Dispute Distribution

Dispute distribution is responsible for ensuring all concerned validators will be aware of a dispute and have the relevant votes.

Design Goals

This design should result in a protocol that is:

  • resilient to nodes being temporarily unavailable
  • make sure nodes are aware of a dispute quickly
  • relatively efficient, should not cause too much stress on the network
  • be resilient when it comes to spam
  • be simple and boring: We want disputes to work when they happen

Protocol

Input

DisputeDistributionMessage

Output

Wire format

Disputes

Protocol: "/polkadot/send_dispute/1"

Request:


#![allow(unused)]
fn main() {
struct DisputeRequest {
  /// The candidate being disputed.
  pub candidate_receipt: CandidateReceipt,

  /// The session the candidate appears in.
  pub session_index: SessionIndex,

  /// The invalid vote data that makes up this dispute.
  pub invalid_vote: InvalidDisputeVote,

  /// The valid vote that makes this dispute request valid.
  pub valid_vote: ValidDisputeVote,
}

/// Any invalid vote (currently only explicit).
pub struct InvalidDisputeVote {
  /// The voting validator index.
  pub validator_index: ValidatorIndex,

  /// The validator signature, that can be verified when constructing a
  /// `SignedDisputeStatement`.
  pub signature: ValidatorSignature,

  /// Kind of dispute statement.
  pub kind: InvalidDisputeStatementKind,
}

/// Any valid vote (backing, approval, explicit).
pub struct ValidDisputeVote {
  /// The voting validator index.
  pub validator_index: ValidatorIndex,

  /// The validator signature, that can be verified when constructing a
  /// `SignedDisputeStatement`.
  pub signature: ValidatorSignature,

  /// Kind of dispute statement.
  pub kind: ValidDisputeStatementKind,
}
}

Response:


#![allow(unused)]
fn main() {
enum DisputeResponse {
  Confirmed
}
}

Vote Recovery

Protocol: "/polkadot/req_votes/1"


#![allow(unused)]
fn main() {
struct IHaveVotesRequest {
  candidate_hash: CandidateHash,
  session: SessionIndex,
  valid_votes: Bitfield,
  invalid_votes: Bitfield,
}

}

Response:


#![allow(unused)]
fn main() {
struct VotesResponse {
  /// All votes we have, but the requester was missing.
  missing: Vec<(DisputeStatement, ValidatorIndex, ValidatorSignature)>,
}
}

Functionality

Distributing disputes needs to be a reliable protocol. We would like to make as sure as possible that our vote got properly delivered to all concerned validators. For this to work, this subsystem won't be gossip based, but instead will use a request/response protocol for application level confirmations. The request will be the payload (the actual votes/statements), the response will be the confirmation. See [above][#wire-format].

Starting a Dispute

A dispute is initiated once a node sends the first DisputeRequest wire message, which must contain an "invalid" vote and a "valid" vote.

The dispute distribution subsystem can get instructed to send that message out to all concerned validators by means of a DisputeDistributionMessage::SendDispute message. That message must contain an invalid vote from the local node and some valid one, e.g. a backing statement.

We include a valid vote as well, so any node regardless of whether it is synced with the chain or not or has seen backing/approval vote can see that there are conflicting votes available, hence we have a valid dispute. Nodes will still need to check whether the disputing votes are somewhat current and not some stale ones.

Participating in a Dispute

Upon receiving a DisputeRequest message, a dispute distribution will trigger the import of the received votes via the dispute coordinator (DisputeCoordinatorMessage::ImportStatements). The dispute coordinator will take care of participating in that dispute if necessary. Once it is done, the coordinator will send a DisputeDistributionMessage::SendDispute message to dispute distribution. From here, everything is the same as for starting a dispute, except that if the local node deemed the candidate valid, the SendDispute message will contain a valid vote signed by our node and will contain the initially received Invalid vote.

Note, that we rely on the coordinator to check availability for spam protection (see below).

