Network Working Group                                      M. Nottingham
Internet-Draft                                                    Fastly
Updates: 7234 (if approved)                             October 30, 2017                            February 13, 2018
Intended status: Standards Track
Expires: May 3, August 17, 2018

                      HTTP Representation Variants
                      draft-nottingham-variants-01
                      draft-nottingham-variants-02

Abstract

   This specification introduces the HTTP "Variants" response header
   field an alternative way to communicate what representations are available a
   secondary cache key for a given HTTP resource, and using the companion HTTP "Variants"
   and "Variant-Key" response header field to
   indicate which representation was selected.  It fields.  Its aim is an augmentation of
   the "Vary" mechanism in to make HTTP caching.
   proactive content negotiation more cache-friendly.

Note to Readers

   _RFC EDITOR: please remove this section before publication_

   The issues list for this draft can be found at
   https://github.com/mnot/I-D/labels/variants [1].

   The most recent (often, unpublished) draft is at
   https://mnot.github.io/I-D/variants/ [2].

   Recent changes are listed at https://github.com/mnot/I-D/commits/gh-
   pages/variants [3].

   See also the draft's current status in the IETF datatracker, at
   https://datatracker.ietf.org/doc/draft-nottingham-variants/ [4].

   There is a prototype implementation of the algorithms herein at
   https://github.com/mnot/variants-toy [5].

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on May 3, August 17, 2018.

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   Copyright (c) 2017 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
   2.  The "Variants" HTTP Header Field  . . . . . . . . . . . . . .   4   5
     2.1.  Relationship to Vary  . . . . . . . . . . . . . . . . . .   6
   3.  The "Variant-Key" HTTP Header Field . . . . . . . . . . . . .   6
   4.  Defining Content Negotiation Using Variants   7
     3.1.  Generating a Normalised Variant-Key . . . . . . . . . . .   7
   5.
   4.  Cache Behaviour . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.   8
     4.1.  Find Available Keys . . . . . . . . . . . . . . . . . . .   9
     5.2.
     4.2.  Check Vary  . . . . . . . . . . . . . . . . . . . . . . .  10
     4.3.  Example of Cache Behaviour  . . . . . . . . . . . . . . .   9
   6.  Example Headers  10
   5.  Origin Server Behaviour . . . . . . . . . . . . . . . . . . .  11
     5.1.  Examples  . . . .  10
     6.1.  Single Variant . . . . . . . . . . . . . . . . . . . .  12
       5.1.1.  Single Variant  .  10
     6.2.  Multiple Variants . . . . . . . . . . . . . . . . . .  12
       5.1.2.  Multiple Variants . .  11
     6.3. . . . . . . . . . . . . . . . .  13
       5.1.3.  Partial Coverage  . . . . . . . . . . . . . . . . . .  13
   6.  Defining Content Negotiation Using Variants . . .  11 . . . . . .  14
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12  14
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  13  15
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  13  15
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13  15
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  13  15
     10.2.  Informative References . . . . . . . . . . . . . . . . .  14  16
     10.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  14  16
   Appendix A.  Variants for Existing Content Negotiation Mechanisms  14  17
     A.1.  Accept-Encoding  Accept  . . . . . . . . . . . . . . . . . . . . .  14 . . . .  17
     A.2.  Accept-Encoding . . . . . . . . . . . . . . . . . . . . .  17
     A.3.  Accept-Language . . . . . . . . . . . . . . . . . . . . .  15  18
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15  19

1.  Introduction

   HTTP proactive content negotiation ([RFC7231], Section 3.4.1) is
   starting to be used more widely again.  The most widely seen use -
   determining a response's content-coding - is being joined by
   seeing renewed
   interest in negotiation interest, both for language existing request headers like
   Content-Language and other, for newer attributes ones (for example, see
   [I-D.ietf-httpbis-client-hints]).

   Successfully reusing negotiated responses that have been stored in a
   HTTP cache requires establishment of a secondary cache key
   ([RFC7234], Section 4.1) using 4.1).  Currently, the Vary header ([RFC7231],
   Section 7.1.4), which identifies the request headers that form the
   secondary cache key for 7.1.4) does this by nominating a given response. set of request headers.

