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Transcript
Wireless Ad hoc networks – Routing Proposed ad hoc Routing Approaches • Conventional wired-type schemes (global routing, proactive): – Distance Vector; Link State • Proactive ad hoc routing: – OLSR, TBRPF • On- Demand, reactive routing: – DSR (Source routing), MSR, BSR – AODV (Backward learning) • Scalable routing : – Hierarchical routing: HSR, Fisheye – OLSR + Fisheye – LANMAR (for teams/swarms) • Geo-routing: GPSR, GeRaF, etc – Motion assisted routing – Direction Forwarding Wireless multihop routing challenges • mobility • need to scale to large numbers (100’s to 1000's) • need to support multimedia applications (QoS) • unreliable radio channel (fading, external interference, mobility, etc) • limited bandwidth • limited power Conventional wired routing limitations • Distance Vector (eg, Bellman-Ford, BGP): – Tables grow linearly with # nodes – routing control O/H linearly increasing with network size – convergence problems (count to infinity); potential loops (mobility?) • Link State (eg, OSPF): – link update flooding O/H caused by network size and frequent topology changes CONVENTIONAL ROUTING DOES NOT SCALE TO SIZE AND MOBILITY Intra-AS Inter-AS DV RIP BGP LS OSPF Proactive ad hoc schemes – OLSR and TBRPF • Link State explodes because of Link State update overhead • Question: how can we reduce the O/H? • Answer: Link State with “Topology reduction” – (1) if the network is “dense”, use fewer forwarding nodes – (2) if the network is dense, advertise only a subset of the links • Two leading IETF Link State schemes enhance scalability in large scale networks: – OLSR : Optimal Link State Routing – TBRPF: Topology Broadcast Reverse Path Routing LSR (Link State Routing) • In LSR protocol a lot of control msg unnecessary duplicated 24 retransmissions to diffuse a message up to 3 hops Retransmission node OLSR (Optimal Link State Routing) • In OLSR only a subset of neighbors (MPR-Multipoint Relay Selectors) retransmit control messages: – Reduce size of control message; – Minimize flooding 11 retransmission to diffuse a message up to 3 hops Retransmission node OLSR Overview • RFC 3626, October 2003 • In LSR protocol a lot of control messages unnecessarily duplicated • In OLSR only a subset of neighbors (MPR-Multipoint Relay Selectors) retransmit control messages – Reduce flooding overhead – Adapted for dense network • OLSR retains all the advantages of LSR: – stable; – Does not depend upon any central entity; – Tolerates loss of control messages; – Supports nodes mobility On-Demand Routing Protocols • Routes are established “on demand” as requested by the source • Only the active routes are maintained by each node • Channel/Memory overhead is minimized • Two leading methods for route discovery: source routing and backward learning (similar to LAN interconnection routing) Existing On-Demand Protocols • • • • • • • • • • Dynamic Source Routing (DSR) -- CMU Multipath Source Routing (MSR) – TJU Backup Source Routing (BSR) – UofO+TJU Ad-hoc On-demand Distance Vector (AODV) Associativity-Based Routing (ABR) Temporarily Ordered Routing Algorithm (TORA) Zone Routing Protocol (ZRP) Location assisted routing (LAR, DREAM) Signal Stability Based Adaptive Routing (SSA) On Demand Multicast Routing Protocol (ODMRP) – UCLA Dynamic Source Routing (DSR) • RFC 4728 – February 2007 • Forwarding: source route driven instead of hop-by-hop route table driven – Mobility ? • No periodic routing update message is sent • The first path discovered is selected as the route • Two main phases – Route Discovery – Route Maintenance DSR - Route Discovery • To establish a route, the source floods a Route Request message with a unique request ID • The Route Request packet “picks up” the node ID numbers • Route Reply message containing path information is sent back to the source either by – the destination, or – intermediate nodes that have a route to the destination • Each node maintains a Route Cache which records routes it has learned and overheard over time DSR - Route Maintenance • Route maintenance performed only while route is in use • Monitors the validity of existing routes by passively listening to acknowledgments of data packets transmitted to neighboring