Internet DRAFT - draft-svensson-profiled-representations
draft-svensson-profiled-representations
Internet Engineering Task Force L. Svensson
Internet-Draft
Intended status: Informational R. Verborgh
Expires: September 10, 2021 Ghent University - imec
H. Van de Sompel
Data Archiving and Networked Services
March 9, 2021
Indicating, Discovering, Negotiating, and Writing Profiled
Representations
draft-svensson-profiled-representations-01
Abstract
This document details approaches for enriching HTTP interactions with
information pertaining to the profiles to which resource
representations conform. It surveys approaches that were recently
introduced to indicate the profile of a resource representation, and
to make representations that conform to a profile discoverable. It
introduces a generally applicable approach to negotiate for a
resource representation that conforms to a profile preferred by a
user agent. That approach leverages the existing content negotiation
mechanism but applies it to the profile dimension to which it was
previously not applied. The document also shows how a server can
convey which profiled representations it is able to accept from a
user agent.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 10, 2021.
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Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Notational Conventions . . . . . . . . . . . . . . . . . 5
2. Indicating Profiled Representations . . . . . . . . . . . . . 5
3. Discovering Profiled Representations . . . . . . . . . . . . 7
4. Negotiating for Profiled Representations . . . . . . . . . . 7
4.1. Profile Negotiation Details . . . . . . . . . . . . . . . 8
4.1.1. Proactive Profile Negotiation . . . . . . . . . . . . 9
4.1.2. Reactive Profile Negotiation . . . . . . . . . . . . 12
4.2. Accept-Profile HTTP Header Syntax . . . . . . . . . . . . 14
5. Writing Profiled Representations . . . . . . . . . . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Normative References . . . . . . . . . . . . . . . . . . 17
9.2. Informative References . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
Any given web resource can typically be represented in a variety of
ways. For example, the same information could be rendered according
to different media types, say, as XML or JSON. But in many cases,
variations in representation other than those inherent to a given
media type are also possible. For example, the same structured data
could be rendered in XML according to different XML Schema
[W3C.REC-xmlschema11-1-20120405]. Or the same RDF graph could be
expressed on the basis of different vocabularies.
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This dimension of variability regarding representations that goes
beyond media types has been acknowledged for quite some time. For
example, in 2000, the Dublin Core Metadata Initiative (DCMI)
introduced "Application Profiles" to explicitly acknowledge that the
same metadata can be represented in various ways and defined them as
"schemas which consist of data elements drawn from one or more
namespaces, combined together by implementers, and optimised for a
particular local application" [HeeryAndPatel].
1.1. Terminology
Inspired by the term used in [RFC6906] to refer to this extra
dimension of variability, this document uses the term "profile" to
mean a description of structural and/or semantic constraints on
representations of resources that apply in addition to the
constraints inherently indicated by their MIME type:
o Profiles can be dependent on a media type. For example, this is
the case for XML, with constraints being expressed using an XML
Schema. This is also the case for JSON that offers a wide range
of options regarding the use of tree structure, keys, and value
types. In these cases, the meaning of the term "profile" intended
by this document coincides with the use of the same term in
[RFC6906].
o Profiles can be independent of media type. For example, this is
the case for RDF graphs that can be rendered according to various
media types, while constraints can be expressed in a manner that
is independent of media type, among others, using SHACL
[W3C.REC-shacl-20170720]. In these cases, the meaning of the term
"profile" intended by this document is a slight extension of the
one intended by [RFC6906].
1.2. Purpose
When it comes to HTTP interactions, profiles have received little
attention despite their de facto existence and the added-value they
can bring for building rich applications. Such applications benefit
from knowledge regarding the nature of a representation that a client
obtains from a server, that a client sends to a server, and that a
server is willing to accept from a client, beyond what is conveyed by
the representation's MIME type. These applications are also be
helped by an ability to discover representations rendered according
to a profile they can handle, or, optimally, an ability to explicitly
request a rendering according to a preferred profile.
A common approach to handle profiles is to register them as a media
type, dedicated to the combination of an actual media type and a
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profile of it. Media types that illustrate this approach include
"application/activity+json", "application/calendar+json", and
"application/calendar+xml. This approach allows conveying all
necessary profile information in HTTP interactions, e.g. using the
"Accept" and "Content-Encoding" HTTP headers and the "type" attribute
for web links. As such it supports indicating, discovering, and
content negotiating (in the media type dimension) for profiled
representations. This registration-based approach may be feasible
for profiles that are expected to be very widely used but is not
practical in case support for many different profiles is required.
