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Versions: 00 01                                                         
Network Working Group                                             E. Pot
Internet-Draft                                              May 02, 2019
Intended status: Standards Track
Expires: November 3, 2019

                 HTTP-client suggested Push Preference


   "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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on November 3, 2019.

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   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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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

   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

   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

   A server must set reasonable restrictions around the number of pushed

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

              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,

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,

              Grigorik, I. and Y. Weiss, "Preload", World Wide Web
              Consortium CR CR-preload-20171026, October 2017,

9.2.  Informative References

   [HAL]      Kelly, M., "JSON Hypertext Application Language", June

              "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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