/*
 * Copyright 2019 Google LLC
 *
 * 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 google.internal.federatedml.v2;

import "google/protobuf/any.proto";
import "google/protobuf/duration.proto";
import "google/rpc/code.proto";
import "fcp/secagg/shared/secagg_messages.proto";

option java_package = "com.google.internal.federatedml.v2";
option java_multiple_files = true;
option java_outer_classname = "FederatedProto";

// `FederatedTrainingApi` provides the protocol between a device and
// cloud servers for federated computations.
//
// This API works as follows:
//
// 1. Clients are associated with *populations*. A population is
//    uniquely identified by an opaque string which indicates the
//    application together with additional information like a specific
//    flavor within this application.
//
// 2. Periodically, clients issue an `EligibilityEvalCheckinRequest` to indicate
//    they are interested in participating in a computation, followed by a
//    `CheckinRequest` to actually be assigned one of the available
//    computations they are compatible with. These requests specify the
//    population for which the requests are issued.
//
// 3. The server decides whether the client shall participate in a computation,
//    and if so returns the data (a computation description and a checkpoint)
//    needed for the client to start executing the computation.
//
// 4. If the client is selected, it performs local execution. When done, the
//    client issues a `ReportRequest`, specifying both population and phase id,
//    and the updated checkpoint.
service FederatedTrainingApi {
  // Initiates client-server communication session (bidirectional stream).
  rpc Session(stream ClientStreamMessage) returns (stream ServerStreamMessage) {
  }
}

// Message sent from a client to a server in a bidirectional stream.
// The message is sent either directly or (for large messages when chunking
// is supported) split into chunk encapsulated into smaller instances of
// ClientStreamMessage.
message ClientStreamMessage {
  // Different kinds of messages.
  oneof kind {
    // Checkin request.
    CheckinRequest checkin_request = 1;

    // Report request.
    ReportRequest report_request = 2;

    // Transfer of ClientStreamMessage in multiple chunks
    ChunkedTransferMessage chunked_transfer = 6;

    // Eligibility evaluation checkin request.
    EligibilityEvalCheckinRequest eligibility_eval_checkin_request = 7;
  }

  // Secure Aggregation messages.  These form a parallel stream of
  // messages, and so are outside the 'kind' oneof.
  fcp.secagg.ClientToServerWrapperMessage secure_aggregation_client_message = 4;

  repeated google.protobuf.Any serialized_side_channel_event = 5;

  // Internal identifier of the connection to the client. This value isn't set
  // by the device (ignored if set), but used by the server when forwarding
  // requests internally within the system.
  string internal_client_id = 3 [deprecated = true];
}

// Message sent from a server to a client in a bidirectional stream.
// The message is sent either directly or (for large messages when chunking
// is supported) split into chunks encapsulated into smaller instances of
// ServerStreamMessage.
message ServerStreamMessage {
  // Different kinds of messages.
  oneof kind {
    // Checkin response.
    CheckinResponse checkin_response = 1;

    // Report response.
    ReportResponse report_response = 2;

    // Transfer of ServerStreamMessage in multiple chunks
    ChunkedTransferMessage chunked_transfer = 4;

    // Ack of a CheckinRequest or an EligibilityEvalCheckinRequest. See {@link
    // CheckinRequest} for details.
    CheckinRequestAck checkin_request_ack = 5;

    // Eligibility evaluation checkin response.
    EligibilityEvalCheckinResponse eligibility_eval_checkin_response = 6;

    // This is expected to be extended in upcoming versions of the protocol
  }

  // Secure Aggregation messages.  These form a parallel stream of
  // messages, and so are outside the 'kind' oneof.
  fcp.secagg.ServerToClientWrapperMessage secure_aggregation_server_message = 3;
}

// Supported levels of compression for chunked blob transfer.
enum CompressionLevel {
  // Compression disabled
  UNCOMPRESSED = 0;
  // zlib compression with default settings; this level uses gzip-6
  // (currently).
  // 'Z_DEFAULT_COMPRESSION requests a default compromise between speed and
  // compression (currently equivalent to level 6).'
  // Source - // http://www.zlib.net/manual.html
  ZLIB_DEFAULT = 1;
  // zlib compression optimized for most compression; this level uses gzip-9.
  // '9 gives best compression' Source - http://www.zlib.net/manual.html
  ZLIB_BEST_COMPRESSION = 2;
  // zlib compression optimized for speed; this level uses gzip-1.
  // '1 gives best speed' Source - http://www.zlib.net/manual.html
  ZLIB_BEST_SPEED = 3;
}

