Revert "Revert "Proto API for LB request/responses""
diff --git a/BUILD b/BUILD
index 72d5caa..31e0f3a 100644
--- a/BUILD
+++ b/BUILD
@@ -182,6 +182,7 @@
     "src/core/client_config/client_config.h",
     "src/core/client_config/connector.h",
     "src/core/client_config/initial_connect_string.h",
+    "src/core/client_config/lb_policies/load_balancer_api.h",
     "src/core/client_config/lb_policies/pick_first.h",
     "src/core/client_config/lb_policies/round_robin.h",
     "src/core/client_config/lb_policy.h",
@@ -242,6 +243,7 @@
     "src/core/json/json_common.h",
     "src/core/json/json_reader.h",
     "src/core/json/json_writer.h",
+    "src/core/proto/grpc/lb/v0/load_balancer.pb.h",
     "src/core/statistics/census_interface.h",
     "src/core/statistics/census_rpc_stats.h",
     "src/core/surface/api_trace.h",
@@ -283,6 +285,10 @@
     "src/core/transport/transport_impl.h",
     "src/core/census/aggregation.h",
     "src/core/census/rpc_metric_id.h",
+    "third_party/nanopb/pb.h",
+    "third_party/nanopb/pb_common.h",
+    "third_party/nanopb/pb_decode.h",
+    "third_party/nanopb/pb_encode.h",
     "src/core/httpcli/httpcli_security_connector.c",
     "src/core/security/base64.c",
     "src/core/security/client_auth_filter.c",
@@ -319,6 +325,7 @@
     "src/core/client_config/connector.c",
     "src/core/client_config/default_initial_connect_string.c",
     "src/core/client_config/initial_connect_string.c",
+    "src/core/client_config/lb_policies/load_balancer_api.c",
     "src/core/client_config/lb_policies/pick_first.c",
     "src/core/client_config/lb_policies/round_robin.c",
     "src/core/client_config/lb_policy.c",
@@ -382,6 +389,7 @@
     "src/core/json/json_reader.c",
     "src/core/json/json_string.c",
     "src/core/json/json_writer.c",
+    "src/core/proto/grpc/lb/v0/load_balancer.pb.c",
     "src/core/surface/alarm.c",
     "src/core/surface/api_trace.c",
     "src/core/surface/byte_buffer.c",
@@ -436,6 +444,9 @@
     "src/core/census/operation.c",
     "src/core/census/placeholders.c",
     "src/core/census/tracing.c",
+    "third_party/nanopb/pb_common.c",
+    "third_party/nanopb/pb_decode.c",
+    "third_party/nanopb/pb_encode.c",
   ],
   hdrs = [
     "include/grpc/grpc_security.h",
@@ -484,6 +495,7 @@
     "src/core/client_config/client_config.h",
     "src/core/client_config/connector.h",
     "src/core/client_config/initial_connect_string.h",
+    "src/core/client_config/lb_policies/load_balancer_api.h",
     "src/core/client_config/lb_policies/pick_first.h",
     "src/core/client_config/lb_policies/round_robin.h",
     "src/core/client_config/lb_policy.h",
@@ -544,6 +556,7 @@
     "src/core/json/json_common.h",
     "src/core/json/json_reader.h",
     "src/core/json/json_writer.h",
+    "src/core/proto/grpc/lb/v0/load_balancer.pb.h",
     "src/core/statistics/census_interface.h",
     "src/core/statistics/census_rpc_stats.h",
     "src/core/surface/api_trace.h",
@@ -585,6 +598,10 @@
     "src/core/transport/transport_impl.h",
     "src/core/census/aggregation.h",
     "src/core/census/rpc_metric_id.h",
+    "third_party/nanopb/pb.h",
+    "third_party/nanopb/pb_common.h",
+    "third_party/nanopb/pb_decode.h",
+    "third_party/nanopb/pb_encode.h",
     "src/core/surface/init_unsecure.c",
     "src/core/census/grpc_context.c",
     "src/core/census/grpc_filter.c",
@@ -601,6 +618,7 @@
     "src/core/client_config/connector.c",
     "src/core/client_config/default_initial_connect_string.c",
     "src/core/client_config/initial_connect_string.c",
+    "src/core/client_config/lb_policies/load_balancer_api.c",
     "src/core/client_config/lb_policies/pick_first.c",
     "src/core/client_config/lb_policies/round_robin.c",
     "src/core/client_config/lb_policy.c",
@@ -664,6 +682,7 @@
     "src/core/json/json_reader.c",
     "src/core/json/json_string.c",
     "src/core/json/json_writer.c",
+    "src/core/proto/grpc/lb/v0/load_balancer.pb.c",
     "src/core/surface/alarm.c",
     "src/core/surface/api_trace.c",
     "src/core/surface/byte_buffer.c",
@@ -718,6 +737,9 @@
     "src/core/census/operation.c",
     "src/core/census/placeholders.c",
     "src/core/census/tracing.c",
+    "third_party/nanopb/pb_common.c",
+    "third_party/nanopb/pb_decode.c",
+    "third_party/nanopb/pb_encode.c",
   ],
   hdrs = [
     "include/grpc/byte_buffer.h",
@@ -1281,6 +1303,7 @@
     "src/core/client_config/connector.c",
     "src/core/client_config/default_initial_connect_string.c",
     "src/core/client_config/initial_connect_string.c",
+    "src/core/client_config/lb_policies/load_balancer_api.c",
     "src/core/client_config/lb_policies/pick_first.c",
     "src/core/client_config/lb_policies/round_robin.c",
     "src/core/client_config/lb_policy.c",
@@ -1344,6 +1367,7 @@
     "src/core/json/json_reader.c",
     "src/core/json/json_string.c",
     "src/core/json/json_writer.c",
+    "src/core/proto/grpc/lb/v0/load_balancer.pb.c",
     "src/core/surface/alarm.c",
     "src/core/surface/api_trace.c",
     "src/core/surface/byte_buffer.c",
@@ -1398,6 +1422,9 @@
     "src/core/census/operation.c",
     "src/core/census/placeholders.c",
     "src/core/census/tracing.c",
+    "third_party/nanopb/pb_common.c",
+    "third_party/nanopb/pb_decode.c",
+    "third_party/nanopb/pb_encode.c",
   ],
   hdrs = [
     "include/grpc/grpc_security.h",
@@ -1441,6 +1468,7 @@
     "src/core/client_config/client_config.h",
     "src/core/client_config/connector.h",
     "src/core/client_config/initial_connect_string.h",
+    "src/core/client_config/lb_policies/load_balancer_api.h",
     "src/core/client_config/lb_policies/pick_first.h",
     "src/core/client_config/lb_policies/round_robin.h",
     "src/core/client_config/lb_policy.h",
@@ -1501,6 +1529,7 @@
     "src/core/json/json_common.h",
     "src/core/json/json_reader.h",
     "src/core/json/json_writer.h",
+    "src/core/proto/grpc/lb/v0/load_balancer.pb.h",
     "src/core/statistics/census_interface.h",
     "src/core/statistics/census_rpc_stats.h",
     "src/core/surface/api_trace.h",
@@ -1542,6 +1571,10 @@
     "src/core/transport/transport_impl.h",
     "src/core/census/aggregation.h",
     "src/core/census/rpc_metric_id.h",
+    "third_party/nanopb/pb.h",
+    "third_party/nanopb/pb_common.h",
+    "third_party/nanopb/pb_decode.h",
+    "third_party/nanopb/pb_encode.h",
   ],
   includes = [
     "include",