Also, the "calendar" examples illustrate that the registration-based
approach is not ideal when a profile applies to multiple media types.
And, the "activity" example illustrates that the approach supports
indicating what the major ingredient of a profiled representation is
(i.e. the ActivityStreams Vocabulary) but becomes problematic when
indications are also needed regarding additional vocabularies used in
representations.
Another approach to handle profiles leverages the ability provided by
[RFC6838] to register parameters when registering a media type. Some
media types have used this capability to register an attribute
dedicated to conveying profiles of the media type. For example, for
"application/ld+json" the "profile" parameter has been registered for
this purpose. The approach provides flexibility for handling many
profiles, including ones that are not yet known when registering the
media type. It also supports indicating, discovering, and content
negotiating (in the media type dimension) for profiled
representations using common approaches. But the approach remains
problematic because it ties profile information to a media type,
depends on registering a parameter to convey profile information when
registering a new media type, and, realistically, on the registration
of the same parameter name (i.e. "profile" as suggested in [RFC6906])
for all media types for which registrants deem that conveying profile
information is important. Additionaly, [RFC6838] discourages
registering parameters for previously registered media types, making
it highly questionable that a uniform attribute to convey profile
information across all media types could retroactively be defined.
Recognizing the importance of profiles and the problems with the
aforementioned approaches to handle them, specifications have started
to introduce alternative approaches to express information about
resource representation profiles in HTTP interactions:
o [RFC6906] introduces the "profile" link relation type that is
generally applicable for indicating the profile of a resource
representation that is sent by a client to a server or by a server
to a client.
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o [I-D.nottingham-link-hint] introduces a capability to make
profiled representations discoverable via web links by using the
"formats" attribute to express the profile of a linked resource.
Section 2 and Section 3 provide a concise overview of the approaches
introduced by [RFC6906] and [I-D.nottingham-link-hint], to
respectively indicate and discover the profile of resource
representations. Section 4 specifies a generally applicable approach
to negotiate for representations that conform to a profile preferred
by a user agent. Section 5 shows how servers can convey which
profiled representations they are able to accept from user agents.
1.3. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
This specification uses the terms "link context" and "link target" as
defined in [RFC8288]. These terms respectively correspond with
"Context IRI" and "Target IRI" as used in [RFC5988]. Although
defined as IRIs, in common scenarios they are also URIs.
In the examples provided in this document, links in the HTTP "Link"
header are shown on separate lines in order to improve readability.
Note, however, that as per Section 3.2 of [RFC7230], line breaks are
not allowed in values for HTTP headers; only whitespaces and tabs are
supported as seperators.
2. Indicating Profiled Representations
As per [RFC6906], a web link with a "profile" link relation type can
be used to indicate the profile of a representation that is exchanged
in HTTP interactions. Figure 1 shows a client requesting a
representation from a server and Figure 2 shows the server
responding. The response includes a "Link" header ([RFC8288]) that
contains a link with the "profile" relation type. The link target
<http://purl.org/dc/terms/> indicates the profile of the response
body (not shown).
Figure 3 shows a client submitting a representation to a server,
using the same approach to express the profile to which that
representation complies. Section 5 describes how a server can convey
the profiles it supports for representations that are submitted by a
user agent.
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GET /some/resource HTTP/1.1
Host: example.org
Connection: close
Figure 1: Client requests a representation
HTTP/1.1 200 OK
Content-Type: application/xml ; charset=utf-8
Link: <http://purl.org/dc/terms/>; rel="profile"
Content-Length: 23364
Connection: close
...
Figure 2: Server indicates the profile of the returned representation
PUT /some/resource HTTP/1.1
Host: example.org
Content-Type: application/xml ; charset=utf-8
Link: <http://purl.org/dc/terms/>; rel="profile"
Content-Length: 23364
Connection: close
...
Figure 3: Client indicates the profile of a representation submitted
to a server
In some cases organisations use separate servers to perform content
negotiation and to deliver resources, e. g. when static content is
served through content delivery networks. The servers that perform
the content negotiation interact with the client requesting the
resource and then typically refer to the correct representation using
a 303 redirect. In those cases the servers that deliver the
representations are not profile aware and thus cannot add the
appropriate "Link" headers to the response. Instead the server
performing the negotiation will have to supply that information.
This is done by adding an "anchor" attribute pointing to the
representation the link header refers to. Figure 4 shows the
response to a Figure 1 where the server performing the negotiation
redirects the client to the resource specified in the "Location"
header and by using the same URI in the "anchor" attribute of the
"Link" header indicates that the information in the "Link" header
does not apply to this response but to the resource redirected to.
