Internet DRAFT - draft-wilde-linkset-link-rel
draft-wilde-linkset-link-rel
Network Working Group E. Wilde
Internet-Draft CA Technologies
Intended status: Informational H. Van de Sompel
Expires: December 15, 2017 Los Alamos National Laboratory
June 13, 2017
Linkset: A Link Relation Type for Link Sets
draft-wilde-linkset-link-rel-02
Abstract
This specification defines the "linkset" link relation type that can
be used to link to a resource that provides a set of links.
Note to Readers
Please discuss this draft on the ART mailing list
(<https://www.ietf.org/mailman/listinfo/art>).
Online access to all versions and files is available on GitHub
(<https://github.com/dret/I-D/tree/master/linkset-link-rel>).
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document authors. All rights reserved.
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Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1. Origin Resource . . . . . . . . . . . . . . . . . . . . . 3
2.2. Link Set Resource . . . . . . . . . . . . . . . . . . . . 3
3. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Third-Party Links . . . . . . . . . . . . . . . . . . . . 3
3.2. Large Number of Links . . . . . . . . . . . . . . . . . . 4
4. The "linkset" Relation Type for Linking to Link Sets . . . . 5
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Links Provided in the Header of the Link Set Resource . . 6
5.2. Link Set Serialized as application/link-format+json . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6.1. Link Relation Type: linkset . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Normative References . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
Resources on the Web often convey typed Web Links [RFC5988] as a part
of resource representations, for example, using the <link> element
for HTML representations, or the "Link" header field in HTTP response
headers for representations of any media type. In some cases,
however, providing links by value is impractical or impossible. In
these cases, an approach to provide links by reference (instead of by
value) can solve the problem. This specification defines the
"linkset" relation type that allows to link resources to sets of
links, thereby making it possible to represent links by reference,
and not by value.
2. Terminology
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 section introduces two types of resources involved in providing
links by reference, as well as the link relation type used to
interlink them.
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2.1. Origin Resource
An "origin resource" is a resource that makes links in which it
participates discoverable by providing a typed link that has a "link
set resource" as the target. From the perspective of the origin
resource, the links in the "link set resource" are therefore provided
by reference.
2.2. Link Set Resource
A "link set resource" is a resource - distinct from the origin
resource, and possibly linked to from the origin resource - that
provides one or more links in which the origin resource participates.
Because the link set resource is distinct from the origin resource,
links provided by the link set resource must represent both the
source and target of each link to allow unambiguous interpretation.
3. Scenarios
The following sections outline some scenarios in which it is useful
to have the ability to separate resources and links pertaining to
them.
These are all scenarios in which providing (some) links by reference
is advantageous or necessary to accomplish certain goals. It is
important to keep in mind that even when using the pattern of "links
by reference", it is still possible to provide links by value,
allowing resources to decide and combine which of the two patterns
they would like to use.
3.1. Third-Party Links
In some cases, it is useful that links pertaining to an origin
resource are provided by a server other than the one that hosts the
origin resource. For example, this allows:
o Providing links in which the origin resource is involved not just
as source but also as target.
o Providing links pertaining to the original resource that the
server hosting that resource is not aware of.
o External management of links pertaining to the origin resource in
a special-purpose link management service.
In such cases, a third-party link set resource provides links
pertaining to the origin resource. This link set resource may be
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managed by the same custodian as the origin resource, or by a third
party.
In order for the server hosting the origin resource to provide an up-
to-date and complete set of links for it, it would need to obtain the
links from the link set resource, and embed them in the origin
resource's representations prior to responding to a client. Doing so
would increase latency and load, which may be unnecessary if a client
is not intent on consuming these links. Providing links by
reference, instead of by value, removes the server-to-server
communication and resulting overhead required to obtain links.
Instead, the consumer of the origin resource can decide if they need
the additional links as context for the resource.
3.2. Large Number of Links
When conveying links in the HTTP "Link" header, it is possible for
the size of the HTTP response header to become unpredictable. This
can be the case when links are determined dynamically dependent on a
range of contextual factors. It is possible to statically configure
a web server to correctly handle large HTTP response headers by
specifying an upper boundary for their size. But when the number of
links is unpredictable, estimating a reliable upper boundary is
challenging.
HTTP [RFC7231] defines error codes related to excess communication by
the user agent ("413 Request Entity Too Large" and "414 Request-URI
Too Long"), but no specific error codes are defined to indicate that
a response header exceeds the upper boundary that can be handled by
the server, and thus it has been truncated. As a result,
applications take counter measures aimed at controlling the size of
the HTTP "Link" header, for example by limiting the links they
provide to those with select relation types, thereby limiting the
value of the HTTP "Link" header to clients. Providing links by
reference, instead of by value, overcomes challenges related to the
unpredictable nature of the extent of HTTP "Link" headers.
