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3. Terminology

  • Persistent connection: a TCP connection that is not closed either by the server after sending the first response nor by the client after receiving the first response.

  • Connection Reuse: the sending of multiple queries and responses over a single TCP connection.

  • Idle DNS-over-TCP session: Clients and servers view application-level idleness differently. A DNS client considers an established DNS-over-TCP session to be idle when it has no pending queries to send and there are no outstanding responses. A DNS server considers an established DNS-over-TCP session to be idle when it has sent responses to all the queries it has received on that connection.

  • Pipelining: the sending of multiple queries and responses over a single TCP connection but not waiting for any outstanding replies before sending another query.

  • Out-of-Order Processing: The processing of queries concurrently and the returning of individual responses as soon as they are available, possibly out of order. This will most likely occur in recursive servers; however, it is possible in authoritative servers that, for example, have different backend data stores.

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