Add some documentation for protocol developers

book/src/SUMMARY.md

@@ -53,3 +53,4 @@
 * [Contributing](./contributing.md)
     * [Development Environment](./setup.md)
 * [FAQs](./faq.md)
+* [Protocol Developers](./developers.md)

book/src/developers.md

@@ -0,0 +1,56 @@
+# For Protocol Developers
+
+_Documentation for protocol developers._
+
+This section lists Lighthouse-specific decisions that are not strictly spec'd and may be useful for
+other protocol developers wishing to interact with lighthouse.
+
+
+
+## Custom ENR Fields
+
+Lighthouse currently uses the following ENR fields:
+
+### Ethereum Consensus Specified
+
+| Field | Description |
+| ---- | ---- |
+| `eth2` | The `ENRForkId` in SSZ bytes specifying which fork the node is on |
+| `attnets` | An SSZ bitfield which indicates which of the 64 subnets the node is subscribed to for
+an extended period of time |
+| `syncnets` | An SSZ bitfield which indicates which of the sync committee subnets the node is
+subscribed to |
+
+
+### Lighthouse Custom Fields
+
+Lighthouse is currently using the following custom ENR fields.
+| Field | Description |
+| ---- | ---- |
+| `quic` | The UDP port on which the QUIC transport is listening on IPv4 |
+| `quic6` | The UDP port on which the QUIC transport is listening on IPv6 |
+
+
+## Custom RPC Messages
+
+The specification leaves room for implementation-specific errors. Lighthouse uses the following
+custom RPC error messages.
+
+### Goodbye Reason Codes
+
+| Code | Message | Description |
+| ---- | ---- | ---- |
+| 128 | Unable to Verify Network | Teku uses this, so we adopted it. It relates to having a fork
+mismatch |
+| 129 | Too Many Peers | Lighthouse can close a connection because it has reached its peer-limit and
+pruned excess peers |
+| 250 | Bad Score | The node has been dropped due to having a bad peer score |
+| 251 | Banned | The peer has been banned and disconnected |
+| 252 | Banned IP | The IP the node is connected to us with has been banned |
+
+
+### Error Codes
+
+| Code | Message | Description |
+| ---- | ---- | ---- |
+| 139 | Rate Limited | The peer has been rate limited so we return this error as a response |