   HTTP's caching model allows a certain amount of latitude in
   normalising those request header fields identified by Vary to match those
   stored in the cache, field values, so as to increase the
   chances of a cache hit while still respecting the semantics of that
   header.  However, this normalisation is often inadequate; even with understanding of not formally defined, leading to
   divergence in cache behaviours.

   Even when the headers' semantics to facilitate such normalisation, are understood, a cache does not
   know enough about the possible alternative representations available
   on the origin server to make an appropriate decision.

   For example, if a cache has stored the following request/response
   pair:

   GET /foo HTTP/1.1
   Host: www.example.com
   Accept-Language: en;q=1.0, fr;q=0.5

   HTTP/1.1 200 OK
   Content-Type: text/html
   Content-Language: fr
   Vary: Accept-Language
   Transfer-Encoding: chunked

   [French content]

   Provided that the cache has full knowledge of the semantics of
   Accept-Language and Content-Language, it will know that a French
   representation is available and might be able to infer that an
   English representation is not available.  But, it does not know (for
   example) whether a Japanese representation is available without
   making another request, thereby incurring possibly unnecessary latency.

   This specification introduces the HTTP Variants response header field
   (Section 2) to enumerate the available variant representations on the
   origin server, to provide clients and caches with enough information
   to properly satisfy requests - either by selecting a response from
   cache or by forwarding the request towards the origin - by following
   an
   the algorithm defined in Section 5. 4.

   Its companion the Variant-Key response header field (Section 3)
   indicates which representation was selected, so that it can be
   reliably reused in the future.

   This mechanism requires that proactive  When this specification is in use,
   the example above might become:

   GET /foo HTTP/1.1
   Host: www.example.com
   Accept-Language: en;q=1.0, fr;q=0.5

   HTTP/1.1 200 OK
   Content-Type: text/html
   Content-Language: fr
   Vary: Accept-Language
   Variants: Accept-Language;fr;de;en;jp
   Variant-Key: fr
   Transfer-Encoding: chunked

   [French content]

   Proactive content negotiation mechanisms that wish to be used with
   Variants need to define how they use it; to do so explicitly; see Section 4.  It 6.  As a
   result, it is best suited for negotiation over request headers that
   are well-understood.  It

   Variants also works best when content negotiation takes place over a
   constrained set of representations; since each variant needs to be
   listed in the header field, it is ill-suited for open-ended sets of
   representations.

   It

   Variants can be seen as a simpler version of the Alternates header
   field introduced by [RFC2295]; unlike that mechanism, Variants does
   not require specification of each combination of attributes, and does
   not assume that each combination has a unique URL.

1.1.  Notational Conventions

   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 specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234] with a list extension, defined in Section 7 of
   [RFC7230], that allows for compact definition of comma-separated
   lists using a '#' operator (similar to how the '*' operator indicates
   repetition).

   Additionally, it uses the "field-name", "OWS" and "token" rules from
   [RFC7230].

2.  The "Variants" HTTP Header Field

   The Variants HTTP response header field indicates what
   representations are available for a given resource at the time that
   the response is produced, by enumerating the request header fields
   that it varies on, along with the values that are available for each.

   Variants        = 1#variant-item
   variant-item    = field-name *( OWS ";" OWS available-value )
   available-value = token

   Each "variant-item" indicates a request header field that carries a
   value that clients might proactively negotiate for; each parameter on
   it indicates a value for which there is an available representation
   on the origin server.

   So, given this example header field:

   Variants: Accept-Encoding;gzip

   a recipient can infer that the only content-coding available for that
   resource is "gzip" (along with the "identity" non-encoding; see
   Appendix A.1). A.2).

   Given:

   Variants: accept-encoding

   a recipient can infer that no content-codings (beyond identity) are
   supported.  Note that as always, field-name is case-insensitive.

   A more complex example:

   Variants: Accept-Encoding;gzip;br, Accept-Language;en ;fr

   Here, recipients can infer that two content-codings in addition to
   "identity" are available, as well as two content languages.  Note
   that, as with all HTTP header fields that use the "#" list rule (see

   [RFC7230], Section 7), they might occur in the same header field or
   separately, like this:

   Variants: Accept-Encoding;gzip;brotli
   Variants: Accept-Language;en ;fr

   The ordering of available-values after the field-name is significant,
   as it might be used by the header's algorithm for selecting a
   response (see (in this example, the first language is the default; see
   Appendix A.1 for an example of this). A.3).

