Priority Placeholders in HTTP/2
draft-bishop-httpbis-priority-placeholder-00

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HTTPbis                                                        M. Bishop
Internet-Draft                                                 Microsoft
Intended status: Standards Track                           July 25, 2017
Expires: January 26, 2018

                    Priority Placeholders in HTTP/2
              draft-bishop-httpbis-priority-placeholder-00

Abstract

   [RFC7540] defines HTTP/2, including a method for communicating
   priorities.  Some implementations have begun using closed streams as
   placeholders when constructing their priority tree, but this has
   unbounded state commitments and interacts poorly with HTTP/QUIC
   ([I-D.ietf-quic-http]).  This document proposes an extension to the
   HTTP/2 priority scheme for both protocols.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on January 26, 2018.

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

   This document is subject to BCP 78 and the IETF Trust's Legal
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   include Simplified BSD License text as described in Section 4.e of

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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.  The Priority Placeholder Extension  . . . . . . . . . . . . .   3
     2.1.  Priority Placeholder Setting  . . . . . . . . . . . . . .   3
       2.1.1.  Mid-session updates . . . . . . . . . . . . . . . . .   3
     2.2.  Frame Modifications . . . . . . . . . . . . . . . . . . .   3
       2.2.1.  Existing Frame Types  . . . . . . . . . . . . . . . .   4
       2.2.2.  PLACEHOLDER_PRIORITY Frame  . . . . . . . . . . . . .   4
     2.3.  Priority Management Logic . . . . . . . . . . . . . . . .   5
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  SETTINGS_PLACEHOLDERS Setting . . . . . . . . . . . . . .   6
     4.2.  PLACEHOLDER_PRIORITY Frame  . . . . . . . . . . . . . . .   6
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Stream Priority is described in [RFC7540], Section 5.3.  Priority is
   communicated using PRIORITY frames and with reference to other
   streams, with stream 0 being the root of the tree.  Each stream
   depends on one other stream with a particular weight; these weights
   represent relative priorities among the multiple children of a
   stream.

   Unfortunately, the scheme as specified encourages servers to actively
   maintain closed streams in the priority tree, since other streams
   might reference them later.  This produces an unbounded state
   commitment on the part of the server if it is to correctly reflect
   any possible reference the client might make.  While priorities are
   only advisory and the server is free to discard as much state as it
   needs to, references to streams which are no longer in the server's
   state are treated as references to the root of the tree.  This can
   result in wildly different conceptions of the priority tree between
   client and server, a situation which all parties would prefer to
   avoid.

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1.1.  Notational Conventions

   The words "MUST", "MUST NOT", "SHOULD", and "MAY" are used in this
   document.  It's not shouting; when they are capitalized, they have
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