Telemetry is an automated communication process by which measurements are collected and transmitted to receivers for monitoring of the data. Model-driven telemetry replaces the need for periodic polling of network elements using SNMP. Instead, it involves a continuous request for information to be delivered to a subscriber or receiver that has an established session with the network device. Data is received either periodically or as objects change via a subscribed set of YANG objects.
RFC 6241: Network Configuration Protocol (NETCONF) explains a YANG push model, which is a subscription and push mechanism for access to YANG databases. The YANG push model encompasses all of the data in the configuration and operational databases using the YANG model on the network device. However, a filter needs to be used for the data because the subscription to all data is not supported.
All sessions in telemetry use NETCONF sessions, which impose any session limitation specific to the NETCONF implementation. High availability in telemetry for NETCONF sessions uses SSH to the active switch or a member in a switch stack. If a NETCONF session is broken, a new NETCONF session must be established, including sessions that carry telemetry subscriptions.
Subscription Explained
To stream data in model-driven telemetry, the client application requests a subscription to a data set in YANG from the network device. A subscription is an agreement between the subscriber and the subscription service that describes the data to be pushed out. The subscription service allows clients to subscribe to the desired YANG data models, and then the network device pushes the data to the receiver per the agreed upon subscription model. There are two types of subscriptions: periodic and on-change publications. We take a look at both of them in the upcoming sections.
Figure 12-13 illustrates subscriptions in model-driven telemetry.

Figure 12-13 Model-Driven Telemetry Subscriptions
Periodic Publication
Subscriptions that are periodic are streamed out to the receivers at specified intervals, such as every 5 seconds. With periodic publications, the network device continuously sends data for the lifetime of that configured subscription. A typical example where this type of periodic subscription is useful is for receiving data from PDU counters on a device.
Figure 12-14 shows sample output from the show telemetry ietf subscription command, using a period of 10 seconds. The period configuration in Figure 12-14 is for time and is in centiseconds, which is 1/100 of a second between updates.

Figure 12-14 Telemetry Subscription Detail from a Switch CLI