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区块链国际资讯

Bitcoin-NG:新一代区块链交易容量解决之道

比特币目前面临的最大挑战恐怕就是如何最大化区块链的交易处理容量了。不过研究者们的目光可不止于此,他们正在考虑如何解决更前沿的问题。

9月18日在蒙特利尔召开的Scaling Bitcoin会议是首届面向比特币相关开发者的重要会议。这次会议汇聚了大量的技术专家,同样也让人们有机会了解他们正在研究的课题,成功解决这些问题将极大地推动区块链技术的进步。

其中,由康奈尔大学博士后Ittay Eyal、博士研究生Adem Efe Gencer、计算机科学专家Emin Gün Sirer和研究员Robbert Van Renesse一同提出的一个方案就在大会上引发了众人关注。这个方案叫做Bitcoin-NG,即下一代(Next Generation)比特币。研究团队该希望这个方案能帮助解决区块链设计中的"内生问题",这些问题既存在于比特币本身,又存在于以太坊(Ethereum)一类的用于取代现有技术的分布式账本之中。

Eyal指出,Bitcoin-NG的设计灵感来源于区块链的中的很多基础问题,因为这些问题会为区块链在扩大应用的过程中带来严峻的挑战。

Eyal表示:"对于证券市场或数字资产交易而言,如果你想要在一个区块链上引入所有这些数据,那么你将需要有效扩大区块链的规模。"

Bitcoin-NG团队提出,分叉风险就是基础设计的严重问题之一。如果区块链上的数据区块规模上升,区块链出现分叉的可能性也会上升,这将可能导致过去交易的公开信息出现相互冲突的版本,也会降低网络沟通的效率。

区块传播

Eyal认为可以把Bitcoin-NG当作是一个练习,用它帮忙找出在区块链网络中验证与传播独立交易(或交易区块)时的益处或可能出现的问题,而这些都是需要在之后的设计中加以保持或维护的。

最终,Bitcoin-NG的研究者们得出结论:区块链上的数据区块有两个独立的功能--第一,选举出一个"领导"来决定主要的区块链上将包括些交易;第二,将回报分配给矿工。

一位研究者解释道:"当你把一个区块放进(比特币的)区块链之中时,就相当于认定了你现在是之前这个区块的'领导',并且决定了接下来这段时间内交易的顺序。"

Bitcoin-NG提议用两种不同类型的区块来构建区块链:"关键区块"用来选择"领导",而"微区块"用来描述未来某一段时间内的交易。

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"领导"矿工将得到区块的所有回报,然后分出一部分回报用于同前一个领导所在关键区块交易的费用。

深度思考

Eyal表明,整个团队花了好一阵来论证这项研究的中心论点。这项研究指出,虽然目前比特币中的分叉现象一天只会出现几次,但若交易区块变大,分叉现象将变得更为普遍。

研究者指出,这个问题不能通过简单地改变已有网络的区块大小或频率来解决,因此扩展区块链需要更大的改进。不过Eyal说,第一步应当是确定分析的指标。

他说:"你需要一个指标来确定你到底想要最优化什么,并且这个指标直观上必须清晰,比如你想要最大化交易被放入区块链的速度,你想要在区块链中看到交易,你想要在区块链中放尽量多的交易等等。"

据Eyal所言,他们在研究过程发明的最重要的指标,就是所谓"一致延迟"的概念,即之前一次大多数节点就区块链当前状态达成共识的时间距今有多久。

"对于区块链中一小时或更久以前的历史信息,所有人都是达成共识的。但一小时以内的信息呢?如果这里有分叉,甚至有很多分叉,那么显然大家是不会达成共识的。"他补充道。

除此以外,研究员还发明了其他指标,例如"删除时间",即矿工了解到他是否在正确的分支(即他正在处理交易的区块链版本)上所需要的时间。Eyal提示,当区块规模扩大时,删除时间也将增加。

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此外还有"采矿权利用率",即最后主区块链上完整区块的比例;以及"获胜时间",即所有矿工一致同意所给出的区块链是最长的所需要的时间,在这之后新的资源将以此为基础继续扩展。

构造区块

作为一个参考,Eyal引用了重量优先贪心子树(Greedy Heaviest-Observed Sub-Tree, GHOST)。这个研究成果展望了如何能使在分叉中选出的主区块变得更好。

GHOST最早由Yonatan Sompolinsky和Aviv Zohar两位研究者于2013年提出,并在开发基于区块链的技术的以太坊时也被考虑过。以太坊是一个服务于分布式应用的网络。

