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

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
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   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
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   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 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
   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].

   This document uses the variable-length integer encoding and frame
   diagram format from [I-D.ietf-quic-transport].

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
   efficiently recover from this situation.  The server can observe that
   the header is missing, but the user agent may not have supported the
   header, or may have chosen not to send it.  Triggering a new request
   in these cases would risk an infinite loop or an unnecessary round-
   trip.

   Conversely, the user agent can observe that a request header appears
   in the Accept-CH (Section 3.1 of [I-D.ietf-httpbis-client-hints]) and
   Vary (Section 7.1.4 of [RFC7231]) response header fields.  However,
   retrying based on this information would waste resources if the
   resource only used the Client Hint as an optional optimization.

   This document introduces critical Client Hints.  These are the Client
   Hints which meaningfully change the resulting resource.  For example,
   a server may use the Device-Memory Client Hint [DEVICE-MEMORY] to
   select simple and complex variants of a resource to different user
   agents.  Such a resource should be fetched consistently across page
   loads to avoid jarring user-visible switches.

   The server specifies critical Client Hints with the Critical-CH
   response header field.  It is a Structured Header
   [I-D.ietf-httpbis-header-structure] whose value MUST be an sf-list
   (Section 3.1 of [I-D.ietf-httpbis-header-structure]) whose members
   are tokens (Section 3.3.4 of [I-D.ietf-httpbis-header-structure]).
   Its ABNF is:

     Critical-CH = sf-list

   For example:

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     Critical-CH: Sec-CH-Example, Sec-CH-Example-2

   Each token listed in the Critical-CH header SHOULD additionally be
   present in the Accept-CH and Vary response headers.

   When a user agent receives an HTTP response containing a Critical-CH
   header, it first processes the Accept-CH header as described in
   Section 3.1 of [I-D.ietf-httpbis-client-hints].  It then performs the
   following steps:

   1.  If the request did not use a safe method (Section 4.2.1 of
       [RFC7231]), ignore the Critical-CH header and continue processing
       the response as usual.

   2.  If the response was already the result of a retry, ignore the
       Critical-CH header and continue processing the response as usual.

   3.  Determine the Client Hints that would have been sent given the
       updated Accept-CH value, incorporating the user agent's local
       policy and user preferences.  See also Section 2.1 of
       [I-D.ietf-httpbis-client-hints].

   4.  Compare this result to the Client Hints which were sent.  If any
       Client Hint listed in the Critical-CH header was not previously
       sent and would now have been sent, retry the request with the new
       preferences.  Otherwise, continue processing the response as
       usual.

   Note this procedure does not cause the user agent to send Client
   Hints it would not otherwise send.

3.1.  Example

   For example, if the user agent loads https://example.com with no
   knowledge of the server's Accept-CH preferences, it may send the
   following response:

     GET / HTTP/1.1
     Host: example.com

     HTTP/1.1 200 OK
     Content-Type: text/html
     Accept-CH: Sec-CH-Example, Sec-CH-Example-2
     Vary: Sec-CH-Example
     Critical-CH: Sec-CH-Example

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   In this example, the server, across the whole origin, uses both Sec-
   CH-Example and Sec-CH-Example-2 Client Hints.  However, this resource
   only uses Sec-CH-Example, which it considers critical.

   The user agent now processes the Accept-CH header and determines it
   would have sent both headers.  Sec-CH-Example is listed in Critical-
   CH, so the user agent retries the request, and receives a more
   specific response.

     GET / HTTP/1.1
     Host: example.com
     Sec-CH-Example: 1
     Sec-CH-Example-2: 2

     HTTP/1.1 200 OK
     Content-Type: text/html
     Accept-CH: Sec-CH-Example, Sec-CH-Example-2
     Vary: Sec-CH-Example
     Critical-CH: Sec-CH-Example

4.  The ACCEPT_CH Frame

   While Critical-CH header provides reliability, it requires a retry on
   some requests.  This document additionally introduces the ACCEPT_CH
   HTTP/2 and HTTP/3 frames as an optimization so the server's Client
   Hint preferences are usually available before the client's first
   request.