In more extreme scenarios it is conceivable that the number of links
pertaining to the origin resource becomes so large that the response
from the associated link set resource becomes too large. This could
be the case for highly popular origin resources, when the link set
includes incoming links as well. In such cases, the link set
resource could deliver responses incrementally, for example, using a
paged resource model that clients could consume as required.
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4. The "linkset" Relation Type for Linking to Link Sets
A link with the "linkset" link relation type has as Context IRI the
IRI of an origin resource, and as Target IRI the IRI of an associated
link set resource.
A link with the "linkset" relation type MAY be provided in the header
and/or the body of the origin resource's representation. It may also
be discovered by other means, such as through client-side
information.
More than one link with a "linkset" relation type MAY be provided.
Multiple such links can refer to the same set of links expressed
using different representations, or to different link sets
(potentially provided by different services).
The use of a link with the "linkset" relation type does not preclude
the provision of links with other relation types, i.e. the origin
resource can provide typed links other than a "linkset" link.
Therefore, the effective set of links pertaining to the origin
resource is the union of the links that the resource itself provides,
and of all links in which it participates which are provided by the
link set resources linked from it via "linkset" links.
The link set resource MAY provide the links that pertain to the
origin resource in its HTTP response header and/or body:
o In cases whereby the link set resource provides these links in its
Link HTTP response header, the payload of that header MUST comply
with the syntax defined in Web Linking [RFC5988]. The media type
of the response body is not constrained.
o In cases whereby the link set resource provides these links in its
response body, the body SHOULD allow a client to determine the
source and target of each provided link. The media type of the
response body is otherwise not constrained.
There is no constraint on the Target IRI of a link with the "linkset"
relation type; designing and using these links is left to the
discretion of implementers.
If an origin resource provides a "linkset" link pointing at a link
set resource, and that link set resource provides a "linkset" link in
turn, then this latter link points at links pertaining to the link
set resource. This means that in the context of the latter link, the
link set resource is an origin resource. This also means that
linkset relations are not transitive; it is up to a client to decide
whether they follow "chains" of linkset links or not.
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5. Examples
Sections Section 5.1 and Section 5.2 show examples whereby the link
set resource provides links pertaining to the origin resource, in its
response header and body, respectively.
5.1. Links Provided in the Header of the Link Set Resource
Figure 1 shows a client issuing an HTTP head request against origin
resource http://example.org/resource1.
HEAD /resource1 HTTP/1.1
Host: example.org
Connection: close
Figure 1: Client HTTP HEAD Request
Figure 2 shows the response to the HEAD request of Figure 1. The
response contains a Link header with a link that uses the "linkset"
relation type. It indicates that links pertaining to the origin
resource are provided by link set resource http://example.com/
links?uri=http%3A%2F%2Fexample.org%2Fresource.
HTTP/1.1 200 OK
Date: Mon, 28 Nov 2016 14:37:51 GMT
Server: Apache-Coyote/1.1
Link: <http://example.com/links?uri=http%3A%2F%2Fexample.org%2Fresource>
; rel="linkset"
; type="text/html"
Content-Length: 5214
Content-Type: text/html;charset=utf-8
Connection: close
Figure 2: Response to HTTP HEAD on Origin Resource
While in this example the IRI of the linkset resource uses a pattern
that represents the IRI of the origin resource, this is opaque to the
client, which simply follows the provided linkset IRI when retrieving
the linkset resource.
Figure 3 shows the client issuing an HTTP GET request against the
link set resource provided in Figure 2.
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GET /links?uri=http%3A%2F%2Fexample.org%2Fresource HTTP/1.1
Host: example.com
Connection: close
Figure 3: Client HTTP GET against the Link Set Resource
Figure 4 shows the response headers to the HTTP GET request of
Figure 3. The links pertaining to the origin resource are provided
in the Link response header of the link set resource. As can be
seen, in order to support an unambiguous determination of the Context
IRI of each link, the "anchor" attribute is provided for each link.
Note that most, but not all, links have the origin resource as
Context IRI (anchor).