Eyal解释道:"论文中有两个要点。第一,GHOST采用一种不同的方法来选择区块链。在比特币之中,最长的链就成为主链,这种方法选出的链俗称孤儿区块。这是一种讹传,它实际上是被剪枝后的分支链。GHOST采用一种不同的方法来选择最长的分支。"

Eyal指出最长的链不一定有最多的区块。在比特币中,一个有着更少区块,却有更多分支的链可能会被选为主链。

"除此以外,他们还发明了包含区块链的概念。在包含区块链,如果你处于一个分支之中,你可以稍后再把你的交易归并入主链之中。"Eyal继续说道。

为了更好地推进这些过程,在Bitcoin-NG的系统中,关键区块采用类似比特币的工作量证明,并且仅有领导可以增加微区块,尽管这些微区块增加的时间间隔更短。在这个方案之中,关键区块每10分钟增加一次,而微区块每10秒钟增加一次。

Eyal说,在这段时间内,60%的采矿回报将交给下一个矿工。这个机制可以激励矿工尽量成为区块链的最后一个人。当前的矿工只获得40%的回报。

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"为什么是40%?因为我们必须假设攻击者的大小,并且我们不希望攻击者受激励去挖矿,这将问题变得复杂。比例可以是10%,但这样一来更大的矿工可能就不乐意将交易放入区块之中。"Eyal在会议上发言。

实现方案

Eyal表明,目前Bitcoin-NG已经被放入了比特币的代码库,并且这个网络已经作为某些实验项目的内容了。

不过他也表示,像Bitcoin-NG这样的方案被植入比特币网络还需要一段时间,主要是因为在这个开源项目中各个利益相关者之间达成一致十分困难。

"理论上说,努力让比特币采用这些新方案来增加对带宽和潜力的扩展性是可能的。不过正如你所见,在区块大小的讨论上这是很难做出改变的,更不必说那个大变动需要所有人达成一致的机制了。"

目前,Bitcoin-NG正在运转一个1000节点的测试网络。Eyal与他的队伍使用康奈尔数据中心的150台电脑运转7个客户端来进行这项实验。

在这之后,他说团队希望能在更大范围上运行这个方案,接着出版一份标准协议,并最后将Bitcoin-NG公诸于众。

虽然测试尚未完成,Eyal对于采用Bitcoin-NG,以及他提议的几个指标来解决当前关于区块大小的辩论依然保持乐观态度。他总结道:

"我们看到,我们能够做到一些你们做不到的事,包括增加带宽与效率。我们打算看看在不改变方案特性的前提下我们还能走多远,我们希望达到至少和比特币一样的安全性,带宽和潜力,甚至更好。"

 

The question of how to best increase the transaction processing capacity of the blockchain may be bitcoin's current crisis, but that isn't stopping researchers from working to solve more forward-looking issues.

Held last weekend in Montreal, the inaugural Scaling Bitcoin marked the first major conference for developers, and as such, it featured a broad sampling of technical experts working on solutions to problems that may come to light as knowledge of blockchain technology advances.

One of the more novel proposals to debut at the event was developed by Cornell post-doc student Ittay Eyal, PhD student Adem Efe Gencer, computer science professor Emin Gün Sirer and research scientist Robbert Van Renesse. Called Bitcoin-NG (the "NG" is short for "next generation"), the proposal is envisioned as a solution to "inherent problems" in blockchain design, both in bitcoin and alternative distributed ledgers such as Ethereum.

Eyal said that behind Bitcoin-NG is the belief that there are more fundamental issues with the design of blockchains that will make scaling any implementation, public or private, a challenge.

Eyal told CoinDesk:"For securities markets, for transacting digital assets, if you want to have all of these on a blockchain, you will need significant scaling."

One of the most pressing problems, the team behind Bitcoin-NG argues, is that as the size of data blocks on a blockchain increases, so does the risk that the blockchain forks, resulting in competing versions of the public record of past transactions and inefficiencies in network communication.

Block propagation

Bitcoin-NG, according to Eyal, was an exercise in identifying issues that arise when individual transactions and blocks of transactions are verified and propagated over a blockchain network, as well as the benefits that should be maintained in any redesign.

Ultimately, the researchers behind Bitcoin-NG came to the conclusion that blocks on a blockchain have two separate functions – electing a 'leader' that decides which transactions are included in the main blockchain and distributing a reward to the miner.

