In this episode of Bitcoin, Explained, hosts Aaron van Wirdum and Sjors Provoost discuss URSFs, which stands for either User Rejected Soft Forks or User Resisted Soft Forks, depending on who you ask. URSFs are a recently introduced tool in Bitcoin’s upgrade mechanism toolkit.

In the first part of the episode, Aaron and Sjors explain that URSFs are best considered the mirror equivalent of UASFs (User Activated Soft Forks) with mandated signaling. Where UASFs will towards the end of a soft fork activation window reject blocks that don’t signal readiness for a soft fork, URSFs will reject blocks that do signal. If both UASF and URSF clients are deployed, they would in principle create a split in the blockchain.

In the second part of the episode, the duo outlines the various soft fork upgrade mechanisms, ranging from MASFs (Miner Activated Soft Forks), flag day activated UASFs and mandated signaling UASFs. Aaron then explains why he believes mandated signaling UASFs are his preferred method of deploying soft forks, and why he thinks URSFs should in the future be offered as an added option for users who prefer to reject the soft fork.

Finally, Sjors lays out the “rough consensus” guidelines as used in context of the Internet Engineering Taskforce (IETF), and how this applies to Bitcoin upgrades.