// Supported compressed file formats for HTTP downloads.
enum HttpCompressionFormat {
  HTTP_COMPRESSION_FORMAT_UNSPECIFIED = 0;
  // Gzip-compressed data. If data is compressed in this way, then the
  // "Content-Type" HTTP response header will have a "+gzip" suffix.
  HTTP_COMPRESSION_FORMAT_GZIP = 1;
}

// A request, sent by the device to check if it should participate
// in the current phase.
message CheckinRequest {
  // The name of the population this client belongs to.
  string population_name = 1;

  // Optional. Retry token (opaque to the client) passed by the server when last
  // participated in the training or rejected. If clients have such a token
  // available, they should provide it. If not, things are still expected to
  // work, but providing this gives server better control on organizing
  // participation.
  //
  // Note that an `EligibilityEvalCheckinRequest` and its subsequent
  // `CheckinRequest` request will both use the same value for this field,
  // since both requests are considered part of the same logical protocol
  // session.
  string retry_token = 2;

  reserved 3;

  // The attestation measurement providing evidence of integrity for this
  // client. The measurement is bound to the population_name and retry_token
  // values in this CheckinRequest.
  //
  // Note that an `EligibilityEvalCheckinRequest` and its subsequent
  // `CheckinRequest` request will both use the same value for this field,
  // since both requests are considered part of the same logical protocol
  // session.
  string attestation_measurement = 4;

  // Protocol options supported by the client.
  ProtocolOptionsRequest protocol_options_request = 5;

  string client_version = 6 ;

  // The client computes this message using the plan returned by a previous
  // `EligibilityEvalCheckinResponse`.
  //
  // If this field is set, it describes to the server which tasks the client is
  // (in)eligible for, out of the set of tasks currently being served by the
  // server. The server must only use this information to decide which task to
  // serve to the client in response to this request, must use this information
  // only during this specific protocol session, and must not persist this
  // information in any way.
  //
  // If this field is unset, it may indicate that the client previously received
  // an `EligibilityEvalCheckinResponse` without an
  // `EligibilityEvalPayload` message (i.e. the population did not
  // have an eligibility-computing task configured at the time of the request).
  // It may also indicate a client for which the eligibility-computing task
  // feature has been disabled, or an old client that does not support this
  // feature yet.
  //
  // If this field is unset but the population has an eligibility-computing task
  // configured, then the server must reject this client, since the server has
  // no way to determine which tasks the client is (in)eligible for.
  //
  // If this field is unset and the population does not have an
  // eligibility-computing task configured, then the server may serve this
  // client any task.
  //
  TaskEligibilityInfo task_eligibility_info = 7;
}

// Describes to the server which tasks a client is eligible for.
message TaskEligibilityInfo {
  // A semantic version describing how the set of eligibility descriptors should
  // be interpreted. This fields enables assigning different semantics for how
  // the server should interpret the descriptors, without having to change the
  // wire format (e.g. different ways of interpreting `TaskWeight.weight`).
  int64 version = 1;

  // A list of task weights, which the server may use when assigning the client
  // a task in response to the current request.
  //
  // If none of the `TaskWeight` messages match a given task, then the client
  // must be considered ineligible for that task, and the server must not serve
  // the client that task.
  //
  // Therefore, if a `TaskEligibilityInfo` message is provided but this field is
  // empty then the client should be considered ineligible for all tasks in the
  // population (although in practice the client will simply close the
  // connection in that case, rather than issue a `CheckinRequest` with such an
  // empty list of weights).
  repeated TaskWeight task_weights = 2;
}

// Describes a weight that should be assigned to a specific task.
message TaskWeight {
  // Name of the task this weight applies to.
  //
  // These correspond to the task names provided by server in the
  // `PopulationEligibilitySpec.TaskInfo.task_name` field (see
  // fcp/protos/population_eligibility_spec.proto and
  // fcp/client/eligibility_decider.cc).
  string task_name = 1;

