On Wed, Sep 11, 2019 at 11:55:04AM -0700, S Moonesamy <sm+ietf@xxxxxxxxxxxx> wrote a message of 18 lines which said: > There is the following recommendation in Section 4: "When returning > a response containing stale records, a recursive resolver MUST set > the TTL of each expired record in the message to a value greater > than 0, with 30 seconds RECOMMENDED". Where does the "30 seconds" > come from? It comes from the fact that it is the upper recommended value between retrials by the resolver (section 5). So, a downstream client has no reason to try again before 30 seconds. The reasons of this choice (why 30) are in section 6: "Other very short TTLs could lead to congestive collapse as TTL-respecting clients rapidly try to refresh. The recommended value of 30 seconds not only sidesteps those potential problems with no practical negative consequences, it also rate limits further queries from any client that honors the TTL, such as a forwarding resolver."