   The ordering of the request header fields themselves indicates
   descending application of preferences; for example, in the headers example above, a cache will serve gzip'd content regardless
   that has all of language if it
   is available. the possible permutations stored will honour the
   client's preferences for Accept-Encoding before honouring Accept-
   Language.

   Origin servers SHOULD consistently send Variant header fields on all
   cacheable (as per [RFC7234], Section 3) responses for a resource,
   since its absence will trigger caches to fall back to Vary
   processing.

   Likewise, servers MUST send the Variant-Key response header field
   when sending Variants. Variants, since its absence means that the stored
   response will not be reused when this specification is implemented.

2.1.  Relationship to Vary

   Caches that fully implement this specification SHOULD ignore request header
   fields in the "Vary" Vary header for the purposes of secondary cache key
   calculation ([RFC7234], Section 4.1) when their semantics are
   implemented as per this specification and their corresponding
   response header field is listed in "Variants". Variants.

   If any member of the Vary header does not have a corresponding
   variant that is understood by the implementation, it is still subject
   to the requirements there.

   See Section 5.1.3 for an example.

   In practice, implementation of Vary varies considerably.  As a
   result, cache efficiency might drop considerably when Variants does
   not contain all of the headers referenced by Vary, because some
   implementations might choose to disable Variants processing when this
   is the case.

3.  The "Variant-Key" HTTP Header Field

   The Variant-Key HTTP response header field is used to indicate the
   value(s) from the Variants header field that identify the
   representation it occurs within.

   Variant-Key     = 1#available-value

   Each value indicates the selected available-value, in the same order
   as the variants listed in the Variants header field.

   Therefore, Variant-Key MUST be the same length (in comma-separated
   members) as Variants, and each member MUST correspond in position to
   its companion in Variants.

   For example:

   Variants: Content-Encoding;gzip;br, Content-Language;en ;fr
   Variant-Key: gzip, fr

   This header pair indicates that the representation is used for
   responses that have has a "gzip"
   content-coding and "fr" content-
   language. content-language.

   Note that the contents of Variant-Key are is only used to indicate what request
   attributes are identified associated with the response containing it; this is
   different from headers like Content-Encoding, which indicate
   attributes of the response. response itself.  In the example above, it might be
   that a gzip'd version of the French content is not available, in
   which case
   it the response will include:

   Variant-Key: gzip, fr

   even though Content-Encoding does not include "Content-Encoding: gzip", but still have "gzip"
   in Variant-Key.

4.  Defining Content Negotiation Using Variants

   To be usable with Variants, proactive content negotiation mechanisms
   need to be specified to take advantage of it.  Specifically, they:

   o  MUST define contain "gzip".

3.1.  Generating a request header field that advertises the clients
      preferences or capabilities, whose field-name SHOULD begin with
      "Accept-".

   o  MUST define the syntax of available-values that will occur in
      Variants and Variant-Key.

   o  MUST define an Normalised Variant-Key

   This algorithm for selecting generates a result.  It MUST return normalised string for Variant-Key,
   suitable for comparison with values generated by Section 4.

   Given stored-headers, a
      list set of available-values that are suitable headers from a stored response, a
   normalised variant-key for that message can be generated by:

   1.  Let variant-key-header be a string, the request, in
      order result of preference, given the value selecting all
       field-values of the request header
      nominated above stored-headers whose field-name is "Variant-Key"
       and an available-values list joining them with a comma (",").

   2.  Remove all whitespace from the Variants
      header.  If the result is an empty list, it implies that the cache
      cannot satisfy the request.

   Appendix A fulfils these requirements for some existing proactive
   content negotiation mechanisms in HTTP.

5. variant-key-header.

   3.  Return variant-key-header.

4.  Cache Behaviour

   Caches that implement the Variants header field and the relevant
   semantics of the field-name it contains can use that knowledge to
   either select an appropriate stored representation, or forward the
   request if no appropriate representation is stored.

