Add per-host connection limits to DestinationRule

Add perHostLimits field to ConnectionPoolSettings to enable per-host connection limits for individual endpoints in a cluster.This aligns with Envoy's per_host_thresholds capability and is useful for preventing endpoint overload in autoscaling scenarios.

Currently only maxConnections field is supported per Envoy's implementation.

Fixes #57697

Author: prashanthjos

kubernetes/customresourcedefinitions.gen.yaml

@@ -594,6 +594,27 @@ message WarmupConfiguration {
 //           interval: 75s
 // ```
 //
+// The following example sets a per-host connection limit of 10 connections
+// for each individual endpoint in the reviews service. This is useful for
+// preventing overload of individual endpoints, especially in autoscaling scenarios.
+// Note: currently only the max_connections field is supported for per-host limits.
+//
+// ```yaml
+// apiVersion: networking.istio.io/v1
+// kind: DestinationRule
+// metadata:
+//   name: reviews-per-host-limits
+// spec:
+//   host: reviews.prod.svc.cluster.local
+//   trafficPolicy:
+//     connectionPool:
+//       tcp:
+//         maxConnections: 100
+//       perHostLimits:
+//         tcp:
+//           maxConnections: 10
+// ```
+//
 message ConnectionPoolSettings {
   // Settings common to both HTTP and TCP upstream connections.
   message TCPSettings {
@@ -701,6 +722,21 @@ message ConnectionPoolSettings {
   TCPSettings tcp = 1;
   // HTTP connection pool settings.
   HTTPSettings http = 2;
+
+  // Per-host connection limits that apply to each individual host in the upstream service.
+  // These limits allow controlling connections to each endpoint independently,
+  // which is useful for preventing overload of individual hosts and managing
+  // concurrency in autoscaling scenarios.
+  //
+  // Note: Currently only the `maxConnections` field is supported for per-host limits,
+  // as per Envoy's circuit breaker implementation.
+  message PerHostLimits {
+    // Per-host TCP settings. Currently only `maxConnections` is supported.
+    TCPSettings tcp = 1;
+  }
+
+  // Optional per-host connection limits.
+  PerHostLimits per_host_limits = 3;
 }
 
 // A Circuit breaker implementation that tracks the status of each

networking/v1/destination_rule_alias.gen.go

@@ -0,0 +1,9 @@
+apiVersion: release-notes/v2
+kind: feature
+area: traffic-management
+issue:
+  - https://github.com/istio/istio/issues/42490
+
+releaseNotes:
+  - |
+    **Added** `perHostSettings` to the `ConnectionPoolSettings` in the `DestinationRule` API to enable configuring per-host connection limits for each individual endpoint in a cluster. This is useful for preventing overload of individual hosts and managing concurrency in autoscaling scenarios. Note: currently only the `maxConnections` field is supported for per-host limits.