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gRPC Network Management Interface (gNMI)

Contributors: Paul Borman, Marcus Hines, Carl Lebsack, Chris Morrow, Anees Shaikh, Rob Shakir

Date: January 30, 2018

Version: 0.6.0

Table of Contents

1 Introduction

This document defines a gRPC-based protocol for the modification and retrieval of configuration from a target device, as well as the control and generation of telemetry streams from a target device to a data collection system. The intention is that a single gRPC service definition can cover both configuration and telemetry - allowing a single implementation on the target, as well as a single NMS element to interact with the device via telemetry and configuration RPCs.

All messages within the gRPC service definition are defined as protocol buffers (specifically proto3). gRPC service definitions are expected to be described using the relevant features of the protobuf IDL. The protobuf definition of gNMI is maintained in the openconfig/gnmi GitHub repository.

The service defined within this document is assumed to carry payloads that contain data instances of OpenConfig YANG schemas, but can be used for any data with the following characteristics:

  1. structure can be represented by a tree structure where nodes can be uniquely identified by a path consisting of node names, or node names coupled with attributes;
  2. values can be serialised into a scalar object.

Currently, values may be serialised to a scalar object through encoding as a JSON string or a Protobuf type - although the definition of new serialisations is possible.

Throughout this specification the following terminology is used:

2 Common Message Types and Encodings

2.1 Reusable Notification Message Format

When a target wishes to communicate data relating to the state of its internal database to an interested client, it does so via means of a common Notification message. Notification messages are reused in other higher-layer messages for various purposes. The exact use of the Notification message is described on a per-RPC basis.

The fields of the Notification message are as follows:

The creator of a Notification message MUST include the timestamp field. All other fields are optional.

2.2 Common Data Types

2.2.1 Timestamps

Timestamp values MUST be represented as the number of nanoseconds since the Unix epoch (January 1st 1970 00:00:00 UTC). The value MUST be encoded as a signed 64-bit integer (int64)1.

2.2.2 Paths

Paths are represented according to gNMI Path Conventions, which defines a structured format for path elements, and any associated key values. Rather than utilising a single string to represent the path - with the / character separating each element of the path, the path is represented by an ordered list of PathElem messages, starting at the root node, and ending at the most specific path element. Each PathElem message contains the name of the node within the data tree, along with any associated keys/attributes that may be required for it. Keys are contained within a map<string, string> where the key of the map is the name of the key element, and the value is the key's value encoded as a string.

A path is represented by the Path message with the following fields:

Each Path element should correspond to a node in the data tree. For example, the path /a/b/c/d is encoded as:

path: <
  elem: <
    name: "a"
  >
  elem: <
    name: "b"
  >
  elem: <
    name: "c"
  >
  elem: <
    name: "d"
  >
>

Where attributes are to be specified, these are encoded alongside the node name within the path element, for example a node specified by /a/e[key=k1]/f/g would have the path encoded as:

path: <
  elem: <
    name: "a"
  >
  elem: <
    name: "e"
    key: <
      key: "key"
      value: "k1"
    >
  >
  elem: <
    name: "f"
  >
  elem: <
    name: "g"
  >
>

The root node (/) is encoded as a zero-length array (slice) of PathElem messages within the elem field.

path: <
>

Paths (defined to be the concatenation of the prefix and path within the message) specified within a message MUST be absolute - no messages with relative paths should be generated.

2.2.2.1 Path Target

The target field within a path is name for the target. This field MUST only ever be present on prefix paths in the corresponding request and response messages. This field is optional for clients. When set in the prefix in a request, GetRequest, SetRequest or SubscribeRequest, the field MUST be reflected in the prefix of the corresponding GetResponse, SetResponse or SubscribeResponse by a server. This field is used to allow a name to be associated with all the data for a given stream if requested by a client. If a client does not set this field in the prefix of a request, it MUST NOT be set in the prefix of the corresponding response messages. The value for target is tied to the context of a client RPC and not persisted or shared among multiple clients.

2.2.3 Node Values

The value of a data node (or subtree) is encoded in a TypedValue message as a oneof field to allow selection of the data type by setting exactly one of the member fields. The possible data types include:

Several native scalar protobuf types are included in the TypedValue message:

Additional defined data types include:

The remaining fields in the TypedValue message define structured data types. Section 2.3 describes these further.

2.3 Structured data types

When structured data is sent by the client or the target in an Update message, it MUST be serialized according to one of the supported encodings listed in the Encoding enumeration. The table below lists the supported encodings and their corresponding TypedValue fields, followed by further details on each encoding type. It should be noted that the target never utilises the Encoding enumeration to declare to the client the type of encoding utilised, hence the client must infer the encoding from the populated TypedValue field.