   They do so by running this algorithm (or its functional equivalent)
   upon receiving a request, incoming-request:

   1.  Let selected-responses be request:

   Given incoming-request, a mapping of field-names to lists of field
   values, and stored-responses, a list of the stored responses suitable for
   reuse as defined in [RFC7234] Section 4, excepting the requirement to
   calculate a secondary cache key. key:

   1.  If stored-responses is empty, return an empty list.

   2.  Order selected-responses stored-responses by the "Date" header field, most recent to
       least recent.

   3.  Let sorted-variants be an empty list.

   4.  If the freshest member of stored-responses (as per [RFC7234],
       Section 4.2) has one or more "Variants" header field(s):

       1.  Select one member of selected_responses stored-responses and let its "Variants"
           header field-value(s) be variants-header.  This SHOULD be the
           most recent response, but MAY be from an older one as long as
           it is still fresh.

       2.  Let sorted-variants be an empty list.

       3.  For each variant in variants-header:

           1.  If variant's field-name corresponds to the response request header
               field identified by a content negotiation mechanism that
               the implementation supports:

               1.  Let request-value be the field-value of the request
                   header field(s) identified by the content negotiation
                   mechanism. field-value(s) associated
                   with field-name in incoming-request.

               2.  Let available-values be a list containing all
                   available-value for the variant.

               3.  Let sorted-values be the result of running the
                   algorithm defined by the content negotiation
                   mechanism with request-value and available-values.

               4.  Append sorted-values to sorted-variants.

           At this point, sorted-variants will be a list of lists, each
           member of the top-level list corresponding to a variant-item
           in the Variants header field-value, containing zero or more
           items indicating available-values that are acceptable to the
           client, in order of preference, greatest to least.

       4.  If any member of sorted-variants is an empty list, stop
           processing and forward the request towards the origin, since
           an acceptable response is not stored in the cache.

   5.  Let sorted-keys be the  Return result of running Find Available Keys (Section 5.1) 4.1) on sorted-variants
       sorted-variants, an empty string and two an empty lists. list.

   This will result in returns a list of lists, where each member of the top-
   level list indicates, in preference order, a key strings suitable for an acceptable
   response comparing to normalised
   Variant-Keys (Section 3.1) that represent possible responses on the request.

   A Cache MAY
   server that can be used to satisfy the request with any response whose Variant-Key
   header corresponds to a member of sorted-keys; when doing so, it
   SHOULD use request, in preference order,
   provided that their secondary cache key (after removing the most preferred available response.

   See also headers
   covered by Variants) matches.  Section 2.1 regarding handling of Vary.

5.1. 4.2 illustrates one way to do
   this.

4.1.  Find Available Keys

   Given sorted-variants, a list of lists, and key-stub, a list string
   representing a partial key, and possible-keys, a list:

   1.  Let sorted-values be the first member of sorted-variants.

   2.  For each sorted-value in sorted-values:

       1.  If key-stub is an empty string, let this-key be a copy of
           sorted-value.

       2.  Otherwise:

           1.  Let this-key be a copy of key-stub.

           2.  Append a comma (",") to this-key.

           3.  Append sorted-value to this-key.

       3.  Let remaining-variants be a copy of all of the members of
           sorted-variants except the first.

       4.  If remaining-variants is empty, append this-key to possible-
           keys.

       5.  Else,  Otherwise, run Find Available Keys on remaining-variants,
           this-key and possible-keys.

       6.

   3.  Return possible-keys.

5.2.

4.2.  Check Vary

   This algorithm is an example of how an implementation can meet the
   requirement to apply the members of the Vary header field that are
   not covered by Variants.

   Given a stored response, stored-response:

   1.  Let filtered-vary be the field-value(s) of stored-response's
       "Vary" header field.

   2.  Let processed-variants be a list containing the request header
       fields that identify the content negotiation mechanisms supported
       by the implementation.

   3.  Remove any member of filtered-vary that is a case-insensitive
       match for a member of processed-variants.

   4.  If the secondary cache key (as calculated in [RFC7234],
       Section 4.1) for stored_response matches incoming-request, using
       filtered-vary for the value of the "Vary" response header, return
       True.

   5.  Return False.

   This returns a Boolean that indicates whether stored-response can be
   used to satisfy the request.