   HTTP/2 and HTTP/3 servers which request Client Hints SHOULD send an
   ACCEPT_CH frame as early as possible.  Connections using TLS
   [RFC8446] which negotiate the Application Layer Protocol Settings
   (ALPS) [I-D.vvv-tls-alps] extension SHOULD include the ACCEPT_CH
   frame in the ALPS value as described in [I-D.vvv-httpbis-alps].  This
   ensures the information is available to the user agent when it makes
   the first request.

   [[TODO: Alternatively, is it time to revive draft-bishop-httpbis-
   extended-settings?]]

4.1.  HTTP/2 ACCEPT_CH Frame

   The HTTP/2 ACCEPT_CH frame type is TBD (decimal TBD) and contains
   zero or more entries, each consisting of a pair of length-delimited
   strings:

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   +-------------------------------+
   |         Origin-Len (16)       |
   +-------------------------------+-------------------------------+
   |         Origin                                              ...
   +-------------------------------+-------------------------------+
   |         Value-Len (16)        |
   +-------------------------------+-------------------------------+
   |         Value                                               ...
   +---------------------------------------------------------------+

   The fields are defined as follows:

   Origin-Len:  An unsigned, 16-bit integer indicating the length, in
      octets, of the Origin field.

   Origin:  A sequence of characters containing the ASCII serialization
      of an origin (Section 6.2 of [RFC6454]) that the sender is
      providing an Accept-CH value for.

   Value-Len:  An unsigned, 16-bit integer indicating the length, in
      octets, of the Value field.

   Value:  A sequence of characters containing the Accept-CH value for
      the corresponding origin.  This value MUST satisfy the Accept-CH
      ABNF defined in Section 3.1 of [I-D.ietf-httpbis-client-hints].

   Clients MUST NOT send ACCEPT_CH frames.  Servers which receive an
   ACCEPT_CH frame MUST respond with a connection error (Section 5.4.1
   of [RFC7540]) of type PROTOCOL_ERROR.

   ACCEPT_CH frames always apply to a single connection, never a single
   stream.  The identifier in the ACCEPT_CH frame MUST be zero.  The
   flags field of an ACCEPT_CH field is unused and MUST be zero.  If a
   user agent receives an ACCEPT_CH frame whose stream identifier or
   flags field is non-zero, it MUST respond with a connection error of
   type PROTOCOL_ERROR.

4.2.  HTTP/3 ACCEPT_CH Frame

   The HTTP/3 ACCEPT_CH frame type is TBD (decimal TBD) and contains
   zero or more entries, each containing an origin and a corresponding
   Accept-CH value.

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   HTTP/3 ACCEPT_CH Entry {
     Origin Length (i),
     Origin (..)
     Value Length (i),
     Value (..),
   }

   HTTP/3 ACCEPT_CH Frame {
     Type (i) = TBD,
     Length (i),
     HTTP/3 ACCEPT_CH Entry (..) ...,
   }

   The fields of each HTTP/3 ACCEPT_CH Entry are defined as follows:

   Origin Length:  A variable-length integer containing the length, in
      bytes, of the Origin field.

   Origin:  A sequence of characters containing the ASCII serialization
      of an origin (Section 6.2 of [RFC6454]) that the sender is
      providing an Accept-CH value for.

   Value Length:  A variable-length integer containing the length, in
      bytes, of the Value field.

   Value:  A sequence of characters containing the Accept-CH value for
      the corresponding origin.  This value MUST satisfy the Accept-CH
      ABNF defined in Section 3.1 of [I-D.ietf-httpbis-client-hints].