nodes • When problem detected, send Route Error packet to original sender to perform new route discovery MSR - Multipath Source Routing • Direct Descendant of DSR • On-demand + Source Routing + Multipath • Probing-based adaptive load balancing among multiple paths • Motivation of MSR – Efficiently using the network resource – Alleviate the oscillation in adaptive single path routing – Fast re-routing – Reducing computing & storage requirement – Exploiting computing power of host instead of link capacity Distributing Traffic among Multiple Paths • Quantities: A heuristic equation • Probing-based adaptive control – Decoupling between transport layer and network layer: SRPing – Cost effective • Scheduling: Packet Weighted Round Robin • TCP out-of-order (re-sequencing) problem Distributing Traffic among Multiple Paths • Heuristic equation – Rationale: Autonomous system, homogeneous assumption, bandwidth-delay product constant W j i j min d max ,U R j d i where , j d i is the delay of route with index i, j d max is the maximum delay of all the routes to the same destination, R is a factor to control the switching frequency between routes. U is a bound value to insure that should not to be too large. MSR Summary • Reduce network congestion • Improve throughput, delay, mobility, fault tolerance (CBR & FTP) • Acceptable routing overhead? – Little more than that of DSR – Route discovery – Route maintenance • Probing (unicast) add little O/H • Good candidate for QoS support – QoS-MSR, reliable-MSR • Acceptable packet out-of-order level ? Backup Source Routing (BSR) • Establish and maintain backup routes that can be utilized after the primary path breaks • Define a new routing metric - route reliability, and use it to provide the basis for the backup path selection • Reduce the frequency of route discovery flooding, which is a major overhead in ondemand protocols • Can improve the performance significantly in more challenging situations of high mobility Simulation Methodology • ns – Wireless extensions by CMU • Adopt methods used in [Broch98, Johnson99] • Two major files: – Movement pattern file – Communication pattern file • 50 mobile hosts placed randomly within a 1500m×300m area • 20 connections • Different traffic types: CBR & FTP • Two set of simulations: Max speed 20m/s & 1m/s Performance Evaluation • MSR vs. DSR vs. BSR • Performance Metrics – Packet delivery ratio – Data throughput – End-to-end delay – Packet drop probability – Queue size Simulation Results with UDP Traffic -- Packet delivery ratio for 20 sources 8 Simulation Results – CBR • End-to-end throughput 200 Throughput, packets/second DSR MSR 150 100 50 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Connection No. Simulation Results with UDP Traffic -- Average end-to-end delay for 20 sources 11 Simulation Results - CBR • Packets dropped at each node DSR MSR # of drops 15 10 5 0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 Node No. Previous Work on Using Multiple Paths • Alternate use (primary and backup) – It works OK for CBR traffic (BSR, Bypass DSR, Node Disjoint M-path AODV, etc) – TCP does not get much benefit. Backup path is used only after timeout; not efficient in mobility/errors.? • Concurrent use (ie, packet scattering) – MSR – TCP does well in a static, error free net with long paths (up to 50% improvement) – With mobility & errors, TCP suffers out-oforder problems because of RTT difference on the two paths “TCP Performance on multiple paths in ad hoc nets..” Liaw et al ICC 2004 Static net, no errors, opt W: max improvement 50%; typical improvement between 8% and 18% Multiple Path TCP with Packet Replicas • TCP data packet duplication on multiple paths – May introduce less O/H than repeated end to end retransmissions • Improve end-to-end route robustness when single route is not stable: – Replicate packet on multiple paths – Combat random, non correlated link losses – Combat path breakage Total Throughput(bits/s) Variable Loss Rate [ 0.05; 0.1; 0.15; 0.2] Mobility(m/s) Original TCP Multipath TCP Where do we stand? • OLSR and TBRPF can dramatically reduce the “state” sent out on update messages • They are very effective in “dense” networks. • However, the state still grows with O(N) • Neither of the above