# BOLT #9: Assigned Feature Flags

This document tracks the assignment of `features` flags in the `init`
message ([BOLT #1](01-messaging.md)), as well as `features` fields in
the `channel_announcement` and `node_announcement` messages ([BOLT
#7](07-routing-gossip.md)).  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(_even_ bits), which will be refused by outdated nodes: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](01-messaging.md#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](11-payment-encoding.md) invoices.

| Bits  | Name                             | Description                                               | Context  | Dependencies      | Link                                  |
|-------|----------------------------------|-----------------------------------------------------------|----------|-------------------|---------------------------------------|
| 0/1   | `option_data_loss_protect`       | Requires or supports extra `channel_reestablish` fields   | IN       |                   | [BOLT #2][bolt02-retransmit]          |
| 3     | `initial_routing_sync`           | Sending node needs a complete routing information dump    | I        |                   | [BOLT #7][bolt07-sync]                |
| 4/5   | `option_upfront_shutdown_script` | Commits to a shutdown scriptpubkey when opening channel   | IN       |                   | [BOLT #2][bolt02-open]                |
| 6/7   | `gossip_queries`                 | More sophisticated gossip control                         | IN       |                   | [BOLT #7][bolt07-query]               |
| 8/9   | `var_onion_optin`                | Requires/supports variable-length routing onion payloads  | IN9      |                   | [Routing Onion Specification][bolt04] |
| 10/11 | `gossip_queries_ex`              | Gossip queries can include additional information         | IN       | `gossip_queries`  | [BOLT #7][bolt07-query]               |
| 12/13 | `option_static_remotekey`        | Static key for remote output                              | IN       |                   | [BOLT #3](03-transactions.md)         |
| 14/15 | `payment_secret`                 | Node supports `payment_secret` field                      | IN9      | `var_onion_optin` | [Routing Onion Specification][bolt04] |
| 16/17 | `basic_mpp`                      | Node can receive basic multi-part payments                | IN9      | `payment_secret`  | [BOLT #4][bolt04-mpp]                 |

## 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](01-messaging.md#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](11-payment-encoding.md) invoice contexts, the features as set in
the [BOLT 11](11-payment-encoding.md) invoice should override those set in the
`node_announcement`. This keeps things consistent with the unknown features
behavior as specified in [BOLT 7](07-routing-gossip.md#the-node_announcement-message).

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.

![Creative Commons License](https://i.creativecommons.org/l/by/4.0/88x31.png "License CC-BY")
<br>
This work is licensed under a [Creative Commons Attribution 4.0 International License](http://creativecommons.org/licenses/by/4.0/).

[bolt02-retransmit]: 02-peer-protocol.md#message-retransmission
[bolt02-open]: 02-peer-protocol.md#the-open_channel-message
[bolt04]: 04-onion-routing.md
[bolt07-sync]: 07-routing-gossip.md#initial-sync
[bolt07-query]: 07-routing-gossip.md#query-messages
[bolt04-mpp]: 04-onion-routing.md#basic-multi-part-payments