   Clients MUST NOT send ACCEPT_CH frames.  Servers which receive an
   ACCEPT_CH frame MUST respond with a connection error (Section 8 of
   [I-D.ietf-quic-http]) of type H3_FRAME_UNEXPECTED.

   ACCEPT_CH frames may only be sent on the control stream.  Clients
   which receive an ACCEPT_CH frame on any other stream MUST respond
   with a connection error of type H3_FRAME_UNEXPECTED.

4.3.  Processing ACCEPT_CH Frames

   The user agent remembers the most recently received ACCEPT_CH frame
   for each HTTP/2 or HTTP/3 connection.  When it receives a new
   ACCEPT_CH frame, either in application data or ALPS, it overwrites
   this value.  As this is an optimization, the user agent MAY bound the
   size and ignore or forget entries to reduce resource usage.

   When the user agent makes an HTTP request to a particular origin over
   an HTTP/2 or HTTP/3 connection, it looks up the origin in the
   remembered ACCEPT_CH, if any.  If it finds a match, it determines

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   additional Client Hints to send, incorporating its local policy and
   user preferences.  See Section 2.1 of
   [I-D.ietf-httpbis-client-hints].

   If there are additional Client Hints, the user agent restarts the
   request with updated headers.  The connection has already been
   established, so this restart does not incur any additional network
   latency.  Note it may result in a different secondary HTTP cache key
   (see Section 4.1 of [RFC7234]) and select a different cached
   response.  If the new cached response does not need revalidation, it
   may not use the connection at all.

   User agents MUST NOT process Client Hint preferences in ACCEPT_CH
   frames corresponding to origins for which the connection is not
   authoritative.  Note the procedure above implicitly satisfies this by
   deferring processing to after the connection has been chosen for a
   corresponding request.  Unauthoritative origins and other unmatched
   entries are ignored.

   [[TODO: Some variations on this behavior we could choose instead:

   *  Do new ACCEPT_CH frames override the whole set or implement some
      kind of update?  Overriding the whole set seems simplest and most
      consistent with an EXTENDED_SETTINGS variant.

   *  Should the user agent reject the ACCEPT_CH frame if there are
      unexpected origins in there?  Deferring avoids needing to worry
      about this, and ignoring the unused ones may interact better with
      secondary certs.

   *  Should ACCEPT_CH frames be deferred or just written to the cache
      when received?  Deferred simplifies reasoning about bad origins,
      predictive connections, etc., but means interactions between
      ACCEPT_CH and Accept-CH are more complex (see below).

   *  How should ACCEPT_CH and Accept-CH interact?  The document
      currently proposes unioning them, which is easy.  Accept-CH first
      would work, but unnecessarily ignore newer connection-level
      ACCEPT_CHs.  ACCEPT_CH would not; a stale connection-level
      preference would get stuck.  Whichever is received earlier would
      also work, but requires tracking timestamps if deferred (see
      above).]]

4.4.  Interaction with Critical-CH

   The ACCEPT_CH frame avoids a round-trip, so relying on it over
   Critical-CH would be preferable.  However, this is not always
   possible:

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   *  The server may be running older software without support for
      ACCEPT_CH or ALPS.

   *  The server's Accept-CH preferences may change while existing
      connections are open.  Those connections will have outdated
      ACCEPT_CH frames.  While the server could send a new frame, it may
      not arrive in time for the next request.  Moreover, if the HTTP
      serving frontend is an intermediary like a CDN, it may not be
      proactively notified of origin server changes.

   *  HTTP/2 and HTTP/3 allow connection reuse across multiple origins
      (Section 9.1.1 of [RFC7540] and Section 3.4 of
      [I-D.ietf-quic-http]).  Some origins may not be listed in the
      ACCEPT_CH frame, particularly if the server used a wildcard X.509
      certificate.

   Thus this document defines both mechanisms.  Critical-CH provides
   reliable Client Hint delivery, while the ACCEPT_CH frame avoids the
   retry in most cases.

