CS541 Advanced Networking 1 Mobile Ad Hoc Networks (MANETs) Neil Tang 02/02/2009

CS541 Advanced Networking 2 Outline  Network Architecture  Characteristics  Applications  Major Concerns  Challenges  Ad Hoc Routing  DSR

CS541 Advanced Networking 3 Network Architecture A B C D E A B C D E

CS541 Advanced Networking 4 Characteristics  Multihop wireless network  Unrestricted mobility  Dynamic node membership  Various physical layer techniques, e.g., directional antenna, cognitive radio

CS541 Advanced Networking 5 Applications  Battle-field communications  Emergency communications  Transportation system

CS541 Advanced Networking 6 Major Concerns  Mobility: link breakage  Power consumption  QoS  Scalability  Security

CS541 Advanced Networking 7 Challenges  MAC protocol design ( DCF): directional antenna, cognitive radio  Routing  End-to-end QoS support: mobility and intra-flow interference.  Multicast/Broadcast Routing

CS541 Advanced Networking 8 Ad Hoc Routing  On-demand (reactive) routing: Upon arrival of a connection request, the source node floods route discovery messages and find a route for packet forwarding. For example, Ad hoc On-demand Distance Vector (AODV) protocol, Dynamic Source Routing (DSR) protocol.  Proactive routing: Nodes flood updates throughout the network whenever the network topology changes. For example, Optimized Link State Routing (OLSR) protocol.  Hybrid routing: Route discovery is basically conducted reactively but link state update is conducted proactively within a certain range, e.g., 2-hop neighborhood of a node. For example, Zone Routing Protocol (ZRP)  IETF MANET group:

CS541 Advanced Networking 9 On-Demand VS. Proactive  On-demand (reactive) routing: Low routing overhead but long route discovery latency.  Proactive routing: High routing overhead especially in the case of high mobility but short route discovery latency.

CS541 Advanced Networking 10 Dynamic Source Routing (DSR)  DSR is an on-demand routing protocol for MANETs.  The whole source-to-destination route is included in every data packet and no routing table is needed for packet forwarding in each node.  Loop freedom is guaranteed.  Large overhead in the packets.

CS541 Advanced Networking 11 Route Discovery  The source node broadcasts a RREQ (request) message to request a path to the destination.  A tuple (SrcID, RequestID) is used to uniquely identify a route request.  A node v receiving the RREQ will, - discard the packet if it is an old or duplicate one - discard the packet if v is already in the route list - send an RREP (reply) packet back to the source through the reverse route if v is the destination. - otherwise, append itself in the route list and re-broadcast the packet.  Both RREQ and RREP will be sent out only once in each node.  If a node has a record in its cache showing how to reach the destination, it can reply an RREP to the source immediately.

CS541 Advanced Networking 12 A B C E D G H F A A A-B A-C A-C-E A-B-DA-B-D-G Route Discovery Route request for A->G Red – RREQ, Green - RREP

CS541 Advanced Networking 13 Route Maintenance  A link-layer hop-by-hop ACK is usually used for reliable transmissions. For example, DCF supports the link-layer ACK.  A RERR will be sent by the end node to the source node if it detects a link breakage. Nodes along the path will then update their caches accordingly and the source node will initiate a new route discovery.

CS541 Advanced Networking 14 Route Maintenance A B C E D G H F G RERR Route Cache (A) G: A, B, D, G F: C, E, F

CS541 Advanced Networking 15 Route Optimization  Route Caching: Each node caches a new route it learns by any means. For example, when A finds route [A,B,D,E,F] to F, A also learns route [A,B,D,E] to E; D forwards data [A,B,D,E,F], D learns route [D,E,F] to F; So a node usually organizes its cache in the format of a shortest path tree with itself as the root.  Avoid RREP Storm Problem: An intermediate node will delay transmitting the route reply for a random period of d. During this period, cancel the route reply if overhearing any packet containing a better route.  Limit the Propagation of RREQ Packets: First, set TTL = 1 for first route request packet. If no route reply is received after some time period, set TTL = maximum for next RREQ..

CS541 Advanced Networking 16 Route Optimization  Reflect Shorter Route: A node can send an unsolicited RREP to the source to inform the shorter route.  Improve Error Handling: exponential backoff is used to limit the rate at which new route discoveries are initiated.  Piggyback Data on RREQs