Herein we present the specification for a decentralized mining pool we name "Braidpool". For background information and general considerations please read General Considerations for Decentralized Mining Pools which has relevant general discussion omitted from this document. The sections below correspond to the sections in that document, describing how Braidpool will solve each of the indicated issues. Orthogonal considerations including encrypted miner communication is being pursued by the StratumV2 project, which Braidpool will build upon.
- Shares and Weak Blocks
- Braid Consensus Mechansim
- Payout Update
- Payout Update and Settlement Signing
- Transaction Selection
A share is a "weak block" that is defined as a standard bitcoin block that
does not meet bitcoin's target difficulty
The share is itself a bearer proof that approximately
Shares or blocks which do not commit to the additional metadata proving that the share is part of the Braidpool must be excluded from the share calculation, and those miners are not "part of" the pool. In other words, submitting a random sha256 header to a Braidpool node must not count as a share contribution unless the ultimate payout for that share, had it become a bitcoin block, would have paid all members of the pool in such a way that all other hashers are paid.
A Braidpool "share" or "bead" is a data structure containing a bitcoin block header, the coinbase transaction, and metadata:
| Field | Description |
|---|---|
blockheader |
Version, Previous Block Hash, Merkle Root, Timestamp, Target, Nonce |
coinbase |
Coinbase Txn, Merkle Sibling, Merkle Sibling, ... |
payout |
Payout Update Txn, Merkle Sibling, Merkle Sibling, ... |
metadata |
Braidpool Metadata (see below) |
un_metadata |
Uncommitted Metadata (see below) |
The first line is a standard Bitcoin block header. The Merkle Siblings in the
second and third line are the additional nodes in the transaction Merkle tree
necessary to verify that the specified Coinbase Transaction and Payout Commitment transactions are included in the Merkle Root. This Coinbase Transaction commits to any additional data needed for the Braidpool's braid
consensus mechansim, in an OP_RETURN output.
While we could commit to this data in a more space-efficient manner (e.g. via a
pubkey tweak), the coinbase is also the location of the extranonce 8-byte
field used by some mining equipment.
The Coinbase Transaction is a standard transaction having no inputs, and
must have the following outputs:
OutPoint(Value:0, scriptPubKey OP_RETURN "BP"+<Braidpool Commitment>+<extranonce>)
OutPoint(Value:<block reward>, scriptPubKey <P2TR pool_pubkey>)
The <block reward> is the sum of all fees and block reward for this halving
epoch, and pool_pubkey is an address controlled collaboratively by the pool in
such a way that the braid consensus mechanism can
only spend it in such a way as to pay all hashers in the manner described by its
share accounting.
The <Braidpool Commitment> is a hash of Braidpool Metadata committing to
additional data required for the operation of the pool. Validation of this share
requires that the PoW hash of this bitcoin header be less than this weak
difficulty target
The Braidpool Metadata is:
| Field | Description |
|---|---|
target |
Miner-selected target difficulty |
payout_pubkey |
P2TR pubkey for this miner's payout |
comm_pubkey |
secp256k1 pubkey for encrypted DH communication with this miner |
miner IP |
IP address of this miner |
[[parent, timestamp], ...] |
An array of block hashes of parent beads and timestamps when those parents were seen |
The Uncommitted Metadata block is intentionally not committed to in the PoW
mining process. It contains:
| Field | Description |
|---|---|
timestamp |
timestamp when this bead was broadcast |
signature |
Signature on the Uncommitted Metadata block using the payout_pubkey |
The purpose of this data is to gather higher resolution timestamps than are possible if the timestamp was committed. All Braidpool timestamps are 64-bit fields as milliseconds since the Unix epoch. When a block header is sent to mining devices, many manufacturers' mining devices do not return for quite some time (10-60 seconds) while they compute the hash, which causes PoW-mined timestamps to be delayed by this amount. Adding timestamps when parents were seen by the node and a timestamp when the bead was broadcast allows the braid to compute bead times with much higher precision. Though the data is uncommitted in the PoW header, it is signed by a key that is committed in the PoW header, so third parties cannot falsify these timestamps.
A great many share payout algorithms have been proposed and used by pools. Because Braidpool will not collect fees and has no source of funds other than block rewards with which pay hashers, it will use the Full Proportional method, meaning that all rewards and fees are fully distributed to hashers proportionally to their contributed shares. Closely related methods like Pay Per Share (PPS) allow the pool operator to earn the fees, but a decentralized mining pool has no operator which could/should be earning these fees. While many projects have inserted a "developer donation", we feel that Braidpool is an open source public good that should be developed and maintained by the community, without the political drama of who and how to pay with a source of funds.