Name Description TypedValue field Encoding Value
JSON A JSON encoded string as per 2.3.1. json_val 0
Bytes An arbitrary sequence of bytes as per 2.3.2. bytes_val 1
Proto A Protobuf encoded message using protobuf.any, as per 2.3.3. any_val 2
ASCII An ASCII encoded string representing text formatted according to a target-defined convention (described in Section 2.3.4). ascii_val 3
JSON_IETF A JSON encoded string as per 2.3.1 using JSON encoding compatible with RFC 7951. json_ietf_val 4

2.3.1 JSON and JSON_IETF

The JSON type indicates that the value is encoded as a JSON string. This format utilises the specification in RFC7159. Additional types (e.g., JSON_IETF) are utilised to indicate specific additional characteristics of the encoding of the JSON data (particularly where they relate to serialisation of YANG-modeled data).

For any JSON encoding:

Using the following example data tree:

 root +
      |
      +-- a +
            |
            +-- b[name=b1] +
                           |
                           +-- c +
                                 |
                                 +-- d (string)
                                 +-- e (uint32)

The following serialisations would be used (note that the examples below follow the conventions for textproto, and Go-style backticks are used for string literals that would otherwise require escaping):

For /a/b[name=b1]/c/d:

update: <
  path: <
    elem: <
      name: "a"
    >
    elem: <
      name: "b"
      key: <
        key: "name"
        value: "b1"
      >
    >
    elem: <
      name: "c"
    >
    elem: <
      name: "d"
    >
  >
  val: <
    json_val: "AStringValue"
  >
>

For /a/b[name=b1]/c/e:

update: <
  path: <
    elem: <
      name: "a"
    >
    elem: <
      name: "b"
      key: <
        key: "name"
        value: "b1"
      >
    >
    elem: <
      name: "c"
    >
    elem: <
      name: "e"
    >
  >
  val: <
    json_val: 10042    // decoded byte array
  >
>

For /a/b[name=b1]/c:

update: <
  path: <
    elem: <
      name: "a"
    >
    elem: <
      name: "b"
      key: <
        key: "name"
        value: "b1"
      >
    >
    elem: <
      name: "c"
    >
  >
  val: <
    json_val: `{ "d": "AStringValue", "e": 10042 }`
  >
>

For /a :

update: <
  path: <
    elem: <
      name: "a'
    >
  >
  val: <
    json_ietf_val: `{ "b": [
                      {
                        "name": "b1",
                        "c": {
                        "d": "AStringValue",
                          "e": 10042
                        }
                       }
                   ]
            }`
  >
>

Note that all JSON values MUST be valid JSON. That is to say, while a value or object may be included in the message, the relevant quoting according to the JSON specification in RFC7159 must be used. This results in quoted string values, and unquoted number values.

JSON_IETF encoded data MUST conform with the rules for JSON serialisation described in RFC7951. Data specified with a type of JSON MUST be valid JSON, but no additional constraints are placed upon it. An implementation MUST NOT serialise data with mixed JSON and JSON_IETF encodings.

Both the client and target MUST support the JSON encoding as a minimum.

2.3.2 Bytes

Data encoded using the BYTES type (i.e., with the byte_val field) contains a byte sequence whose semantics is opaque to the protocol.

2.3.3 Protobuf

Data encoded using the PROTOBUF type (i.e., within the any_val field) contains a serialised protobuf message using protobuf.Any. Note that in the case that the sender utilises this type, the receiver must understand the schema (and hence the type of protobuf message that is serialised) in order to decode the value. Such agreement is not guaranteed by the protocol and hence must be established out-of-band.

2.3.4 ASCII

The ASCII type indicates that the data contains system-formatted ASCII encoded text. For configuration data, for example, this may consist of semi-structured CLI configuration data formatted according to the target platform. The gNMI protocol does not define the format of the text – this must be established out-of-band.

2.4 Use of Data Schema Paths

2.4.1 Path Prefixes

In a number of messages, a prefix can be specified to reduce the lengths of path fields within the message. In this case, a prefix field is specified within a message - comprising of a valid path encoded according to Section 2.2.2. In the case that a prefix is specified, the absolute path is comprised of the concatenation of the list of path elements representing the prefix and the list of path elements in the path field.

