diff --git a/metropolis/proto/api/configuration.proto b/metropolis/proto/api/configuration.proto
new file mode 100644
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+++ b/metropolis/proto/api/configuration.proto
@@ -0,0 +1,36 @@
+// Copyright 2020 The Monogon Project Authors.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+syntax = "proto3";
+package metropolis.proto.api;
+option go_package = "source.monogon.dev/metropolis/proto/api";
+
+// NodeParameters is the data with which a Node is set booted. It contains the
+// configuration required for a node to either bootstrap a new cluster, or
+// register into an existing one.
+// It is serialized into a proto message and supplied to Metropolis in an
+// implementation-specific way (currently: either on ESP partition or via qemu
+// fw_cfg).
+message NodeParameters {
+    message ClusterBootstrap {
+    }
+    message ClusterRegister {
+    }
+    oneof cluster {
+        ClusterBootstrap cluster_bootstrap = 1;
+        ClusterRegister cluster_register = 2;
+    }
+}