HTTP                                                              K. Oku
Internet-Draft                                                    Fastly
Intended status: Standards Track                               L. Pardue
Expires: January 22, 2020                                     Cloudflare
                                                           July 21, 2019

                     The Priority HTTP Header Field


   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

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

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   This Internet-Draft will expire on January 22, 2020.

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

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   ( in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

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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
   5.  Considerations  . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Why use an End-to-End Header Field? . . . . . . . . . . .   8
     5.2.  Why do Urgencies Have Meanings? . . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  11
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .  11
     B.1.  Since draft-kazuho-httpbis-priority-00  . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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.

   HTTP/2 ([RFC7540]) provides such a prioritization scheme.  A client
   sends a series of PRIORITY frames to communicate to the server a
   "priority tree"; this represents the client's preferred ordering and

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   weighted distribution of the bandwidth among the HTTP responses.
   However, the design has shortcomings:

   o  Its complexity has led to varying levels of support by HTTP/2
      clients and servers.

   o  It is hard to coordinate with server-driven prioritization.  For
      example, a server, with knowledge of the document structure, might
      want to prioritize the delivery of images that are critical to
      user experience above other images, but below the CSS files.  But
      with the HTTP/2 prioritization scheme, it is impossible for the
      server to determine how such images should be prioritized against
      other responses that use the client-driven prioritization tree,
      because every client builds the HTTP/2 prioritization tree in a
      different way.

   o  It does not define a method that can be used by a server to
      express the priority of a response.  Without such a method,
      intermediaries cannot coordinate client-driven and server-driven

   o  The design cannot be ported cleanly to HTTP/3
      ([I-D.ietf-quic-http]).  One of the primary goals of HTTP/3 is to
      minimize head-of-line blocking.  Transmitting the evolving
      representation of a "prioritization tree" from the client to the
      server requires head-of-line blocking.

   Based on these observations, this document defines the Priority HTTP
   header field that can be used by both the client and the server to
   specify the precedence of HTTP responses in a standardized,
   extensible, protocol-version- independent, end-to-end format.  This
   header-based prioritization scheme can act as a substitute for the
   HTTP/2 frame-based prioritization scheme (see Section 4).

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   The terms sh-token and sh-boolean are imported from

   Example HTTP requests and responses use the HTTP/2-style formatting
   from [RFC7540].

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2.  The Priority HTTP Header Field

   The Priority HTTP header field can appear in requests and responses.
   A client uses it to specify the priority of the response.  A server
   uses it to inform the client that the priority was overwritten.  An
   intermediary can use the Priority information from client requests
   and server responses to correct or amend the precedence to suit it
   (see Section 3).

   The value of the Priority header field is a Structured Headers
   [I-D.ietf-httpbis-header-structure] Dictionary.  Each dictionary
   member represents a parameter of the Priority header field.  This
   document defines the "urgency" and "progressive" parameters.  Values
   of these parameters MUST always be present.  When any of the defined
   parameters are omitted, or if the Priority header field is not used,
   their default values SHOULD be applied.

   Unknown parameters MUST be ignored.

2.1.  urgency

   The "urgency" parameter takes an integer between -1 and 6 as shown

            |         Urgency | Definition                    |
            |              -1 | prerequisite (Section 2.1.1)  |
            |               0 | default (Section 2.1.2)       |
            | between 1 and 5 | supplementary (Section 2.1.3) |
            |               6 | background (Section 2.1.4)    |

                            Table 1: Urgencies

   The value is encoded as an sh-integer.  The default value is zero.

   A server SHOULD transmit HTTP responses in the order of their urgency
   values.  The lower the value, the higher the precedence.

   The following example shows a request for a CSS file with the urgency
   set to "-1":

   :method = GET
   :scheme = https
   :authority =
   :path = /style.css
   priority = urgency=-1

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   The definition of the urgencies and their expected use-case are
   described below.  Endpoints SHOULD respect the definition of the
   values when assigning urgencies.

2.1.1.  prerequisite

   The prerequisite urgency (value -1) indicates that the response
   prevents other responses with an urgency of prerequisite or default
   from being used.

   For example, use of an external stylesheet can block a web browser
   from rendering the HTML.  In such case, the stylesheet is given the
   prerequisite urgency.

2.1.2.  default

   The default urgency (value 0) indicates a response that is to be used
   as it is delivered to the client, but one that does not block other
   responses from being used.

