3. Terminology
DSO: DNS Stateful Operations.
connection: a bidirectional byte (or message) stream, where the bytes (or messages) are delivered reliably and in order, such as provided by using DNS-over-TCP [RFC1035] [RFC7766] or DNS-over-TLS [RFC7858].
session: the unqualified term "session" in the context of this document refers to a persistent network connection between two endpoints that allows for the exchange of DNS messages over a connection where either end of the connection can send messages to the other end. (The term has no relationship to the "session layer" of the OSI "seven-layer model".)
DSO Session: a session established between two endpoints that acknowledge persistent DNS state via the exchange of DSO messages over the connection. This is distinct from a DNS-over-TCP session as described in the previous specification for DNS-over-TCP [RFC7766].
close gracefully: a normal session shutdown where the client closes the TCP connection to the server using a graceful close such that no data is lost (e.g., using TCP FIN; see Section 5.3).
forcibly abort: a session shutdown as a result of a fatal error where the TCP connection is unilaterally aborted without regard for data loss (e.g., using TCP RST; see Section 5.3).
server: the software with a listening socket, awaiting incoming connection requests, in the usual DNS sense.
client: the software that initiates a connection to the server's listening socket, in the usual DNS sense.
initiator: the software that sends a DSO request message or a DSO unidirectional message during a DSO Session. Either a client or server can be an initiator.
responder: the software that receives a DSO request message or a DSO unidirectional message during a DSO Session. Either a client or a server can be a responder.
sender: the software that is sending a DNS message, a DSO message, a DNS response, or a DSO response.
receiver: the software that is receiving a DNS message, a DSO message, a DNS response, or a DSO response.
service instance: a specific instance of server software running on a specific host (Section 9.1).
long-lived operation: an outstanding operation on a DSO Session where either the client or server, acting as initiator, has requested that the responder send new information regarding the request, as it becomes available.
early data: a TLS 1.3 handshake containing data on the first flight that begins a DSO Session (Section 2.3 of the TLS 1.3 specification [RFC8446]). TCP Fast Open [RFC7413] is only permitted when using TLS.
DNS message: any DNS message, including DNS queries, responses, updates, DSO messages, etc.
DNS request message: any DNS message where the QR bit is 0.
DNS response message: any DNS message where the QR bit is 1.
DSO message: a DSO request message, DSO unidirectional message, or DSO response to a DSO request message. If the QR bit is 1 in a DSO message, it is a DSO response message. If the QR bit is 0 in a DSO message, it is a DSO request message or DSO unidirectional message, as determined by the specification of its Primary TLV.
DSO response message: a response to a DSO request message.
DSO request message: a DSO message that requires a response.
DSO unidirectional message: a DSO message that does not require and cannot induce a response.
Primary TLV: the first TLV in a DSO request message or DSO unidirectional message; this determines the nature of the operation being performed.
Additional TLV: any TLVs that follow the Primary TLV in a DSO request message or DSO unidirectional message.
Response Primary TLV: in a DSO response, any TLVs with the same DSO-TYPE as the Primary TLV from the corresponding DSO request message. If present, any Response Primary TLV(s) MUST appear first in the DSO response message, before any Response Additional TLVs.
Response Additional TLV: any TLVs in a DSO response that follow the (optional) Response Primary TLV(s).
inactivity timer: the time since the most recent non-keepalive DNS message was sent or received (see Section 6.4).
keepalive timer: the time since the most recent DNS message was sent or received (see Section 6.5).
session timeouts: the inactivity timer and the keepalive timer.
inactivity timeout: the maximum value that the inactivity timer can have before the connection is gracefully closed.
keepalive interval: the maximum value that the keepalive timer can have before the client is required to send a keepalive (see Section 7.1).
resetting a timer: setting the timer value to zero and restarting the timer.
clearing a timer: setting the timer value to zero but not restarting the timer.