Sending of messages

Starting and participating in a dispute are pretty similar from the perspective of dispute distribution. Once we receive a SendDispute message we try to make sure to get the data out. We keep track of all the parachain validators that should see the message, which are all the parachain validators of the session where the dispute happened as they will want to participate in the dispute. In addition we also need to get the votes out to all authorities of the current session (which might be the same or not and may change during the dispute). Those authorities will not participate in the dispute, but need to see the statements so they can include them in blocks.

We keep track of connected parachain validators and authorities and will issue warnings in the logs if connected nodes are less than two thirds of the corresponding sets. We also only consider a message transmitted, once we received a confirmation message. If not, we will keep retrying getting that message out as long as the dispute is deemed alive. To determine whether a dispute is still alive we will issue a DisputeCoordinatorMessage::ActiveDisputes message before each retry run. Once a dispute is no longer live, we will clean up the state accordingly.

Reception & Spam Considerations

Because we are not forwarding foreign statements, spam is less of an issue in comparison to gossip based systems. Rate limiting should be implemented at the substrate level, see #7750. Still we should make sure that it is not possible via spamming to prevent a dispute concluding or worse from getting noticed.

Considered attack vectors:

  1. Invalid disputes (candidate does not exist) could make us run out of resources. E.g. if we recorded every statement, we could run out of disk space eventually.
  2. An attacker can just flood us with notifications on any notification protocol, assuming flood protection is not effective enough, our unbounded buffers can fill up and we will run out of memory eventually.
  3. An attacker could participate in a valid dispute, but send its votes multiple times.
  4. Attackers could spam us at a high rate with invalid disputes. Our incoming queue of requests could get monopolized by those malicious requests and we won't be able to import any valid disputes and we could run out of resources, if we tried to process them all in parallel.

For tackling 1, we make sure to not occupy resources before we don't know a candidate is available. So we will not record statements to disk until we recovered availability for the candidate or know by some other means that the dispute is legit.

For 2, we will pick up on any dispute on restart, so assuming that any realistic memory filling attack will take some time, we should be able to participate in a dispute under such attacks.

Importing/discarding redundant votes should be pretty quick, so measures with regards to 4 should suffice to prevent 3, from doing any real harm.

For 4, full monopolization of the incoming queue should not be possible assuming substrate handles incoming requests in a somewhat fair way. Still we want some defense mechanisms, at the very least we need to make sure to not exhaust resources.

The dispute coordinator will notify us on import about unavailable candidates or otherwise invalid imports and we can disconnect from such peers/decrease their reputation drastically. This alone should get us quite far with regards to queue monopolization, as availability recovery is expected to fail relatively quickly for unavailable data.

Still if those spam messages come at a very high rate, we might still run out of resources if we immediately call DisputeCoordinatorMessage::ImportStatements on each one of them. Secondly with our assumption of 1/3 dishonest validators, getting rid of all of them will take some time, depending on reputation timeouts some of them might even be able to reconnect eventually.

To mitigate those issues we will process dispute messages with a maximum parallelism N. We initiate import processes for up to N candidates in parallel. Once we reached N parallel requests we will start back pressuring on the incoming requests. This saves us from resource exhaustion.

To reduce impact of malicious nodes further, we can keep track from which nodes the currently importing statements came from and will drop requests from nodes that already have imports in flight.

Honest nodes are not expected to send dispute statements at a high rate, but even if they did:

  • we will import at least the first one and if it is valid it will trigger a dispute, preventing finality.
  • Chances are good that the first sent candidate from a peer is indeed the oldest one (if they differ in age at all).
  • for the dropped request any honest node will retry sending.
  • there will be other nodes notifying us about that dispute as well.
  • honest votes have a speed advantage on average. Apart from the very first dispute statement for a candidate, which might cause the availability recovery process, imports of honest votes will be super fast, while for spam imports they will always take some time as we have to wait for availability to fail.

So this general rate limit, that we drop requests from same peers if they come faster than we can import the statements should not cause any problems for honest nodes and is in their favor.

