```
  BIP: 434
  Layer: Peer Services
  Title: Peer Feature Negotiation
  Authors: Anthony Towns <aj@erisian.com.au>
  Status: Draft
  Type: Specification
  Assigned: 2026-01-14
  License: BSD-2-Clause
  Discussion: 2025-12-19: https://gnusha.org/pi/bitcoindev/aUUXLgEUCgGb122o@erisian.com.au/T/#u
              2020-08-21: https://gnusha.org/pi/bitcoindev/20200821023647.7eat4goqqrtaqnna@erisian.com.au/
  Version: 0.1.0
```

## Abstract

This BIP defines a peer-to-peer (P2P) message that can be used for
announcements and negotiation related to support of new peer-to-peer
features.

## Motivation

Historically, new peer-to-peer protocol changes have been tied to
bumping the protocol version, so that nodes know to only attempt
feature negotiation with peers that support the feature. Coordinating
the protocol version across implementations, when different clients may
have different priorities for features to implement, is an unnecessary
burden in the upgrade process for P2P features that do not require
universal support. And at a more philosophical level, having the P2P
protocol be [permissionlessly extensible][permless-extensible], with no
coordination required between implementations or developers, seems ideal
for a decentralized system.

Many earlier P2P protocol upgrades were implemented as new messages
sent after a peer connection is set up (ie, after receipt of a `verack`
message by both sides). See [BIP 130 (sendheaders)][BIP130], [BIP 133
(feefilter)][BIP133], and [BIP 152 (compact blocks)][BIP152] for some
examples. However, for some P2P upgrades, it is helpful to perform
feature negotiation prior to a connection being fully established
(ie, prior to the `verack` being received by both sides). [BIP 155
(addrv2)][BIP155] and [BIP 339 (wtxid-relay)][BIP339] are examples of
this approach, which involves sending and receiving a single new message
(`sendaddrv2` and `wtxidrelay` respectively), in between `version` and
`verack` to indicate support of the new feature.

In all these cases, sending new messages on the network raises the
question of what non-implementing software will do with such messages. The
common behavior observed on the network was for software to ignore
unknown messages received from a peer, so these proposals posed minimal
risk of potential network partitioning. In fact, supporting protocol
extensibility in this manner was given as an explicit reason to ignore
unknown messages in Bitcoin's [first release][0.1-extensibility].

However, if nodes respond to unknown messages by disconnecting, then
the network might partition in the future as incompatible software is
deployed. And in fact, some clients on the network have historically
discouraged or disallowed unknown messages, both between `version`
and `verack` (eg, Bitcoin Core discouraged such messages between
[PR#9720][PR#9720] and [PR#19723][PR#19723], and btcd disallowed
such messages until [PR#1812][btcd#1812], but see also discussion in
[#1661][btcd#1661]), as well as after `verack`.

To maximise compatibility with such clients, most of these BIPs require
that peers bump the protocol version:

 * [BIP 130][BIP130] requires version 70012 or higher,
 * [BIP 133][BIP133] requires version 70013 or higher,
 * [BIP 152][BIP152] recommends version 70014/70015 or higher, and
 * [BIP 339][BIP339] requires version 70016 or higher.

And while [BIP 155][BIP155] does not specify a minimum protocol version,
implementations have [added][PR#20564] a de facto requirement of version
70016 or higher.

In this BIP, we propose codifying and generalising the mechanism used by
[BIP 339][BIP339] for future P2P upgrades, by adding a single new feature
negotiation message that can be reused for advertising arbitrary new
features, and requiring that implementing software ignore unknown features
that might be advertised. This allows future upgrades to negotiate new
features by exchanging messages prior to exchanging `verack` messages,
without concerns of being unnecessarily disconnected by a peer which
doesn't understand the messages, and without needing to coordinate
updating the protocol version.

## Specification

The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in RFC 2119.

For the purposes of this section, `CompactSize` refers to the
variable-length integer encoding used across the existing P2P protocol
to encode array lengths, among other things, in 1, 3, 5 or 9 bytes. Only
`CompactSize` encodings which are minimally-encoded (ie the shortest
length possible) are used by this specification.

Nodes implementing this BIP:

 * MUST advertise a protocol version number `>= 70017`,
 * MUST NOT send `feature` messages to peers that advertise a protocol
   version number `< 70017`,
 * MUST accept `feature` messages received after the `version` message
   and before the `verack` message, and
 * MUST NOT send `feature` messages after sending the `verack` message.

In addition, nodes implementing this BIP:

 * SHOULD ignore unknown messages received after the `version` message
   and before the `verack` message,
 * MAY ignore `feature` messages sent after `verack`, and
 * MAY disconnect peers who send `feature` messages after `verack`.

