Protocol-Level Pruning

It occurred to me that we could push the classic concept of pruning even further: we could significantly shrink the blockchain as well as reduce the amount of network traffic during initial block download by doing something I would call protocol-level pruning. This would, as of today, reduce the size of the blockchain by a factor of 50, hence enabling massive on-chain scaling.

The idea behind PLP is to serialize the UTXO set in a standardized way, and publish a hash of it in the block header so that the blockchain commits to it. Since hashing and verifying it is a moderately intensive operation, perhaps the UTXO set hash should be published only once every 576 blocks (4 days).

When a new Bitcoin node joins the network, it would download the block headers only (not the block data), it would identify the most recent block containing the UTXO set hash, and download the UTXO set from peers. From that point on, it downloads and verifies all blocks as normal.

Every 576 blocks, nodes serialize and verify that their UTXO set hash matches the one published in the blockchain. Doing so becomes a new part of consensus rules. The last 576 blocks could then be permanently discarded as they are no longer useful.

Today the serialized UTXO set is about 3GB and the blockchain is about 150GB. Therefore PLP would cut down the amount of data stored by full nodes by a factor of ~50 as they would have to store only the UTXO set plus at most 576 blocks.

One trivial optimization is possible: to avoid hashing the entire UTXO set every 576 blocks (which would take multiple seconds even on a fast machine), the UTXO set serialization could be a sparse merkle tree <https://eprint.iacr.org/2016/683.pdf> which would allow on-the fly recomputation of the hash as new blocks are mined: when a UTXO is added to (or removed from) the tree, only a small number of hash operations are needed to recalculate the UTXO set merkle tree root hash.

Maybe we don't even need sparse merkle trees, but a regular merkle tree would suffice: the tree leaves would be small groups of UTXOs (some bits in the ID/hash of a UTXO would determine which leaf it belongs to.)

Unlike classic pruning mode, *ALL* full nodes on the network could switch to PLP. There is no need for any node to archive the entire blockchain any more.

I can think of one downside of PLP: nodes would no longer be able to handle reorgs that go further back than the last UTXO set hash published on the chain (since previous blocks have been discarded). So, perhaps keeping around the last N*576 blocks (N=10?) would be a sufficient workaround.

Thoughts?

-Marc

It's not clear to me if you are have looked at the previous UTXO set commitment proposals.

some utxo set commitment bookmarks (a little old) http://diyhpl.us/~bryan/irc/bitcoin/utxo-commitments-or-fraud-proofs.stdout.txt

TXO bitfields http://diyhpl.us/wiki/transcripts/sf-bitcoin-meetup/2017-07-08-bram-cohen-merkle-sets/

delayed TXO commitments https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2016-May/012715.html

TXO commitments do not need a soft-fork to be useful https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-February/013591.html

rolling UTXO set hashes https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-May/014337.html

lotta other resources available, including source code proposals..

- Bryan http://heybryan.org/ 1 512 203 0507

Ah, thanks, I suspected the idea was too simple and must have been discussed before, but somehow I missed these proposals. I've got some reading to do.

-Marc

On Thu, Nov 16, 2017 at 11:14 AM, Bryan Bishop <kanzure@gmail•com> wrote:

> It's not clear to me if you are have looked at the previous UTXO set
> commitment proposals.
>
> some utxo set commitment bookmarks (a little old)
> http://diyhpl.us/~bryan/irc/bitcoin/utxo-commitments-or-
> fraud-proofs.stdout.txt
>
> TXO bitfields
> http://diyhpl.us/wiki/transcripts/sf-bitcoin-meetup/
> 2017-07-08-bram-cohen-merkle-sets/
>
> delayed TXO commitments
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/
> 2016-May/012715.html
>
> TXO commitments do not need a soft-fork to be useful
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/
> 2017-February/013591.html
>
> rolling UTXO set hashes
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/
> 2017-May/014337.html
>
> lotta other resources available, including source code proposals..
>
> - Bryan
> http://heybryan.org/
> 1 512 203 0507 <(512)%20203-0507>
>

Bryan Bishop via bitcoin-dev <bitcoin-dev@lists•linuxfoundation•org> writes:

> It's not clear to me if you are have looked at the previous UTXO set
> commitment proposals.
>
> some utxo set commitment bookmarks (a little old)
> http://diyhpl.us/~bryan/irc/bitcoin/utxo-commitments-or-fraud-proofs.stdout.txt
>
> TXO bitfields
> http://diyhpl.us/wiki/transcripts/sf-bitcoin-meetup/2017-07-08-bram-cohen-merkle-sets/
>
> delayed TXO commitments
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2016-May/012715.html
>
> TXO commitments do not need a soft-fork to be useful
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-February/013591.html
>
> rolling UTXO set hashes
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-May/014337.html
>
> lotta other resources available, including source code proposals..

Thanks!

Has anyone categoried list discussions by topic like this? It seems a lot of this stuff is scattered between mailing lists, irc conversations, etc and can be hard to know whats floating out there.