Re: [Bitcoin-development] Proposed additional options for pruned nodes

[Jeff Garzik <jgarzik@bitpay.com>]

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One general problem is that security is weakened when an attacker can DoS a
small part of the chain by DoS'ing a small number of nodes - yet the impact
is a network-wide DoS because nobody can complete a sync.


On Tue, May 12, 2015 at 12:24 PM, gabe appleton <gappleto97@gmail.com>
wrote:

> 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@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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>
>
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-- 
Jeff Garzik
Bitcoin core developer and open source evangelist
BitPay, Inc.      https://bitpay.com/

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