Client Hint Reliability
draft-davidben-http-client-hint-reliability-01

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HTTP                                                         D. Benjamin
Internet-Draft                                                Google LLC
Updates: ietf-httpbis-client-hints (if                   August 27, 2020
         approved)
Intended status: Experimental
Expires: February 28, 2021

                        Client Hint Reliability
             draft-davidben-http-client-hint-reliability-01

Abstract

   This document defines the Critical-CH HTTP response header, and the
   ACCEPT_CH HTTP/2 frame to allow HTTP servers to reliably specify
   their Client Hint preferences, with minimal performance overhead.

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 February 28, 2021.

Copyright Notice

   Copyright (c) 2020 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
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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   2
   3.  The Critical-CH Response Header Field . . . . . . . . . . . .   3
     3.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  The ACCEPT_CH HTTP/2 Frame  . . . . . . . . . . . . . . . . .   5
     4.1.  Processing ACCEPT_CH Frames . . . . . . . . . . . . . . .   6
     4.2.  Interaction with Critical-CH  . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   [I-D.ietf-httpbis-client-hints] defines a response header, Accept-CH,
   for servers to advertise a set of request headers used for proactive
   content negotiation.  This allows user agents to send request headers
   only when used, improving their performance overhead as well as
   reducing passive fingerprinting surface.

   However, on the first HTTP request to a server, the user agent will
   not have received the Accept-CH header and may not take the server
   preferences into account.  More generally, the server's configuration
   may have changed since the most recent HTTP request to the server.
   This document defines a pair of mechanisms to resolve this:

   1.  an HTTP response header, Critical-CH, for the server to instruct
       the user agent to retry the request

   2.  an alternate delivery mechanism for Accept-CH in HTTP/2
       [RFC7540], which can avoid the performance hit of a retry in most
       cases

2.  Conventions and Definitions

   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.

   This document uses the Augmented Backus-Naur Form (ABNF) notation of
   [RFC5234].

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3.  The Critical-CH Response Header Field

   When a user agent requests a resource based on a missing or outdated
   Accept-CH value, it may not send a desired request header field.
   Neither user agent nor server has enough information to reliably and