"When you place a block in the [bitcoin] blockchain, then you implicitly say I'm the leader from the previous block until now and that's the order of the transactions I decide for this period," he explained.

Bitcoin-NG proposes composing a blockchain of two separate types of blocks: 'key blocks' that decide a 'leader' and 'microblocks' that feature transactions for a specific amount of time in the future.

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Leader miners would be awarded the entirety of the block reward, while splitting fees on transactions between keyblocks with the previous leader.

Considerations

Eyal indicated that it took the team a while to hone in on a central thesis for its research, that forks, while currently happening a few times a day in bitcoin, will be more common should transaction blocks be larger.

The researcher suggested that this problem can't be solved by changing the block size or frequency of a given network, and that scalability will require greater changes. First, however, Eyal said metrics for the analysis needed to be identified.

"You need metrics to realize what you're trying to optimize and the metrics are intuitively very clear, you want a transaction to be placed in a blockchain as fast as possible, you want to see it there and you want to be able to place as many transactions as possible," he said.

The most important metric developed during the process, according to Eyal, was the idea of "consensus delay", or how long into the past most nodes agree on the state of the blockchain.

"Everyone agrees on everything that happened in the blockchain history from one hour ago and back. But what about inside one hour? If there are forks and there are a lot of forks, then surely we don't agree," he continued.

Additional metrics used by the researchers included "time to prune", or the time it takes for miners whether they are on the correct "branch" or version of the blockchain they are processing transactions. As block sizes increase, Eyal suggested time to prune increases.

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Also considered were mining power utilization, or the ratio of completed blocks that end up on the main chain, and 'time to win', the period of time before all miners agree that a given version of the blockchain is the longest so as to expend resources accordingly.

Building blocks

As for inspirations, Eyal cited Greedy Heaviest-Observed Sub-Tree (GHOST), a research proposal that envisions how a main chain could be better selected from the variety of competing forks.

Proposed by researchers Yonatan Sompolinsky and Aviv Zohar in 2013, GHOST was also considered during the development of the alternative blockchain Ethereum, which aims to serve as a network for distributed applications.

"There were two points made in the paper," Eyal explained. "GHOST has a different way of choosing chains. In bitcoin, you choose the longest chain and it becomes the main chain. This is popularly known as an orphan block. This is the wrong word, it's a pruned branch. GHOST has a different way of choosing the longest branch."

Eyal indicated that the longest chain doesn't necessarily have the most blocks, and that in bitcoin, one with less blocks, but more branches, could be selected.

"The other thing they add is the idea of inclusive blockchain where if you have a branch you can later merge transactions back into the main chain," Eyal continued.

To better facilitate these processes, Bitcoin-NG's system is one in which keyblocks use proof-of-work like bitcoin and only leaders generate microblocks, though they come in shorter intervals. In the proposal, keyblocks are generated every 10 minutes, while microblocks are generated every 10 seconds.

Sixty percent of mining fees generated during this time go forward to the next miner, Eyal said, motivating it to place itself in the chain as last as possible. The current leaders get 40% of fees.

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"Why 40%? Because we have to make some assumptions about the size of the attacker, and we don't want the attacker to be motivated about mining multiple blocks, it becomes complicated. It could be 10%, but then larger miners might be motivated not to place transactions in blocks," Eyal said at the conference.

Implementation

To date, Eyal said that Bitcoin-NG has already been implemented on bitcoin's code case, and that this network has been the subject of some experimentation.

Still, he said it would likely be some time before anything like Bitcoin-NG is implemented on the bitcoin network, mostly due to the difficulty of reaching consensus given the disparate stakeholders in the open-source project.

"In theory, it's possible to just fork bitcoin or hard fork bitcoin to use this new protocol and increase scalability for much better latency and bandwidth. As you saw with the blocksize discussion it is difficult to make changes, let alone major changes with this consensus mechanism," he said.

Currently, Bitcoin-NG is operating as a testbed running 1,000 nodes. Eyal and his team are using a Cornell data center for the experiment, with 150 machines running seven clients.

Going forward, he said the team hopes to conduct its work on a larger scale, publish a white paper and ultimately release Bitcoin-NG to the public.

Though testing needs to be done, Eyal was optimistic Bitcoin-NG could contribute to the current blocksize debate through the metrics it used to determine its design, concluding:

"We saw that we were able to do something that you don't get to do a lot, improving bandwidth and efficiently. We're trying to see how far we can get without changing the properties, we want the same level of security, bandwidth and latency [as bitcoin], or better."


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