5.  Security Considerations

   Request header fields may expose sensitive information about the
   user's environment.  Section 4.1 of [I-D.ietf-httpbis-client-hints]
   discusses some of these considerations.  The document augments the
   capabilities of Client Hints, but does not change these
   considerations.  The procedure described in Section 3 does not result
   in the user agent sending request headers it otherwise would not
   have.

   The ACCEPT_CH frame does introduce a new way for HTTP/2 or HTTP/3
   connections to make assertions about origins they are not
   authoritative for, but the procedure in Section 4.3 defers processing
   until after the user agent has decided to use the connection for a
   particular request (Section 9.1.1 of [RFC7540] and Section 3.4 of
   [I-D.ietf-quic-http]).  The user agent will thus only use information
   from an ACCEPT_CH frame if it considers the connection authoritative
   for the origin.

6.  IANA Considerations

   This specification adds an entry to the "HTTP/2 Frame Type" registry
   [RFC7540] with the following parameters:

   *  Frame Type: ACCEPT_CH

   *  Code: TBD

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   *  Allowed in ALPS: Yes

   *  Reference: [[this document]]

   This specification adds an entry to the "HTTP/3 Frame Type" registry
   [I-D.ietf-quic-http] with the following parameters:

   *  Frame Type: ACCEPT_CH

   *  Code: TBD

   *  Allowed in ALPS: Yes

   *  Reference: [[this document]]

   [[TODO: As of writing, the Frame Type registries do not include
   Allowed in ALPS columns, but [I-D.vvv-httpbis-alps] adds them.  This
   document should be updated as that design evolves.]]

7.  References

7.1.  Normative References

   [I-D.ietf-httpbis-client-hints]
              Grigorik, I. and Y. Weiss, "HTTP Client Hints", Work in
              Progress, Internet-Draft, draft-ietf-httpbis-client-hints-
              15, 3 July 2020, <http://www.ietf.org/internet-drafts/
              draft-ietf-httpbis-client-hints-15.txt>.

   [I-D.ietf-httpbis-header-structure]
              Nottingham, M. and P. Kamp, "Structured Field Values for
              HTTP", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-header-structure-19, 3 June 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-httpbis-
              header-structure-19.txt>.

   [I-D.ietf-quic-http]
              Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-32, 20 October 2020, <http://www.ietf.org/
              internet-drafts/draft-ietf-quic-http-32.txt>.

   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", Work in Progress, Internet-Draft,
              draft-ietf-quic-transport-32, 20 October 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-quic-
              transport-32.txt>.

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   [I-D.vvv-httpbis-alps]
              Vasiliev, V., "Using TLS Application-Layer Protocol
              Settings (ALPS) in HTTP", Work in Progress, Internet-
              Draft, draft-vvv-httpbis-alps-00, 6 July 2020,
              <http://www.ietf.org/internet-drafts/draft-vvv-httpbis-
              alps-00.txt>.

   [I-D.vvv-tls-alps]
              Benjamin, D. and V. Vasiliev, "TLS Application-Layer
              Protocol Settings Extension", Work in Progress, Internet-
              Draft, draft-vvv-tls-alps-01, 21 September 2020,
              <http://www.ietf.org/internet-drafts/draft-vvv-tls-alps-
              01.txt>.

   [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/info/rfc2119>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <https://www.rfc-editor.org/info/rfc6454>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <https://www.rfc-editor.org/info/rfc7234>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [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/info/rfc8174>.

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   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

7.2.  Informative References

   [DEVICE-MEMORY]
              Panicker, S., "Device Memory", n.d.,
              <https://w3c.github.io/device-memory/>.

Acknowledgments

   This document has benefited from contributions and suggestions from
   Ilya Grigorik, Nick Harper, Matt Menke, Aaron Tagliaboschi, Victor
   Vasiliev, Yoav Weiss, and others.

Author's Address

   David Benjamin
   Google LLC

   Email: davidben@google.com

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