Bitcoin XT

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Bitcoin XT
The identifying marks of an XT client
Original authorsGavin Andresen
Mike Hearn
Bitcoin XT is a fork of Bitcoin Core that aims to make transactions reliable, inexpensive, and accessible. It achieved significant support after adopting BIP 0101, making it an important proponent of the block size limit controversy.

After the inauguration of Wladimir van der Laan to the position of Core maintainer, Bitcoin XT was organized[when?] by Gavin Andresen and Mike Hearn to address controversial ideas lacking the consensus required to be implemented in Bitcoin Core.[1]

Once 75% of the last 1000 blocks are observed to be in support of XT, a hardfork will occur and XT nodes will separate from the rest of the network, implementing several changes, most notably an increase in block size.

Mission statement

The XT mission statement defines what the project believes is important: commitment to these principles are what differentiates us from Bitcoin Core. We try to follow Satoshi's original vision, as it is that vision which brought the Bitcoin community together.

  • Scaling the network up to handle user demand is important, even if that means the network changes along the way. It's what Satoshi wanted[2] and the idea of a global system used by ordinary people is what motivated many of us to join him.
  • XT provides people with information they need, even if using it requires them to make risk based decisions. For example:
    • We believe unconfirmed transactions are important. Many merchants want or need to accept payments within seconds rather than minutes or hours. XT accepts this fact and does what it can to minimise the risk, then help sellers judge what remains. It is committed to the first seen rule[3]. We will not adopt changes that make unconfirmed transactions riskier.
    • Lightweight wallets are important. Most users cannot or will not run a fully verifying node. Most of the world population does not even own a computer: they will experience the internet exclusively via smartphones. These users must sacrifice some security in order to participate, so XT supports whatever technical tradeoffs wallet developers wish to explore.
  • Decision making is quick and clear. Decisions are made according to a leadership hierarchy. The XT software encodes decisions that follow the above principles: people who disagree are welcome to use different software, or patch ours. We do not consider writing principled software to be centralising and do not refuse to select reasonable defaults.
  • The Bitcoin XT community is friendly, pragmatic, cares about app developers and considers the user experience in everything we do. We value professionalism in technical approach and communication. We run a moderated mailing list and do not tolerate troublemakers.

You can read more about the code changes in Bitcoin XT.

Block size hard fork

Many years ago, a capacity limit was introduced into Bitcoin by Satoshi. He intended it to be removed once lightweight wallets were developed, however, this was never done. It is expected that soon Bitcoin will be out of capacity and experiencing reliability problems as a result. The Bitcoin Core developers are currently unwilling to increase the block chain's capacity, so miners and users who disagree with them must either switch to Bitcoin XT, or apply the bigblocks patches[4] manually.

There has been much community debate on this topic. You can read analysis and explanations for why we think raising the block size limit is important here:


Miners that side with Bitcoin XT will produce blocks with a new version number. This indicates to the rest of the network that they support XT. When 75% of the last 1000 blocks are new-version blocks, a decision will have been reached to start building larger blocks that will be rejected by Bitcoin Core nodes. This will begin after a waiting period of two weeks to allow news of the new consensus to spread and allow anyone who hasn't upgraded yet to do so. During this time, existing Bitcoin Core nodes will be printing a message notifying the operators about the availability of an upgraded version.

If the hard fork occurs and you are still mining with Bitcoin Core, your node will reject the first new block that is larger than one megabyte in size. At that point there is a risk your newly mined coins will not be accepted at major exchanges or merchants.

Users and merchants

If insufficient mining hash power runs XT to reach supermajority then nothing will happen. If enough does, you will follow the new chain and things will continue as normal.

Additionally, XT has a useful feature: double spend monitoring and relaying. By running XT you help propagate information about double spends across the network, making it harder for payment fraudsters to steal from sellers by broadcasting two conflicting transactions simultaneously.

See Also