   For example, when a user using a web browser navigates to a new HTML
   document, the request for that HTML is given the default urgency.
   When that HTML document uses a custom font, the request for that
   custom font SHOULD also be given the default urgency.  This is
   because the availability of the custom font is likely a precondition
   for the user to use that portion of the HTML document, which is to be
   rendered by that font.

2.1.3.  supplementary

   The supplementary urgency indicates a response that is helpful to the
   client using a composition of responses, even though the response
   itself is not mandatory for using those responses.

   For example, inline images (i.e., images being fetched and displayed
   as part of the document) are visually important elements of an HTML
   document.  As such, users will typically not be prevented from using
   the document, at least to some degree, before any or all of these
   images are loaded.  Display of those images are thus considered to be
   an improvement for visual clients rather than a prerequisite for all
   user agents.  Therefore, such images will be given the supplementary

   Values between 1 and 5 are used to represent this urgency, to provide
   flexibility to the endpoints for giving some responses more or less
   precedence than others that belong to the supplementary group.
   Section 3 explains how these values might be used.

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   Clients SHOULD NOT use values 1 and 5.  Servers MAY use these values
   to prioritize a response above or below other supplementary

   Clients MAY use values 2 to indicate that a request is given
   relatively high priority, or 4 to indicate relatively low priority,
   within the supplementary urgency group.

   For example, an image certain to be visible at the top of the page,
   might be assigned a value of 2 instead of 3, as it will have a high
   visual impact for the user.  Conversely, an asynchronously loaded
   JavaScript file might be assigned an urgency value of 4, as it is
   less likely to have a visual impact.

   When none of the considerations above is applicable, the value of 3
   SHOULD be used.

2.1.4.  background

   The background urgency (value 6) is used for responses of which the
   delivery can be postponed without having an impact on using other

   As an example, the download of a large file in a web browser would be
   assigned the background urgency so it would not impact further page
   loads on the same connection.

2.2.  progressive

   The "progressive" parameter takes an sh-boolean as the value that
   indicates if a response can be processed progressively, i.e. provide
   some meaningful output as chunks of the response arrive.

   The default value of the "progressive" parameter is "0".

   A server SHOULD distribute the bandwidth of a connection between
   progressive responses that share the same urgency.

   A server SHOULD transmit non-progressive responses one by one,
   preferably in the order the requests were generated.  Doing so
   maximizes the chance of the client making progress in using the
   composition of the HTTP responses at the earliest moment.

   The following example shows a request for a JPEG file with the
   urgency parameter set to "3" and the progressive parameter set to

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   :method = GET
   :scheme = https
   :authority =
   :path = /image.jpg
   priority = urgency=3, progressive=?1

3.  Merging Client- and Server-Driven Parameters

   It is not always the case that the client has the best understanding
   of how the HTTP responses deserve to be prioritized.  For example,
   use of an HTML document might depend heavily on one of the inline
   images.  Existence of such dependencies is typically best known to
   the server.

   By using the "Priority" response header, a server can override the
   prioritization hints provided by the client.  When used, the
   parameters found in the response header field overrides those
   specified by the client.

   For example, when the client sends an HTTP request with

   :method = GET
   :scheme = https
   :authority =
   :path = /menu.png
   priority = urgency=3, progressive=?1

   and the origin responds with

   :status = 200
   content-type = image/png
   priority = urgency=1

   the intermediary's understanding of the urgency is promoted from "3"
   to "1", because the server-provided value overrides the value
   provided by the client.  The progressiveness continues to be "1", the
   value specified by the client, as the server did not specify the
   "progressive" parameter.

4.  Coexistence with HTTP/2 Priorities

   Standard HTTP/2 ([RFC7540]) endpoints use frame-based prioritization,
   whereby a client sends priority information in dedicated fields
   present in HEADERS and PRIORITY frames.  A client might instead
   choose to use header-based prioritization as specified in this

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   To improve communication of the client's intended prioritization
   scheme, this document specifies a new HTTP/2 SETTINGS parameter with
   the name "SETTINGS_HEADER_BASED_PRIORITY".  The value of the
   parameter MUST be 0 or 1; the initial value is 0.  Frame-based
   prioritization is respected when the value is 0, or when the server
   does not recognize the setting.