With PPS-type methods, most centralized pool operators are taking a risk on paying immediately for shares, therefore absorbing the variance risk involved in "luck". For hashers that desire immediate payout this can be achieved using any third party willing to buy their shares and take on the risk management of "luck" and fee variance. It's not necessary or desirable for Braidpool itself to subsume this risk management function. It is logical to allow professional risk management firms to take it on by directly buying shares. We envision that existing pools might run on top of Braidpool and continue to perform this risk management function for their clients.
Other payout algorithms such as Pay Per Last N Shares (PPLNS) were created primarily to discourage pool hopping. We don't feel that this is needed in the modern day and a smoothing function applied to payouts interferes with the notion of using shares as a hashrate derivative instrument.
A purely work-weighted proportional algorithm would work for a pure-DAG blockchain, however we have the problem that some of the beads are blocks in a parent blockchain, and the parent blockchain has the property that some blocks can be orphans and receive no reward. We must dis-incentivize the creation of blocks which might become orphans. One component of this solution is the difficulty retarget algorithm which maximizes throughput while minimizing the number of simultaneous beads.
However simultaneous beads will happen naturally due to the faster bead time,
latency, and attackers. Within a time window
where
Therefore shares within a cohort containing 2 or more beads must be weighted by
Uncommitted Metadata.
(FIXME: Is it appropriate to apply this factor to even blockchain-like beads?)
As
Summing it all up, the number of shares
Where
At first glance this algorithm might seem to "punish" lower-target (higher work)
miners given miner-selected difficulty, however
because it is directly proportional to work
The sum of shares generated by this formula must be equal to the actual work going into the blockchain, minus work lost due to orphans (communications latency).
The consensus algorithm we choose is inspired by simply extending Nakamoto consensus to a Directed Acyclic Graph. We call nodes in this DAG "beads" and the overall structure a "braid" so as to distinguish it from the bitcoin blocks and chain. Some of the beads in the DAG are bitcoin blocks.
We call this structure a "braid" because it contains an extra restriction relative to a general DAG: beads must not name as parents other beads which are ancestors of another parent. Naming a parent that is an ancestor of another parent conveys no useful information, since ancestors of each parent are already implied when ordering the DAG and including transactions. Visually this means that a braid will never have triangles or some other higher order structures.
A DAG can be totally ordered in linear time using either Kahn's algorithm or a modified depth-first search which terminates when a bead is found that is a common ancestor to all of a bead's parents, which defines a "graph cut" and a point of global consensus on all ancestors. We define the set of beads between two graph cuts to be a "cohort". Within a cohort it is not possible to total order the contained beads using graph structure alone. The cohort can be defined as a set of beads having the same set of oldest common descendants and youngest common ancestors.
It should be noted that within a braid we keep all beads with a valid PoW, regardless of whether they are considered invalid in other ways, or contain conflicting transactions. Transaction conflict resolution within the Braid is decided by the work weighting algorithm and doing so requires retaining both sides of the conflict. It is generally possible for new work to change which beads are considered in the "main chain", just as in Bitcoin new work can cause a reorganization of the chain ("reorg"), which makes a block that was previously an orphan be in the main chain.
We have considered the PHANTOM proposal which has many similarities to ours and should be read by implementors. We reject it for the following reasons:
-
The k-width heuristic is somewhat analogous to our cohorts, but has the property that it improperly penalizes naturally occurring beads. If for example we target the bead rate such that 40% of the cohorts have 2 or more beads, this means that approximately 2.5% of cohorts would have 4 or more beads. The red/blue algorithm of PHANTOM would improperly penalize all but the first three of the beads in this cohort.
-
It is impossible in practice to reliably identify "honest" and "attacking" nodes. There is only latency, which we can measure and take account of. Even in the absence of attackers, cohorts exceeding the k-width happen naturally and cannot be prevented.
Within Bitcoin, the "Longest Chain Rule" determines which tip has the most work among several possible tips. The "Longest Chain Rule" only works at constant difficulty and the actual rule is a "Highest Work" rule when you consider difficulty changes.
Therefore we require an algorithm to calculate the total work for each bead. This total work can then be used to select the highest work tips as well as to select transactions within beads which have more work than other beads for transaction conflict resolution.
