Network Routing

💡 The Plain-English Definition

Network routing is the process by which a Lightning payment finds its path from sender to recipient through a network of payment channels — hopping through one or more intermediate nodes to reach its destination even without a direct channel between the two parties.

🤔 But Why Though?

If Lightning required a direct payment channel (a funded two-party connection) between every pair of users who wanted to transact, it would need billions of channels and billions of on-chain transactions to set them up. Instead, Lightning routes payments through existing channels — a payment from Alice to Dave can travel through Alice→Bob→Carol→Dave if those channels exist, even though Alice and Dave have no direct connection.

Finding this route is non-trivial. Each node maintains its own local copy of the channel graph (the public map of Lightning Network channels and their capacities), but doesn’t know the current balance distribution within each channel — only the total capacity. A channel with 1 BTC capacity might have 0.99 BTC on one side and 0.01 BTC on the other, making it effectively unusable in one direction, but nothing in the graph reveals this. Routing algorithms have evolved significantly to handle this. Early Lightning used source routing (the sender calculated the full path). Modern implementations use techniques like probing (sending trial payments to learn actual channel balances), probabilistic pathfinding (estimating likely balances from available data and adjusting with each success or failure), and multipath payments (splitting the payment across multiple routes when no single route can carry the full amount). The reliability of Lightning routing has improved dramatically from its early years. Payments today succeed at much higher rates than in 2019, driven by better pathfinding algorithms, a denser channel graph with more well-capitalised nodes, and AMP (Atomic Multipath Payments — the technique that splits a single payment across multiple simultaneous paths) for larger amounts.

🌍 The Real-World Analogy

Think of Lightning routing like a postal system without direct routes between every city. A letter from a small village to a remote town travels through a series of sorting centres — each one only knows to pass it to the next centre in the chain, not the full journey. The postal network’s routing algorithm determines the sequence of handoffs. If a particular sorting centre is overwhelmed (channel balance depleted), the algorithm finds an alternative route. The letter arrives without the sender knowing or caring which sorting centres handled it.