Aren't one heavy site equivalent to multiple light sites?


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bramas
b
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Hi everyone,
I'm a french CS researcher and I started recently to look into blockchain (and DAG-based) technologies.
I read the Tangle white paper and found it very interesting (with a lots of challenges).
I have a question about the evaluation of the probability for an attacker to create a chain that is longer/heavier than the main chain.
In paper version 1.4.1, section 4, page 17 the calculus is done with one site that has possibly unbounded weight and the observation is that the attacker has a non-negligible probability to generate a site with weight greater that the cumulative weight of the main chain. Thus "the situation where the maximum own weight is capped at a value of 1" is considered. My question is in the title: isn't creating 3^n sites of weight 1 as difficult as creating a single site with weight 3^n, by definition ? So that there is no need to do the analyze in this case as the previous analyze can be applied here. Moreover, it may even be easier to create a large number of sites with weight 1 as the granularity is smaller so that the chance of creating a branch with cumulative weight greater than in the main chain is maybe even greater.
Maybe I missed something and in this case I would be happy to read your explanation.

Thanks a lot for this forum allowing public discussion on this amazing topic!

Quentin.
pauldouglas
pauldouglas
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I think that there are a couple of notable differences. A single heavy site can be added somewhat atomically, but propagation delays are amplified by for many light sites, and becomes non-linear in the high-load regime. Also, while an attacker may have unlimited bandwidth, individual nodes likely have an upper bound to their bandwidth, and when it saturates, it takes 3 times the work to replace a tx in queue to broadcast. If we limit the own weight for purposes of tip selection, then it may be possible to assume there to be some limit the maximum rate at which nodes in a cluster see an increase of cumulative weight of some particular subtangle.
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