diff --git a/pw_software_update/BUILD.bazel b/pw_software_update/BUILD.bazel
index ce2c69a..861302e 100644
--- a/pw_software_update/BUILD.bazel
+++ b/pw_software_update/BUILD.bazel
@@ -46,6 +46,26 @@
     ],
 )
 
+pw_cc_library(
+    name = "update_backend",
+    hdrs = ["public/pw_software_update/update_backend.h"],
+    includes = ["public"],
+    deps = [
+        ":update_bundle_proto",
+        "//pw_status",
+    ],
+)
+
+pw_cc_library(
+    name = "update_service",
+    srcs = ["service.cc"],
+    hdrs = ["public/pw_software_update/service.h"],
+    includes = ["public"],
+    deps = [
+        ":update_bundle_proto",
+    ],
+)
+
 pw_cc_test(
     name = "update_bundle_test",
     srcs = ["update_bundle_test.cc"],
diff --git a/pw_software_update/BUILD.gn b/pw_software_update/BUILD.gn
index 16ba387..6f431d7 100644
--- a/pw_software_update/BUILD.gn
+++ b/pw_software_update/BUILD.gn
@@ -16,41 +16,40 @@
 
 import("$dir_pw_docgen/docs.gni")
 import("$dir_pw_protobuf_compiler/proto.gni")
+import("$dir_pw_third_party/protobuf/protobuf.gni")
 import("$dir_pw_unit_test/test.gni")
 
-config("default_config") {
+config("public_include_path") {
   include_dirs = [ "public" ]
+  visibility = [ ":*" ]
 }
 
-pw_proto_library("protos") {
-  sources = [
-    "tuf.proto",
-    "update_bundle.proto",
-  ]
-  prefix = "pw_software_update"
-  python_package = "py"
-}
-
-pw_source_set("update_bundle") {
-  public_configs = [ ":default_config" ]
-  public_deps = [
-    dir_pw_blob_store,
-    dir_pw_kvs,
-  ]
-  deps = [ dir_pw_log ]
-  public = [
-    "public/pw_software_update/config.h",
-    "public/pw_software_update/update_bundle.h",
-  ]
-}
-
-pw_test("update_bundle_test") {
-  sources = [ "update_bundle_test.cc" ]
-  public_deps = [
-    ":update_bundle",
-    "$dir_pw_kvs:fake_flash",
-    "$dir_pw_kvs:fake_flash_test_key_value_store",
-  ]
+if (dir_pw_third_party_protobuf != "") {
+  pw_proto_library("protos") {
+    deps = [
+      "$dir_pw_protobuf:common_protos",
+      "$dir_pw_third_party/protobuf:wellknown_types",
+    ]
+    sources = [
+      "service.proto",
+      "tuf.proto",
+      "update_bundle.proto",
+    ]
+    prefix = "pw_software_update"
+    python_package = "py"
+  }
+} else {
+  # placeholder target to allow py package to build
+  pw_proto_library("protos") {
+    deps = [ "$dir_pw_protobuf:common_protos" ]
+    sources = [
+      "service.proto",
+      "tuf.proto",
+      "update_bundle.proto",
+    ]
+    prefix = "pw_software_update"
+    python_package = "py"
+  }
 }
 
