The Priority HTTP Header Field
draft-kazuho-httpbis-priority-02

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HTTP                                                              K. Oku
Internet-Draft                                                    Fastly
Intended status: Standards Track                               L. Pardue
Expires: January 24, 2020                                     Cloudflare
                                                           July 23, 2019

                     The Priority HTTP Header Field
                    draft-kazuho-httpbis-priority-02

Abstract

   This document describes the Priority HTTP header field.  This header
   field can be used by endpoints to specify the absolute precedence of
   an HTTP response in an HTTP-version-independent way.

Status of This Memo

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   This Internet-Draft will expire on January 24, 2020.

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   Copyright (c) 2019 IETF Trust and the persons identified as the
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Oku & Pardue            Expires January 24, 2020                [Page 1]
Internet-Draft       The Priority HTTP Header Field            July 2019

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  The Priority HTTP Header Field  . . . . . . . . . . . . . . .   4
     2.1.  urgency . . . . . . . . . . . . . . . . . . . . . . . . .   4
       2.1.1.  prerequisite  . . . . . . . . . . . . . . . . . . . .   5
       2.1.2.  default . . . . . . . . . . . . . . . . . . . . . . .   5
       2.1.3.  supplementary . . . . . . . . . . . . . . . . . . . .   5
       2.1.4.  background  . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  progressive . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Merging Client- and Server-Driven Parameters  . . . . . . . .   7
   4.  Coexistence with HTTP/2 Priorities  . . . . . . . . . . . . .   7
     4.1.  The SETTINGS_HEADER_BASED_PRIORITY SETTINGS Parameter . .   8
   5.  Considerations  . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Why use an End-to-End Header Field? . . . . . . . . . . .   8
     5.2.  Why do Urgencies Have Meanings? . . . . . . . . . . . . .   9
     5.3.  Reprioritization  . . . . . . . . . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  12
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .  12
     B.1.  Since draft-kazuho-httpbis-priority-01  . . . . . . . . .  12
     B.2.  Since draft-kazuho-httpbis-priority-00  . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   It is common for an HTTP ([RFC7230]) resource representation to have
   relationships to one or more other resources.  Clients will often
   discover these relationships while processing a retrieved
   representation, leading to further retrieval requests.  Meanwhile,
   the nature of the relationship determines whether the client is
   blocked from continuing to process locally available resources.  For
   example, visual rendering of an HTML document could be blocked by the
   retrieval of a CSS file that the document refers to.  In contrast,
   inline images do not block rendering and get drawn progressively as
   the chunks of the images arrive.

   To provide meaningful representation of a document at the earliest
   moment, it is important for an HTTP server to prioritize the HTTP
   responses, or the chunks of those HTTP responses, that it sends.
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