   Note that implementation of the Vary header field varies in practice,
   and the algorithm above illustrates only one way to apply it.  It is
   equally viable to forward the request if there is a request header
   listed in Vary but not Variants.

4.3.  Example of Cache Behaviour

   For example, if the selected variants-header was:

   Variants: Accept-Language;en;fr,de, Accept-Encoding;gzip,br

   and the request contained the headers:

   Accept-Language: fr;q=1.0, en;q=0.1
   Accept-Encoding: gzip

   Then the sorted-variants would be:

 [
   ["fr", "en"]           // prefers French, will accept English
   ["gzip", "identity"]   // prefers gzip encoding, will accept identity
 ]

   Which means that the sorted-keys would be:

   [
     ['fr', 'gzip'],
     ['fr', 'identity'],
     ['en', 'gzip'],
     ['en', 'identity']
     'fr gzip',
     'fr identity',
     'en gzip',
     'en identity'
   ]

   Representing a first preference of a French, gzip'd response.  Thus,
   if a cache has a response with:

   Variant-Key: fr, gzip

   it could be used to satisfy the first preference.  If not, responses
   corresponding to the other keys could be returned, or the request
   could be forwarded towards the origin.

6.  Example Headers

6.1.

5.  Origin Server Behaviour

   Origin servers that wish to take advantage of Variants will need to
   generate both the Variants (Section 2) and Variant-Key (Section 3)
   header fields in all cacheable responses for a given resource.  If
   either is omitted and the response is stored, it will have the effect
   of disabling caching for that resource until it is no longer stored
   (e.g., it expires, or is evicted).

   Likewise, origin servers will need to assure that the members of both
   header field values are in the same order and have the same length,
   since discrepancies will cause caches to avoid using the responses
   they occur in.

   The value of the Variants header should be relatively stable for a
   given resource over time; when it changes, it can have the effect of
   invalidating previously stored responses.

   As per Section 2.1, the Vary header is required to be set
   appropriately when Variants is in use, so that caches that do not
   implement this specification still operate correctly.

   Origin servers are advised to carefully consider which content
   negotiation mechanisms to enumerate in Variants; if a mechanism is
   not supported by a receiving cache, it will "downgrade" to Vary
   handling, which can negatively impact cache efficiency.

5.1.  Examples

   The operation of Variants is illustrated by the examples below.

5.1.1.  Single Variant

   Given a request/response pair:

   GET /foo /clancy HTTP/1.1
   Host: www.example.com
   Accept-Language: en;q=1.0, fr;q=0.5

   HTTP/1.1 200 OK
   Content-Type: image/gif
   Content-Language: en
   Cache-Control: max-age=3600
   Variants: Content-Language;en;de
   Variant-Key: en
   Vary: Accept-Language
   Transfer-Encoding: chunked

   Upon receipt of this response, the cache knows that two
   representations of this resource are available, one with a "Content-
   Language" Content-
   Language of "en", and another whose "Content-Language" Content-Language is "de".

   Subsequent requests (while this response is fresh) will cause the
   cache to either reuse this response or forward the request, depending
   on what the selection algorithm determines.

   So, if a request with "en" in "Accept-Language" Accept-Language is received and its
   q-value indicates that it is acceptable, the stored response is used.
   A request that indicates that "de" is acceptable will be forwarded to
   the origin, thereby populating the cache.  A cache receiving a
   request that indicates both languages are acceptable will use the
   q-value to make a determination of what response to return.

   A cache receiving a request that does not list either language as
   acceptable (or does not contain an Accept-Language at all) will
   return the "en" representation (possibly fetching it from the
   origin), since it is listed first in the "Variants" Variants list.

   Note that "Accept-Language" Accept-Language is listed in Vary, to assure backwards-
   compatibility with caches that do not support "Variants".

6.2. Variants.

5.1.2.  Multiple Variants

   A more complicated request/response pair:

   GET /bar /murray HTTP/1.1
   Host: www.example.net
   Accept-Language: en;q=1.0, fr;q=0.5
   Accept-Encoding: gzip, br

   HTTP/1.1 200 OK
   Content-Type: image/gif
   Content-Language: en
   Content-Encoding: br
   Variants: Content-Language;en;jp;de
   Variants: Content-Encoding;br;gzip
   Variant-Key: en, br
   Vary: Accept-Language, Accept-Encoding
   Transfer-Encoding: chunked

   Here, the cache knows that there are two axes that the response
   varies upon; "Content-Language" Content-Language and "Content-Encoding". Content-Encoding.  Thus, there are
   a total of six nine possible representations for the resource, resource (including
   the identity encoding), and the cache needs to consider the selection
   algorithms for both axes.

