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Splicing allows spending the current funding transaction to replace it with a new one that changes the capacity of the channel, allowing both peers to add or remove funds to/from their channel balance. Splicing takes place while a channel is quiescent, to ensure that both peers have the same view of the current commitments. We don't want channels to be unusable while waiting for transactions to confirm, so channel operation returns to normal once the splice tx has been signed and we're waiting for it to confirm. The channel can then be used for payments, as long as those payments are valid for every pending splice transactions. Splice transactions can be RBF-ed to speed up confirmation. Once one of the pending splice transactions confirms and reaches acceptable depth, peers exchange `splice_locked` to discard the other pending splice transactions and the previous funding transaction. The confirmed splice transaction becomes the channel funding transaction. Nodes then advertize this spliced channel to the network, so that nodes keep routing payments through it without any downtime.
111 lines
9.6 KiB
Markdown
111 lines
9.6 KiB
Markdown
# BOLT #9: Assigned Feature Flags
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This document tracks the assignment of `features` flags in the `init`
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message ([BOLT #1](01-messaging.md)), as well as `features` fields in
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the `channel_announcement` and `node_announcement` messages ([BOLT
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#7](07-routing-gossip.md)). The flags are tracked separately, since
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new flags will likely be added over time.
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Some features were introduced and became so widespread they are `ASSUMED` to be present by all nodes, and can be safely ignored (and the semantics are only defined in prior revisions of this spec).
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Flags are numbered from the least-significant bit, at bit 0 (i.e. 0x1,
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an _even_ bit). They are generally assigned in pairs so that features
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can be introduced as optional (_odd_ bits) and later upgraded to be compulsory
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(_even_ bits), which will be refused by outdated nodes:
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see [BOLT #1: The `init` Message](01-messaging.md#the-init-message).
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Some features don't make sense on a per-channels or per-node basis, so
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each feature defines how it is presented in those contexts. Some
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features may be required for opening a channel, but not a requirement
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for use of the channel, so the presentation of those features depends
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on the feature itself.
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The Context column decodes as follows:
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* `I`: presented in the `init` message.
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* `N`: presented in the `node_announcement` messages
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* `C`: presented in the `channel_announcement` message.
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* `C-`: presented in the `channel_announcement` message, but always odd (optional).
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* `C+`: presented in the `channel_announcement` message, but always even (required).
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* `9`: presented in [BOLT 11](11-payment-encoding.md) invoices.
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* `B`: presented in the `allowed_features` field of a blinded path.
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| Bits | Name | Description | Context | Dependencies | Link |
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|-------|-----------------------------------|-----------------------------------------------------------|----------|-----------------------------|-----------------------------------------------------------------------|
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| 0/1 | `option_data_loss_protect` | ASSUMED | | | |
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| 4/5 | `option_upfront_shutdown_script` | Commits to a shutdown scriptpubkey when opening channel | IN | | [BOLT #2][bolt02-open] |
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| 6/7 | `gossip_queries` | Peer has useful gossip to share | | | |
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| 8/9 | `var_onion_optin` | ASSUMED | | | |
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| 10/11 | `gossip_queries_ex` | Gossip queries can include additional information | IN | | [BOLT #7][bolt07-query] |
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| 12/13 | `option_static_remotekey` | ASSUMED | | | |
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| 14/15 | `payment_secret` | Node supports `payment_secret` field | IN9 | | [Routing Onion Specification][bolt04] |
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| 16/17 | `basic_mpp` | Node can receive basic multi-part payments | IN9 | `payment_secret` | [BOLT #4][bolt04-mpp] |
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| 18/19 | `option_support_large_channel` | Can create large channels | IN | | [BOLT #2](02-peer-protocol.md#the-open_channel-message) |
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| 22/23 | `option_anchors` | Anchor commitment type with zero fee HTLC transactions | IN | | [BOLT #3][bolt03-htlc-tx], [lightning-dev][ml-sighash-single-harmful] |
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| 24/25 | `option_route_blinding` | Node supports blinded paths | IN9 | | [BOLT #4][bolt04-route-blinding] |
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| 26/27 | `option_shutdown_anysegwit` | Future segwit versions allowed in `shutdown` | IN | | [BOLT #2][bolt02-shutdown] |
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| 28/29 | `option_dual_fund` | Use v2 of channel open, enables dual funding | IN | | [BOLT #2](02-peer-protocol.md) |
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| 34/35 | `option_quiesce` | Support for `stfu` message | IN | | [BOLT #2][bolt02-quiescence] |
