Multicast Subtree Group Announcement Frame Format
Jeff Harris (jeff@lightweb.net)
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:
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.
SubtreeGroupAnnounce — a periodic 64-byte control datagram sent by block assemblers announcing that a given
SubtreeIDis a member of a given group.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.
Site (0x05)
FF05::B:FFFC
Org (0x08)
FF08::B:FFFC
Global (0x0E)
FF0E::B:FFFC
Operators MAY override the IANA group-id (0x000B) via the -mc-group-id configuration flag for private deployments; the resulting addresses change accordingly.
SubtreeGroupAnnounce Wire Format (MsgType 0x30) — 64 bytes
MsgType 0x30) — 64 bytes0
4
Network Magic
0xE3E1F3E8 (BSV mainnet P2P magic)
4
2
Protocol Ver
0x02BF (703, BSV large-block baseline)
6
1
MsgType
0x30 (SubtreeGroupAnnounce)
7
1
Flags
Reserved; must be 0x00
8
32
SubtreeID
SHA-256 subtree root hash (from BRC-124 frame header)
40
16
GroupID
128-bit group identifier, big-endian
56
4
Epoch
Unix timestamp (uint32 BE) when this announcement was created
60
2
TTL
Validity in seconds (uint16 BE); 0 = use listener default
62
2
Reserved
Must be 0x0000
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
For each (SubtreeID, GroupID) pair, construct a 64-byte SubtreeGroupAnnounce datagram.
Send the datagram as a UDP packet to the SubtreeGroupAnnounce control group on the configured multicast scope(s).
Repeat at the configured announcement interval (recommended: 10–30 seconds).
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
Join the SubtreeGroupAnnounce control-plane multicast group on the configured scope(s).
For each received datagram, validate the magic, ProtoVer, MsgType, and length.
If a sender include/exclude filter is configured, check the UDP source address before processing.
If the GroupID matches a subscribed group, record
(SubtreeID → expiry)in the group registry.Periodically evict entries whose expiry has passed.
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.
Example
SubtreeGroupAnnounce for SubtreeID A3F1...9247, GroupID A1B2C3D4E5F6A7B8C9D0E1F2A3B4C5D6, Epoch 1746800000, TTL 0 (use listener default):
Constants Reference
MagicBSV
3823236072
0xE3E1F3E8
BSV mainnet P2P magic
ProtoVer
703
0x02BF
Protocol version
MsgTypeSubtreeGroupAnnounce
48
0x30
SubtreeGroupAnnounce datagram type
GroupSubtreeGroupAnnounce
65532
0xFFFC
Control-plane subtree group announce group
GroupBeacon
65533
0xFFFD
Control-plane beacon group (BRC-126)
SubtreeGroupAnnounceSize
64
—
Fixed datagram size in bytes
DefaultAnnounceTTL
900
—
Recommended listener default TTL (s)
DefaultAnnounceInterval
10
—
Recommended sender re-announce interval (s)
References
BRC-124: Multicast Transaction Frame Format — Data-plane frame format; defines
SubtreeID
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