Feature specifications based on this BIP:

 * MUST forbid sending messages it introduces after `verack` to a peer
   that has not indicated support for the feature via a `feature`
   message.

### `feature` message

The payload of the `feature` message contains exactly the following data:

| Type        | Name          | Description |
| ----------- | ------------- | ----------- |
| string      | `featureid`   | Unique identifier for the feature |
| byte-vector | `featuredata` | Feature-specific configuration data |

The `featureid` is encoded in the usual way, that is, as a `CompactSize`
specifying the string length, followed by that many bytes. The string
length MUST be between 4 and 80, inclusive. The string SHOULD include
only printable ASCII characters (ie, each byte should have a value
between 32 and 126, inclusive).

Likewise, `featuredata` is encoded as a `CompactSize` specifying the
byte-vector size, followed by that many bytes. How these bytes are
interpreted is part of the feature's specification. The byte-vector size
MUST NOT be more than 512 bytes. Note that the `featuredata` field is not
optional, so if no data is required, an empty vector should be provided,
ie serialized as `CompactSize` of 0.

Nodes implementing this BIP MUST ignore `feature` messages specifying a
`featureid` they do not support, so long as the payload conforms to the
requirements above.

Nodes implementing this BIP MAY disconnect peers that send `feature`
messages where the `feature` message's payload cannot be correctly
parsed (including having missing or additional data), even if they do
not recognise the `featureid`.

The `featureid` MUST be a globally unique identifier for the feature.
For features published as a BIP, the `featureid` SHOULD be the assigned
BIP number, eg "BIP434", or be based on the BIP number (eg, "BIP434v2"
where the "v2" suffix covers versioning, or "BIP434.3" where the ".3"
suffix covers part 3 of the BIP). For experimental features that do not
(yet) have a BIP number assigned, some other unique identifier MUST be
chosen, such as a URL to the repository where development is taking place,
or the sha256 digest of some longer reference.

#### `feature` message 1-byte identifier

Nodes implementing both this BIP and [BIP 324 (v2 P2P encrypted
transport)][BIP324] MUST treat a message with a 1-byte `message_type`
equal to `37` that is received prior to `verack` as the `feature` message.

### Feature negotiation

It is RECOMMENDED that feature negotiation be designed and implemented
as follows:

 * all `feature` messages and the `verack` message should be sent
   immediately on receipt of the peer's `version` message
 * any negotiation calculations should be performed immediately on
   receipt of the peer's `verack` message

This structure is fairly easy to implement, and avoids introducing any
significant latency that might result from more interactive negotiation
methods.

Feature specifications defining a `featureid` MAY make use of the
following approaches:

#### Feature advertisement:

 1. Send a `feature` message advertising the `featureid` unconditionally
 2. Accept messages related to the feature unconditionally
 3. Only send messages defined by the feature if the peer sent
    a valid `feature` message for the `featureid`.

This approach is appropriate for many simple features that define
new messages, particularly where an implementation might only
implement sending or receiving a message, but not both, eg [BIP 35
(mempool)][BIP35].

#### Feature coordination:

 1. Send a `feature` message advertising the `featureid` unconditionally
 2. Check if the peer sends the same `feature` message (or a compatible
    one), and enable the feature for this peer if so.
 3. Only send/accept messages or encode data items according to the
    feature's specification if the feature is enabled for this peer.

This approach is appropriate for upgrades to data encoding in
P2P messages, eg [BIP 339 (wtxidrelay)][BIP339] or [BIP 155
(addrv2)][BIP155].

#### Feature versioning:

 1. Send `feature` messages for multiple incompatible features, eg
    `BIP434v3`, `BIP434v2`, `BIP434v1`, ordered from most preferred
    to least.
 2. Track the corresponding `feature` messages from your peer.
 3. If you were the listening peer, enable your highest preference feature
    that your peer also supports.
 4. If you were the initiating peer, enable the first feature that your
    peer announced, that you also support.
 5. For example if the listening peer sends `BIP434v3`, `BIP434v2`,
    `BIP434v1`, and the initiating peer sends `BIP434v1`, `BIP434v2`,
    then the listening peer should select `BIP434v2` when `verack`
    is received, and the initiating peer should select `BIP434v2`
    as soon as `feature BIP434v2` is received.
 6. Conversely, if the initiating peer sends `BIP434v3`, `BIP434v2`,
    `BIP434v1`, and the listening peer sends `BIP434v1`, `BIP434v2`,
    then the listening peer should select `BIP434v1` when `verack`
    is received, and the initiating peer should select `BIP434v1`
    as soon as `feature BIP434v1` is received.
 7. In most cases, implementations should simply advertise incompatible
    features in order from most recent to oldest, on the basis that
    the only reason to make incompatible updates is because there are
    significant improvements. Exceptions to that may occur when two
    incompatible features are both receiving active development, or
    when an implementation has only partially implemented the latest
    spec, and the older spec is better supported (and thus should be
    listed first, as the preferred protocol to adopt).

