docs(dot/parachain): design for availability recovery

docs/docs/design/availability-recovery.md

@@ -0,0 +1,30 @@
+## Availability Recovery Subsystem Design
+
+The design for the Availability Recovery Subsystem is based on the Polkadot Host specification as detailed in the
+[Polkadot Parachain Host Implementers' Guide](https://paritytech.github.io/polkadot-sdk/book/node/availability/availability-recovery.html).
+This subsystem is responsible for recovering the data made available via the Availability Distribution subsystem,
+neccessary for candidate validation during the approval/disputes processes. Additionally, it is also being used by
+collators to recover PoVs in adversarial scenarios where the other collators of the para are censoring blocks. The
+implementation is composed of the following components:
+
+- **Create skeleton for Availability Recovery subsystem**: Create skeleton code for the implementation of this 
+  subsystem utilizing the subsystem pattern that has been used in other gossamer subsystems.  The details for this 
+  implementation is described in [issue #4297](https://github.com/ChainSafe/gossamer/issues/4297). 
+- **Implement logic for determing and launching recovery strategy**: This component is responsible for determining 
+  the recovery strategy to be used based on the system configuration, parameters and session info.  It launches the 
+  `RecoveryTask` using the determined strategy. The details for this implementation is described in [issue #4298](https://github.com/ChainSafe/gossamer/issues/4298).
+- **Implement code for FullFetch recovery strategy**: This component is responsible for implementing the logic for 
+  the FullFetch recovery strategy. This strategy tries requesting the full available data from the validators in the 
+  backing group to which the node is already connected. They are tried one by one in a random order. It is very 
+  performant if there's enough network bandwidth and the backing group is not overloaded. The costly reed-solomon 
+  reconstruction is not needed. See [issue #4299](https://github.com/ChainSafe/gossamer/issues/4299) for more details.
+- **Implement code for FetchSystematicChunks recovery strategy**: This component is responsible for implementing the 
+  logic for the FetchSystematicChunks recovery strategy.  Very similar to FetchChunks below but requests from the
+  validators that hold the systematic chunks, so that we avoid reed-solomon reconstruction. Only possible if
+  `node_features::FeatureIndex::AvailabilityChunkMapping` is enabled and the core_index is supplied (currently only for
+  recoveries triggered by approval voting). See [issue #4300](https://github.com/ChainSafe/gossamer/issues/4300) for more details.
+- **Implement code for FetchChunks recovery strategy**: This component is responsible for implementing the logic for 
+  the FetchChunks recovery strategy.  The least performant strategy but also the most comprehensive one. It's the 
+  only one that cannot fail under the byzantine threshold assumption, so it's always added as the last one in the 
+  recovery_strategies queue. Performs parallel chunk requests to validators. When enough chunks were received, do 
+  the reconstruction. In the worst case, all validators will be tried. See [issue #4301](https://github.com/ChainSafe/gossamer/issues/4301) for more details.

docs/docs/design/availability-subsystems.md

@@ -11,13 +11,13 @@ Make availability data (PoV and Persisted Validation Data) widely available with
 
 Distribute  erasure-coded chunks of availability data, and track distribution using signed bitfields.
 Enabling reassembling of complete PoV when required, for example when approval checker needs to validate a parachain block
-
 
 Availability is handled by these subsystems:
+
 - Availability Distribution subsystem
 - Bitfield Signing subsystem
-- Bitfield Distribution subsystem
-- Availability Recovery subsystem
+- [Bitfield Distribution subsystem](./bitfield-distribution.md)
+- [Availability Recovery subsystem](./availability-recovery.md)
 - Availability Store subsystem 
 
 ## Availability Distribution subsystem
@@ -45,7 +45,7 @@ This implements a gossip system.
 
 ![](../assets/img/availability-thresholds.png)
 
-## Availability Recovery subsystem
+## [Availability Recovery subsystem](./availability-recovery.md)
 - Responsible for recovering data made available via the availability distribution subsystem.
 - Necessary for candidate validation during approval/disputes process
 Also used by collators to recover PoVs in adversarial scenarios where other collators of the parachain are censoring blocks.