Internet DRAFT - draft-pot-prefer-push
draft-pot-prefer-push
Network Working Group E. Pot
Internet-Draft May 02, 2019
Intended status: Standards Track
Expires: November 3, 2019
HTTP-client suggested Push Preference
draft-pot-prefer-push-01
Abstract
"Prefer-Push" is a HTTP header that a client may use to request that
a server uses HTTP/2 Push to send related resources as identified by
their link relationships.
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1. Introduction
HTTP/2 [RFC7540] allows a server to push request and response pairs
to HTTP clients. This can save round-trips between server and client
and reduces the total time required for a client to retrieve all
requested resources.
This mechanism is completely controlled by the server, and it is up
to implementors of services to anticipate what resources a client
might need next.
This specification defines a new HTTP header that allows a client to
inform a server of resources they will require next based on a link
relation type [RFC8288].
2. Rationale
Many HTTP-based services provide some mechanism to embed the HTTP
response bodies of resources into other HTTP resource. A common
example of this is when a resource is structured as a "collection of
resources". Examples of this include:
o The Atom Syndication Format [RFC4287] that encodes "ATOM:entry"
XML elements for each subordinate.
o The [HAL] format, which provides an "_embedded" element to
embedding bodies of resources in other resources.
o The [JSON-API] format, which provides a "included" property to
embed resources.
Embedding resource responses in other resources has two major
peformance advantages:
1. It reduces the number of roundtrips. A client can make a single
HTTP request and get many responses.
2. Generating a related set of resources can often be implemented on
a server to be less time consuming than generating each response
individually.
These mechanisms also pose an issue. To HTTP clients and
intermediaries such as proxies and caches resources are opaque. They
are not aware of a concept of embedded resources.
One example where this might fail is if a client recieves a resource,
embedded in another resource, a cache might not be aware of this
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resource and serve a stale, older version when this resource is
requesed directly.
To keep the performance advantage of being able to generate a related
set of HTTP responses together, HTTP/2 Push could be an alternative
to embedding.
HTTP/2 Push allows the server to initiate a request and response pair
and send them to the client early if the server thinks it will need
them. Another advantage of HTTP/2 push over embedding is that it
allows resources of mixed mediatypes to be pushed.
Servers can however not always anticipate which resources a client
might want pushed. To avoid guessing, this specification introduces
a "Prefer-Push" header that allows a client to inform a server which
resources they will need next.
In many REST apis, sub-ordiniate or embedded resources are identified
by their link relation. By using the link relation, it will be
possible for a client to indicate to a server which links they intent
to follow, allowing a server to only push the resources that the
client knows it will need.
3. The header format
This format should the "List" Data Type from the Structured Headers
specification [I-D.ietf-httpbis-header-structure].
GET /articles HTTP/1.1
Prefer-Push: item, author, "https://example.org/custom-rel"
Each item in the list is a link relationship, as described in Web
Links [RFC8288].
4. Handling a Prefer-Push request
When a server receives the "Prefer-Push" header, it can choose to
push the related resources. It's up to the discretion of the
implementor to decide which resources to push. A server is also free
to ignore push-requests.
If a server chooses to act on an item in the "Prefer-Push" list, the
Link Relationship should exist at the target resource. This
specification does not require that the link relationships get
returned as HTTP "Link" headers. The "Link" may be defined as
"<link>" HTML element, or as a JSON property. How the link is
serialized is dependent on the media type.
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5. Using with "preload" relationship types
[W3C.CR-preload-20171026] defines a "preload" relationship type.
This relationship type can be used by an origin to inform a client or
intermediate to start fetching a resource, or a proxy to initiate a
HTTP/2 push.
A distinct difference between "preload" and "Prefer-Push" is that
"preload" can be used by origin servers to inform clients and
intermediates to fetch and potentially push resources optimistically,
but fundamentally "Prefer-Push" is a completely client-driven
mechanism.
These features can co-exist, but a wide adoption of client-driven
suggestions for pushes might eventually make "preload" unnecceary as
in most cases clients will have a better knowledge of the resources
they need.
6. Security considerations
The Prefer-Push mechanism can potentially result in a large number of
resources being pushed. This can result in a Denial-of-Service
attack.
A server must set reasonable restrictions around the number of pushed
resources.
7. IANA considerations
This document defines the "Prefer-Push" HTTP request fields and
registers them in the Permanent Message Header Fields registry.
7.1. Prefer-Push
o Header field name: Prefer-Push
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: IETF
o Specification document(s): Section 7.1 of this document
o Related information: for Client Hints
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8. Acknowledgements
9. References
9.1. Normative References
[I-D.ietf-httpbis-header-structure]
Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
draft-ietf-httpbis-header-structure-10 (work in progress),
April 2019.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/info/rfc8288>.
[W3C.CR-preload-20171026]
Grigorik, I. and Y. Weiss, "Preload", World Wide Web
Consortium CR CR-preload-20171026, October 2017,
<https://www.w3.org/TR/2017/CR-preload-20171026>.
9.2. Informative References
[HAL] Kelly, M., "JSON Hypertext Application Language", June
2012,
<https://tools.ietf.org/html/draft-kelly-json-hal-00>.
[JSON-API]
"JSON:API", n.d., <https://jsonapi.org/format/>.
[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
December 2005, <https://www.rfc-editor.org/info/rfc4287>.
Appendix A. Example
A server serves a document with a JSON-based media-type. The
following example document might represent a list of articles:
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HTTP/1.1 200 OK
Content-Type: application/vnd.example.links+json
{
"links": [
{ "rel": "item", "href": "/article/1" },
{ "rel": "item", "href": "/article/2" },
{ "rel": "item", "href": "/article/3" },
{ "rel": "item", "href": "/article/4" },
{ "rel": "item", "href": "/article/5" }
]
"total" : 5,
}
A "Prefer-Push"-enabled client knows it will want to receive the full
representations of all articles. When the client receives the list
of articles via a "GET" request, it can indicate this preference with
the "Prefer-Push" header:
GET /article HTTP/1.1
Accept: application/vnd.example.links+json
Prefer-Push: item
Upon recieving this request, server may immediately generate the
request and response pairs for every "item" link in the collection
and initiate push streams for each.
Appendix B. Changelog
B.1. Changes since -00
o Added an abstract
o Updated rationale section significantly.
Author's Address
Evert Pot
Email: me@evertpot.com
URI: https://evertpot.com/
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