rfc-017-abci++-vote-extension-propag.md

RFC 017: ABCI++ Vote Extension Propagation

Changelog

• 11-Apr-2022: Initial draft (@sergio-mena).
• 15-Apr-2022: Addressed initial comments. First complete version (@sergio-mena).
• 09-May-2022: Addressed all outstanding comments.

Abstract

According to the ABCI++ specification (as of 11-Apr-2022), a validator MUST provide a signed vote extension for each non-nil precommit vote of height h that it uses to propose a block in height h+1. When a validator is up to date, this is easy to do, but when a validator needs to catch up this is far from trivial as this data cannot be retrieved from the blockchain.

This RFC presents and compares the different options to address this problem, which have been proposed in several discussions by the Tendermint Core team.

Document Structure

The RFC is structured as follows. In the Background section, subsections Problem Description and Cases to Address explain the problem at hand from a high level perspective, i.e., abstracting away from the current Tendermint implementation. In contrast, subsection Current Catch-up Mechanisms delves into the details of the current Tendermint code.

In the Discussion section, subsection Solutions Proposed is also worded abstracting away from implementation details, whilst subsections Feasibility of the Proposed Solutions and Current Limitations and Possible Implementations analize the viability of one of the proposed solutions in the context of Tendermint's architecture based on reactors. Finally, Formalization Work briefly discusses the work still needed demonstrate the correctness of the chosen solution.

The high level subsections are aimed at readers who are familiar with consensus algorithms, in particular with the one described in the Tendermint (white paper), but who are not necessarily acquainted with the details of the Tendermint codebase. The other subsections, which go into implementation details, are best understood by engineers with deep knowledge of the implementation of Tendermint's blocksync and consensus reactors.

Background

Basic Definitions

This document assumes that all validators have equal voting power for the sake of simplicity. This is done without loss of generality.

There are two types of votes in Tendermint: prevotes and precommits. Votes can be nil or refer to a proposed block. This RFC focuses on precommits, also known as precommit votes. In this document we sometimes call them simply votes.

Validators send precommit votes to their peer nodes in precommit messages. According to the ABCI++ specification, a precommit message MUST also contain a vote extension. This mandatory vote extension can be empty, but MUST be signed with the same key as the precommit vote (i.e., the sending validator's). Nevertheless, the vote extension is signed independently from the vote, so a vote can be separated from its extension. The reason for vote extensions to be mandatory in precommit messages is that, otherwise, a (malicious) node can omit a vote extension while still providing/forwarding/sending the corresponding precommit vote.

The validator set at height h is denoted valset_h. A commit for height h consists of more than 2n_h/3 precommit votes voting for a block b, where n_h denotes the size of valset_h. A commit does not contain nil precommit votes, and all votes in it refer to the same block. An extended commit is a commit where every precommit vote has its respective vote extension attached.

Problem Description

In the version of ABCI present up to Tendermint v0.35, for any height h, a validator v MUST have the decided block b and a commit for height h in order to decide at height h. Then, v just needs a commit for height h to propose at height h+1, in the rounds of h+1 where v is a proposer.

In ABCI++, the information that a validator v MUST have to be able to decide in h does not change with respect to pre-existing ABCI: the decided block b and a commit for h. In contrast, for proposing in h+1, a commit for h is not enough: v MUST now have an extended commit.

When a validator takes an active part in consensus at height h, it has all the data it needs in memory, in its consensus state, to decide on h and propose in h+1. Things are not so easy in the cases when v cannot take part in consensus because it is late (e.g., it falls behind, it crashes and recovers, or it just starts after the others). If v does not take part, it cannot actively gather precommit messages (which include vote extensions) in order to decide. Before ABCI++, this was not a problem: full nodes are supposed to persist past blocks in the block store, so other nodes would realise that v is late and send it the missing decided block at height h and the corresponding commit (kept in block h+1) so that v can catch up. However, we cannot apply this catch-up technique for ABCI++, as the vote extensions, which are part of the needed extended commit are not part of the blockchain.