For example, again considering the data tree shown in Section 2.3.1 if a Notification message updating values, a prefix could be used to refer to the /a/b[name=b1]/c/d and /a/b[name=b1]/c/e data nodes:

notification: <
  timestamp: (timestamp)      // timestamp as int64
  prefix: <
    elem: <
        name: "a"
      >
    elem: <
      name: "b"
      key: <
          key: "name"
         value: "b1"
        >
      >
     elem: <
       name: "c"
     >
  >
  update: <
    path: <
      elem: <
        name: "d"
      >
    >
    value: <
      val: <
        json_val: "AStringValue"
      >
    >
  >
  update: <
    path: <
      elem: <
       name: "e"
      >
    >
    val: <
      json_val: 10042      // converted to int representation
    >
  >
>

2.4.2 Path Aliases

In some cases, a client or target MAY desire to utilise aliases for a particular path - such that subsequent messages can be compressed by utilising the alias, rather than using a complete representation of the path. Doing so reduces total message length, by ensuring that redundant information can be removed.

Support for path aliases MAY be provided by a target. In a case where a target does not support aliases, the maximum message length SHOULD be considered, especially in terms of bandwidth utilisation, and the efficiency of message generation.

A path alias is encoded as a string. In order to avoid valid data paths clashing with aliases (e.g., a in the above example), an alias name MUST be prefixed with a # character.

The means by which an alias is created is defined on a per-RPC basis. In order to delete an alias, the alias name is sent with the path corresponding to the alias empty.

Aliases MUST be specified as a fully expanded path, and hence MUST NOT reference other aliases within their definition, such that a single alias lookup is sufficient to resolve the absolute path.

2.4.3 Interpretation of Paths Used in RPCs

When a client specifies a path within an RPC message which indicates a read, or retrieval of data, the path MUST be interpreted such that it refers to the node directly corresponding with the path and all its children. The path refers to the direct node and all descendent branches which originate from the node, recursively down to each leaf element. If specific nodes are expected to be excluded then an RPC MAY provide means to filter nodes, such as regular-expression based filtering, lists of excluded paths, or metadata-based filtering (based on annotations of the data schema being manipulated, should such annotations be available and understood by both client and target).

For example, consider the following data tree:

root +
     |
     +-- childA +
     |          |
     |          +-- leafA1
     |          +-- leafA2
     |          +-- childA3 --+
     |                        |
     |                        +-- leafA31
     |                        +-- leafA32
     |
     +-- childB +
                |
                +-- leafB1
                +-- leafB2

A path referring to "root" (which is represented by a path consisting of an empty set of elements) should result in the nodes childA and childB and all of their children (leafA1, leafA2, leafB1, leafB2, childA3, leafA31 and leafA32) being considered by the relevant operation.

In the case that the RPC is modifying the state of data (i.e., a write operation), such recursion is not required - rather the modification operation should be considered to be targeted at the node within the schema that is specified by the path, and the value should be deserialized such that it modifies the content of any child nodes if required to do so.

2.5 Error handling

Errors MUST be represented by a canonical gRPC error code (Java, Go, C++). The entity generating the error MUST specify a free-text string which indicates the context of the error, allowing the receiving entity to generate log entries that allow a human operator to understand the exact error that occurred, and its context, and MAY supply structured data encoded as protobuf.Any. Each RPC defines the meaning of the relevant canonical error codes within the context of the operation it performs.

The canonical error code that is chosen MUST consider the expected behavior of the client on receipt of the message. For example, error codes which indicate that a client may subsequently retry SHOULD only be used where retrying the RPC is expected to result in a different outcome.

Where the client or target wishes to indicate an error, it MUST use the Status message in the RPC return trailers.

The Status message consists of three fields:

2.6 Schema Definition Models

The data tree supported by the target is expected to be defined by a set of schemas. The definition and format of these models is out of scope of this specification (YANG-modeled data is one example). In the case that such schema definitions are used, the client should be able to determine the models that are supported by the target, so that it can generate valid modifications to the data tree, and interpret the data returned by Get and Subscribe RPC calls.

Additionally, the client may wish to restrict the set of models that are utilised by the target so that it can validate the data returned to it against a specific set of data models. This is particularly relevant where the target may otherwise add new values to restricted value data elements (e.g., those representing an enumerated type), or augment new data elements into the data tree.

In order to allow the client to restrict the set of data models to be used when interacting with the target, the client MAY discover the set of models that are supported by the target using the Capabilities RPC described in Section 3.2. For subsequent Get and Subscribe RPCs, the client MAY specify the models to be used by the target. The set of models to use is expressed as a ModelData message, as specified in Section 2.6.1.

If the client specifies a set of models in a Get or Subscribe RPC, the target MUST NOT utilize data tree elements that are defined in schema modules outside the specified set. In addition, where there are data tree elements that have restricted value sets (e.g., enumerated types), and the set is extended by a module which is outside of the set, such values MUST NOT be used in data instances that are sent to the client. Where there are other elements of the schema that depend on the existence of such enumerated values, the target MUST NOT include such values in data instances sent to the client.