For conflict resolution, we choose the Simple Sum of Descendant Work (SSDW), which is the sum of work among descendants for each bead, disregarding any graph structure. This is the direct analog of Nakamoto's "longest chain/highest work" rule. Graph structure is manipulable at zero cost, therefore we must have a conflict resolution algorithm that is independent of graph structure, lest we create a game which can be played to give a non-work advantage to an attacking miner which he could use to reverse transactions. The SSDW work is:
where
The SSDW can be optimized by first applying the Cohort algorithm, since all beads in a parent cohort have all beads in all descendant cohorts added to their work. Therefore, the only thing that matters for conflict resolution is descendant work within a cohort.
In the event that two beads containing conflicting transactions have exactly the same SSDW, the one with the lower hash ("luck") will be selected.
The cohort time
where
where
Given a starting value for
| Parameter | Description |
|---|---|
| Number of beads | |
| Number of cohorts | |
| Cohort time | |
| Bead time |
This function has a minimum at
This minimum corresponds to the fastest possible cohort time, and the most
frequent global consensus achievable in a braid. For smaller target difficulty
This gives us a zero-parameter retargeting algorithm. At any time we can
evaluate
Braid Retargeting Algorithm contains the full analysis that results in this formula including code to reproduce this result.
Within the Braid we wish to allow different miners to select their difficulty
and to target for constant variance among miners by allowing a small miner to
use a lower difficulty than a larger miner. Miners may select any difficulty
between the maximum target
Braidpool will automatically select an appropriate target difficulty based on the miner's observed hashrate. Larger miners will see a higher target selected while smaller miners will see a lower target, where we will seek that each miner is expected to produce on average one bead per bitcoin block. For miners smaller than this they will be allocated to a Sub-Pool.
Note that this equal-variance target is not enforceable by consensus. A miner could choose to run multiple Braidpool instances or just change the code to select a different target, and the Braidpool software-selected target is an unenforceable recommendation. The consequence of a miner ignoring this recommendation would be to decrease a single miner's variance at the expense of producing more beads in the braid for the same amount of work. This slows down the braid and increases the bead time. Accepting this equal-variance target allows Braidpool to accommodate the maximum number of miners, the most work, and the fastest possible bead time without resorting to allocating more miners to Sub-Pools.
The Payout Update is a separate transaction from the coinbase transaction that aggregates previous coinbase outputs into a single new output. This output contains all funds from the block reward and fees in this and all past blocks. This payout must commit to the share payout structure as calculated at the time the block is mined. In other words, it must represent and commit to the consensus of the decentralized mining pool's share accounting. This transaction has two inputs and one output
Input (1): <existing Braidpool payout update UTXO>
Input (2): <block N-100 coinbase output>
Outputs (1): <new Braidpool payout update UTXO>
Validating the output of the consensus mechanism is well beyond the capability of bitcoin script. Therefore generally one must find a mechanism such that a supermajority (Byzantine Fault Tolerant subset) of Braidpool participants can sign the output, which is essentially reflecting the consensus about share payments into bitcoin. This is done by having a single P2TR public key which is controlled by a Byzantine fault tolerant supermajority of miners which must cooperate to sign the output.
The payout is a rolling commitment that spends the previous payout update
output and creates a new one including rewards and fees from the new block. This
must be signed with SIGHASH_ANYONECANPAY so that the output amount is
uncommitted in the sighash. Since each miner may choose different transactions,
the exact amount of the fee reward in this block cannot be known until a block
is successfully mined, and we cannot commit to this value.
Since newly created coinbase outputs cannot be spent for 100 blocks due to a bitcoin consensus rule, the Payout Update transaction is always 100 blocks in arrears. The transaction that must be included in a bead spends the most recent Braidpool coinbase that's at least 100 blocks old into a new Payout Commitment output.
This rolling set of payout updates is an on-chain version of the Eltoo
protocol. By spending the previous Payout
Update, we automatically invalidate the previous UHPO payout tree, and replace
it with a new one. Old UHPO settlement transactions can no longer be broadcast
as they would be double-spends. Relative to the Eltoo diagram above,
Note that the most common discussions around Eltoo revolve around holding the update transactions off-chain using a NOINPUT or ANYPREVOUT flag. In our case, there is really no good reason to hold these updates off-chain nor wait for these transaction flags to be deployed. These update transactions must be held by Braidpool nodes between the time that they are signed and mined into a block.
For the payout commitment we present a new and simple record accounting for shares. Consider the consensus mechanism as a UTXO-based blockchain analagous to bitcoin. The "UTXO set" of the consensus mechanism is the set of payment outputs for all hashers, with amounts decided by the recorded shares and consensus mechanism rules.
