An HTTP Status Code for Indicating Hints
draft-ietf-httpbis-early-hints-02

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HTTP Working Group                                                K. Oku
Internet-Draft                                            DeNA Co., Ltd.
Intended status: Experimental                               May 16, 2017
Expires: November 17, 2017

                An HTTP Status Code for Indicating Hints
                   draft-ietf-httpbis-early-hints-02

Abstract

   This memo introduces an informational HTTP status code that can be
   used to convey hints that help a client make preparations for
   processing the final response.

Note to Readers

   Discussion of this draft takes place on the HTTP working group
   mailing list (ietf-http-wg@w3.org), which is archived at
   https://lists.w3.org/Archives/Public/ietf-http-wg/ .

   Working Group information can be found at https://httpwg.github.io/ ;
   source code and issues list for this draft can be found at
   https://github.com/httpwg/http-extensions/labels/early-hints .

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 17, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  103 Early Hints . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Changes . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Since draft-ietf-httpbis-early-hints-01 . . . . . . . . .   4
     6.2.  Since draft-ietf-httpbis-early-hints-00 . . . . . . . . .   4
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   It is common for HTTP responses to contain links to external
   resources that need to be fetched prior to their use; for example,
   rendering HTML by a Web browser.  Having such links available to the
   client as early as possible helps to minimize perceived latency.

   The "preload" ([Preload]) link relation can be used to convey such
   links in the Link header field of an HTTP response.  However, it is
   not always possible for an origin server to generate a response
   header block immediately after receiving a request.  For example, the
   origin server might need to query a database before generating a
   response, or it might delegate a request to an upstream HTTP server
   running at a distant location.

   The dilemma here is that even though it is preferable for an origin
   server to send some headers as soon as it receives a request, it
   cannot do so until the status code and the full headers of the final
   HTTP response are determined.

   HTTP/2 ([RFC7540]) server push can be used as a solution to this
   issue, but has its own limitations.  The responses that can be pushed

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   using HTTP/2 are limited to those belonging to the same origin.
   Also, it is impossible to send only the links using server push.
   Finally, sending HTTP responses for every resource is an inefficient
   way of using bandwidth, especially when a caching server exists as an