2.6.1 The ModelData message

The ModelData message describes a specific model that is supported by the target and used by the client. The fields of the ModelData message identify a data model registered in a model catalog, as described in [MODEL_CATALOG_DOC] (the schema of the catalog itself - expressed in YANG - is described in [MODEL_CATALOG_YANG]). Each model specified by a ModelData message may refer to a specific schema module, a bundle of modules, or an augmentation or deviation, as described by the catalog entry.

Each ModelData message contains the following fields:

The combination of name, organization, and version uniquely identifies an entry in the model catalog.

2.7 Extensions to gNMI

Each top-level RPC message (e.g., SubscribeRequest and SubscribeResponse for the Subscribe RPC) defines an extensions field which can be used to carry additional parameters for a gNMI RPC. [GNMI-EXT] defines the mechanisms to define such extensions. It must be noted that the base operation of the RPCs as described in this specification MUST NOT be modified by an extension.

3 Service Definition

A single gRPC service is defined - future revisions of this specification MAY result in additional services being introduced, and hence an implementation MUST NOT make assumptions that limit to a single service definition.

The service consists of the following RPCs:

A target implementing the gNMI service SHOULD register against the gRPC server reflection service to allow clients to determine that gNMI is available on the target.

3.1 Session Security, Authentication and RPC Authorization

The session between the client and server MUST be encrypted using TLS - and a target or client MUST NOT fall back to unencrypted sessions. The target and client SHOULD implement TLS >= 1.2.

New connections are mutually authenticated -- each entity validates the X.509 certificate of the remote entity to ensure that the remote entity is both known, and authorized to connect to the local system.

If the target is expected to authenticate an RPC operation, the client MUST supply a username and password in the metadata of the RPC message (e.g., SubscribeRequest, GetRequest or SetRequest). If the client supplies username/password credentials, the target MUST authenticate the RPC per its local authentication functionality.

Authorization is also performed per-RPC by the server, through validating client-provided metadata. The client MAY include the appropriate AAA metadata, which MUST contain a username, and MAY include a password in the context of each RPC call it generates. If the client includes both username and password, the target MUST authenticate and authorize the request. If the client only supplies the username, the target MUST authorize the RPC request.

A more detailed discussion of the requirements for authentication and encryption used for gNMI is in [GNMI-AUTH].

3.2 Capability Discovery

A client MAY discover the capabilities of the target using the Capabilities RPC. The CapabilityRequest message is sent by the client to interrogate the target. The target MUST reply with a CapabilityResponse message that includes its gNMI service version, the versioned data models it supports, and the supported data encodings. This information is used in subsequent RPC messages from the client to indicate the set of models that the client will use (for Get, Subscribe as described in Section 2.6) , and the encoding to be used for the data.

When the client does not specify the models it is using, the target SHOULD use all data schema modules that it supports when considering the data tree to be addressed. If the client does not specify the encoding in an RPC message, it MUST send JSON encoded values (the default encoding).

3.2.1 The CapabilityRequest message

The CapabilityRequest message is sent by the client to request capability information from the target. The CapabilityRequest message carries a single repeated extension field, which is used as per the definition in Section 2.7.

3.2.2 The CapabilityResponse message

The CapabilityResponse message has the following fields:

3.3 Retrieving Snapshots of State Information

In some cases, a client may require a snapshot of the state that exists on the target. In such cases, a client desires some subtree of the data tree to be serialized by the target and transmitted to it. It is expected that the values that are retrieved (whether writeable by the client or not) are collected immediately and provided to the client.

The Get RPC provides an interface by which a client can request a set of paths to be serialized and transmitted to it by the target. The client sends a GetRequest message to the target, specifying the data that is to be retrieved. The fields of the GetRequest message are described in Section 3.3.1.

Upon reception of a GetRequest, the target serializes the requested paths, and returns a GetResponse message. The target MUST reflect the values of the specified leaves at a particular collection time, which MAY be different for each path specified within the GetRequest message.

The target closes the Get RPC following the transmission of the GetResponse message.

3.3.1 The GetRequest Message

The GetRequest message contains the following fields:

Since the data tree stored by the target may consist of different types of data (e.g., values that are operational in nature, such as protocol statistics) - the client MAY specify that a subset of values in the tree are of interest. In order for such filtering to be implemented, the data schema on the target MUST be annotated in a manner which specifies the type of data for individual leaves, or subtrees of the data tree.

The types of data currently defined are:

If the type field is not specified, the target MUST return CONFIG, STATE and OPERATIONAL data fields in the tree resulting from the client's query.

