6. Protocol Operation

6. Protocol Operation

The DNS Push Notification protocol is a session-oriented protocol and makes use of DNS Stateful Operations (DSO) [RFC8490].

For details of the DSO message format, refer to the DNS Stateful Operations specification [RFC8490]. Those details are not repeated here.

DNS Push Notification clients and servers MUST support DSO. A single server can support DNS Queries, DNS Updates, and DNS Push Notifications (using DSO) on the same TCP port.

A DNS Push Notification exchange begins with the client discovering the appropriate server, using the procedure described in Section 6.1, and then making a TLS/TCP connection to it.

After making the TLS/TCP connection to the server, a typical DNS Push Notification client will then immediately issue a DSO Keepalive operation to establish the DSO session and request a session timeout and/or keepalive interval longer than the 15-second default values, but this is not required. A DNS Push Notification client MAY issue other requests on the session first, and only issue a DSO Keepalive operation later if it determines that to be necessary. Sending either a DSO Keepalive operation or a Push Notification subscription request over the TLS/TCP connection to the server signals the client's support of DSO and serves to establish a DSO session.

In accordance with the current set of active subscriptions, the server sends relevant asynchronous Push Notifications to the client. Note that a client MUST be prepared to receive (and silently ignore) Push Notifications for subscriptions it has previously removed, since there is no way to prevent the situation where a Push Notification is in flight from server to client while the client's UNSUBSCRIBE message canceling that subscription is simultaneously in flight from client to server.