Softforks and hardforks are usually defined in terms of block validity (BIP99): 
making valid blocks invalid is a softfork, making invalid blocks valid is a 
hardfork, and SFs are usually considered as less disruptive as it is considered 
to be “opt-in”. However, as shown below this technical definition could be very 
misleading. Here I’m trying to redefine the terminology in terms of software 
upgrade necessity and difficulty.

Softforks are defined as consensus rule changes that non-upgraded software will 
be able to function exactly as usual, as if the rule changes have never happened

Hardforks are defined as consensus rule changes that non-upgraded software will 
cease to function or be severely handicapped

SFs and HFs under this definitions is a continuum, which I call it 
“hardfork-ness”. A pure softfork has no hardfork-ness.

*Mining node

Under this definitions, for miners, any trivial consensus rule changes is 
somewhat a hardfork, as miners can’t reliably use non-upgraded software to 
create blocks. However, there is still 3 levels of “hardfork-ness”, for example:

1. Those with lower hardfork-ness would be the SFs that miners do not need to 
upgrade their software at all. Instead, the minimum requirement is to setup a 
boarder node with latest rules to make sure they won’t mine on top of an 
invalid block. Examples include CSV and Segwit

2. Some SFs have higher hardfork-ness, for example BIP65 and BIP66. The minimum 
actions needed include setting up a boarder node and change the block version. 
BIP34 has even higher hardfork-ness as more actions are needed to follow the 
new consensus.

3. Anything else, ranging from simple HFs like BIP102 to complete HFs like 
spoonnet, or soft-hardfork like forcenet, have the highest hardfork-ness. In 
these cases, boarder nodes are completely useless. Miners have to upgrade their 
servers in order to stay with the consensus.

*Non-mining full node

Similarly, in terms of non-mining full node, as the main function is to 
fully-validate all applicable rules on the network, any consensus change is a 
hardfork for this particular function. However, a technical SF would have much 
lower hardfork-ness than a HF, as a border node is everything needed in a SF. 
Just consider a company has some difficult-to-upgrade software that depends on 
Bitcoin Core 0.8. Using a 0.13.1+ boarder node will make sure they will always 
follow the latest rules. In case of a HF, they have no choice but to upgrade 
the backend system.

So we may use the costs of running a boarder node to further define the 
hardfork-ness of SFs, and it comes to the additional resources needed:

1. Things like BIP34, 65, 66, and CSV involves trivial resources use so they 
have lowest hardfork-ness.

2. Segwit is higher because of increased block size.

3. Extension block has very high hardfork-ness as people may not have enough 
resources to run a boarder node.

* Fully validating wallets

In terms of the wallet function in full node, without considering the issues of 
validation, the hardfork-ness could be ranked as below:

1. BIP34, 65, 66, CSV, segwit all have no hardfork-ness for wallets. 
Non-upgraded wallets will work exactly in the same way as before. Users won’t 
notice any change at all. (In some cases they may not see a new tx until it has 
1 confirmation, but this is a mild issue and 0-conf is unsafe anyway)

2. Extension block, as presented in my January post (
 ), has higher hardfork-ness, as users of legacy wallets may find it difficult 
to receive payments from upgraded wallet. However, once they got paid, the user 
experience is same as before

3. Another extension block proposal ( 
<> ) has very high 
hardfork-ness for wallets, as legacy wallets will frequently and suddenly find 
that incoming and outgoing txs becoming invalid, and need to sign the 
invalidated txs again, even no one is trying to double spend.

4. Hardfork rule changes have highest hardfork-ness for full node wallets

I’ll explain the issues with extension block in a separate post in details

* Real SPV wallet

The SPV wallets as proposed by Satoshi should have the ability to fully 
validate the rules when needed, so they could be somehow seen as fully 
validating wallets. So far, real SPV wallet is just vapourware.

* Fake SPV wallet, aka light wallet

All the so-called SPV wallets we have today are fake SPV according to 
whitepaper definition. Since they validate nothing, the hardfork-ness profile 
is very different:

1. BIP34, 65, 66, CSV, segwit has no hardfork-ness for light wallets. Block 
size HF proposals (BIP10x) and Bitcoin Unlimited also have no hardfork-ness 
(superficially, but not philosophically). Along the same line, even an 
inflation hardfork has no hardfork-ness for light wallets.

2. Extension block has the same kind of hardfork-ness issue as I mentioned.

3. HFs that deliberately breaks light wallets, such as spoonnet, is a complete 

While some people try to leverage weakness of light wallets, the inability to 
validate any important rules like block size, double spending, and inflation is 
a serious vulnerability.


Before I finish, I’d also like to analyse some other interesting cases.

1. Soft-hardfork: which requires miners to mine empty blocks with 0 reward, and 
put the tx merkle tree in the legacy coinbase (e.g. 
<> ). This 
allows most hardfork-ing changes including block size and inflation. In terms 
of block validity this is a softfork. But with the definition I presented, 
soft-hardforks are clearly hardforks for every practical purposes.

2. On-chain KYC, blacklist, account freezing: technically softforks, but all 
are very disruptive hardforks in terms of user experience.

3. Lightning network and side chains are not consensus rule changes, and they 
could provide new features without any hardfork-ness.

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