lightning-bolts/09-features.md

BOLT #9: Assigned Feature Flags

This document tracks the assignment of features flags in the init message (BOLT #1), as well as features fields in the channel_announcement and 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can be introduced as optional (odd bits) and later upgraded to be compulsory (even bits), which will be refused by outdated nodes: see BOLT #1: The init Message.

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 init message.
• N: presented in the node_announcement messages
• C: presented in the channel_announcement message.
• C-: presented in the channel_announcement message, but always odd (optional).
• C+: presented in the channel_announcement message, but always even (required).
• 9: presented in BOLT 11 invoices.

Bits	Name	Description	Context	Dependencies	Link
0/1	option_data_loss_protect	Requires or supports extra channel_reestablish fields	IN		BOLT #2
3	initial_routing_sync	Sending node needs a complete routing information dump	I		BOLT #7
4/5	option_upfront_shutdown_script	Commits to a shutdown scriptpubkey when opening channel	IN		BOLT #2
6/7	gossip_queries	More sophisticated gossip control	IN		BOLT #7
8/9	var_onion_optin	Requires/supports variable-length routing onion payloads	IN9		Routing Onion Specification
10/11	gossip_queries_ex	Gossip queries can include additional information	IN	gossip_queries	BOLT #7
12/13	option_static_remotekey	Static key for remote output	IN		BOLT #3
14/15	payment_secret	Node supports payment_secret field	IN9	var_onion_optin	Routing Onion Specification
16/17	basic_mpp	Node can receive basic multi-part payments	IN9	payment_secret	BOLT #4
18/19	option_support_large_channel	Can create large channels	IN		BOLT #2
20/21	option_anchor_outputs	Anchor outputs	IN	option_static_remotekey	BOLT #3
22/23	option_anchors_zero_fee_htlc_tx	Anchor commitment type with zero fee HTLC transactions	IN		BOLT #3, lightning-dev
26/27	option_shutdown_anysegwit	Future segwit versions allowed in shutdown	IN		BOLT #2

Requirements

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 init Message.

Rationale

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 node_announcement 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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