A 2015 draft BIP suggested using a "hard fork bit":
The most significant bit in nVersion is defined as the hardfork bit. Currently, blocks with this header bit setting to 1 are invalid, since BIP34 interprets nVersion as a signed number and requires it to be >=2 (with BIP66, >=3). Among the 640 bits in the block header, this is the only one which is fixed and serves no purpose, and therefore the best way to indicate the deployment of a hardfork.
If bit indexes increase from left to right, then I imagine the hard fork bit occupying index 31, marked as
* below (
nVersion is a 32-bit value).
0 1 2 3 0|1|2|3|4|5|6|7|8|9|0|1|2|3|4|5|6|7|8|9|0|1|2|3|4|5|6|7|8|9|0|1 *
According to the proposal, setting bit 31 of
nVersion should cause a block to be rejected as invalid.
BIP-9 places further constraints on the available bits, specifically, bits 0-2 must be set as [0, 0, 1]:
Blocks in the STARTED state get an nVersion whose bit position bit is set to 1. The top 3 bits of such blocks must be 001, so the range of actually possible nVersion values is [0x20000000...0x3FFFFFFF], inclusive.
In addition, BIP-9 arranges version bits in increasing order from right to left. That is, version bit 0 is
nVersion bit 31, version bit 1 is
nVersion bit 30, and so on.
But if this were the case, then version bit 0 would be off-limits. BIP-9 says nothing about this, and in fact BIP-68 was signaled using version bit 0:
This BIP is to be deployed by "versionbits" BIP9 using bit 0.
My questions are simple:
- Given my diagram above, which
nVersionbit index is the "hard fork bit?"
- If I'm correct in my interpretation (I don't believe I am), then how could BIP-68 have been signaled without causing the blocks doing so to be rejected as invalid?
Update: the source of my confusion was thinking that
nVersion bit indexing and version bit indexing ran in opposite directions. The don't. To signal readiness for a proposal using version bit 1, for example,
nVersion bit 1 is set. BIP-9's rules activate when an
nVersion signature is detected. This signature is bits 29-31 set as
0, respectively. This leaves bit 31 unset.
Peter's answer addresses question (1) above.
As for question (2) BIP-68 signaling didn't cause block rejection because bit 0 was being set, not bit 31.