3.3.2 The GetResponse message

The GetResponse message consists of:

3.3.3 Considerations for using Get

The Get RPC is intended for clients to retrieve relatively small sets of data as complete objects, for example a part of the configuration. Such requests are not expected to put a significant resource burden on the target. Since the target is expected to return the entire snapshot in the GetResponse message, Get is not well-suited for retrieving very large data sets, such as the full contents of the routing table, or the entire component inventory. For such operations, the Subscribe RPC is the recommended mechanism, e.g. using the ONCE mode as described in Section 3.5.

Another consideration for Get is that the timestamp returned is associated with entire set of data requested, although individual data items may have been sampled by the target at different times. If the client requires higher accuracy for individual data items, the Subscribe RPC is recommended to request a telemetry stream (see Section 3.5.2).

3.4 Modifying State

Modifications to the state of the target are made through the Set RPC. A client sends a SetRequest message to the target indicating the modifications it desires.

A target receiving a SetRequest message processes the operations specified within it - which are treated as a transaction (see Section 3.4.3). The server MUST process deleted paths (within the delete field of the SetRequest), followed by paths to be replaced (within the replace field), and finally updated paths (within the update field). The order of the replace and update fields MUST be treated as significant within a single SetRequest message. If a single path is specified multiple times for a single operation (i.e., within update or replace), then the state of the target MUST reflect the application of all of the operations in order, even if they overwrite each other. A SetRequest specifying an empty set of paths MUST NOT be treated as an error by the target. For example, a SetRequest message carrying only extensions is valid.

In response to a SetRequest, the target MUST respond with a SetResponse message. For each operation specified in the SetRequest message, an UpdateResult message MUST be included in the response field of the SetResponse. The order in which the operations are applied MUST be maintained such that UpdateResult messages can be correlated to the SetRequest operations. In the case of a failure of an operation, the status of the UpdateResult message MUST be populated with error information as per the specification in Section 3.4.7. In addition, the status of the SetResponse message MUST be populated with an error message indicating the success or failure of the set of operations within the SetRequest message (again using the error handling behavior defined in Section 3.4.7).

3.4.1 The SetRequest Message

A SetRequest message consists of the following fields:

The semantics of "updating" versus "replacing" content are defined in Section 3.4.4.

A re-usable Update message is utilised to indicate changes to paths where a new value is required. The Update message contains two fields:

3.4.2 The SetResponse Message

A SetResponse consists of the following fields:

3.4.3 Transactions

All changes to the state of the target that are included in an individual SetRequest message are considered part of a transaction. That is, either all modifications within the request are applied, or the target MUST rollback the state changes to reflect its state before any changes were applied. The state of the target MUST NOT appear to be changed until such time as all changes have been accepted successfully. Hence, telemetry update messages MUST NOT reflect a change in state until such time as the intended modifications have been accepted.

As per the specification in Section 3.4, within an individual transaction (SetRequest) the order of operations is delete, replace, update.

As the scope of a "transaction" is a single SetRequest message, a client desiring a set of changes to be applied together MUST ensure that they are encapsulated within a single SetRequest message.

3.4.4 Modes of Update: Replace versus Update

Changes to read-write values on the target are applied based on the replace and update fields of the SetRequest message.

For both replace and update operations, if the path specified does not exist, the target MUST create the data tree element and populate it with the data in the Update message, provided the path is valid according to the data tree schema. If invalid values are specified, the target MUST cease processing updates within the SetRequest method, return the data tree to the state prior to any changes, and return a SetResponse status indicating the error encountered.

For replace operations:

For example, consider the following tree, where a schema defines the boolean b as having a default value of True and boolean c as having no default value. Both b and c have been previously set as False.

root +
     |
     + a --+
     |     |
     |     +-- b
     |     |
     |     +-- c
     |
     |
     + d --+
           +-- e
           |
           +-- f

A replace operation is issued where e and f are set, and all other elements are omitted. In this scenario, b MUST be reverted to its default setting of True and the configuration of c MUST be deleted from the tree, and returned to its original un-configured setting.

For update operations, only the value of those data elements that are specified explicitly should be treated as changed.

3.4.5 Modifying Paths Identified by Attributes

The path convention defined in Section 2.2.2 allows nodes in the data tree to be identified by a unique set of node names (e.g., /a/b/c/d) or paths that consist of node names coupled with attributes (e.g., /a/e[key=10]). In the case where where a node name plus attribute name is required to uniquely identify an element (i.e., the path within the schema represents a list, map, or array), the following considerations apply:

For example, consider a tree corresponding to the examples above, as illustrated below.

root +
     |
     + a --+
             |
             +-- f[k1=10][k2=20] --+
             |                     |
             |                     +-- k1 = 10
             |                     +-- k2 = 20
             |
             +-- f[k1=10][k2=21] --+
                                   |
                                   +-- k1 = 10
                                   +-- k2 = 21

In this case, nodes k1 and k2 are standalone nodes within the schema, but also correspond to attribute values for the node "f". In this case, an update or replace message specifying a path of /a/f[k1=10][k2=20] setting the value of k1 to 100 MUST be considered erroneous, and a status code of InvalidArgument (3) specified.

