- Author(s): @abhay, @mrpatrick1991, et al
- Start Date: 2022-04-02
- Category: Technical
- Original HIP PR: #381
- Tracking Issue: #384
The Proof-of-Coverage incentive model is crucial to the ongoing growth of the Helium Network. As the network grows, efforts to prevent Proof-of-Coverage gaming must be bolstered in order to encourage honest Hotspot deployment. One known gaming technique works by altering the information provided by beacon and witness packets to increase reported distances. This can make it appear that Hotspots provide coverage where they do not. An effective way to reduce the impact of this attack is to place an upper limit on the distance at which witnesses for Hotspots will be considered valid by the blockchain and thus be considered for rewards.
When PoCv11 was developed, a witness distance limit feature was written but never activated. If
approved, this proposal would impose a 100 km limit to Proof-of-Coverage witnessing and this value
can be modified further through governance in the future. Witnesses where the asserted distance is
greater than 100 km would be marked invalid by the Challenger. This limit is further supported by
the fact that most sensor deployments have a practical maximum distance of 30-50km. Although a
Helium Hotspot may be able to see PoC beacons greater than 100 km, there are no current use cases at
this range.
Currently there is no sanity check on the reported distances at which witnesses for a Hotspot are considered valid. Since there's a continuous FSPL curve that's used, witness receipts can be manipulated to make it appear that a particular Hotspot falsely provides immense (hundreds, or thousands of kilometers) of coverage. In addition, it is harder to determine a realistic FSPL value at longer distances. This problem results in disproportionate rewards being given to those Hotspots, and subsequently fewer overall rewards sent to Hotspots which provide legitimate coverage.
A hard limit on the reported distances which generate rewards can be applied in a way which does not significantly affect legitimate Hotspots. This is because LoRaWAN works when there is a clear line of sight between the transmitter and the receiver. For example, the vast majority (> 99.9 percent based on 10,000 random witness reports) of Hotspots provide coverage within a radius of less than 100 kilometers. It can also be shown that of the exceptional Hotspots providing legitimate coverage on this scale, any reduction in their expected rewards will be insignificant.
There are two expected outcomes. The first is that gaming techniques which operate on reported distances above the selected threshold will be made impossible. The second outcome is that there will be no measurable reduction in the rewards received by legitimate Hotspots, even for exceptionally high performers such as tower deployments.
All Hotspot owners are affected by this proposal. Other network participants (Validators, Routers, etc) are not affected.
Data transfer remains unaffected.
The distance limit must be chosen so as to not reduce the rewards for legitimate deployments. This can be seen in the following graph, which shows a cumulative histogram of witness distances for a random sample of 10,000 current (< 1 week old) witness receipts from the blockchain.
Almost all of the valid witnesses occur over distances of less than 50 kilometers. The selection of an upper limit can be balanced against the requirement to not affect rewards from legitimate deployments by examining the effect of hypothetical cutoffs on existing witness data. The following is a histogram showing the number of received beacons by given Hotspot (sample size n=10000) expressed as a percentage as a function of the distance to the received beaconer. Lines are drawn at a distance of 100 kilometers and a percentage 0.5 percent.
Fewer than 1.6 percent of beacons are witnessed at distances of greater than 100 kilometers based on this sample. The y axis is shown on a log scale to make the small population at distances greater than 100 kilometers visible.
We show that of the highest performing Hotspots, few of their rewards are due to witnessing beacons from distances above the 100 kilometer cutoff. This ensures that high performing operators, such as those with antennas on towers and other high locations will not be de-incentivised from providing exceptional coverage. Define a high performing Hotspot as one which is in the top 20 percent of total witnessers in the sample. The histogram of witness distances for these Hotspots is:
Of the total number of beacons received by this high performing group, only 1.4 percent were from distances of greater than the threshold of 100 kilometers.
Suppose that 0.1 percent of rewards are currently being distributed to gaming Hotspots who produce false witness reports with distances that fall above the threshold. When these rewards are distributed to legitimate deployments, the 80th percentile witnessers will see a net increase in their rewards, even though they will not be rewarded for witnessing beacons at distances above the threshold. This effect increases if the percentage of gaming at these distances is higher than we assumed. Ordinary Hotspots, who witness beacons at shorter distances, will be unaffected.
The Hotspots that are most impacted in terms of the relative loss on their POC rewards are those that have a significant percentage of witness receipts surpassing the distance threshold. As shown in the following figure, only about 7,000 gateways (out of nearly 700,000 at the time of writing) garner more than 10% of their witness activity from beacons further than 100km away. It also turns out that denylisted Hotspots show a disproportionately high rate of extraordinarily far witness paths; of the gateways that stand to lose more than 50% of their witness activity, nearly a third are already on the denylist.
Further, if we consider the potential effect of the HIP in absolute terms, the difference between legitimate and gamed deployments is even more stark. The below figure plots each Hotspot's total number of witness receipts against the number of witness receipts exceeding the distance threshold (using a 4,000-block sample of network-wide POC activity). Clearly, a relatively small number of denylisted Hotspots mine a significant amount of POC rewards by reporting extreme witness events. It is unrealistic for real-world gateways to witness at such a high rate, almost exclusively at 100km+ distances. While these illegitimate actors can be placed on the denylist, it is more efficient for anti-gaming purposes to build this hard limit directly into the POC algorithm.
This proposal realigns Proof of Coverage closer to effective distances of existing devices and LoRaWAN use cases. Although we acknolwedge that there are a small set of legitimate Hotspots who would be affected by this proposal at 100 km, this wouldn't be a significant impact on their witnessing activity and not affect them at all at lower ranges.
This adjustment to the network is one of many changes that should be made to continue to incentivize good LoRaWAN coverage creation.
- The majority of current Hotspot owners will see no effect.
- The highest performing Hotspots may see an increase in rewards, and at minimum, will not see a decrease.
- Gaming Hotspots will be forced to work with falsely asserted distances under 100 kilometers. This will reduce their earnings, especially in the most egregious cases of witnesses being reported at multiple thousands of kilometers.
- Existing documentation will need to reflect that an upper limit on witness distance exists.
The success of this design can be verified by community members and Hotspot owners. Community members can verify that no Hotspots are being rewarded for distances of greater than the threshold using explorer or the Helium API. Hotspot owners may compute their rewards specifically due to witnesses at larger than the threshold from before this HIP is implemented and verify that they are either zero, or insignificant compared to their rewards from witnesses at distances below the threshold.