Cases to Address

Before we tackle the description of the possible cases we need to address, let us describe the following incremental improvement to the ABCI++ logic. Upon decision, a full node persists (e.g., in the block store) the extended commit that allowed the node to decide. For the moment, let us assume the node only needs to keep its most recent extended commit, and MAY remove any older extended commits from persistent storage. This improvement is so obvious that all solutions described in the Discussion section use it as a building block. Moreover, it completely addresses by itself some of the cases described in this subsection.

We now describe the cases (i.e. possible runs of the system) that have been raised in different discussions and need to be addressed. They are (roughly) ordered from easiest to hardest to deal with.

• (a) Happy path: all validators advance together, no crash.

  This case is included for completeness. All validators have taken part in height h. Even if some of them did not manage to send a precommit message for the decided block, they all receive enough precommit messages to be able to decide. As vote extensions are mandatory in precommit messages, every validator v trivially has all the information, namely the decided block and the extended commit, needed to propose in height h+1 for the rounds in which v is the proposer.

  No problem to solve here.

• (b) All validators advance together, then all crash at the same height.

  This case has been raised in some discussions, the main concern being whether the vote extensions for the previous height would be lost across the network. With the improvement described above, namely persisting the latest extended commit at decision time, this case is solved. When a crashed validator recovers, it recovers the last extended commit from persistent storage and handshakes with the Application. If need be, it also reconstructs messages for the unfinished height (including all precommits received) from the WAL. Then, the validator can resume where it was at the time of the crash. Thus, as extensions are persisted, either in the WAL (in the form of received precommit messages), or in the latest extended commit, the only way that vote extensions needed to start the next height could be lost forever would be if all validators crashed and never recovered (e.g. disk corruption). Since a correct node MUST eventually recover, this violates Tendermint's assumption of more than 2n_h/3 correct validators for every height h.

  No problem to solve here.

• (c) Lagging majority.

  Let us assume the validator set does not change between h and h+1. It is not possible by the nature of the Tendermint algorithm, which requires more than 2n_h/3 precommit votes for some round of height h in order to make progress. So, only up to n_h/3 validators can lag behind.

  On the other hand, for the case where there are changes to the validator set between h and h+1 please see case (d) below, where the extreme case is discussed.

• (d) Validator set changes completely between h and h+1.

  If sets valset_h and valset_h+1 are disjoint, more than 2n_h/3 of validators in height h should have actively participated in conensus in h. So, as of height h, only a minority of validators in h can be lagging behind, although they could all lag behind from h+1 on, as they are no longer validators, only full nodes. This situation falls under the assumptions of case (h) below.

  As for validators in valset_h+1, as they were not validators as of height h, they could all be lagging behind by that time. However, by the time h finishes and h+1 begins, the chain will halt until more than 2n_h+1/3 of them have caught up and started consensus at height h+1. If set valset_h+1 does not change in h+2 and subsequent heights, only up to n_h+1/3 validators will be able to lag behind. Thus, we have converted this case into case (h) below.

• (e) Enough validators crash to block the rest.

  In this case, blockchain progress halts, i.e. surviving full nodes keep increasing rounds indefinitely, until some of the crashed validators are able to recover. Those validators that recover first will handshake with the Application and recover at the height they crashed, which is still the same the nodes that did not crash are stuck in, so they don't need to catch up. Further, they had persisted the extended commit for the previous height. Nothing to solve.

  For those validators recovering later, we are in case (h) below.

• (f) Some validators crash, but not enough to block progress.

  When the correct processes that crashed recover, they handshake with the Application and resume at the height they were at when they crashed. As the blockchain did not stop making progress, the recovered processes are likely to have fallen behind with respect to the progressing majority.

  At this point, the recovered processes are in case (h) below.

• (g) A new full node starts.

  The reasoning here also applies to the case when more than one full node are starting. When the full node starts from scratch, it has no state (its current height is 0). Ignoring statesync for the time being, the node just needs to catch up by applying past blocks one by one (after verifying them).

  Thus, the node is in case (h) below.

• (h) Advancing majority, lagging minority

  In this case, some nodes are late. More precisely, at the present time, a set of full nodes, denoted L_hp, are falling behind (e.g., temporary disconnection or network partition, memory thrashing, crashes, new nodes) an arbitrary number of heights: between h_s and h_p, where h_s < h_p, and h_p is the highest height any correct full node has reached so far.