3.4.6 Deleting Configuration

Where a path is contained within the delete field of the SetRequest message, it should be removed from the target's data tree. In the case that the path specified is to an element that has children, these children MUST be recursively deleted. If a wildcard path is utilised, the wildcards MUST be expanded by the target, and the corresponding elements of the data tree deleted. Such wildcards MUST support paths specifying a subset of attributes required to identify entries within a collection (list, array, or map) of the data schema.

In the case that a path specifies an element within the data tree that does not exist, these deletes MUST be silently accepted.

3.4.7 Error Handling

When a client issues a SetRequest, and the target is unable to apply the specified changes, an error status MUST be reported to the client. The error is specified in multiple places:

The RPC status supplied with the SetResponse message MUST reflect the overall result of the transaction.

In the case that any operation within the SetRequest message fails, then (as per Section 3.4.3), the target MUST NOT apply any of the specified changes, and MUST consider the transaction as failed. The target SHOULD set the status code of the SetResponse message to Aborted (10), along with an appropriate error message, and MUST set the message field of the UpdateResult corresponding to the failed operation to an Error message indicating failure. In the case that the processed operation is not the only operation within the SetRequest the target MUST set the message field of the UpdateResult messages for all other operations, setting the code field to Aborted (10).

For the operation that the target is unable to process, the code indicated in the status message returned within the RPC response MUST be set to a specific error code indicating the reason for failure based on the following mappings to canonical gRPC error codes:

3.5 Subscribing to Telemetry Updates

When a client wishes to receive updates relating to the state of data instances on a target, it creates a subscription via the Subscribe RPC. A subscription consists of one or more paths, with a specified subscription mode. The mode of each subscription determines the triggers for updates for data sent from the target to the client.

All requests for new subscriptions are encapsulated within a SubscribeRequest message - which itself has a mode which describes the longevity of the subscription. A client may create a subscription which has a dedicated stream to return one-off data (ONCE); a subscription that utilizes a stream to periodically request a set of data (POLL); or a long-lived subscription that streams data according to the triggers specified within the individual subscription's mode (STREAM).

The target generates messages according to the type of subscription that has been created, at the frequency requested by the client. The methods to create subscriptions are described in Section 3.5.1.

Subscriptions are created for a set of paths - which cannot be modified throughout the lifetime of the subscription. In order to cancel a subscription, the client cancels the Subscribe RPC associated with the subscription, or terminates the entire gRPC session.

Subscriptions are fundamentally a set of independent update messages relating to the state of the data tree. That is, it is not possible for a client requesting a subscription to assume that the set of update messages received represent a snapshot of the data tree at a particular point in time. Subscriptions therefore allow a client to:

Based on the fact that subsequent update messages are considered to be independent, and to ensure that the efficiencies described above can be achieved, by default a target MUST NOT aggregate values within an update message.

In some cases, however, elements of the data tree may be known to change together, or need to be interpreted by the subscriber together. Such data MUST be explicitly marked in the schema as being eligible to be aggregated when being published. Additionally, the subscribing client MUST explicitly request aggregation of eligible schema elements for the subscription - by means of the allow_aggregation flag within a SubscriptionList message. For elements covered by a subscription that are not explicitly marked within the schema as being eligible for aggregation the target MUST NOT coalesce these values, regardless of the value of the allow_aggregation flag.

When aggregation is not permitted by the client or the schema each update message MUST contain a (key, value) pair - where the key MUST be a path to a single leaf element within the data tree (encoded according to Section 2.2.2). The value MUST encode only the value of the leaf specified. In most cases, this will be a scalar value (i.e., a JSON value if a JSON encoding is utilised), but in some cases, where an individual leaf element within the schema represents an object, it MAY represent a set of values (i.e., a JSON object or Protobuf message).

Where aggregation is permitted by both the client and schema, each update message MUST contain a key value pair, where the key MUST be the path to the element within the data tree which is explicitly marked as being eligible for aggregation. The value MUST be an object which encodes the children of the data tree element specified. For JSON, the value is therefore a JSON object, and for Protobuf is a binary-encoded Protobuf message (along with any child messages).

3.5.1 Managing Subscriptions

3.5.1.1 The SubscribeRequest Message

A SubscribeRequest message is sent by a client to request updates from the target for a specified set of paths.

