Expand and clarify Listeners definition

Explicitly define Listener compatibility as definitively routable and
allowed on the same address by the implementation.

Narrow language about collapsing compatible Listeners to mean assigning
multiple Gateways the same addresses if their Listeners are compatible,

Expand Listeners examples to include mixed TCP and UDP and indicate that
they are not compatible if an implementation does not support that
combination.

Add language about Gateway-level conditions for incompatible Listeners.

apis/v1beta1/gateway_types.go

@@ -85,38 +85,52 @@ type GatewaySpec struct {
 	//
 	// Port and protocol combinations not listed above are considered Extended.
 	//
-	// An implementation MAY group Listeners by Port and then collapse each
-	// group of Listeners into a single Listener if the implementation
-	// determines that the Listeners in the group are "compatible". An
-	// implementation MAY also group together and collapse compatible
-	// Listeners belonging to different Gateways.
-	//
-	// For example, an implementation might consider Listeners to be
-	// compatible with each other if all of the following conditions are
-	// met:
-	//
-	// 1. Either each Listener within the group specifies the "HTTP"
-	//    Protocol or each Listener within the group specifies either
-	//    the "HTTPS" or "TLS" Protocol.
-	//
-	// 2. Each Listener within the group specifies a Hostname that is unique
-	//    within the group.
-	//
-	// 3. As a special case, one Listener within a group may omit Hostname,
-	//    in which case this Listener matches when no other Listener
-	//    matches.
-	//
-	// If the implementation does collapse compatible Listeners, the
-	// hostname provided in the incoming client request MUST be
-	// matched to a Listener to find the correct set of Routes.
-	// The incoming hostname MUST be matched using the Hostname
-	// field for each Listener in order of most to least specific.
-	// That is, exact matches must be processed before wildcard
+	// A Gateway's Listeners are considered "compatible" if:
+	//
+	// 1. The implementation can serve them in compliance with the Addresses
+	//    requirement that all Listeners are available on all assigned
+	//    addresses.
+	// 2. No Listeners sharing the same Port share the same Hostname value,
+	//    including the empty value, if this would prevent the implementation
+	//    from matching an inbound request to a specific Listener.
+	//
+	// Compatible combinations in Extended support are expected to vary across
+	// implementations. A combination that is compatible for one implementation
+	// may not be compatible for another.
+	//
+	// If this field specifies multiple Listeners that are not compatible, the
+	// implementation MUST raise a true "Conflicted" condition in the Listener
+	// Status.
+	//
+	// Implementations MAY choose to still accept a Gateway with conflicted
+	// Listeners if they accept a partial Listener set that contains no
+	// incompatible Listeners. They MUST set a "ListenersNotValid" condition
+	// the Gateway Status when the Gateway contains incompatible Listeners
+	// whether or not they accept the Gateway.
+	//
+	// For example, the following Listener scenarios may be compatible
+	// depending on implementation capabilities:
+	//
+	// 1. Multiple Listeners with the same Port that all use the "HTTP"
+	//    Protocol that all have unique Hostname values.
+	// 2. Multiple Listeners with the same Port that use either the "HTTPS" or
+	//    "TLS" Protocol that all have unique Hostname values.
+	// 3. A mixture of "TCP" and "UDP" Protocol Listeners, where no Listener
+	//    with the same Protocol has the same Port value.
+	//
+	// An implementation that cannot serve both TCP and UDP listens on the same
+	// address, or cannot mix HTTPS and generic TLS listens on the same port
+	// would not consider those cases compatible.
+	//
+	// Implementations using the Hostname value to select between same-Port
+	// Listeners MUST match inbound request hostnames from the most specific
+	// to least specific Hostname values to find the correct set of Routes.
+	// Exact matches must be processed before wildcard matches, and wildcard
+	// matches must be processed before fallback (empty Hostname value)
 	// matches.
 	//
-	// If this field specifies multiple Listeners that have the same
-	// Port value but are not compatible, the implementation must raise
-	// a "Conflicted" condition in the Listener status.
+	// Implementations MAY collapse separate Gateways onto a single set of
+	// Addresses if the Listeners across all Gateways are compatible.
 	//
 	// Support: Core
 	//