Header-Only Mining

Header-only mining (HOM) is the Stratum V2 operating mode used on standard channels, in which a hashing device works exclusively on the 80-byte block header and never touches transactions at all. The upstream node or pool selects the transactions, builds the coinbase, and computes the merkle root; the device receives that merkle root pre-baked inside a NewMiningJob message and simply grinds the header fields it is permitted to change. It is the cleanest expression of Stratum V2's design philosophy: push complexity up the stack, keep the embedded device small, fast, and dumb.

What the device actually rolls

On a standard channel the job arrives as a channel ID, a job ID, a version field, and a complete 32-byte merkle root — nothing else to assemble. The device's search space is exactly three knobs: the 32-bit nonce, the nTime timestamp (within the window the upstream allows), and the general-purpose version bits defined by BIP320, exercised through version rolling. When a share clears the target, the device answers with SubmitSharesStandard, reporting just the nonce, ntime, and full rolled version it used. Contrast this with V1's mining.notify, where every device receives coinbase fragments and a merkle branch and must reconstruct the merkle root itself for each extranonce it tries.

The 280-terahash budget

Restricting the device this way bounds the work available per job. With a fixed nTime, one standard job offers roughly 2⁴⁸ header candidates — the 32 nonce bits times 16 rollable version bits — which is about 280 terahashes of search space. A modern ASIC hashing in the hundreds of terahashes per second exhausts that in around a second or less, so the upstream must keep a pipeline of fresh jobs with new merkle roots flowing, and Stratum V2's future-job mechanism (jobs delivered ahead of time and activated by SetNewPrevHash) exists exactly to keep hardware saturated across block changes. Slower devices, like a Bitaxe-class open miner, sip from the same job for far longer, which is why header-only work suits low-power hardware so well: minimal bandwidth, minimal memory, minimal firmware surface. The nTime field adds headroom on top: the upstream allows the timestamp to be rolled forward within consensus limits, stretching each job's useful life for devices that would otherwise exhaust it between updates.

Dumb device, sovereign stack

Header-only mining does not mean surrendering transaction choice — it means relocating it. In a self-hosted Stratum V2 deployment, transaction selection happens at the operator's own node and Job Declarator, upstream of the device; a proxy running an extended Stratum mining channel receives the coinbase prefix, suffix, and merkle path, builds jobs locally, and feeds plain standard jobs to each ASIC. The merkle root a device grinds is ultimately derived from a template built by the operator's Template Provider and negotiated through the Job Declaration Protocol. The device stays simple; sovereignty over block content lives one layer up, on hardware the operator controls.

Why the split matters

This division of labor pays off three ways. Firmware gets simpler and easier to audit — a hashing device that only rolls header fields has less code to hide bugs or surprises in. Bandwidth collapses, since a 32-byte merkle root replaces full transaction sets, which matters for miners behind rural links or mesh backhaul. And the architecture scales down as gracefully as up: the same standard-channel contract serves a container full of Antminers and a single USB-powered open miner on a shelf. Firmware support for the full V2 stack remains the gate — BraiinsOS+ on industrial Antminers and AxeOS/ESP-Miner v2.14.0+ on the Bitaxe ship it natively today — and header-only mining is the piece of that stack every hashing device, however small, participates in.