 pw_test_group("tests") {
@@ -60,3 +59,55 @@
 pw_doc_group("docs") {
   sources = [ "docs.rst" ]
 }
+
+if (dir_pw_third_party_nanopb != "" && dir_pw_third_party_protobuf != "") {
+  pw_source_set("rpc_service") {
+    public_configs = [ ":public_include_path" ]
+    public_deps = [ ":protos.nanopb_rpc" ]
+    deps = [ dir_pw_log ]
+    sources = [
+      "public/pw_software_update/service.h",
+      "service.cc",
+    ]
+  }
+
+  pw_source_set("update_backend") {
+    public_configs = [ ":public_include_path" ]
+    public_deps = [
+      ":protos.nanopb",
+      dir_pw_status,
+    ]
+    sources = [ "public/pw_software_update/update_backend.h" ]
+  }
+
+  pw_source_set("update_bundle") {
+    public_configs = [ ":public_include_path" ]
+    public_deps = [
+      ":update_backend",
+      dir_pw_blob_store,
+      dir_pw_kvs,
+    ]
+    deps = [ dir_pw_log ]
+    public = [
+      "public/pw_software_update/config.h",
+      "public/pw_software_update/update_bundle.h",
+    ]
+  }
+} else {
+  group("rpc_service") {
+  }
+  group("update_bundle") {
+  }
+}
+
+pw_test("update_bundle_test") {
+  enable_if =
+      dir_pw_third_party_nanopb != "" && dir_pw_third_party_protobuf != ""
+  sources = [ "update_bundle_test.cc" ]
+  public_deps = [
+    ":rpc_service",
+    ":update_bundle",
+    "$dir_pw_kvs:fake_flash",
+    "$dir_pw_kvs:fake_flash_test_key_value_store",
+  ]
+}
diff --git a/pw_software_update/public/pw_software_update/service.h b/pw_software_update/public/pw_software_update/service.h
new file mode 100644
index 0000000..4caedea
--- /dev/null
+++ b/pw_software_update/public/pw_software_update/service.h
@@ -0,0 +1,17 @@
+// Copyright 2021 The Pigweed Authors
+//
+// 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
+//
+//     https://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.
+
+#pragma once
+
+#include "pw_software_update/service.rpc.pb.h"
diff --git a/pw_software_update/public/pw_software_update/update_backend.h b/pw_software_update/public/pw_software_update/update_backend.h
new file mode 100644
index 0000000..856df65
--- /dev/null
+++ b/pw_software_update/public/pw_software_update/update_backend.h
@@ -0,0 +1,88 @@
+// Copyright 2021 The Pigweed Authors
+//
+// 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
+//
+//     https://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.
+
+#pragma once
+
+#include "pw_software_update/update_bundle.pb.h"
+#include "pw_status/status.h"
+
+namespace pw::software_update {
+
+// TODO(pwbug/478): update documentation for backend api contract
+class BundledUpdateBackend {
+ public:
+  virtual ~BundledUpdateBackend() = default;
+
+  // Perform any product-specific abort tasks before mark as aborted in bundled
+  // updater.
+  // This should set any downstream state to a default no-update-pending state.
+  virtual Status BeforeUpdateAbort() { return OkStatus(); };
+
+  // Perform any product-specific tasks needed before starting update sequence.
+  virtual Status BeforeUpdateStart() { return OkStatus(); };
+
+  // Perform any product-specific tasks needed before starting verification.
+  virtual Status BeforeUpdateVerify() { return OkStatus(); };
+
+  // Perform any product-specific bundle verification tasks (e.g. hw version
+  // match check), done after TUF bundle verification process.
+  virtual Status VerifyMetadata(
+      [[maybe_unused]] const pw_software_update_Manifest& manifest) {
+    return OkStatus();
+  };
+
+  // Perform product-specific tasks after all bundle verifications are complete.
+  virtual Status AfterBundleVerified() { return OkStatus(); };
+
+  // Optionally verify that the instance/content of the target file in use
+  // on-device matches the metadata in the given manifest, called before apply.
+  // (e.g. by checksum, if failed abort partial update and wipe/mark-invalid
+  // running manifest)
+  virtual Status VerifyTargetFile(
+      [[maybe_unused]] const pw_software_update_Manifest& manifest,
+      [[maybe_unused]] std::string_view target_file_name) {
+    return OkStatus();
+  };
+
+  // Perform any product-specific tasks before apply sequence started
+  virtual Status BeforeUpdateApply() { return OkStatus(); };
+
+  // Get status information from update backend. This will not be called when
+  // BundledUpdater is in a step where it has entire control with no operation
+  // handed over to update backend.
+  virtual int64_t GetStatus() { return 0; }
+
+  // Update the specific target file on the device.
+  virtual Status ApplyTargetFile(
+      [[maybe_unused]] std::string_view target_file_name,
+      [[maybe_unused]] stream::Reader& target_payload) {
+    return OkStatus();
+  };
+
+  // Perform any product-specific tasks needed after completion of the update.
+  virtual Status AfterUpdateComplete() { return OkStatus(); };
+
+  // Get reader of the device's root metadata.
+  virtual Status GetRootMetadataReader([[maybe_unused]] stream::Reader* out) {
+    return Status::Unimplemented();
+  };
+
+  // Use a reader that provides a new root metadata for the device to save.
+  virtual Status UpdateRootMetadata(
+      [[maybe_unused]] stream::Reader& root_metadata) {
+    return OkStatus();
+  };
+};
+
+}  // namespace pw::software_update
\ No newline at end of file
diff --git a/pw_software_update/public/pw_software_update/update_bundle.h b/pw_software_update/public/pw_software_update/update_bundle.h
index 7ab889b..5f56a21 100644
--- a/pw_software_update/public/pw_software_update/update_bundle.h
+++ b/pw_software_update/public/pw_software_update/update_bundle.h
@@ -17,13 +17,13 @@
 #include <cstddef>
 
