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Network Working Group                                             E. Pot
Internet-Draft                                         December 02, 2018
Intended status: Standards Track
Expires: June 5, 2019

                 HTTP-client suggested Push Preference



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   This Internet-Draft will expire on June 5, 2019.

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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 that generating each response

   These mechanism also poses an issue.  HTTP clients and intermediaries
   are not aware of these embedded resources, because there was never a
   true HTTP request.

   By leveraging HTTP/2 push instead of format-specific embedding
   mechanisms, it's possible for services to push subordinate resources
   as soon as possible, generate HTTP responses as a "set" all while

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   still taking advantage of existing HTTP infrastructure.  Another
   advantage of HTTP/2 push over embedding it that it allows resources
   of mixed mediatypes to be pushed.

   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 uses the "List" format from the Structured Headers
   format [I-D.ietf-httpbis-header-structure].

   GET /articles HTTP/1.1
   Prefer-Push: item, author, "https://example.org/custom-rel"

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.

   [RFC8288] defines Web Links as an abstract concept that can be
   specified in a variety of ways.  It defines the HTTP "Link" header as
   a specific serialization.  Like [RFC8288], this specification is not
   dependent on the serialization of the Web Link.

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

   As such, these features can co-exist.

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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 amount of pushes
   it sends.  In the case of N-Depth pushes, servers SHOULD also set
   restrictions around the depth it supports.

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

8.  Acknowledgements

9.  References

9.1.  Normative References

              Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
              draft-ietf-httpbis-header-structure-09 (work in progress),
              December 2018.

   [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,

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              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:

   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

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   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.

Author's Address

   Evert Pot

   Email: me@evertpot.com
   URI:   https://evertpot.com/

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