EIP: Implement eth_chainId method in JSON-RPC

EIPS/eip-rpc_eth_chain_id.md

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+## Preamble
+
+    EIP: <to be assigned>
+    Title: Create `eth_chainId` method for JSON-RPC
+    Author: Isaac Ardis: isaac.ardis@gmail.com, Wei Tang: hi@that.world, [@tcz001](https://github.com/tcz001)
+    Type: Standard Track
+    Category: Interface
+    Status: Draft
+    Created: 2017-08-21
+
+
+## Simple Summary
+Include `eth_chainId` method in `eth_`-namespaced JSON-RPC methods.
+
+## Abstract
+The `eth_chainId` method should return a single STRING result
+for an integer value in hexadecimal format, describing the
+currently configured "Chain Id" value used for signing replay-protected transactions,
+introduced via EIP-155.
+
+## Motivation
+Currently although we can use net_version RPC call to get the
+current network ID, there's no RPC for querying the chain ID. This
+makes it impossible to determine the current actual blockchain using
+the RPC.
+
+## Specification
+
+----
+
+### eth_chainId
+
+Returns the currently configured chain id, a value used in replay-protected transaction
+signing as introduced by EIP-155.
+
+##### Parameters
+none
+
+##### Returns
+
+`QUANTITY` - big integer of the current chain id. Defaults are mainnet=61, morden=62.
+
+##### Example
+```js
+curl -X POST --data '{"jsonrpc":"2.0","method":"eth_chainId","params":[],"id":1}'
+
+// Result
+{
+  "id":83,
+  "jsonrpc": "2.0",
+  "result": "0x3d" // 61
+}
+```
+
+----
+
+## Rationale
+An ETH/ETC client can accidentally connect to an ETC/ETH RPC
+endpoint without knowing it unless it tries to sign a transaction or
+it fetch a transaction that is known to have signed with a chain
+ID. This has since caused trouble for application developers, such as
+MetaMask, to add multi-chain support.
+
+Please note related links:
+
+- [Parity PR](https://github.com/paritytech/parity/pull/6329)
+- [Geth Classic PR (merged)](https://github.com/ethereumproject/go-ethereum/pull/336)
+
+
+## Backwards Compatibility
+Not relevant.
+
+## Test Cases
+Not currently implemented.
+
+## Implementation
+Would be good to have a test to confirm that expected==got.
+
+## Copyright
+Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).