  The correct full nodes that reached h_p were able to decide for h_p-1. Therefore, less than n_hp-1/3 validators of h_p-1 can be part of L_hp, since enough up-to-date validators needed to actively participate in consensus for h_p-1.

  Since, at the present time, no node in L_hp took part in any consensus between h_s and h_p-1, the reasoning above can be extended to validator set changes between h_s and h_p-1. This results in the following restriction on the full nodes that can be part of L_hp.

  • ∀ h, where h_s ≤ h < h_p, | valset_h ∩ L_hp | < n_h/3

  If this property does not hold for a particular height h, where h_s ≤ h < h_p, Tendermint could not have progressed beyond h and therefore no full node could have reached h_p (a contradiction).

  These lagging nodes in L_hp need to catch up. They have to obtain the information needed to make progress from other nodes. For each height h between h_s and h_p-2, this includes the decided block for h, and the precommit votes also for deciding h (which can be extracted from the block at height h+1).

  At a given height h_c (where possibly h_c << h_p), a full node in L_hp will consider itself caught up, based on the (maybe out of date) information it is getting from its peers. Then, the node needs to be ready to propose at height h_c+1, which requires having received the vote extensions for h_c. As the vote extensions are not stored in the blocks, and it is difficult to have strong guarantees on when a late node considers itself caught up, providing the late node with the right vote extensions for the right height poses a problem.

At this point, we have described and compared all cases raised in discussions leading up to this RFC. The list above aims at being exhaustive. The analysis of each case included above makes all of them converge into case (h).

Current Catch-up Mechanisms

We now briefly describe the current catch-up mechanisms in the reactors concerned in Tendermint.

Statesync

Full nodes optionally run statesync just after starting, when they start from scratch. If statesync succeeds, an Application snapshot is installed, and Tendermint jumps from height 0 directly to the height the Application snapshop represents, without applying the block of any previous height. Some light blocks are received and stored in the block store for running light-client verification of all the skipped blocks. Light blocks are incomplete blocks, typically containing the header and the canonical commit but, e.g., no transactions. They are stored in the block store as "signed headers".

The statesync reactor is not really relevant for solving the problem discussed in this RFC. We will nevertheless mention it when needed; in particular, to understand some corner cases.

Blocksync

The blocksync reactor kicks in after start up or recovery (and, optionally, after statesync is done) and sends the following messages to its peers:

• StatusRequest to query the height its peers are currently at, and
• BlockRequest, asking for blocks of heights the local node is missing.

Using BlockResponse messages received from peers, the blocksync reactor validates each received block using the block of the following height, saves the block in the block store, and sends the block to the Application for execution.

If blocksync has validated and applied the block for the height previous to the highest seen in a StatusResponse message, or if no progress has been made after a timeout, the node considers itself as caught up and switches to the consensus reactor.

Consensus Reactor

The consensus reactor runs the full Tendermint algorithm. For a validator this means it has to propose blocks, and send/receive prevote/precommit messages, as mandated by Tendermint, before it can decide and move on to the next height.

If a full node that is running the consensus reactor falls behind at height h, when a peer node realises this it will retrieve the canonical commit of h+1 from the block store, and convert it into a set of precommit votes and will send those to the late node.

Discussion

Solutions Proposed

These are the solutions proposed in discussions leading up to this RFC.

• Solution 0. Vote extensions are made best effort in the specification.

  This is the simplest solution, considered as a way to provide vote extensions in a simple enough way so that it can be part of v0.36. It consists in changing the specification so as to not require that precommit votes used upon PrepareProposal contain their corresponding vote extensions. In other words, we render vote extensions optional. There are strong implications stemming from such a relaxation of the original specification.

  • As a vote extension is signed separately from the vote it is extending, an intermediate node can now remove (i.e., censor) vote extensions from precommit messages at will.
  • Further, there is no point anymore in the spec requiring the Application to accept a vote extension passed via VerifyVoteExtension to consider a precommit message valid in its entirety. Remember this behavior of VerifyVoteExtension is adding a constraint to Tendermint's conditions for liveness. In this situation, it is better and simpler to just drop the vote extension rejected by the Application via VerifyVoteExtension, but still consider the precommit vote itself valid as long as its signature verifies.

