Toms Blog

Where I talk about Bitcoin and Technology

Tom Zander

Last weekend we had the On-chain scaling conference which was a big success with lots of excellent speakers and a large amount of visitors asking questions and showing interest.

I understand that a lot of people didn't manage to see them live for various reasons. Not the least of them being time-zone differences.

When my talk went onto YouTube I decided to put up a transcript here. The actual talk can be found on youtube.


About 10 years ago I worked at Trolltech, a small Norwegian company making tools for developers to build beautiful applications. I worked there for some years when we got acquired by Nokia. Nokia wanted our development libraries to work fast and smooth on their phones as well as on traditional desktops.

As this was a developer-driven company many developers came up with solutions on how to reach that goal. These developers showing off their ideas meant we had lots of awesome projects to chose from. The projects can roughly be split into 2 sides. The "optimization" side which easily fixes a hundred little speed problems . And a "Big-Changes" side. These are projects that require much more time because they change some fundamental ideas, but they also have a lot larger return on investment.

I would argue that Bitcoin is in the same position now. We have to get better and we have to scale Bitcoin for many more expected users coming in over the next years.

My experience with this process in Trolltech was that all the small, low-risk changes to make it faster were a huge success. They were needed, and they made a huge difference.

On the other hand, the larger "Big-Changes" inevitably introduced more risk and they forced people using our software to change their behavior to actually use the new concepts and ideas. For those reasons their effort ended up taking several years to bear fruit.

In the end both groups pulled it off, and the results are in my personal opinion simply unique and amazing.

Some lessons I learned from those projects at Trolltech are;

This conference presents various "optimization-level" changes that will make Bitcoin scale much better than it does now. With very little pain, and from experience I expect quite a big gain. We call this on-chain scaling, because whatever your belief is on which path leads to Bitcoins success, if we want to scale, we need on-chain transactions to scale beyond the current 3 transactions per second.

For the next years some changes are being prepared which by the nature of them being fundamental changes in the way that Bitcoin will end up being used, will take much longer to create and rollout. I've seen plans like Segregated Witness and the Lightning Network, interesting ideas, for sure. Various teams are working on that and I wish them all the luck. It is already clear that we can't expect those scaling solutions to have any reasonable impact without on-chain scaling solutions being rolled out as well.

This makes sense off-course, when you think about it, you need the foundation to be strong when building on top of it.

Bitcoin, in a nutshell

Now, enough with the high level talk, lets see some actual ideas.

This is the idea of Bitcoin. We have a single computer that receives transactions and groups them all in its memory pool. What it does at the same time is Bitcoin checks each of those transactions for validity and after some time those transactions are combined into a block.

Every single transaction entering this Bitcoin computer is checked that the transaction is a legal one. For instance, a payment from person A to person B can only be created by person A. We use cryptographic proof to check that the person initiating the transaction is actually the owner of the coins he or she he sending.

Bitcoin also checks that people don't try to send the digital coins to two people at the same time. The Bitcoin application holds all transactions in storage and if one comes in that tries to spend already spent money, it is simply rejected.

When a block comes in it makes the transactions contained in that block permanent and Bitcoin will merge that block into its database. The transactions it had in memory are now removed if they were already in the block, or no longer valid because of other transactions in the block.

Bitcoin is essentially fully operational with just one node, but it works with duplication of its functionality over different nodes. Here on screen are 5 nodes.

When a transaction comes in, the node validates it and if its valid the node will forward it to all its neighbors. Which do the same and forward it to their neighbors. In less than 1 second 50% of the network will have the transaction, in less than 4s practically all of them will have it.

These nodes are in many respects exact duplicates, so technically there is little reason to have more than one node, except maybe failover. Politically this duplication means we avoid centralized decision making. We avoid one operator being able to tell the rest of the world what Bitcoin will look like today. This is assuming we make the system work fast enough.

Blocks, the big collections of transactions, get copied between nodes as well. The propagation path is similar to transactions. Currently we send blocks as a complete chunk of data in one go. This is then validated by a node to follow the Bitcoin rules and forwarded to its neighbors.

A healthy network has a large number of Bitcoin nodes to ensure operation and to allow some of them to be removed without harming Bitcoin as a whole. Damage to any part of the network will just get routed around.

Naturally, with a large number of nodes, how they are networked together becomes more important and that networking layer itself becomes a source of failure.