HTTP/1.1 200 OK
Date: Mon, 28 Nov 2016 14:40:02 GMT
Server: Apache-Coyote/1.1
Link: <http://authors.example.net/johndoe>
; rel="author"
; type="application/rdf+xml"
; anchor="http://example.org/resource1",
<http://authors.example.net/janedoe>
; rel="author"
; type="application/rdf+xml"
; anchor="http://example.org/resource1",
<http://example.org/resource1/items/AF48EF.pdf>
; rel="item"
; type="application/pdf"
; anchor="http://example.org/resource1",
<http://example.org/resource1/items/CB63DA.html>
; rel="item"
; type="text/html"
; anchor="http://example.org/resource1",
<http://example.net/resource41/>
; rel="related"
; type="application/pdf"
; anchor="http://example.org/resource1/items/AF48EF.pdf"
Content-Type: text/html
Content-Length: 3018
Figure 4: Response to HTTP GET against the Link Set Resource
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5.2. Link Set Serialized as application/link-format+json
Figure 5 is an example of a client issuing an HTTP head request
against origin resource http://example.org/article?id=10.1371/
journal.pone.0167475
HEAD article?id=10.1371/journal.pone.0167475 HTTP/1.1
Host: example.org
Connection: close
Figure 5: Client HTTP HEAD Request
Figure 6 shows the response to the HEAD request of Figure 5. The
response contains a Link header with a link that has the "linkset"
relation type. It indicates that links pertaining to the origin
resource are provided by link set resource
http://example.com/links/10.1371/journal.pone.0167475, which provides
a representation with vendor media type application/
vnd.example.org.linkset+json.
HTTP/1.1 200 OK
Date: Mon, 28 Nov 2016 14:37:51 GMT
Server: Apache-Coyote/1.1
Link: <http://example.com/links/10.1371/journal.pone.0167475>
; rel="linkset"
; type="application/vnd.example.org.linkset+json"
Content-Length: 236
Content-Type: text/html;charset=utf-8
Connection: close
Figure 6: Response to HTTP HEAD on Origin Resource
In this example, the IRI of the linkset resource does not directly
represent the IRI of the origin resource anymore. There still is an
association possible through a IRI pattern that is including DOI
information, but as in the example above, the linkset IRI is opaque
to the client which simply accesses the IRI to retrieve the linkset
resource.
Figure 7 shows the client issuing an HTTP GET request against the
link set resource provided in Figure 6.
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GET /links/10.1371/journal.pone.0167475 HTTP/1.1
Host: example.com
Accept: application/vnd.example.org.linkset+json
Connection: close
Figure 7: Client HTTP GET against the Link Set Resource
Figure 8 shows the response headers to the HTTP GET request of
Figure 7. The links pertaining to the origin resource are provided
in the response body of the link set resource and are serialized
according to the vendor media type application/
vnd.example.org.linkset+json.
HTTP/1.1 200 OK
Date: Mon, 28 Nov 2016 14:40:02 GMT
Server: Apache-Coyote/1.1
Content-Type: application/vnd.example.org.linkset+json
Content-Length: 729
[{"anchor":"http://example.org/article?id=10.1371/journal.pone.0167475","href":"http://authors.example.net/johndoe","rel":"author","type":"application/rdf+xml"},
{"anchor":"http://example.org/article?id=10.1371/journal.pone.0167475","href":"http://authors.example.net/janedoe","rel":"author","type":"application/rdf+xml"},
{"anchor":"http://example.org/article?id=10.1371/journal.pone.0167475","href":"http://example.org/resource1/items/AF48EF.pdf","rel":"item","type":},
{"anchor":"http://example.org/article?id=10.1371/journal.pone.0167475","href":,"rel":,"type":"application/pdf"},
{"anchor":"http://example.org/resource1/items/AF48EF.pdf","href":"http://example.net/resource41/","rel":"related","type":"application/pdf"}]
Figure 8: Response to HTTP GET against the Link Set Resource
6. IANA Considerations
The link relation type below has been registered by IANA per
Section 6.2.1 of RFC 5988 [RFC5988]:
6.1. Link Relation Type: linkset
Relation Name: linkset
Description: The Target IRI of a link with the "linkset" relation
type provides a set of links that pertain to the Context IRI of
the link.
Reference: [[ This document ]]
7. Security Considerations
...
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8. 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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
DOI 10.17487/RFC5988, October 2010,
<http://www.rfc-editor.org/info/rfc5988>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<http://www.rfc-editor.org/info/rfc6690>.
[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,
<http://www.rfc-editor.org/info/rfc7231>.
Authors' Addresses
Erik Wilde
CA Technologies
Email: erik.wilde@dret.net
URI: http://dret.net/netdret/
Herbert Van de Sompel
Los Alamos National Laboratory
Email: herbertv@lanl.gov
URI: http://public.lanl.gov/herbertv/
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