• Solution 1. Include vote extensions in the blockchain.

  Another obvious solution, which has somehow been considered in the past, is to include the vote extensions and their signatures in the blockchain. The blockchain would thus include the extended commit, rather than a regular commit, as the structure to be canonicalized in the next block. With this solution, the current mechanisms implemented both in the blocksync and consensus reactors would still be correct, as all the information a node needs to catch up, and to start proposing when it considers itself as caught-up, can now be recovered from past blocks saved in the block store.

  This solution has two main drawbacks.

  • As the block format must change, upgrading a chain requires a hard fork. Furthermore, all existing light client implementations will stop working until they are upgraded to deal with the new format (e.g., how certain hashes calculated and/or how certain signatures are checked). For instance, let us consider IBC, which relies on light clients. An IBC connection between two chains will be broken if only one chain upgrades.
  • The extra information (i.e., the vote extensions) that is now kept in the blockchain is not really needed at every height for a late node to catch up.
    • This information is only needed to be able to propose at the height the validator considers itself as caught-up. If a validator is indeed late for height h, it is useless (although correct) for it to call PrepareProposal, or ExtendVote, since the block is already decided.
    • Moreover, some use cases require pretty sizeable vote extensions, which would result in an important waste of space in the blockchain.

• Solution 2. Skip propose step in Tendermint algorithm.

  This solution consists in modifying the Tendermint algorithm to skip the send proposal step in heights where the node does not have the required vote extensions to populate the call to PrepareProposal. The main idea behind this is that it should only happen when the validator is late and, therefore, up-to-date validators have already proposed (and decided) for that height. A small variation of this solution is, rather than skipping the send proposal step, the validator sends a special empty or bottom (⊥) proposal to signal other nodes that it is not ready to propose at (any round of) the current height.

  The appeal of this solution is its simplicity. A possible implementation does not need to extend the data structures, or change the current catch-up mechanisms implemented in the blocksync or in the consensus reactor. When we lack the needed information (vote extensions), we simply rely on another correct validator to propose a valid block in other rounds of the current height.

  However, this solution can be attacked by a byzantine node in the network in the following way. Let us consider the following scenario:

  • all validators in valset_h send out precommit messages, with vote extensions, for height h, round 0, roughly at the same time,
  • all those precommit messages contain non-nil precommit votes, which vote for block b
  • all those precommit messages sent in height h, round 0, and all messages sent in height h, round r > 0 get delayed indefinitely, so,
  • all validators in valset_h keep waiting for enough precommit messages for height h, round 0, needed for deciding in height h
  • an intermediate (malicious) full node m manages to receive block b, and gather more than 2n_h/3 precommit messages for height h, round 0,
  • one way or another, the solution should have either (a) a mechanism for a full node to tell another full node it is late, or (b) a mechanism for a full node to conclude it is late based on other full nodes' messages; any of these mechanisms should, at the very least, require the late node receiving the decided block and a commit (not necessarily an extended commit) for h,
  • node m uses the gathered precommit messages to build a commit for height h, round 0,
  • in order to convince full nodes that they are late, node m either (a) tells them they are late, or (b) shows them it (i.e. m) is ahead, by sending them block b, along with the commit for height h, round 0,
  • all full nodes conclude they are late from m's behavior, and use block b and the commit for height h, round 0, to decide on height h, and proceed to height h+1.

  At this point, all full nodes, including all validators in valset_h+1, have advanced to height h+1 believing they are late, and so, expecting the hypothetical leading majority of validators in valset_h+1 to propose for h+1. As a result, the blockhain grinds to a halt. A (rather complex) ad-hoc mechanism would need to be carried out by node operators to roll back all validators to the precommit step of height h, round r, so that they can regenerate vote extensions (remember vote extensions are non-deterministic) and continue execution.

• Solution 3. Require extended commits to be available at switching time.

  This one is more involved than all previous solutions, and builds on an idea present in Solution 2: vote extensions are actually not needed for Tendermint to make progress as long as the validator is certain it is late.

  We define two modes. The first is denoted catch-up mode, and Tendermint only calls FinalizeBlock for each height when in this mode. The second is denoted consensus mode, in which the validator considers itself up to date and fully participates in consensus and calls PrepareProposal/ProcessProposal, ExtendVote, and VerifyVoteExtension, before calling FinalizeBlock.