What I've skipped over so far is where the blocks come from. A block is created by a miner. See, here is one in the bottom left corner.

It creates a block and sends it over the network. Each node turning blue means they received the data.

Now, because a block is actually a substantial amount of data, it will take longer than the one second that a single transaction took. reports between 10 seconds and a minute to propagate a 1MB block to practically the whole network.

If we start making bigger blocks, this time will go up. More data means slower propagation times.

As the mining difficulty is adjusted every 2 weeks to make sure that the time between blocks stays on average 10 minutes, the time it takes to send an entire block is not a big deal for end users. As long as they can download blocks faster than they get created, things will continue working. Not optimally, but it works.

There are many people using Bitcoin that have higher requirements than that, though, and the current way that Bitcoin works leaves a lot of space for improvement.

How does this work for miners?

I'm currently focussing on miners. How to make sure that miners get the best out of the system. To explain what I'm working on I will first show what happens with miners using the Bitcoin software.

I'm going to draw the time as shown from the perspective of two miners. One at the top of the screen, and another miner in another physical location at the bottom of the screen.

Two miners are mining and one finds a nonce that makes his block valid. So this miner is happy, he just made several thousands of dollars worth of money. At least, as long as he can convince the rest of the world to use this block he just completed to build on top off for following Bitcoin blocks. This implies his block is seen by the rest of the network before any competitors block have finished their block.

To do so, he sends this block over the network to another miner. The mean propagation time measured by Peter Rizun for blocks between 900kb and 1MB shows about 4 seconds, but as soon as you hop over the great firewall of China this rises to a mean of 17.4 seconds. I'll use the real-time example of 10 seconds here mostly because I don't talk so fast anyway.

Now, after the miner on the bottom received the block, he will be able to start mining on top of that.

What you may notice is that the miner on the bottom is essentially doing work for about 10 seconds that is useless. He was still competing in the competition for block 1 while that race has already been won by another. He just didn't know that yet.

Lets take a look at my proposal using "Optimistic mining", which tries to fix this specific little issue.

Same situation, one miner finds a block. Here is a difference. I split up the block into two parts, one is teeny tiny and send over the network in close to the speed of light and the other is the actual block, unchanged from before.

While the entire block is being sent, the miner at the bottom is now all of a sudden able to mine on top of this new block. Therefore no longer losing money doing work that is useless. Because the full block with all the transactions hasn't arrived yet he can't know which transactions were included in it, and thus which transactions he can safely include in his block. The solution to this is for his block to be empty.

The miner switches to mining a full block as soon as he received the whole block with transactions and is able to decide which transactions to include in his block.

The 10 seconds highlighted here is the propagation time of a full block. This time can be influenced by many things. Bigger blocks can make it take longer. Network attacks may make it essential to ask the original miner for more details, slowing down the propagation time. And most importantly, as we currently see with the great-firewall of china, some countries may just not have such a great connection to the rest of the world. I don't think its a good idea that a miner in Australia loses money just because he has a slower connection to the rest of the world.

Optimistic mining means a miner is made immune to any propagation delays of the transactions in a block. This only can mean good things to avoid centralization of mining power.

In my designs the teeny-tiny part of the block a miner sends out as soon as he finds a block will be just a block header making it approximately 100 bytes. A node can validate this with a cheap check and then forward it.

Two details are important here; the data is forwarded by any node to its peers and thats it. There is no more network traffic that a peer generates based on this. It is just a way to broadcast a new piece of data as fast as possible, as far as possible.

As I promised, these changes are mostly boring and really not noble price worthy. I still get excited from changes that have a substantial benefit, though. But I'm just that kind of geek. These changes do help a lot when you add many of them together and end up with a massively better scaling system than what we have today.

What to work on next

If we look at what it means to scale Bitcoin today we look at all the obvious resources that any computer program needs. Most of them are really not an issue. For instance it would take 75 years to fill up a mainstream harddrive of 3TB with todays block size. People run the client on a raspberry Pi. And it copes just fine.

In Bitcoin the biggest scaling issue we have today is the inefficient use of bandwidth and the poor peer to peer network. This is demonstrated best by the usage of a centralized relay system which operates outside of the Bitcoin network that miners use to send blocks to each other as fast as possible. As long as those centralizes systems are needed we will have a permissioned system where a new player needs permission to enter. New player that don't get permission to use the relay system that all the other miners are on, will not be able to compete. We don't want a system where the relay operators get to decide what miners can and can not do, under threat of losing their access and subsequently their business.

