The Purpose Declined HTTP Status Code

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Table of Contents

1. Introduction

[FETCH] introduces a mechanism whereby HTTP [HTTP] user agents can speculatively request a representation of a resource, in order to improve perceived performance.¶

In some circumstances, a server might have information that leads it to believe that sending a full response will not improve performance, and could have negative impacts.¶

When this happens, it is common practice to use a 503 (Service Unavailable) status code. However, this has been observed to cause confusion: a server operator who sees a spike in that status code being sent tends to draw the conclusion that there is a server-side operational issue.¶

While other status codes (e.g., 403 (Forbidden)) could be used, they can also suffer (to varying degrees) from the same problem: being confused with an error, operational problem, or other condition.¶

This specification defines a new status code to specifically address this situation, making servers' behavior more legible to their operators. It does not introduce any new capability.¶

2. The 419 (Purpose Declined) Status Code

The 419 (Purpose Declined) status code indicates that the server is refusing a request based upon its declared purpose.¶

A request's declared purpose is indicted by its Sec-Purpose header field [FETCH].¶

This indication is only applicable to the associated request; future requests with the same purpose might or might not succeed.¶

Because responses with this status code are not intended to be displayed to a user, they SHOULD have zero-length content, and any content that is sent SHOULD be discarded.¶

Both origin servers and gateways acting on their behalf (e.g., Content Delivery Networks and so-called "reverse proxies") MAY generate this status code. Proxies (who are not acting on behalf of the origin server) SHOULD NOT generate it.¶

This status code is not heuristically cacheable (see Section 15.1 of [HTTP]). To avoid responses with this status code being reused by a cache, they SHOULD NOT be cacheable (see Section 3 of [HTTP-CACHING]).¶

3. IANA Considerations

The following entry should be registered in the "HTTP Status Codes" registry:¶

• Code: 419¶

• Description: Purpose Declined¶

• Specification: RFC nnnn (this document)¶

4. Security Considerations

The security considerations of [HTTP] and [FETCH] apply. Conceivably, the use of this status code could leak information about the internal state of the server; caution should be exercised to assure that it does not.¶

5. Normative References

[FETCH]

WHAT Working Group, "Fetch", n.d., <https://fetch.spec.whatwg.org/>.

[HTTP]

Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, June 2022, <https://www.rfc-editor.org/rfc/rfc9110>.

[HTTP-CACHING]

Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Caching", STD 98, RFC 9111, DOI 10.17487/RFC9111, June 2022, <https://www.rfc-editor.org/rfc/rfc9111>.

[RFC2119]

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/rfc/rfc2119>.

[RFC8174]

Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

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