  The catch-up mode does not need vote extension information to make progress, as all it needs is the decided block at each height to call FinalizeBlock and keep the state-machine replication making progress. The consensus mode, on the other hand, does need vote extension information when starting every height.

  Validators are in consensus mode by default. When a validator in consensus mode falls behind for whatever reason, e.g. cases (b), (d), (e), (f), (g), or (h) above, we introduce the following key safety property:

  • for every height h_p, a full node f in h_p refuses to switch to catch-up mode until there exists a height h' such that:
    • p has received and (light-client) verified the blocks of all heights h, where h_p ≤ h ≤ h'
    • it has received an extended commit for h' and has verified:
      • the precommit vote signatures in the extended commit
      • the vote extension signatures in the extended commit: each is signed with the same key as the precommit vote it extends

  If the condition above holds for h_p, namely receiving a valid sequence of blocks in the f's future, and an extended commit corresponding to the last block in the sequence, then node f:

  • switches to catch-up mode,
  • applies all blocks between h_p and h' (calling FinalizeBlock only), and
  • switches back to consensus mode using the extended commit for h' to propose in the rounds of h' + 1 where it is the proposer.

  This mechanism, together with the invariant it uses, ensures that the node cannot be attacked by being fed a block without extensions to make it believe it is late, in a similar way as explained for Solution 2.

Feasibility of the Proposed Solutions

Solution 0, besides the drawbacks described in the previous section, provides guarantees that are weaker than the rest. The Application does not have the assurance that more than 2n_h/3 vote extensions will always be available when calling PrepareProposal at height h+1. This level of guarantees is probably not strong enough for vote extensions to be useful for some important use cases that motivated them in the first place, e.g., encrypted mempool transactions.

Solution 1, while being simple in that the changes needed in the current Tendermint codebase would be rather small, is changing the block format, and would therefore require all blockchains using Tendermint v0.35 or earlier to hard-fork when upgrading to v0.36.

Since Solution 2 can be attacked, one might prefer Solution 3, even if it is more involved to implement. Further, we must elaborate on how we can turn Solution 3, described in abstract terms in the previous section, into a concrete implementation compatible with the current Tendermint codebase.

Current Limitations and Possible Implementations

The main limitations affecting the current version of Tendermint are the following.

• The current version of the blocksync reactor does not use the full light client verification algorithm to validate blocks coming from other peers.
• The code being structured into the blocksync and consensus reactors, only switching from the blocksync reactor to the consensus reactor is supported; switching in the opposite direction is not supported. Alternatively, the consensus reactor could have a mechanism allowing a late node to catch up by skipping calls to PrepareProposal/ProcessProposal, and ExtendVote/VerifyVoteExtension and only calling FinalizeBlock for each height. Such a mechanism does not exist at the time of writing this RFC.

The blocksync reactor featuring light client verification is being actively worked on (tentatively for v0.37). So it is best if this RFC does not try to delve into that problem, but just makes sure its outcomes are compatible with that effort.

In subsection Cases to Address, we concluded that we can focus on solving case (h) in theoretical terms. However, as the current Tendermint version does not yet support switching back to blocksync once a node has switched to consensus, we need to split case (h) into two cases. When a full node needs to catch up...

• (h.1) ... it has not switched yet from the blocksync reactor to the consensus reactor, or

• (h.2) ... it has already switched to the consensus reactor.

This is important in order to discuss the different possible implementations.

Base Implementation: Persist and Propagate Extended Commit History

In order to circumvent the fact that we cannot switch from the consensus reactor back to blocksync, rather than just keeping the few most recent extended commits, nodes will need to keep and gossip a backlog of extended commits so that the consensus reactor can still propose and decide in out-of-date heights (even if those proposals will be useless).

The base implementation － for which an experimental patch exists － consists in the conservative approach of persisting in the block store all extended commits for which we have also stored the full block. Currently, when statesync is run at startup, it saves light blocks. This base implementation does not seek to receive or persist extended commits for those light blocks as they would not be of any use.

