gRPC

In gNMI service, the client discovers the capabilities of the gRPC server through a Capability-Discovery RPC, which consists of ‟CapabilityRequest” and ‟CapabilityResponse” messages.

During this message exchange, the gRPC server informs the client about following attributes:

• supported gNMI version

• supported models

• supported encodings

The SR OS server announces the supported models based on the configuration in the following context.

configure system management-interface yang-modules

The supported models includes the Nokia YANG or OpenConfig (OC) models.

The advertised module names and organizations are as follows:

• nokia-conf, org = "Nokia"

• nokia-state, org = "Nokia"

• openconfig, org = "OpenConfig working group" (as specified by the 'organization' in the YANG models)

• version - the version number is be defined as follows:

  • for Nokia YANG models, the version number corresponds to an SR OS release number, for example, "16.0.R1"

  • for OC YANG models, the version number corresponds to a version number defined in "oc-ext:openconfig-version" that is included in the respective YANG models

  • for OC-YANG models, including Nokia deviations, the version number corresponds to an SR OS release number, for example, "16.0.R1"

The following is an example of a ‟Capabilities Response Message”:

Going to send message of type gnmi.CapabilityResponse:
 .gnmi_version: 0.4.0
 .supported_encodings (1):
  .encoding: 0 = JSON
 .supported_models (47):
  { .name: 'nokia-conf', .organization: 'Nokia', .version: '16.0.R1' }
  { .name: 'nokia-state', .organization: 'Nokia', .version: '16.0.R1' }
  { .name: 'openconfig-
bgp', .organization: 'OpenConfig working group', .version: '4.0.1' }
<snip>
  { .name: 'nokia-sr-openconfig-if-ethernet-deviations', .organization: 'Nokia', 
.version: '16.0.R1' }
  { .name: 'nokia-sr-openconfig-if[[-ip-deviations', .organization: 'Nokia', 
.version: '16.0.R1'..."

SR OS supports multiple encoding formats for gNMI operations. The selected encoding format determines how data is serialized and exchanged between the gRPC client and the server.

Information is retrieved from the NE using GET RPC messages, which consist of ‟GetRequest” and ‟GetResponse” messages. The client asks for information by specifying the following:

• a set of paths

  All rules to a path definition apply, as specified in the gNMI specification.

• type

  Consists of configuration, state, or operational data.

• encoding

  This is in accordance with the server advertisement during capability discovery.

• use_models

  This message is ignored.

See Encoding support by RPC type for information about supported encodings for Get and Set operations.

The ‟GetResponse” message size is limited to 100 MB. If the limit is exceeded, the SR OS gRPC server responds with an error message.

The Set gRPC message is used to modify the information in an NE element. This gRPC supports the following transactions:

• delete

• replace

• update

With a gNMI Set RPC, SR OS checks the YANG tree and authorizes all configuration changes.

When a container is deleted, the child containers in the hierarchy that are authorized for deletion are deleted along with their content. The children containers that are not authorized for deletion are retained. For example, upon deletion of configure system, configure system security is not deleted because the deletion of that child container is not authorized.

The Subscribe RPC is part of the telemetry support in gNMI.

The gRPC client sends a Subscribe RPC that contains a "SubscribeRequest" message to the gRPC server, and initiates a subscription. A prefix can be specified in the "SubscribeRequest". If a prefix is present, it is prepended to every path to provide a full path.

A subscription contains the following information:

• List of one or more paths with the following conditions:

  • A path represents the data tree as a series of repeated strings and elements. Each element represents a data tree node name and its associated attributes.

  • A path must be syntactically valid within the set of schema modules that the gRPC server supports.

  • The path list cannot be modified during the lifetime of the subscription.

  • If the subscription path is to a container node, all child leafs of that container node are subscribed to.

  • Any specified path must be unique within the list; paths cannot be repeated within the list. An error is returned if the same path is used more than once in a single subscription.

  • A specified path does not need to pre-exist within the current data tree on the gRPC server. If a path does not exist, the gRPC server continues to monitor for the existence of the path. Assuming that the path exists, the gRPC server transmits telemetry updates.

  • The gRPC server does not send data for a non-existent path; for example, if a path is non-existent at the time of subscription creation or if the path was deleted after the subscription was established.

  • The maximum number of paths for all subscriptions is 14400. A path using a wildcard is considered a single path.
• Encoding where subscribe operations support JSON, JSON-IETF, BYTES, and PROTO encodings. See gNMI encoding for information about encoding support and TypedValue field mapping.

• Subscription mode of one of the following types:

  • ONCE mode

    The server returns only one notification containing all the information to which the client has subscribed. In general, use telemetry to retrieve large amounts of information from the NE: ‟SubscribeRequest” message with ONCE subscription type.

  • ON_CHANGE mode

    The server returns notifications only when the value of the subscribed field changes. See ON_CHANGE subscription mode for more information.

  • SAMPLE mode

    The gRPC server sends notifications at the specified sampling interval. The first notification is sent at the next sampling interval, not immediately.