The fields of the SubscribeRequest are as follows:

In order to create a new subscription a client MUST initiate a Subscribe RPC with a SubscribeRequest message specifying the subscribe field. The SubscriptionList may create a one-off subscription, a poll-only subscription, or a streaming subscription. In the case of ONCE subscriptions, the RPC MUST be closed following the initial response generation with the relevant status code.

Subscriptions are set once, and subsequently not modified by a client. If a client wishes to subscribe to additional paths from a target, it MUST do so by sending an additional Subscribe RPC call, specifying a new SubscriptionList message. In order to end an existing subscription, a client simply cancels the Subscribe RPC that relates to that subscription. If an RPC is initiated with a SubscribeRequest message that does not specify a SubscriptionList message with the request field, the target MUST consider this an error. If an additional SubscribeRequest message specifying a SubscriptionList is sent via an existing RPC, the target MUST respond to this message with SubscribeResponse message indicating an error status, with a code of InvalidArgument (4); other Subscribe RPCs on the gRPC session MUST not be modified or terminated.

If a client initiates a Subscribe RPC with a SubscribeRequest message which does not contain a SubscriptionList message, this is an error. A SubscribeResponse message with the status indicating a error code of InvalidArgument MUST be sent, and the RPC closed. The status message SHOULD indicate that an out-of-order operation was requested on a non-existent subscription.

3.5.1.2 The SubscriptionList Message

A SubscriptionList message is used to indicate a set of paths for which common subscription behavior are required. The fields of the message are:

A client generating a SubscriptionList message MUST include the subscription field - which MUST be a non-empty set of Subscription messages, all other fields are optional.

3.5.1.3 The Subscription Message

A Subscription message generically describes a set of data that is to be subscribed to by a client. It contains a path, specified as per the definition in Section 2.2.2.

There is no requirement that the path specified in the message must exist within the current data tree on the server. While the path within the subscription SHOULD be a valid path within the set of schema modules that the target supports, subscribing to any syntactically valid path within such modules MUST be allowed. In the case that a particular path does not (yet) exist, the target MUST NOT close the RPC, and instead should continue to monitor for the existence of the path, and transmit telemetry updates should it exist in the future.

For POLL and STREAM subscriptions, a client may optionally specify additional parameters within the Subscription message. The semantics of these additional fields are described in the relevant section of this document.

3.5.1.4 The SubscribeResponse Message

A SubscribeResponse message is transmitted by a target to a client over an established Subscribe RPC. The message contains the following fields:

SubscribeResponse messages with the update field set to a Notification are not required to be sent by the target in timestamp order, and client implementations MUST NOT assume that these messages will be received in timestamp order.

3.5.1.5 Creating Subscriptions

3.5.1.5.1 ONCE Subscriptions

A subscription operating in the ONCE mode acts as a single request/response channel. The target creates the relevant update messages, transmits them, and subsequently closes the RPC.

In order to create a one-off subscription, a client sends a SubscribeRequest message to the target. The subscribe field within this message specifies a SubscriptionList with the mode field set to ONCE. Updates corresponding to the subscription are generated as per the semantics described in Section 3.5.2.

Following the transmission of all updates which correspond to data items within the set of paths specified within the subscription list, a SubscribeResponse message with the sync_response field set to true MUST be transmitted, and the RPC via which the SubscribeRequest was received MUST be closed.

3.5.1.5.2 STREAM Subscriptions

Stream subscriptions are long-lived subscriptions which continue to transmit updates relating to the set of paths that are covered within the subscription indefinitely.

A STREAM subscription is created by sending a SubscribeRequest message with the subscribe field containing a SubscriptionList message with the type specified as STREAM. Each entry within the Subscription message is specified with one of the following modes:

3.5.1.5.3 POLL Subscriptions

Polling subscriptions are used for on-demand retrieval of data items via long-lived RPCs. A poll subscription relates to a certain set of subscribed paths, and is initiated by sending a SubscribeRequest message with encapsulated SubscriptionList. Subscription messages contained within the SubscriptionList indicate the set of paths that are of interest to the polling client.

To retrieve data from the target, a client sends a SubscribeRequest message to the target, containing a poll field, specified to be an empty Poll message. On reception of such a message, the target MUST generate updates for all the corresponding paths within the SubscriptionList. Updates MUST be generated according to Section 3.5.2.3.

