. By incrementing the sequence number, it prevents any predecessors (nodes that use the current node as a suc- cessor) from replying and maintains loop-freedom. DYMO also uses distance labels and sequence numbers to maintain loop-freedom. All RREQ broadcasts must be answered by the destination node, and the destination will increase a route sequence number if the requested sequence number is larger than the stored number, or the reply path length is longer than the requested path length. Jaffe and Moss [7] made a key observation in the study of loop-free distance vector routing protocols. They noted that a node may independently add a new successor to a destination if the new distance does not exceed the current distance. If the distance increases, then the node must coordinate with other nodes through some mechanism. The coordination must ensure both that the new successor path is loop-free and that the new distance at the node is not out-of-order with respect to any predecessors. The conditions in DUAL [8] generalize the work of Jaffe and Moss and provide a reliable mechanism to reset the ordering information at predecessors through a diffusing computation [9]. Reliable diffusing computations, however, are impractical in a wireless ad hoc network due to their overhead and latency problems timing out non-existent links. Loop-free ad hoc routing protocols address the reset condi- tion several ways. One approach is to use source routing, such as in DSR and variations on it. Another approach consists of relying on reliable internodal coordination based on the values of distances to destinations reported by nodes (e.g., DUAL [8], LPA [10], ROAM [11]). Yet another approach, used in proto- cols similar to AODV, consists of having nodes use a distance label (D) and a sequence number (SN) to reset distance values. In this case, The ordered pair (SN, D) constitutes a lexico- graphic order. AODV manipulates the pair such that when there is a link break a node requests the next higher SN. This 1-4244-0222-0/06/$20.00 (c)2006 IEEE This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the Proceedings IEEE Infocom.