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BRC-135: correct emitter HashKey group index to 0xFFFA; SSM egress note #174

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BRC-135: correct emitter HashKey group index to 0xFFFA; SSM egress note #174

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17 changes: 12 additions & 5 deletions transactions/0135.md
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Original file line number Diff line number Diff line change
Expand Up @@ -65,6 +65,13 @@ The egress group-id is set independently of the fabric group-id. This ensures
BRC-135 frames reach only downstream consumers, not peer fabric subscribers or
retry endpoints on the ingress fabric.

The egress address is shown in ASM (`FF05`) form; under SSM, substitute the
`FF3x` prefix per [BRC-129](./0129.md). The frame format is unchanged. The
frame's source is the emitter (`emitterIPv6`), which stamps its own identity, so
downstream consumers `(S,G)`-join per emitter. Under SSM the multi-emitter
reliability pattern uses distinct per-emitter sources (or VRRP active-standby);
anycast or shared-source emission is not supported (BRC-129).

### Frame Header Format (92 bytes)

The BRC-135 header is layout-identical to BRC-124. All infrastructure components
Expand All @@ -78,7 +85,7 @@ All multi-byte integers are big-endian.
| 6 | 1 | — | Frame Version | 0x07 — BRC-135 block header |
| 7 | 1 | — | Reserved | 0x00 |
| 8 | 32 | 8-byte | BlockHash | SHA256d of the 80-byte block header (internal byte order) |
| 40 | 8 | 8-byte | HashKey | XXH64(emitterIPv6 ∥ 0xFFFE ∥ zeros); stamped by emitter |
| 40 | 8 | 8-byte | HashKey | XXH64(emitterIPv6 ∥ 0xFFFA ∥ zeros); stamped by emitter |
| 48 | 8 | 8-byte | SeqNum | Monotonic per-emitter counter; stamped by emitter |
| 56 | 32 | 8-byte | LayoutPad32 | All zeros (no subtree scope for block headers) |
| 88 | 4 | — | PayloadLen | 0x00000050 (80 = fixed block header size, uint32 BE) |
Expand Down Expand Up @@ -121,12 +128,12 @@ same block.
A stable per-emitter flow identifier computed as:

```
HashKey = XXH64(emitterIPv6 [16 bytes] ∥ 0x0000FFFE [4 bytes BE] ∥ zeros [32 bytes])
HashKey = XXH64(emitterIPv6 [16 bytes] ∥ 0x0000FFFA [4 bytes BE] ∥ zeros [32 bytes])
```

The `0xFFFE` group index is `GroupBlockBroadcast`, the same control-plane index
used by BRC-131/133/134. The 32 zero bytes correspond to the absent SubtreeID.
The HashKey is constant for the lifetime of the emitter process.
The `0xFFFA` group index is `GroupBlockHeader`, the emitter multicast egress
channel (see the group table above). The 32 zero bytes correspond to the absent
SubtreeID. The HashKey is constant for the lifetime of the emitter process.

A value of `0` indicates the field is unset.

Expand Down