Channel announcements
Channel announcements are advertisements that a channel is available to forward payments. The advertisements are relayed through the LN gossip network.
Many LN nodes choose to announce their existence and their channels to encourage other nodes to use them for routing payments. Other nodes choose to remain private, using unannounced channels.
As of this writing, many developers call the currently deployed BOLT7 announcement protocol “gossip v1”. One key feature of gossip v1 is that channel announcements and updates must be signed by a key belonging to a P2WSH UTXO corresponding to a 2-of-2 multisig script (the type of script used for v1 LN funding transactions). This means that a channel announcement or update message can only be created after someone has paid the onchain fees necessary to get a P2WSH output confirmed, providing built-in denial-of-service (DoS) protection against fake channel announcements that would waste bandwidth and potentially result in failed routing attempts across non-existent channels.
This approach has two major downsides:
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● Restricts upgrades: the requirement in v1 gossip to use P2WSH UTXOs with a specific script prevents announced LN channels from using alternative scripts and output types. Anyone experimenting with new ideas for LN, such as simple taproot channels, is forced to use unannounced channels.
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● Linkability: it’s expected that channels will be announced using the specific UTXO that is jointly controlled by the two nodes. Although this provides DoS protection through an efficient reuse of something the nodes already need to pay for, it reduces privacy by publicly linking node and channel activity to a specific UTXO.
A proposed upgrade to LN gossip, sometimes called “v1.5 gossip”, would allow using P2TR outputs with keypath-signed channel announcements and updates, addressing the problem of restricted upgrades.
A more ambitious proposed upgrade to LN gossip, sometimes called “v2 gossip”, proposes to address both linkability and restricted upgrades by allowing signatures for a broad range of UTXOs to be used—not just UTXOs that match an LN template. This would allow breaking the connection between channels and UTXOs in several ways, including allowing owners of UTXOs to sell non-spending signatures for them to LN node operators who want maximal privacy. It would also allow some non-LN UTXO owners to broadcast fake channel announcements as part of a potential DoS attack, but the amount of gossip that could be created would still be limited by the cost to create and hold a UTXO.
A version of gossip v2 was first proposed in 2019, with an updated version being proposed in 2022 that allowed using a wide range of UTXOs. Concerns about the use of non-LN UTXOs have been debated since then, with gossip v1.5 being proposed as a less ambitious alternative. A compromise version called v1.75 was proposed at an LN developer meeting.
Primary code and documentation
Optech newsletter and website mentions
2024
- LND #8044 adds new message types for the new v1.75 gossip protocol compatible with taproot channels
- Proving UTXO set inclusion in zero knowledge for more private channel announcement messages
- New anonymous usage tokens proposed that could be used to improve channel announcement privacy
- Disclosure of two past vulnerabilities in LND gossip handling
2023
- LN developer discussion about updated channel announcements
- LDK #2222 allows accepting gossip without verifying it first
- LDK #2198 increases the amount of time before gossiping that a channel is down
2022
- LN fee rate cards proposed to reduce fee-related channel update gossip
- Core Lightning #5239 improves its gossip handling code
- LN developer meeting discussion about gossip updates
- LN gossip rate limiting for use with Erlay-like set reconciliation
- Continued discussion about updated gossip proposal
- Updated gossip proposal
2021
- C-Lightning #4639 adds experimental support for gossiping liquidity advertisements
- Blockstream Satellite begins broadcasting LN gossip data
2019
- C-Lightning #3064 begins limiting gossip updates to once per day
- Request for comments on limiting LN gossip updates to once per day
- Eclair #954 adds a gossip sync whitelist
- Eclair #899 implements extended gossip queries as proposed in BOLTs #557
- LND #3359 adds an
ignore-historical-filtersconfiguration option for ignoring some gossip filters - Gossip update proposed
- LND #2985 waits to relay gossip announcements until there are there are at least ten
- LND #2740 implements a new gossiper subsystem which puts its peers into two buckets
- LND #2690 puts more gossip traffic in a queue to allow prioritizing urgent information
See also
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