User Agent Client Hints
draft-west-ua-client-hints-00

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HTTP                                                             M. West
Internet-Draft                                                    Google
Intended status: Standards Track                       November 29, 2018
Expires: June 2, 2019

                        User Agent Client Hints
                     draft-west-ua-client-hints-00

Abstract

   This document defines a set of Client Hints that aim to provide
   developers with the ability to perform agent-based content
   negotiation when necessary, while avoiding the historical baggage and
   passive fingerprinting surface exposed by the venerable "User-Agent"
   header.

Status of This Memo

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   This Internet-Draft will expire on June 2, 2019.

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   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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West                      Expires June 2, 2019                  [Page 1]
Internet-Draft           User Agent Client Hints           November 2018

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
   2.  User Agent Hints  . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  The 'Sec-CH-Arch' Header Field  . . . . . . . . . . . . .   5
     2.2.  The 'Sec-CH-Model' Header Field . . . . . . . . . . . . .   5
     2.3.  The 'Sec-CH-Platform' Header Field  . . . . . . . . . . .   6
     2.4.  The 'Sec-CH-UA' Header Field  . . . . . . . . . . . . . .   6
     2.5.  Integration with Fetch  . . . . . . . . . . . . . . . . .   7
   3.  Security and Privacy Considerations . . . . . . . . . . . . .   8
     3.1.  Secure Transport  . . . . . . . . . . . . . . . . . . . .   8
     3.2.  Delegation  . . . . . . . . . . . . . . . . . . . . . . .   8
     3.3.  Access Restrictions . . . . . . . . . . . . . . . . . . .   9
   4.  Implementation Considerations . . . . . . . . . . . . . . . .   9
     4.1.  The 'User-Agent' Header . . . . . . . . . . . . . . . . .   9
     4.2.  GREASE-like UA Strings  . . . . . . . . . . . . . . . . .   9
     4.3.  The 'Sec-CH-' prefix  . . . . . . . . . . . . . . . . . .  10
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  'Sec-CH-Arch' Header Field  . . . . . . . . . . . . . . .  10
     5.2.  'Sec-CH-Model' Header Field . . . . . . . . . . . . . . .  10
     5.3.  'Sec-CH-Platform' Header Field  . . . . . . . . . . . . .  11
     5.4.  'Sec-CH-UA' Header Field  . . . . . . . . . . . . . . . .  11
     5.5.  'User-Agent' Header Field . . . . . . . . . . . . . . . .  11
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Changes  . . . . . . . . . . . . . . . . . . . . . .  13
     A.1.  draft-west-ua-client-hints-00 . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Today, user agents generally identify themselves to servers by
   sending a "User-Agent" HTTP request header field along with each
   request (defined in Section 5.5.3 of [RFC7231]).  Ideally, this
   header would give servers the ability to perform content negotiation,
   sending down exactly those bits that best represent the requested
   resource in a given user agent, optimizing both bandwidth and user
   experience.  In practice, however, this header's value exposes far
   more information about the user's device than seems appropriate as a
   default, on the one hand, and intentionally obscures the true user
   agent in order to bypass misguided server-side heuristics, on the
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