HTTP                                                         D. Benjamin
Internet-Draft                                                Google LLC
Updates: ietf-httpbis-client-hints (if                     July 13, 2020
Intended status: Experimental
Expires: January 14, 2021

                        Client Hint Reliability


   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

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   This Internet-Draft will expire on January 14, 2021.

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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.  Client Behavior . . . . . . . . . . . . . . . . . . . . .   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  . . . . . . . . . . . . . . . . .   9
   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 clients 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 client 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 client 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

2.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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

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3.  The Critical-CH Response Header Field

   When a client requests a resource based on a missing or outdated
   Accept-CH value, it may not send a desired request header field.
   Neither client nor server has enough information to reliably and
   efficiently recover from this situation.  The server can observe that
   the header is missing, but the client 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-

   Conversely, the client 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
   clients.  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:

     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.

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   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

   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

   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 with no
   knowledge of the server's Accept-CH preferences, it may send the
   following response:

     GET / HTTP/1.1

     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

   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.

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     GET / HTTP/1.1
     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 HTTP/2 Frame

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

   [[TODO: Alternatively, is it time to revive draft-bishop-httpbis-

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

   |         Origin-Len (16)       |
   |         Origin                                              ...
   |         Accept-CH-Len (16)    |
   |         Accept-CH-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.

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

   Accept-CH-Value:  A sequence of characters containing the Accept-CH
      value for the corresponding origin.  This value MUST satisfy the

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      Accept-CH ABNF defined in Section 3.1 of

   HTTP/2 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 client when it makes the first

   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
   client receives an ACCEPT_CH frame whose stream identifier or flags
   field is non-zero, it MUST respond with a connection error of type

4.1.  Client Behavior

   The client remembers the most recently received ACCEPT_CH frame for
   each 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 client MAY bound the size and ignore or forget
   entries to reduce resource usage.

   When the client makes an HTTP request to a particular origin over an
   HTTP/2 connection, it looks up the origin in the remembered
   ACCEPT_CH, if any.  If it finds a match, it treats the union of that
   value and any saved Accept-CH response header as the server's Client
   Hint preferences.  It then proceeds as in Section 2.1 of

   Clients 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:

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   o  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.

   o  Should the client 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.

   o  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).

   o  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

4.2.  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

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

   o  The server's Accept-CH preferences may change while clients have
      existing connections open.  In this case, the connection-level
      settings may be out of date.  While the server could send a new
      ACCEPT_CH frame, the frame may not arrive in time for the client's
      next request.  Moreover, if the HTTP serving frontend is an
      intermediary like a CDN, it may not be proactively notified of
      origin server changes.

   o  HTTP/2 allows connection reuse across multiple origins
      (Section 9.1.1 of [RFC7540]).  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.

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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

   The ACCEPT_CH frame does introduce a new way for HTTP/2 connections
   to make assertions about origins they are not authoritative for, but
   the procedure in Section 4.1 defers processing until after the client
   has decided to use the connection for a particular request
   (Section 9.1.1 of [RFC7540]).

6.  IANA Considerations

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

   o  Frame Type: ACCEPT_CH

   o  Code: TBD

   o  Allowed in ALPS: Yes

   o  Reference: [[this document]]

   [[TODO: As of writing, the HTTP/2 Frame Type registry does not
   include an Allowed in ALPS column.  [I-D.vvv-httpbis-alps], as of
   writing, will add it.  This document should be updated as that design

7.  References

7.1.  Normative References

              Grigorik, I. and Y. Weiss, "HTTP Client Hints", draft-
              ietf-httpbis-client-hints-15 (work in progress), July

              Nottingham, M. and P. Kamp, "Structured Field Values for
              HTTP", draft-ietf-httpbis-header-structure-19 (work in
              progress), June 2020.

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              Vasiliev, V., "Using TLS Application-Layer Protocol
              Settings (ALPS) in HTTP", draft-vvv-httpbis-alps-00 (work
              in progress), July 2020.

              Vasiliev, V., "TLS Application-Layer Protocol Settings
              Extension", draft-vvv-tls-alps-00 (work in progress), June

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,

   [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,

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

7.2.  Informative References

              Panicker, S., "Device Memory", n.d.,

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

Author's Address

   David Benjamin
   Google LLC


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