BOLT #9: Assigned Feature Flags
This document tracks the assignment of
features flags in the
message (BOLT #1), as well as
features fields in
node_announcement messages (BOLT
#7). The flags are tracked separately, since
new flags will likely be added over time.
Flags are numbered from the least-significant bit, at bit 0 (i.e. 0x1,
an even bit). They are generally assigned in pairs so that features
can be introduced as optional (odd bits) and later upgraded to be compulsory
(even bits), which will be refused by outdated nodes:
see BOLT #1: The
Some features don't make sense on a per-channels or per-node basis, so each feature defines how it is presented in those contexts. Some features may be required for opening a channel, but not a requirement for use of the channel, so the presentation of those features depends on the feature itself.
The Context column decodes as follows:
I: presented in the
N: presented in the
C: presented in the
C-: presented in the
channel_announcementmessage, but always odd (optional).
C+: presented in the
channel_announcementmessage, but always even (required).
9: presented in BOLT 11 invoices.
||Requires or supports extra
||Sending node needs a complete routing information dump||I||BOLT #7|
||Commits to a shutdown scriptpubkey when opening channel||IN||BOLT #2|
||More sophisticated gossip control||IN||BOLT #7|
||Requires/supports variable-length routing onion payloads||IN9||Routing Onion Specification|
||Gossip queries can include additional information||IN||
||Static key for remote output||IN||BOLT #3|
||Routing Onion Specification|
||Node can receive basic multi-part payments||IN9||
||Can create large channels||IN||BOLT #2|
||Anchor commitment type with zero fee HTLC transactions||IN||BOLT #3, lightning-dev|
The origin node:
- If it supports a feature above, SHOULD set the corresponding odd bit in all feature fields indicated by the Context column unless indicated that it must set the even feature bit instead.
- If it requires a feature above, MUST set the corresponding even feature bit in all feature fields indicated by the Context column, unless indicated that it must set the odd feature bit instead.
- MUST NOT set feature bits it does not support.
- MUST NOT set feature bits in fields not specified by the table above.
- MUST set all transitive feature dependencies.
The requirements for receiving specific bits are defined in the linked sections in the table above.
The requirements for feature bits that are not defined
above can be found in BOLT #1: The
There is no even bit for
initial_routing_sync, as there would be little
point: a local node can't determine if a remote node complies, and it must
interpret the flag, as defined in the initial spec.
Note that for feature flags which are available in both the
and BOLT 11 invoice contexts, the features as set in
the BOLT 11 invoice should override those set in the
node_announcement. This keeps things consistent with the unknown features
behavior as specified in BOLT 7.
The origin must set all transitive feature dependencies in order to create a well-formed feature vector. By validating all known dependencies up front, this simplifies logic gated on a single feature bit; the feature's dependencies are known to be set, and do not need to be validated at every feature gate.
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