In Bitcoin we currently have what I'd label a first generation p2p network. Think about a system that students typically set up in class during computer science as a play thing. Its making all the traditional mistakes.

Further research

The Bitcoin network uses unlabeled binary blobs to send over the wire. This implies that developers can never fix a message because old clients would not understand it and likely fail spectacularly. Tagged formats, the most well known of which is JSON, solved this many years ago. What we need is like HTML where an old browser can still read the newest format without giving errors. It just ignores the parts it doesn't know.

When I worked for a financial company sending out half a million stock-price quotes every second, I learned a lot about how to do a binary protocol properly. I've done some extended testing already with a new on-the-wire protocol that has looks like its saving on bandwidth and speed quite substantially. But the real benefit is that it can be extended because the fields are labeled much like JSON or XML do. Its values are also strongly typed and extensible. What that means is that the protocol itself includes the information that says a value is an int, a bool or a string.

Making such a chance would be very beneficial to make the protocol more maintainable by allowing bugfixes to be made that are backwards compatible in a clean manner.

Another issue we have with our peer to peer system is based on how an individual node finds other nodes to connect to. We have some code that decides based on the IP-address numbers whom to connect to. The logic seems to be that an IP address is a good indication for location, so it searches to connect to many different locations. Unfortunately, this is overstating the correlation between addresses and distance so this is only slightly better than pure randomly choosing whom to connect to.

Why is this important? Lets take an example of 2000 nodes, all around the world that want to get a message as fast as possible to all of those nodes, my thinking is that we each node should measure the distance to another node by seeing how long it takes to send a message and the message to come back to us.

We can then order the sending of the messages so longer distance messages are send first and shorter distance messages are send later.

Think about it like this, you could send a message by courier to the next village and he sends it to the next and so on. Or you send a courier by plane to the next big city and he starts sending couriers out from there. The idea is that a message would get to the the other side of the globe in 5 hops instead of 50.

To be fair, there are really not a lot of systems out there that create a peer to peer system of this scale that have no central server and have to send messages through the entire system as fast as possible. But this lack of competition just means its going to be a nice challenge to find what works and what doesn't.

Some other details

We have some work done that fixes the problem that while downloading and validating a block that node can't relay transactions.

For our protocol to get to a more professional level we'd also need message priority. A node pushing something to the top of the message-queue to be sent.

We'd likely want to support both TCP and UDP queues where UDP is used for higher-priority but smaller messages. That would be ideal for the optimistic mining teeny-tiny block message we want to go around the world in near lightspeed.

And last we'd really need to set a maximum size for a single message. The telco's switched to packet switching in the 90s, we're long overdue. We now send a single message of one or many megabytes, experience shows that splitting that up into multiple messages or maybe 50kb, and re-assembling it on the other side will have a very positive effect on throughput because you can send bigger data to two peers at the same time, using only twice the wall-time in optimal situations. But when you are sending a large block to a peer that is very slow in receiving it, this message chunk-ING will stop slowing down your sending speed to another peer at the same time.

I've implemented this in another system where the overhead per 50KB message ended up being only 14 bytes. So I really don't see any reason to not do this.

These are useful changes that I'm hoping to see being worked on, if not by me, then someone else.

Other issues we will bump into as we get bigger blocks and higher throughput are going to be found, and fixed as we move forward. I have no doubt about that. That is the nature of this game, there is always some optimization that we haven't done yet.

Beyond the network layer are a lot of other things that show up as being useful to work on in order to allow more data to go through Bitcoin cheaper and faster. Its still young software and I don't think its been bothered with profilers or other speed measurements very often. Gavin started a benchmark suite that never really saw uptake by any of the other developers. So please expect a huge amount of benefits to happen in the next years alone for throughput as its easy pickings for optimizations when its not been done for years.

I would say that within a year the network would be safely able to handle between 20 and 50 MB blocks.

Creating a Bitcoin that, on-chain, can grow and support more users will be the thing that actually creates more value for the Bitcoin ecosystem. So while our opponents are discussing how to distribute the money of the rich differently, until they inevitably run out of money, we are working to actually create new value to increase your wealth as well as ours.

Bitcoin has a bright future.