This approach may be appropriate when making substantial changes to a
deployed protocol and backwards compatibility is desirable on a short-term
basis, or when there is disagreement amongst implementations or users
as to which approach is most desirable.

## Considerations

The advantage this approach has over bumping the protocol version
number when introducing new P2P messages or data structures, is that no
coordination is required (that is, there is no longer a question whether
version "n+1" belongs to Alice's new feature, or Bob's new feature),
and there is no implication that supporting each new feature means all
prior features are also supported.

The advantage this approach has over defining new messages for each
feature is that the `featureid` can be much longer (at up to 80
bytes) than a message type id (which are limited to 12 bytes). With a
[BIP 324][BIP324] one-byte `message_type`, the overhead compared to that
approach is also kept small.

This approach is largely equivalent to adding a [payload to the `verack`
message][verack-payload] (eg, a vector of `featureid`, `featuredata`
pairs). It was chosen because:

 * it retains compatibility with any implementations that expect `verack`
   to have no payload;
 * it allows peers to process each feature request individually, rather than
   having to first load the configuration information for all features into
   memory at once (in order to validate the message's checksum), and then
   deal with each feature's configuration;
 * limiting the maximum message payload size you accept (eg to 4MB)
   does not limit the number of features you can accept; and
 * we have experience with negotiating features with individual messages,
   but no experience with doing so via `verack` payload.

A mild disadvantage compared to using a `verack` payload is that this
approach allows the possibility of interactive feature negotiation prior
to `verack`. However interactive feature negotiation is always possible
simply by having the initiating peer disconnect and reconnect after
discovering the listening peer's supported features.

This specification attempts to maximise compatibility with implementations
that prefer to fully validate each message received:

 * `feature` messages, even for unknown features, must always be fully
   parseable into a `featureid` and `featuredata`
 * Ignoring unknown messages prior to `verack` is only a recommendation,
   not a requirement, so compliant implementations may disconnect on an
   unknown message that cannot be validated.
 * Sending unknown messages after `verack` is explicitly forbidden,
   in so far as that is possible.

## Backward compatibility

Clients specifying a version number prior to `70017` remain fully
compatible with this change.

Clients specifying a version number of `70017` or higher that do not
implement this BIP remain fully compatible provided they do not disconnect
peers upon receiving unexpected messages received between `version` and
`verack`.

## Acknowledgements

Much of the logic here, and much of the text in the motivation section,
is based on Suhas Daftuar's 2020 post on [Generalizing feature
negotiation][suhas-draft].

## Copyright

This BIP is licensed under the 2-clause BSD license.

[BIP130]: https://github.com/bitcoin/bips/blob/master/bip-0130.mediawiki
[BIP133]: https://github.com/bitcoin/bips/blob/master/bip-0133.mediawiki
[BIP152]: https://github.com/bitcoin/bips/blob/master/bip-0152.mediawiki
[BIP155]: https://github.com/bitcoin/bips/blob/master/bip-0155.mediawiki
[BIP339]: https://github.com/bitcoin/bips/blob/master/bip-0339.mediawiki
[BIP35]:  https://github.com/bitcoin/bips/blob/master/bip-0035.mediawiki
[BIP324]:  https://github.com/bitcoin/bips/blob/master/bip-0324.mediawiki
[verack-payload]: https://gnusha.org/pi/bitcoindev/B514142F-382B-4D49-B68D-0115ECBD1D79@voskuil.org/
[PR#20564]: https://github.com/bitcoin/bitcoin/pull/20564
[PR#9720]: https://github.com/bitcoin/bitcoin/pull/9720
[PR#19723]: https://github.com/bitcoin/bitcoin/pull/19723
[btcd#1812]: https://github.com/btcsuite/btcd/pull/1812
[btcd#1661]: https://github.com/btcsuite/btcd/issues/1661
[permless-extensible]: https://github.com/bitcoin/bitcoin/pull/20564#issuecomment-738456560
[0.1-extensibility]: https://github.com/benjiqq/bitcoinArchive/blob/master/bitcoin0.1/src/main.cpp#L2035-L2039
[suhas-draft]: https://gnusha.org/pi/bitcoindev/CAFp6fsE=HPFUMFhyuZkroBO_QJ-dUWNJqCPg9=fMJ3Jqnu1hnw@mail.gmail.com/