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| 38/39 | `option_onion_messages` | Can forward onion messages | IN | | [BOLT #7](04-onion-routing.md#onion-messages) |
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| 42/43 | `option_provide_storage` | Can store other nodes' encrypted backup data | IN | | [BOLT #1](01-messaging.md#peer-storage) |
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| 44/45 | `option_channel_type` | Node supports the `channel_type` field in open/accept | IN | | [BOLT #2](02-peer-protocol.md#the-open_channel-message) |
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| 46/47 | `option_scid_alias` | Supply channel aliases for routing | IN | | [BOLT #2][bolt02-channel-ready] |
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| 48/49 | `option_payment_metadata` | Payment metadata in tlv record | 9 | | [BOLT #11](11-payment-encoding.md#tagged-fields) |
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| 50/51 | `option_zeroconf` | Understands zeroconf channel types | IN | `option_scid_alias` | [BOLT #2][bolt02-channel-ready] |
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| 60/61 | `option_simple_close` | Simplified closing negotiation | IN | `option_shutdown_anysegwit` | [BOLT #2][bolt02-simple-close] |
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| 62/63 | `option_splice` | Allows replacing the funding transaction with a new one | IN | | [BOLT #2](02-peer-protocol.md#channel-splicing) |
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## Requirements
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The origin node:
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* If it supports a feature above, SHOULD set the corresponding odd
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bit in all feature fields indicated by the Context column unless
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indicated that it must set the even feature bit instead.
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* If it requires a feature above, MUST set the corresponding even
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feature bit in all feature fields indicated by the Context column,
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unless indicated that it must set the odd feature bit instead.
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* MUST NOT set feature bits it does not support.
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* MUST NOT set feature bits in fields not specified by the table above.
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* MUST NOT set both the optional and mandatory bits.
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* MUST set all transitive feature dependencies.
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* MUST support:
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* `var_onion_optin`
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The receiving node:
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* if both the optional and the mandatory feature bits in a pair are set,
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the feature should be treated as mandatory.
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The requirements for receiving specific bits are defined in the linked sections in the table above.
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The requirements for feature bits that are not defined
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above can be found in [BOLT #1: The `init` Message](01-messaging.md#the-init-message).
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## Rationale
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Note that for feature flags which are available in both the `node_announcement`
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and [BOLT 11](11-payment-encoding.md) invoice contexts, the features as set in
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the [BOLT 11](11-payment-encoding.md) invoice should override those set in the
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`node_announcement`. This keeps things consistent with the unknown features
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behavior as specified in [BOLT 7](07-routing-gossip.md#the-node_announcement-message).
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The origin must set all transitive feature dependencies in order to create a
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well-formed feature vector. By validating all known dependencies up front, this
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simplifies logic gated on a single feature bit; the feature's dependencies are
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known to be set, and do not need to be validated at every feature gate.
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<br>
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This work is licensed under a [Creative Commons Attribution 4.0 International License](http://creativecommons.org/licenses/by/4.0/).
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[bolt02-retransmit]: 02-peer-protocol.md#message-retransmission
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[bolt02-open]: 02-peer-protocol.md#the-open_channel-message
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[bolt02-simple-close]: 02-peer-protocol.md#closing-negotiation-closing_complete-and-closing_sig
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[bolt03-htlc-tx]: 03-transactions.md#htlc-timeout-and-htlc-success-transactions
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[bolt02-shutdown]: 02-peer-protocol.md#closing-initiation-shutdown
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[bolt02-quiescence]: 02-peer-protocol.md#channel-quiescence
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[bolt02-channel-ready]: 02-peer-protocol.md#the-channel_ready-message
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[bolt04]: 04-onion-routing.md
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[bolt07-sync]: 07-routing-gossip.md#initial-sync
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[bolt07-query]: 07-routing-gossip.md#query-messages
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[bolt04-mpp]: 04-onion-routing.md#basic-multi-part-payments
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[bolt04-route-blinding]: 04-onion-routing.md#route-blinding
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[ml-sighash-single-harmful]: https://lists.linuxfoundation.org/pipermail/lightning-dev/2020-September/002796.html
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