   Upon a subsequent request, if both selection algorithms return a
   stored representation, it can be served from cache; otherwise, the
   request will need to be forwarded to origin.

6.3.

5.1.3.  Partial Coverage

   Now, consider the previous example, but where only one of the Vary'd
   axes is listed in "Variants": Variants:

   GET /bar HTTP/1.1
   Host: www.example.net
   Accept-Language: en;q=1.0, fr;q=0.5
   Accept-Encoding: gzip, br

   HTTP/1.1 200 OK
   Content-Type: image/gif
   Content-Language: en
   Content-Encoding: br
   Variants: Content-Encoding;br;gzip
   Variant-Key: br
   Vary: Accept-Language, Accept-Encoding
   Transfer-Encoding: chunked
   Here, the cache will need to calculate a secondary cache key as per
   [RFC7234], Section 4.1 - but considering only "Accept-Language" Accept-Language to be
   in its field-value - and then continue processing "Variants" Variants for the
   set of stored responses that the algorithm described there selects.

6.  Defining Content Negotiation Using Variants

   To be usable with Variants, proactive content negotiation mechanisms
   need to be specified to take advantage of it.  Specifically, they:

   o  MUST define a request header field that advertises the clients
      preferences or capabilities, whose field-name SHOULD begin with
      "Accept-".

   o  MUST define the syntax of available-values that will occur in
      Variants and Variant-Key.

   o  MUST define an algorithm for selecting a result.  It MUST return a
      list of available-values that are suitable for the request, in
      order of preference, given the value of the request header
      nominated above and an available-values list from the Variants
      header.  If the result is an empty list, it implies that the cache
      cannot satisfy the request.

   Appendix A fulfils these requirements for some existing proactive
   content negotiation mechanisms in HTTP.

7.  IANA Considerations

   This specification registers two values in the Permanent Message
   Header Field Names registry established by [RFC3864]:

   o  Header field name: Variants

   o  Applicable protocol: http

   o  Status: standard

   o  Author/Change Controller: IETF

   o  Specification document(s): [this document]

   o  Related information:

   o  Header field name: Variant-Key

   o  Applicable protocol: http
   o  Status: standard

   o  Author/Change Controller: IETF

   o  Specification document(s): [this document]

   o  Related information:

8.  Security Considerations

   If the number or advertised characteristics of the representations
   available for a resource are considered sensitive, the "Variants" Variants
   header by its nature will leak them.

   Note that the "Variants" Variants header is not a commitment to make
   representations of a certain nature available; the runtime behaviour
   of the server always overrides hints like "Variants". Variants.

9.  Acknowledgments

   This protocol is conceptually similar to, but simpler than,
   Transparent Content Negotiation [RFC2295].  Thanks to its authors for
   their inspiration.

   It is also a generalisation of a Fastly VCL feature designed by
   Rogier 'DocWilco' Mulhuijzen.

   Thanks to Hooman Beheshti for his review and input.

10.  References

10.1.  Normative References

   [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>.

   [RFC4647]  Phillips, A. and M. Davis, "Matching of Language Tags",
              BCP 47, RFC 4647, DOI 10.17487/RFC4647, September 2006,
              <https://www.rfc-editor.org/info/rfc4647>.

   [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>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [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>.

   [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>.

10.2.  Informative References

   [I-D.ietf-httpbis-client-hints]
              Grigorik, I., "HTTP Client Hints", draft-ietf-httpbis-
              client-hints-04
              client-hints-05 (work in progress), April 2017. January 2018.

   [RFC2295]  Holtman, K. and A. Mutz, "Transparent Content Negotiation
              in HTTP", RFC 2295, DOI 10.17487/RFC2295, March 1998,
              <https://www.rfc-editor.org/info/rfc2295>.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,
              <https://www.rfc-editor.org/info/rfc3864>.