We will term the set of hasher payments the Unspent Hasher Payment Output (UHPO) set. This is the "UTXO set" of the decentralized mining pool, and calculation and management of the UHPO set is the primary objective of the decentralized mining pool.
The UHPO set can be simply represented as a transaction which has as inputs all unspent coinbases mined by the pool, and one output for each unique miner with an amount decided by his share contributions subject to the consensus mechanism rules.
In p2pool this UHPO set was placed directly in the coinbase of every block, resulting in a large number of very small payments to hashers. One advantage of traditional pools is that the aggregate these payments over multiple blocks so that the number of withdrawals per hasher is reduced. A decentralized mining pool should do the same. The consequence of this was that in p2pool, the large coinbase with small outputs competed for block space with fee-paying transactions.
The commitment to the UHPO set in the coinbase output is a mechanism that allows all hashers to be correctly paid if the decentralized mining pool shuts down or fails after this block. As such, the UHPO set transaction(s) must be properly formed, fully signed and valid bitcoin transactions that can be broadcast. See Payout Authorization for considerations on how to sign/authorize this UHPO transaction.
We don't ever want to actually have to broadcast this UHPO set transaction except in the case of pool failure. Similar to other optimistic protocols like Lightning, we will withhold this transaction from bitcoin and update it out-of-band with respect to bitcoin. With each new block we will update the UHPO set transaction to account for any new shares since the last block mined by the pool.
Furthermore a decentralized mining pool should support "withdrawal" by hashers. This would take the form of a special message or transaction sent to the pool (and agreed by consensus within the pool) to remove a hasher's output from the UHPO set transaction, and create a new separate transaction which pays that hasher, authorizes it, and broadcasts it to bitcoin. Miners may not cash out for the current difficulty adjustment window (because the share/BTC price is not yet decided), but may only cash out for the last (and older) difficulty adjustment window(s).
The share value for the current difficulty adjustment epoch is encoded proportionally in the UHPO transactions, however this is only for use in the case of catastrophic failure of Braidpool. During normal operation, the UHPO transaction is fixed at the end of the difficulty adjustment window when the share/BTC price for that epoch is known.
Once a bitcoin block is mined by the pool, Braidpool will kick off a signing ceremony to create a new Payout Commitment and UHPO settlement transaction.
It is impossible or impractical to sign the payout update and UHPO set transactions prior to mining a block, because the extranonce used by mining devices changes the coinbase txid, we can't sign this transaction until its Input(2) txid is known.
After the RCA transaction is signed, and its corresponding UHPO transaction is
signed, spending the RCA's output, Braidpool nodes will delete the
corresponding key shares and keys associated with signing these. As long as
FIXME should update and settlement keys be different here?
FIXME use a tapscript for the UHPO payment. Happy path is RCA, and just a Schnorr signature.
FIXME Can we authorize the tapscript UHPO in any other way? Can we verify a PoW hash for instance?
FIXME pre-kegen and ROAST parallel signing
FIXME use nlocktime or CSV? CSV would separate the update and settlement transactions.
FIXME what do we do with any coinbases mined by Braidpool after the settlement tx is broadcast? CSV and let the miner take it all?
FIXME from eltoo paper: "The use of different key-pairs prevents an attacker from simply swapping out the branch selection and reusing the same signatures for the other branch." This should still be possible with tapscript. An attacker can know the pubkey tweak and adapt an update signature to be a settlement signature and v/v. (CHECK THIS)
The script
If the decentralized mining pool supports transactions of its own, one could "send shares" to another party. This operation replaces one party's address in the UHPO set transaction with that of another party. In this way unpaid shares can be delivered to an exchange, market maker, or OTC desk in exchange for immediate payment (over Lightning, for example) or as part of a derivatives contract.
The reason that delivery of shares can constitute a derivative contract is that they are actually a measurement of hashrate and have not yet settled to bitcoin. While we can compute the UHPO set at any point and convert that to bitcoin outputs given the amount of bitcoin currently mined by the pool, there remains uncertainty as to how many more blocks the pool will mine before settlement is requested, and how many fees those blocks will have.
A private arrangement can be created where one party buys future shares from another in exchange for bitcoin up front. This is a futures contract, where the counterparty to the miner is taking on pool "luck" risk and fee rate risk.
