Client Hint Reliability
draft-davidben-http-client-hint-reliability-02

Document Type Active Internet-Draft (individual)
Author David Benjamin 
Last updated 2020-11-30
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HTTP                                                         D. Benjamin
Internet-Draft                                                Google LLC
Updates: ietf-httpbis-client-hints (if approved)        30 November 2020
Intended status: Experimental                                           
Expires: 3 June 2021

                        Client Hint Reliability
             draft-davidben-http-client-hint-reliability-02

Abstract

   This document defines the Critical-CH HTTP response header, and the
   ACCEPT_CH HTTP/2 and HTTP/3 frames to allow HTTP servers to reliably
   specify their Client Hint preferences, with minimal performance
   overhead.

Discussion Venues

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

   Source for this draft and an issue tracker can be found at
   https://github.com/davidben/http-client-hint-reliability.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 3 June 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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
   and restrictions with respect to this document.  Code Components
   extracted from this document must 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  The Critical-CH Response Header Field . . . . . . . . . . . .   3
     3.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  The ACCEPT_CH Frame . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  HTTP/2 ACCEPT_CH Frame  . . . . . . . . . . . . . . . . .   5
     4.2.  HTTP/3 ACCEPT_CH Frame  . . . . . . . . . . . . . . . . .   6
     4.3.  Processing ACCEPT_CH Frames . . . . . . . . . . . . . . .   7
     4.4.  Interaction with Critical-CH  . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

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]
       and HTTP/3 [I-D.ietf-quic-http], which can avoid the performance
       hit of a retry in most cases

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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
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