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116 lines
9.3 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(_even_ bits), which will be refused by outdated nodes: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` | Requires or supports extra `channel_reestablish` fields | IN | | [BOLT #2][bolt02-retransmit] |
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| 3 | `initial_routing_sync` | Sending node needs a complete routing information dump | I | | [BOLT #7][bolt07-sync] |
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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` | More sophisticated gossip control | IN | | [BOLT #7][bolt07-query] |
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| 8/9 | `var_onion_optin` | Requires/supports variable-length routing onion payloads | IN9 | | [Routing Onion Specification][bolt04] |
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| 10/11 | `gossip_queries_ex` | Gossip queries can include additional information | IN | `gossip_queries` | [BOLT #7][bolt07-query] |
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| 12/13 | `option_static_remotekey` | Static key for remote output | IN | | [BOLT #3](03-transactions.md) |
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| 14/15 | `payment_secret` | Node supports `payment_secret` field | IN9 | `var_onion_optin` | [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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| 20/21 | `option_anchor_outputs` | Anchor outputs | IN | `option_static_remotekey` | [BOLT #3](03-transactions.md) |
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| 22/23 | `option_anchors_zero_fee_htlc_tx` | Anchor commitment type with zero fee HTLC transactions | IN | `option_static_remotekey` | [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 | `var_onion_optin` | [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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| 38/39 | `option_onion_messages` | Can forward onion messages | IN | | [BOLT #7](04-onion-routing.md#onion-messages) |
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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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## Definitions
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We define `option_anchors` as `option_anchor_outputs || option_anchors_zero_fee_htlc_tx`.
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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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There is no _even_ bit for `initial_routing_sync`, as there would be little
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point: a local node can't determine if a remote node complies, and it must
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interpret the flag, as defined in the initial spec.
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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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[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-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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