The mempool is each node’s holding area for valid unconfirmed transactions. Miners (and other nodes) choose what to relay and mine based on policy and fees — not every consensus-valid transaction is welcome under default policy.
Complete Step 1 // Required reading in the study plan above, then continue here. This lesson closes the Bitcoin Basics chapter.
Step 2 — Core idea: auction, policy, bumping
Users experience Bitcoin through confirmation delays and fee spikes. Wallets that cannot bump fees create support nightmares.
Analogy: An auction-house lobby. Anyone can submit a bid (transaction). Higher fee rates tend to get seated (mined) first when space is scarce. Some items are rejected at the door for policy reasons even if they are “legal.”
const feeRateSatVb = feeSats / vsize;Estimate using node RPCs (estimatesmartfee) or public observers, then expose slow/medium/fast targets in UX.
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- RBF (BIP-125): replace an unconfirmed tx with a higher-fee variant (signals required)
- CPFP: spend an unconfirmed output with a high-fee child that incentivizes mining both
Done when: You can explain when RBF vs CPFP is the right bump tool.
Step 3 — Try it: fee math + bump choice (lab)
bitcoin-cli -regtest estimatesmartfee 6
# Or after a send: feeSats / vsize from getrawtransaction ... truePick a stuck-payment scenario (e.g. low fee, no RBF signal, or you control a change output). Write which bump you would use and why.
Common mistakes
- Broadcasting without RBF signaling when users may need to bump
- Overpaying dramatically due to stale fee estimates
- Creating long unconfirmed chains that hit ancestor/descendant limits
Done when: Lab evidence (fee calc + RBF/CPFP choice + source link) is complete. Complete the chapter quiz when prompted.
Next lesson
P2P Network (Protocol Depth) — how transactions and blocks actually propagate.