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HTTP/1.1 300 See other
Location: https://static.example.org/other/resource
Link: <http://purl.org/dc/terms/>; rel="profile"
; anchor="https://static.example.org/other/resource"
...
Figure 4: Server indicates the profile of the representation referred
to in the 'Location' header
3. Discovering Profiled Representations
The link hints capability introduced in [I-D.nottingham-link-hint],
can be leveraged by a server to make profiled representations
discoverable by including a "formats" attribute on web links.
Figure 5 shows a response to the client request of Figure 1 in which
the server uses this technique. The "Link" header indicates the
profile of the returned representation but also points at two
alternative representations, each of which conforms to another
profile.
HTTP/1.1 200 OK
Content-Type: application/xml
Link: <http://purl.org/dc/terms/> ; rel="profile" ,
</some/other_resource_1>
; rel="alternate" ; type="application/xml"
; formats="http://example.org/our_internal_xml_profile" ,
</some/other_resource_2>
; rel="alternate"
; formats="http://example.org/our_community_profile"
Content-Length: 23364
Connection: close
...
Figure 5: Server makes profiled representations discoverable
4. Negotiating for Profiled Representations
Section 1.2 describes two approaches for conveying profile
information in HTTP interactions that make it possible to negotiate
for profiled representations by applying content negotiation in the
media type dimension. It also indicates the restricted applicability
of these approaches, in both cases a result of their direct
dependence on the media type registration process.
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This section describes an approach that applies content negotiation
in a dimension that it was previously not applied to. The profile
negotiation approach introduced here is generally applicable for
resource representations, irrespective of media type. These are its
core aspects:
o In order to allow a user agent to inform a server about its
preferences regarding profiles for resource representations, the
"Accept-Profile" HTTP header introduced here is used. A user
agent can specify several profiles and use quality indicators
(q-values) to indicate preferences.
o In order to allow a server to express its support for profile
negotiation the "Vary" HTTP header is used, in this case, with the
"Accept-Profile" value.
o In order to allow a server to convey the profile of a
representation delivered to a user agent, rather than introducing
a server-side counterpart to the client-side "Accept-Profile"
header, the existing "profile" link approach introduced by
[RFC6906] is used. If a representation conforms to multiple
profiles, a distinct "profile" link is used per profile; the order
in which these links are provided has no relevance.
o In order to allow a server to convey the profiles it supports, web
links with the "formats" link hint introduced in
[I-D.nottingham-link-hint] are used to convey the profiles of the
link target resources. This approach uniformely applies to
reponses to HTTP HEAD/GET/PUT/POST/PATCH.
o Throughout the profile negotiation approach, a profile MUST be
referred to by a URI. This is the case for the content of the
"Accept-Profile" HTTP header, for the target of web links with the
"profile" link relation, and for the content of the "formats" link
hint for web links.
4.1. Profile Negotiation Details
Profile negotiation uses the content negotiation processes described
in Section 3.4 of RFC 7231 [RFC7231] but applies them to the profile
dimension. Both proactive negotiation and reactive negotiation for
profiles can be supported by servers. Both are described in more
detail in the remainder of this section.
In profile negotiation, a profile MUST be referred to by a URI that,
from here onwards, is named a profile URI. If the profile URI is
dereferencable it SHOULD lead to a document that details the profile.
If the profile URI is not dereferencable (e.g. a URN [RFC8141] or an
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info-URI [RFC4452]) facilities SHOULD be available to allow user
agents and servers to understand their meaning, e.g. community
registries of profiles.
4.1.1. Proactive Profile Negotiation
In proactive profile negotiation, the user agent uses the "Accept-
Profile" HTTP header to inform the server about the agent's
preference regarding profiles to be used for representing a resource
in the server's response. In case a user agent wants to express a
preference for a single profile, the value of the header is that
profile's URI. In case a user agent wants to express a preference
for multiple profiles, the value of the header is a list containing
each profile's URI, seperated by commas. Alternatively, multiple
"Accept-Profile" HTTP headers can be used, each conveying a single
profile URI. Quality indicators (q-values) MAY be used to rank
profile preferences. The order in which profile URIs are conveyed or
the duplicate mentioning of a same profile URI MUST NOT be
interpreted as significant.
