Internet DRAFT - draft-nottingham-variants
draft-nottingham-variants
Network Working Group M. Nottingham
Internet-Draft Fastly
Updates: 7234 (if approved) February 13, 2018
Intended status: Standards Track
Expires: August 17, 2018
HTTP Representation Variants
draft-nottingham-variants-02
Abstract
This specification introduces an alternative way to communicate a
secondary cache key for a HTTP resource, using the HTTP "Variants"
and "Variant-Key" response header fields. Its aim is to make HTTP
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The most recent (often, unpublished) draft is at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
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
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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 August 17, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 4
2. The "Variants" HTTP Header Field . . . . . . . . . . . . . . 5
2.1. Relationship to Vary . . . . . . . . . . . . . . . . . . 6
3. The "Variant-Key" HTTP Header Field . . . . . . . . . . . . . 7
3.1. Generating a Normalised Variant-Key . . . . . . . . . . . 7
4. Cache Behaviour . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Find Available Keys . . . . . . . . . . . . . . . . . . . 9
4.2. Check Vary . . . . . . . . . . . . . . . . . . . . . . . 10
4.3. Example of Cache Behaviour . . . . . . . . . . . . . . . 10
5. Origin Server Behaviour . . . . . . . . . . . . . . . . . . . 11
5.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1.1. Single Variant . . . . . . . . . . . . . . . . . . . 12
5.1.2. Multiple Variants . . . . . . . . . . . . . . . . . . 13
5.1.3. Partial Coverage . . . . . . . . . . . . . . . . . . 13
6. Defining Content Negotiation Using Variants . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
8. Security Considerations . . . . . . . . . . . . . . . . . . . 15
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
10.1. Normative References . . . . . . . . . . . . . . . . . . 15
10.2. Informative References . . . . . . . . . . . . . . . . . 16
10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Appendix A. Variants for Existing Content Negotiation Mechanisms 17
A.1. Accept . . . . . . . . . . . . . . . . . . . . . . . . . 17
A.2. Accept-Encoding . . . . . . . . . . . . . . . . . . . . . 17
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A.3. Accept-Language . . . . . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
HTTP proactive content negotiation ([RFC7231], Section 3.4.1) is
seeing renewed interest, both for existing request headers like
Content-Language and for newer 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). Currently, the Vary header ([RFC7231],
Section 7.1.4) does this by nominating a set of request headers.
HTTP's caching model allows a certain amount of latitude in
normalising those request header field values, so as to increase the
chances of a cache hit while still respecting the semantics of that
header. However, normalisation is not formally defined, leading to
divergence in cache behaviours.
Even when the headers' semantics 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, incurring possibly unnecessary latency.
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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
the algorithm defined in Section 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. 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 to do so explicitly; see Section 6. As a
result, it is best suited for negotiation over request headers that
are well-understood.
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.
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.
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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.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
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[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 (in this example, the first language is the default; see
Appendix A.3).
The ordering of the request header fields themselves indicates
descending application of preferences; in the example above, a cache
that has all of 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, since its absence means that the stored
response will not be reused when this specification is implemented.
2.1. Relationship to Vary
Caches that implement this specification SHOULD ignore request header
fields in the 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.
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.
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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 has a "gzip"
content-coding and "fr" content-language.
Note that Variant-Key is only used to indicate what request
attributes are associated with the response containing it; this is
different from headers like Content-Encoding, which indicate
attributes of the response itself. In the example above, it might be
that a gzip'd version of the French content is not available, in
which case the response will include:
Variant-Key: gzip, fr
even though Content-Encoding does not contain "gzip".
3.1. Generating a Normalised Variant-Key
This algorithm generates a normalised string for Variant-Key,
suitable for comparison with values generated by Section 4.
Given stored-headers, a set of 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 result of selecting all
field-values of stored-headers whose field-name is "Variant-Key"
and joining them with a comma (",").
2. Remove all whitespace from variant-key-header.
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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:
Given incoming-request, a mapping of field-names to lists of field
values, and stored-responses, a list of stored responses suitable for
reuse as defined in [RFC7234] Section 4, excepting the requirement to
calculate a secondary cache key:
1. If stored-responses is empty, return an empty list.
2. Order 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 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. For each variant in variants-header:
1. If variant's field-name corresponds to the request header
field identified by a content negotiation mechanism that
the implementation supports:
1. Let request-value be the field-value(s) associated
with field-name in incoming-request.
2. Let available-values be a list containing all
available-value for variant.
3. Let sorted-values be the result of running the
algorithm defined by the content negotiation
mechanism with request-value and available-values.
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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.
5. Return result of running Find Available Keys (Section 4.1) on
sorted-variants, an empty string and an empty list.
This returns a list of strings suitable for comparing to normalised
Variant-Keys (Section 3.1) that represent possible responses on the
server that can be used to satisfy the request, in preference order,
provided that their secondary cache key (after removing the headers
covered by Variants) matches. Section 4.2 illustrates one way to do
this.
4.1. Find Available Keys
Given sorted-variants, a list of lists, and key-stub, a 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. Otherwise, run Find Available Keys on remaining-variants,
this-key and possible-keys.
3. Return possible-keys.
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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:
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[
["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'
]
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.
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
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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 /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 of "en", and another whose 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 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 list.
Note that Accept-Language is listed in Vary, to assure backwards-
compatibility with caches that do not support Variants.
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5.1.2. Multiple Variants
A more complicated request/response pair:
GET /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 and Content-Encoding. Thus, there are
a total of nine possible representations for the 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.
5.1.3. Partial Coverage
Now, consider the previous example, but where only one of the Vary'd
axes is listed in 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
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Here, the cache will need to calculate a secondary cache key as per
[RFC7234], Section 4.1 - but considering only Accept-Language to be
in its field-value - and then continue processing 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
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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
header by its nature will leak them.
Note that the 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.
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>.
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[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-05 (work in progress), 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
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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 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 variants, as per
[RFC7231] Section 5.3.4.
To perform content negotiation for Accept-Encoding given a request-
value and available-values:
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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. If a coding lacks an explicit weight, an
implementation MAY assign one.
3. If "identity" is not a member of preferred-codings, append
"identity".
4. Append "identity" to available-values.
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 to preferred-available.
6. Return 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 variants, as per
[RFC7231] Section 5.3.5.
To perform content negotiation for Accept-Language given 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). If a language-range lacks a weight, an
implementation MAY assign one.
3. If the first member of available-values is not a member of
preferred-langs, append it to preferred-langs (thus making it the
default).
4. For each preferred-lang in preferred-langs:
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1. If any member of available-values matches preferred-lang,
using either the Basic or Extended Filtering scheme defined
in [RFC4647] Section 3.3, append those members of available-
values to preferred-available (preserving their order).
5. Return preferred-available.
Author's Address
Mark Nottingham
Fastly
Email: mnot@mnot.net
URI: https://www.mnot.net/
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