Liquidity and channel management

Wonder

1. Why does opening a channel with your own funds grant you the ability to send but not to receive?
2. What mechanism allows two parties to change the size of an existing channel without broadcasting a force-close or losing their routing history?
3. How does a small, locked reserve on each side of a channel ensure that cheating remains economically unviable?

Explore

1. A Lightning channel is directional. The balance split determines utility: outbound capacity allows sending payments, while inbound capacity allows receiving them. When a user opens a channel by broadcasting a funding transaction with their own coins, they acquire outbound capacity (less the reserve and commitment fees) but no inbound capacity. This asymmetry means the user can immediately spend within that channel, but cannot accept any new value from the counterparty until the balance shifts.
2. Acquiring inbound liquidity requires external action or internal movement. A user may request another node to open a channel toward them, effectively buying receiving capacity. Alternatively, the user can initiate payments through existing channels; as funds move out of their outbound side and into the counterparty’s, the user’s inbound capacity for that specific peer increases. This spending-first approach naturally builds receivability over time without new on-chain transactions.
3. Lightning service providers (LSPs) address this friction commercially. These entities open channels to users in exchange for a fee. An LSP injects capital into the user’s node, creating immediate inbound capacity that would otherwise require waiting for organic balance shifts. This service is critical for merchants who must maintain constant receivability to avoid losing sales during high-volume periods.
4. Rebalancing manages directional imbalances across a user’s channel graph. If one channel runs out of outbound capacity while another has excess, funds can be routed in a circle through the network to shift balances. This process consumes routing fees and requires sufficient liquidity at each hop. Splicing, deployed from 2023 in implementations like Core Lightning and Eclair, offers an alternative by resizing a channel on-chain without closing it, keeping the channel and its history; its public short channel ID changes, and the network is told of the new one.
5. Each side of a channel maintains a reserve, typically around one percent of the total capacity. The reserve means each party always has funds at stake: someone who had spent their whole balance would have nothing to lose by broadcasting an old state that paid them more, so the protocol keeps a slice in reserve that a penalty can take.
6. Channel management carries tangible economic costs. Opening and closing channels require on-chain transactions, exposing users to variable mempool fees that spike during periods of high network congestion. Rebalancing incurs routing fees for every hop traversed.

Key facts

Exercise nine, six, three, one

Fact check

Dates, figures and claims checked against the published sources listed below.

• A. M. Antonopoulos, O. Osuntokun & R. Pickhardt, Mastering the Lightning Network (2021)
• BOLT #2: The Bitcoin Lightning Network - Peer Networking Protocol
• Core Lightning splicing documentation
• Eclair splicing documentation (ACINQ)