 #include "pw_blob_store/blob_store.h"
+#include "pw_software_update/update_backend.h"
 
 namespace pw::software_update {
 
 // TODO(pwbug/456): Place-holder declaration for now. To be imlemented
 // and moved elsewhere.
 class ElementPayloadReader {};
-class BundledUpdateHelper {};
 class Manifest;
 
 // UpdateBundle is responsible for parsing, verifying and providing
@@ -76,8 +76,8 @@
   // update_bundle - The software update bundle data on storage.
   // helper - project-specific BundledUpdateHelper
   UpdateBundle(blob_store::BlobStore& update_bundle,
-               BundledUpdateHelper& helper)
-      : bundle_(update_bundle), helper_(helper) {}
+               BundledUpdateBackend& backend)
+      : bundle_(update_bundle), backend_(backend) {}
 
   // Opens and verifies the software update bundle, using the TUF process.
   //
@@ -120,7 +120,7 @@
 
  private:
   blob_store::BlobStore& bundle_;
-  BundledUpdateHelper& helper_;
+  BundledUpdateBackend& backend_;
 };
 
 }  // namespace pw::software_update
\ No newline at end of file
diff --git a/pw_software_update/service.cc b/pw_software_update/service.cc
new file mode 100644
index 0000000..8ad1afd
--- /dev/null
+++ b/pw_software_update/service.cc
@@ -0,0 +1,17 @@
+// Copyright 2021 The Pigweed Authors
+//
+// 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
+//
+//     https://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.
+
+#include "pw_software_update/service.h"
+
+namespace pw_software_update {}
\ No newline at end of file
diff --git a/pw_software_update/service.proto b/pw_software_update/service.proto
new file mode 100644
index 0000000..b8ad924
--- /dev/null
+++ b/pw_software_update/service.proto
@@ -0,0 +1,99 @@
+// Copyright 2021 The Pigweed Authors
+//
+// 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
+//
+//     https://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 pw_software_update;
+
+import "pw_software_update/tuf.proto";
+import "pw_software_update/update_bundle.proto";
+import "pw_protobuf_protos/common.proto";
+import "google/protobuf/any.proto";
+
+message BundledUpdateState {
+  enum State {
+    UNKNOWN = 0;
+    INACTIVE = 1;
+    READY_FOR_UPDATE = 2;
+    VERIFYING_UPDATE_BUNDLE = 3;
+    VERIFIED_AND_READY_TO_APPLY = 4;
+    APPLYING_UPDATE = 5;
+  }
+
+  State manager_state = 1;
+
+  // This is the percentage of estimated progress for the current update
+  // state in hundreths of a percent. (e.g. 5.00% = 500u)
+  optional uint32 current_state_progress_hundreth_percent = 2;
+}
+
+message OperationResult {
+  BundledUpdateState state = 1;
+  optional google.protobuf.Any extended_status = 2;
+}
+
+message PrepareUpdateResult {
+  OperationResult result = 1;
+  optional uint32 transfer_endpoint = 2;
+}
+
+// TODO(pwbug/478): add documentation for details of api contract
+service BundledUpdateService {
+  // Abort any current software update in progress.
+  //
+  // Safe to call at any point.
+  rpc Abort(pw.protobuf.Empty) returns (OperationResult) {};
+
+  // Get current state of software update.
+  //
+  // Safe to call at any point.
+  rpc SoftwareUpdateState(pw.protobuf.Empty) returns (OperationResult) {};
+
