Multicast Subtree Group Announcement Frame Format

Abstract

This BRC specifies a periodic multicast announcement protocol that maps BRC-124 SubtreeID values to stable 128-bit group identifiers. Senders broadcast compact 64-byte datagrams on a reserved control-plane multicast group, allowing listeners to dynamically learn which subtree IDs belong to which groups and filter incoming transaction traffic accordingly — without static configuration.

Copyright

This BRC is licensed under the Open BSV License.

Motivation

BRC-124 frames carry a 32-byte SubtreeID that identifies the transaction subtree to which each frame belongs. Subtree IDs are derived deterministically by each block assembler and change with every new work candidate. Static subtree ID lists are impractical for listeners that want to subscribe to a logical class of transactions (a subtree group) across block boundaries.

This BRC introduces:

1. Subtree groups — stable 128-bit identifiers that logically group related subtree IDs. A group ID is an XXH3-128 hash of a canonical group name, or any operator-assigned 128-bit value.
2. SubtreeGroupAnnounce — a periodic 64-byte control datagram sent by block assemblers announcing that a given SubtreeID is a member of a given group.
3. Dynamic listener filtering — listeners join a dedicated control-plane multicast group, process SubtreeGroupAnnounce datagrams, maintain a time-bounded registry, and pass transactions based on group membership.

Subtrees are temporal: they are valid only for the duration of a work candidate. Announcements are sent periodically; entries not refreshed within their TTL are evicted automatically.

Specification

Control-Plane Group

SubtreeGroupAnnounce datagrams are sent to the reserved control-plane multicast group index 0xFFFC (GroupSubtreeGroupAnnounce), one slot below the BRC-126 beacon group (0xFFFD).

Control-plane addresses use the IANA-aligned layout: bytes 0–1 carry the scope prefix, bytes 2–11 are zero (IANA 96-bit boundary), bytes 12–13 carry the IANA Bitcoin group-id (0x000B), and bytes 14–15 carry the group index.

Operators MAY override the IANA group-id (0x000B) via the -mc-group-id configuration flag for private deployments; the resulting addresses change accordingly.

Announce-group addresses are shown in ASM (FF0x) form; under SSM, substitute the FF3x prefix per BRC-129 (FF3E::B:FFFC for inter-domain scope, which is SSM-only per RFC 8815). The datagram format is unchanged. This is a control-plane group whose source — the announcing block assembler — is not discoverable from within the group, so SSM listeners (S,G)-join using the bootstrap source lists defined in BRC-129; these are the same source addresses a listener may configure in the include filter (see Sender Filtering).

SubtreeGroupAnnounce Wire Format (MsgType 0x30) — 64 bytes

Total: 64 bytes. Power-of-two message size; GroupID at offset 40 is 8-byte aligned.

Flags (byte 7)

All bits are reserved and must be zero. Future versions of this protocol may define flag bits.

GroupID

A GroupID is a 128-bit value uniquely identifying a logical subtree group. It is recommended to derive GroupIDs using XXH3-128 applied to a canonical group name string, as this provides hardware-accelerated, collision-resistant derivation. GroupIDs may also be assigned directly as opaque 128-bit values.

TTL and Expiry

A TTL value of 0 instructs the listener to apply its locally configured default TTL. Listeners SHOULD set the default TTL to at least 900 seconds (15 minutes) to accommodate block creation intervals. Senders SHOULD re-announce each (SubtreeID, GroupID) pair at an interval well below the effective TTL (recommended: every 10–30 seconds).

Multi-Group Membership

A single SubtreeID may belong to multiple groups. The sender emits one SubtreeGroupAnnounce datagram per (SubtreeID, GroupID) pair per announcement interval.

Sender Behaviour

1. For each (SubtreeID, GroupID) pair, construct a 64-byte SubtreeGroupAnnounce datagram.
2. Deliver the datagram to the SubtreeGroupAnnounce control group on the configured multicast scope(s). A sender on the multicast fabric sends it as a UDP packet directly. A sender that is not on the fabric, or that has no multicast egress of its own, MAY instead submit the same 64 bytes to an ingress proxy over its submission transport and have the proxy forward them to the control group unchanged. The datagram on the control group is identical either way, so a listener cannot tell the two apart and needs no knowledge of which was used.
3. Repeat at the configured announcement interval (recommended: 10–30 seconds).
4. On work reset (new block), stop announcing old SubtreeIDs. Listeners will evict them when the TTL expires.

No retransmission or sequencing is required. Periodic re-announcement provides eventual consistency.

Listener Behaviour

1. Join the SubtreeGroupAnnounce control-plane multicast group on the configured scope(s).
2. For each received datagram, validate the magic, ProtoVer, MsgType, and length.
3. If a sender include/exclude filter is configured, check the UDP source address before processing.
4. If the GroupID matches a subscribed group, record (SubtreeID → expiry) in the group registry.
5. Periodically evict entries whose expiry has passed.
6. On data-plane frame receipt, allow the frame if its SubtreeID is present in any subscribed group's registry.

Sender Filtering

Listeners MAY configure include and exclude filters on announcement source addresses (IPv6 addresses or CIDR prefixes). The evaluation order is: exclude first, then include. An empty include list accepts all non-excluded sources.

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Example

SubtreeGroupAnnounce for SubtreeID A3F1...9247, GroupID A1B2C3D4E5F6A7B8C9D0E1F2A3B4C5D6, Epoch 1746800000, TTL 0 (use listener default):

E3E1F3E8                                  // Network Magic
02BF                                      // Protocol Version
30                                        // MsgType = SubtreeGroupAnnounce
00                                        // Flags = 0
A3F1E8B04C7D2A9156E3F08B4D7C2E9A          // SubtreeID bytes 0–15
1B5F3D8E6C0A4279B1D5E8F3C6A09247          // SubtreeID bytes 16–31
A1B2C3D4E5F6A7B8C9D0E1F2A3B4C5D6          // GroupID (128-bit)
67E27D80                                  // Epoch = 1746800000
0000                                      // TTL = 0 (use listener default)
0000                                      // Reserved

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Constants Reference

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References

• BRC-124: Multicast Transaction Frame Format — Data-plane frame format; defines SubtreeID
• BRC-129: Multicast Group Address Assignments — SSM/ASM addressing, scopes, and (S,G) source discovery