schemes can handle large scale nets from 10’s to thousands of nodes • What to do? Hierarchical Routing The previous schemes reduce control traffic O/H but do not significantly reduce routing table size Solution: use hierarchical routing to reduce table size In the process, reduce also control traffic O/H Proposed hierarchical schemes include: – Hierarchical State Routing (HSR) – Fisheye State Routing (FSR) – Landmark Routing – Zone routing (hybrid scheme) Routing • Current MANET solutions have limitations: – (a) proactive routing solutions (eg, Optimal Links State -OLSR) do not scale because of table size and control traffic overhead – (b) on demand routing cannot handle high mobility and dense traffic patterns – (c) explicit hierarchical routing introduces excessive address maintenance O/H in high mobility • MANET protocols do not scale • UCLA approach: LANMAR – Exploit implicit hierarchy induced by group mobility Solution: Landmark Routing Overlay • Main assumption: nodes move in groups • Groups are predefined or dynamically recognized • Node address: < group ID , Host address> • Landmark elected in each group • Landmarks advertisements maintain the landmark overlay Landmark Logical Subnet LANMAR Overlay Routing (cont) • Builds upon existing MANET protocols – (1) “local ” routing algorithm that keeps accurate routes within local scope < k hops (e.g., OLSR) – (2) Landmark routes advertised to all mobiles using DSDV Landmark Logical Subnet LANMAR Overlay Routing (cont) • Packet Forwarding: – A packet to “local” destination is routed directly using local tables – A packet to remote destination is routed to Landmark corresponding to logical addr. – Once the landmark is “in sight”, the direct route to destination is found in local tables • Benefits: low storage, low update traffic O/H Landmark Logical Subnet Landmark Routing In action LM1 Landmark LM2 LM3 Logical Subnet dest local routing Long haul routing source 1. Node address = {subnet ID, Host ID} 2. Look up local routing table to locate dest fail 3. Look up landmark table to find destination subnet LM1 4. Send a packet toward LM1 Link Overhead of LANMAR • Dramatic O/H reduction from linear to O(N) to O (sqrtN) LANMAR enhances MANET routing schemes We compare: (a) MANET routing schemes: DSDV, OLSR and FSR; and (b) same MANET schemes, BUT with LANMAR overlay on top Delivery Ratio LANMAR-DSDV LANMAR-FSR OLSR LANMAR-OLSR FSR DSDV • DSDV and FSR decrease quickly when number of nodes increases • OLSR generates excessive control packets, cannot exceed 400 nodes Georouting - Key Idea • Each node knows its geo-coordinates (eg, from GPS or Galileo) • Source knows destination geo-coordinates; it stamps them in the packet • Geo-forwarding: at each hop, the packet is forwarded to the neighbor closest to destination • Options: – Each node keeps track of neighbor coordinates – Nodes know nothing about neighbor coordinates Greedy Perimeter Stateless Routing for Wireless Networks (GPSR) • Greedy forwarding – Each nodes knows own coordinates – Source knows coordinates of destination – Greedy choice – “select” the most forward node Finding the most forward neighbor • Beaconing: periodically each node broadcasts to neighbors own {MAC ID, IP ID, geo coordinates} • Each data packet piggybacks sender coordinates • Alternatively (for low energy, low duty cycle ops) the sender solicits “beacons” with “neighbor request” packets Greedy Perimeter Forwarding D is the destination; x is the node where the packet enters perimeter mode; forwarding hops are solid arrows; Got stuck? Perimeter forwarding > Greedy forwarding failure. x is a local maximum in its geographic proximity to D; w and y are farther from D. > Node x’s void with respect to destination D GPSR vs DSR Throughput (Kbps) TCP over GPSR, AODV, DSR and DSDV Speed(m/s) GPSR commentary • Very scalable: – small per-node routing state – small routing protocol message complexity – robust packet delivery on densely deployed, mobile wireless networks • TCP is extremely sensitive to path breakage (timeout) -- It does very well with georouting • Outperforms DSR and AODV • Drawback: it requires knowledge of dest geo coordinates (explicit forwarding node address) – Beaconing overhead – nodes may go to sleep (on and off)