3.5.1.6 Client-defined Aliases within a Subscription

When a client wishes to create an alias that a target should use for a path, the client should send a SubscribeRequest message specifying the aliases field. The aliases field consists of an AliasList message. An AliasList specifies a list of aliases, each of which consists of:

Where a target is unable to support a client-defined alias it SHOULD respond with a SubscribeResponse message with the status code indicating an error of the following types:

Thus, for a client to create an alias corresponding to the path /a/b/c/d[id=10]/e with the name shortPath, it sends a SubscribeRequest message with the following fields specified:

subscriberequest: <
  aliases: <
    alias: <
      path: <
        elem: <
          name: "a"
        >
        elem: <
          name: "b"
        >
        elem: <
          name: "c"
        >
        elem: <
          name: "d"
          key: <
            name: "id"
            value: "10"
          >
        elem: <
          name: "e"
        >
      >
      alias: "#shortPath"
    >
  >
>

If the alias is acceptable to the target, subsequent updates are transmitted using the #shortPath alias in the same manner as described in Section 3.5.2.2.

3.5.2 Sending Telemetry Updates

3.5.2.1 Bundling of Telemetry Updates

Since multiple Update messages can be included in the Notification message of a SubscribeResponse, it is possible for a target to bundle messages such that fewer messages are sent to the client. The advantage of such bundling is clearly to reduce the number of bytes on the wire (caused by message overhead); however, since only Notification messages contain the timestamp at which an event occurred, or a sample was taken, such bundling assigns a single timestamp for all bundled Update values. As such, it has the downside of negatively affecting the sample accuracy and freshness to the client, and as a result, on the client's ability to react to events on the target.

Since it is not possible for the target to infer whether its clients are sensitive to the latency introduced by bundling, if a target implements optimizations such that multiple Update messages are bundled together, it MUST provide an ability to disable this functionality within the configuration of the gNMI service. Additionally, a target SHOULD provide means by which the operator can control the maximum number of updates that are to be bundled into a single message, This configuration is expected to be implemented out-of-band to the gNMI protocol itself.

3.5.2.2 Target-defined Aliases within a Subscription

Where the use_aliases field of a SubscriptionList message has been set to true, a target MAY create aliases for paths within a subscription. A target-defined alias MUST be created separately from an update to the corresponding data item(s).

To create a target-defined alias, a SubscribeResponse message is generated with the update field set to a Notification message. The Notification message specifies the aliased path within the prefix field, and a non-null alias field, specified according to Section 2.4.2.

Thus, a target wishing to create an alias relating to the path /a/b/c[id=10] and subsequently update children of the c[id=10] entity must:

subscriberesponse: <
  update: <
    timestamp: (timestamp)
    prefix: <
      elem: <
        name: "a"
      >
      elem: <
        name: "b"
      >
      elem: <
        name: "c"
        key: <
          name: "id"
          value: "10"
        >
      >
    >
    alias: "#42"
  >
>

subscriberesponse: <
  update: <
    timestamp: (timestamp)
    prefix: <
      element: "#42"
    >
    update: <
      path: <
        elem: <
          name: "child1"
        >
      >
      value: <
        val: <
          int_val: 32
        >
      >
    >
  >
>

3.5.2.3 Sending Telemetry Updates

When an update for a subscribed telemetry path is to be sent, a SubscribeResponse message is sent from the target to the client, on the RPC associated with the subscription. The update field of the message contains a Notification message as per the description in Section 2.1. The timestamp field of the Notification message MUST be set to the time at which the value of the path that is being updated was collected from the underlying data source, or the event being reported on (in the case of ON_CHANGE occurred).

Where a leaf node's value has changed, or a new node has been created, an Update message specifying the path and value for the updated data item MUST be appended to the update field of the message.

Where a node within the subscribed paths has been removed, the delete field of the Notification message MUST have the path of the node that has been removed appended to it.

To replace the contents of an entire node within the tree, the target populates the delete field with the path of the node being removed, along with the new contents within the update field.

When the target has transmitted the initial updates for all paths specified within the subscription, a SubscribeResponse message with the sync_response field set to true MUST be transmitted to the client to indicate that the initial transmission of updates has concluded. This provides an indication to the client that all of the existing data for the subscription has been sent at least once. For STREAM subscriptions, such messages are not required for subsequent updates. For POLL subscriptions, after each set of updates for individual poll request, a SubscribeResponse message with the sync_response field set to true MUST be generated.

In the case where the updates_only field in the SubscribeRequest message has been set, a sync_response is sent as the first message on the stream, followed by any updates representing subsequent changes to current state. For a POLL or ONCE mode, this means that only a sync_response will be sent. The updates_only field allows a client to only watch for changes, e.g. an update to configuration.

4 Appendix: Current Protobuf Message and Service Specification

The latest Protobuf IDL gNMI specification is found in GitHub at openconfig/gnmi.

5 Appendix: Current Outstanding Issues/Future Features

6 Copyright

Copyright 2016 Google Inc.

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

7 Revision History

Notes

  1. This matches the types that Go UnixNano and Java TimeUnit toNanos return, and hence is used rather than an unsigned integer. ↩