(sorry if this is duplicated - the submission from my mail client was rejected I think)

Hi Silas,

 

Thanks for this write-up; I found it helpful.

 

All,

 

I hope nobody minds my joining this discussion but I think the framing of the proposed contribution of Besu provides the opportunity for a timely technical/architectural debate. In particular, I understand that the basis of the proposed contribution (the ‘net-new capability this contribution would make to Hyperledger’, so to speak) is the Besu client’s ability to talk to the public Ethereum network – and that, by implication, this would be of value to a project whose mission is to: “create an enterprise grade, open source distributed ledger framework and code base, upon which users can build and run robust, industry-specific applications, platforms and hardware systems to support business transactions.”

 

I regularly see claims online that connecting a “private” or “permissioned” blockchain to a “mainnet” can provide benefits but I’ve always struggled to understand exactly what those benefits would be. The problem, I think, is that Tweets and Medium articles aren’t really the place for nuanced technical discussion.  Hence why this very narrowly focused proposal in this forum is such an opportunity.

 

I know there can be emotion and I am obviously not unbiased so in what follows I’ll try to keep the discussion generic as I try to outline what I don’t understand – and ask for help in figuring out what I’m missing.  

 

First – let’s check we have similar mental models.  For me, the whole permissioned/permissionless thing is solely about transaction confirmation. Specifically: who are the entities that take place in the consensus forming process for a network and how is their participation decided? We often call these entities validators but I think that’s unhelpful… their primary role is transaction ordering/confirmation, not validation… validation is the responsibility of a much larger group (‘full nodes’ in most architectures). Assuming this focus on “who decides if a transaction gets confirmed or not” is at the absolute heart of things then we can tease out some distinctions.

 

In a ‘permissioned’ chain such as Fabric or Corda, traditional consensus theory is applied. The participants in the network agree to utilise the services of some number of actors who will collaborate to confirm transactions. These actors could be assumed to be entirely trustworthy or BFT-style assumptions may be made that some proportion could be malicious. The key point is that the number of actors is known and the network participants go to lengths to ensure the actors are not sybils, etc.  And, as a result, ‘traditional’ consensus research can be applied. This approach is not without its downsides (of course) but it has an important property of importance to business: once a transaction is marked as confirmed it will stay confirmed. And it is a fault of the system if this property does not hold. 

 

The starting point for ‘permissionless’ blockchains, by contrast – and we can trace this all the way back to Bitcoin – is that agreeing on a known set of transaction confirmers makes it impossible to build censorship-resistant networks. Bluntly, if you know who the transaction confirmers are, then so do the authorities. And so they can be shut down or pressured to censor/delete transactions.  So if you have a desire to build a solution where those in authority can not prevail on transaction confirmers to bend to their will, you need a design where the set of transaction confirmers is unknown – unknown in terms of who they are and how many there are, and where they can come and go at will. This means that the consensus algorithms developed over previous decades can’t be used and Satoshi’s genius was to devise a totally new way of solving the problem. However, the rules of mathematics and computer science didn’t change with the advent of Bitcoin. Instead, a requirement was softened. That requirement was finality.  If we are willing to accept the probability that a transaction could sometimes go from “confirmed” to “unconfirmed” then a whole world of possibility opens up and techniques such as Proof of Work become possible.  This was an amazing insight and most of us are probably in this space today because of it and its implications.

 

But… this probabilistic finality situation is annoying, of course. Like I said to John Wolpert of ConsenSys on a podcast a while back, if there wasa permissionless system that truly gave me finality and over which I could reason about concentration/collusion risk, etc., I’d probably be abuyer of such a thing.  But there isn’t and so I can’t.  It’s as if we’re in this annoying situation where there’s something we’d all like to be true… but the universe is conspiring against us to make it just not so!

 

Now… assuming my (hopefully not over-simplistic) model is broadly OK then we can observe a few things.