A server that supports proactive profile negotiation for the resource
that a user agent interacts with:
o MUST include a "Vary" HTTP header containing the value "accept-
profile" in its response to the user agent.
o MUST include a "Link" HTTP header containing a link with the
"profile" relation type that has as link target the profile URI of
the resource representation returned to the user agent. In case
the representation conforms to additional profiles known to the
server, such a "profile" link SHOULD be included for each.
o SHOULD convey the availability of alternate profiled
representations of the resource by using the link hint approach
described in Section 4.1.2.
These requirements for servers that support proactive profile
negotiation also apply when:
o The user agent expressed a profile preference in its request by
using an "Accept-Profile" header but the server cannot return a
representation that conforms to a preferred profile.
o The user agent did not express a profile preference using an
"Accept-Profile" header in its request.
A user agent SHOULD interpret the abscence of a "Vary" HTTP header
with an "accept-profile" value in a response from a server as the
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lack of support for profile negotiation for the resource the user
agent interacts with.
A server SHOULD consider representations that do not conform to any
of the profiles listed by a user agent in an "Accept-Profile" header
as non-interpretable by the agent. As such, honoring the user agent
preferences in the profile dimension SHOULD take precedence over
honoring content negotiation in other dimensions.
In Figure 6 a user agent requests an RDF serialization from a server
and expresses preference for two media types using the "Accept"
header and two profiles using the "Accept-Profile" header. It uses
q-values to express a preference for the profile with profile URI
<http://example.org/shapes/shape-1> over the one with profile URI
<http://example.org/shapes/shape-2>.
GET /document HTTP/1.1
Host: example.org
Accept: text/turtle, application/rdf+xml
Accept-Profile: "http://example.org/shapes/shape-1" ; q=0.8 ,
"http://example.org/shapes/shape-2" ; q=0.5
Connection: close
Figure 6: Client expresses a preference for two profiles
Figure 7 shows the server's response to the request of Figure 6. By
means of the "Vary" header, the server expresses support for
negotiation in both the media type and profile dimensions. The
"profile" link with link target <http://example.org/shapes/shape-2>
indicates that the server was able to honor the user agent's second
profile preference. Another "profile" link shows that the delivered
representation also conforms to a profile with profile URI
<http://example.org/shapes/shape-3>. Furthermore, using an
"alternate" link, the server indicates support for another profile
with <http://example.org/shapes/shape-4> as profile URI. Note that,
even if the user agent does not express profile preferences using the
"Accept-Profile" header and the server's "Vary" header would be the
same, the "Link" header would still include a "profile" link to
indicate the profile of the representation returned by the server.
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HTTP/1.1 200 OK
Content-Type: text/turtle
Vary: Accept, Accept-Profile
Link: <http://example.org/shapes/shape-2>
; rel="profile" ,
<http://example.org/shapes/shape-3>
; rel="profile" ,
<http://example.org/document>
; rel="alternate"
; type="text/turtle"
; formats="http://example.org/shapes/shape-4"
Content-Length: 8724
Connection: close
...
Figure 7: Response honors a user agent's preference
Figure 8 shows the response to the request of Figure 6 in case the
server supports profile negotiation for the resource at hand but can
not return a representation that conforms to a profile preferred by
the user agent. The server has chosen to nevertheless return a
representation that conforms to profile <http://example.org/shapes/
shape-4>, which is not among the ones preferred by the user agent.
The server also reveals the existence of a representation that
conforms to profile <http://example.org/shapes/shape-5>. The server
could also choose not to return a default representation in which
case it would return a "406 Not Acceptable" HTTP response code and no
response body. It would not provide any "profile" links but might
use "alternate" links with a "formats" attribute to indicate the
existence of supported profiles.
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HTTP/1.1 200 OK
Content-Type: text/turtle
Vary: Accept, Accept-Profile
Link: <http://example.org/shapes/shape-4>
; rel="profile" ,
<http://example.org/document>
; rel="alternate"
; type="text/turtle"
; formats="http://example.org/shapes/shape-5"
Content-Length: 6333
Connection: close
...
Figure 8: Response does not honor a user agent's preference but
includes default representation
Figure 9 shows the response to the request of Figure 6 in case the
server does not support profile negotiation for the resource
<http://example.org/document>. It does support negotiation in the
media type dimension and has honoured one of the user agent's
preferences with that regard, as can be seen by the "Vary" and
"Content-Type" headers.
HTTP/1.1 200 OK
Content-Type: text/turtle
Vary: Accept
Content-Length: 8724
Connection: close
...