10.3.  URIs

   [1] https://github.com/mnot/I-D/labels/variants

   [2] https://mnot.github.io/I-D/variants/

   [3] https://github.com/mnot/I-D/commits/gh-pages/variants

   [4] https://datatracker.ietf.org/doc/draft-nottingham-variants/

   [5] https://github.com/mnot/variants-toy

Appendix A.  Variants for Existing Content Negotiation Mechanisms

   This appendix defines the required information to use existing
   proactive content negotiation mechanisms (as defined in [RFC7231],
   Section 5.3) with the "Variants" Variants header field.

A.1.  Accept

   This section defines handling for Accept variants, as per [RFC7231]
   Section 5.3.2.

   To perform content negotiation for Accept given a request-value and
   available-values:

   1.  Let preferred-available be an empty list.

   2.  Let preferred-types be a list of the types in the request-value,
       ordered by their weight, highest to lowest, as per [RFC7231]
       Section 5.3.2 (omitting any coding with a weight of 0).  If
       "Accept" is not present or empty, preferred-types will be empty.
       If a type lacks an explicit weight, an implementation MAY assign
       one.

   3.  If the first member of available-values is not a member of
       preferred-types, append it to preferred-types (thus making it the
       default).

   4.  For each preferred-type in preferred-types:

       1.  If any member of available-values matches preferred-type,
           using the media-range matching mechanism specified in
           [RFC7231] Section 5.3.2 (which is case-insensitive), append
           those members of available-values to preferred-available
           (preserving the precedence order implied by the media ranges'
           specificity).

   5.  Return preferred-available.

   Note that this algorithm explicitly ignores extension parameters on
   media types (e.g., "charset").

A.2.  Accept-Encoding

   This section defines handling for "Accept-Encoding" Accept-Encoding variants, as per
   [RFC7231] Section 5.3.4.

   To perform content negotiation for Accept-Encoding given an a request-
   value and available-values:

   1.  Let preferred-available be an empty list.

   2.  Let preferred-codings be a list of the codings in the request-
       value, ordered by their weight, highest to lowest, as per
       [RFC7231] Section 5.3.1 (omitting any coding with a weight of 0).
       If "Accept-Encoding" is not present or empty, preferred-codings
       will be empty.

   2.  If a coding lacks an explicit weight, an
       implementation MAY assign one.

   3.  If "identity" is not a member of preferred-codings, append
       "identity".

   3.

   4.  Append "identity" to available-values.

   4.  Remove any

   5.  For each preferred-coding in preferred-codings:

       1.  If there is a case-insensitive, character-for-character match
           for preferred-coding in available-values, append that member
           of available-values not present in preferred-
       codings, comparing in a case-insensitive fashion.

   5. to preferred-available.

   6.  Return available-values.

A.2. preferred-available.

   Note that the unencoded variant needs to have a Variant-Key header
   field with a value of "identity".

A.3.  Accept-Language

   This section defines handling for "Accept-Language" Accept-Language variants, as per
   [RFC7231] Section 5.3.5.

   To perform content negotiation for Accept-Language given an a request-
   value and available-values:

   1.  Let preferred-available be an empty list.

   2.  Let preferred-langs be a list of the language-ranges in the
       request-value, ordered by their weight, highest to lowest, as per
       [RFC7231] Section 5.3.1 (omitting any language-range with a
       weight of 0).

   2.  If preferred-langs a language-range lacks a weight, an
       implementation MAY assign one.

   3.  If the first member of available-values is empty, not a member of
       preferred-langs, append "*".

   3.  Filter it to preferred-langs (thus making it the
       default).

   4.  For each preferred-lang in preferred-langs:

       1.  If any member of available-values matches preferred-lang,
           using preferred-langs with either the Basic or Extended Filtering scheme defined
           in [RFC4647] Section 3.3.1, or the
       Lookup scheme defined in Section 3.4 of that document.  Use the
       first member 3.3, append those members of available-values as the default.

   4. available-
           values to preferred-available (preserving their order).

   5.  Return available-values. preferred-available.

Author's Address

   Mark Nottingham
   Fastly

   Email: mnot@mnot.net
   URI:   https://www.mnot.net/