- Author(s): @macpie, @michaeldjeffrey, @Vagabond, @abhay, et al
- Start Date: 2022-02-21
- Category: Technical
- Original HIP PR: helium#366
- Tracking Issue: helium#369
This HIP describes an update to the dispute mechanism for State Channels that would improve blockchain scaling, better reconcile Data Transfer activity and rewards for Hotspots, and address recent issues around large block sizes.
State Channel disputes provide a mechanism to validate the integrity of a State Channel. Previously, disputes triggered by participating Hotspots contained redundant information, and increased block size. On a couple of occasions the number of disputes caused some large blocks to halt chain activity due to Validators not being able to consume them. This proposal retains the State Channel integrity check by any participant, but also updates the dispute mechanism to prevent disputes from halting chain activity.
We propose that the first valid instance of a disputed State Channel that is accepted by Validators becomes canonical and does the following:
- All other disputes are marked as invalid and are rejected by the Consensus Group.
- No rewards are distributed to any participant of the State Channel.
- The State Channel owner will still be slashed of its Data Credits and its Data Credit overage. As a note, this point is the current implementation that will not be changed with this proposal.
We believe these three steps (with the changes included) will reduce the impact of State Channel disputes on the overall blockchain and not inadvertently reward (potentially) colluding actors who could be in control of both the Hotspot and the Router. This would allow the blockchain to increase the State Channel Grace Period (described below) and better reward Hotpots in the majority of cases where Routers are able to close State Channels promptly and Hotspots do not need to file disputes.
Failures in Routers closing State Channels within the network-specified block interval has caused participating Hotspots to file disputes on chain leading to bloated blocks resulting in a few chain halts in the last few months. These halts are due to Validators not being able to ingest the series of large dispute transactions. Routers may occasionally fail to close their State Channels and thus trigger a cascade of events that may eventually lead to a stream of disputes. To improve the resiliency of the blockchain, we believe that it's appropriate to be intolerant to potentially misbehaving Routers and/or Hotspots.
All Router operators and Hotspot owners are directly affected by this HIP. When a State Channel is validly disputed, Router Operators will be slashed their Data Credits, and participating Hotspots will not be rewarded.
Routers open and close State Channels in order to buy packets on the LoRaWAN network. You can read
more about how State Channels work in this blog post. The Router is given a grace period,
defined as the sc_grace_blocks chain variable, to close their State Channel after it is supposed
to expire. Once the Router submits the close transaction, the current dispute mechanism allows for
any Hotspot to file a State Channel close transaction as a dispute, if they disagree with the
initial close of the State Channel. Since Routers accept/attest that they will buy a packet before
the Hotspot sends it to them, the Hotspot has all the information it needs to dispute the State
Channel.
State Channel close transactions are already fairly large since they can contain up to 2000 Hotspot
addresses, controlled by the sc_max_actors chain variable. A disputed close transaction can
potentially be twice this size. In the past few months, this has become problematic as having
several of these disputes in a single block can cause block absorption issues and chain halts.
Moreover, it complicates rewards calculations as Validators will attempt to reconcile these disputes
and will, most of the time, not be able to reward all Hotspots in the State Channel equitably.
This proposal includes the following changes to insure that disputes do not cause the chain to slow down or halt:
- As soon as one dispute is filed by a Hotspot, subsequent dispute transactions will be marked invalid and rejected. This will avoid the creation of massive blocks containing too many State Channel close transactions.
- If a dispute is filed against a State Channel close transaction, no rewards will be distributed to any participant of this state channel.
- The State Channel owner will still, like before, lose its Data Credits and the Overage.
It is safer to not reward any participant for several reasons. It can be computationally expensive to distribute all rewards to each participant involved if the dispute and original close have a large discrepancy. Also, a Hotspot and Router could conspire to publish a fake dispute and assign all rewards to one or multiple Hotspots. By potentially slashing the Router's Data Credits, this approach avoids that risk.
All of these changes will be enabled by a chain variable: sc_dispute_strategy_version.
This proposal is currently implemented in blockchain-core#1229 and blockchain-core#1238.
As soon as a State Channel close is disputed no rewards will be distributed for this Data Transfer activity.
Normal users should not expect to see a measurable impact, other than improvements to chain performance.