docs: document communication layer architecture challenges and solution

docs/Protocol_DEV.md

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+# MetaMask SDK Protocol Development: Addressing Architectural Challenges
+
+## Core Problem: Single Communication Layer Package
+
+The original design of `sdk-communication-layer` as a single package, shared between dApps and the mobile wallet, was initially efficient. However, evolving platform requirements, particularly for React Native (mobile), have created critical issues:
+
+1.  **Dependency Conflicts:**
+    *   Mobile requires `eciesjs@^0.3.16` due to React Native limitations.
+    *   The SDK uses `eciesjs@^0.4.11`.
+    *   These versions are incompatible, blocking the use of the latest communication layer in mobile and breaking the development workflow (`sdk-comm-layer-mobile-overwrite.sh` fails with `@ecies/ciphers/aes` resolution errors).
+
+2.  **Bundle Size & Complexity:**
+    *   The mobile wallet is forced to include dApp-specific code, increasing bundle size.
+    *   Managing dependencies and debugging across environments is overly complex.
+
+## Required Solution: Package Splitting
+
+The necessary path forward is to **split the communication layer package**:
+
+
+```
+sdk-communication-layer/
+├── core/       # Shared core logic & types
+├── wallet/     # Wallet-specific code
+│   └── mobile/ # Mobile-optimized implementation (using compatible dependencies like eciesjs 0.3.x)
+└── dapp/       # dApp-specific implementation (using latest dependencies like eciesjs 0.4.x)
+```
+
+**Benefits:**
+
+*   **Resolves Dependency Conflicts:** Allows mobile and dApp implementations to use appropriate dependency versions.
+*   **Optimizes Bundle Size:** Mobile only includes necessary code.
+*   **Simplifies Development:** Easier dependency management, debugging, and platform-specific optimizations.
+*   **Unblocks Development:** Enables the mobile wallet to use an updated (but compatible) communication layer.
+
+**This architectural change is critical to address the current development blockers and ensure the maintainability and performance of the SDK across all platforms.**
+
+---
+
+*(Optional: Include simplified Architecture/Workflow sections below if needed for context)*
+
+## Architecture Components (Brief)
+
+*   **Mobile Wallet:** React Native app (`metamask-mobile`).
+*   **Communication Layer:** Currently single package (`sdk-communication-layer`), needs splitting.
+*   **Backend:** Socket server (`sdk-socket-server-next`).
+*   **dApp Examples:** Test environments (`devnext`, `playgroundnext`).
+
+## Detailed Development Workflow
+
+### Setting Up the Environment
+
+1.  Clone the repositories:
+    ```bash
+    git clone https://github.com/MetaMask/metamask-sdk
+    git clone https://github.com/MetaMask/metamask-mobile
+    ```
+
+2.  Configure environment:
+    *   Create `.env` file in SDK root (`metamask-sdk/`) with:
+        ```
+        MM_MOBILE_PATH=/path/to/metamask-mobile
+        ```
+
+### Development Process (Currently Broken)
+
+1.  **Modify Communication Layer:**
+    ```bash
+    # In metamask-sdk/
+    cd packages/sdk-communication-layer
+    # Make your changes
+    yarn build
+    ```
+
+2.  **Update Mobile Implementation (Fails):**
+    ```bash
+    # From SDK root (metamask-sdk/)
+    ./scripts/sdk-comm-layer-mobile-overwrite.sh
+    ```
+    *   **What it *should* do:** Remove old layer, copy new build, run `rn-nodeify`.
+    *   **Current State:** Fails due to the ECIES version incompatibility mentioned above. Mobile development must use the older `[email protected]`.
+
+3.  **Configure Mobile Socket Server:**
+    *   For testing against a local backend (`sdk-socket-server-next`), configure the Mobile Wallet:
+        *   Set `SDK_COMMLAYER_URL` environment variable, OR
+        *   Modify `socketServerUrl` directly in the mobile codebase (e.g., within `SDKConnect.ts`).