The Preliminary Request Denied HTTP Status Code
draft-ietf-httpbis-pre-denied-00

Network Working Group                                      M. Nottingham
Internet-Draft                                                Cloudflare
Intended status: Standards Track                            3 April 2026
Expires: 5 October 2026

            The Preliminary Request Denied HTTP Status Code
                    draft-ietf-httpbis-pre-denied-00

Abstract

   This specification defines a HTTP status code to indicate that the
   server is denying a prefetch or preload request.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at https://httpwg.org/
   http-extensions/draft-ietf-httpbis-pre-denied.html.  Status
   information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-httpbis-pre-denied/.

   Discussion of this document takes place on the HTTP Working Group
   mailing list (mailto:ietf-http-wg@w3.org), which is archived at
   https://lists.w3.org/Archives/Public/ietf-http-wg/.  Working Group
   information can be found at https://httpwg.org/.

   Source for this draft and an issue tracker can be found at
   https://github.com/httpwg/http-extensions/labels/pre-denied.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 5 October 2026.

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Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  The 4xx (Preliminary Request Denied) Status Code  . . . . . .   3
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   5.  Normative References  . . . . . . . . . . . . . . . . . . . .   3
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   4

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 shown 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.

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1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  The 4xx (Preliminary Request Denied) Status Code

   The 4xx (Preliminary Request Denied) status code indicates that the
   server is refusing a preliminary request.

   A preliminary request is one that contains a Sec-Purpose header field
   [FETCH] containing the value "prefetch".

   This indication is only applicable to the associated request; future
   preliminary requests might or might not succeed.

3.  IANA Considerations

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

   *  Code: 4xx

   *  Description: Preliminary Request Denied

   *  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", 2026,
              <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>.

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   [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>.

Author's Address

   Mark Nottingham
   Cloudflare
   Melbourne
   Australia
   Email: mnot@mnot.net
   URI:   https://www.mnot.net/

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