RFC 7540, "Hypertext Transfer Protocol Version 2 (HTTP/2)", May 2015Source of RFC: httpbis (app)
Errata ID: 4666
Status: Held for Document Update
Reported By: Vasiliy Faronov
Date Reported: 2016-04-13
Held for Document Update by: Alexey Melnikov
Date Held: 2017-02-23
Section 9.1.2 says:
A 421 response is cacheable by default, i.e., unless otherwise indicated by the method definition or explicit cache controls (see Section 4.2.2 of [RFC7234]).
It should say:
The HTTP cache key (RFC 7234 Section 2) is based on the request URI, not on properties of the connection. Therefore, if a client were to cache a 421 response, it would then use this cached 421 to satisfy further requests to the same URI, before it has a chance to connect to an authoritative server.
Mark Nottingham: As discussed on list, I think the best we can do here is to note that in many cases, it'd be desireable to mark this as explicitly uncacheable.
With this paragraph removed, a 421 response is not cacheable by default, per RFC 7231 Section 6.1.