  // The weight that should be applied to the specified task.
  //
  // Must be >0.
  //
  // This weight may (or may not) be used by the server to implement some form
  // of task or client prioritization.
  //
  // NOTE: While this is a float field, it is currently only used to convey a
  // boolean eligibility signal (0.0 = ineligible, 1.0 = eligible). See
  // fcp/client/eligibility_decider.cc for how this field is used and populated.
  float weight = 2;
}

// Response to the checkin request, sent to the device.
message CheckinResponse {
  // One of two outcomes, depending on server's decision on participation of the
  // client.
  oneof checkin_result {
    // If the client joined the phase with this call, information how
    // to proceed.
    AcceptanceInfo acceptance_info = 1;

    // If the client was not accepted, information how to proceed.
    RejectionInfo rejection_info = 2;
  }

  // Instructions from server to the client how to execute protocol.
  // While, conceptually, chunked transfer is a symmetric protocol with respect
  // to peers (both server and client act like senders and receivers), the
  // protocol handshake and configuration part is intentionally skewed towards
  // the server driving the decisions on how chunking are performed on the wire,
  // so we have a centralized way of controlling the feature.
  //
  // Note that if a client receives more than one `ProtocolOptionsResponse` over
  // the life of a protocol session (e.g. in
  // `EligibilityEvalCheckinResponse` as well as `CheckinResponse`)
  // then the client will use the most recently-received value for further
  // communications with the server.
  ProtocolOptionsResponse protocol_options_response = 4;

  reserved 3;
}

// Acknowledgement for a `CheckinRequest` or an `EligibilityEvalCheckinRequest`
// from the client. This happens almost instantenously for all clients (that
// request an ack using ProtocolRequestOptions#should_ack_checkin) as soon as
// they issue either request, and happens *before* either a `CheckinResponse`
// or a `EligibilityEvalCheckinResponse` is returned to the client.
message CheckinRequestAck {
  // Retry window to use for the next checkin attempt if this attempt ends up
  // being subsequently accepted by the server, as in the client received a
  // CheckinResponse with an AcceptanceInfo.
  RetryWindow retry_window_if_accepted = 1;

  // Retry window to use if this checkin attempt is not accepted by the server,
  // as in the client doesn't receive a CheckinResponse with an AcceptanceInfo.
  RetryWindow retry_window_if_rejected = 2;
}

// A request, sent by the device to request the eligibility-computing plan for
// the population. This plan is run by the client to generate a
// `TaskEligibilityInfo` proto result, which is then included with a subsequent
// `CheckinRequest` (within the same protocol session) to inform the server
// which tasks the client is eligible for.
//
// The use of an `EligibilityEvalCheckinRequest` is optional (i.e. clients
// may simply issue a `CheckinRequest` without a preceding
// `EligibilityEvalCheckinRequest`, in which case the
// `CheckinRequest.task_eligibility_info` field will be left unset).
message EligibilityEvalCheckinRequest {
  // The name of the population this client belongs to.
  string population_name = 1;

  // Optional. This field has the same semantics as
  // `CheckinRequest.retry_token`, see that field for details.
  string retry_token = 2;

  // This field has the same semantics as
  // `CheckinRequest.attestation_measurement`.
  // See that field for details.
  string attestation_measurement = 4;

  // Protocol options supported by the client.
  ProtocolOptionsRequest protocol_options_request = 5;

  // This field has the same semantics as `CheckinRequest.client_version`. See
  // that field for details.
  string client_version = 6 ;

  // The client's capabilities when downloading and running Eligibility Eval
  // tasks.
  EligibilityEvalTaskCapabilities eligibility_eval_task_capabilities = 7;
}

// The client's capabilities for determining task eligibility.
message EligibilityEvalTaskCapabilities {
  // Whether the client supports multiple task assignment
  // (/TaskAssignments.PerformMultipleTaskAssignments). If false, the client
  // will not be provided information about tasks that require multiple task
  // assignment.
  bool supports_multiple_task_assignment = 1;