Figure 9: Response indicates lack of support for proactive profile
negotiation
4.1.2. Reactive Profile Negotiation
In reactive profile negotiation, the user agent selects the profiled
representation that best meets its preferences on the basis of a list
of possible representations it obtains from the server. A server
that supports reactive profile negotiation MUST provide such a list
of supported profiled representation as a set of links in the "Link"
header. Each of these links:
o SHOULD have the "alternate" relation type.
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o MUST use the "formats" link hint to convey the profile URI of the
profile to which the resource that is the link target conforms.
o SHOULD use the "allow" link hint to convey the HTTP methods that
are supported by the resource that is the link target.
Figure 10 shows a user agent issuing a HTTP HEAD on a resource in
order to determine whether profiled representations are available for
it. Figure 11 shows the response of a server that supports reactive
profile negotiation. By means of "alternate" links in the "Link"
header, the server indicates support for two profiled representations
for the resource at hand, and, for each, indicates the URI at which
they can be accessed, as well as their respective profile URIs, media
types, and supported HTTP methods. On the basis of this response,
the client can decide whether any of the linked resources conform to
a preferred profile, and, if so, access the respective link target.
Figure 12 shows the response to an HTTP HEAD issued on the link
target <http://example.org/bibrecord/1/DC> of the first "alternate"
link.
HEAD /bibrecord/1 HTTP/1.1
Host: example.org
Accept: application/xml
Connection: close
Figure 10: Client determines support for profiles
HTTP/1.1 200 OK
Content-Type: text/plain
Link: <http://example.org/bibrecord/1/DC>
; rel="alternate"
; type="application/xml"
; formats="http://purl.org/dc/terms/"
; allow="HEAD,GET,PATCH" ,
<http://example.org/bibrecord/1/BIBFRAME>
; rel="alternate"
; formats="http://id.loc.gov/ontologies/bibframe/"
; allow="HEAD,GET"
Content-Length: 200
Connection: close
Figure 11: Server supports two profiles
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HTTP/1.1 200 OK
Content-Type: application/xml
Link: <http://purl.org/dc/terms/>
; rel="profile" ,
<http://example.org/bibrecord/1/BIBFRAME>
; rel="alternate"
; formats="http://id.loc.gov/ontologies/bibframe/"
; allow="HEAD,GET"
Allow: HEAD, GET, PATCH
Accept-Patch: application/xml-patch+xml
Content-Length: 458
Connection: close
Figure 12: Response to a client accessesing a profiled representation
4.2. Accept-Profile HTTP Header Syntax
Figure 13 describes the syntax of the "Accept-Profile" HTTP header,
using the grammar defined in RFC 5234 [RFC5234] and the rules defined
in Section 3.2 of RFC 7230 [RFC7230]. The definitions of "URI-
reference" and "weight" are imported from RFC 7230 [RFC7230] and RFC
7231 [RFC7231], respectively.
Accept-Profile = "Accept-Profile" ":"
OWS (accept-value) *(OWS "," OWS accept-value) OWS
accept-value = "<" URI-reference ">" [weight] | accept-value-ext
Figure 13: ABNF for the "Accept-Profile" HTTP header
5. Writing Profiled Representations
A user agent that wants to submit a profiled representation to a
server can use the reactive negotiation approach to determine the
nature of a server's support with this regard.
A server that allows user agents to submit profiled representations
SHOULD follow the directions for reactive negotiation described in
Section 4.1.2.
A client that submits a representation that complies to a profile
that was not advertised by the server by means of the reactive
negotiation approach, SHOULD assume that the server is not able to
process it.
A server that fails a submission request due to receiving a payload
with a profile that it does not support MUST respond with a "422
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Unprocessable Entity" HTTP status code and SHOULD use the approach
described in Section 4.1.2 to convey profiles that are supported.
Continuing from Figure 12, Figure 14 shows a user agent issuing a
HTTP PATCH against resource <http://example.org/bibrecord/1/DC> in
order to update it. It uses the "Content-Encoding" header to convey
the XML Patch media type of its message body and a link with a
"profile" relation type in the "Link" header to indicate the profile
to which it conforms. Figure 15 shows the server's response,
indicating that the patch was applied successfully; the server
returns the updated representation as message body.
PATCH /bibrecord/1/DC HTTP/1.1
Host: example.org
Content-Encoding: application/xml-patch+xml
Link: <http://purl.org/dc/terms/>
; rel="profile"
Accept: application/xml
Content-Length: 321
Connection: close
...