  • TARGET_DEFINED mode

    This type means ON_CHANGE for all states that support ON_CHANGE notifications and SAMPLE mode for all other objects in the YANG tree.

• A sample interval for each path:

  • If a sample interval of less than 1 s is specified, the gRPC server returns an error.
  • If the sample interval is set to 0, the default value of 10 s is used.
  • A sample interval is specified in nanoseconds (10 000 000 000 by default).

When a subscription is successfully initiated on the gRPC server, ‟SubscribeResponse” messages are sent from the gRPC server to the gRPC client. The ‟SubscribeResponse” message contains update notifications about the subscription path list.

An update notification contains the following:

• a timestamp of the statistics collection time, in nanoseconds

• a prefix

  • If a prefix is present, it is logically appended to the start of every path to provide the full path.

  • The presence of a prefix in the ‟SubscribeResponse” message is not related to the presence of a prefix in the original ‟SubscribeRequest” message. The prefix in the ‟SubscribeResponse” message is optimized by the gRPC server.

• a list of updates (path and value pairs)

  • A path represents the data tree path as a series of repeated strings or elements, where each element represents a data tree node name and its associated attributes. See Schema paths for more information.

  • The ‟TypedValue” message represents the value of the data tree node, where the encoding is ‟JSON”, ‟JSON_IETF”, ‟Bytes, or ‟Protobuf” depending on the information in the ‟SubscribeRequest” message.

Multiple notification messages can be combined in a single ‟SubscribeResponse” message. This bundling minimizes overhead, which improves the efficiency of telemetry data transport; however, this may delay some notifications, and timestamps may be less accurate. The following commands control message bundling:

• max-time-granularity

  This parameter controls the maximum time during which notifications can be bundled.

• max-msg-count

  This parameter controls the maximum number of notifications that can be bundled.

A synchronization response notification is sent once, after the gRPC server sends all updates for the subscribed-to paths. The synchronization response must be set to ‟true” for the gRPC client to consider that the stream has synced one time. A synchronization response is used to signal the gRPC client that it has a full view of the subscribed-to data.

The gRPC server sends an error, if required. The error contains a description of the problem.

The SR OS supports ON_CHANGE subscription mode. This subscription mode indicates that notification messages are sent as follows:

• after the ‟SubscriptionRequest” message is received

• every time the corresponding leaf value is changed

The notification message, as a response to an ON_CHANGE subscription, always contains the new value of the corresponding leaf, as defined in the gNMI specification.

The ON_CHANGE subscription is supported for all configuration leafs as well as for selected state leafs. Use the following command to display all state leafs supporting the ON_CHANGE subscription.

tools dump system telemetry on-change-paths

ON_CHANGE subscription is accepted for all valid paths. The server sends ON_CHANGE notifications only for leafs within the specified subtree that support ON_CHANGE notifications.

With dial-out telemetry, where the SR OS node is the gRPC client instead of the gRPC server, the SR OS node sends a Publish RPC with a ‟SubscribeResponse” message to the gRPC server. See Dial-out telemetry for more information.

Because the current gnmi.proto definition does not support dial-out mode, a protobuf definition is introduced, with a separate gRPC service, as follows:

NOKIA-DialOut.proto
      option (dialout_service) = 0.1.0
      service gMIDialOut {
              rpc Publish(stream SubscribeResponse) returns (stream PublishResponse)
      }
      message PublishResponse {
      }

The preceding proto file definition reuses the ‟SubscribeResponse” message defined for dial-in telemetry, in accordance with the gNMI specification.

Telemetry subscriptions include a set of schema paths to identify which data nodes are of interest to the collector.

The paths in Telemetry Subscribe RPC requests follow the basic conventions described in the OpenConfig gnmi-path-conventions.md published on github.com (version 0.4.0, published June 21, 2017).

A path consists of a set of path segments, often shown with a ‟/” character as a delimiter.

/configure/router[router-name=Base]/interface[interface-name=my-interface1]/description

["configure", "router[router-name=Base]", "interface[interface-name=my-interface1]", "description"]

Telemetry is a network monitoring and fault management framework. Telemetry is driven by the need to use fresh data obtained from the network to make fast networking decisions, such as traffic optimization and preventive troubleshooting.

The following figure shows NMS configuration and information retrieval using the gNMI service.

In the context of gNMI, every Set RPC appears as an single commit operation, regardless of the number of paths included in the message. Nokia and OpenConfig models are supported by gNMI Get or Set RPCs.