  // Whether the client supports native evaluation of Eligibility Eval Tasks.
  bool supports_native_eets = 2;
}

// Response to the `EligibilityEvalCheckinRequest`, sent to the
// device.
message EligibilityEvalCheckinResponse {
  // Each response will contain one of the following results.
  oneof checkin_result {
    // If the population has an eligibility-computing plan configured, and if
    // the client is compatible with that plan, then this field will be set,
    // containing the plan's payload. The client should run the plan and include
    // its `TaskEligibilityInfo` result in the subsequent `CheckinRequest`.
    EligibilityEvalPayload eligibility_eval_payload = 1;

    // If the population does not have an eligibility-computing plan configured,
    // then this field will be set. The client should continue by issuing a
    // `CheckinRequest` without the `task_eligibility_info` field set.
    NoEligibilityEvalConfigured no_eligibility_eval_configured = 2;

    // If the population has an eligibility-computing plan configured, but the
    // client is incompatible with that plan, then this field will be set.
    RejectionInfo rejection_info = 3;
  }

  // This field has the same semantics as
  // `CheckinResponse.protocol_options_response`. See that field for details.
  ProtocolOptionsResponse protocol_options_response = 4;
}

// Contains the eligibility evaluation plan payload.
message EligibilityEvalPayload {
  oneof init_checkpoint_type {
    // A blob representing the checkpoint to start execution from.
    bytes init_checkpoint = 1;

    // A URI and other metadata of the checkpoint to start execution from.
    UriResource init_checkpoint_resource = 4;
  }

  oneof plan_type {
    // A blob representing the plan to be used for execution.
    bytes plan = 2;

    // A URI and other metadata of the plan to be used for execution.
    UriResource plan_resource = 5;
  }

  oneof population_eligibility_spec_type {
    // A serialized PopulationEligibilitySpec describing the eligibility
    // criteria for tasks in the population.
    bytes population_eligibility_spec = 6;

    // A URI and other metadata of the population eligibility spec to be used.
    UriResource population_eligibility_spec_resource = 7;
  }

  // The opaque id of the eligibility evaluation plan payload the client is
  // being given. This is a string generated by the server and used by the
  // client for logging purposes. This id MUST NOT contain any information that
  // could be used to identify a specific device.
  // Also see the similar `AcceptanceInfo.execution_phase_id`.
  string execution_id = 3;
}

// Currently-empty message describing the case where a population does not have
// an eligibility-computing plan configured.
message NoEligibilityEvalConfigured {}

// Per-aggregand dynamic configuration information for side channel protocols.
message SideChannelExecutionInfo {
  // Dynamic configuration for SecureAggregation side channels.
  message SecureAggregandExecutionInfo {
    // Bitwidth for secure-aggregation.  This must be wide enough to
    // encode the sum of all client inputs, given the current number of clients
    // participating in the protocol.
    //
    // This field is deprecated; use modulus instead.
    int32 output_bitwidth = 1 [deprecated = true];

    // Modulus for secure aggregation.
    //
    // The secure aggregation protocol will compute the sum modulo this modulus.
    //
    // To achieve equivalence with non-modular summation, the modulus must be
    // larger than the sum of all client inputs, given the number of clients
    // participating in the aggregation shard.
    //
    // If modulus is missing but output_bitwidth is specified, modulus will
    // will be taken to be 2**output_bitwidth (for backwards compatibility).
    uint64 modulus = 2;
  }

  // What type of side channel is used.
  oneof type {
    // Dynamic configuration for Secure Aggregation side channels.
    SecureAggregandExecutionInfo secure_aggregand = 1;
  }
}

// Per-protocol options information for side channel protocols.
message SideChannelProtocolOptionsRequest {
  // Options for SecureAggregation side channels.
  message SecureAggregation {
    // Protocol versions available.
    repeated fcp.secagg.ClientVariant client_variant = 2;
  }

  // Protocol options for Secure Aggregation side channels.
  SecureAggregation secure_aggregation = 1;
}

