xds features per release

doc/grpc_xds_features.md

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+# xDS Features in gRPC
+
+This document lists the [xDS](https://github.com/envoyproxy/data-plane-api/tree/master/envoy/api/v2)
+features supported in various gRPC language implementations and versions.
+
+Note that a gRPC client will simply ignore the configuration of a feature it
+does not support. The gRPC client does not generate a log
+to indicate that some configuration was ignored. It is impractical to generate
+a log and keep it up-to-date because xDS has a large number of APIs that gRPC
+does not support and the APIs keep evolving too. We recommend reading the
+[first gRFC](https://github.com/grpc/proposal/blob/master/A27-xds-global-load-balancing.md)
+on xDS support in gRPC to understand the design philosophy.
+
+The EDS policy will *not* support
+[overprovisioning](https://www.envoyproxy.io/docs/envoy/latest/intro/arch_overview/upstream/load_balancing/overprovisioning),
+which is different from Envoy.  Envoy takes the overprovisioning into
+account in both [locality-weighted load balancing](https://www.envoyproxy.io/docs/envoy/latest/intro/arch_overview/upstream/load_balancing/locality_weight)
+and [priority failover](https://www.envoyproxy.io/docs/envoy/latest/intro/arch_overview/upstream/load_balancing/priority),
+but gRPC assumes that the xDS server will update it to redirect traffic
+when this kind of graceful failover is needed.  gRPC will send the
+[`envoy.lb.does_not_support_overprovisioning` client
+feature](https://github.com/envoyproxy/envoy/pull/10136) to the xDS
+server to tell the xDS server that it will not perform graceful failover;
+xDS server implementations may use this to decide whether to perform
+graceful failover themselves.
+
+The EDS policy will not support per-endpoint stats; it will report only
+per-locality stats.
+
+An [`lb_endpoint`](https://github.com/envoyproxy/envoy/blob/master/api/envoy/api/v2/endpoint/endpoint_components.proto)
+is ignored if the `health_status` is not HEALTHY or UNKNOWN.
+The optional `load_balancing_weight` is always ignored.
+
+Initially, only `google_default` channel creds will be supported
+to authenticate with the xDS server.
+
+Features | gRFCs  | [C++, Python,<br> Ruby, PHP, C#](https://github.com/grpc/grpc/releases) | [Java](https://github.com/grpc/grpc-java/releases) | [Go](https://github.com/grpc/grpc-go/releases)
+---------|--------|--------------|------|------
+**xDS Infrastructure:**<br>LDS->RDS->CDS->EDS flow,<br>ADS stream,<br>Client-side Load reporting via [LRS](https://github.com/envoyproxy/data-plane-api/blob/master/envoy/service/load_stats/v2/lrs.proto), | [A27](https://github.com/grpc/proposal/blob/master/A27-xds-global-load-balancing.md) | v1.30.0  | v1.30.0 | v1.30.0 |
+**Load Balancing:**<br>Virtual host matching,<br>Only default path ("" or "/") matching,<br>Priority-based weighted round-robin locality picking,<br>Round-robin endpoint picking within locality,<br>Cluster route action| [A27](https://github.com/grpc/proposal/blob/master/A27-xds-global-load-balancing.md) | v1.30.0  | v1.30.0 | v1.30.0 |