[bitcoin-dev] SPV Mining reveals a problematic incentive issue.

Tier Nolan tier.nolan at gmail.com
Sat Jul 11 13:17:53 UTC 2015

On Sat, Jul 11, 2015 at 1:09 PM, Nathan Wilcox <nathan at leastauthority.com>

> If there's any cost to non-SPV mining, and the chance that an incoming
> block contains only valid transactions is very high, isn't there still an
> incentive to make this timeout longer and longer?

The benefit drops off pretty quickly as the timeout increases (and
eventually goes negative).

You could look at it that headers having 4 states.

1) Valid
2) Probably Valid
3) Probably Invalid
4) Invalid

SPV mining puts newly received headers into the "probably valid" category.

It builds empty (coinbase only) blocks on top of probably valid headers and
build empty blocks on the header.

Once it receives the full block, it can change the state to Valid.  At that
point, it can build full blocks on top of the header.

As time passes without the full block being received/validated, it becomes
less and less likely that the block is actually valid.

The timeout is to recognize that fact.  Making the timeout 24 hours is not
likely to give the miner much benefit over making it 1-2 minutes.

Setting the timeout to low means that the miner sometimes switches away
from a header that turns out to be valid.

Setting it to high means that the miner ends up staying to long on a header
that turns out to be invalid.

At some point, the second effect is stronger than the first effect.  The
timeout needs to be high enough so switching away from valid headers is
rare but low enough so that it doesn't stay on invalid headers for ages.

If 99% of full blocks are received (and validated) within 30 seconds of the
header arriving, then it is reasonable for the miner to assume that if the
full block hasn't arrived within 60 seconds of the header arriving, then
the header is for an invalid block.

Yes. If it's rare enough, then skipping transaction validation saves more
> cost than the expected cost of mining atop an invalid block.  ... but if
> everyone follows that logic, the chance is no longer rare.

SPV miners don't actually produce invalid blocks though (other than
building on invalid blocks).  The full blocks they produce are still fully
checked blocks.

> SPV-mining is to prevent hashing hardware from having to waste power when
>> it isn't needed.
> It may be less of a problem if (when?) electricity costs dominate hardware
>> capital costs.
> Oh.  This is a different explanation than Peter Todd's I linked to above,
> which claimed it was network latency in receiving transactions.

SPV mining is mainly to protect against latency.  The reason that matters
is that latency means that hashers end up building on blocks even though a
new block has been found.

You can look at it as wasting hashing power due to latency.

In the world where minting fees are very low, there is no point in SPV

I assume at the point, the memory pool/queue is a few blocks deep.  This
means that the pool can create a full block without having to wait for new
transactions to be sent in.

It still needs to wait for the new full block before it knows which
transactions to remove from its memory pool.

Pools have to pay their hashers for hashing power.  When minting fees are
tiny, pools only get income only from tx fees.

There is no point in creating empty blocks, since they don't pay anything.

Between when the block is found and the pool has a new block ready to mine,
there is no incentive for the pool to give out new work.  The stratum
protocol could be modified so pools can say.  (It might already support

<Send work to hashers>

-- block is found by some other pool --

<Cancel work for miners>

-- pool builds new block --

<Send new work to hashers>

The cancel command says that the pool will not accept any of the work that
has been made obsolete.

This gives a window of 20-30 seconds after each block where the pool has
invalidated the old work, but does not send new work.  Hashers' hardware
would be stalled.

On the other hand, the pool that found the block could create a new block
straight away.  There is an incentive for hashers to have a connection to
multiple pools.

Pools might go pure pay per share.  The protocol would say how much they
are willing to pay for a share and the local mining proxy would pick the
most profitable pool.  This eliminates pools having lots of ways of saying
how they charge fees, you just connect to lots of pools and go with the one
that pays the most.

More interestingly, the average fee per byte for transactions in the memory
pool is likely to increase as time passes since the last block.

When two blocks are found very fast one after another, the second block is
likely to have lower average fees.  This is because the first block would
have included most of the high value transactions and there wasn't enough
time for new ones to arrive before the second block was found.

Hashers would end up getting variable payouts based on how long since the
last block was received.  If some miners increase/decrease their output
based on the fees the pools offer, then as time passes since the last
block, the hashing rate would increase.  This reduces the variation in
block to block times.

For example, if there was 1.5MB of transactions in the memory pool and they
all paid the same fee per byte, then the block would be a full block at
that rate.  However, there would only be 0.5MB of transactions left.  This
means that the next block would be half full and only be able to pay out
50% of the fee, but the difficulty would be the same.  The pay per hash
would be 50% lower.  Once 0.5MB of new transactions arrive, the fee would
be back up to the same as the previous block.

If there are major SHA256 altcoins (or side chains), then miners might end
up switching between coins.  The longer a coin went without a new block
being found, the more tx fees available in the memory pool, so the more
hashing power would decide to switch to that coin.

There could be a situation where adding a few more transactions to the
memory pool of a coin would cause a 100X increasing in hashing until the
block was found and then it stalling again as the hashing power switches
away.  This is similar to alt coins getting blasted by coin switching pools
and then dropping to almost no power.
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