// Negotiated settings for side channel protocols. Side channel options may not
// be set for channels which will not be used in the ReportRequest.
message SideChannelProtocolOptionsResponse {
  // Server-directed secure aggregation options to apply.
  message SecureAggregation {
    // The client variant to use.
    fcp.secagg.ClientVariant client_variant = 1;
  }

  // SecureAggregation protocol options.  Only set if a SecureAggregation plan
  // will be used.
  SecureAggregation secure_aggregation = 1;
}

// Per-protocol dynamic configuration information for side channel protocols.
message SideChannelProtocolExecutionInfo {
  // Dynamic configuration for SecureAggregation side channels.
  message SecureAggregationProtocolExecutionInfo {
    // Number of clients that a client may exchange data with while running
    // Secure Aggregation protocol. In the case of a full graph SecAgg protocol
    // this is a total number of clients that started the protocol.
    // In the case of subgraph SecAgg protocol this is a number of neighbours
    // that each client has.
    int32 expected_number_of_clients = 1;

    // Secure Aggregation client completion threshold. This is a parameter
    // communicated by the server side of Secure Aggregation protocol to each
    // client to establish Shamir sharing of secrets.
    // Additionally, at least `minimum_surviving_clients_for_reconstruction` out
    // of the initial `expected_number_of_clients` must 'survive' in order for
    // the protocol to continue on the client side; otherwise the client will
    // abort its connection.
    int32 minimum_surviving_clients_for_reconstruction = 2;

    // The minimum number of clients' values that must be aggregated together
    // before the server can gain access to the aggregate,
    // even transiently (e.g. in RAM).
    // This isn't needed by Secure Aggregation protocol on the client side but
    // shared by the server with clients for transparency or policy reasons.
    int32 minimum_clients_in_server_visible_aggregate = 3;
  }

  // Dynamic configuration for Secure Aggregation side channels.
  SecureAggregationProtocolExecutionInfo secure_aggregation = 1;
}

// When client (device) supports HTTP download of resources, this message
// is used to carry information about each individual downloadable resource.
message UriResource {
  // Resource URI e.g. fully qualified URL.
  string uri = 1;

  // Stable identifier for this resource, used by the client cache
  // implementation. If this field is not set, the client should not attempt to
  // cache the resource referenced by `uri`.
  string client_cache_id = 2;

  // The maximum duration for how long the resource should be cached by the
  // client. Not set if `client_cache_id` is not set.
  google.protobuf.Duration max_age = 3;
}

// When client (device) is accepted for the current phase, this
// data structure carries information necessary to begin training.
message AcceptanceInfo {
  // The opaque id of the phase the client has joined. This is a string
  // generated by the server and used by the client for logging purposes.
  // This id MUST NOT contain any information that could be used to identify
  // a specific device.
  string execution_phase_id = 1;

  // The name identifying the task for which the client was accepted.
  string task_name = 10;

  oneof init_checkpoint_type {
    // A blob representing the checkpoint to start execution from.
    bytes init_checkpoint = 2;

    // A URI and other metadata of the checkpoint to start execution from.
    UriResource init_checkpoint_resource = 8;
  }

  // Note: Plan fields below should be unset when included in a JoinResponse
  // from the aggregator to the selector, and then set by the selector before
  // sending on to the client.
  oneof plan_type {
    // A blob representing the plan to be used for execution.
    bytes plan = 3;

    // A URI and other metadata of the plan to be used for execution.
    UriResource plan_resource = 9;
  }

  // Per-aggregand dynamic configuration information for side channel protocols.
  // The keys in this map are the names of side channels, aligning with
  // CheckpointOp.side_channel_tensors in plan.proto.
  map<string, SideChannelExecutionInfo> side_channels = 4;

  // Per-protocol dynamic configuration information for side channel protocols.
  // This configuration applies to all aggregands configured to use each
  // protocol.
  SideChannelProtocolExecutionInfo side_channel_protocol_execution_info = 5;

  reserved 6, 7;

