SIMD-0313: Drop unchained merkle shreds

Author: AshwinSekar

proposals/0313-drop-unchained-merkle-shreds.md

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+---
+simd: '0313'
+title: Drop unchained merkle shreds
+authors:
+  - Ashwin Sekar
+category: Standard
+type: Core
+status: Review
+created: 2025-06-30
+feature:
+  - 5KLGJSASDVxKPjLCDWNtnABLpZjsQSrYZ8HKwcEdAMC8
+  - https://github.com/anza-xyz/feature-gate-tracker/issues/80
+---
+
+## Summary
+
+As the new shred format is being sent by all clients, this feature deprecates
+the old shred format.
+
+## Motivation
+
+It is inconvinient to support both formats, and upcoming consensus changes (alpenglow)
+expect all shreds to be of the chained variety.
+
+## New Terminology
+
+N/A
+
+## Detailed Design
+
+The chained merkle shred variant, adds an additional field to Data and Coding shreds.
+It adds the Merkle root of the previous erasure batch after the data buffer or
+erasure coded shard respectively, before the merkle proof:
+
+```
+// Layout: {common, data} headers | data buffer
+//     | [Merkle root of the previous erasure batch if chained]
+//     | Merkle proof
+//     | [Retransmitter's signature if resigned]
+// The slice past signature till the end of the data buffer is erasure coded.
+// The slice past signature and before the merkle proof is hashed to generate
+// the Merkle tree. The root of the Merkle tree is signed.
+
+// Layout: {common, data} headers | data buffer
+//     | [Merkle root of the previous erasure batch if chained]
+//     | Merkle proof
+//     | [Retransmitter's signature if resigned]
+// The slice past signature till the end of the data buffer is erasure coded.
+// The slice past signature and before the merkle proof is hashed to generate
+// the Merkle tree. The root of the Merkle tree is signed.
+```
+
+Additionally in the common shred header, the first 4 bits of the shred variant
+field are reserved to specify the shred variant. The second bit indicates if this
+shred is of the chained merkle shred variant.
+
+If `drop_unchained_merkle_shreds: 5KLGJSASDVxKPjLCDWNtnABLpZjsQSrYZ8HKwcEdAMC8`
+is active, then any shred with the second bit of the shred variant as zero will
+be dropped on ingest.
+
+## Alternatives Considered
+
+None
+
+## Impact
+
+Any clients still producing blocks using the old shred format will have their
+shreds ignored on ingest.
+
+## Security Considerations
+
+None
+
+## Backwards Compatibility
+
+This feature is not backwards compatible.