+  // Get the manifest of the software currently active on the device.
+  //
+  // Safe to call at any point.
+  rpc GetCurrentManifest(pw.protobuf.Empty) returns (stream Manifest) {};
+
+  // Verify the manifest of the software currently active on device. Do any
+  // device-specific checks of device contents as needed.
+  //
+  // Safe to call at any point.
+  rpc VerifyCurrentManifest(pw.protobuf.Empty) returns (OperationResult) {};
+
+  // Get the manifest of any verified and staged update.
+  //
+  // Safe to call at any point.
+  rpc GetStagedManifest(pw.protobuf.Empty) returns (Manifest) {};
+
+  // Prepare for software update. Do any device-specific tasks needed to be
+  // ready for update. Open pw_transfer channel used for staging bundle. Device
+  // UpdateState set to READY_FOR_UPDATE.
+  //
+  // Device UpdateState should be INACTIVE when calling, will otherwise be
+  // rejected.
+  rpc PrepareForUpdate(pw.protobuf.Empty) returns (OperationResult) {};
+
+  // Verify the bundle that has been transferred to the staging area. Close the
+  // pw_transfer channel used for staging bundle.
+  //
+  // Device UpdateState should be READY_FOR_UPDATE when calling, will otherwise
+  // be rejected.
+  rpc VerifyStagedBundle(pw.protobuf.Empty) returns (OperationResult) {};
+
+  // Trigger the application of the device, which might result in a device
+  // becoming slow to respond and possibly reboot.
+  //
+  // Device UpdateState should be VERIFIED_AND_READY_TO_APPLY when calling, will
+  // otherwise be rejected.
+  rpc ApplyUpdate(pw.protobuf.Empty) returns (OperationResult) {};
+}
diff --git a/pw_software_update/update_bundle_test.cc b/pw_software_update/update_bundle_test.cc
index 9f2ba3f..4522cac 100644
--- a/pw_software_update/update_bundle_test.cc
+++ b/pw_software_update/update_bundle_test.cc
@@ -17,6 +17,7 @@
 #include "gtest/gtest.h"
 #include "pw_kvs/fake_flash_memory.h"
 #include "pw_kvs/test_key_value_store.h"
+#include "pw_software_update/update_backend.h"
 
 namespace pw::software_update {
 namespace {
@@ -31,27 +32,27 @@
   UpdateBundleTest()
       : blob_flash_(kFlashAlignment),
         blob_partition_(&blob_flash_),
-        bunble_blob_("TestBundle",
+        bundle_blob_("TestBundle",
                      blob_partition_,
                      nullptr,
                      kvs::TestKvs(),
                      kBufferSize) {}
 
-  blob_store::BlobStoreBuffer<kBufferSize>& bunble_blob() {
-    return bunble_blob_;
+  blob_store::BlobStoreBuffer<kBufferSize>& bundle_blob() {
+    return bundle_blob_;
   }
 
-  BundledUpdateHelper& helper() { return helper_; }
+  BundledUpdateBackend& backend() { return backend_; }
 
  private:
   kvs::FakeFlashMemoryBuffer<kSectorSize, kSectorCount> blob_flash_;
   kvs::FlashPartition blob_partition_;
-  blob_store::BlobStoreBuffer<kBufferSize> bunble_blob_;
-  BundledUpdateHelper helper_;
+  blob_store::BlobStoreBuffer<kBufferSize> bundle_blob_;
+  BundledUpdateBackend backend_;
 };
 
 }  // namespace
 
-TEST_F(UpdateBundleTest, Create) { UpdateBundle(bunble_blob(), helper()); }
+TEST_F(UpdateBundleTest, Create) { UpdateBundle(bundle_blob(), backend()); }
 
 }  // namespace pw::software_update