   An HTTP/2 client that uses header-based priority SHOULD send a
   "SETTINGS_HEADER_BASED_PRIORITY" parameter with a value of 1 when
   connecting to a server.

   parameter with a value of 1 for a connection it establishes when, and
   only when, all the requests to be sent over that connection originate
   from a client that utilizes this header-based prioritization scheme.
   Otherwise this settings parameter SHOULD be set to 0.

   A client or intermediary MUST NOT send a
   "SETTINGS_HEADER_BASED_PRIORITY" parameter with the value of 0 after
   previously sending a value of 1.

   A server MUST NOT send a "SETTINGS_HEADER_BASED_PRIORITY" parameter.
   Upon receipt, a client that supports header-based prioritization MUST
   close the connection with a protocol error.  Non-supporting clients
   will ignore this extension element (see [RFC7540], Section 5.5).

5.  Considerations

5.1.  Why use an End-to-End Header Field?

   Contrary to the prioritization scheme of HTTP/2 that uses a hop-by-
   hop frame, the Priority header field is defined as end-to-end.

   The rationale is that the Priority header field transmits how each
   response affects the client's processing of those responses, rather
   than how relatively urgent each response is to others.  The way a
   client processes a response is a property associated to that client
   generating that request.  Not that of an intermediary.  Therefore, it
   is an end-to-end property.  How these end-to-end properties carried
   by the Priority header field affect the prioritization between the
   responses that share a connection is a hop-by-hop issue.

   Having the Priority header field defined as end-to-end is important
   for caching intermediaries.  Such intermediaries can cache the value
   of the Priority header field along with the response, and utilize the
   value of the cached header field when serving the cached response,

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   only because the header field is defined as end-to-end rather than

   It should also be noted that the use of a header field carrying a
   textual value makes the prioritization scheme extensible; see the
   discussion below.

5.2.  Why do Urgencies Have Meanings?

   One of the aims of this specification is to define a mechanism for
   merging client- and server-provided hints for prioritizing the
   responses.  For that to work, each urgency level needs to have a
   well-defined meaning.  As an example, a server can assign the highest
   precedence among the supplementary responses to an HTTP response
   carrying an icon, because the meaning of "urgency=1" is shared among
   the endpoints.

   This specification restricts itself to defining a minimum set of
   urgency levels in order to provide sufficient granularity for
   prioritizing responses for ordinary web browsing, at minimal

   However, that does not mean that the prioritization scheme would
   forever be stuck to the eight levels.  The design provides
   extensibility.  If deemed necessary, it would be possible to
   subdivide any of the eight urgency levels that are currently defined.
   Or, a graphical user-agent could send a "visible" parameter to
   indicate if the resource being requested is within the viewport.

   A server can combine the hints provided in the Priority header field
   with other information in order to improve the prioritization of
   responses.  For example, a server that receives requests for a font
   [RFC8081] and images with the same urgency might give higher
   precedence to the font, so that a visual client can render textual
   information at an early moment.

6.  Security Considerations


7.  IANA Considerations

   This specification registers the following entry in the Permanent
   Message Header Field Names registry established by [RFC3864]:

   Header field name:  Priority

   Applicable protocol:  http

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   Status:  standard

   Author/change controller:  IETF

   Specification document(s):  This document

   Related information:  n/a

   This specification registers the following entry in the HTTP/2
   Settings registry established by [RFC7540]:


   Code:  0xTBD

   Initial value:  0

   Specification:  This document

8.  References

8.1.  Normative References

              Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
              draft-ietf-httpbis-header-structure-11 (work in progress),
              July 2019.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

8.2.  Informative References

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              Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", draft-ietf-quic-http-22 (work in progress),
              July 2019.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,

   [RFC8081]  Lilley, C., "The "font" Top-Level Media Type", RFC 8081,
              DOI 10.17487/RFC8081, February 2017,

8.3.  URIs



Appendix A.  Acknowledgements

   Roy Fielding presented the idea of using a header field for
   representing priorities in
   slides-83-httpbis-5.pdf [1].  In
   prioritization-proposal [2], Patrick Meenan advocates for
   representing the priorities using a tuple of urgency and concurrency.

   Many thanks to Robin Marx, Patrick Meenan and Ian Swett for their

Appendix B.  Change Log

B.1.  Since draft-kazuho-httpbis-priority-00

   o  Expand urgency levels from 3 to 8.

Authors' Addresses

   Kazuho Oku


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   Lucas Pardue


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