The following are examples of the Get RPC and Set RPC commands, including the response messages from the gRPC server.

gNMI_rpc - INFO - SENT::GetRequest GET140550212650064
path {
  elem {
    name: "configure"
  }
  elem {
    name: "system"
  }
  elem {
    name: "location"
  }
}
type: CONFIG
2017-12-06 12:17:28,639 - gMI_rpc - INFO - 
 RCVD::GetResponse GET140550212650064
notification {
  timestamp: 1512559048634751055
  update {
  path {
    elem {
      name: "configure"
    }
    elem {
      name: "system"
    }
    elem {
      name: "location"
    }
  }
  val {
    json_val: "zurich"
  }
 }
}

 gNMI_rpc - DEBUG - SENT::SetRequest
prefix {
}
update {
  path {
    elem {
      name: "configure"
    }
    elem {
      name: "system"
    }
 }
 val {
   json_val: {"location": "zurich"}
 }
}
gMI_rpc - DEBUG - RCVD::SetResponse
prefix {
}
response {
  path {
    elem {
      name: "configure"
    }
    elem {
      name: "system"
    }
  }
  op: UPDATE
} 

RPC GetCertificates provide information to the controller about all active certificates on the server (SR OS node). The following figure shows the message flow.

The RPC GetCertificates messages include a GetCertificateRequest and a GetCertificateResponse message. The GetCertificatesResponse message shown in the preceding figure includes the following information:

• certificate_id

  The SR OS uses a certificate filename as the certificate ID.

• CertificateType

  This is always set to X509 because it is the only type that the SR OS supports.

• endpoint

  Indicates the CERT profiles in the SR OS node that use this certificate; if multiple CERT profiles use the certificate, the names are concatenated with the separation character ‟/”.

The RPC CanGenerateCSR message can be used to determine whether the gRPC server (SR OS node) can generate a Certificate Signing Request (CSR). It is a simple request and response operation, as shown in the following figure.

The SR OS supports only RSA keys and X509 certificates, so it responds positively only when those values are present in the respective fields. The key size must be between 512 and 8192. In all other cases, the SR OS responds negatively to the CanGenerateCSRRequest message.

RPC Rotate allows the controller to rotate an active certificate on the server. After the rotation is completed, a new certificate can be used without affecting existing TLS connections.

The following cases are supported for a certificate rotation:

• server capable of generating a CSR (see RPC Rotate message flow for CSRs generated on the SR OS node)

• server not capable of generating a CSR (see RPC Rotate message flow when CSRs are not generated on the SR OS node)

The SR OS supports both scenarios, although it is assumed that in most cases the CSR is generated on SR OS node.

The following steps apply to both scenarios:

1. Generate the CSR.

2. Sign the CSR by the Certificate Authority (CA).

3. Load the new certificate on the server.

4. Verify the new certificate by creating a new connection.

5. Finalize by confirming that the new certificate is being used.

After the RPC Rotate is completed, all new connections use new keys.

From the perspective of the interaction of the controller and the server (SR OS) two stages are the most important:

• message exchange to generate the CSR

• message exchange to load the new certificates on the server

The following figure shows a detailed content of the messages that are exchanged for CSR generation. The SR OS accepts requests only for the X509 certificate type, RSA key type, and a minimum key length of 512 bits.

For RPC Rotate, the certificate_id points to an existing certificate on the node. All the other parameters in the GenerateCSRRequest message are not checked by the SR OS software explicitly. They are used by the internal API to generate the CSR and that result is transparently passed to the controller.

After the CA signs the certificates, the files are loaded to the server using LoadCSRRequest and LoadCSRResponse message exchange, as shown in the following figure. If this message exchange is used in the context of RPC Rotate, the certificate_id should not be present in LoadCSRRequest message. When the SR OS receives the message, it performs all the necessary steps to load this certificate, including storing the certificate and key files on the disk.

The controller is responsible for verifying the connection with the new certificate (Step 4 in RPC Rotate message flow for CSRs generated on the SR OS node and RPC Rotate message flow when CSRs are not generated on the SR OS node); SR OS treats this as an optional step.

After the whole RPC is successfully closed, the system can use the new certificate to start new TLS connections.

The controller can use RPC Install to install a new certificate on the server. After the certificate is installed, the server must be configured (assign a certificate and key files in the CERT profile) before the new certificate can be used.

The following two possible cases are supported for installing a certificate:

• server capable of generating a CSR (see RPC install message flow for CSRs generated on the SR OS node)

• server is not capable of generating a CSR (see RPC install message flow if CSRs are not generated on the SR OS node)

The SR OS supports both scenarios, although it is assumed that in most cases the CSR is generated on the SR OS node.

Both scenarios require the following steps:

1. Generate the CSR.

2. Sign the CSR by the Certificate Authority (CA).

3. Load the new certificate on the server.

The message exchange during phases 1 and 3 is the same as shown in GenerateCSR message flow and LoadCSRRequest/Response message flow. The only difference, in the case of RPC Install, is that a new certificate_id is used.

After new certificates are installed, the system must be configured before it can be used. Configuration is supported using the following methods:

• an existing gRPC session

• a CLI session, SNMP, or NETCONF

The purpose of the RPC RevokeCertificates is to make the existing certificate unusable by any client. In cases where the certificate being revoked by the client does not exist on the SR OS node, the corresponding RPC silently succeeds. The following figure shows the message flow.