Figure 14: Client submits profiled representation
HTTP/1.1 200 OK
Content-Type: application/xml
Content-Location: http://example.org/bibrecord/1/DC
Link: <http://example.org/bibrecord/1/DC>
; rel=profile" ,
<http://example.org/bibrecord/1/BIBFRAME>
; rel="alternate"
; formats="http://id.loc.gov/ontologies/bibframe/"
; allow="HEAD,GET"
Allow: HEAD, GET, PATCH
Accept-Patch: application/xml-patch+xml
Content-Length: 592
Connection: close
...
Figure 15: Server indicates successful submission of profiled
representation by client
If, in Figure 14, the user agent would have issued an HTTP PATCH on
resource <http://example.org/bibrecord/1/DC> indicating that the
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profile URI of the patch was <http://id.loc.gov/ontologies/
bibframe/>, the response would have a 422 HTTP status code to express
that the profile is not supported by the resource at hand. Such a
response is shown in Figure 16.
HTTP/1.1 422 Unprocessable Entity
Content-Type: text/plain
Link: <http://example.org/bibrecord/1/DC>
; rel="alternate"
; type="application/xml"
; formats="http://purl.org/dc/terms/"
; allow="HEAD,GET,PATCH"
Content-Length: 110
Connection: close
...
Figure 16: Server indicates unsuccessful submission of profiled
representation by client
6. IANA Considerations
This memo requires IANA to register the Accept-Profile HTTP header
defined in Section 4.2 in the appropriate IANA registry:
o Header Field Name: Accept-Profile
o Applicable Protocol: Hypertext Transfer Protocol (HTTP)
o Status: Informational
o Author/Change controller: IETF
o Specification document(s): this document
7. Security Considerations
8. Acknowledgments
Many thanks to our colleagues for feedback: Enno Meijers, Michael L.
Nelson
9. References
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9.1. Normative References
[I-D.nottingham-link-hint]
Nottingham, M., "HTTP Link Hints", draft-nottingham-link-
hint-02 (work in progress), March 2020.
[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>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[RFC6906] Wilde, E., "The 'profile' Link Relation Type", RFC 6906,
DOI 10.17487/RFC6906, March 2013,
<https://www.rfc-editor.org/info/rfc6906>.
[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>.
9.2. Informative References
[HeeryAndPatel]
Heery, R. and M. Patel, "Application Profiles: Mixing and
Matching Metadata Schemas", 2000,
<http://www.ariadne.ac.uk/issue25/app-profiles>.
[RFC4452] Van de Sompel, H., Hammond, T., Neylon, E., and S. Weibel,
"The "info" URI Scheme for Information Assets with
Identifiers in Public Namespaces", RFC 4452,
DOI 10.17487/RFC4452, April 2006,
<https://www.rfc-editor.org/info/rfc4452>.
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[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
DOI 10.17487/RFC5988, October 2010,
<https://www.rfc-editor.org/info/rfc5988>.
[RFC8141] Saint-Andre, P. and J. Klensin, "Uniform Resource Names
(URNs)", RFC 8141, DOI 10.17487/RFC8141, April 2017,
<https://www.rfc-editor.org/info/rfc8141>.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/info/rfc8288>.
[W3C.REC-shacl-20170720]
Knublauch, H. and D. Kontokostas, "Shapes Constraint
Language (SHACL)", World Wide Web Consortium
Recommendation REC-shacl-20170720, July 2017,
<https://www.w3.org/TR/2017/REC-shacl-20170720>.
[W3C.REC-xmlschema11-1-20120405]
Gao, S., Sperberg-McQueen, M., Thompson, H., Mendelsohn,
N., Beech, D., and M. Maloney, "W3C XML Schema Definition
Language (XSD) 1.1 Part 1: Structures", World Wide Web
Consortium Recommendation REC-xmlschema11-1-20120405,
April 2012,
<https://www.w3.org/TR/2012/REC-xmlschema11-1-20120405>.
Authors' Addresses
Lars G. Svensson
Email: lars.svensson@web.de
URI: https://orcid.org/0000-0002-8714-9718
Ruben Verborgh
Ghent University - imec
Sint-Pietersnieuwstraat 41
Ghent 9000
Belgium
Phone: +32 9 331 49 10
Email: ruben.verborgh@ugent.be
URI: https://ruben.verborgh.org/
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Herbert Van de Sompel
Data Archiving and Networked Services
Anna van Saksenlaan 51
The Hague 2593 HW
Netherlands
Email: herbert.van.de.sompel@dans.knaw.nl
URI: https://orcid.org/0000-0002-0715-6126
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