Notes
Slide Show
Outline
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)