Then, we modify the blocksync reactor so that peers always send requested full blocks together with the corresponding extended commit in the BlockResponse messages. This guarantees that the block store being reconstructed by blocksync has the same information as that of peers that are up to date (at least starting from the latest snapshot applied by statesync before starting blocksync). Thus, blocksync has all the data it requires to switch to the consensus reactor, as long as one of the following exit conditions are met:

• The node is still at height 0 (where no commit or extended commit is needed)
• The node has processed at least 1 block in blocksync

The second condition is needed in case the node has installed an Application snapshot during statesync. If that is the case, at the time blocksync starts, the block store only has the data statesync has saved: light blocks, and no extended commits. Hence we need to blocksync at least one block from another node, which will be sent with its corresponding extended commit, before we can switch to consensus.

As a side note, a chain might be started at a height h_i > 0, all other heights h < h_i being non-existent. In this case, the chain is still considered to be at height 0 before block h_i is applied, so the first condition above allows the node to switch to consensus even if blocksync has not processed any block (which is always the case if all nodes are starting from scratch).

When a validator falls behind while having already switched to the consensus reactor, a peer node can simply retrieve the extended commit for the required height from the block store and reconstruct a set of precommit votes together with their extensions and send them in the form of precommit messages to the validator falling behind, regardless of whether the peer node holds the extended commit because it actually participated in that consensus and thus received the precommit messages, or it received the extended commit via a BlockResponse message while running blocksync.

This solution requires a few changes to the consensus reactor:

• upon saving the block for a given height in the block store at decision time, save the corresponding extended commit as well
• in the catch-up mechanism, when a node realizes that another peer is more than 2 heights behind, it uses the extended commit (rather than the canoncial commit as done previously) to reconstruct the precommit votes with their corresponding extensions

The changes to the blocksync reactor are more substantial:

• the BlockResponse message is extended to include the extended commit of the same height as the block included in the response (just as they are stored in the block store)
• structure bpRequester is likewise extended to hold the received extended commits coming in BlockResponse messages
• method PeekTwoBlocks is modified to also return the extended commit corresponding to the first block
• when successfully verifying a received block, the reactor saves its corresponding extended commit in the block store

The two main drawbacks of this base implementation are:

• the increased size taken by the block store, in particular with big extensions
• the increased bandwith taken by the new format of BlockResponse

Possible Optimization: Pruning the Extended Commit History

If we cannot switch from the consensus reactor back to the blocksync reactor we cannot prune the extended commit backlog in the block store without sacrificing the implementation's correctness. The asynchronous nature of our distributed system model allows a process to fall behing an arbitrary number of heights, and thus all extended commits need to be kept just in case a node that late had previously switched to the consensus reactor.

However, there is a possibility to optimize the base implementation. Every time we enter a new height, we could prune from the block store all extended commits that are more than d heights in the past. Then, we need to handle two new situations, roughly equivalent to cases (h.1) and (h.2) described above.

• (h.1) A node starts from scratch or recovers after a crash. In thisy case, we need to modify the blocksync reactor's base implementation.
  • when receiving a BlockResponse message, it MUST accept that the extended commit set to nil,
  • when sending a BlockResponse message, if the block store contains the extended commit for that height, it MUST set it in the message, otherwise it sets it to nil,
  • the exit conditions used for the base implementation are no longer valid; the only reliable exit condition now consists in making sure that the last block processed by blocksync was received with the corresponding commit, and not nil; this extended commit will allow the node to switch from the blocksync reactor to the consensus reactor and immediately act as a proposer if required.
• (h.2) A node already running the consensus reactor falls behind beyond d heights. In principle, the node will be stuck forever as no other node can provide the vote extensions it needs to make progress (they all have pruned the corresponding extended commit). However we can manually have the node crash and recover as a workaround. This effectively converts this case into (h.1).

Formalization Work

A formalization work to show or prove the correctness of the different use cases and solutions presented here (and any other that may be found) needs to be carried out. A question that needs a precise answer is how many extended commits (one?, two?) a node needs to keep in persistent memory when implementing Solution 3 described above without Tendermint's current limitations. Another important invariant we need to prove formally is that the set of vote extensions required to make progress will always be held somewhere in the network.

References

• ABCI++ specification
• ABCI as of v0.35
• Vote extensions issue
• Light client verification