  // Info for how to generate URIs for fetching slices at runtime.
  FederatedSelectUriInfo federated_select_uri_info = 11;

  reserved 12, 13;
}

// Info for how to generate URIs for fetching slices that the task might request
// to be downloaded at runtime.
//
// When one or more slices are requested by the task, the template specified
// here should be used to form a URI from which the client can download the
// slice data, by replacing the "{served_at_id}" and "{key_base10}" substrings
// with the `google.internal.federated.plan.SlicesSelector.served_at_id` and the
// base-10 representation of the `SlicesSelector.keys` value. The client must
// not perform any URI escaping to the values that the substrings are replaced
// with.
message FederatedSelectUriInfo {
  // The URI template to use for fetching slices.
  //
  // This template must always start with "https://".
  //
  // This template must contain the following substrings: "{served_at_id}" and
  // "{key_base10}", as per the above documentation.
  string uri_template = 1;
}

// This is sent when client (device) is rejected for the participation.
message RejectionInfo {
  // Optional. A suggestion to the client when to retry next connection to the
  // service
  // Deprecated in favor of `CheckinRequestAck`. If a client supports
  // `CheckinRequestAck` then this value is ignored.
  RetryWindow retry_window = 4 [deprecated = true];

  reserved 1, 2, 3;
}

// This is sent by the client after the client finishes local (on-device)
// training.
//
// If secure aggregation side channel is used, this must accompany the
// secure aggregation commit message in the same ClientStreamMessage.
message ReportRequest {
  // The name of the population this client belongs to.
  string population_name = 1;

  // The id of the execution phase this client participates in.
  string execution_phase_id = 2;

  // The report.
  Report report = 3;
}

// This is sent by the server as the final message in the reporting protocol.
message ReportResponse {
  // Optional. A suggestion to the client when to retry next connection to the
  // service
  // Deprecated in favor of `CheckinRequestAck`. If a client supports
  // `CheckinRequestAck` then this value is ignored.
  RetryWindow retry_window = 1 [deprecated = true];
}

// A report with results of local (on-device) training.
message Report {
  // A blob representing the updated checkpoint, if any.  The content
  // is dependent of the execution method.
  bytes update_checkpoint = 1;

  // Status code reported by client.
  // Code.OK indicates that client execution completed successfully and produced
  // report. Any other code indicates unsuccessful execution and train events
  // below might contain detailed diagnostic information.
  google.rpc.Code status_code = 5;

  reserved 3;

  // A serialized ClientExecutionStats field about stats produced during a
  // client side execution of the plan.
  repeated google.protobuf.Any serialized_train_event = 4;

  reserved 2;

  reserved 6;
}

// This message is used to report duration to the server.
message ClientExecutionStats {
  // The time spent on running the plan (includes I/O such as reading examples,
  // but does not include time spent on the network for retrieving the plan
  // or uploading results).
  google.protobuf.Duration duration = 2;

  reserved 1;
}

// A suggestion to the client when to retry the connection to the service next
// time
message RetryWindow {
  // Optional. If set, the server offers the client to call back in
  // an interval [delay_min .. delay_max].
  // The client must pass this token back to the server to identify he is
  // retrying. If this is not set, the client can retry for another phase at a
  // time of his choosing.
  string retry_token = 1;

  // Required (if retry_token is set).
  // The suggested minimal duration after which the client should
  // retry. If the client retries earlier, it is likely he will be rejected
  // again.
  google.protobuf.Duration delay_min = 2;

  // Required. The suggested maximal duration after which the client should
  // retry, provided scheduling conditions allow. The client is supposed to make
  // a best effort to callback in the min..max window, and should avoid
  // calling before min. If he calls after max, the likelihood to be rejected
  // again is higher.
  google.protobuf.Duration delay_max = 3;
}

// Intermediate Representation for checkpoints after side channels and
// quantization have been applied.  This is the input and post-aggregation
// output of the transport-and-aggregate protocols.
message Checkpoint {
  // An aggregand is a (flattened) collection of checkpoint variables
  // that will be aggregated using the same transport-and-aggregate protocol.
  message Aggregand {
    repeated uint64 values = 1 [packed = true];
    int32 bitwidth = 2;
  }

  // Checkpoint variables are partitioned into multiple aggregands,
  // each of which can use a different transport-and-aggregate protocol.
  //
  map<string, Aggregand> aggregands = 1;
}

