4. Applicability
DNS Stateful Operations are applicable to several known use cases and are only applicable on transports that are capable of supporting a DSO Session.
4.1. Use Cases
Several use cases for DNS Stateful Operations are described below.
4.1.1. Session Management
In one use case, establishing session parameters such as server-defined timeouts is of great use in the general management of persistent connections. For example, using DSO Sessions for stub-to-recursive DNS-over-TLS [RFC7858] is more flexible for both the client and the server than attempting to manage sessions using just the edns-tcp-keepalive EDNS(0) Option [RFC7828]. The simple set of TLVs defined in this document is sufficient to greatly enhance connection management for this use case.
4.1.2. Long-Lived Subscriptions
In another use case, DNS-based Service Discovery (DNS-SD) [RFC6763] has evolved into a naturally session-based mechanism where, for example, long-lived subscriptions lend themselves to 'push' mechanisms as opposed to polling. Long-lived stateful connections and server-initiated messages align with this use case [Push].
A general use case is that DNS traffic is often bursty, but session establishment can be expensive. One challenge with long-lived connections is sustaining sufficient traffic to maintain NAT and firewall state. To mitigate this issue, this document introduces a new concept for the DNS -- DSO "keepalive traffic". This traffic carries no DNS data and is not considered 'activity' in the classic DNS sense, but it serves to maintain state in middleboxes and to assure the client and server that they still have connectivity to each other.
4.2. Applicable Transports
DNS Stateful Operations are applicable in cases where it is useful to maintain an open session between a DNS client and server, where the transport allows such a session to be maintained, and where the transport guarantees in-order delivery of messages on which DSO depends. Two specific transports that meet the requirements to support DNS Stateful Operations are DNS-over-TCP [RFC1035] [RFC7766] and DNS-over-TLS [RFC7858].
Note that in the case of DNS-over-TLS, there is no mechanism for upgrading from DNS-over-TCP to DNS-over-TLS mid-connection (see Section 7 of the DNS-over-TLS specification [RFC7858]). A connection is either DNS-over-TCP from the start, or DNS-over-TLS from the start.
DNS Stateful Operations are not applicable for transports that cannot support clean session semantics or that do not guarantee in-order delivery. While in principle such a transport could be constructed over UDP, the current specification of DNS-over-UDP [RFC1035] does not provide in-order delivery or session semantics and hence cannot be used. Similarly, DNS-over-HTTP [RFC8484] cannot be used because HTTP has its own mechanism for managing sessions, which is incompatible with the mechanism specified here.
Only DNS-over-TCP and DNS-over-TLS are currently defined for use with DNS Stateful Operations. Other transports may be added in the future if they meet the requirements set out in the first paragraph of this section.