In order to form hashrate derivatives, it must be posible to deliver shares across two different difficulty adjustment windows. Shares in one difficulty adjustment window have a different value compared to shares in another window, due to the difficulty adjustment itself. If one can compute the derivative
then derivative instruments such as options and futures can be constructed by private contract, where shares from different difficulty adjustment epochs are delivered to the derivative contract counterparty in exchange for BTC, possibly with time restrictions. We do not describe further how to achieve this, here we are only pointing out that the sufficient condition for the decentralized mining pool to support private contract derivative instruments are:
- The ability to send shares to another party
- The ability to settle shares into BTC at a well defined point in time with respect to the difficulty adjustment (for instance after the adjustment, for the previous epoch)
- The ability transact shares across two difficulty adjustment windows.
It may be tempting to turn a decentralized mining pool into a full DeFi market place with an order book. We caution that the problem of Miner Extractable Value (MEV) is a serious one that destroys fairness and confidence in the system, and should be avoided here. The only operations we consider here are (a) sending shares to another party and (b) requesting payout in BTC for shares.
Finally let us note that the value of a "share" is naturally fixed after each difficulty adjustment. Within one two-week difficulty adjustment window, each sha256d hash attempt has a fixed value in terms of BTC, but the exact amount of BTC is unknown until the next difficulty adjustment. Therefore, the 2-week difficulty adjustment window is a natural point to automatically broadcast the UHPO tree for the last epoch and settle out all shares from the previous epoch.
In Payout Commitment we described a simple mechansim to represent shares and share payouts as decided by the Consensus Mechansim on shares at any point in time. However, bitcoin is incapable of evaluating the logic of the pool's consensus mechanism and we must find a simpler way to represent that share payout consensus to bitcoin, such that the coinbase outputs cannot be spent in any other way than as decided by the pool's consensus.
Probably the most straightforward way to authorize the share payouts and signing of coinbase outputs is to use a large threshold multi-signature. The set of signers can be any pool participant running the pool's consensus mechanism and having availability of all data to see that consensus mechanism's chain tip. We assume that in the weak block metadata, the pool participants include a pubkey with which they will collaboratively sign the payout authorization.
FIXME -- choose a subset of nodes who submitted shares using a hash function to "elect" them. Those nodes must then submit proof that their shares were valid by broadcasting the transaction tree in their share. If validation fails, the miner's shares are invalidated. This allows us to spot-check all hashers, mitigate block withholding attacks, and keep the signing subset small.
The most logical set of signers to authorize the coinbase spends are the set of
miners who have already successfully mined a bitcoin block. We want to avoid
having any single miner having unilateral control over a coinbase and the
ability to steal the funds without paying other hashers. As such the minimum
number of signers is four, using the
After the first 4 blocks, we assume that 66%+1 miners who have previously mined a block must sign the coinbase output(s), paying into the UHPO set transaction.
This is probably the biggest unsolved problem in building a decentralized mining pool -- how to coordinate a large number of signers. If we assume that shares are paid out onto bitcoin with every difficulty adjustment, this is 2016 blocks and up to 1345 signers that must collaborate to make a threshold multi-signature. This is a very large number and generally well beyond the capabilities of available signing algorithms such as FROST, ROAST, MP-ECDSA, or Lindell's threshold Schnorr algorithm.
Below we discuss threshold Schnorr in more detail, but this may not be the only way to commit to and then authorize spending of coinbases into the UHPO tree. We encourage readers to find alternative solutions to this problem. The very large drawback to all signing algorithms we are able to find is that they are intolerant to failures.
We have reviewed a large amount of literature on threshold Schnorr algorithms.
They all generally involve a Distributed Key Generation (DKG) phase using a variant of Pedersen's DKG, often augmenting it with polynomial commitments introduced by Feldman to achieve a Verifiable Secret Sharing scheme (VSS). There are many papers with variations on this idea, each focusing on organizing rounds of communication, assumptions about communication (such as whether a broadcast channel exists) and security proofs.
Participants in the threshold signature each contribute entropy in the DKG phase by creating and secret sharing their contribution to all other participants. In this way a key can be created with entropy input from all participants, such that no participant knows the key, but at the end of the DKG, all participants hold shares of it such that a t-of-n threshold number of shares must be Lagrange interpolated to reconstruct the secret.
These secret shares are then used to compute a signature. Instead of directly reconstructing the secret key (which would give unilateral spending control to the party doing the reconstruction) one computes the signature using the secret share as the private key, and then Lagrange interpolation is performed on the resulting set of signatures instead.