 

First: notice how the permissionless approach to consensus says nothing about security. It is primarily about censorship-resistance. This is especially important to note because the last ten years have shown that, whilst it is possible to build consensus systems where the participants can come and go at will without permission, the economics are such that the relevant participants are usually relatively few in number…. possibly far fewer than would participate in a robust traditional BFT cluster!   Note: this is not a comment about the security of any given client implementation (I fully agree that code such as geth has clearly been battle-tested). The security question is one about how many people you have to hack to take over the consensus forming process.  If there are 21 unique participants in a Fabric BFT cluster and five miners with 80+% of the hashing power for a PoW blockchain, I would submit that it would be easier to subvert the latter chain than the former. (Note: you don’t need to hack the Fabric or Geth clients to take over confirmation processing for a network… you simply need to gain control of whichever systems are controlling the miners’ infrastructures… a Linux kernel vulnerability or somebody willing to kidnap the miners’ families might be all you need).   So I’m unpersuaded by security arguments for or against a permissioned or permissionless approach in terms of compromises to transaction confirmation integrity.  It ultimately all comes down to whether you need censorship resistance or not.

 

Secondly: experience has shown us that whilst we’d like to believe the probability of a transaction reversal on permissionless chains trends asymptotically to zero quickly, in reality there are occasional events far out in the tail that make analysis and engineering extremely difficult…. eg the Ethereum Classic 100+ block reorg a while back. 

 

So the piece I’ve always struggled with – and which I hope this discussion can help open my eyes to is:  if I don’t have the need for censorship-resistance in transaction confirmation for my business problem – or if it’s an anti-requirement for my problem, what’s the argument for why I would use a permissionless chain?  It strikes me that there are lots of downsides and no obvious upsides.

 

That said, the discussions below seem mostly to be about bridging or integrating the permissionless and permissioned worlds, as opposed to enabling business-focused use-cases to be deployed directly onto a permissionless chain.  But if the discussion is primarily only about integration/bridging that then makes me scratch my head even harder. If I look at Silas’s use-cases (thanks again btw… this is the first time I’ve seen them enumerated so clearly), I’ll disregard 1) and 3) for the purposes of this discussion since I’m only really qualified to talk about the business problems I see in my work, none of which involve usage of eth.

 

Which leaves me with: announcements and interchain-connectivity.

 

With respect to interchain-connectivity (proving A happened before B on different networks), isn’t that almost a perfect example of where a permissionless chain would be the wrong choice?  The two independent transactions could be reordered in the event of a reorg and it would not be a fault… it would be the system working as designed.  But if you sought to overcome this by making one transaction depend on the other to force an ordering then you already have your proof of ordering and so why do you need the blockchain?

 

With respect to announcements, is the idea here that the public chain is being used as a way to reassure yourself that there is no newer announcement from any given party… ie that you’re looking at the most current publication about a certain topic from somebody?  If so, I agree that’s a highly desirable service to have as part of a solution design if one were available. But, again, isn’t it something that a permissionless chain is singularly bad at providing? If your threat model admits the possibility that the publisher would try to deceive you about the most currently published document, wouldn’t a platform that includes probabilistic finality and treats reorgs as a normal part of business be the perfect tool to let them perform their dastardly deeds? It would seem that the correct architectural solution would be something like a widely-witnessed proof-of-existence service, such as Guardtime’s offering that publishes merkle roots in a national newspaper.

 

Like I said near the start, I know my position as CTO of a firm with an open source permissioned blockchain means what I write can’t be seen as unbiased. So I’ve tried my hardest to write objectively and constructively and to avoid setting up strawmen (hence the laboured permissioned/permissionless section so you can ‘see my working’). 

 

There has been so much noise and ‘people talking past each other’ at the boundary of permissioned and permissionless chains… and so, done right, this debate could be something we look back on as a milestone in really nailing some of these concepts.  

 

Richard

 

Richard G Brown | R3. | Chief Technology Officer

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