BIP 68 Relative Lock-Time Explained for Bitcoin Transactions

BIP 68 relative lock-time explained: learn sequence-number encoding, height and time delays, version-two transactions and mining-node checks.

Absolute and relative lock-time

An absolute lock says a transaction cannot confirm before a particular chain height or time. BIP 68 instead gives selected nSequence bits a relative meaning: an input may require that its spent output has aged by a number of blocks or time intervals. This allows a pre-signed transaction to remain delayed relative to an event whose exact date is unknown.

Start with the validating node, because the ASIC only hashes the candidate header it receives. Record the node release and the pool component that assembled the block. If those facts are unknown, the operator cannot show which rules were actually applied before electricity was committed to the work.

Version two and the disable flag

BIP 68 applies consensus semantics to transactions with version two or higher. The highest sequence bit disables relative lock-time for that input, preserving a path for sequence values used without the delay. Wallets must construct the complete 32-bit field deliberately. Copying a sequence value from an unrelated fee-bump example can silently change whether a delay is active.

Treat status dashboards as observations, not as the source of truth. Compare them with an independently operated node and retain the raw deployment or template response. Period boundaries, chain reorganisations and cached pool pages can otherwise make a correct-looking percentage describe the wrong state.

Height-based sequence locks

When the type flag is clear, the low 16 bits specify a delay measured in blocks. The spend becomes eligible after the referenced output has aged by the encoded interval under the specification’s off-by-one conventions. Height locks are easy to audit from chain data. But their wall-clock duration changes with actual block production and cannot guarantee a precise calendar deadline.

Build the failure response before the boundary arrives. Define which rejection messages trigger an alert, who can pause a template source and how failover is prevented from returning miners to the same faulty validation stack. A second hostname is not independent when both endpoints share one node.

Time-based sequence locks

When the type flag is set, the low 16 bits count units of 512 seconds. Time is evaluated using median time past associated with the relevant blocks, not the local machine clock. The granularity keeps useful ranges inside limited bits. Operators should present an approximate human duration while retaining the exact encoded interval for validation and recovery procedures.

Separate readiness, signalling and enforcement in the operating log. Readiness is a claim about software and process, signalling is data carried by blocks, and enforcement is a validation result. Combining them into a single supported or unsupported label hides the point at which revenue is actually at risk.

Relationship to BIP 112 and BIP 113

BIP 68 defines transaction-level sequence-lock consensus. BIP 112 exposes compatible relative constraints to Script through CHECKSEQUENCEVERIFY. At the same time, BIP 113 makes absolute transaction locktime use the previous block’s median time past. They activated together through CSV deployment. The BIPs are related but each solves a distinct layer. So implementation evidence should name all applicable rules.

Map responsibility across the full path: validating node, template server, pool protocol, proxy, firmware and ASIC. For each layer, state what it can alter and what it merely relays. This prevents a version-bit setting in firmware from being mistaken for complete consensus-rule support. Relate that responsibility map to the pool and job-control boundary in our Stratum V2 guide.

Mempool and template behaviour

Nodes evaluate sequence locks against the next candidate block context. A transaction that is valid in principle may be non-final for the current height and excluded until its delay expires. Reorganisations can change the age context. Template servers must recalculate eligibility on each tip and should not retain a stale ready flag from a prior branch or cached wallet estimate.

Test the primary and failover paths with the same checks. Compare chain tip, chainwork, deployment state, required rules and template age, then save the result with a timestamp. The process should be repeatable by another operator without relying on an undocumented pool conversation.

Testing relative delays

Create version-two spends with height and time flags on regtest. Exercise the disable bit, boundary values, multiple inputs and a reorganisation. Compare wallet reporting with testmempoolaccept and block proposal validation. Record the funding confirmation height and median-time-past values. For real recovery transactions, rehearse fee funding as well as maturity because a valid delayed spend still needs relay incentive.

Turn the conclusion into a business decision. State which chain and settlement venues the operation intends to serve, the maximum acceptable stale-block exposure and the point at which mining pauses. This connects protocol evidence to electricity cost, pool revenue and payout finality.

Operator decision record

A concise decision record for BIP 68 relative lock time should name the source documents, their dates, the node release tested, the responsible pool or template provider and the exact trigger for action. Include screenshots or machine-readable output for the deployment state. But keep the raw node response as the stronger evidence.

State whether a change affects policy, block construction or consensus validity, because those layers have different failure costs.

Run the check on every production and failover path. Confirm that monitoring alerts on stale templates, unexpected chain tips, rejected proposals and a rise in stale shares. Keep rollback instructions for node and pool configuration. But do not roll back across an active consensus boundary without understanding the rules the older release enforces.

If the evidence conflicts, pause the affected path and investigate before committing more electricity to uncertain work.

For related background, read our plain-English BIP-110 guide and technical BIP-110 review. Those articles use a modern proposal to show why signalling, activation, template construction and accepted chain history must be examined separately.

Conclusion

Bip 68 relative lock time is best understood as a defined interaction between validating software, mining infrastructure and economic acceptance. The safest operator does not infer consensus from a dashboard percentage or a pool slogan. They verify the rule source, the activation boundary, the template fields and the chain their payouts ultimately settle on.

That discipline reduces the chance of hashing an invalid or commercially unwanted block.