// Protocol options sent from client to server inside
// `EligibilityEvalCheckinRequest` and `CheckinRequest`.
message ProtocolOptionsRequest {
  // True if client supports chunked blob transfer protocol.
  bool supports_chunked_blob_transfer = 1;

  // Chunked blob transfer compression levels supported by this client.
  repeated CompressionLevel supported_compression_levels = 2;

  // Per-protocol configuration option information for side channel protocols.
  // These options apply to all aggregands configured to use each protocol.
  SideChannelProtocolOptionsRequest side_channels = 3;

  // When this is set in a {@link CheckinRequest} or {@link
  // EligibilityEvalCheckinRequest} message, the server should ack using
  // a {@link CheckinRequestAck} sent back to the client.
  //
  // Note that if a client previously issued a
  // `EligibilityEvalCheckinRequest` (for which this field is always set
  // to true), then this field will not be set to true for the subsequent
  // `CheckinRequest`.
  bool should_ack_checkin = 4;

  // True if client supports download of resources via HTTP.
  bool supports_http_download = 5;

  // True if client supports download of Eligibility Eval resources via HTTP.
  bool supports_eligibility_eval_http_download = 6;

  // HTTP download compression formats supported by this client. All clients
  // that support HTTP downloads are assumed to support uncompressed payloads.
  repeated HttpCompressionFormat supported_http_compression_formats = 7;
}

// Protocol options sent from server to client inside
// `EligibilityEvalCheckinResponse` and `CheckinResponse`.
message ProtocolOptionsResponse {
  // Tells client what chunk size to use for uploading data, default 8192.
  int32 chunk_size_for_upload = 1;

  // Tells client how many chunks to send ahead of receiving ack, default: 2
  int32 max_pending_chunks = 2;

  // Indicates desired compression level
  CompressionLevel compression_level = 4;

  // Negotiated side channel protocol options; side channel options may not
  // be set for channels which will not be used in the ReportRequest.
  SideChannelProtocolOptionsResponse side_channels = 5;
}

// Allows to transmit large ClientStreamMessage or ServerStreamMessage
// (depending on a direction of a transfer) as a stream of multiple small chunks
// with optional compression flow control.
message ChunkedTransferMessage {
  // Supported types of compression scheme. Deprecated, replaced by
  // CompressionLevel. Do not add new values.
  enum CompressionType {
    // Compression disabled
    UNCOMPRESSED = 0;
  }

  // Initiation of the chunked transfer. Transmitting party starts sending
  // chunks (Data messages) right after sending Start.
  message Start {
    // Uncompressed size of the blob, in bytes.
    int32 uncompressed_size = 2;

    // Level of compression.
    CompressionLevel compression_level = 3;

    // Size of the blob transferred over the wire, in bytes.
    // This field may not be set by older clients, so readers should check the
    // uncompressed_size field if this value is zero.
    int32 blob_size_bytes = 4;

    reserved 1;
  }

  // Carries a chunk of data. Receiving party assembles all the chunks to get
  // a serialized form of a large ClientStreamMessage or ServerStreamMessage
  // depending on a direction of a transfer.
  message Data {
    // 0-based index of the chunk
    // All chunk messages must be ordered, the index is included for diagnostics
    // purposes.
    int32 chunk_index = 1;

    // Next chunk of the blob.
    bytes chunk_bytes = 2;
  }

  // Acknowledgement of receiving of Data message.
  // Must be sent in a response of a successful receiving of a chunk. It is used
  // for the purposes of the flow control: transmitting party limits number of
  // of chunks speculatively sent
  message Ack {
    // 0-based index of received chunk
    // All ack messages must be ordered, the index is included for diagnostic
    // purposes.
    int32 chunk_index = 1;
  }

  // Completion of the chunked transfer.
  message End {
    // Total number of chunks transferred.
    int32 chunk_count = 1;

    // TODO(team): Add checksum.
  }

  // Kind of chunked message.
  oneof kind {
    // Start message, sent by transmitter
    Start start = 1;

    // Data message (a single chunk), sent by transmitter.
    Data data = 2;

    // Ack of receiving of Data message, sent by receiver.
    Ack ack = 3;

    // End of the transmission, sent by transmitter.
    End end = 4;
  }
}