Both ECDSA and Schnorr signatures require a nonce
The Stratum V2 project is focusing on a model where hashers are responsible for constructing the block and selecting transactions. This is an improvement over Stratum V1 where the (centralized) pool chooses the block and transactions.
The risk here is that the pool either censors valid transactions at the direction of a government entity, or prioritizes transactions through out-of-band payment, risking the "censorship resistant" property of the system.
In the Weak Blocks section we did not indicate how transaction selection was done. This is a factorizable problem, and for a decentralized mining pool we also assume that individual hashers are constructing blocks, and the pool places no further restrictions on the transaction content of a block mined by a participating hasher. In fact, for weak blocks which do not meet bitcoin's difficulty threshold, it is probably best to elide the transaction set entirely for faster verification of shares. This introduces a problem that a hasher could construct a block with invalid transactions, but this would be easily discovered if that hasher ever mined a block, and his shares could invalidated.
A transaction selection mechanism using both a decentralized mining pool and Stratum V2 should be able to easily slot into the block structure required by the decentralized mining pool as indicated in weak blocks, as long as Stratum V2 is tolerant of the required coinbase and metadata structure.
In our opinion simply allowing hashers to do transaction selection is insufficient, as centralized pools can simply withhold payment unless hashers select transactions according to the rules dictated by the pool. A full solution that restores bitcoin's censorship resistance requires decentralized payment as well.
Because signing very large threshold Schnorr outputs is impractical, it is
necessary to keep the number of signers
Therefore there exists the possibility that just due to luck, the same (large)
miner might mine all
The UHPO root must be signed by t-of-n of the most recent distinct miners who successfully mined bitcoin blocks.
We might also consider including hashers who have not won bitcoin blocks. In order to do this we might select a random subset of recent shares, and require that those hashers prove the entire bitcoin block committed to in their share. Upon successful validation of their share, they are included in the signing subset for future blocks. Consensus on this signing subset would be included in beads.
If a hasher is elected for UHPO signing, fails to provide proof of his
The largest unsolved problem here is that of the Payout Authorization. While off-the-shelf algorithms are available such as ROAST, they require fixing the set of signers and are intolerant to failure in either the nonce generation phase, the signing phase, or both. A threshold number of participants must be chosen, and must all remain online through the keygen and signing phase. If any participant fails, a different subset must be chosen and the process restarted. There does exist an approach due to Joshi et al at the cost of an extra preprocessing step, which makes the final signature aggregation asynchronous assuming the nonce generation was successful, though the setup phases are still intolerant to failure.
The fact that both ECDSA and Schnorr signatures require a nonce
In practice if no new algorithm is found and an existing Schnorr threshold signature is used (something involving a DKG and Shamir sharing), a balance must be struck between having so many signers that payouts cannot be signed in a reasonable time, and so few signers that the system is insecure and coinbases could be stolen by a small subset.
An approach that might be considered is to sub-sample the set of signers, and somehow aggregate signatures from subsets. As the resultant signatures would have different nonces, they cannot be straightforwardly aggregated, but this is the same problem as aggregating different signatures within a transaction or block, and approaches to Cross Input Signature Aggregation (CISA) might be used here and might indicate the desirability of a future soft fork in this direction.
One might take the UHPO set transaction and convtert it to a tree structure, using covenants to enforce the structure of the tree in descendant transactions. This is often done in the context of covenant-based soft fork proposals so that one party can execute his withdrawal while not having to force everyone else to withdraw at the same time.
Because a decentralized mining pool is an active online system, it seems better to use an interactive method to write a new transaction for a withdrawal, than to allow broadcasting part of a tree. If part of a tree were broadcast, this must also be noticed by all miners and the share payouts updated.
In our opinion the only reason the whole UHPO set transaction(s) would be broadcast is in a failure mode or shutdown of the pool, in which case the tree just increases the on-chain data load for no benefit.
Since a consensus system cannot achieve consensus faster than the global latency, this is an improvement in share size of at most about 1000x. In order to support even smaller hashers, one might consider "chaining" the decentralized mining pool to create a sub-pool.
Instead of coinbase UTXOs as inputs to its UHPO set, a sub-pool would have UHPO set entries from a parent pool as entries in its UHPO set. With a separate consensus mechanism from its parent, a chain of two decentralized mining pools could allow hashers 1000000x smaller to participate. A pool could in principle dynamically create and destroy sub-pools, moving miners between the sub-pools and main pool dependent on their observed hashrate, so as to target a constant variance for all hashers.