The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.
BIP9 introduced a mechanism for doing parallel soft forking deployments based on repurposing the block nVersion field. Activation is dependent on near unanimous hashrate signalling which may be impractical and result in veto by a small minority of non-signalling hashrate. Super majority hashrate based activation triggers allow for accelerated activation where the majority hash power enforces the new rules in lieu of full nodes upgrading. Since all consensus rules are ultimately enforced by full nodes, eventually any new soft fork will be enforced by the economy. This proposal combines these two aspects to provide eventual flag day activation after a reasonable time, as well as for accelerated activation by majority of hash rate before the flag date.
Due to using timestamps rather than block heights, it was found to be a risk that a sudden loss of significant hashrate could interfere with a late activation.
Block time is somewhat unreliable and may be intentionally or unintentionally inaccurate, so thresholds based on block time are not ideal. Secondly, BIP9 specified triggers based on the first retarget after a given time, which is non-intuitive. Since each new block must increase the height by one, thresholds based on block height are much more reliable and intuitive and can be calculated exactly for difficulty retarget.
# The '''bit''' determines which bit in the nVersion field of the block is to be used to signal the soft fork lock-in and activation. It is chosen from the set {0,1,2,...,28}.
# The '''timeoutheight''' specifies a block height at which the miner signalling ends. Once this height has been reached, if the soft fork has not yet locked in (excluding this block's bit state), the deployment is considered failed on all descendants of the block.
# The '''lockinontimeout''' boolean if set to true, blocks are required to signal in the final period, ensuring the soft fork has locked in by timeoutheight.
# '''name''' should be selected such that no two softforks, concurrent or otherwise, ever use the same name. For deployments described in a single BIP, it is recommended to use the name "bipN" where N is the appropriate BIP number.
# '''bit''' should be selected such that no two concurrent softforks use the same bit. The bit chosen should not overlap with active usage (legitimately or otherwise) for other purposes.
# '''startheight''' should be set to some block height in the future when a majority of economic activity is expected to have upgraded to a software release including the activation parameters. Some allowance should be made for potential release delays. It should be rounded up to the next height which begins a retarget period for simplicity.
# '''timeoutheight''' should be set to a block height when it is considered reasonable to expect the entire economy to have upgraded by, probably at least 1 year, or 52416 blocks (26 retarget intervals) after '''startheight'''.
# '''lockinontimeout''' should be set to true for any softfork that is expected or found to have political opposition from a non-negligible percent of miners. (It can be set after the initial deployment, but cannot be cleared once set.)
'''startheight''' and '''timeoutheight''' must be an exact multiple of 2016 (ie, at a retarget boundary), and '''timeoutheight''' must be at least 4096 blocks (2 retarget intervals) after '''startheight'''.
# '''MUST_SIGNAL''' for one retarget period prior to the timeout, if LOCKED_IN was not reached and '''lockinontimeout''' is true.
# '''LOCKED_IN''' for one retarget period after the first retarget period with STARTED (or MUST_SIGNAL) blocks of which at least threshold have the associated bit set in nVersion.
The nVersion block header field is to be interpreted as a 32-bit little-endian integer (as present), and bits are selected within this integer as values (1 << N) where N is the bit number.
During the MUST_SIGNAL phase, if '''(2016 - threshold)''' blocks in the retarget period have already failed to signal, any further blocks that fail to signal are invalid.
Note that when '''lockinontimeout''' is true, the LOCKED_IN state will be reached no later than at a height of '''timeoutheight''', and ACTIVE will be reached no later than at a height of '''timeoutheight + 2016'''.
Implementations should be careful not to ban peers that send blocks that are invalid due to not signalling (or blocks that build on those blocks), as that would allow an incompatible chain that is only briefly longer than the compliant chain to cause a split of the p2p network. If that occurred, nodes that have not set ''lockinontimeout'' may not see new blocks in the compliant chain, and thus not reorg to it at the point when it has more work, and would thus not be following the valid chain with the most work.
Implementations with ''lockinontimeout'' set to true may potentially follow a lower work chain than nodes with ''lockinontimeout'' set to false for an extended period. In order for this not to result in a net split nodes with ''lockinontimeout'' set to true, those nodes may need to preferentially connect to each other. Deployments proposing that implementations set ''lockinontimeout'' to true should either use parameters that do not risk there being a higher work alternative chain, or specify a mechanism for implementations that support the deployment to preferentially peer with each other.
To support upgrade warnings, an extra "unknown upgrade" is tracked, using the "implicit bit" mask = (block.nVersion & ~expectedVersion) != 0. Mask will be non-zero whenever an unexpected bit is set in nVersion. Whenever LOCKED_IN for the unknown upgrade is detected, the software should warn loudly about the upcoming soft fork. It should warn even more loudly after the next retarget period (when the unknown upgrade is in the ACTIVE state).
===getblocktemplate changes===
The template request Object is extended to include a new item:
{| class="wikitable"
!colspan=4| template request
|-
! Key !! Required !! Type !! Description
|-
| rules || No || Array of Strings || list of supported softfork deployments, by name
|}
The template Object is also extended:
{| class="wikitable"
!colspan=4| template
|-
! Key !! Required !! Type !! Description
|-
| rules || Yes || Array of Strings || list of softfork deployments, by name, that are active state
|-
| vbavailable || Yes || Object || set of pending, supported softfork deployments; each uses the softfork name as the key, and the softfork bit as its value
|-
| vbrequired || No || Number || bit mask of softfork deployment version bits the server requires enabled in submissions
|}
The "version" key of the template is retained, and used to indicate the server's preference of deployments.
If versionbits is being used, "version" MUST be within the versionbits range of [0x20000000...0x3FFFFFFF].
Miners MAY clear or set bits in the block version WITHOUT any special "mutable" key, provided they are listed among the template's "vbavailable" and (when clearing is desired) NOT included as a bit in "vbrequired".
Softfork deployment names listed in "rules" or as keys in "vbavailable" may be prefixed by a '!' character.
Without this prefix, GBT clients may assume the rule will not impact usage of the template as-is; typical examples of this would be when previously valid transactions cease to be valid, such as BIPs 16, 65, 66, 68, 112, and 113.
If a client does not understand a rule without the prefix, it may use it unmodified for mining.
On the other hand, when this prefix is used, it indicates a more subtle change to the block structure or generation transaction; examples of this would be BIP 34 (because it modifies coinbase construction) and 141 (since it modifies the txid hashing and adds a commitment to the generation transaction).
A client that does not understand a rule prefixed by '!' must not attempt to process the template, and must not attempt to use it for mining even unmodified.
BIP8 and BIP9 deployments should not share concurrent active deployment bits. Nodes that only implement BIP9 will not activate a BIP8 soft fork if hashpower threshold is not reached by '''timeoutheight''', however, those nodes will still accept the blocks generated by activated nodes.
