Add Backend Protocol Section to implementators guide kubernetes-sigs#…

…2453

site-src/guides/implementers.md

@@ -355,3 +355,35 @@ or both.
 Implementations that support cross-namespace references MUST watch ReferenceGrant
 and reconcile any ReferenceGrant that points to an object that's referred to by
 an in-scope Gateway API object.
+
+## Backend Protocol
+
+Not all implementations of Gateway API support automatic protocol selection. In some cases protocols are disabled without an explicit opt-in.
+
+When a Route's backend references a Kubernetes Service, application developers can specify the protocol using ServicePort appProtocol field.
+
+Reference [Backend Protocol Selection](https://gateway-api.sigs.k8s.io/guides/backend-protocol/) within the user guide.
+
+### Supporting Protocols
+
+If a Route is not able to send traffic to the backend using the specified protocol then the backend is considered invalid. Implementations MUST set `ResolvedRefs` condition to `False` with the Reason `UnsupportedProtocol`.
+
+Implementations MAY support the following combinations below:
+
+ServicePort Protocol | ServicePort AppProtocol | Route Type | Supported
+-|-|-|-
+`TCP` | `kubernetes.io/h2c` | `GRPCRoute` | Yes [1]
+`TCP` | `kubernetes.io/h2c` | `HTTPRoute` | Yes
+`TCP` | `kubernetes.io/ws`  | `HTTPRoute` | Yes
+`TCP` | `kubernetes.io/wss` | `TLSRoute`  | Yes
+
+1. GRPC works over h2c - so a GRPCRoute should be able to connect to an h2c backend
+
+Implementations MAY support the following combinations below:
+
+ServicePort Protocol | ServicePort AppProtocol | Route Type | Supported
+-|-|-|-
+ServicePort Protocol | ServicePort AppProtocol | Route Type | Supported
+`TCP`  | `kubernetes.io/wss` | `HTTPRoute` | Conditional [1]
+
+1. Only if there is a corresponding `BackendTLSPolicy` - see [GEP-1897](/geps/gep-1897)