1	Implementation vs. Simulation: Evaluating a MANET Multicast Protocol Thomas Kunz Systems and Computer Engineering Carleton University http://kunz-pc.sce.carleton.ca/ tkunz@sce.carleton.ca
2	Motivation Ad hoc networks: infrastructure-less, rapid deployment Many applications for ad hoc networks require one-to-many and many-to-many communication Multicast protocols are intended to efficiently support such communication patterns Multicasting well researched in fixed networks (i.e., the Internet), building efficient distribution structures (typically a multicast tree) Ad hoc networks: dynamic topology makes it harder to maintain distribution structure with low overhead
3	Motivation (cont.) Previous work: studied performance of multicast and broadcast protocols in NS2, determined that broadcast protocols can support one-to-many and many-to-many communications well in a MANET Choose BCAST as starting point, a protocol that utilizes 2-hop neighborhood information to dynamically prune re-broadcasts However, even for “best” broadcast protocol, Packet Delivery Ratio (PDR) can be reduced when Mobility increases Number of multicast sender (i.e., traffic) increases
4	“Reliable” Protocol: Key Design Alternatives Reliability Mechanism? Forward Error Correction: Overhead with each packet, design often based on worst-case assumptions Retransmissions: detect packet loss and recover Ack-based Nack-based Which Protocol Layer? Transport Layer Routing Layer Flow Control, Security, etc. (not considered)
5	Reliable BCAST Each node keeps cache of recently transmitted packets (FIFO, small) Each node, upon receiving packet X from sender S, checks whether it received packet X-1 from that sender If not, broadcast retransmission request (NACK) to 1-hop neighbors Neighbors listen to overhear other retransmission and cancels theirs First set of experiments revealed that under high traffic load, too many NACKs were issued, flooding the network and resulting in overall worse performance (70% PDR for 10 sender scenarios) Added feature: NACK throttle Results: consistent improvement in PDR compared to best-effort version, at cost of slightly higher packet latency (all performance differences statistically significant at 99% confidence level).
6	But are simulation results true….? Idea: compare simulation results with testbed Implemented BCAST on Linux laptops and IEEE 802.11a/b/g Netgear wireless cards Compare results Static network topologies Dynamic network topologies Emulate node movement Used MNE (Mobile Network Emulator from NRL)
7	Comparison 1: Static Network
8	Comparison 2: Dynamic Networks
9	Analysis of Dynamic Network Results Not the same quantitative results (was expected) However, also not the same qualitative results! Reliable BCAST does not increase the PDR for low traffic loads (independent of the mobility rate): Under light traffic load, the single-sender scenario has a lower PDR than the multiple-sender scenario, yet the reliable protocol version does not recover from the single-transmitter losses (as static network results show, PDR of 100% is possible) Reliable BCAST performs worse under heavy traffic loads (many multicast sender sending lots of data) Under heavy traffic load, the increase in the number of senders caused severe congestion, producing a significant drop in PDR for all mobility rates and protocol versions.
10	Conclusions and Future Work Implemented a multicast/broadcast protocol both in a popular network simulator (NS2) and on Linux laptops Compared performance under a range of scenarios Quantitative and qualitative differences Quantitative differences expected Qualitative differences not expected and cause for concern Future work: Include additional simulators (GloMoSim) and tests without MNE Analyze observed differences closer Isolate the effect of MNE Additionally, work on core protocol (congestion control, security)