Introduce System version to RuntimeVersion object (#42)

This RFC proposes adding `system_version` and remove `state_version`
from `RuntimeVersion` object. This makes it possible to use
`StateVersion::V1` for both Storage and Extrinsic root derivations

text/0042-extrinsics-state-version.md

@@ -0,0 +1,102 @@
+# RFC-0042: Add System version that replaces StateVersion on RuntimeVersion
+
+|                 |                                             |
+|-----------------|---------------------------------------------|
+| **Start Date**  | 25th October 2023                           |
+| **Description** | Add System Version and remove State Version |
+| **Authors**     | Vedhavyas Singareddi                        |
+
+## Summary
+
+At the moment, we have `system_version` field on `RuntimeVersion` that derives which state version is used for the
+Storage.
+We have a use case where we want extrinsics root is derived using `StateVersion::V1`. Without defining a new field
+under `RuntimeVersion`,
+we would like to propose adding `system_version` that can be used to derive both storage and extrinsic state version.
+
+## Motivation
+
+Since the extrinsic state version is always `StateVersion::V0`, deriving extrinsic root requires full extrinsic data.
+This would be problematic when we need to verify the extrinsics root if the extrinsic sizes are bigger. This problem is
+further explored in https://github.com/polkadot-fellows/RFCs/issues/19
+
+For `Subspace` project, we have an enshrined rollups called `Domain` with optimistic verification and Fraud proofs are
+used to detect malicious behavior.
+One of the `Fraud proof` variant is to derive `Domain` block extrinsic root on `Subspace`'s consensus chain.
+Since `StateVersion::V0` requires full extrinsic data, we are forced to pass all the extrinsics through the Fraud proof.
+One of the main challenge here is some extrinsics could be big enough that this variant of Fraud proof may not be
+included in the Consensus block due to Block's weight restriction.
+If the extrinsic root is derived using `StateVersion::V1`, then we do not need to pass the full extrinsic data but
+rather at maximum, 32 byte of extrinsic data.
+
+## Stakeholders
+
+- Technical Fellowship, in its role of maintaining system runtimes.
+
+## Explanation
+
+In order to use project specific StateVersion for extrinsic roots, we proposed
+an [implementation](https://github.com/paritytech/polkadot-sdk/pull/1691) that introduced
+parameter to `frame_system::Config` but that unfortunately did not feel correct.
+So we would like to [propose](https://github.com/paritytech/polkadot-sdk/pull/1968) adding this change to
+the `RuntimeVersion`
+object. The system version, if introduced, will be used to derive both storage and extrinsic state version.
+If system version is `0`, then both Storage and Extrinsic State version would use V0.
+If system version is `1`, then Storage State version would use V1 and Extrinsic State version would use V0.
+If system version is `2`, then both Storage and Extrinsic State version would use V1.
+
+If implemented, the new `RuntimeVersion` definition would look something similar to
+
+```rust
+/// Runtime version (Rococo).
+#[sp_version::runtime_version]
+pub const VERSION: RuntimeVersion = RuntimeVersion {
+		spec_name: create_runtime_str!("rococo"),
+		impl_name: create_runtime_str!("parity-rococo-v2.0"),
+		authoring_version: 0,
+		spec_version: 10020,
+		impl_version: 0,
+		apis: RUNTIME_API_VERSIONS,
+		transaction_version: 22,
+		system_version: 1,
+	};
+```
+
+## Drawbacks
+
+There should be no drawbacks as it would replace `state_version` with same behavior but documentation should be updated
+so that chains know which `system_version` to use.
+
+## Testing, Security, and Privacy
+
+AFAIK, should not have any impact on the security or privacy.
+
+## Performance, Ergonomics, and Compatibility
+
+These changes should be compatible for existing chains if they use `state_version` value for `system_verision`.
+
+### Performance
+
+I do not believe there is any performance hit with this change.
+
+### Ergonomics
+
+This does not break any exposed Apis.
+
+### Compatibility
+
+This change should not break any compatibility.
+
+## Prior Art and References
+
+We [proposed](https://github.com/paritytech/polkadot-sdk/pull/1691) introducing a similar change by introducing a
+parameter to `frame_system::Config` but did not feel that
+is the correct way of introducing this change.
+
+## Unresolved Questions
+
+I do not have any specific questions about this change at the moment.
+
+## Future Directions and Related Material
+
+IMO, this change is pretty self-contained and there won't be any future work necessary.