Ctrlk
For the complete documentation index, see llms.txt. This page is also available as Markdown.

Multicast Transaction Frame Format

Jeff Harris (jeff@lightweb.net)

Abstract

This BRC specifies the wire format for transporting BSV transactions over IPv6 multicast and TCP/UDP unicast. The format extends the legacy BRC-12 frame with additional fields for flow identification, sequence tracking, and subtree identification. All fields are 8-byte aligned for efficient memory access on 64-bit architectures.

Copyright

This BRC is licensed under the Open BSV License.

Motivation

As BSV transaction throughput scales to billions of transactions per second, efficient distribution infrastructure becomes critical. IPv6 multicast provides a scalable one-to-many transport mechanism, but globally scalable transaction distribution requires:

  1. Deterministic sharding — routing transactions to specific multicast groups based on transaction ID

  2. Sequence tracking — detecting and recovering from frame loss via hash-chain NACK-based retransmission

  3. Subtree identification — filtering batches of related transactions at the network layer

  4. Flow identification — stable per-flow hashing for gap tracking and NACK-based retransmission

This frame format addresses these requirements while maintaining compatibility with the existing BRC-12 transaction payload format. It uses a 92-byte header that provides the metadata necessary for high-throughput multicast distribution with best-effort reliability, complementing the legacy 44-byte header defined in BRC-12.

Conceptual Attribution: The IPv6 multicast transaction broadcast architecture from which this software draws inspiration was articulated by Dr. Craig S. Wright in Multicast Within Multicast: Anycast, Sharded Resends, and Hierarchical Distribution for Transaction and Block Propagation.

Specification

Frame Structure Overview

A frame consists of a 92-byte header followed by a variable-length payload containing a BRC-12 raw transaction. All multi-byte integers are big-endian. All fields from offset 8 onward are 8-byte aligned.

Header Format (92 bytes)

Offset
Size
Alignment
Field
Description

0

4

—

Network Magic

0xE3E1F3E8 (BSV mainnet P2P magic)

4

2

—

Protocol Version

0x02BF (703, BSV large-block baseline)

6

1

—

Frame Version

0x02

7

1

—

Reserved

Must be 0x00

8

32

8-byte

Transaction ID

Raw 256-bit TXID (internal byte order)

40

8

8-byte

HashKey

Stable per-flow XXH64 identifier; 0 = unset

48

8

8-byte

SeqNum

Monotonic per-flow counter; 0 = unset

56

32

8-byte

Subtree ID

32-byte batch identifier; zeros = unset

88

4

8-byte

Payload Length

uint32 BE

92

*

—

Payload

BRC-12 raw transaction bytes

Field Definitions

Network Magic (bytes 0–3)

The value 0xE3E1F3E8 (BSV mainnet P2P network magic). This enables standard BSV firewall rules and network monitoring tools to correctly classify shard frames. Frames with incorrect magic are rejected.

Protocol Version (bytes 4–5)

The value 0x02BF (703 in decimal), representing the BSV node protocol version that introduced the large-block policy. This field is informational; receivers do not validate it.

Frame Version (byte 6)

The value 0x02 for frames following this specification. The legacy format defined in BRC-12 uses 0x01 (see Compatibility section). Any other value causes the frame to be rejected.

Reserved (byte 7)

Must be 0x00. Reserved for future protocol extensions.

Transaction ID (bytes 8–39)

The 32-byte transaction hash in internal byte order (as used in BSV P2P protocol), not the reversed display order used by block explorers.

HashKey (bytes 40–47)

A 64-bit unsigned integer (big-endian) containing a stable per-flow identifier computed as:

The 52-byte hash input combines the sender's IPv6 address, the derived multicast group index, and the subtree identifier to produce an 8-byte key that is constant for all frames in a flow. This enables per-flow gap tracking, NACK dispatch, and cache lookup without carrying the full 52 bytes in every frame.

This field is stamped in-place by the ingress proxy (bitcoin-shard-proxy) before multicast forwarding. Senders (generators) set it to 0; a value of 0 indicates the frame has not been stamped and gap tracking is skipped.

SeqNum (bytes 48–55)

A 64-bit unsigned integer (big-endian) containing a monotonic counter per flow, starting at 1. Incremented by 1 for each frame in a (sender, group, subtree) flow. Used by receivers to detect gaps: a gap is detected when the incoming SeqNum advances by more than 1 from the last-seen value for the same HashKey.

The pair (HashKey, SeqNum) forms the 16-byte cache key used by retry endpoints for NACK-based retransmission.

This field is stamped in-place by the ingress proxy alongside HashKey. A value of 0 indicates the frame has not been stamped.

Subtree ID (bytes 56–87)

A 32-byte opaque batch identifier. A subtree is an ordered set of related transactions sharing a common batch context. This field enables downstream subscribers to filter frames by batch at the application layer. All-zero bytes indicate the field is unset.

Payload Length (bytes 88–91)

A 32-bit unsigned integer (big-endian) specifying the number of payload bytes immediately following the header. It is up to the application to determine the maximum allowed payload size.

Payload (byte 92 onward)

Raw serialized BSV transaction in BRC-12 format: version (4 bytes LE) + input vector + output vector + locktime (4 bytes LE). No additional envelope wraps the transaction.

Alignment Verification

Field
Offset
Offset % 8

Transaction ID

8

0 ✓

HashKey

40

0 ✓

SeqNum

48

0 ✓

Subtree ID

56

0 ✓

Payload Length

88

0 ✓

Payload

92

4

Compatibility with BRC-12 Legacy Format

Legacy BRC-12 frames (Frame Version 0x01) use a 44-byte header with no HashKey, SeqNum, or subtree fields:

Offset
Size
Field

0

4

Network Magic

4

2

Protocol Version

6

1

Frame Version (0x01 = legacy)

7

1

Reserved

8

32

Transaction ID

40

4

Payload Length

44

*

Payload

TCP Transport

When transported over TCP, frames are concatenated end-to-end with no additional envelope. The reader:

  1. Reads 44 bytes (sufficient for legacy header or BRC-124 header start)

  2. Inspects byte 6 (Frame Version)

    • Version 0x01 (legacy): header complete; read PayLen at bytes 40–43

    • Version 0x02 (BRC-124): read 48 more bytes; read PayLen at bytes 88–91

  3. Reads exactly PayLen bytes

  4. Processes the complete frame

Frame Hex Dump

A frame following this specification carrying a 200-byte transaction:

Legacy Frame Hex Dump

The same transaction in legacy (BRC-12) format:

References

Constants Reference

Name
Value
Hex
Description

MagicBSV

3823236072

0xE3E1F3E8

BSV mainnet P2P magic

ProtoVer

703

0x02BF

Protocol version

FrameVerLegacy

1

0x01

Legacy BRC-12 frame version

FrameVerBRC124

2

0x02

Current BRC-124 frame version

HeaderSizeLegacy

44

0x2C

Legacy header size in bytes

HeaderSize

92

0x5C

BRC-124 header size in bytes

PreviousSubTree Unified Merkle Path (STUMP) FormatNextMulticast Transaction NACK Retransmission Protocol

Last updated