Size of N: The larger N the better we can handle distributed flood attacks (see previous paragraph), but we also get potentially more availability recovery processes happening at the same time, which slows down the individual processes. And we rather want to have one finish quickly than lots slowly at the same time. On the other hand, valid disputes are expected to be rare, so if we ever exhaust N it is very likely that this is caused by spam and spam recoveries don't cost too much bandwidth due to empty responses.

Considering that an attacker would need to attack many nodes in parallel to have any effect, an N of 10 seems to be a good compromise. For honest requests, most of those imports will likely concern the same candidate, and for dishonest ones we get to disconnect from up to ten colluding adversaries at a time.

For the size of the channel for incoming requests: Due to dropping of repeated requests from same nodes we can make the channel relatively large without fear of lots of spam requests sitting there wasting our time, even after we already blocked a peer. For valid disputes, incoming requests can become bursty. On the other hand we will also be very quick in processing them. A channel size of 100 requests seems plenty and should be able to handle bursts adequately.

Node Startup

On startup we need to check with the dispute coordinator for any ongoing disputes and assume we have not yet sent our statement for those. In case we find an explicit statement from ourselves via DisputeCoordinatorMessage::QueryCandidateVotes we will pretend to just have received a SendDispute message for that candidate.

Backing and Approval Votes

Backing and approval votes get imported when they arrive/are created via the dispute coordinator by corresponding subsystems.

We assume that under normal operation each node will be aware of backing and approval votes and optimize for that case. Nevertheless we want disputes to conclude fast and reliable, therefore if a node is not aware of backing/approval votes it can request the missing votes from the node that informed it about the dispute (see Resiliency

Resiliency

The above protocol should be sufficient for most cases, but there are certain cases we also want to have covered:

  • Non validator nodes might be interested in ongoing voting, even before it is recorded on chain.
  • Nodes might have missed votes, especially backing or approval votes. Recovering them from chain is difficult and expensive, due to runtime upgrades and untyped extrinsics.

To cover those cases, we introduce a second request/response protocol, which can be handled on a lower priority basis as the one above. It consists of the request/response messages as described in the [protocol section][#vote-recovery].

Nodes may send those requests to validators, if they feel they are missing votes. E.g. after some timeout, if no majority was reached yet in their point of view or if they are not aware of any backing/approval votes for a received disputed candidate.

The receiver of a IHaveVotesRequest message will do the following:

  1. See if the sender is missing votes we are aware of - if so, respond with those votes.
  2. Check whether the sender knows about any votes, we don't know about and if so send a IHaveVotesRequest request back, with our knowledge.
  3. Record the peer's knowledge.

When to send IHaveVotesRequest messages:

  1. Whenever we are asked to do so via DisputeDistributionMessage::FetchMissingVotes.
  2. Approximately once per block to some random validator as long as the dispute is active.

Spam considerations: Nodes want to accept those messages once per validator and per slot. They are free to drop more frequent requests or requests for stale data. Requests coming from non validator nodes, can be handled on a best effort basis.

Considerations

Dispute distribution is critical. We should keep track of available validator connections and issue warnings if we are not connected to a majority of validators. We should also keep track of failed sending attempts and log warnings accordingly. As disputes are rare and TCP is a reliable protocol, probably each failed attempt should trigger a warning in logs and also logged into some Prometheus metric.

Disputes for non available candidates

If deemed necessary we can later on also support disputes for non available candidates, but disputes for those cases have totally different requirements.

First of all such disputes are not time critical. We just want to have some offender slashed at some point, but we have no risk of finalizing any bad data.

Second, as we won't have availability for such data, the node that initiated the dispute will be responsible for providing the disputed data initially. Then nodes which did the check already are also providers of the data, hence distributing load and making prevention of the dispute from concluding harder and harder over time. Assuming an attacker can not DoS a node forever, the dispute will succeed eventually, which is all that matters. And again, even if an attacker managed to prevent such a dispute from happening somehow, there is no real harm done: There was no serious attack to begin with.