shortchannelid

Shortchannelid, often written as short_channel_id, is a compact 64-bit identifier used in the Lightning Network to refer to a specific payment channel. It is primarily used in routing and gossip messages to identify channels without transmitting the full funding transaction data. The concept is defined in Lightning Network protocol specifications and is widely adopted by implementations such as LND, c-lightning, and Eclair.

The short_channel_id encodes three fields into 64 bits: block_height, transaction_index within that block, and the output_index of the funding transaction. Specifically, 24 bits are allocated for the block height, 24 bits for the transaction index, and 16 bits for the output index. The common textual representation is block_height:tx_index:output_index, though it can also be used as a decimal 64-bit value. For example, a channel funded in block 680000, as the 12th transaction, output 0 would have a short_channel_id formatted as 680000:12:0.

A short_channel_id is derived from the funding transaction that opens the channel and becomes valid once that transaction is confirmed. It serves as a concise reference to identify the channel in messages that inform nodes about network topology and available routes. The long-term, cryptographic reference for the channel is the channel_id, derived from the funding outpoint, which remains stable for the channel even if reorgs occur; the short_channel_id is more ephemeral and can be more sensitive to blockchain reorganizations.

Limitations and considerations include the finite bit allocation (block height up to 2^24−1) and potential changes in rare chain reorg scenarios. Despite these caveats, short_channel_id remains a core, efficient mechanism for channel references in the Lightning Network.