Two Different Timers Govern Neighbor Detection in Link-State Ad Hoc Routing

Author

Dina Hisham Ahmad

Abstract

 OLSRv1 ties the validity time in each HELLO message to the HELLO interval; both the OLSR specification and the ns-3 model set it to three times that interval. We show that this single setting hides two separate timers: one governs how fast a node notices a neighbor leaving, the other how fast it notices one returning. In ns-3.49 timestamp traces, a departed neighbor is purged when the validity time of its last HELLO runs out, with observed and predicted purge times differing by 0.03 s on average across 1,440 observations. Departure detection takes about the hold time minus half a HELLO interval, whereas a returning neighbor is found after about 1.5 intervals whatever the hold time. This explains why a gap-triggered adaptive HELLO controller gave no gain over stock OLSR in 600 runs. We separated the two parameters (HELLO every 3 s, hold time 6 s) and tested them in a prespecified study of 900 runs. Against stock OLSR, control bytes fell by about 10% under churn and node outage and by 7% in stable networks, with no measurable loss in delivery ratio. On fading channels the setting cost about 8 points of delivery ratio and should be avoided.

Keywords

OLSR, HELLO interval, Neighbor hold time, MANET, ns-3, Neighbor discovery

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References

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