[Bitcoin-development] Proposed additional options for pruned nodes

[gabe appleton]

0, 1, 3, 4, 5, 6 can be solved by looking at chunks chronologically. Ie,
give the signed (by sender) hash of the first and last block in your range.
This is less data dense than the idea above, but it might work better.

That said, this is likely a less secure way to do it. To improve upon that,
a node could request a block of random height within that range and verify
it, but that violates point 2. And the scheme in itself definitely violates
point 7.
On May 12, 2015 3:07 PM, "Gregory Maxwell" <gmaxwell at gmail.com> wrote:

> It's a little frustrating to see this just repeated without even
> paying attention to the desirable characteristics from the prior
> discussions.
>
> Summarizing from memory:
>
> (0) Block coverage should have locality; historical blocks are
> (almost) always needed in contiguous ranges.   Having random peers
> with totally random blocks would be horrific for performance; as you'd
> have to hunt down a working peer and make a connection for each block
> with high probability.
>
> (1) Block storage on nodes with a fraction of the history should not
> depend on believing random peers; because listening to peers can
> easily create attacks (e.g. someone could break the network; by
> convincing nodes to become unbalanced) and not useful-- it's not like
> the blockchain is substantially different for anyone; if you're to the
> point of needing to know coverage to fill then something is wrong.
> Gaps would be handled by archive nodes, so there is no reason to
> increase vulnerability by doing anything but behaving uniformly.
>
> (2) The decision to contact a node should need O(1) communications,
> not just because of the delay of chasing around just to find who has
> someone; but because that chasing process usually makes the process
> _highly_ sybil vulnerable.
>
> (3) The expression of what blocks a node has should be compact (e.g.
> not a dense list of blocks) so it can be rumored efficiently.
>
> (4) Figuring out what block (ranges) a peer has given should be
> computationally efficient.
>
> (5) The communication about what blocks a node has should be compact.
>
> (6) The coverage created by the network should be uniform, and should
> remain uniform as the blockchain grows; ideally it you shouldn't need
> to update your state to know what blocks a peer will store in the
> future, assuming that it doesn't change the amount of data its
> planning to use. (What Tier Nolan proposes sounds like it fails this
> point)
>
> (7) Growth of the blockchain shouldn't cause much (or any) need to
> refetch old blocks.
>
> I've previously proposed schemes which come close but fail one of the
> above.
>
> (e.g. a scheme based on reservoir sampling that gives uniform
> selection of contiguous ranges, communicating only 64 bits of data to
> know what blocks a node claims to have, remaining totally uniform as
> the chain grows, without any need to refetch -- but needs O(height)
> work to figure out what blocks a peer has from the data it
> communicated.;   or another scheme based on consistent hashes that has
> log(height) computation; but sometimes may result in a node needing to
> go refetch an old block range it previously didn't store-- creating
> re-balancing traffic.)
>
> So far something that meets all those criteria (and/or whatever ones
> I'm not remembering) has not been discovered; but I don't really think
> much time has been spent on it. I think its very likely possible.
>
>
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