[bitcoin-dev] BIP159 - NODE_NETWORK_LIMITED service bits, extendability

[Sjors Provoost]

I came across the proposed Bitcoin Core implementation of BIP159 [0] in this PR [1]. The goal is to allow pruned nodes to "serve a limited number of historical blocks" (as opposed to none at all).

It contains a counter-measure for peer fingerprinting. I'm trying to understand how that impacts extendibility.

> Peers may have different prune depths (depending on the peers configuration,
> disk space, etc.) which can result in a fingerprinting weakness (finding the
> prune depth through getdata requests). NODE_NETWORK_LIMITED
> supporting peers SHOULD avoid leaking the prune depth and therefore
> not serve blocks deeper then the signaled NODE_NETWORK_LIMITED
> thresholds.

This means pruned nodes can only serve the last 288 blocks:

> If signaled, the peer MUST be capable of serving at least the last 288 blocks (~2 day

As the blockchain keeps growing there will be ever more pruned nodes (perhaps offset by new nodes with more storage).  Although a strict improvement over todays situation, it seems a bit wasteful to have a node with 10-100 GB of storage only be able to share the most recent 288 blocks.

It would be nice if a future extension of this BIP allows more flexibility. To limit the ability to fingerprint nodes, we could limit the number of choices to e.g. 288 + 1000 * 2^n. That yields only 8 possibilities at the current chain size. A slightly better formula could take into account typical hard drive size increments, leaving enough space for the OS and other data. Node operators could opt-in to this if they think the increased fingerprint risk outweighs their desire to share archived blocks.

I can also imagine - but not implement :-) - a future scenario where nodes prune a random subset of their chain, meaning that even nodes with little storage can be of help during Initial Blockchain Download (IBD) of other nodes.


How would such extension be signaled for? Would we need a whole new version bit?

Would upgraded nodes need a new message type to communicate the chosen prune depth? Or can that information tag along some existing message?

Jonas Schnelli pointed out on the Github discussion that waiting for BIP150 would be appropriate. Can you explain how this is related? Although I can see why whitelisted peers can be exempted from the anti-fingerprinting measure, I would not want to restrict it to just those.


Some minor suggestions for improving the BIP itself:
* add link to mailinglist discussion(s) in reference section
* explain that 288 is not just the minimum limit for Bitcoin Core, but also the bulk of traffic (as I understand from earlier discussion [2])

Cheers,

Sjors

[0] https://github.com/bitcoin/bips/blob/master/bip-0159.mediawiki
[1] https://github.com/bitcoin/bitcoin/pull/10387
[2] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-May/thread.html#14315
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